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SYNTHESIS AND SOLUTION PROPERTIES OF LINEAR-
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LESLIE MICHELE PASSENO

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2/05 p;/CIRC/Dale0ue.indd~p.1

 

SYNTHESIS AND SOLUTION PROPERTIES OF DENDRIMERS AND LINEAR-
DENDRIMER DIBLOCK COPOLYMERS

By

Leslie Michele Passeno

A DISSERTATION
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
DOCTOR OF PHILOSOPHY
Chemistry

2006

ABSTRACT

SYNTHESIS AND SOLUTION PROPERTIES OF DENDRIMERS AND LINEAR-
DENDRIMER DIBLOCK COPOLYMERS

By

Leslie Michele Passeno

Dendrimer — linear diblock copolymers have potential applications as molecular
sensor and drug delivery vehicles, among other uses, by manipulation of the dendrimer
block’s size and conformation. We synthesized and investigated the conformation of
these diblocks in solution, with particular emphasis on probing the location of the linear
chain with respect to the dendrimer in a variety of environmental conditions.

Neutron scattering from a poly(styrene)-poly(benzyl ether) (PS PBE) linear-
dendrimer diblock revealed the system underwent a single molecule phase transition. The
dendrimer expelled the linear polymer from its cavities to the surrounding solvent and
changed shape as the molecular weight of the linear polymer increased. This unusual
behavior demonstrates the potential for this class of architecturally asymmetric molecules
to perform as molecular machines. Since we believe it is the relative size of the linear
polymer to the dendron that is responsible for this transition a second system was studied
to test this hypothesis.

We investigated a poly(ethy1ene oxide)-poly(amido amine) (PEG-PAMAM) linear -
dendrimer diblock since the PAMAM dendrimer can undergo a significant size change
that depends on solvent environment and molecular weight (generation number). The
PAMAM dendron was synthesized from a 2-methoxyethylamine core by exhaustive

Michael addition with methyl acrylate followed by arnidation with ethylene-diamine to

create the first generation. These steps were repeated until the desired generation was
obtained. Small angle neutron scattering experiments of PAMAM solutions revealed that
the fourth generation dendrimer had the greatest volume change as expected from a
simple scaling theory. In addition, fluorescence measurements of dansyl-cored PAMAM
di-dendrons reveal a change in morphology at generation 4 from an extended to a
globular morphology. This generation is anticipated to have the most utility as a
molecular machine and was the target for the synthesis of PEG-PAMAM linear
dendrimer diblocks. Dynamic light scattering of the PEO-PAMAM hybrids in water
demonstrates that the hydrodynamic radius (Rh) goes through a transition as the
molecular weight of the PEO chain increases. At low hybrid molecular weights the Rh is
smaller than a PEO chain with analogous molecular weight. When the molecular weight
of the linear PEO chain is increased the Rh of the hybrid is equal to that of an analogous
PEO chain. This transition parallels that of the PS-PBE system, demonstrating that the
relative molecular weight of the linear and dendrimer blocks, along with the dendrimers’
ability to change morphology in response to its surrounding environment, allows the

hybrid systems to change conformation.

ACKNOWLEDGEMENTS

I had a lot of help over the years all of which is very much appreciated. I am very
grateful for Dr. Baker’s patience and guidance, who taught me when I didn’t know I was
learning and helped me think in an innovative way. Thanks to Dr. Mackay who gave me
an appreciation of learning new things, including everything I know about neutron
scattering, and also being there to answer all of my questions. Thanks to my guidance
committee, Dr. Bruening and Dr. Maleczka for helping me throughout this process.

Thanks to all of my group members, Bao, Ying, Ping, Erin V., quei, Feng, DJ,
Sampa, Anish, Krishnan, Erin P., Tiffany, Melissa, Dave and Qin for opening bottles,
showing me how to fix things, answering questions and most importantly making this
fun.

I am very grateful for my family for their support and encouragement. Thank you
mom for answering millions of questions I had growing up. Thanks sister (Liz) for
making me laugh and being my roommate. Thanks Papa and Jojo for always being

supportive. Thanks Tom for the endless patience you have with me.

iv

TABLE OF CONTENTS

LIST OF TABLES ............................................................................................................ vii
LIST OF FIGURES ......................................................................................................... viii
LIST OF SCHEMES ....................................................................................................... xvi
ABBREVIATIONS ........................................................................................................ xvii
CHAPTER ONE: Introduction ........................................................................................... 1
Anatomy of a Dendrimer ................................................................................................ 1
A Brief History of Dendrimers ....................................................................................... 3
Iterative Growth of Dendrimers ...................................................................................... 5
Applications of Dendrimers .......................................................................................... 11
Motivation of work ....................................................................................................... 16
References ..................................................................................................................... 17
CHAPTER TWO: Scattering Theory ............................................................................... 20
Small Angle Neutron Scattering (SANS) ......................................................................... 21
Dynamic Light Scattering ............................................................................................. 28
References ..................................................................................................................... 30

CHAPTER THREE: Conformation of Poly(styrene)-Poly(benzyl ether) Linear-

Dendrimer Diblock Copolymers in Dilute Solution ......................................................... 32
Introduction ................................................................................................................... 32
Experimental ................................................................................................................. 37

Materials. .................................................................................................................. 37
SANS. ....................................................................................................................... 37
Dynamic Light Scattering (DLS). ............................................................................. 40
Results and Discussion ................................................................................................. 41
Conformation of PBE Dendrimer ............................................................................. 41
Conformation of Dendrimer Block ........................................................................... 45
Conformation of Linear Block .................................................................................. 57
Conclusion .................................................................................................................... 63
References ..................................................................................................................... 64

CHAPTER FOUR: Synthesis and Solution Behavior of PAMAM di-dendrons .............. 67
Introduction ................................................................................................................... 67
Experimental ................................................................................................................. 75

Materials. .................................................................................................................. 75
PAMAM Synthesis ................................................................................................... 75
SANS. ....................................................................................................................... 77
Dynamic Light Scattering (DLS). ............................................................................. 79
Results and Discussion ................................................................................................. 80
Synthesis of MBA dendrons ..................................................................................... 80

Size of MBA G3.0-G5.0 Dendrons in D20 ............................................................... 81

Effect of Solvent Quality on the Dimensions of MBA-G40 in Solution ................. 89
Comparison of the Solution Behavior of MBA G40 and MBA G5.0 ...................... 95
Influence of Core-Functionality on the Behavior of PAMAM Dendrimers in
Solution ..................................................................................................................... 97
Effect of Ionic Strength on the Morphology and Solution Behavior of MBA
Dendrons ................................................................................................................. 101
Conclusion .................................................................................................................. 104
References ................................................................................................................... 106
CHAPTER FIVE: Synthesis and Characterization of Dansyl-Cored PAMAM Di- ...... 109
Dendrons ......................................................................................................................... 109
Introduction ................................................................................................................. 109
Experimental ............................................................................................................... 1 16
Materials. ................................................................................................................ 1 16
Dansyl-PAMAM Synthesis .................................................................................... 116
Fluorescence Measurements. .................................................................................. 120
Results and Discussion ............................................................................................... 120
Solvent Dependence of the Emission Spectra of (Dansyl-EDA) (2) ...................... 120
Fluorescence of Dansyl-PAMAM .......................................................................... 124
Conclusion .................................................................................................................. 143
References ................................................................................................................... 145

CHAPTER SIX: Synthesis and Solution Properties of PEG-PAMAM Linear-Dendrimer

Diblock Copolymer ......................................................................................................... 147
Introduction ................................................................................................................. 147
Experimental ............................................................................................................... 152

Materials ................................................................................................................. 152
Synthesis of PEO-NH; (2 kDa) ............................................................................... 153
Synthesis for PEO-NH; (MW = 750 Da, 5 kDa, 9 kDa, 20 kDa) ........................... 154
Dynamic Light Scattering (DLS). ........................................................................... 161
Results and Discussion ............................................................................................... 162
Synthesis of PEO-PAMAM .................................................................................... 162
Dynamic Light Scattering ....................................................................................... 165

Conclusion ...................................................................................................................... 1 76

References ....................................................................................................................... 177

APPENDICIES ............................................................................................................... 179

APPENDIX A ................................................................................................................. 180

APPENDIX B ................................................................................................................. 201

APPENDIX C ................................................................................................................. 221

vi

LIST OF TABLES

Table 3.1. Sample codes and number average molecular mass (M) of compounds used in
this study” .......................................................................................................................... 38

Table 3.2. Values for R8 and Rc determined from Guinier analyses described in the text at
a concentration of 50 mg/mL. ........................................................................................... 43

Table 3.3. Experimental QI(O) values were set equal to the numerical prefactor of the
Guinier expression for a finite sized cone, eq 3.5, with various R values representing the
radius of the cone’s base. .................................................................................................. 57

Table 4.1. Sample codes and theoretical molecular mass (M) of MEA compounds ....... 78

Table 4.2. R8 values for MEA G3.0-G5.0 determined from the Guinier analysis
described in the text at a concentration of 4 mg/mL. ........................................................ 86

Table 4.3. Rg values for MEA 64.0 and G5.0 in D(CD2)mOD determined from the
Guinier analysis described in the text at a concentration of 4 mg/mL .............................. 94

Table 4.4. R8 values for Tetra G4.0 in D20 and d'O-butanol determined from the Guinier
analysis described in the text at a concentration of 4 mg/mL. ........................................ 100

Table 5.1. Theoretical molecular mass (M) of the dansyl-cored PAMAM dendrimers. 114

Table 5.2. Solvents and their dielectric constants (a) used in this study. ....................... 121
Table 5.3. km“ (nm) data for dansyl-PAMAM dendrimers ........................................... 127
Table 6.1. Sample codes and number average molecular mass (M) of compounds used in

this study” ........................................................................................................................ 166
Table 6.2. Hydrodyamic radii (nm) of PEO and PEG-PAMAM samples ...................... 166

vii

LIST OF FIGURES
Figure 1.1. Anatomy of a dendrimer .................................................................................. 1
Figure 1.2. Elements of a generation 2, poly(amido amine) (PAMAM) dendrimer. ......... 2

Figure 1.3. Flory’s branched polymer architecture created by the reaction of AB; type
monomers ............................................................................................................................ 4

Figure 1.4. Example of a divergent synthesis of a dendrimer with a 1—)2 branching
scheme and a tri-fiinctional core. ........................................................................................ 6

Figure 1.5. The synthesis of poly(propylene imine) dendrimers. ....................................... 7

Figure 1.6. Possible defects and imperfections that can result during the divergent
synthesis of a poly(amido amine) dendrimer. ..................................................................... 8

Figure 1.7. Example of a convergent synthesis of a dendrimer with 1—)2 branching
scheme and a di-functional core. ........................................................................................ 9

Figure 1.8. A linear-dendrimer diblock copolymer (hybrid) where a poly(L-lysine) linear
chain is covalently attached to the focal point of a carboxylic acid terminated PAMAM

dendrimer. ......................................................................................................................... 14
Figure 1.9. A dendronized polymer consisting of a fourth generation hydroxyl terminated
polyester dendrimer attached to the repeat unit of poly(L-lysine). ................................... 15
Figure 2.1. Schematic of a small angle neutron scattering experiment. ........................... 22

Figure 2.2. Cartoon representing information that can be obtained from a series of

contrast match experiments ............................................................................................... 26
Figure 2.3. Set-up of a dynamic light scattering experiment. ........................................... 30
Figure 3.1. Three potential conformational states of linear-dendrimer diblocks in solution:
a. knitted coil b. encapsulated dendrimer c. random coil .................................................. 35
Figure 3.2. Guinier analysis for PBE G4 dendrons in d6-benzene and ds-THF at a

concentration of 50 mg/mL, data fit for a compact (sphere-like) object. ......................... 42
Figure 3.3. Guinier analysis for PBE-dPS 20kDa system in dB-THF. .............................. 46

Figure 3.4. Rod-like Guinier analysis for PBE-dPS 45kDa hybrid in dB-THF at a
concentration of 50 mg/mL. .............................................................................................. 48

viii

Figure 3.5. Rod-like Guinier analysis for PBE-dPS IOOkDa hybrid in ds-THF at

SOmg/mL. .......................................................................................................................... 49
Figure 3.6. Holtzer plot (01 vs. Q) of G4-PBE in ails-benzene at 50 mg/mL. ................... 51
Figure 3.7. Holtzer plot (Q1 vs. Q) of G4-PBE in ds-THF at 50 mg/mL .......................... 52
Figure 3.8. Holtzer plot (QI vs. Q) of G4-dPS-20 kDa in dB-THF at 50 mg/mL .............. 53
Figure 3.9. Holtzer (Q1 vs. Q) plot of G4-dPS-45 kDa in (Is-THF at 50 mg/mL .............. 54
Figure 3.10. Holtzer (01 vs. Q) plot of G4-dPS-100 kDa in (IS-THF at 50 mg/mL .......... 55
Figure 3.11. A plot of radius of gyration versus molecular weight for both the dPSblock
of the hybrids and dPS in 79:21 h":aé benzene at 4 mg/mL. ............................................ 59
Figure 3.12. A plot of radius of gyration versus molecular weight for G4-PBE-PS hybrids
and PS in ds-THF at a concentration of 4 mg/mL. ........................................................... 60
Figure 3.13. A plot of hydrodynamic radius versus molecular weight for G4-PBE-PS
hybrids and PS in THF at a concentration of 4 mg/mL. ................................................... 61
Figure 4.1.SANS data for MBA G 3.0 dendrons in D20. ................................................. 82
Figure 4.2. SANS data for MEA G 3.5 dendrons in D20. ................................................ 82
Figure 4.3. SANS data for MBA G 4.0 dendrons in D20. ................................................ 83
Figure 4.4. SANS data for MEA G 4.5 dendrons in D20 ................................................. 83
Figure 4.5. SANS data for MEA G 5.0 dendrons in D20. ............................................... 84
Figure 4.6. R1; and volume of MEA dendrons in D20. .................................................... 85
Figure 4.7. Radius of gyration vs. molecular weight for the MEA dendrons G 3.0—G 5.0 in
D20 at a concentration of 4 mg/mL. ................................................................................. 89
Figure 4.8. SANS of MBA G4.0 dendrons in aid-methanol. ............................................ 91
Figure 4.9. SANS of MBA (34.0 dendrons in id‘s-ethanol ................................................. 91
Figure 4.10. SANS of MEA G4.0 dendrons in dlo-butanol. ............................................. 92
Figure 4.11. R3 of MEA G4 vs. solvent type (m) of D(CD2)m0D. .................................. 92
Figure 4.12. SANS of MBA 05.0 in d'O-butanol ............................................................. 96

ix

Figure 4.13. SANS of Tetra G 4.0 dendrons in D20. ...................................................... 98

Figure 4.14. SANS of Tetra G 4.0 in d'O-butanol. ............................................................ 99
Figure 4.15. Percent mass vs. hydrodynamic radius for a MEA G4.0 dendron in 1 M

HCl. ................................................................................................................................. 102
Figure 4.16. I(Q) vs. Q for MBA G4.0 in D20 at a concentration of 25 mg/mL ........... 103

Figure 4.17. Percent mass vs. hydrodynamic radius of an MEA G4.0 dendron in 1 M HCl
and 0.100 M NaCl at a concentration of 4 mg/mL. ........................................................ 104

Figure 5.1. Normalized fluorescence emission spectra of (2-aminoethyl)-dansylamide. 122

Figure 5.2. Amax vs. dielectric constant for dansyl-EDA. ................................................ 123
Figure 5.3. Normalized emission spectra of dansyl-PAMAM full generations (GI-G5) in
methanol .......................................................................................................................... 125
Figure 5.4. Normalized fluorescence emission spectra of dansyl-PAMAM half
generations in methanol. ................................................................................................. 126
Figure 5.5. km, vs. generation for dansyl-cored PAMAM dendrimer in methanol. ...... 127
Figure 5.6. Normalized fluorescence emission spectra of full generation dansyl-PAMAM
dendrimers in ethanol. ..................................................................................................... 130
Figure 5.7. Normalized fluorescence emission spectra of dansyl-PAMAM half generation
dendrimers in ethanol. ..................................................................................................... 131
Figure 5.8. km“ vs. generation for dansyl-cored PAMAM dendrimer in ethanol. ......... 132
Figure 5.9. Normalized fluorescence emission spectra of dansyl-PAMAM full
generations, G1-G4, in butanol. ...................................................................................... 133
Figure 5.10. Normalized fluorescence emission spectra of dansyl-PAMAM half
generation dendrimers in butanol .................................................................................... 134
Figure 5.11. km, vs. generation for dansyl—cored PAMAM dendrimer in butanol. ....... 135

Figure 5.12. Normalized fluorescence emission spectra of dansyl-PAMAM full
generation dendrimers in chloroform .............................................................................. 136

Figure 5.13. Normalized fluorescence emission spectra of dansyl-PAMAM half
generation dendrimers in chloroform .............................................................................. 137

Figure 5.14. km“ vs. generation for dansyl-cored PAMAM dendrimer in chloroform. 138

Figure 5.15. Normalized fluorescence emission spectra of dansyl-PAMAM full
generations in acetonitrile. .............................................................................................. 140

Figure 5.16. Normalized fluorescence emission spectra of dansyl-PAMAM half
generations in acetonitrile. .............................................................................................. 141

Figure 5.17. 3. max vs. generation for dansyl-cored PAMAM dendrimer in acetonitrile. 142

Figure 6.1. Cartoon showing the conformational changes of a linear-dendrimer diblock
copolymer. ...................................................................................................................... 15 1

Figure 6.2. Dynamic light scattering results for PEO 750 Da-G4 in Milli Q water. ...... 166
Figure 6.3. Dynamic light scattering results for PEO 2 kDa-G4 in Milli Q water. ........ 167
Figure 6.4. Dynamic light scattering results for PEO 5 kDa-G4 in Milli Q water. ........ 167
Figure 6.5. Dynamic light scattering results for PEO 9 kDa-G4 in Milli Q water. ........ 168

Figure 6.6. Dynamic light scattering results for PEO 20 kDa-G4 in Milli Q water. ...... 168

Figure 6.7. Dynamic light scattering results for PEO 2 kDa in Milli Q water. .............. 169
Figure 6.8. Dynamic light scattering results for PEO 5 kDa in Milli Q water. .............. 170
Figure 6.9. Dynamic light scattering results for PEO 8 kDa in Milli Q water. .............. 170
Figure 6.10. Dynamic light scattering results for PEO 20 kDa in Milli Q water. .......... 171
Figure 6.11. Dynamic light scattering results for PEO 36 kDa in Milli Q water. .......... 171
Figure 6.12. Hydrodynamic radius vs. molecular weight of PEO and PEO-PAMAM

hybrids in Milli Q water at concentrations of 10 mg/mL. .............................................. 172
Figure 6.13. Dynamic light scattering results for G4 in Milli Q water ........................... 174
Figure A]. 1H NMR MEA GO.5 .................................................................................... 181
Figure A.2. 13C NMR MEA G05 ................................................................................... 182
Figure A.3. ‘H NMR MEA G1.0 .................................................................................... 183
Figure A.4. 13C NMR MEA (31.0 ................................................................................... 184

xi

Figure A.5. 1H NMR MEA (31.5 .................................................................................... 185

Figure A.6. l3c NMR MEA (31.5 ................................................................................... 186
Figure A.7. ‘H NMR MEA (32.0 .................................................................................... 187
Figure A.8. 13C NMR MEA (32.0 ................................................................................... 188
Figure A.9. 1H NMR MEA (32.5 .................................................................................... 189
Figure A.10. 13C NMR MEA (32.5 ................................................................................. 190
Figure A.11. ‘HNMRMEA (33.0 .................................................................................. 191
Figure A.12. 13C NMR MEA G30 ................................................................................. 192
Figure A.13. 'HNMRMEA (33.5 .................................................................................. 193
Figure A.14. 13C NMR MEA (33.5 ................................................................................. 194
Figure A.15. ‘H NMR MEA (34.0 .................................................................................. 195
Figure A.16. 13C NMR MEA (34.0 ................................................................................. 196
Figure A.17. ‘H NMR MEA (34.5 .................................................................................. 197
Figure A.18. 13C NMR MEA (34.5 ................................................................................. 198
Figure A.19. ‘H NMR MEA (35.0 .................................................................................. 199
Figure A.20. 13c NMR MEA (35.0 ................................................................................. 200
Figure 3.1. 1H NMR Dansyl EDA ................................................................................. 202
Figure 13.2. ‘H NMR Dansyl (30.5 ................................................................................. 203
Figure B.3. 13C NMR Dansyl (30.5 ................................................................................ 204
Figure B.4. ‘H NMR Dansyl 1.0 .................................................................................... 205
Figure 13.5. 13C NMR Dansyl (31.0 ................................................................................ 206
Figure 8.6. 1H NMR Dansyl (31.5 ................................................................................. 207
Figure 13.7. 13c NMR Dansyl (31.5 ................................................................................ 208

xii

Figure B.8. 1H NMR Dansyl G2.0 ................................................................................. 209

Figure 3.9. 13C NMR Dansyl 02.0 ................................................................................ 210
Figure 3.10. ‘H NMR Dansy1G2.5 ............................................................................... 211
Figure 3.11. 13C NMR Dansyl G2.5 .............................................................................. 212
Figure 3.12. ‘H NMR Dansy1G3.0 ............................................................................... 213
Figure 3.13. l3CNMRDausyi G30 .............................................................................. 214
Figure 3.14. 'HNMRDansyl G35 ............................................................................... 215
Figure 3.15. l3CNMRDansy1G3.5 .............................................................................. 216
Figure 3.16. ‘H NMR Dansyl G40 ............................................................................... 217
Figure 3.17. 13C NMR Dansyl G40 .............................................................................. 218
Figure 3.18. ‘H NMR Dansyl G45 ............................................................................... 219
Figure 3.19. 13C NMR Dansyl G45 .............................................................................. 220
Figure C.1. ‘H NMR 330 750 Da 0Ms ........................................................................ 222
Figure C.2. 1H NMR 330 750 Da phth ......................................................................... 223
Figure CB. ‘3 NMR PEO 750 Da NH2 ......................................................................... 224
Figure C.4. 1H NMR PEO 750 G05 .............................................................................. 225
Figure C5. '3 NMR 330 750 Da G10 ........................................................................ 226
Figure C6. ‘3 NMR 330 750 Da G15 ........................................................................ 227
Figure C.7. 1H NMR PEO 750 Da G20 ........................................................................ 228
Figure C8. ‘3 NMR PEO 750 Da G25 ........................................................................ 229
Figure C9. ‘3 NMR PEG 750 Da G30 ........................................................................ 230
Figure cm. ‘H NMR 330 750 Da G35 ...................................................................... 231
Figure C11. ‘H NMR PEG 2 kDa OMs ........................................................................ 232

xiii

Figure C.12.
Figure 013.
Figure C.14.
Figure C.15.
Figure C.16.
Figure C.17.
Figure C.18.
Figure C.19.
Figure C.20.
Figure C.21.
Figure C.22.
Figure C.23.
Figure C.24.
Figure C.25.
Figure C.26.
Figure C.27.
Figure C.28.
Figure C.29.
Figure C.30.
Figure C. 31.
Figure C.32.
Figure C.33.

Figure 034.

‘H NMR PEG 2 kDa phth ......................................................................... 233

‘H NMR PEG 2 kDa NH2 ......................................................................... 234
'HNMRPEO 2 kDa GO.5 ........................................................................ 235
1HNMRFEQ 2 kDa G10 ........................................................................ 236
‘H NMR 330 2 kDa G15 ........................................................................ 237
‘H NMR PEO 2 kDa G20 ........................................................................ 238
1H NMR P30 2 kDa G25 ........................................................................ 239
‘H NMR PEG 2 kDa G30 ........................................................................ 240
1HNMRPEOZkDaG35 ........................................................................ 241
1H NMR PEG 2 kDa G40 ........................................................................ 242
‘H NMR PEG 5 kDa OMs ........................................................................ 243
‘H NMR PEO 5 kDa phth ......................................................................... 244
1H NMR PEG 5 kDa NH2 ......................................................................... 245
‘H NMR 330 5 kDa GO.5 ........................................................................ 246
‘H NMR PEo 5 kDa G10 ........................................................................ 247
lHNMRPEQ 5 kDa G15 ........................................................................ 248
‘H NMR PEG 5 kDa G20 ........................................................................ 249
‘H NMR PEG 5 kDa G25 ........................................................................ 250
‘HNMRPEOSkDaGBD ....................................................................... 251
‘HNMRPEO 5 kDa G35 ....................................................................... 252
1H NMR PEO 5 kDa G40 ........................................................................ 253
1H NMR PEG 9 kDa OMS ........................................................................ 254
‘H NMR PEG 9 kDa phth ......................................................................... 255

xiv

Figure 035.
Figure C.36.
Figure C.37.
Figure C.38.
Figure C.39.
Figure C.40.
Figure C.41.
Figure C.42.
Figure C.43.
Figure C.44.
Figure 045.
Figure C.46.
Figure C.47.
Figure 048.
Figure C.49.
Figure C.50.
Figure C.51.
Figure C.52.

Figure C53.

‘H NMR PEG 9 kDa NH2 ......................................................................... 256
1H NMR PEG 9 kDa G05 ........................................................................ 257
1HNMRPE09kDaG1.O ........................................................................ 258
‘HNMRPE09kDa G15 ........................................................................ 259
1H NMR PEG 9 kDa G20 ........................................................................ 260
‘H NMR 330 9 kDa G25 ........................................................................ 261
‘H NMR PEO 9 kDa G30 ........................................................................ 262
‘H NMR PEG 9 kDa G35 ........................................................................ 263
‘H NMR 330 9 kDa G40 ........................................................................ 264
1H NMR 330 20 kDa 0Ms ...................................................................... 265
‘11 NMR P30 20 kDa NH2 ....................................................................... 266
‘H NMR PEO 20 kDa G05 ...................................................................... 267
‘HNMR PEO 20 kDaG1.0 ...................................................................... 268
‘H NMR 330 20 kDa Gl.5 ...................................................................... 269
‘H NMR PEO 20 kDa G20 ...................................................................... 270
‘H NMR P30 20 kDa G25 ...................................................................... 271
'H NMR PEO 20 kDa G30 ...................................................................... 272
‘H NMR PEO 20 kDa G3.5 ...................................................................... 273
‘H NMR P30 20 kDa G40 ...................................................................... 274

XV

LIST OF SCHEMES

Scheme 3.1. PBE-G4-PS dendrimer-linear diblock copolymer ........................................ 36
Scheme 4.1. Synthesis of MEA-PAMAM dendrons. ....................................................... 73
Scheme 4.2. MEA-G40. ................................................................................................... 74
Scheme 5.1. Synthesis of dansyl-cored PAMAM dendrimer. ........................................ 114
Scheme 5.2. Generation 4 dansyl-cored PAMAM dendrimer. ....................................... 115
Scheme 6.1. Synthesis of methoxy-amine terminated poly(ethy1ene oxide). ................. 163
Scheme 6.2. Synthesis of PEG-PAMAM diblock copolymers ....................................... 164

xvi

dansyl-EDA
DLS
dPS

P(Q)

ABBREVIATIONS

solid angle element defined by the size of a detector pixel
the square of the difference of the solvent and the scatterer SLD
differential scanning cross-section

effective Stokes radius of a sphere

area of the particle

spectral amplitude

background signal

coherent neutron scattering length of nucleus 1'

baseline
correlation function

bulk density of the molecule,

diffusion coefficient

preparation of (2-aminoethyl)-dansylamide

dynamic light scattering

deuterated poly(styrene)

difference in scattering length density of the sample and medium
volume fraction of scattering centers

multiplicity of the branching units

core functionality

detector efficiency

viscosity of the solvent
intensity at zero wave vector

intensity fluctuations

incident flux of neutrons

magnitude of the scattering vector
Boltzmann’s constant

incident wavevector

scattered wavevector

wavelength

rod length

distance between sample and detector in SANS
molecular weight of the molecule
2-methoxyethylarnine

number concentration of scattering centers

Avogadro’s number
form factor

xvii

PAMAM
PBE
PEO

PS

IQI 01' Q

poly(amido amine)

poly(benzyl ether) dendrimer

poly(ethy1ene oxide)

poly(styrene)

modulus of Q, independent variable in SANS
scattering vector

mass fraction of solvent within the scatterer
scattering length density

homogeneous sphere radius
root-mean-square of the distances of all of the atoms in a cross-
section of Rf

radial distance between sample and scattered neutrons
radius of gyration

hydrodynamic radius

visciometric radius

scattering length density of the dendrimer

scattering length density of the scatterers

scattering length density of the solvent

structure factor

small-angle neutron diffractometer

scattering length density

Single Photon Counting Module

neutron transmission of the sample

time for the diffusion step to occur

duration of a dynamic light scattering experiment
absolute temperature

Tetra-functional PAMAM G4.0

tetrahydrofuran
sheet thickness
volume of one scattering center

volume
hard-core volume
volume of each scattering center

van der Waals volume

xviii

CHAPTER ONE: Introduction
Anatomy of a Dendrimer

Dendrimers are globular, nano-sized macromolecules synthesized in a controlled
manner to result in a highly branched structure with three distinct regions: (i) a central
focal point, core, which is a single atom or an atomic group with at least two identical
chemical moieties, (ii) branches emanating from the focal point, consisting of repeat units
that have at least one branch junction or split resulting in a series of radially concentric
layers called generations and (iii) multiple end groups that have identical functionality
(Figure 1.1). A segment of a dendrimer, a dendron, is shown in Figure 1.1 along with the
basic anatomy of a dendrimer, with the parts of the dendrimer labeled. An example of a
second generation poly(amido amine) (PAMAM) dendrimer is displayed in Figure 1.2
where the focal point is tri-functional, a tri-dendron, meaning it has three sites of

reactivity for dendrimer growth.

Dendron

 

Figure 1.1. Anatomy of a dendrimer.

Dendron and dendrimer are represented by the solid lines.
The broken lines beginning from the center of the generation three dendrimer represent
the core, first, second and third generations respectively.

NH: ”ZN / end group

 

Figure 1.2. Elements of a generation 2, poly(amido amine) (PAMAM) dendrimer.

The constituents of generation one are circled.

These three elements of a dendrimer are critical to the morphology of these
molecules in solution, and altering any element has a direct influence on their properties.
For example modifying the functionality of the core, i.e. how many dendrons emanate
from the focal point, directly affects the steric bulk of the growing dendrimer as well the
number of end groups at each generation. The flexibility of the branching units plays a
key role in the dynamics of the dendrimers in solution. Rigid dendrimers have been
synthesized with a highly conjugated branching system and have different properties than
those synthesized with more flexible connecting groups. In addition, the chemical
functionality of the end groups greatly influences the interaction of these molecules both
with themselves and their surrounding medium. The properties of dendrimers are also
controlled by the interaction of the different dendritic elements, i.e. hydrogen bonding.

A Brief History of Dendrimers

The evolution of the construction of macromolecules possessing a branched
architecture has three general eras.1 The first period was from the late 1860’s to the early
1940’s when branched impurities were thought to be the insoluble byproducts formed in
polymerization reactions, however purification and separation techniques were too
primitive at the time to prove the structures of these impurities. In the second period of
dendrimer history, from the early 1940’s to the late 1970’s, branched polymer structures
were considered mostly from the view of polymer physicists who were developing theory
that described the behavior of macromolecules and branched species. The synthesis of
these branched structures was attempted using differentiated monomers in a traditional
one-pot synthesis used for the preparation of linear polymer chains. Flory noted in the

1950’s,2 “The breadth of the distribution coupled with the impossibility of selectively

fractionating ‘branching’ and ‘molecular weight’ separately make this approach
impractical. Attempts to investigate ‘branching by such means consequently have been

notably fruitless.” However, he did examine the scaling properties of branched polymers

where he envisioned a branched system synthesized with an AB2 type monomer resulting

in a l—> 2 branching system (Figure 1.3). It wasn’t until the late 1970’s and early 1980’s
when dendrimers were successfully synthesized using a controlled iterative method of
growth. The first example of this approach was reported by V('5gtel,3 who named the
process a “cascade synthesis.” Soon after, Denkewalter and K0104 published a series of
US patents describing the synthesis of L-lysine-based dendrimers made by an iterative

controlled growth, however little characterization of theses molecules was reported.

Figure 1.3. Flory’s branched polymer architecture created by the reaction of AB2 type
monomers.

Iterative Growth of Dendrimers

Two distinct approaches for iterative grth have been developed for the
synthesis of dendrimers: a divergent approach to make “starburst” and “arboral”
structures by T omalias’ 6 and Newkome7 respectively, and the convergent approach

9

characterized by the synthesis of poly(benzyl ether) dendrons by Fréchet.8’ Later,

'0'” convergently produced phenylacetylene dendrimers. The main

Moore’s group
difference between the two methods is the site of reactivity. For the divergent scheme the
end groups are the reactive species, whereas for the convergent approach, the focal point
is the site of reactivity. Both methods require the same number of theoretical steps to
produce each generation.

The divergent approach (Figure 1.4) begins with a core or focal point and via
sequential monomer addition, extends the branches from the focal point towards the
periphery of the molecule. Since numerous reactions are being done on the same
molecule, each reaction has to be very selective and proceed to high conversion to ensure
the integrity of the final product. For example, there are 64 end groups in a fifth
generation poly(propylene imine) dendrimer requiring 248 reactions to occur on one
molecule to achieve a perfect dendrimer.l3 If the average selectivity of each reaction is
99.5% the yield of defect-free dendrimer produced is only 0.995248 = 29%. Since every
new generation is very difficult to purify, the presence of a small number of statistical
defects cannot be avoided (Figure 1.5). The divergent synthesis of poly(amido amine)
dendrimers may lead to structural defects including ring formation and incomplete

grth of dendritic arms (Figure 1.6) The number of such defects is minimized by

controlling the reaction conditions and stoichiometry.” The purity of the molecules

produced by the divergent method is controlled by statistics and thus it is virtually

impossible to produce perfect dendrimers beyond generation five or six. ‘5

 

o
x H,
)——x
x
core
1. o——->x
o
2. 3—(
o

 

B B
H: D 2 B D >—e—<B DH
G1 DH
D (32

3 o
o—(D o 0
G3

Figure 1.4. Example of a divergent synthesis of a dendrimer with a 1—>2 branching

scheme and a tri-functional core.

NH
1/\CN

NH N

r

2. H2 Raney Co f
NH
H2“ A ’

Figure 1.5. The synthesis of poly(propylene imine) dendrimers.

Product A is the desired first generation and products B and C are unwanted impurities. B
results from incomplete Michael addition or a retro-Michael reaction and C is the result
of an undesired cyclization reaction. Both impurities have properties similar to the
desired product, thus complicating purification.

o 0 H

0 (Y N.
WN/VN OCH3 = N
H H H OCH3

 

OCH3
O

HZN /\/NH2

0
O (Y0 OA/lLN/me
WNNN HN\/\NH2 —_’ H NH
H Room 0

0
Figure 1.6. Possible defects and imperfections that can result during the divergent
synthesis of a poly(amido amine) dendrimer.

Both ring formation and retro-Michael reactions can be avoided by controlling the
reaction conditions.

The convergent approach (Figure 1.7) entails synthesizing the branches of the
dendrimers and attaching them to core and is often called the “outside-in” approach
(Figure 1.3). It begins by attaching two terminal units, each containing one reactive
group, X, to one monomer possessing a protected functionality Z, resulting in the first
generation (G1). The protected focal point, Z, is converted to functionality X, and then
reacted with 0.5 equivalents of the monomer to obtain G2. The reactivity point in the
convergent approach is the focal point of the dendron and thus there is increased steric
crowding at the site of reactivity with each subsequent generation and this results in a
synthesis that is increasingly challenging with increasing generation. Dendrimers
synthesized by the convergent approach can be nearly pure, and mass spectroscopy data

l6, 17

on Fréchet-type dendrimers and Moore’s phenylacetylene dendrimersl8 display

limited amounts of impurities.

 

 

N
(b
.o
i
m m
>
V
N

Figure 1.7. Example of a convergent synthesis of a dendrimer with 1—>2 branching
scheme and a di-functional core.

Both synthetic methods provide the unique highly symmetric branched
architecture, characteristic of dendrimers. The number of end groups (Ne) of a dendrimer

is defined as‘9

Mada-W (m

where fC is the functionality of the core, fbu is the functionality of the branching unit and
g is the generation number. Similarly, the molecular mass of a dendrimer is19

134

fliiflwt +fbi. (1.2)
bu

Mg =Mc +fi|:Mbu[

where MC, Mbu and Mt are the molecular weights of the core, branching unit and terminal

group respectfully. Simplifying eq 1.2, demonstrates the molecular mass scales as

M ~ f.(f... _1), (1.3)
The PAMAM and poly(benzyl ether) (PBE) dendrimers studied here both have a

multiplicity at the branch junction points (be) of 2 and both are di-dendrons, giving a
value of fc=2. Thus molecular mass scales as

M ~ 2g (1.4)
The volume of dendrimers (V) is approximated as

V~g3 as
which is quite different from the volume grth of linear chains which follow the trend

V~RU3’2 do

where RU is the number of repeat units. The grth properties of dendrimers determine

their solution properties and will be discussed in greater detail in Chapter 3, where the

10

solution properties of a di-functional PAMAM dendrimer were measured with small
angle neutron scattering (SANS) and dynamic light scattering (DLS). It should be noted
that the solution properties of dendrimers are a direct consequence of their molecular
architecture, which results in the occupied volume of a single molecule to increase
cubically with generation, whereas its mass increases exponentially. This makes their
solution properties deviate from linear polymers, especially at higher generations. Unlike
linear polymers, whose intrinsic viscosity increases with increasing molecular weight, the
intrinsic viscosity of dendrimers reaches a maximum at a certain dendrimer generation,
which is determined by the functionality of the core and branching units.
Applications of Dendrimers

The ability to control the chemical functionality at the focal point, branching units
and end groups of dendrimers enhances the diversity of their material properties and thus
a wide array of applications of these molecules has appeared in the literature over the past

25 years. Groups have fine tuned the properties of dendrimers for uses such as drug

20-23 24, 25

delivery, reaction catalysis, transfection agents,26 process additives,27 calibration
standards,28 electrokinetic chromatography” and light harvesting.3o Medicinal
application of dendrimers has gained recent attention with researchers taking advantage
of the numerous functional groups of these molecules as well as their ability to sequester
guests. Dendrimers have advantages over linear chains for polymer therapeutics since
drug manufacturers desire a drug to be composed of a single, defined species, whose
identity (and impurities) can be specified using validated techniques and whose

pharrnacokinetics and therapeutic index (activity and toxicity) can be precisely defined.

The heterogeneity of linear polymers and their conjugates has a profound effect on the

11

biological activity of pharmaceuticals. While it has been possible to define acceptable
product specifications using linear polymers in therapies methods are being sought to
minimize the polymer heterogeneity thereby simplifying the process of bringing polymer-
drug conjugates to market.

Dendrimers have potential as drug delivery vehicles, since their chemistry can be
altered to create water soluble compounds with targeting peripheral groups and shielding
branching units. The determining factor for the commercialization of dendrimers will be
the cost-effectiveness of their complex synthesis as well as their challenging
characterization. Despite these challenges, many researchers have developed promising
dendrimer-drug conjugates. Glycodendrimers with an L-lysine cores were synthesized by

1.31. 32

Roy et a with various carbohydrates at the end groups. Compared to a

monofunctional compound they show an enhanced binding capability in a direct enzyme-

l.33’ 3" coupled peptides to a dendritic core to be used as

linked lectin assay. Tam et a
multiple antigen peptides. When injected into mice or rabbits, the immunoresponse was
greater than for conventional peptide carrier conjugates, and was attributed to the high
antigen content of the modified dendrimers. More recently, Qualmann et al.35’ 36 have
introduced antigen selectivity in boron neutron capture therapy,37 a cancer treatment
where cytotoxic and energetic products from nuclear fission reactions of low-energy
neutrons and 10B nuclei are used to destroy malignant cells. Coupling a lysine-based
boronated dendrimer to antibody fragments leads to better stability compared to borate
coated polystyrene beads.

To expand the range of applications of dendrimers many groups have synthesized

and characterized dendrimer-linear conjugates. Three distinct classes of these adducts

12

include (i) attachment of linear chains to the end groups of dendrimers, (ii) placement of
a linear chain at the focal point of a dendrimer, resulting in a linear-dendrimer diblock
(hybrid) and (iii) attachment of a dendron to the repeat unit of a linear polymer
(dendronized polymer). Combining the properties of a linear chain to that of a dendrimer
greatly increases the diversity of the material properties of these systems.

Harada et a]. recently published the synthesis and polycation behavior of a linear-
dendrimer diblock copolymer where a poly(L-lysine) (PLL) linear chain is covalently
attached to the focal point of a carboxylic acid terminated PAMAM dendrimer (Figure
1.8).38 The hybrid was synthesized by polymerization from the focal point of the
PAMAM dendrimer where the resulting diblock showed a 102 fold higher transfection
efficiency to cells compared to the PLL alone. Fréchet and Lee39 recently synthesized a
dendronized polymer based on a poly(L—lysine) backbone with a fourth generation
polyester dendrimer (Figure 1.9). The dendronized polypeptide backbones are helical at
lower generations, but upon increasing dendon size undergo a conformation change from
(it-helical to disordered as observed by scanning force microscopy. The dendrons
encapsulate the linear chain and because of the unique backbone conformation are the

first known macromolecules to behave as single molecule glasses."0

13

OH

H
NH; O
+
O NH 0 0"

Figure 1.8. A linear-dendrimer diblock copolymer (hybrid) where a poly(L-lysine) linear
chain is covalently attached to the focal point of a carboxylic acid terminated PAMAM
dendrimer.

14

H

HO )(N CH

HO\9\(0 YQ/OH
o o

o
n
0
HO No HN o 0% OH
Ho\>\IYo o E o oW/QOH
° Eur mg °

0 o
o o o o o o
o 0% £0 0
J; o o o o E
OH OH OH OH
OH OH OH OH

Figure 1.9. A dendronized polymer consisting of a fourth generation hydroxyl terminated
polyester dendrimer attached to the repeat unit of poly(L-lysine).

15

Motivation of work

This dissertation will focus on the properties of linear-dendrimer diblock
copolymers where a linear polymer chain is covalently bonded to the focal point of a
dendron. We have studied two different systems where the first consists of a Fréchet-
type PBE dendron with a poly(styrene) (PS) chain connected to its focal point. Extensive
SANS experiments on partially deuterated molecules were conducted to determine the
relative location of the linear chain and dendrimer block and will be discussed in Chapter
3. The ability of the dendrimer block to change size and shape will also be discussed.
The second system is water soluble and consists of a linear poly(ethy1ene oxide) (PEO)
chain connected to the focal point of a PAMAM di-dendron. The morphology of this
system in dilute solution is discussed in Chapter 6. The properties of the dendrimer block
of this system are discussed in Chapter 4, where we describe the synthesis of a dendrimer
as well as its solution properties. We used SANS and DLS experiments to investigate the
dynamics of PAMAM dendrimers with respect to solvent quality and found that
functionality of the core as well as chemistry of the end groups greatly influences the
dimensions of these molecules in solution. To further look at the morphology of the
dendrimer block of the PEO-PAMAM system, we synthesized a di-functional PAMAM
dendron with a solvatochromic probe at its focal point. The behavior of the fluorescence
of this molecule as a function of generation and solvent is discussed in Chapter 5. The
chapters are organized in order of the thought process I used to answer the questions (1)
How can the morphology of linear-dendrimer diblock copolymers be probed and proven?
(2) What factors of the dendrimer block, if any, influence the behavior of these

molecularly asymmetric molecules in solution?

16

References

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Tomalia, D.; Baker, H.; Dewald, J .; Hall, M.; Kallos, G.; Martin, S.; Roeck, J .;
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Tomalia, D. A. B., H.; Martin, S.; J .; Hall, M.; Kallos, G.; Roeck, J .; Ryder, J .;
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Hawker, C. J .; Fréchet, J. M. J ., J. Am. Chem. Soc. 1990, 112, 7638-7647.
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Xu, 2.; Moore, J. S., Angew. Chem. Int. Ed. Eng. 1993, 32, 246-248.

Kawaguchi, T.; Walker, K. L.; Wilkins, C. L.; Moore, J. S., J. Am. Chem. Soc.
1995, 11 7, 2159-2165.

Bosman, A. W.; J anssen, H. M.; Meijer, E. W., Chem. Rev. 1999, 99, 1665-1688.

Tomalia, D. A.; Smith, P. R; Hall, M. J .; Martin, S. J ., A Characterization of the
Structure and Synthetic Reactions of Polyamidoamine "Starburst" Polymers.
Hanser: Munich, 1987.

Bosman, A. W.; Janssen, H. M.; Meijer, E. W., Chem. Rev. 1999, 99, 1665-1688.
Leon, J. W. F., J. M. J ., Polym. Bull. 1995, 35, 449-455.
Pollak, K. W.; Sanford, E. M.; Frechet, J. M. J., Mater. Chem 1998, 8, 519-527.

Walker, K. L.; Kahr, M. S.; Wilkins, C. L.; Xu, Z.; Moore, J. S., J. Am. Mass.
Spectrom. 1994, 5, 731-739.

Tomalia, D. A.; Naylor, A. M.; Goddard, W. A., Angew. Chem. Int. Ed. Engl.
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Majoros, I. J .; Myc, A.; Thomas, T.; Mehta, C. R; Baker, J. R.,
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Majoros, I. J .; Thomas, T. P.; Mehta, C. B.; Baker, J. R., J. Med. Chem. 2005, 48,
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Knapen, J. W. J.; van der Made, A. W.; de Wilde, J. C.; van Leeuwen, P. W. M.;
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DeLong, R.; Stephenson, K.; Loflus, T.; Fisher, M.; Alahari, S.; Nolting, A.;
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Mackay, M. E.; Hong, Y.; Hong, 8.; Russell, T. P.; Hawker, C. J .; Vestberg, R.;
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18

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19

CHAPTER TWO: Scattering Theory

The size of polymers in solution can be probed with light, neutron or x-ray
scattering experiments. It has become increasingly popular in polymer characterization
to not only be concerned with molecular weight, but also with the dimensions of
macromolecules and how environmental factors can alter their morphology. There are
two basic types of scattering experiments, static (elastic) scattering and dynamic (quasi-
elastic) scattering. Elastic scattering experiments provide information such as molecular

weight, and with collection of the time-averaged intensity over a range of wave vector
(Q), can provide us the radius of gyration (Rg) and second virial coefficient."3 Quasi-

elastic scattering experiments monitor the random (Brownian) motion of polymers in
solution by collecting a time average change in intensity to obtain a diffusion coefficient,
which can be used to determine the size and shape of polymers via the Stokes Einstein
- 4
equatron.
I have used both neutron and light scattering experiments to analyze dendrimers

and their copolymers in solution. Small angle neutron scattering experiments (SANS)

were conducted to determine the Rg of linear polymers, dendrimers and linear-dendrimer

diblock copolymers in dilute solution. The hydrodynamic radius (Rh) of samples was

determined from quasi-elastic light scattering, also known as dynamic light scattering
(DLS) or photon correlation spectroscopy. The field of polymer characterization using
SANS and DLS has increased in popularity over the years and implementation of both
DLS"'6 and SANS3’ 7'” have been discussed in great detail elsewhere. Here only a brief

outline of the two methods is presented.

20

Small Angle Neutron Scattering (SANS)

Small Angle Neutron Scattering is a technique that enables one to probe the size
and shape of molecules and their assemblies both in solution and in the bulk. It is
comparable to other diffraction techniques, such as Small Angle X—Ray scattering
(SAXS) and static light scattering, and while often the same information can be drawn
from all three techniques, a key advantage of SANS for the polymer chemist is that
hydrogen and deuterium scatter differently, thus allowing interrogation of selected parts
of macromolecules by deuterium labeling chemistry.2

SANS uses slow (long-wavelength) neutrons that range from 0.15 to 2.5 nm.
Neutrons leave the radiation source and are collimated to an appropriate size and shape
through the use of neutron-absorbing material. This is analogous to focusing a beam of
light. The neutrons hit the sample and are scattered with equal intensity in all directions.
The lack of angular dependence of intensity is due to the wavelength of the neutrons
being orders of magnitude larger than the size of the nucleus from which they are being
scattered, thus in neutron scattering, nuclei may be considered as ‘point scatters.’ This is
contrary to scattered light and X-ray radiation where there is pronounced angular
dependence in intensity due to the radiation being scattered by electrons surrounding the
nucleus and the wavelength of radiation is closer in size to the scatterer.

The geometry of a SANS experiment is shown in Figure 2.1 where neutrons with

a wavelength, A, are scattered by the nuclei of the sample at an angle, 0. The scattered

neutrons hit a detector at a distance, Lsd, from the sample. The scattering vector, Q,

describes the relationship between the incident, ki, and scattered, ks, wavevectors, where

21

Q = k, - ks. The modulus of Q, IQI, is the independent variable in a SANS experiment

and has dimensions of (length)".2

Area detector

   
 

Scattering
center

 

 

Figure 2.1. Schematic of a small angle neutron scattering experiment.

22

There are two main types of SANS instruments that differ based on the neutron
source. At a reactor source a fixed-wavelength instrument is most common, whereas at a
spallation source a fixed-geometry instrument is used. Use of the fixed-wavelength

instrument entails choosing a value of A, and varying the Q range by changing the
sample-detector distance, Lsd. A fixed-geometry instrument has set values of Lsd and Ida-
The Q range is varied by the range of wavelengths hitting the detector and the range in
wavelength sets the Q range of the experiment.

The dependent variable in SANS is the differential scanning cross-section
(BE/d9) (Q), which typically has units of cm'l and gives information about the size and
shape of the sample. A common misinterpretation of (82/89) (Q) is that it is the same as
the intensity of scattering, I(Q). (BE/BS2) (Q) is determined from the intensity of
scattering given by2

I(Q) = 16004 S2 1180») TM) Vs (323/39) (Q) (2.1)
where 10 is the incident flux of neutrons, A $2 is the solid angle element defined by the

size of a detector pixel, nd is the detector efficiency, T is the neutron transmission of the

sample and VS is the volume of the sample illuminated by the neutron beam. The

incident flux, solid angle element and detector efficiency are all instrument specific and
typically determined by the instrument scientist. The transmission is sample specific and
is measured before each scattering run. The volume of sample is known, thus leaving
only the differential scanning cross section to be determined during data reduction.

The differential scanning cross section is then used to determine the structural

features of the sample by2

23

(82/89) = W2 (Ap)2 P(Q)S(Q) + B (2.2)
where N is the number concentration of scattering centers, V is the volume of one
scattering center, (Ap)2 is the contrast between the sample and the solvent/medium, P(Q)
is the form factor, S(Q) is the structure factor and B is the background signal.

The contrast between the sample and the solvent, (Ap)2, allows a SANS
experiment to take place. Here, p represents scattering length density and Ac is the

difference between the scattering length density of the sample and the solvent. The

scattering length density typically has the units 1010 cm'2 or 10'6 A"2 and can be found by2

M
.0 = N'Zb. = MA 'Zbr (2.3)

where 5 is the bulk density of the molecule, NA is Avogadro’s number, M is the

 

molecular weight of the molecule and b; is the coherent neutron scattering length of

nucleus i. It is important to note that only coherently scattered neutrons, where phase is
conserved, carry any structural information about the sample.

Scattering length density (SLD) varies irregularly between atoms. The magnitude
of scattering length is determined by the quantum mechanics of the neutron-nucleus
interaction. The bulk density of the sample has a great influence on the scattering length
density of a sample. For polymers, the scattering length density is calculated by using the
molecular formula of the monomer, since the monomer represents a scattering center.
The contrast, (Ap)2, is the square of the difference between the sample and the
solvent/medium. When the SLD of the solvent matches that of the sample, no scattering

will occur and this is called a contrast match. Contrast matching ideally allows one to

24

look at a specific portion of a two component system. For a diblock copolymer where
one of the blocks is deuterated, the solvent can be matched to the deuterated portion of
the molecule and scattering that occurs will be representative of the hydrogenated portion
of the molecule.

An example of a contrast match experiment is shown in Figure 2.2, where a linear
chain (gray in color) has a different SLD than the dendrimer diblock (black in color).
Three different experiments can be conducted; the first allows the size and shape of the
entire molecule to be obtained by placing the linear-dendrimer diblock in a solvent that
has a SLD different from both blocks. Information on the dimensions of the linear block
is obtained by a second experiment where the scattering length density of the solvent
matches that of the dendrimer, thus the neutrons only scatter off the linear chain. Thirdly,
conformational information regarding the dendrimer block is obtained by placing the

system in a solvent that has a scattering length density equal to that of the linear chain.

25

 

A B
c D

Figure 2.2. Cartoon representing information that can be obtained from a series of
contrast match experiments.

 

A. A linear-dendrimer diblock copolymer where the linear block, red in color, has a
different scattering length density than the blue colored dendrimer block. B. The solvent
has a different scattering length density than both the linear and dendrimer block and
neutrons scatter off the entire molecule. C. The scattering length density of the solvent
matches that of the dendrimer block, thus size and shape information of the linear block
is obtained from SANS. D. The scattering length density of the solvent matches that of
the linear chain, thus neutrons only scatter from the dendrimer. Images in this
dissertation are presented in color.

26

In addition to the contrast, the differential scanning cross section is related to the
form factor, P(Q). P(Q) is a dimensionless function that describes how the differential
scanning cross section, more commonly referred to as the intensity of scattering, is
changed by neutrons scattered from different parts within the same molecule. This
makes P(Q) a function of the size and shape of the molecule. Many common shapes have
analytical expressions for the form factor. The two most important form factors used by

a polymer chemist is the homogeneous sphere with a radius R, given by

 

P(Q) = [3(sin(QR) — QR cos(QR)) ]

 

(2.4)
(QR)3
and a Gaussian coil characterized by a radius of gyration, Rg given by
2<exp(—QZRZ ) + QZRZ -1)
P(Q) = g g (2.5)

(Q21? ,3 )2
where P(Q) deals with the scattering of neutrons from the same molecule, S(Q), the
structure factor, describes how neutrons are scattered by different molecules within the
sample. S(Q) is a dimensionless function which is dependent on the local organization
within a sample and the interaction between two molecules in a sample. As a sample
becomes more dilute and the probability of two molecules interacting with one another
decreases, the structure factor goes to unity and is given by

47W

S(Q)=1+ QV.

 

jlgm -‘1irSi11(Qr)dr (2.6)
0

Here, g(r) is a density distribution function related to the radial distribution function in

statistical mechanics and is usually a damped oscillatory function whose maxima

27

corresponds to a distance, r, of each nearest-neighbor coordination shell. The structure
factor is calculated from eq 2.2, by scattering at two different concentrations, assuming
that the form factor is not concentration dependent.
Dynamic Light Scattering

Dynamic light scattering, also referred to as quasielastic light scattering or photon
correlation spectroscopy, is a noninvasive probe of diffusion in complex fluids." 6 DLS
measures the diffusion coefficient and when applied to dilute monodisperse solutions it is
used to estimate the size of the molecules in solution. In a light scattering experiment,
light from a laser passes through a polarizer and then through the sample. The electric
field of the light induces an oscillating polarization of the electrons in the molecules. The
molecules then scatter the light in a way that is characteristic of their size and shape. The
basic geometry of the dynamic light scattering experiment I conducted is displayed in
Figure 2.3, where light from a laser is passed though a polarizer and hits the sample. The
light scattered at 90° is guided via a fiber optic cable to an actively quenched, solid state
Single Photon Counting Module (SPCM). The photons are then converted to electrical
pulses and correlated. Light is scattered from a homogeneous solution due to Brownian
motion, which moves the solute molecules in a semi-random manner. When the solute
molecules move, the light paths and phases of the waves change. These changes are
directly related to the diffusion of the solute. To obtain constructive scattering, two
particles need to diffuse far enough that light paths differ by about half a light
wavelength. This distance is called a diffusion step and the time for the diffusion step to

take place is called 1 ,equal to6
—1 2
t = DQ (2.7)

28

where 1: is the time scale for the experiment, D is the diffusion coefficient of the solute
species and Q is the magnitude of the scattering vector. The time scale of the experiment
is important. Due to the random movement of the solute particles, the intensity at the
detector is also changing randomly. Therefore intensity may either decrease or increase
for small changes in time. To obtain an accurate recording of the intensity, the intensity

data is subjected to the correlation function,
T
C(a‘r) = Idt1(t)l(t+ a) (2.8)
0

where T is the duration of the experiment and C(51) is the correlation function. The
intensity fluctuations, I(t), are sent to a digital correlator which produces the correlation

function for that sample, C(t), given by

C(t) = A, exp(—2Dk2t) + 3, (29,
where A, is the spectral amplitude, D is the diffusion coefficient, k is the magnitude of

the scattering vector and BI is the baseline. The diffusion coefficient can be related to the

size of the solute via the Stokes-Einstein equation

_ 135..
6mm

D (2.10)

D is the translational diffusion coefficient, k3 is Boltzmann’s constant, Ta is the absolute

temperature, 11 is the viscosity of the solvent and or is the effective Stokes radius of a

sphere.

29

 

 

7 kl = ZWAi
I Incident laser lightJ—-' :

 

 

90° scattered

 

 

 

 

()=I('-kI light I data analysis 7
q=(411/A,)sin(G/2) f
5 Lcorrelator
k, = 27r/As

Figure 2.3. Set-up of a dynamic light scattering experiment.

30

References

l.

10.

ll.

Dawkins, J. V., Developments in Polymer Characterization. 5 ed.; Elsevier
Science: New York, 1986; p 1-94.

King, S. M., Small-angle neutron scattering. In Modern Techniques for Polymer
Characterization, Pethrick, R. A.; Dawkins, J. V., Eds. John Wiley & Sons: New
York, 1999.

Higgins, J. S.; Benoit, H. C., Polymers and Neutron Scattering. Clarendon Press:
Oxford, 2002.

Beme, B. J.; Pecoria, R., Dynamic Light Scattering. John Wiley & Sons, Inc.:
New York, 1976.

Dahneke, B. E., Measurement of Suspended Particles by QuasiElastic Light
Scattering. Wiley-Interscience: New York, 1983.

Phillies, G. D. J.,Anal. Chem. 1990, 62, 1049A-1057A.

Arrighi, V.; Higgins, J. S., Plastics Rubber and Composites 2004, 33, 313-329.
Oberthur, R. C., Revue Phys. Appl. 1984, 19, 663-670.

Richards, R., Adv. Poly. Sci. 1985, 71, 1-39.

Richards, R., J. Macrom. Sci. Chem. 1989, 26, 787-815.

Fei gin, L. A.; Svergun, D. 1., Structure Analysis by Small-Angle X-ray and
Neutron Scattering. Plenum Press: New York, 1987; p 335.

31

CHAPTER THREE: Conformation of Poly(styrene)-Poly(benzyl ether) Linear-
Dendrimer Diblock Copolymers in Dilute Solution

Introduction
Dendrimers or dendrons are a unique class of macromolecules that have a specific
molecular weight, large number of terminal groups and well-defined architecture

consisting of branched units emanating from a focal point. "3

These unique
characteristics have triggered numerous studies to use dendrimers in applications such as
drug delivery,4 light harvesting5 and reaction catalysis.6 It is clear that the dendrimer
molecular architecture dictates its physical properties and thus enables such diverse
applications to be explored. In addition, synthetic modification of dendrimers at both the
focal point7' 8 and the end group9"' has proven successful in expanding their utility.

Further, block copolymers consisting of a linear polymer attached to the focal pointlz’z’

have promising applications due to their architectural asymmetry.’2"5

In order to take full advantage of the properties of linear-dendrimer diblock
copolymers their conformation in both solution and the bulk must be understood. In prior
studies, the phase behavior of these molecules in the bulk has been proven to be different
from traditional linear-linear diblock copolymers due to the architectural differences
between each block.’2’2’ It was found that both the generation number as well as the
molecular weight of the linear chain dictated which morphology the hybrid diblock
copolymer adopted.

The solution behavior of these diblocks has also been shown to be unique and
significantly affected by the molecular asymmetry between the two blocks. For example,

Gitsov and Fréchet” have investigated the solution properties of poly(benzyl ether)

(PBE)-poly(ethy1ene oxide) (PEO) diblocks which are dominated by this molecular

32

asymmetry. Since the dendrimer and linear blocks have different solubility parameters,
they examined how the solvent quality affected the intrinsic viscosity and solubility of
the system as compared to linear PEO analogs. The solution behavior of a generation 4
(G4) PBE dendrimer - PEO chain was reported to be a function of the PEO molecular
weight, and for PEO molecular weights of 26kDa and 46kDa, the intrinsic viscosity
values were lower than that of pure PEO when in THF. This is a marginal solvent for
PEO and a good solvent for the dendrimer, thus indicating the entire molecule was in a
collapsed state compared to the linear chain. When the system was placed in a
methanol/water solution, a good solvent for the linear block and a poor solvent for the
dendrimer block, the solubility of the diblocks was dependent on the molecular weight of
the linear chain being large enough to allow unimolecular micelle formation. These
results indicate that the hybrids’ conformations are a function of both the chain molecular
weight and dendrimer generation.

A hybrid system containing a PBE dendron block was studied via intrinsic
viscosity by Jeong et al.22 Here, a G4 PBE dendrimer with a linear chain attached to its
focal point was investigated and showed interesting behavior with an increase of the PS
chain molecular weight. In particular, when compared to the intrinsic viscosity of linear
PS, the smaller molecular weight hybrids had an intrinsic viscosity lower than a PS chain
of equal molecular weight. However, a sudden intrinsic viscosity increase was observed
for the hybrids between molecular weights of 60 kDa and 80 kDa to that of linear PS
having molecular weights greater than 80 kDa. This suggests the overall molecular
conformation of the hybrid changes with respect to the relative molecular weight of the

two blocks.

33

While the intrinsic viscosity studies hint that the PBE-PS hybrids are capable of a
conformational transformation, it is difficult to infer if one or both of the blocks changes
conformation. All studies of linear-dendrimer diblock copolymers in solution thus far
have focused on the conformation of the entire system.” ’9' 22 A full understanding of the
system necessitates pinpointing the relative locations and morphology of each block.
Figure 3.1 illustrates three possible linear-dendrimer diblock conformational states in
solution for a system where the linear and dendrimer blocks are compatible with each
other. Each conformational state pertains to different relative locations of the two
blocks.23 The first state is a knitted coil, where the linear chain weaves in and out of the
dendrimer, thus the dendrimer shields or partially shields the linear polymer from the
surrounding environment. For this to occur, the dendrimer must have enough free volume
to accommodate the linear block. A second hybrid conformation consists of the linear
chain wrapping itself around the dendrimer, thus shielding the dendrimer from the
surroundings and forming a unimolecular micelle. This encapsulated dendrimer state
would occur when the linear block is compatible with the surrounding medium and the
dendrimer less compatible, resulting in a core-shell morphology. A third potential
molecular arrangement consists of the linear chain completely expelled from the cavities
within the dendrimer. This could occur if there is not enough free volume within the
dendrimer to contain the entire linear chain and each component has similar solubility in

the solvent.22

34

"Q © “0483
Figure 3.1. Three potential conformational states of linear-dendrimer diblocks in
solution: a. knitted coil b. encapsulated dendrimer c. random coil.

Distinguishing between these three states requires knowledge of the relative block
locations, which was performed for the G4-PBE-PS hybrid, Scheme 3.1, in the present
work. We studied three different molecular weight hybrids, 20kDa, 45kDa and 100kDa,
which include the G4 PBE dendrimer (M = 3288g/mol) and linear chain molecular
weights, and thus the only difference between the three hybrids is the molecular weight
of the linear block. The PS chain was also deuterated (dPS) to allow small angle neutron
scattering (SANS) to individually visualize each block though a series of contrast

matching experiments.24 This information can be used to ascertain the architectural

structure of the two blocks.

35

Scheme 3.1. PBE-G4-PS dendrimer-linear diblock copolymer.

36

Experimental

Materials. Poly (benzyl ether) dendrimers were prepared according to a previous
procedure.2 The third generation (G3) bromides were coupled with a 3,5-
dihydroxylbenzyl-functionalized alkoxyamine initiator.25 The purified fourth generation
(G4) dendritic initiators were then used to prepare the hybrid dendrimer-linear diblock
copolymers by standard living free radical conditions. The linear polystyrene block
degree of polymerization is controlled by the molar ratio of dendritic initiator to
monomer. Deuterated solvents were purchased from Aldrich Chemical Co. (St. Louis,
MO) and used as received.

SANS. Measurements were taken at the small-angle neutron diffractometer (SAND) at
Argonne’s Intense Pulsed Neutron Source. A sample to area detector distance of 2.0 m
was used. The radiation had a wavelength range of 1-14 A making the total wave vector
(Q) range covered 0.0034-O.6 A", where Q = 47tsin(0/2)/A with 0 and A defined as the
scattering angle and radiation wavelength, respectively. Measurements were taken at
room temperature. Low concentrations were used for each sample to generate sufficient
scattering in the allotted time while minimizing intermolecular scattering effects.” Hence
consistent power laws were not observed for plots of radius versus molecular mass due to
concentration effects, particularly for the higher molecular weight samples. Note the
effect of concentration on the size of polymers has been extensively studied before, 26“"
here the radius was compared at a particular (low) concentration. Samples were filtered
through a 0.111m Whatman Anodisc 13 filter and sonicated for 20 minutes and placed in a
hermetically sealed quartz cell for the scattering experiments. A list of samples is given

in Table 3.1.

37

Table 3.1. Sample codes and number average molecular mass (M) of compounds used in

this study“

 

 

 

 

 

 

 

 

 

 

 

 

 

 

M-dendrimer(Da) M-dendrimer(Da)
Sample code lM-polystyrene PDIb Sample code lM-polystyrene 1’le
(kDa)/M (kDa) (kDa)/M (kDa)

G4 3288/0/3288 ~l dPS-156k 0/ 156/ 156 1.19
G4-dPS-20k 3288/16.7/20 1.06 G4-PS46k 3288/42.7/46 1.09
G4-dPS-45k 3288/41.7/45 1.08 G4-PS-70k 3288/66.7/46 1.1 1

G4-dPS- 100k 3288/96.7/100 1.12 G4-PS-9 l k 3288/87.7/9l 1.15
dPS-21k 0/21/21 1.04 PS-44k 0/44.1/44.1 1.07
dPS-635k O/63.5/63.5 1.10 PS-75k 0/75.2/75.2 1.17
dPS-83k 0/83/83 1.16 PS-115 0/115.3/115.3 1.08

 

 

a M—dendrimer represents the mass of the dendrimer, M-polystyrene, the mass of the
polystyrene and M, the total mass. b The polydispersity index, PDI, is defined as the

weight to number-average molecular mass ratio.

When the scattering length density (SLD) of the solvent matches that of the
scatterer, there is a contrast match, and no coherent scattering occurs. Deuterating the
linear chain makes the two blocks’ SLD quite different, with the dendrimer block having
a SLD of 2.03><10*S A '2 and the dPS, a SLD of 6.42x10'6 A ‘2. When the G4-dPS hybrid
is placed in dB-THF (SLD = 6.35x104’ A‘Z) the dPS chain is essentially contrast matched
with the solvent and scattering only occurs from the dendrimer block. The linear block
was investigated using a solvent combination of 79:21 h6 benzenezd6 benzene (SLD of
2.07x104’ A '2) which provides a contrast match to the G4 PBE dendrimer and thus
scattering only occurs from the linear block. Performing both of these contrast match
experiments on the three molecular weight hybrids and comparing the behavior of each
block to that of a native PBE dendrimer or dPS chain in the same solvent, allowed the
conformation of each block to be determined as well as the overall conformation of the

diblock.

38

Neutron scattering data for hybrids were analyzed in the Guinier regime.29 This
analysis allowed for quick determination of the shape and size of the dendrimer as
models for compact-objects, rods, and sheets were applied to all samples. The scattering
intensity, I(Q), in the low Q regime was evaluated using the expression for compact

(‘sphere-like’) objects with a given radius of gyration (Rg),30

22 22

I(le¢VS(Ap)2 exp “Q3 8 EI(O)exp ‘Q—3L (3.1)

 

where (t) is volume fraction of scattering centers, V,, the volume of each scattering center,
(Ap)2, the square of the difference of the solvent and the scatterer SLD and 1(0), the
intensity at zero wave vector. The slope resulting from a plot of ln(I(Q)) versus Q2 in the

region where QRgzl allows Rg to be obtained. The radius of a homogeneous, constant

density sphere is determined from the Rg by R = 0.775ng.

A modified Guinier analysis for rod-like objects approximates the scattering

intensity in the low Q regime by30

V A 2 _ 2R2 _ 2R2
QI(Q)==”¢ (LP) exp _g2_._ -=-QI<0>exp —%—‘~‘- (,2,

 

where L is the rod length and Rc is the root-mean-square of the distances of all of the
atoms in a cross—section of the rod to the centroid of this cross-section determined from

the slope in a plot of ln(Q(I(Q)) versus Q2, in the region of Q where QRCEI. The radius of

the rod (R,) is obtained by RC\/2. A sheet-like particle, which is much smaller in one

dimension than the other two, has, 30

39

 

, 22,2111 A 2 —Q2T,2 , —Q2T,2
Q I(Q):= A p) CXP[T]EQ 1 (0)6XP[—12—) (3.3)

where A is the area of the particle and Ts, the sheet thickness and is valid when QTS<\/12.

To determine the goodness of each Guinier fit, the experimental value of 1(0) or
QI(0) was used to determine the amount of solvent within each sample and check that the
mass density of the swollen dendrimer is reasonable. The numerical prefactors of
equations 3.1 and 3.2 were set equal to the experimental value of 1(0) and QI(0)

respectively and then solved for (Ap)2. Using this value of (Ap)2 and the scattering length

density of the solvent (psojv) we calculated the scattering length density of the scatterers

(pscal), which includes solvent and dendrimer. We calculated the amount of solvent
within the dendrimer by,

(pscat) = [0.66180 + [l-leosamptel (3.4)
where 0 is the mass fraction of solvent within the scatterer and psample is the scattering

length density of the dendrimer.

Neutron scattering data was also analyzed by fitting 1 vs Q to the form factor of a
solid sphere or Gaussian coil when appropriate. We also used Holtzer plots3| to reveal
coil-like and rod-like behavior.

Dynamic Light Scattering (DLS). Measurements were performed with a Protein
Solutions Dyna Pro-MS/X system with temperature control. All samples were filtered as
described above and allowed to equilibrate in the instrument for 25 minutes at 25 °C

before measurements were taken resulting in calculation of the hydrodynamic radius

(Rh). The sample is illuminated by a semi-conductor laser with ~ 830 nm wavelength.

The light scattered at an angle of 90° is collected and guided via a fiber optic cable to an

40

actively quenched, solid state Single Photon Counting Module (SPCM), where the
photons are converted to electrical pulses and correlated. Autocorrelation is used to
analyze the time scale of the scattered light intensity fluctuations. The uniformity of the
sample sizes is determined by a monomodal curve fit, cumulants, which assumes a single
particle size with a Gaussian distribution.
Results and Discussion
Conformation of PBE Dendrimer

The molecular morphology of the G4-PBE dendron samples was investigated in
both dé-benzene and dB-THF using the three Guinier analyses mentioned above to
determine the influence of solvent on the size and shape of the dendrimer block. The
dendrimers behave as sphere-like objects in both d6-benzene and dB-THF due to the
observed negatively sloped straight line in the Guinier plots for compact objects (Figure
3.2). It is evident from the Guinier fits in Figure 3.2 that the dendrimer is larger in d6-
benzene than in ds-THF, with radii values of 1.73 t 0.03 nm and 1.35 i 0.04 nm,
respectively. These sizes agree reasonably well with radii values reported by Tande et
al.'2 of 1.93 i 0.1 nm and 1.50 i 0.1 nm in ab-benzene and dB-THF, respectively. Using
the experimental value of 1(0) (Table 3.2) the mass fraction of solvent (0) within the
dendrimer was calculated by eq 3.4 as described above and found to be 0.45 and 0.51 for

(is-THF and db-benzene, respectively.

41

 

 

 

 

 

 

 

 

 

.E
cl»
0 d6-benzene E}
_1 2 -l- dB-THF jf
ii
Rg=1.04 :1: 0.03nm ::
A :- R =1.35 :1: 0.04nm ::
TE -1 .4 I" r / 2i
0 n‘fi; - "
: I!=' ",5”: I SI
2 gill i: - I I it
‘5' -1.6 ‘1 ~ 4:}
1 8 r i ii
' 39:1.33 :1: 0.02 nm 5;
R=1.73:1:0.03 nm ;:
5 EE

-2.0 ' i _3

2 4 6 8 10x10

(12 (A")

Figure 3.2. Guinier analysis for PBE G4 dendrons in 116-benzene and dB-TI-IF at a
concentration of 50 mg/mL, data fit for a compact (sphere-like) object.

42

Table 3.2. Values for Rg and RC determined from Guinier analyses described in the text
at a concentration of 50 mg/mL.

Symbols are explained in the text while Qmax is the maximum Q-value used in the

 

 

 

 

 

 

 

 

 

 

 

regression.
Sample code G4 G4 G4-dPS- G4-dPS- G4-dPS-
—-> 20k 45k 100k
SOlveflt (lg-THF d6-benzene d8 -T1-IF (lg-THF 218111}:
scattering from dendrimer dendrimer dendrimer dendri mer dendrimer
R11 (“my 1.04 r 0.03 1.33 :1; 0.02 1.40 e 0.01 nab na
[(0) m" (cm'l) 0.241 :1; 0.002 0.245 :1; 0.002 0.062 1 0.003 na na
fifth 0.919 1.13 1.35 na "3
Re (nm)d na" na na 2.70:0.28 2.62:0.29
01(0),”,f (25033.4) (3.71250)
(A'lcm'l) na na na x10 x10
me*Rc na na na 1 .006 0.976
08 0.45 0.51 0.56 0.82 0.78

 

 

 

 

 

 

 

 

a Radius of gyration for a sphere-like object. b Attempts to fit data to a sphere-like object
failed for these samplesc Experimental value of 1(0) from a sphere-like object Gunier plot
(eql). dCross sectional radius for a rode Attempts to fit data to a rod-like object failed for
these samples. f Experimental value of QI(0) from a rod-like object Gunier fit (eq 3.2). g
0 is the mass fraction of solvent within the scatterers.

43

The G4 PBE dendrimer is larger than its minimum solution radius, 1.09 nm in
chloroform, which is close to the value expected if the dendrimer collapses to its bulk
density,22 indicating the dendrimer is in an expandedgstate in both dé-benzene and d8-
THF. A solvent dependence on size is a well known property of linear polymers and has

been previously observed in dendrimers.22

While the dendrimer size has changed with
solvent quality, the shape has remained ‘sphere-like.’ Further contrast variation studies of
the dendrons need be conducted to determine the overall segment distribution and
whether this distribution changes with solvent.32'3"

These structural changes may be delineated in part by measuring the

hydrodynamic radius, Rh, of the G4-PBE dendrimers in both dis-benzene and ds-THF. An

Rh of 1.70 i 0.02 nm was obtained in both solvents making the ratio of Rg/Rh = 0.79 :I:
0.03 for ds-THF and 1.02 i 0.03 for dis-benzene. This ratio is a quantitative indicator of
molecular conformation,3S where an Rg/Rh ratio of \/(3/5) 2 0.775 indicates a constant

density, homogeneous sphere, 1.50 describes a monodisperse Gaussian coil in a theta

36, 37

solvent and 1.78 represents linear polymers in a good solvent although experiments

show values of 1.16 and 1.27 for linear polymers in theta solvents.” 38 The Rg/Rh ratios

for the dendrimer suggest a more spherical nature in both dB-THF and d6-benzene
indicating a significantly different structure in solution as compared to random-coil linear
polymers under analogous conditions. The ratio also suggests that the dendron behaves as
a constant density (expanded) sphere in dig-THF implying backfolding of end groups.” 40

It should be noted that this effect is not due to poor dendrimer-solvent interactions as the

solubility parameter of the dendrimer has been found to be similar to that of benzene and

THF while the radius of the dendrimer is larger in both solvents than the minimum
possible value of 1.09 nm. 22
Conformation of Dendrimer Block

The dendrimer block conformation within the hybrid block copolymers was
determined by dissolving the G4-PBE-dPS diblocks in dB-THF, which is a contrast match
for the dPS block. Two different concentrations were analyzed, 4 mg/mL and 50 mg/mL,
for the hybrids with molecular weights of 20kDa, 45kDa and 100kDa, noting there are
insignificant differences observed for the dendrimer block size at these two
concentrations.

By fitting the scattering data of the G4-PBE-dPS 20kDa hybrid in dB-THF at a
concentration of 50 mg/mL with a Guinier analysis for compact objects (eq. 3.1), Figure
3.3, a radius of 1.82 i 0.01 nm for the dendrimer block was found, which is appreciably
larger that the radius of the ‘free’ dendrimer in the same solvent, 1.35 :t 0.04 nm. The
mass fraction of solvent and dPS within the dendrimer is 0.56 (Table 3.2), which is
higher than the value for the corresponding free dendrimer in dS-THF, 0.45. Since the
hybrids’ dendrimer block is more swollen than the corresponding free dendrimer, there is
more mass, i.e. linear dPS block and solvent, occupying the free volume within the
dendrimer. Attempts to fit scattering data for the dendron as a rod-like object (eq 3.2)
failed as shown in the inset of Figure 3.3. The sheet-like architecture (eq 3.3) also did not

represent the scattering data for any of the samples and will not be mentioned further.

45

a

+ PBE-d PS 20kDa

 

0 'I'I'I'I'l'l

 

 

 

 

 

 

 

"i
.1 E: J:
3 9 II
n a m
6 ' ..
:r ..
E 2i
.3 5"r- 4. _.
E i I
_4 39:1.40 e 0.01 nm .22
F1 = 1.82 :1: 0.01 nm 1:

.5 q: _3

2 4 6 8 10x10

0’ (A4)

Figure 3.3. Guinier analysis for PBE-dPS 20kDa system in dig-THF.

Dendrimer block is scattering as a sphere-like object. Inset is a
modified Guinier analysis for a rod-like object, which does not represent the data.

46

The dendrimer block of the next higher molecular weight G4-PBE-dPS hybrid
(45kDa) was subsequently studied in dB-THF at a concentration of 50 mg/mL using a
Guinier analysis for compact objects, see Figure 3.4. Attempts to make a linear fit with
the ‘sphere-like’ morphology failed, as evidenced by the non-linear behavior at small Q
as displayed in the inset of Figure 3.4. The Guinier analysis for rod-like objects (eq 3.2)

proved successful, Figure 3.4, with a substantial increase in the dendron size and an RC

value of 3.81 z 0.39 nm, which is comparable to the maximum possible radius of ~3.6
nm and corresponding to the dendrons in an a fully extended state. This is a much larger
dendrimer radius as compared to both the ‘free’ dendrimer radius, 1.35 i 0.04 nm, and
the dendrimer block of the 20kDa hybrid, 1.82 :1: 0.01 nm, and will be discussed in
greater detail below.

We performed a Guinier analysis for the G4—PBE-dPS 100kDa hybrid in dig-THF
at a concentration of 50 mg/mL and found that the dendrimer block behaves analogously
to that of the 45kDa system, scattering as a rod-like object as opposed to a sphere, Figure
3.5. The model fit results in a dendrimer radius of 3.40 t 0.38 nm, which matches the size
of the 45kDa dendrimer block considering experimental error. As for the 45kDa system,
attempts to fit the 100kDa hybrid’s data to a Guinier analysis for a sphere failed due to

non-linear behavior at small values of Q, inset of Figure 3.5.

47

+ G4 PBE dPS 45kDa

 

 

 

 

 

I'I'I'I'I'ITI

-4 2.- —
a 5 _ firing

75 a £13-

V
7 -5 C -
'53 E 1.. ._
A

9 11135131111!!!

1 2 -2

-7 Flc=2.70 : 0.28 nm
R = 3.81: 0.39 nm

,Ltnslstrlsstlslslslslatslrsels.

02 0.4 0.6 0.8 10 1.2 141.6x10'3
(12 (A4)

 

 

 

 

Figure 3.4. Rod-like Guinier analysis for PBE-dPS 45kDa hybrid in dS-THF at a
concentration of 50 mg/mL.

Inset shows the attempted representation of data to Guinier fit for compact object.

48

L A A A A A A A A A A A A A A A A A n A - A A -

115
i
—.l
d
d
.1
—(
d
d
-
_
d
d
d
—
d
.-
d
_
d
I:
1
i
d
-
.

A44

+ G4 PBE dPS 100 kDa

 

 

 

 

 

 

 

 

-3ii
:: fiiiiii i 1 i1
15 2
1: '5
7a .41;- 3 hat I
1p A g;;;
g I: O - -1
'.' 3F :r
-< 1: E _ -'
z 5:: — Irljirlrlrl
g ' i: """"""
" 2 2
9e :E} 9 (A 1
-51: g E
{E Fic=2.62 a: 0.29 nm
-73=_ Fl: 3.40: 0.38 nm
,-

 

llllllllllllllllllllJlll

v v *7 v v 1 v v v v v v v v v 1 '7 v v v v v v v -

Figure 3.5. Rod-like Guinier analysis for PBE-dPS 100kDa hybrid in ds-THF at
50mg/mL.
Inset is the Guinier analysis for compact-object, which is a poor fit due to curvature.

49

These contrast match experiments revealed that the dendrimer block’s size and
shape is dependent upon the linear block molecular weight. The G4-PBE-dPS-20kDa
hybrid has a dendrimer block where the branches adopt a ‘sphere-like’ shape, Figure 3.3,
just as the free dendrimer in solution, Figure 3.2, yet the size of this sphere is
approximately 35% larger in the hybrid system (> 2x change in volume). The difference
in size between the hybrid’s dendrimer block and the free dendrimer in solution is
probably due to the linear chain occupying space within the dendrimer, resulting in
swelling and possible development of the knitted coil morphology shown in Figure 3.1.
The two higher molecular weight systems, G4-PBE-dPS 45kDa, Figure 3.4, and G4-
PBE-dPS 100kDa, Figure 3.5, did not show the expanded sphere-like morphology and
instead these two systems have dendrimer blocks that are ‘rod-like’ in shape (eq 3.2).
According to the model, the cylinder radius for the 45kDa and 100kDa systems has
values of 3.81 :t 0.39 nm and 3.40 i 0.38 nm, respectively, making the cylinder length of
order 0.1 nm suggesting a disc-like morphology. Fitting the I vs. Q data with the disc
form factor, similar in form to eq 3.3, was unsuccessful.

Evidence of the dendrimer’s change in structure from a sphere-like object to a
more expanded ‘rod-like’ structure is supported by fitting the scattering data to a Holtzer

l

plot,3 a plot of Q x I vs. Q where a peak in this plot indicates a coil or sphere and a

plateau indicates a rod-like object or a more expanded structure. Holtzer plots of the G4-
PBE dendron in both (to-benzene (Figure 3.6) and dB-THF (Figure 3.7) result in a peak
indicating a more compact coil-like structure, which agrees with the Guinier analyses that
shows these dendrimers are spherical. The Holtzer plot of G4-PBE-dPS 20kDa in da-

THF (Figure 3.8), which is only scattering from dendrimer, also shows a peak and agrees

50

with the Guinier analysis that the dendrimer block is a sphere-like object. The Holtzer
plots of the G4-PBE-dPS 45kDa and G4-PBE-dPS 100kDa in dB-THF, Figures 3.9 and

3.10 respectively show no peak, indicating the dendrimer blocks of these hybrids are in a

more expanded state.

 

0.020 I
0.018 ~ ° G4-PBE

0.016 ~ 1 i i
0.014 '7
0.012 f
0.010 1‘
0.008
0.006
0.004
0.002

0,000 1 1 r 1
0.02 0.22 0.42 0.62

    

 

cum) (cm" x A ")

 

 

(1(A")

Figure 3.6. Holtzer plot (QI vs. Q) of G4-PBE in d6-benzene at 50 mg/mL.
A peak indicates that the dendrimer is in a coil-like or sphere-like state.

51

 

0.020
0.018 °G4-PBE
0.016 ‘
0.014
0.012 2
0.010 .
0.003 ‘
0.006
0.004
0.002
0.000 .
0.02 0.22 0.42 0.62

a ( A")

 

0*! (cm" x A")

 

 

 

Figure 3.7. Holtzer plot (QI vs. Q) of G4-PBE in d8-THF at 50 mg/mL.
A peak indicates that the dendrimer is in a coil-like or sphere-like state.

52

 

0.008
0.007 f
0.006 1 I
0.005 4
0.004 1 i I f
0.003 4’ if; I I I f
0.002 fig, :1! ;

0.001 I I I I

; I
0.000 . . . .

0.02 0.22 0.42 0.62
a ( A“)

0 G4-dPS-20 kDa

 

 

Q’l (cm'1 x A“)

 

 

Figure 3.8. Holtzer plot (QI vs. Q) of G4-dPS-20 kDa in d8-THF at 50 mg/mL.
A peak indicates that the dendrimer is in a coil-like or sphere-like state.

53

 

 

 

 

0.0030
0 G4-dPS-45 kDa
0.0025 2
3; 0.0020 2 Kg
x I {I I I
.. I H I
'E 0.0015 * {El-{Eifqfi I 1 x 1 !
3 I i I
r - I I
0 0.0010
0.0005 2
0.0000 . T
0.02 0.07 0.12

O ( A")

Figure 3.9. Holtzer (QI vs. Q) plot of G4-dPS-45 kDa in d8-THF at 50 mg/mL.
There is no peak in this plot indicating the dendrimer block of the hybrid is in a more
expanded or stretched out state as opposed to a coil or sphere.

54

0.0050

 

 

 

 

0.0045 ° G4-dPS-100 kDa
0.0040 '
3; 0.0035
g 0.0030
g 0.0025’ gfiifllifii I 1! I I
; 0.0020“ {1‘35 ‘1! n“,
a 0.0015“ 1
0.0010 2
0.0005 i
0.0000 . f r
0.02 0.07 0.12 0.17
0(A“)

Figure 3.10. Holtzer (QI vs. Q) plot of G4-dPS-100 kDa in dS-THF at 50 mg/mL.
There is no peak in the plot indicating the dendrimer block of the hybrid is in a more
expanded or stretched out state as opposed to a coil or sphere.

55

Further, due to the large solvent mass fractions of 0.82 and 0.78 calculated from
the rod-like Guinier fit of the dendron blocks of the 45kDa and 100kDa systems
respectively, see Table 3.2, the dendrimer molecule it is not expected to adopt a rod-like
conformation. Rather, it is believed that the actual conformation of the dendrimer block
for the 45kDa and 100kDa hybrids is that of a distorted wedge or “ice-cream cone.” This
molecular arrangement explains why a Guinier analysis for sheet-like objects failed and
morphological equivalence between the cone and rod is expected to follow eq 3.2. In fact
the results given by Hjelm41 show the Guinier expression for a finite size cone should

follow (see Hjelm’s eq 19)

 

01(0) .. 02¢R2 (ApY exp

—taR3]
2 (3.5)

for Q < UL and where R is the radius of the cone’s base and [R3] represents a “weight”

average radius. The numerical prefactor in eq 3.5 reduces the value of QI(0) to numbers
of order 10'3 ,4"ch in better agreement with the experimental values than those given in
Table 3.2 for a true rod. To justify the cone-shaped dendrimer block, we set the
experimental values of QI(0) equal to the numerical prefactor of eq 3.5 by using known

values of (b and estimated value of R to find (Ap)2 and 0, listed in Table 3.3. The resulting

0 values are more reasonable than those from the rod-like fit (Table 3.2) indicating the
dendrimer block is more cone-like. While the exact dendron shape is difficult to fully
ascertain, the data support the conclusion that the shape of the free dendrimer and 20kDa
hybrid’s dendron block is quite different from that of the 45 and 100 kDa hybrid’s

dendrimer block.

56

Table 3.3. Experimental QI(0) values were set equal to the numerical prefactor of the
Guinier expression for a finite sized cone, eq 3.5, with various R values representing the
radius of the cone’s base.

 

 

 

 

 

 

 

(A9)2 (cm4)
Sample Code i R (nm)" b 0"

2 8.68x10'3 0.32

G4-dPS-45k" 2.5 5.56x10'3 0.45

3 3.86x10'3 0.55

2 1.28x1013 0.17

G4-dPS -100k" 2.5 8.22x10'3 0.34
3 5.71x10'3 0.45

 

 

 

 

 

 

“ Radius of cone’s base, various values tried. bExperimental value of the difference in
scattering length density between the scatterer and the solvent found by equating

experimental QI(0) values to numerical prefactor in eq 3.5. c Mass fraction of solvent and
possibly linear dPS within dendrimer, eq 3.4.
Conformation of Linear Block

It is apparent that the relative molecular weights of the linear and dendrimer
blocks play a key role in the subsequent morphology of the dendrimer block, yet
complete information on the location of the two blocks in solution cannot be made
without characterizing the linear block of the hybrids. This was done by placing the G4-
PBE-dPS hybrids in a solution of 79:21 h6-benzene: tic-benzene, which is a contrast
match for the dendrimer. The concentration used for these experiments was 4 mg/mL and
the radius of gyration of the linear block was obtained by plotting I(Q) versus (Q) and

L32 To compare the behavior of the

fitting the profile to the form factor of a Gaussian coi
linear block to that of ‘free’ dPS in the same solvent, we ran scattering experiments on

dPS with molecular weights of l8kDa, 63kDa, 83kDa and 155kDa under the same

57

conditions with similar data interpretation. A plot of the radius of gyration versus
molecular weight was used to compare the size difference between the linear block of the
hybrids to that of a free dPS chain of comparable molecular weight. Figure 3.11 shows
that the two lower molecular weight hybrids, 20kDa and 45kDa, have a linear block that
falls below the size of the analogous ‘free’ dPS chain, indicating the dPS block of the
hybrid is in a more compact state for these two systems. The 100kDa hybrid’s linear
block is approximately the same size as the ‘free’ linear chain, suggesting that it expands
in solution to the size of the corresponding dPS chain. These results match the intrinsic
viscosity data of Jeong et al.22 who studied the analogous non-deuterated forms of the
hybrids in benzene.

To verify that the observations from the contrast match experiments are
independent of solvent type and deuteration effects we performed two additional series of
experiments. First, the size of G4-PBE-PS hybrids with molecular weights of 46 kDa, 70
kDa and 91 kDa in dS-THF was compared to PS in dB-THF, all solutions were at a
concentration of 4 mg/mL, see Figure 3.12. The radius of gyration for the G4-PBE-PS
hybrid and the ‘free’ linear PS was again found by fitting the I(Q) — Q data to the form

factor for a Gaussian coil.32 This value is similar to that found by using a compact sphere
Gunier analysis. As seen in the figure the 46 kDa hybrid has a smaller Rg than a PS chain
of an equal molecular weight, indicating that the hybrid is in a collapsed state. The 70
kDa and 91 kDa hybrids have a Rg that is equal to or greater than its corresponding “free”

PS chain. These results agree with those in Figure 3.11. Both indicate that hybrids below

a molecular weight of ~ 45 kDa are more collapsed than those above ~ 70 kDa, and this

58

collapsed state transition correlates with the transition in dendron block conformation

Further, the two solvents do not affect the transition for these systems.

 

2-511555'I11T1111I' I l l I 'l‘U'UtTVV"
p

: . G4-PBE-dPS f I
. 0 dPS

 

 

 

2 3 456789

M (Da)

Figure 3.11. A plot of radius of gyration versus molecular weight for both the dPSblock
of the hybrids and dPS in 79:21 h6zd" benzene at 4 mg/mL.

The two lower molecular weight hybrids fall below the size of the corresponding
dPS linear chains, indicating a collapsed state for the two hybrids’ linear block.

59

 

IIIIYYVIIYITIIUUUU' I l I V

I G4-PBE-PS
0 PS

 

R9 (nm)

105

I I ll“ L l
vvvv vv vv v v

M (Da)

Figure 3.12. A plot of radius of gyration versus molecular weight for G4—PBE-PS
hybrids and PS in (IS-THF at a concentration of 4 mg/mL.
The 46 kDa G4-PBE-PS hybrid’s size is below that of the corresponding PS linear
chain, indicating a collapsed state. The Rg’s of the 70 kDa and 91 kDa G4-PBE-PS
hybridsare approximately equal to that of a corresponding linear PS chain indicating a

non-collapsed state for these systems.

60

AlALle
vvv v

i

A A A
v

vr

vv

 

To rule out the possibility of deuteration effects between the linear block and the

solvent, we studied G4-PBE-PS hybrids and linear PS in THF at a concentration of 4

mg/mL via DLS to determine Rh, see Figure 3.13. The 46 kDa hybrid has a smaller Rh
than a corresponding PS chain, again demonstrating it is in a collapsed state. The 70 kDa

and 91 kDa hybrids have an Rh that is larger than the corresponding PS chain similar to

the Rg results for the deuterated material.

 

 

1o IIIIIIIIIIIIIIIIII II I I I I I I _I_

v

9E I G4-PBE-PS i ::

: 0 PS i -'

.3 -.

A 71‘ i 1:

E . ,.

r: - ..
v ..

as“

 

M (Da)

Figure 3.13. A plot of hydrodynamic radius versus molecular weight for G4-PBE-PS
hybrids and P8 in THF at a concentration of 4 mg/mL.
The 46 kDa G4-PBE-PS hybrid’s size is below that of the corresponding PS linear
chain, indicating a collapsed state. The 70 kDa and 91 kDa G4-PBE-PS hybrids’ Rh
is larger than the corresponding linear PS chain.

61

We hypothesized that the 20kDa hybrid has the conformation of a knitted coil for
the conditions studied (Figure 3.1). This hypothesis is supported by data that show the
dendrimer block is a spherical object with a radius about 1 nm larger that the
corresponding ‘free’ dendrimer, as well as a linear block that is more contracted than its
native PS chain. A more contracted linear block potentially indicates that the linear chain
is weaving in and out of the dendrimer free volume, thus occupying a smaller space than
if it were a free chain in solution. Comparing the size of the linear block, 3.75 z 0.25 nm,
to the size of the dendrimer block, 1.82 i 0.01 nm, shows that some of the linear block
can be shielded by the dendrimer. Our data for this system do not support an encapsulated
dendrimer conformation since attempts to fit the SANS data of this system to a core-shell
morphology failed.

In the 45kDa system, the linear block is in a more compact form than a free dPS
chain, similar to the 20 kDa system. However, the dendrimer has adopted an extended
form rather than a sphere. Further, the 100kDa hybrid adopted a form where the linear
block is essentially equivalent to an unattached linear molecule with a cone-like
dendrimer attached to it. We believe that the increase in molecular weight triggered a
change from the spherical to an extended/cone-like morphology allowing the linear chain
to maximize the system entropy. This is the hypothesis advanced by Jeong et al.22
following the ideas of Skvortsov et al.“'2 Basically, a chain tethered in a high energy
region (dendrimer) next to a lower energy one (free solvent) will adopt a random walk if
its tethered far from the interface and is of small molecular weight. However, increasing

the molecular mass allows greater sampling of the lower energy region until the chain

straightens within the high energy space and adopts a random walk in the low energy

62

region. This appears to be similar to what we observe except the dendrimer (high energy
region) is flexible and changes shape to minimize contact with the linear macromolecule.
Skvortsov et al. 42 clearly demonstrate that the key to this transition is the relative size of
the chain to tethering distance and so it is expected that the dendrimer generation will
influence the molecular weight at which the morphological change occurs.
Conclusion

The conformation of linear-dendrimer diblock copolymers in solution can be
probed by deuterating one block and performing a series of SANS contrast match
experiments. It was found for the G4-PBE-dPS system that there is a transition from a
knitted to random coil conformation upon increasing the relative molecular weight of the
linear block to the dendrimer block. Further, the dendron changes shape from spherical to
an extended, cone-like structure. This unexpected transition is expected to be driven by
the linear chain maximizing its entropy at the expense of the dendrons. We believe this
shape change, if triggered through external stimuli, can be exploited in the design of a
range of advanced materials ranging from sensors to molecular machines. Future

research will examine these issues and the development of a viable system.

63

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12. T ande, B. M.; Wagner, N. J .; Mackay, M. E.; Hawker, C. J .; Vestberg, R.; Jeong,
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13. Chapman, T. M.; Hillyer, G. L.; Mahan, E. J .; Schaffer, K. A., J. Am. Chem. Soc.
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16. vanHest, J. C. M.; Delnoye, D. A. P.; Baars, M. W. P. L.; vanGenderen, M. H. P.;
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17. Santini, C. M. B.; Johnson, M. A.; Boedicker, J. Q.; Hatton, T. A.; Hammond, P.
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Mackay, M. B.; Jeong, M.; Hong, Y.; Hong, S.; Gido, S. P.; Tande, B.; Wagner,
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66

CHAPTER FOUR: Synthesis and Solution Behavior of PAMAM Di-dendrons
Introduction

Dendrimers are a distinctive class of macromolecules consisting of branches
emanating from a focal point to a precise number of end groups.1 They differ from
hyperbranched polymers in that they are synthesized in a controlled manner resulting in
molecules that have a specific molecular weight and a well defined architecturez’ 3
Applications of dendrimers are often dependent on the knowledge of the internal segment
density of their branches and if certain environmental factors can govern this density
distribution. Of particular interest is the design of smart materials that are capable of
delivering a drug or signaling a disease, which would occur by a system conformational
change. Meijer’s dendritic box4 has shown the capability of dendrimers to sequester
small molecules within their core, which implies potential applications to drug delivery.

Most work done on the behavior of dendrimers in solution has focused on generations
five and larger with tri-functional and tetra-functional cores where results on the
morphology varies vastly between systems. Theoretical and experimental work
investigated the morphology of dendrimers with respect to generation number as well as
solvent quality. One of the earliest theoretical investigations of dendrimers in solution
was performed by de Gennes and Hervet,5 who used self-consistent field analysis. They
assumed the monomers of each generation lie in a concentric shell and that the end
groups lie at the surface of the molecule, a dense shell morphology. These assumptions
resulted in a maximum generation limit due to steric crowding of the end groups,

however no other theoretical results have matched this theory to date.

67

Experimental evidence supporting the ‘dense-shell’ theory of de Gennes and Hervet5
was shown by Topp et al.6 who used small angle neutron scattering (SANS) to study the

spatial distribution of the terminal groups of poly(amido amine) (PAMAM) dendrimers
by labeling the end groups with deuterium. They found that the radius of gyration (Rg) of

the deuterated end groups of generation seven (G7.0) is nearly 0.5 nm larger than that of
the entire dendrimer, and from this concluded that the end groups are located at the

periphery of the molecule. Since then it has been shown that making such a conclusion on
Rg results alone is invalid.7

Muthukumar and Lesanec8 published the first theory to contradict the work of
deGennes,5 where they presented a kinetic growth model of a starburst dendrimer with
tri-functional branch points and flexible spacers between pairs of the branch points.
From an algorithm that allowed the molecule to grow in a self-avoiding walk, they
concluded that the end groups of the dendrimer are not at the surface of the molecule but
rather buried within the cavities of the dendrimer, causing the focal point to have the
highest molecular density which decays monotonically to the edge of the molecule. This

theory also predicted the scaling of the radius of gyration with respect to molecular

weight to be RgocMo'5 for G30 and lower, whereas for higher generations (G = 5.0-7 .0)

RgocMo’Oz'tO'02 was proposed.

Experimental evidence has been published showing that the end groups of dendrimers
are capable of back-folding to the interior of the molecule, supporting the dense-core
model. Wooley et al.9 used rotational-echo double-resonance NMR on poly(benzyl ether)
dendrimers containing isotope labels of '3 C at the end groups and 19F at the focal point.

The intramolecular distances between the focal point and end group were found by the

68

13C-19F dipolar coupling and were approximately 1.2 nm for generations 3-5. They found
decreased interpenetration of the end groups towards the focal point for larger
generations, which is consistent with the geometric constraints of the larger generations.
Ballauff et al.10 used contrast variation studies on fourth generation urea-fimctionalized
poly(propyleneamine) dendrimers and showed that the end groups are dispersed
throughout the dendrimer with a broad maximum located at a radial distance that is
approximately half of the overall size of the molecule.

In addition to the earlier work of Muthukumar and Lesanec,8 theoretical calculations
have shown that dendrimer end groups are capable of ‘back-folding’ into the interior of
the molecule. Mansfield and Klushinll allowed PAMAM-type dendrimers to come into
equilibrium using Monte Carlo calculations and discovered that for generations greater
than four, the end groups of the dendrimer are back-folded towards the focal point. They
concluded that crowding in the outer layers for the larger generations forces the density to
go through a minimum near the core of the molecule, resulting in a slight hollowness or
lower density near the dendrimer’s core, which is in agreement with the results of
Ballauff et al.‘0 For low generations, the density was greatest near the center of the
molecule and decreased monotonically to the periphery, demonstrating the change in
morphology of theses molecules as a function of generation.

Murat and Grest12 did theoretical studies on PAMAM type dendrimers and showed an
increase in internal segment density when the dendrimer is in a poor solvent, which
resulted in a ~45% decrease in size. They also concluded that lower generations have
3

stretched out branches, causing a lower density near the core. Welch and Muthukumar1

did Monte Carlo simulations on the effect of ionic strength and pH on the size of

69

dendrimers and found that the size can increase by as much as 180% as the ionic
concentration and pH are changed. The variance in size of the molecule occurred with a
new density distribution where the maximum density changed fi'om the core to the

1. ‘4 modeled

periphery as the solvent conditions were altered. More recently, Goddard eta
the behavior of tetrafunctional PAMAM dendrimers at various protonation levels in a
good solvent and found the molecular dimensions of the dendrimer varying up to 16%
from high to low pH. Goddard and coworkers expanded on this work by adding Cl'
counterions for neutral and low pH conditions, which neutralizes the charge of the
dendrimer.15 They found the presence of the counterions causes the dendrimer to swell in
size, particularly at high protonation levels.

In addition to the theoretical studies on the effect of solvent on the dimensions of

dendrimers, experimental investigations have shown radii changes, varying from 30% to

no change, depending on the system studied. SANS studies of tetrafunctional PAMAM
dendrimers were done by Amis et al.“5 in dilute solutions of D(CH2)mOD (with m=0, 1,

2, 4). PAMAM G5.0 dendrimers decreased in size by ~10% when the solvent was
changed from D20 to dlo-butanol. A greater transformation of the radius was observed
by Elmer and Stechrnesser17 who used holographic relation spectroscopy to determine the
hydrodynamic radius of a tetrafunctional PAMAM dendrimer in both butanol and water.
A nearly 30% reduction in size was observed for G6.0 upon changing from a good
solvent, water, to a poor solvent, butanol. The influence of solution ionic strength on the
dimensions of a PAMAM G4.0 dendrimer with an ammonia core was studied via
intrinsic viscosity by Tomalia et al.18 who found a 15% reduction in visciometric radius

(Rn) when changing from H20 to a 1M HCl aqueous solution. The smaller Rn when the

70

dendrimer is in acid was attributed to protonation of the amines that disturbs the
intermolecular hydrogen bonding, and thus decreasing the viscosity and Rn- Nisato et

al.19 used SANS to study a tetrafunctional PAMAM G8.0 dendrimer and found contrary
results; the ionic strength of the solution had no effect on the size of the molecule. In this
study the pH was lowered to 4.7 without the dendrimer changing size, yet Tomalia et al.‘8
demonstrated that no change in size occurs until the pH is below 2, which is required for
protonation of the internal tertiary amines. However, earlier SANS studies by Topp et al.
on PAMAM G5.0 dendrimers showed that the addition of acid to dendrimer solutions in
D20 resulted in liquid-like ordering which were attributed to interparticle interactions
that increase ionization of the terminal amine moieties.20 When NaCl was added to the
solutions, the charges of the amine groups were screened resulting in the elimination of
local ordering of the molecules. The difference in these two studies is the generation of
dendrimer used. The lower generations are less sterically hindered and more capable of
changing size. However the G80 system used to study the size change was too large to
exhibit significant variation in dimensions with respect to solvent type.

Extensive experimental studies have shown that as the generation number
increases the morphology of the dendrimer transforms from an extended to a globular
morphology. Evidence of this transformation has been shown by the intrinsic viscosity of
poly(benzyl ether) dendrimers as a function of generation, which goes through a
transition at generation 4. The intrinsic viscosity of the early generations increases with
molecular weight, which is also typical of linear polymers.”’ 22 A decrease in viscosity is

observed for higher generations due to increased steric crowding of the dendritic arms

71

and the adoption of a more globular shape. A maximum in intrinsic viscosity was also
reported for PAMAM tri-dendrons near generation four.”’ 24

Globular transition of dendrimers with increasing generation has been shown via
other experimental methods. Hawker et al.25 studied the microenvironment of Fréchet-
type dendrimers containing a polarity sensitive solvatochromic probe at its focal point,

which is sensitive to the polarity of the surroundings, and found a discontinuity in the

emission Am“ when going from G3.0 to G 4.0, suggesting the transition from an extended
to a globular state. We have observed a similar transition in Am for a PAMAM dendron

with a dansyl group attached at the focal point (see Chapter 5). This result is also
analogous to those of Tomalia et al.,26 who used photoinduced electron-transfer to detect
a transition in PAMAM structure from extended to globular between G30 and G4.0.
Earlier work by Tomalia et al.27 established that the amount of solvent within tri-
functional PAMAM dendrimers reaches a maximum at generations 5 and greater
suggesting a dendrimer structure that changes with increasing generation and reaches a
constant morphology at large generations. Prosa et al.28 used small angle X-ray
scattering to investigate tetra-functional cored PAMAM dendrimers from G3.0—G10.0
and found that generations 5 and higher have a more dense structure, with nearly uniform
internal segment densities.

In this study, we used SANS and dynamic light scattering (DLS) experiments to
investigate the effect of solvent quality on the molecular dimensions of PAMAM
dendrons, G3.0-G5.0, with a di-functional core. Solvents used for the neutron scattering
experiments include dm-butanol, dis-ethanol and (la-methanol and D20. PAMAM

dendrons were synthesized by exhaustive Michael addition of a 2-methoxyethylamine

72

(MEA) core to methyl acrylate followed by transamidation with ethylenediarnine to result
in G1.0, Scheme 4.1. These steps were repeated until G5.0 was reached with thorough
characterization and purification of each intermediate generation. We also investigated
the impact of the functionality of the core on the behavior of PAMAM dendrons in
solution, where we used SANS and DLS to characterize tetra-functional cored G4.0
PAMAM dendrons (Tetra-G40) in D20 and d'O-butanol. In addition we looked at the
effect of ionic strength on the size of a di-functional, 2-methoxyethylamine cored

generation 4 (MBA G4.0) via DLS experiments.

 

 

O O
O /
/0\/\NH 3,er \AO/ : / \/\N O
2
35°C 2 days
1 MeOH O O/
94% yield
2
O
> H
55°C 2 days i
90% yield H
3

Scheme 4.1. Synthesis of MEA-PAMAM dendrons.

73

Experimental

Materials. All chemicals were purchased from Aldrich Chemical Co., Saint Louis, MO.
Tetra-functional PAMAM G4.0, (ethylenediamine core) was donated by Dow Chemical
Co., Midland, MI. Ethylenediamine and 2-methoxyethylarnine were distilled under
nitrogen prior to use. Methyl acrylate was distilled from KOH to remove the inhibitor.
1H NMR and 13 C NMR spectra were obtained using a Varian 300 or 500 MHz
instrument, as indicated, using the residual proton signals fiom the solvent was as the
reference peak.

PAMAM Synthesis

2-Methoxyethylamine-PAMAM (MEA-PAMAM) G0.5 (2): A solution of 1.00g
(0.013 mol) of 2-methoxyethylamine (MBA) in 25 mL of methanol was slowly dripped
into 34.3 g (0.400 mol) of uninhibited methyl acrylate. The reaction was stirred for two
days at 35 °C under nitrogen. Methanol and unreacted methyl acrylate was removed in
vacuo, resulting in 3.14 g of 2 as a clear oil (98% yield). 1H NMR (CDC13, 300 MHz) 8:
3.46 (s, 6H), 3.23 (t, 2H, J = 6.0 Hz), 3.13 (s, 2H), 2.63 (t, 4H, J = 7.2 Hz), 2.45 (t, 2H, J
= 6.0 Hz), 2.26 (t, 4H, J = 7.2 Hz) 13C NMR (CDC13, 300 MHz) 173.2, 71.0, 59.0, 51.2,
49.9, 32.1.

MEA-PAMAM G 1.0 (3): A solution of 0.97g (3.8 mmol) of 2 in 20 mL methanol
was slowly dripped into 6.85g (0.114 mol) of freshly distilled ethylenediamine. The
reaction was stirred at 55 °C for two days. Methanol and unreacted ethylenediamine was
removed under vacuum. The remaining residue was dissolved in octanol (100 mL) and
stirred under vacuum (5 mTorr) at 40 °C for 2 days to remove residual amounts of

ethylenediamine. The octanol mixture was washed with 200 mL solution of 50:50

75

hexanezwater. The water layer was collected and washed with hexanes (3 x 100 mL),
then concentrated and dried under vacuum (5 mTorr) at 40 °C, resulting in 1.03 g of 3 as
a clear oil, (90% yield). 1H NMR (CDC13, 300MHz); 5 7.4 (br s, 2H), 3.4 (t, 2H, J = 5.4
Hz), 3.25 (s, 3H), 3.10 (quartet, 4H, J = 6.3 Hz), 2.80-2.60 (m, 8H), 2.54 (t, 2H, J = 5.4
Hz), 2.70 (t, 4H, J = 6.6 Hz), 2.14 (s, 4H). 13C NMR (CDC13, 300 MHz)
8 172.4, 70.2, 58.6, 53.1, 50.1, 42.0, 41.1, 34.0

General Procedure for synthesizing MEA-PAMAM dendron
Conditions for the formation of subsequent half and whole generations were identical to
that for the formation of 2 and 3 respectively. To avoid impurities, it is imperative to
remove all residual monomer before proceeding to the next step.
MEA-PAMAM G 1.5: 1H NMR (300 MHz, CDC13) 8: 3.64 (s, 12H), 3.50 (t, 2H, J = 6.0
Hz), 3.21 (s, 3H), 3.07 (t, 4H, J = 5.7 Hz), 2.80 (m, 12H), 2.72 (t, 2H, J = 6.0 Hz), 2.51 (t,
4H, J = 5.7 Hz), 2.45 (t, 8H, J = 7.2 Hz), and 2.41 (t, 4H, J = 7.2 Hz). 13C NMR (300
MHz, CDC13) 5: 174.6, 172.6, 70.4, 58.7, 52.7, 52.6, 51.6, 50.3, 49.9, 49.1, 37.0, 32.5.
MEA-PAMAM 02.0: ‘H NMR (300 MHz, (id-methanol) 5: 3.49 (t, 2H, J = 5.4 Hz), 3.24
(s, 3H), 3.17 (t, 12H, J = 6.6 Hz), 2.86-2.60 (m, 22H), 2.54 (t, 4H, J = 6.6 Hz), and 2.32
(t, 12H, J = 6.6 Hz). 13C NMR (300 MHz, (Ii-methanol) 5: 175.2, 175.0, 72.0, 60.0, 53.8,
53.2, 51.2, 49.9, 43.0, 42.0, 38.7, 35.0, 34.5.
MEA-PAMAM G2.5 1H NMR (500 MHz, CDC13) 5: 3.50 (s, 24H), 3.29 (t, 2H, J = 5.4
Hz), 3.15 (s, 3H), 3.10 (q, 12H, J = 6.0 Hz), 2.63 (t, 12H, 1 = 7.0 Hz), 2.58 (t, 16H, J =
7.0 Hz), 2.51 (t, 2H, J = 5.4 Hz), 2.42-2.36 (m, 12H), 2.27 (t, 16H, J = 7.0 Hz), and 2.19
(t, 12H, J = 7.0 Hz). 13(3 NMR (500 MHz, CDC],) 5: 172.8, 172.4, 172.3, 70.0, 59.0,

52.4, 52.2, 51.3, 49.8, 48.8, 37.9, 37.0, 34.0, 33.5, 32.1.

76

MEA-PAMAM 03.0 1H NMR (500 MHz, CDCl3) 5: 8.05 (t, 4H, J = 7.8 Hz), 7.98 (t,
2H, J = 7.8 Hz), 7.75 (1, 8H, J = 7.8 Hz), 3.41 (t, 2H, J = 5.4 Hz), 3.28 (s, 28H), 3.26-3.16
(m, 28H), 2.54-2.40 (m, 42H), 2.40-2.20 (m, 12H), and 2.40-2.20 (m, 28H). ”C NMR
(500 MHz, CDC13) 6: 175.1, 174.9, 72.0, 59.9, 53.5, 51.2, 49.9, 43.0, 42.0, 37.2, 34.9,
34.7.

MEA-PAMAM 03.5 1H NMR (500 MHz, CDC13) 5: 3.60 (s, 48H), 3.36 (t, 2H, J = 5.4
Hz), 3.20 (m, 31H), 2.80-2.64 (m, 62H), 2.60-2.45 (m, 28H), and 2.40—2.25 (m, 60H). ”C
NMR (500 MHz, CDCl3) 5:1729, 172.3, 172.1, 74.0, 59.1, 53.7, 53.5, 53.4, 51.8, 50.4,
50.1, 49.9, 49.7.

MEA-PAMAM 04.0 1H NMR (500 MHz, di-methanol) 5: 3.50 (t, 2H, J = 5.4 Hz), 3.37
(s, 3H), 3.34-3.10 (m, 60H), 2.90-2.66 (m, 92H), 2.62 (m, 26H), and 2.48-2.20 (m, 60H).
”C NMR (500 MHz, dd-methanol) 5: 173.1, 172.6, 67.0, 59.0, 52.6, 52.4, 52.1, 50.3,
42.1, 41.2, 37.8, 37.6, 34.2, 33.8.

MEA-PAMAM 04.5 1H NMR (500 MHz, CDC13) 5: 3.42 (s, 96H), 3.20 (t, 2H, J = 5.4
Hz), 3.12 (s, 3H), 3.10-2.90 (m, 60H), 2.60-2.50 (m, 124H), 2.37-2.28 (m, 60H), 2.20 (t,
64H, J = 7.0 Hz), and 2.10-1.90 (m, 60H). ”C NMR (500 MHz, CDC13) 5: 172.4,
171.82, 171.75, 70.0, 57.6, 52.3, 52.0, 51.8, 51.0, 49.5, 49.3, 48.5, 37.0, 36.7, 33.2, 32.1.
MEA-PAMAM 05.0 ‘H NMR (500 MHz, (Id-methanol) 5: 3.42 (1, 2H, J 5.4 Hz), 3.36
(s, 3H), 3.30-3.10 (m, 124H), 2.82-2.64 (m, 188H), 2.60-2.50 (m, 60H), and 2.40-2.20
(m, 128H). 13C NMR (500 MHz, 114-methanol) 6: 175.2, 174.9, 70.0, 59.9, 54.4, 54.2,
53.9, 51.5, 42.3, 41.9, 38.4, 34.5.

SANS. Measurements were taken at the small-angle neutron diffractometer (SAND) at

Argonne’s Intense Pulsed Neutron Source. A sample to area detector distance of 2.0 m

77

was used. The radiation had a wavelength range of 1-14 A making the total wave vector
(Q) range covered 0.0034-0.6 A ’1, where Q = 4nsin(0/2)/?e with 0 and A defined as the
scattering angle and radiation wavelength, respectively. Measurements were taken at
room temperature. Low concentrations were used for each sample to generate sufficient
scattering in the allotted time while minimizing intermolecular scattering effects.
Samples were filtered through a 0.1um Whatman Anodisc 13 filter and sonicated for 20
minutes and placed in a hermetically sealed quartz cell for the scattering experiments. A

list of samples is given in Table 4.1.

Table 4.1. Sample codes and theoretical molecular mass (M) of MEA compounds.

 

 

 

 

 

 

 

Sample Code M (Da) # end groups end group functionality
MEA 03.0 1671 8 ‘ 1° amine
MEA G35 3047 16 methyl ester
MEA G40 3261 16 1° amine
MEA G45 6247 32 methyl ester
MEA G50 7143 32 1° amine
Tetra G40 6872 32 1o amine

 

 

 

 

 

 

Neutron scattering data for dendrons was analyzed in the Guinier regime.29 This
analysis allowed for quick determination of the shape and size of the dendrimer, with all
samples in this study fitting to the compact-objects (sphere) model. The scattering
intensity, I(Q), in the low Q regime was evaluated using the expression for compact
(‘sphere-like’) objects with a given radius of gyration (Rg),30

22 22

1(Q)~¢5V(A,0)2 exp -Q3 8 E1(0)<’«XP - 3 g (4.1)

 

 

78

where 0 is volume fiaction of scattering centers, V, the volume of each scattering center,
(Ap)2, the square of the difference of the solvent and the scatterer scattering length
density (SLD) and 1(0), the intensity at zero wave vector. The slope resulting from a plot

of ln(I(Q)) versus Q2 in the region where QRgzl allows Rg to be obtained. The radius of

a homogeneous, constant density sphere is determined from the Rg by R = Rg/0.775. We

also analyzed the scattering data of the dendrons in the Guinier regime using the Guinier
approximation for both rods and sheet-like objects, however neither of these
approximations resulted in a good fit indicating the dendrons are always sphere-like for
our experimental conditions.

To determine the goodness of each Guinier fit, the experimental value of 1(0) was
used to determine the amount of solvent within each sample and check that the mass
density of the swollen dendrimer is reasonable. The numerical prefactor of eq 4.1 was set

equal to the experimental value of 1(0) and then solved for (Ap)2. Using this value of

(Ap)2 and the scattering length density of the solvent (psolv); we calculated the scattering

length density of the scatterers (pscat), which includes solvent and dendrimer. We

calculated the amount of solvent within the dendrimer by,
(pscat) = [psolv]Xe + [l'e]xpsample (4-2)

where 0 is the mass fraction of solvent within the scatterer and psample is the scattering

length density of the dendrimer.
Dynamic Light Scattering (DLS). Measurements were performed with a Protein
Solutions Dyna Pro-MS/X system with temperature control. All samples were filtered as

described above and allowed to equilibrate in the instrument for 25 minutes at 25 °C

79

before measurements were taken that resulted in calculation of the hydrodynamic radius
(Rh). Samples for agglomeration studies were pre-equilibrated at 25 °C before filtering.

The sample is illuminated by a semi-conductor laser with ~ 830 nm wavelength. The
light scattered at an angle of 90° is collected and guided via a fiber optic cable to an
actively quenched, solid state Single Photon Counting Module (SPCM), where the
photons are converted to electrical pulses and correlated. Autocorrelation is used to
analyze the time scale of the scattered light intensity fluctuations. The uniformity of the
sample sizes is determined by a monomodal curve fit, cumulants, which assumes a single
particle size with a Gaussian distribution.
Results and Discussion
Synthesis of MEA dendrons

Dendrons were synthesized fi'om a 2-methoxyethylamine core, 1, via exhaustive
Michael addition to methyl acrylate resulting in methyl-ester terminated MEA-PAMAM
G0.5 (MEA G05), 2 (Scheme 4.1). Excess methyl acrylate was used to ensure di-
alkylation of the primary amine moiety and was removed after the reaction was complete
by stirring under vacuum (5 mTorr) at 40 °C for 24 hours, resulting in a clear oil. We
used 1H and '3 C NMR to characterize purity of dendrons. '3 C NMR has been shown by
Tomalia et al. to be a useful technique to detect dendron impurities.18 Detailed NMR
spectra are in Appendix A. MEA G0.5 was reacted with excess ethylenediamine to afford
primary amine terminated MEA G10, 3. After thorough removal of excess diamine, the
primary amine end groups of the full generation allow for these two steps to be repeated
to obtain MEA G2.0. We continued the synthesis to MBA G5.0. Scheme 4.2 displays

the MEA G4.0 dendron, which has 16 primary amine end groups. The number of

80

dendron end groups and the molecular weight of these dendrons doubles with each
successive full generation, Table 4.1.
Size of MEA G3. 0-GS.0 Dendrons in D20

The molecular morphology of MBA dendrons G3.0-G5.0 was studied using the
Guinier analysis of a compact object, sphere, mentioned above,29 to determine their size
and shape as a function of generation. SANS experiments on 4 mg/mL dendron solutions
in D20 showed that all generations studied behave as sphere-like objects due to the
negatively sloped straight line obtained from a Guinier fit, Figures 4.1a-4.5a. The
intensity of scattering as a function of Q (Figures 4.1b-4.5b) is shown to demonstrate the

valid Q range used for the Guinier analyses. The firll generation dendrons, which have
primary amine end groups show an increase in R8 with increasing generation, with R3
values of 1.28 i 0.03 nm for MBA G3.0, 2.59 i 0.13 nm for MEA G40 and 3.18:1: 0.05
nm for MEA G5.0. The methyl-ester terminated half generation dendrons, MEA G35

and MEA G4.5, have Rg values of 0.563 t 0.04 nm and 0.736 d: 0.012nm respectively.

81

 

 

 

 

 

 

 

 

 

 

 

2IIIIIIIII'IIIIII' .1 IIII] 'fifirl'us
0 0 MEA (33 a 10 '6 MEA G3 _.
2 _2 D2034 mg/mL ‘ D20:4 mg/mL
E -4
§ -6
5' '8 Rg=1.281:0.03 nm 104 _ _,
-10 R=1.661 0.04 nm
'12 '5 13:33! ----3-:-n
2 4 6 810x10° 1° 5 67 0.1 2 3
Q2 (A'z) Q (A4)

Figure 4.1.SAN S data for MEA G 3.0 dendrons in D20.

a. Guinier analysis for MBA G3 dendrons in D20 at a concentration of 4 mg/mL, data fit
for a compact (sphere-like) object. b. I vs Q plot of MBA G3 dendrons in D20 at a
concentration of 4 mg/mL.

 

 

 

    
 

 

 

 

 

 

 

 

 

 

 

.2 ITIIIIIIIIIIIIIIIIIIII. 100 I ['Il'l'vvu I I,
1- + MEA 63.5 1 O MEA 63.5 b
A -3_- D2(33""'T‘9/'T"— -_ 10-1 _ D2024mg/mL ..
lg 41- i F; .2
z " . . 9'10 ‘5 I um
01 : i A
z . . a _3
‘5 '5." '2: =10 7 7
— 1. Rg=0.563:l:0.04nm..
t R: 0.731 e 0.06 nm :: .4
.6- 10 _ '—
:se!esee!sus!!sashes; - sass-3!::::3 55
10 20 30 4050x10° 4 ° 801 2
2 -2 '

Figure 4.2. SANS data for MBA G 3.5 dendrons in D20.
3. Guinier analysis for MBA G3.5 dendrons in DzO at a concentration of 4 mg/mL, data

fit for a compact (sphere-like) object. b. I vs Q plot of MBA G3.5 dendrons in DzO at a
concentration of 4 mg/mL.

82

        
   

 

 

 

 

 

 

 

 

 

4 .
1
+ MEAG4 a, 1 MEA 04 b
0 020: 4 mg/mL j 10 D20: 4 mg/mL
.2 = 10°
IE 3 YA .
3 -4 £1 5 101
g ‘1; i Q 10'2
E -8 Rg=2.59:|:0.13nm 1 10'3 - a
R=3.353:0.16nm5 _,
2 10 _' l a
.12 . ‘. u... I I nun 4-4-1 1 L
4 8 12X“ 0.01 2 4 a 0.1 2 4
.1
02(A'2) Q(A )

Figure 4.3. SANS data for MEA G 4.0 dendrons in D20.

a. Guinier analysis for MBA G4.0 dendrons in D20 at a concentration of 4 mg/mL, data
fit for a compact (sphere-like) object. b. I vs Q plot of MEA G4.0 dendrons in D20 at a
concentration of 4 mg/mL, where the circled region indicates agglomeration, thus low Q-
regime was not used in the Guinier approximation.

  

 

    
  

  

O MEAG4.5
020: 4 mg/mL

a

 
  

 

 

 

 

A -2 i

8 -3 , —j 'E

z - - 3.
G I : A
Z '4 C" .1 g
75 . R°=0.736 r 0.012 nm . —

" 5 I R = 0.956 :1: 0.016 nm 3 °-

 

 

IIUU

 

-(
6 u[1111111111unnlrrnrlrrnrlrr
. fi fi

20 40 60x10‘3 _
Q’ (11*) o (A")
Figure 4.4. SANS data for MBA G 4.5 dendrons in D20.
3. Guinier analysis for MEA G4.5 dendrons in DzO at a concentration of 4 mg/mL, data

fit for a compact (sphere-like) object. b. I vs Q plot of MEA G4.5 dendrons in D20 at a
concentration of 4 mg/mL.

83

 
 
 

 

     
  
   

 

 

 

 

 

 

 

 

 

 
 

 

ut‘ ' ' ' U I 4' I I U I II ' ' ‘ ' I
2 u MEAGS a E o MEAG5 b
0 020: 4mg/mL _;j 10.1 D20: 4 mg/mL _
.1
'6': .2 .5 A
g 4 “I‘M H u ‘5 TE
6 : I:
:5 4i R,=3.18:r 0.05 nm 1 g
5 a R=4.14:t0.06 nrn _2
E 104 1- -
-10 -_
12 : -5 .L5 L 1111;113:211
0.5 1.0 1.5 2.0 2.5 3.01110" 10 2 3 ‘ 5 '7 01 2
0’ 1A") a (A")

Figure 4.5. SANS data for MBA G 5.0 dendrons in D20.
a. Guinier analysis for MBA G5.0 dendrons in D20 at a concentration of 4 mg/mL, data
fit for a compact (sphere-like) object. b. I vs Q plot of MBA G5.0 dendrons in D20 at a
concentration of 4 mg/mL.
The size of the dendrons as a fitnction of generation is displayed in Figure 4.6a.

The half generation dendrons have significantly smaller dimensions in solution compared

to the full generations. Despite the nearly two-fold increase in molecular weight between

MEA G30 and MBA G3.5, the R8 of the molecule has decreased from 1.28 i 0.03 nm
for MBA G3.0 to 0.568 i 0.06 nm for MBA G3.5. The same trend is observed for MEA
G40 and MBA G4.5, with Rg decreasing from 2.59 i 0.13 nm for MBA G4.0 to 0.736 i

0.012 nm for MBA G4.5. The full generation dendrons have primary amine end groups
that can act as both hydrogen bond donors and acceptors, whereas the half generation
dendrons have methyl ester end groups which are less polar and cannot act as hydrogen
bond donors. The decrease in size for the half generations is due to the poor interaction
of the end groups with the solvent, D20, demonstrating that the functionality of the end-

group is critical to the behavior of the PAMAM di-dendrons in solution.

84

 

 

 

 

 

 

 

 

4 .q.-.....-. M) .. ”""l‘“”*' l .. . ...+ l l w l 1
a b

5 i I ““1”” '-

“g E I

g 3

011,111+ M j i 1 j

3.0 3.5 4.0 4.5 5.0 3.0 3.5 4.0 4.5 5.0

Generation Generation

Figure 4.6. R8 and volume of MBA dendrons in DzO.

a. Rg vs. generation for MBA dendrons in D20 at a concentration of 4 mg/mL. b.
Volume/MW vs. generation for firll generation, amine terminated, MEA dendrons in D20
at a concentration of 4 mg/mL, where volume was calculated using the volume of a

sphere and R8 as the radius.

Using the experimental value of 1(0) (Table 4.2) the mass fraction of solvent (0)
within the dendrimer was calculated using eq 4.2 as described above and was 0.28 and
0.30 for MEA G35 and MBA G4.5, respectively. The 0 values for the full generations
were 0.38 for MBA G3.0, 0.64 for MBA G40 and 0.61 for MBA G5.0. The large
increase in the amount of solvent contained within the dendrimer from MEA G3.0 to
MBA G4.0 can be explained by the system transitioning from an extended to a globular
structure as the steric requirements of the dendritic branches increase. Similar transitions
have been observed for poly(benzyl ether) dendrons via intrinsic viscosity 2" 22’ 25 and

solvatochromism25 studies as well as for carboxylate-terminated starburst dendrons via

photo-induced electron transfer measurements.26

85

Table 4.2. Rg values for MEA G3.0-G5.0 determined from the Guinier analysis
described in the text at a concentration of 4 mg/mL.
Symbols are explained in the text while Qmax is the maximum Q-value used in the

 

 

 

 

 

 

 

 

 

 

 

 

 

 

regression.
Sample MEA G3.0 MEA G3.5 MEA G4.0 MEA G4.5 MEA G5.0
solvent D20 D20 D20 D20 D20
a
Rg(llm) 1.28i0.03 0.5631004 2.59:0.13 0.736i0.01 3.18i0.05
b -1

[(0) exp (cm ) 0.0091001 0010100011 0.028i0.0015 0.051i0.017 0.05221:0.005
anx*Rg 1.211 1.228 1.42 1.703 1.180

0 C 0.38 0.28 0.64 0.30 0.61
Rb(nm)d 0.90:0.059 0.30i0‘05 e 1.90:0.05 1.40:0.05 2.30:0.10
Rg/Rh 1.421008 0.70:0.05 1.36:0.04 0.53:1:0.02 1.38:0.06

 

 

a Radius of gyration for a sphere-like object. b Experimental value of 1(0) from a sphere-
like object Gunier plot (eq 1). c 0 is the mass fraction of solvent within the scatterers.

Hydrodynamic radius of dendrons determined from dynamic light scattering. e This
value is on the lower edge of the instruments size detection limit, 1 nm.

Dynamic light scattering experiments were performed for MBA G3.0-G5.0 in

Milli Q water at a concentration of 4 mg/mL to determine the hydrodynamic radius (Rh).
Values of Rh are reported in Table 4.2. Information on the internal segment density of

the dendrons in solution can be gained from the ratio of Rngh (Table 4.2).31 An Rg/R;1
ratio of \/(3/5) 2 0.775 is indicative of a constant density, homogeneous sphere. A value
of 1.50 is descriptive of a monodisperse Gaussian coil in a theta solvent and 1.78
represents linear polymers in a good solvent,“ although experiments show values of 1.16
32.33

and 1.27 for linear polymers in theta solvents.

The half generation dendrons, MEA G35 and MEA G4.5 have Rg/Rh ratios of

0.70 .1: 0.05 and 0.53 i 0.02 respectively. These values are lower than expected for a

constant density sphere indicating the segmental distribution of the branches is not

86

uniform from the center to the edge of the molecule. A sphere whose maximum density
is at the center and decreases linearly to outer radius has a theoretical Rg/Rh ratio of

1(2/5) = 0.63. A sphere whose minimum density is at the center and increases linearly to
the outer radius has a theoretical Rg/Rh ratio of N/(22/35) = 0.80. The Rg/Rh values for the

half generation MEA dendrons indicate the maximum density is neither at the center nor

ll, 12

at the edge of the molecule, which matches theoretical calculations as well as

experimental results.10
The full generation dendrons have Rg/Rh ratios that are higher than the half
generations, demonstrating they have a different segment distribution in solution. The

lowest generation studied, MEA G3.0, has an Rg/R)1 ratio of 1.42 i 0.04, which is within
experimental error of the Rg/Rh ratios of MEA G4.0 and MBA G5.0, 1.36 i 0.04 and

1.45 i: 0.06 respectively. None of the full generation dendrons adopt the morphology of a
constant density sphere when in D20, but rather are in a more expanded state. Within the
sensitivity of our experiments, the segmental distribution of dendritic arms of the hill
generations does not change with generation.

A plot of V/M vs. generation of the full generation dendrons is shown in Figure
4.6b. This graph displays a maximum at generation 4.0, corresponding to a minimum in

.4
213and

density, which has been observed previously for poly(benzyl ether) dendrons
PAMAM tri-dendrons.3’ 23 This maximum at generation 4 can be explained by the

difference in growth of the molecular weight and volume of dendrimers in solution. The

molecular mass scales as
M ~ f.(fb. -1)g (4.3)

87

Where fc is the core functionality and fl,u is the multiplicity of the branching units. MEA
dendrimers have multiplicity of 2 at the branch junction points (fbu) and a core

functionality of 2 (fc=2). Thus molecular mass scales as

M ~ 2g (4.4)

We assume the dendrimers are sphere-like, which is appropriate according to our SANS

results, and each generation contributes linearly to the radius resulting in a volume that
scales as g3. This model predicts a maximum in theV/M ratio at a g=3/ln(fc(fbu-l)) or

between the fourth and fifth generations for the MEA-PAMAM system. A linear
increase in radius with increasing generation assumes the dendrimer is in a good solvent.

Tomalia et al.23 studied PAMAM tri-dendrons and found a linear increase in radius for
generations three and greater. The plot of Rg vs. generation (Figure 4.6a) shows an

approximately linear increase in the size of the full generation dendrons with increasing
generation, confirming the assumption is valid for our system. The maximum V/M ratio

for G4.0 indicates that MEA G4.0 has the greatest potential for changing size in solution.
Using the theoretical molecular weights listed in Table 4.1 and the R3 values in

Table 4.2, a log—log plot, Figure 4.7, demonstrates that the scaling of the MEA dendron
with respect to molecular weight is non linear between MEA G3.0 and G5.0 As the
dendrimer becomes larger, the mass increases at a rate faster than the size, corresponding

21, 22 and

to the maximum in intrinsic viscosity found for both poly(benzyl ether)
trifunctional cored PAMAM dendrimers.” 24 While more data are needed to thoroughly
explain the scaling behavior of these molecules, it is clear that the radius increases at a

decreasing rate with respect to generation number, agreeing with theoretical work of

88

Muthukumar and Lesance8 as well as more recent results of Scheler and Fritzinger35 who
studied the scaling behavior of carboxylate terminated tetra-fimctional cored PAMAM

dendrons in solutions of methanol.

 

 

 

 

E
U I v
. .
. .
1
.
I
‘ l - y t 1 l O O '
. . . . . . . . . .
‘ i 1 r l v I 0 - ‘
4 ‘ . . . . y . r . '
- .,_.-......_.-...._.-.--.,._---...---..,...-._....-.-......-.-....-._,-_.-,--.,..-,,
A l I I I l I 1 l r
4 A l I I l I I l l
. 1 1 1 1 1 1 i 1 .
. . . . . . . .
I i
I U
. .
I r
I I I l I I
i , . . r 1 o
' 1 . . . . . .
3- .‘,-.-.. ._..-.-....-.-....j..-.-_..._._...:_.._._....‘..-.....'.-.-..:..-.-.:....:.-..
. . . . . .
' 1 1 1 I 1 1 1
1 | I Q l I I t
‘ l I I 1 I I I
. .
. .
. .
A ; .
I
.
I
.
c i : 2 : : : : : : 1
’ l A I I I I I l I
v 1 r . . . a . 1 . I
. , , .
2.. .,_._.. .. .._._._ ..-._.. ..:._._.. .._.-.. :. ..-._.. ..-.-.. ..,' ...... 5..-.-1. .. ..-..', .....
f ‘ I I P i I . I I ‘
. . . . . . . . . .
. . . . . . . . . .
. I r I 1 v I t t
. . . . . .
I I I O l '
I I l i 1 A
. . . . r ,
l I I 1 I 4
2 2 3 ' . .
z : . : z 1
.
l i r 2 1 '
1 1 1 1 1 .
. . . . . l
. . . . . .
l l l I I I
. . . . . .
. . . . r
I I I I O u
. . . . . ,
1 1 1 1 1 .
. , . . . .
. . . . ,
‘ 1 i I = 1 i
- 2 1 l 1 1 z
.
. i 1 - e 2 .
. .
_ a ..........L......... ...'.-.... ' 1, I a
. . . . . .
‘ n V I U l .
. . . . . . .
n n n I I l D 0
v
I l l l l l l l I I

Figure 4.7. Radius of gyration vs. molecular weight for the MEA dendrons G 3.0-G 5.0
in D20 at a concentration of 4 mg/mL.

Effect of Solvent Quality on the Dimensions of MEA-G4. 0 in Solution

The magnitude of size change for MBA G4.0, Scheme 4.2, in solution was
determined by performing SANS experiments in a series of solvents, D(CD2)mOD, with
m = 1, 2, and 4. The scattering data were fit to a Guinier analysis for compact objects,
spheres, to determine the R8 for each system (Figures 4.8a-4.10a). The graphs of

intensity of scattering as a function of wave vector (Figures 4.8b-4.10b) were used to

determine the valid Q range for the Guinier analysis.

89

The Rg of MBA G4.0 dendrons is sensitive to the solvent quality, as shown in a

plot of R8 versus m, from m=0 (D20) to m=4 (rim-butanol), Figure 4.10. The values of R8

are also reported in Tables 4.2 and 4.3. The size of the MEA G4.0 dendrons decreases as
the polarity of the solvent decreases. The dendron radius shrinks from 2.59 i 0.13 nm to
0.91 i 0.06 nm when the solvent is changed from D20 to dm-butanol, a 67% decrease in
radius, that corresponds to a nearly 96% decrease in volume (Figure 4.11). These results
contradict those obtained by Amis et al.,16 who observed only a 10% reduction in the size
of a 68.0 PAMAM tetra-dendron when changing the solvent from D20 to dIO-butanol.
The greater size change of the MEA G4.0 system compared to the G8.0 tetra-dendrons
can be attributed to the disparity in functionality of the core; MEA G4.0 has a di-
functional core which is less sterically hindered than the tetra-functional core of the
system studied by Amis et al.‘6 Also our results (Figure 4.6b) indicate that G4.0 has the
largest volumezmolecular mass ratio and thus the greatest potential for changing size in

solution.

90

 

 

    

 

 

 

 

 

 

 

 

 

 

5:IlIllI'IIFIIIIIIIIIIII'q ""I I IIIII‘l""I
- a w o b
t . MEAG4 : 10 " O MEAG4 '-
== 0— d4-methanol " 4
~.- - ~ .1 . d-methanol
g ' I ‘7" 10
v 5 - g
6 ' "‘3 I 10'2
E i : 9,
c _ q _
" -1o : Rg=1.76 1 0.08 nm 2 10-3
: R=2.30 10.10nm:
.15pilllllllllelllllJllljl‘ a 104 A 11 1 .-1
5 1o 15 20251110 2 ‘ “0.1 2

o2 (A") o (A")

Figure 4.8. SANS of MBA G4.0 dendrons in d4-methanol.

a. Guinier analysis for MBA G4.0 dendrons in d4-methanol at a concentration of 4
mg/mL, data fit for a compact (sphere-like) object. b. I vs Q plot of MBA 4.0 dendrons

in d4-methanol at a concentration of 4 mg/mL.

 

 

 

   

 

 

 

 

IIIIIIIIIIIIIIIIIIIIII- 100 ""I I I llllll'b"‘
: . MEAG4 : 0 MEAG4
0_— dis-ethanol: 6
'5 5p : '5
v '. '5‘. z:
9. - I - 9.
-1o:— Rg=1.391:0.05 nm-E
- R=1.81i0.07nm -
:ssllss-Ihsseh-lsll:

 

 

 

 

1o 20 30 40x10“
0’ (A4) 0 (Ah

 

Figure 4.9. SANS of MBA G4.0 dendrons in d6-ethanol.
a. Guinier analysis for MBA G4.0 dendrons in d6-ethanol at a concentration of 4 mg/mL,

data fit for a compact (sphere-like) object. b. I vs Q plot of MBA 4.0 dendrons in d6-
ethanol at a concentration of 4 mg/mL.

91

 

  
  

OITTIIIIIIIIIIIIIIT—ITT
0.1

   

O MEA G4 b
dw-butanol

 
   

0.01

IIIIIIIIIII

  

I IIIIIIII I .7

 
 

I(Q) (cm-1)

 

0.001

In( I(Q) (cm"»

 

 

 

 

Rg=0.91i0.06nm:
'3 R=1.18:I:0.07 nm j
II IIIIIIIIIIIIIIIII

 

 

1o 20 30 40x10'3 0.01 0.1
2 -2 .
Q (A ) Q (A 1)
Figure 4.10. SANS of MBA G4.0 dendrons in dlo-butanol.
a. Guinier analysis for MBA G4.0 dendrons in d l0-butanol at a concentration of 4

mg/mL, data fit for a compact (sphere-like) object. b. I vs Q plot of MBA 4.0
dendrons in d I -butanol at a concentration of 4 mg/mL.

 

1 1 1 1 1 1 1
...-......-...,..-..............-.....‘V........—..,........-..,...........,.

1 1 1
1 1 1 1 1 1

I 1 1 1 1 1
I 1 I 1 I 1 1
I I 1 1 1 I
I 1 - 1 I 1 I
I 1 1 l I

1 1 1 1 . . 1
1 . . 1 1 1 1
1 1 1 1 . 1 .
1 1 . 1 . . .
I I I I I 1 I
1 1 1 1 . 1 .
. 1 . 1 1 1 1
. . . 1 1 . 1

..,............,.......................,......<.....,............._...........
1 1 1 1 1 1
I I I n I
I t I I I
1 1 1 1
O u v I 1 I
I I 1

1 1

R9[An gstrom s]
IN

1 1 q 1 1 1
I 1 1 1 1
1 1 1 1 1
I I I t I I
1 1 1 1 . I
1 1 1 1 1 1
1 1 1 1 1 - 1
1 1 1 1 1 . 1

..,---...-.-...,..--..-..-. ..v.....-.... .........<,...........,.... .......‘.
1 1 1 1 1 1
1 1 1
1 1 1 1
1 1 1 1 1 1
1 1 1 1
1 1 1 1
I I I I I I

1 1 1 1 1 1
1 1 1 . . 1
1 1 1 1 1 1
1 1 1 1 1 1 1
I I I I I I I
. 1 1 1 . 1 1
1 1 1 1 1 1 1
>...-......-...,.._.........-.1V.......‘............,........A.. ......--..-‘.

- 1 1 1 1 1
1 1 1 1 1 1
1 1 1 1 . 1
1 1 1 1 1 1
. 1 1 . 1

I 1 1 - 1 1 1
I 1 - 1 1 1 1
1 1 1 . 1 I

I 1 I 1 1 1 I

-1 0 1 2 3 4 5

m [Solvent D(CD2)mOD)

 

 

 

Figure 4.11. R8 of MBA G4 vs. solvent type (m) of D(CD2)mOD.

All solutions were at a concentration of 4 mg/mL.

92

To ensure that the MEA G4.0 dendron is capable of collapsing to the size
measured in rim-butanol, Rg = 0.91 i 0.06 nm, Rh = 1.40 i 0.05 nm, we calculated the
radius of a MBA G4.0 dendron in the bulk state. The reported bulk density (8) of these
dendrons is 1.22 g/cm3 .16 Using this value for the density and the molecular weight of
the G4.0 dendron, (Table 4.1) the minimum volume was calculated from V=M/8. The
minimum radius was found to be ~1 nm, close to the hydrodynamic radius of the MEA
G4.0 dendron in butanol, 1.40 i 0.05 nm.

The mass fraction of solvent, 0, contained within the MEA G4.0 dendrimer as a
function of solvent quality was extracted from the experimental value of 1(0) using
equation 4.2 as described above; these values are listed in Table 4.3. As the quality of the
solvent decreases the amount of solvent located within the dendrimer also decreases with
0 values of 0.46, 0.45 and 0.40 for MBA G4.0 in 114-methanol, d6-ethanol and dlo-butanol

respectively. These values are all smaller than the value for MBA G4.0 in D20, 0.64.

We performed DLS experiments on the MEA G4.0 dendrons in methanol, ethanol

and butanol at a concentration of 4 mg/mL to obtain Rh as a fianction of solvent quality,

Table 4.3. The MBA G4.0 dendron has an Rh in methanol of 1.50 i 0.05 nm and 1.40 i

0.05 nm when in either ethanol or butanol. To gain more information about the

conformational changes of the MEA dendrons as a function of solvent quality, the ratio
of Rg/Rh was calculated and these values are listed in Table 4.3. This ratio decreases
from 1.17 i 0.04 in dd-methanol to 0.99 i 0.04 in d6-ethanol to 0.65 i 0.03 in dlo-butanol,
indicating the dendron segmental distribution is largely influenced by the quality of

solvent. All of these values are larger than 1.36 i 0.04, which is the Rg/Rh value for the

93

MEA G4.0 dendron in water. We observed no difference in Rh values for dendrons in
D20 and H20, indicating negligible deuteration effects for this system. When in
dé-ethanol and dm-butanol, the dendrimer is in a compact state, and the Rg/Rh values are
similar to those found for poly(benzyl ether) G4.0 dendron in solution.36 The large
differences in the Rg/R11 ratios for dendrons placed in different polarity environments

demonstrate that MEA dendrons are capable of changing both size and internal segment

distribution.

Table 4.3. Rg values for MBA G4.0 and G5.0 in D(CD2)mOD determined from the
Guinier analysis described'in the text at a concentration of 4 mg/mL.

Symbols are explained in the text while Qmam is the maximum Q value used in the

 

 

 

 

 

 

 

 

regression

Sample Code —) MEA G4.0 MEA G4.0 MEA G4.0 MEA G5.0

solvent dI-methanol do-ethanol d1 U-butanol div-butanol

R£(nm) a 1.76i0.08 l.39i0.05 0.91i0.06 1.23i0.02

1(0).,p”(cm") 0025:0002 0024232000 0034:0001 0.065i0.0006

meing 2.34 2.47 1.87 2.44

9 c 0.46 0.45 0.40 0.52

Rh(nm)d l.50:t.05 1.40i'.05 1.401105 2.001105
_R—g/R|_. l .17i.0.04 0.9912004 0.6512003 0.6li.0.02

 

 

 

 

 

 

0' Radius of gyration for a sphere-like object. b Experimental value of I(O) from a sphere-

like object Gunier plot (eq 1). c 9 is the mass fraction of solvent within the scatterers.
Hydrodynamic radius of dendrons determined from dynamic light scattering.

94

 

Comparison of the Solution Behavior of MEA G4.0 and MEA G5.0

Since the MEA G4.0 dendron has a morphology that undergoes substantial
changes as a result of its interactions with the surrounding medium, we wanted to
confirm that MEA G4.0 is more dynamic in solution than MEA G5.0 as suggested by the
maximum V/M ratio displayed in Figure 4.6b. We used SANS and DLS to study the
MEA G5.0 dendron in 4mg/mL solutions of D20 and d'O-butanol. The radius of gyration
was determined from the Guinier approximation for compact objects, eq 4.1. The
Guinier plots of the MEA 65.0 dendron in D20 and d'O-butanol are displayed in Figures
4.5a and 4.12a respectively and indicate that the dendrons are sphere-like due to the
negatively sloped straight line. The plots of the intensity of scattering as a function of
wave vector for both the D20 and dIO-butanol system (Figures 4.5b and 4.12b) were used

to determine the valid Q range to use in the Guinier approximation.

The MBA GS.0 dendron has an Rg value of 3.18 i 0.05 nm in D20 (Table 4.2),

which decreases to 1.23 i 0.20 nm when the dendron is in dm-butanol (Table 4.3). This
61% decrease in radius is smaller than the 67% change in radius observed for the MEA
G4.0 system going through the same solvent change. This confirms that the generation 4
has the greatest potential to change size. Despite MEA G5.0 dendron having twice the
molecular weight and number of end groups of MBA G4.0 still changes size with respect

to its surroundings.

95

 

 

IIIIIIIIIIIIIIIIIIIITIIIIIu '"I T 1 IIIHI ""l
‘1

I b .
01 o MEA G5

dm-butanol

  
 
 

 

 

d1 o—butanol

 

111111111111111

     
   

  

 

Rg=1.23 :I: 0.020 nm
R= 1.60 :t 0.026 nm

In( I(Q) (cm"))
in

 

 

 

11111111111111]

 

-7 0001
-8 T
10 20 30x10“ 2 ‘ ° 801 2
02 (A4) Q (A4)

Figure 4.12. SANS of MBA 65.0 in le-butanol.

a. Guinier analysis for MBA G5.0 dendrons in d 10-butanol at a concentration of 4 mg/mL,

data fit for a compact (sphere-like) object. b. I vs Q plot of MBA 5.0 dendrons in d10-
butanol at a concentration of 4 mg/mL.

The mass fraction of solvent contained within the MEA 65.0 dendrimer was
calculated from the experimental 1(0) value, eq 4.2, was 0.61 in D20 and 0.52 in d")-
butanol. There is less solvent within the dendrimer when it is in dIO-butanol, since the
dendrimer has smaller dimensions and is more contracted. This reduction in the mass
fraction of solvent (0) within the dendrimer was also observed for the MEA G4.0 system,
where 9 decreased from 0.64 for D20 (Table 4.2) to 0.40 when in rim-butanol (Table 4.3).
The lower generation can collapse to a more compact state as indicated by the smaller 0
value of 0.40 when in dlo-butanol. The MBA G4.0 dendron has a slightly larger amount
of solvent within its cavities compared to MBA G5.0, suggesting it has more potential to

change size in solution.

96

We used DLS to determine the hydrodynamic radius of the MEA 65.0 dendron in

water and butanol and found values of 2.30 i 0.10 and 2.00 i 0.05 nm respectively
(Tables 4.2 and 4.3). The Rg/Rh ratio for the MEA 65.0 dendron in water is 1.38 i 0.06
and 0.61 i 0.02 when in butanol, where both of these values within experimental error of
the Rg/Rh ratios of the MEA 64.0 systems. The change in Rg/Rh with solvent type, also
observed for the MEA 64.0 system, is a quantitative indicator that the segmental
distribution is a function of the surrounding medium. The matching Rg/Rh ratios for the

MEA 64.0 and MBA 65.0 dendrons point to similar spatial arrangements of the dendritic
arms of the MEA 64.0 and MBA 65.0 molecules, despite the approximate two-fold
difference in molecular weight. This suggests that the density distribution of the
molecule is not generation dependent, but rather a function of the interaction of the
dendrimers with their surrounding environment for the systems we studied, however
further experiments are necessary to verify these results. Despite the similarities in the
molecular arrangement of MBA 64.0 and MBA 65.0 it is important to point out that
MEA 64.0 has the most potential to change size as shown in Figure 4.6b and discussed
above.
Influence of Core-Functionality on the Behavior of PAMAM Dendrimers in Solution
Thus far, all dendrons studied in this work have di-functional cores, which differ

l.16 37 and Nisato et

from the tri and tetra-functional cores studied by Tomalia,18 Topp et a
al.20’ 38 To investigate how the functionality of the core dictates the behavior of PAMAM
dendrons in solution, we studied a tetra-functional cored PAMAM 64.0 dendron in 4

mg/mL solutions of both D20 and dIO-butanol. This molecule has four dendritic wedges

97

connected to the focal point and is substantially more sterically hindered than the di-

functional MEA dendrons.
The SANS data were analyzed using the Guinier analysis for compact spheres
resulting in an R8 of 2.14 i 0.04 nm and 1.79 i 0.03 for the Tetra 64.0 dendrimers in

D20 and d‘O-butanol respectively, Figures 4.13a and 4.14a. This is only a 16% difference
in size which is much smaller than the 67% difference we observed for the MEA G4.0
dendrons undergoing the same solvent change. It is clear from these results that the
functionality of the core has a substantial impact on the ability of the dendrons to alter

dimension in solution, as it determines how the mass scales with generation, eq 4.3.

 

oVIIIIIIIIIIIIIIIIIIIIIIV

 

 

 

 

 

 

 

 

a A Tetra G4 .1
A“? D20: 4 mg/mL 1°
'7 ‘3‘ -2
5'4 g 10
21.5 @10'3
E Rg=2.14 :l: 0.04 nm —
' _8 R = 2.78 a 0.50 nm .4 O Tetra G4.0
10 " DZO "
.10 U—o—o-HJ—A—t—l—J—l—l-IJ—o—o-a
2 4 6 8 102x103 0.01 2 ‘ 2 20.1
2 -2

Figure 4.13. SANS of Tetra 6 4.0 dendrons in D20.
a. Guinier analysis for Tetra G4.0 dendrons in D20 at a concentration of 4 mg/mL, data

fit for a compact (sphere-like) object. b. I vs Q plot of Tetra 4.0 dendrons in D20 at a
concentration of 4 mg/mL.

98

 

         
   
     

 

 

 

 

 

 

 

0 .
a I
o Tetra G4 . 0.1
.2 '2 dm-butanol '1
£4 - F, 0.01
G I
E 6 5" g
E ' Rg=1.79 r 0.03 nm - ‘ o 001
R = 2.32 1 0.032 nm 2 ' o Tetroa G4.0
'8 '. d -butanol
5 1o 15 20x103 2 4 6 80.1 2
.1
0’ (A") Q (A )

Figure 4.14. SANS of Tetra G 4.0 in d‘ O-butanol.
a. Guinier analysis for Tetra 64.0 dendrons in d IO-butanol at a concentration of 4 mg/mL,

data fit for a compact (sphere-like) object. b. I vs Q plot of Tetra 4.0 dendrons in d10-
butanol at a concentration of 4 mg/mL.

We measured the hydrodynamic radius of the Teta—64.0 dendrimers using

dynamic light scattering and found Rh values of 2.00 i 0.05 nm and 1.80 i 0.05 run when
in water and butanol, respectively (Table 4.4). The Rg/Rh ratios for the Tetra-64.0

dendrons are 1.07 i 0.03 when in water and 0.99 i 0.04 in butanol, which are equivalent

within experimental error. The Tetra 64.0 dendrimers segmental distribution in solution

is less sensitive to the surrounding environment than the MEA 64.0 dendron, where the
Rg/Rh ratio changed from 1.36 i 0.04 to 0.65 i 0.03 from butanol to water. The steric
hindrance of the tetra-functional cored system increases the difficulty of the dendritic
arms to adapt to the surrounding medium, explaining the equal Rg/Rh ratio for the butanol

and water systems. The segmental distribution of tetra-functional PAMAMs is similar to

99

that of di-fimctional cored poly(benzyl ether) in benzene, where an Rg/Rh ratio of 1.02 i

0.03 was reported in d‘s-benzene.36

Table 4.4. R8 values for Tetra 64.0 in D20 and d1 0-butanol determined from the Guinier
analysis described in the text at a concentration of 4 mg/mL.
Symbols are explained in the text while Qmam is the maximum Q-value used in the

 

 

 

 

 

 

 

 

regression

Sample Code—) Tetra Tetra

solvent D20 dw-butanol

R,(nm)" 2.14:004 1.78:003

I(0)epr(cm'l) 0.038 0050:0001

9mm, 2.34 2.49

9 c 0.47 0.43
Adnm)‘, 2.001.005 1.80:.005

Rg/RL, 1.07:003 099:.004

 

 

 

 

 

a Radius of gyration for a sphere-like object. b Experimental value of 1(0) from a sphere-
like object Gunier plot (eq 1). c 0 is the mass fraction of solvent within the scatterers.

Due to the inability of the Tetra-64.0 dendrimer to alter its morphology in
solution, there is little change in the mass fraction of solvent contained within the
dendrimer when the solvent is changed from D20 to dw-butanol (Table 4.4). Using the
experimental value of 1(0), eq 4.2, 0 was calculated as described above to be 0.47 and
0.43 for D20 and dlo-butanol respectively. This contrasts with the change in 0 for MEA
64.0 from 0.65 when in D20 to 0.40 in dlo-butanol, and emphasizes the importance of
the core fiinctionality on the ability of the dendritic arms to hold guest molecules. The

Tetra-64.0 system is only capable of altering its conformation in solution by small

amounts, as opposed to the di-functional cored MEA 64.0 dendrons.

100

E jfect of Ionic Strength on the Morphology and Solution Behavior of MEA Dendrons
To determine the effect of ionic strength on the behavior of MEA dendrons in
solution, we conducted dynamic light scattering experiments on MEA 64.0 dendrons in
solutions of 1 M HCl, and 1 M HCl with 0.100 M NaCl at concentrations of 4 mg/mL.
For the acidic solution with no salt added, the primary and tertiary amine groups of the
dendrons are protonated and charged.39 The hydrodynamic radius of the MEA G4.0

dendron in 1 M HCl is 1.70 i 0.05 nm, 10% smaller than when the dendron is in water,

1.90 i 0.05 nm. This size change parallelsthe results of Tomalia et 31.18

A decrease in size with decreasing pH was also observed by Newkome et al. for
amine terminated cascade polymers.40 Figure 4.15a shows a graph of percent mass vs. R,
of MEA 64.0 in 1 M HCl measured 5 minutes afier preparing the sample.
Agglomeration of the sample occurs, with a second peak of particle size ~30 nm,
appearing 25 minutes afier preparing the sample, Figure 4.15b. Attempts were made to
conduct further measurements, but the scattering intensity was too high to analyze
without risking damage to the detector. We observed evidence of agglomeration of the
MEA dendrons in aqueous salt-free solutions via SANS experiments as well, with
Figures 4.3b and 4.16 showing I(Q) vs. Q for the MEA 64.0 in D20 at concentrations of
4 mg/mL and 25 mg/mL respectively. In both graphs, there is a sharp upward change in
I(Q) for Q values lower than 0.02 A". It appears that the entire sample does not
agglomerate at the concentrations studied allowing the data to be analyzed in the Q

region greater than 0.02 A".

101

 

 

__--_.7777-----_____-__

% Mass
% Mass
8

 

 

 

 

 

 

 

 

Rn (nm) Rh (nm)

Figure 4.15. Percent mass vs. hydrodynamic radius for a MBA 64.0 dendron in
1 M HCl.

a. Measurements were taken 5 minutes after the 4 mg/mL.
sample was prepared, the peak corresponds to an R1, of 1.70 i 0.05 nm. b. Percent mass
vs. hydrodynamic radius for a MBA 6 4.0 dendron in l M HCl at a concentration of 4
mg/mL. This measurement was taken 25 minutes after the sample was prepared and
shows agglomeration.

102

 

 

 

 

llll‘l l lillllllllll l

ff

'2 4 68' 2

 

‘11-

Q (A")

Figure 4.16. I(Q) vs. Q for MBA 64.0 in D20 at a concentration of 25 mg/mL.

The negatively sloped line at Q values below 0.02 A'1 indicates significant agglomeration
of the dendrons in salt free conditions.

To determine the cause of agglomeration, we added NaCl to the solutions to
eliminate the electrostatic interactions of the protonated amines and conducted DLS
experiments on the solutions to determine Rh. The graph of percent mass vs. Rh (Figure
4.17a) shows the size of the dendron is 4.10 i 0.05 run when the salt is introduced to the
acidic solution, a greater than two-fold increase compared to the salt free conditions. We
saw no signs of agglomeration in the salt solution, as shown by Figure 4.17b, which was
taken approximately 36 hours afier the sample was made. Our results are comparable to

those of Russo et al.41 who showed Rh values of tri-functional cored PAMAMs to

increase with increased salt concentration before reaching a plateau at ~75 mM NaCl.

103

They also observed Rh values that were greatly dependent on the salt concentration at low

salt levels at neutral pH.

 

 

 

 

 

 

 

 

 

 

100 100

90 + ------------------ 90 + -------------
80+ __a_' 80+ ————————————————— b———
704 -------- —————————— 70+ --------------------
3 60+ eeeeeeeeeeeeeeee 3 60+ ---------------------
3 50+ --------------- - 5 50+ —————————————————————
a: 40+ ~~~~~~~~~~~~~~~~~~~~ a: 40+ --------------------
30« 30+ ~~~~~~~~~~ ~~~~~
20+ --------------- 20+ —————————————————————
104 —--——---.-- - - 10*in ——————————— - ————————

O T j I o f r
0 20 40 60 0 20 40 60

R..(nm) R..(nm)

Figure 4.17. Percent mass vs. hydrodynamic radius of an MBA 64.0 dendron in 1 M
HCl and 0.100 M NaCl at a concentration of 4 mg/mL.

a. Measurement taken 25 minutes after the sample was made; the peak corresponds to an
Rh of 4.10 i 0.05 nm. b. Measurement taken 36 hours afier the sample was made. The

addition of salt prevents agglomeration of dendrons in solution.

Agglomeration of the MEA dendrons is eliminated by shielding the electrostatic
interactions of the amine groups. At the same time the size of the molecule increases as
the ionic strength increases, suggesting an increase in solvent quality. The unique
intermolecular hydrogen bonding capabilities of the PAMAM dendron serves as a handle
to control their morphology and apply them to unique materials.
Conclusion

A generation 4 MBA dendron has the largest volumezmolecular weight ratio,
which demonstrates it is most capable of changing size as a function of the surrounding
medium. We used SANS and DLS experiments to demonstrate that the size of di-

functional MEA dendrons is a function of the quality and ionic strength of the solvent.

104

These molecules are dynamic in solution and can change size, with the smallest size in
dlo-butanol and the largest size in a solution of 1 M BC] with 0.100 M NaCl. The
functionality of the core strongly governs the change in size and density distribution of
dendrimers in solution. A tetra-functional PAMAM dendrimer only decreases in size by
16% when going fi'om D20 to dlo-butanol, whereas the size of a di-functional MEA
dendron decreases by 67%. The steric constraints caused by the increased functionality
of the core decreases the ability of the molecules to change size in solution at a given
generation. PAMAM dendrons have potential to be used for novel “smart” materials

since their molecular dimensions are sensitive to the surrounding environment.

105

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108

CHAPTER FIVE: Synthesis and Characterization of Dansyl-Cored PAMAM Di-
Dendrons

Introduction

2’ 3 are a unique type

Cascade molecules,1 more commonly known as dendrimers
of macromolecule whose branched molecular architecture provides the diversity of
functionalization either at the focal point, the branching units or the periphery. The
chemical modification of dendrimers is a rapidly growing field aimed to understand both
the fundamental properties of these molecules as well as their potential technological
applications. Implementation of dendrimers in molecular sensing and drug delivery
schemes requires a thorough understanding of the behavior of these molecules in
solution, in the bulk or at an interface.

The polarity of microenvironrnents within a dendrimer can be probed using
solvatochromic compounds, whose absorption and emission spectra vary with solvent
polarity. There have been several studies of dendrimers with fluorescent probes located
at either the periphery or the focal point of the molecule. One of the first examples of

functionalization of a dendrimer with a solvatochromic compound was done by Hawker

et al.4 who covalently linked 4-(N, N-dimethylamino)-l-nitrobenzene to the focal point of
a poly(benzyl ether) dendrimer. They observed a discontinuity in the shifiing of km for

the chromophore from generation 3 to generation 4, which they explained by the
conversion of the dendrimer architecture from an extended to a globular state. More
recently Bright and coworkerss’ 6 investigated dendrimers with a fluorescent dansyl group
at the focal point to determine its host-guest relationships and molecular sensing
properties. They placed a dansyl group at the focal point of a carboxylic acid terminated

dendrimer and via photophysical measurements, showed that the dansyl residue is

109

progressively shielded from the solvent as the dendrimer generation increases.5 They
also showed that access to the dansyl moiety at the core is impeded with increasing
dendrimer generation by determining the equilibrium binding constant of B-cyclodextrin
to dansyl as a function of generation. In firrther work, Bright and coworkers6 attached a
pyrene residue to the tertiary amine at the focal point of asymmetric poly(amido)
dendrimers with carboxylate end groups and investigated the accessibility of the core to a
series of neutral and ionic (iodide and cupric ions) quenchers. They found that the
approach of neutral and ionic quenching agents is increasingly hindered as the generation
of the dendrimer increases.

Similar ion detection schemes were reported by Baker and coworkers7 who placed
a pyrene moiety at the focal point of carboxylate terminated poly(amido) dendrons to
detect the permeability of Pb2+ to the core of the molecule as a function of pH and
generation. For aqueous solutions near neutral pH, the dendrimer-pyrene scheme has a
detection limit below the level defined by the World Health Organization for safe
drinking water, 10 ppb, which is encouraging for the utility of dendrimers as sensing
devices. Gawley et al.8 placed a fluorescent dansyl moiety at the core of Newkome-type
dendrimers and attached a portion of the dendrons’ carboxylic end groups to the amine
functionality on a solid-phase protein synthesis bead. The unreacted end groups were
then covalently bound to rhodamine dye, and the ability of the dye to penetrate through
the dendrimer was signaled by the dansyl group.

Numerous groups have attached fluorescent chromophores to the end groups of
dendrimers. Wei and coworkers9 attached phenyl, naphthyl, pyrenyl and dansyl

chromophores to the periphery of poly(amido amine) (PAMAM) dendrimers to study the

110

self-organization of these materials in aqueous solutions. Using fluorescence
spectroscopy, they found aggregates form for 61 to 63 where the size and aggregation
number of the vesicles decreases with increasing generation. The PAMAM dendrimers
60-65 showed a broad structure less fluorescence peak at 468 nm, which was attributed
to the 1r—7t interaction between adjacent parallel-oriented pyrene chromophores.9 The
excimer emission increased from 60 to 63, suggesting that higher generations favor
parallel alignment of the pyrene rings, yet upon reaching 64 the emission intensity
decreased indicating the rings are no longer able to adopt such an arrangement. This
implies that the dendritic arms for 64 and 65 are more sterically crowded and the
molecular arrangement of the dendron changes from 63 to 64 preventing pyrene

alignment. For the dendrimer fimctionalized with dansyl groups at the periphery, the
lm emission shifted from 445 nm for 62 to 512 nm for 63.9 This shifi is due to

transforming the dansyl chromophore from a locally excited state (coplanar) to an
intramolecular charge transfer excited state (twisted)10 due to the changes in the dendritic
architecture.9 Wei et al. H functionalized PAMAM tetra-dendrons end groups with l-
(naphthalenyl)-2-phenyldiazene to study the aggregation behavior with respect to
generation number. They found organized aggregates to occur in a predictable fashion
for 63 and lower, but generations higher than 4 did not self-assemble into ordered
structures. The lack of self assembly at higher generations illustrates the change in
dendrimer morphology between 63 and 64 from an extended to a globular state.

Schluter and coworkers12 designed a targeting drug delivery device by modifing a
2,3,-D/L-diaminopropionic acid dendrimer, placing dansyl groups on a certain number of

the end groups and a cleavable peptide spacer on the remaining end groups. The peptide

11]

chain was used as the spacer between the fluorescent dendrimer and potentially a
cleavable anti-cancer drug. Fluorescence microscopy displayed the rapid uptake of
dendrimers into human HeLa cells and a localized dendrimer concentration next to the
cell nucleus after 18 hours of incubation.

13’ '4’ '5 chemically linked dansyl groups to the periphery of

Balzani and coworkers
poly(propylene amine) dendrimers and measured fluorescence quenching as a function of
Co2+ ion concentration. They showed that metal ions coordinate to the interior of the
dendrimers in a fully reversible fashion. An increase in the fluorescence quenching
efficiency with increasing dendrimer size leads to strong amplification of the signal and
thus an increased sensitivity of the sensor.

The host-guest behavior of poly(lysine) dendrimers and lanthanide ions was
studied by V6gtle et al.16 using dansyl terminated poly(lysine) dendrimers and measuring
the fluorescent quenching as a function of ion concentration. Formation of metal
complexes accompanies quenching of the fluorescent excited state of the dansyl units.
This effect is very large for Nd3+ and Eu3+ and may be exploited for a variety of
applications from sensors to fluoroimmunoassays. Using a similar system, Vogtle and
coworkers17 studied the host-guest properties of a dansyl terminated poly(propylene
amine) dendrimer and eosin dye molecules. The ability of the dye to penetrate into
dendritic cavities is dependent on the size of the dendrimer, since the number of dye
molecules capable that can be held by the dendrimer increases with increasing
generation. Vogtle et al.‘8 synthesized a dendrimer with a cyclam metal ligand core and

naphthyl terminal groups to further study the ability of dendrimers to sequester metal

cations. Complexation of the cyclam core with Zn2+ was shown by an increased i the

112

naphthyl fluorescence intensity. They found that two cyclam-cored dendrons coordinate
with one zinc cation and suggested the ability of the dendrimer to coordinate in a
controlled fashion with Zn2+ may play an important role in catalysis and medical
diagnosis.

Work where dendrimers were modified a dendrimer with a fluorophore was
motivated by the dendron acting as a molecular sensor, with little emphasis on the actual
morphology of the dendrimer as a function of environment or generation. Understanding
the dendritic morphology should simplify the design of such systems and enhance the
efficiency of sensing devices. Depending on the application, it may be desirable to have
either an accessible or more hidden probe, which would alter its selectivity. To
investigate the molecular domains or solvent penetrability near the focal point of a
PAMAM dendrimer, we synthesized a PAMAM di-dendron that contains a
solvatochromic dansyl moiety at its focal point, Scheme 4.1. The synthesis was
continued to 65.0 and the theoretical molecular weight of each generation is listed in
Table 5.1. The fluorescence emission of the dansyl moiety was measured as a function of
generation number and solvent conditions to determine how the solvent quality

influences the behavior of these molecules in solution.

113

0

DO - 0O 1...,
CHZCIZ

l

V

 

028\Cl 0°C to RT OZS\N/\/NH2 MeOH, 35°C 48 hrs
96% H 98%
1 2
\N/ |
°j° M 66 fife
MeOH, N2, 48 hrs
OZS\N/\/N\/\n/O\ 90% 025 N/\/N \/\n/N\/\NH2

3 4 0

Scheme 5.1. Synthesis of dansyl-cored PAMAM dendrimer.

Table 5.1. Theoretical molecular mass (M) of the dansyl-cored PAMAM dendrimers.

 

 

 

 

 

 

Sample Code M (Da) Sample Code M (Da)
dansyl 60.5 465 dansyl 63.0 1889
dansyl 61.0 521 dansyl 63.5 3265
dansyl 61.5 865 dansyl 64.0 3713
dansyl 62.0 977 dansyl 64.5 6465
dansyl 62.5 1665 dansyl 65.0 7361

 

 

 

 

 

 

114

Scheme 5.2. Generation 4 dansyl-cored PAMAM dendrimer.

115

Experimental

Materials. All chemicals were purchased from Aldrich Chemical Co., Saint Louis, MO.
Ethylenediamine was distilled under nitrogen prior to use. Methyl acrylate was distilled
from KOH to remove the inhibitor. 1H and 13 C NMR spectra were acquired on a Varian
300 or 500 MHz instrument, as indicated, with the residual proton signal from the solvent
used as the reference peak. NMR spectra and assigned chemical shifts are displayed in
Appendix B.

Preparation of (2-aminoethyl)-dansylamide (dansyl-EDA), 2.19 (Scheme 4.1) A
solution of dansyl chloride (2.50 g, 9.27 mmol) in dichloromethane (25 mL) was dropped
into a stirred solution of ethylenediamine (62 mL, 55.7 g, 0.927 mol) cooled to 0 °C by an
ice bath. When the addition was complete, the ice bath was removed and the mixture
warmed to room temperature with continued stirring. The bright yellow/ green reaction
mixture was divided into two portions and each portion was acidified by slowly adding 6
M HCl. The pale green acidified solution was extracted with dichloromethane (2 x 200
mL). The aqueous layers were adjusted to pH 9 by slowly adding 10 M NaOH and again
extracted with dichloromethane (2 x 200 mL). The combined organic layers were dried
with MgSO4, filtered and concentrated to give 2.62 g (96% yield) of (2-aminoethyl)-
dansylamide (2) a yellow/green solid. 1H NMR (300 MHz, CDCl3) 8: 8.50 (d, 1H), 8.23
(m, 2H), 7.50 (m, 2H), 7.15 (d, 1H), 2.80 (m, 8H), and 2.64 (t, 2H).

Dansyl-PAMAM Synthesis

Dansyl-PAMAM 60.5 (3). A stirred solution of (2-aminoethyl)-dansylamide (2), (0.541
g, 1.84 mmol) in methanol (25 mL) was slowly dripped into 125 eq methyl acrylate (21

mL, 19.80 g, 0.23 mol) at 35 °C. The reaction was stirred for two days at 35 °C under

116

nitrogen and then methanol and un-reacted methyl acrylate was removed in vacuo, to
provide 0.822 g (98% yield) of 3 as a pale green solid.

1H NMR (300 MHz, CDC13) 6: 8.48 (d, 1H, J = 11.4 Hz), 8.36 (d, 1H, J = 9 Hz), 8.21 (d,
1H, J = 7.2 Hz), 7.54 (m, 2H), 7.13 (d, 1H, J = 8.4 Hz), 3.64 (s, 6H), 2.93
(q, 2H, J = 5.4 Hz), 2.52 (t, 4H, J = 6.6 Hz), 2.39 (t, 2H J = 5.1 Hz), and 2.47

(t, 4HJ=6.6 Hz). ”CNMR (CDC13,3OOI\/II-Iz)8172.8,151.8,135.0, 130.1, 129.9

129.6, 129.4, 128.0, 123.1, 119.2, 115.0, 52.9, 51.7, 48.7, 45.3, 40.6, 32.1.

Dansyl 61.0 (4). A solution of dansyl-PAMAM 60.5 (3) (0.358 g, 0.769 mmol) was
dissolved in 25 mL methanol and slowly dripped into a stirred solution of 200 eq
ethylenediamine (10.30 mL, 9.24 g, 0.154 mol) 55 °C. Afier stirring for two days,
methanol and unreacted ethylenediamine was removed in vacuo. To remove residual
amounts of ethylenediamine, the residue was dissolved in octanol (100 mL) and stirred
under vacuum (5 mTorr) at 40 °C for 2 days. The octanol mixture was washed with
50:50 hexanezwater. The aqueous layer was collected and washed three times with
hexanes (3 x 150 mL), then concentrated and dried under vacuum at 40 °C. The dried
crude product was dissolved in methanol and placed in a dialysis bag (MWCO = 200
g/mol) for two days. The contents of the bag were dried under vacuum providing 0.361 g
(90% yield) of 4 as a yellow/green colored oil'H NMR (300 MHz, til-methanol) 8: 8.43
(d, 1H, J = 11.4 Hz), 8.30 (d, 1H, J = 9.0 Hz), 8.15 (d, 1H, J = 7.2 Hz ), 7.50 (m, 2H),
7.20 (d, 1H, J = 8.4 Hz), 3.18 (t, 4H, J = 6.0 Hz), 2.85 (t, 2H, J = 6.0 Hz), 2.78 (s, 6H),
2.65 (t, 4H, J = 6.0 Hz), 2.54 (t, 4H, J = 6.9 Hz), 2.37 (t, 2H, J = 6.6 Hz), and 2.18 (t, 4H,
J = 6.9 Hz). ”C NMR (dd-methanol, 300MHz) 8: 174.84, 152.8, 135.0, 130.8, 130.5,

130.3, 129.8, 129.1, 124.1, 120.0, 116.1, 53.6, 50.3, 49.0, 45.5, 42.6, 41.6, 34.1.

117

Synthesis of Dansyl-PAMAM

Formation of subsequent half and full generation PAMAM dendrimers was done by
following the above methods for the formation of dansyl-60.5 and dansyl 61.0
respectfirlly.

Dansyl G1.5. 1H NMR (300 MHz, (rt-methanol) 8: 8.52 (d, 1H, , J = 11.3 Hz), 8.33
(d, 1H, J = 9.0 Hz), 8.18 (d, 1H, J = 7.2 Hz), 7.56 (m, 2H), 7.23 (d, 1H, J = 8.4 Hz), 3.58
(s, 12H), 3.20 (t, 4H, J = 6.6 Hz), 2.92 (t, 2H, J = 6.0 Hz), 2.83 (s, 6H), 2.69 (t, 8H, J =
6.0 Hz), 2.61 (t, 4H, J = 6.9 Hz), 2.49-2.39 (m, 14H), and 2.22 (t, 4H, J = 6.9 Hz). 13C
NMR (d4-methanol, 300MHz) 8: 173.4, 173.3, 151.8, 135.7, 131.0, 129.8. 129.6, 128.7,
127.9, 123.0, 119.5, 115.9, 52.5, 52.2, 50.8, 49.3, 49.1, 44.5, 50.7, 37.1, 33.1, 32.2.
Dansyl 62.0. 1H NMR (300 MHz, rid-methanol) 8: 8.50 (d, 1H, J = 11.3 Hz), 8.30 (d, 1H,
J = 9.0 Hz), 3.18 (d, 1H, J = 7.2 Hz), 7.54 (m, 2H), 7.22 (d, 1H, J = 8.4 Hz), 3.24-3.24
(m, 12H), 2.90 (t, 2H, J = 6.0 Hz), 2.82 (s, 6H), 2.78-2.64 (m, 16H), 2.62-2.50 (m, 8H),
2.48-2.40 (m, 2H), 2.30 (t, 8H, J = 6.6 Hz), and 2.20 (t, 4H, J = 6.6 Hz). 13(3 NMR (d4-
methanol, 300MHz) 8: 174.9, 174.5, 131.2, 153.0, 138.0, 131.3, 131.2, 130.9, 130.1,
129.2, 124.4, 120.5, 116.5, 53.4, 51.2, 50.6, 45.2, 42.9, 41.9, 38.6, 34.8, 34.3.

Dansyl (22.5. ‘H NMR (300 MHz, at methanol) 8: 8.52 (d, 1H, J = 11.3 Hz), 8.32 (d, 1H,
J = 9.0 Hz), 8.18 (d, 1H, J = 7.2 Hz), 7.56 (m, 2H), 7.24 (d, 1H, J = 8.4 Hz), 3.62
(s, 24H), 3.30-3.14 (t, 12H, J = 6.0 Hz), 2.92 (t, 2H), 2.84 (s, 6H), 2.82-2.66 (m, 24H),
2.42-2.52 (m, 8H), 2.50 (t, 10H, J = 6.3 Hz), and 2.42-2.10 (m, 24H). 13C NMR
(dd-methanol, 300MHz) 8: 174.9, 153.5, 136.0,131.4, 131.2, 130.9, 130.4, 1205,1247,

120.9, 116.7, 54.0, 53.6, 52.4, 51.3, 50.7, 46.2, 42.3, 38.7, 38.7, 35.0, 34.6, 33.8.

118

Dansy163.0. 1H NMR (300 MHz, (fl-methanol) 8: 8.60 (d, 1H, J = 11.3 Hz), 8.40 (d,
1H, J = 9.0 Hz), 8.25 (d, 1H, J = 7.2 Hz), 7.64 (m, 2H), 7.31 (d, 1H, J = 8.4 Hz), 3.40-
3.15 (m, 28H), 3.00 (t, 2H), 2.91 (s, 6H), 2.90-2.70 (m, 32H), 2.69-2.48 (m, 22H), and
2.47-2.10 (m, 28H). ”C NMR (dd-methanol, 300MHz) 8: 173.8, 173.4, 151.9, 130.2,
130.0, 129.8, 129.6, 128.8, 127.9, 123.1, 119.0, 114.2, 52.1, 49.8, 49.2, 44.9, 44.5, 41.1,
40.6, 37.3, 33.4, 33.0.

Dansyl G3.5. ‘H NMR (300 MHz, all-methanol) 8: 8.50 (d, 1H, J = 11.3 Hz), 8.30 (d, 1H,
J = 9.0 Hz), 8.10 (d, 1H, J = 7.2 Hz), 7.54 (m, 2H), 7.20 (d, 1H, J = 8.4 Hz), 3.59 (s,
48H), 3.18 (m, 28H), 2.88 (m, 2H), 2.84-2.44 (m, 96H), and 2.42 (2.18, 60H). 13C NMR
(rt-methanol, 300MHz) 8:174.6, 174.5, 153.2, 131.5, 131.1, 131.1, 130.9, 130.2, 129.3,
124.5, 120.6, 116.5, 53.7, 53.2, 52.2, 50.9, 50.4, 45.9, 38.4, 34.5, 33.5, 30.7.

Dansyl 04.0 ‘H NMR (300 MHz, f-methanol) 8: 8.47 (d, 1H, J = 11.3 Hz), 8.26 (d, 1H,
J = 9.0 Hz), 8.12 (d, 1H, J = 7.2 Hz), 7.50 (m, 2H), 7.20 (d, 1H), 3.40-3.30 (m, 60H),
2.88 (t, 2H), 2.85 (s, 6H), 2.85-2.65 (m, 88H), 2.65-2.50 (m, 34H), and 2.50-2.20 (m,
60H). 13C NMR (cf-methanol, 300MHz) 8: 176.3, 175.2, 174.7, 153.3, 136.9, 131.3,
131.2, 131.2, 130.9, 130.1, 129.3, 124.5, 120.5, 117.5, 54.6, 53.4, 51.1, 46.3, 45,8, 42.6,
41.9, 40.4, 39.9, 38.6, 37.4, 36.4, 34.7.

Dansyl 64.5. 1H NMR (300 MHz, (Ii-methanol) 8: 8.50 (d, 1H J = 11.3 Hz), 8.30 (d, 1H
J = 9.0 Hz), 8.14 (d, 1H J = 7.2 Hz), 7.56 (m, 2H), 7.20 (d, 1H J = 8.4 Hz), 3.60 (m,
96H), 3.20 (m, 6H), 2.85 (s, 60H), 2.90 (m, 2H), 2.84 (s, 6H), 2.82-2.64 (m, 132H), 2.63
(m, 62H), and 2.40-2.20 (m, 152H). 13C NMR (dd-methanol, 300MHz) 8: 174.7, 174.5,
153.2, 131.5, 131.1, 131.1, 130.9, 130.2, 129.3, 124.5, 120.6, 116.5, 53.7, 53.2, 52.2,

50.9, 50.4, 45.9, 38.4, 34.5, 33.5, 30.7.

119

Dansyl (25.0 'H NMR (300 MHz, d4-methanol) 8: 8.50 (d, 1H J = 11.3 Hz), 8.30 (d, 1H
J = 9.0 Hz), 8.14 (d, 1H J = 7.2 Hz), 7.56 (m, 2H), 7.20 (d, 1H J = 8.4 Hz), 3.30—3.10(m,
124H), 2.82-2.64 (m, 196H), 2.60-2.50 (m, 60H), and 2.40-2.20 (m, 128H). 13C NMR
(500 MHz, til-methanol) 8: 175.2, 174.9, 70.0, 153.2, 131.5, 131.1, 131.1, 130.9, 130.2,
129.3, 124.5, 120.6, 116.5, 54.4, 54.2, 53.9, 51.5, 45.9, 42.3, 41.9, 38.4, 34.5.

Fluorescence Measurements. Dansyl-PAMAM dendrimer samples were dissolved in

the appropriate solvent with concentrations ranging from 5 x 10'5 to 1 x 10'3 M and we
observed no change in lnm as a function of concentration at these concentrations.

Fluorescence measurements were performed using a F luorolog-3 instrument from
Instruments S.A., Inc. The excitation optics consisted of a 450W Xe lamp followed by a
330 nm ruled grating. The detection optics consisted of a 530 nm ruled grating and
multi-alkali photo multiplier tube at 950V (Hamamatsu R928). Data processing was
performed using the Datamax (version 2.2) software package supplied with the
instrument. The excitation beam was centered at 360 nm with a 1 nm bandpass and the
emission monochromator was scanned fi'om 370 to 800 nm in lnm steps.
Results and Discussion
Solvent Dependence of the Emission Spectra of (Dansyl-EDA) (2)

The dansyl group is solvatochromic with the location (wavelength) of its emission
and absorption a function of solvent polarity. It undergoes a significant bathochromic
(red) shift with increasing solvent polarity. We studied the emission of dansyl-EDA (2),

which is generation 0 (dansyl 60), in methanol, ethanol, butanol, chloroform and
acetonitrile and found a significant change in maximum emission wavelength (km) with

respect to solvent, Figure 5 .1. The dielectric constants of the solvents are listed in Table

120

5.2. There is an approximately linear relationship between the dielectric constant and
km for Dansyl-EDA (2), Figure 5.2. The deviation from linearity occurring at the

highest dielectric constant, acetonitrile, is due to the poor hydrogen bonding capabilities
of this solvent. The excited state of the dansyl group (1r*) has a higher dipole moment
than the ground state and is stabilized by hydrogen bonding in addition to increased
solvent polarity. Although methanol is less polar than acetonitrile, it is a better hydrogen
bonding solvent. In this case hydrogen bonding by methanol overcomes the polarity
difference between the two solvents stabilizing the excited state of the dansyl group (shift
to a lower energy, higher wavelength) more effectively than acetonitrile. This
emphasizes the sensitivity of dansyl’s fluorescence emission to both solvent polarity and

hydrogen bonding.

Table 5.2. Solvents and their dielectric constants (8) used in this study.

 

 

 

 

 

 

 

 

Solvent 8
chloroform 5.5
butanol 17.8
ethanol 24.6
methanol 32.6
acetonitrile 36

 

 

121

 

 
 
 
 
 
 
 
 
 

 

2.00E+07
+ methanol
1.80E+07 - o acetonitrile
0 ethanol .......
1.SOE+07 - A butanol """""""""""
x chloroform ''''''''''
1.4OE+07 -
1.20E+07 a

1.00E+07 .

 

8.00E+06

6.00E+06

Rolatlvo lntemlty (counts/s)

4.00E+06

2.00E+06 -

 

 

 

0.00E+OO r r fl r
450 470 490 510 530 550

2. (nm)

Figure 5.]. Normalized fluorescence emission spectra of (2-aminoethyl)-dansylamide.

122

 

l I I 1 l
i i l i l
9 I l I I
I l I I I

Am, (nm)

 

 

1+
4?-_............. .-
—

480 +

 

Figure 5.2. km, vs. dielectric constant for dansyl-EDA.

123

Fluorescence of Dansyl-PAMAM

The fluorescence emission spectra of dansyl cored PAMAM dendrimer
generations 1-5 were measured in solvents of various polarities, Table 5.2. The
fluorescence emission spectra of the fill] generations are shown in Figure 5.3 and the

fluorescence emission spectra of the half generation dendrimers is displayed in Figure

5.4. A plot of lnm vs. generation (Figure 5.5, Table 5.3) shows lnm slowly increasing
with increasing generation until dansyl 64.0 where there is a sudden red shift in 71mm,

7cm, then decreases at dansyl 64.5 and increases once more at 65.0. These oscillations
are consistent with a significant difference in morphology between full and half
generation dendrimers. The increase in kmax at generation 4 corresponds to the onset of a

globular morphology, where solvent has less access to the dansyl moiety located at the
focal point of the dendrimer. The amines of the dendrimers, which are good hydrogen
bond participators, stabilize the dansyl group and induce the shift in Amax.

The large difference between the fluorescence emission of dansyl 64.0 (primary
amine end groups) and dansyl 64.5 (methyl ester end groups) demonstrates the
importance of the end group functionality on the overall morphology with primary amine
end groups favoring a globular dendritic state at generations higher than 4. Our neutron
scattering studies of similar PAMAM di-dendrons show that the behavior of the full and

half generations are quite different, with the half generations having a much smaller
radius of gyration (Rg) than the fill] generations in methanol. The decrease in lnm from

dansyl 64.0 to dansyl 64.5 signals a decrease in polarity near the dansyl moiety and

124

could be explained by the increased number of methyl ester groups near the focal point,

which are both less polar and less capable of hydrogen bonding than primary amines.

 

 

1.20E+07 <

1.10E+07 .

1.00E+07 2

9.00E+06 ~

8.00E+06 .

 

7.00E+06 2

Relative lntenslty (countsls)

6.00E+06 ~

 

5.00E+06

 

 

 

4.00E+06 l l 7 l T I T I l' I
470 480 490 500 510 520 530 540 550 560 570
1. (nm)
Figure 5.3. Normalized emission spectra of dansyl-PAMAM full generations (GI-65) in
methanol.

125

 

1.40E+07

  
 
 
 
  

1.ZOE+O7 -

1.00E+07 2

8.00E+06 ~

 

Relatlvo Intensity (counts/s)

 

 

 

6.00E+O6
A 63.5
4.00E+O6 ~
x 62.5
2.00E+06 - o 61.5
0.00E+OO . a . 4 . . . . 4
470 480 490 500 510 520 530 540 550 560 570

1. (nm)

Figure 5.4. Normalized fluorescence emission spectra of dansyl-PAMAM half
generations in methanol.

126

hm“ (nm)

523

522

521

520

519

518

517

516

 

 

v
4
l
. '
s
1
v
I
u

l

_. .1..-

I

2

3

Generation

methanol

1.11

 

01

Figure 5.5. Amax vs. generation for dansyl-cored PAMAM dendrimer in methanol.

Table 5.3. lnm (nm) data for dansyl-PAMAM dendrimers.

 

generation number

 

solvent 1

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

 

butanol

506.5

507

505

507

507.5

506

504

511

508

 

chloroforrnm

491

491.5

491.5

492

492

493

493.5

498.5

494.5

 

ethanol

511

512

511.5

513

514

514

513

516.5

514

 

methanol

5165

517.5

517

517.5

517.5

518

518

520.5

517

 

acetonitrile

 

 

514.5

 

516

 

515

516.5

 

516

 

 

515

 

514.5

 

510.5

 

514

 

 

“dendrimer insoluble in solvent.

127

 

The fluorescence of dansyl cored PAMAM dendrimers displayed the same trend
in ethanol, butanol and chloroform, as methanol. We obtained km from fluorescence
emission spectra of fill] generation (Figure 5.6) and half generation (Figure 5.7) dansyl-

PAMAM dendrimers in ethanol. A plot of km against generation (Figure 5.8) displays a
sudden increase in 71mm, at generation 4. (Dansyl 65.0 is not adequately soluble in
ethanol to obtain fluorescence data.) The increase in 7mm between dansyl-EDA (60) and

dansyl 64.0 is 5.5 nm in ethanol, slightly smaller than the 6.5 nm increase in Amax we

observed for the dendrimers in methanol.
Dansyl-PAMAM full generations were dissolved in butanol and their

fluorescence emission spectra was collected. The emission spectra of full generation
dendrons (Figure 5.9) show a significant shift in kmax with increasing generation at
dansyl 64.0 whereas the emission maximum of the dansyl-PAMAM half generation
dendrimers (Figure 5.10) is relatively constant with increasing generation. A plot of 71mm,
as a function of generation of the dansyl-PAMAMS in butanol (Figure 5.11) shows a

large red shift of lmax at dansyl 64.0, again demonstrating the onset of a globular

dendritic state. A 4.5 nm escalation in km occurs from dansyl-60 to dansyl-64, which

is a smaller increase than we see in methanol and ethanol. Based on our neutron
scattering and dynamic light scattering experiments on similar dendrimers, we know that
butanol is a poorer solvent for the dendrimers than ethanol and methanol, and the

dendrimer Rg decreases as the hydrophobicity increases. Despite butanol’s poor quality

as a solvent, increasing from 64.0 to 64.5 results in a large blue shift in kmax, as was

128

seen in the ethanol and methanol systems. These data demonstrate the significance of end

group functionality on stabilization of dansyl’s excited state.

For ethanol and butanol there appears to be an oscillating trend in km for the
half generations (Figures 5.8 and 5.11). The trend is more pronounced in the poorer
solvent, butanol (Figure 5.11). For both cases there is a red shift in 71m from 61.5 to
62.5. This could be due to the difference in distance between the dansyl core and the
methyl ester end groups for the two generations which are less able to stabilize the dansyl

excited state than butanol. For 61.5, the length of the dendritic arms is half that of 62.5,

perhaps causing the methyl ester end groups to be closer to the dansyl core and a lower

lmax. The red shift in 7mm seen from 61.5 to 62.5, suggests the methyl ester end groups

are filrther away from the dansyl core. A large blue shift in km from 62.5 to 63.5

suggests methyl esters are near the dansyl core. This could indicate a more globular
dendrimer. The observed red shift from 63.5 to 64.5 implies the dansyl core is more
accessible to butanol in 64.5. This could be due to the dendrimer reaching a globular
state and being able to encapsulate more solvent molecules. These hypotheses could be
tested by investigating the behavior of the half generations in ethanol and butanol.

We looked at the fluorescence behavior of the dansyl-PAMAM dendrimers in
chloroform, less polar than the alcohol solvents, but a moderate hydrogen bond donor.
The fluorescence emission spectra of the full generations (dansyl 61.0-64.0) show a

gradual red shift with increasing generation until dansyl 64.0 where there is a significant
increase in km (Figure 5.12). The fluorescence emission spectra for the methyl ester
terminated half generations also display a red shift with increasing generation, however

there is no discontinuity (Figure 5.13). A plot of Kmax vs. generation for the chloroform

129

system shows a 7.5 nm red shift in km between dansyl 60 and dansyl 64.0, which is the

largest change observed. This suggests the onset of the dendritic globular state at dansyl
64.0 encapsulates the dansyl moiety in an environment that is significantly more polar

and thus more capable of stabilizing dansyl’s charged excited state, than chloroform. The

large shift in lmax is due to the greatest polarity difference between chloroform and the

dendritic cage.

 

8.00E+06

7.00E+06 2

6.00E+06 2

5.00E+06 2

 

 

4.00E+06 2

 

3.00906

Relative Intensity (countele)

2.00906 - a 32

1.00906 . x G1

 

 

 

0.00E+OO r r . fl . . r r r
470 480 490 500 510 520 530 540 550 560 570

1. (nm)

Figure 5.6. Normalized fluorescence emission spectra of full generation dansyl-PAMAM
dendrimers in ethanol.

130

Relative Intensity (counts/s)

 

9.00E+06

8.00E+06 2

7.00E+06 2

6.00E+06 2

5.00E+06 2

 

4.00906 4 , '1

. b
. v
4 s
\

3.00906

 

 

2.00E+06 2 0 G25

1.00E+06 2 x 61.5

 

 

 

0.00E+OO r r r r
470 490 510 530 550 570

1. (nm)

Figure 5.7. Normalized fluorescence emission spectra of dansyl-PAMAM half
generation dendrimers in ethanol.

131

 

518 +-----

u§ ethahol

1 a i a a

514 +Ieu-e

31““(nm)
I
I

i i 1 i i
I I 1 l I

 

 

2 3 4

Generation

 

o
_L
01

Figure 5.8. Amax vs. generation for dansyl-cored PAMAM dendrimer in ethanol.

132

Relaflve Intensity (cannula)

 

1.60E+07

 
 
 
 
 
  

1.40E+07 2

1.20E+07 2

1.00E+07 2

 

8.00906 '

 

O)
8
m
+
C
a)
i

4.00E+06 2

2.00E+06 2

 

 

 

o o 00E+OO l J Y I l l l I l
470 480 490 500 510 520 530 540 550 560 570

1. (nm)

Figure 5.9. Normalized fluorescence emission spectra of dansyl-PAMAM full
generations, 61-64, in butanol.

133

Relative Intensity (counts/a)

1.60E+07

 

1.40E+07 ~
1.20E+07 -

1.00E+07 2

l {5.3“
8.00906 .5?

c.

 

 
 
 
 
 
 
 
  
 
 

 

6.00E+06 2
I 64.5
4.00E+06 2
- 63.5
x G2.5
2.00E+06 2 a (31.5
0.00E+00 + r r r
470 490 510 530 550

1. (nm)

Figure 5.10. Normalized fluorescence emission spectra of dansyl-PAMAM half

generation dendrimers in butanol.

134

 

 

 

512 .F... 1 l 111

I butariol

I I I I
l I I Q
I I I I
I I | I
I I I I
I I l
I I I I
I l I I
I I I I
.

0 ~ I 1 , .
I I - y 1 I
I u I 1 e .
I l I I I I
n r r I n r
508 ' l I ' I I
.-..-...-...., .-. v. -. .-. .- ..,-...-.--... ...... ....»-4.. -.~.‘uvuv. .--.-‘
I l e I I u
. I - r
. l . I
I I I I

r A .

hm“ (nm)
I

I p r r
l I l l
r u I r
I s r r .
I I I u I
I v r I r
—.-. .... .-. .. .5 .-....e.-. ....6_......-. .... .... .-. .. .. . .4 .. ... .. .......4

I I I I

u . 1 r

I . r I r
I I I I l I
I s I I l l

n . o 1 r

.

1 i i L 1 1
0 1 2 3 4 5
Generation

 

Figure 5.11. 71mm, vs. generation for dansyl-cored PAMAM dendrimer in butanol.

135

Relative Intensity (counts/s)

 

1 .4OE+07

1.20E+07 2

1.00E+07 2

8.00E+06 2

 

 

6.00E+06 . ,: '

 

4.00906

2.00E+06 2

 

 

 

0.00E+00 r r r r
450 470 490 51 0 530 550

)1. (nm)

Figure 5.12. Normalized fluorescence emission spectra of dansyl-PAMAM full
generation dendrimers in chloroform.

136

Relative Intensity (counts/s)

 

 

1.60E+07
o 64.5
1.40E+07 2 4 G3.5
1: G2.5

1.20E+07 2

1.00E+07 2

8.00E+06 2

 

 

6.00906 -

 

4.00E+06

2.00E+06 2

 

 

 

0.00E+OO r r r r
450 470 490 510 530 550

1. (nm)

Figure 5.13. Normalized fluorescence emission spectra of dansyl-PAMAM half
generation dendrimers in chloroform.

137

 

! 1 ! 1 1 l

' I gchlorqfomr i

I I I I l I
. . Z I T .
1 a r l I I

Am“ (nm)

5 a 5 E 1 :

492 —.._
1;-

 

 

0 1 2 3 4 5

Generation

 

Figure 5.14. kmax vs. generation for dansyl-cored PAMAM dendrimer in chloroform.

138

To test the interaction of the dansyl-PAMAM dendrimers with a solvent that is
more polar than methanol but less capable of hydrogen bonding, we collected the
emission spectra of the dansyl-cored dendrons in acetonitrile. The half
generationsemission spectra showed little change with increasing generation, comparable

to the results of the other solvents (Figure 5.16). The full generation fluorescence
emission spectra showed a different trend than the other solvents. No change in 71mm, was
observed from dansyl 61.0 to dansyl 63.0 (Figure 5.15) and instead of the red shift of
lnm at dansyl 64.0 seen in methanol, ethanol, butanol and chloroform, we observed a
significant blue shift (Figure 5.17). The discontinuity at dansyl G4.0 again corresponds
to the transition from an extended to globular state. The decrease in km at the globular

state is consistent with a dendritic environment that is less polar than acetonitrile and thus
less capable of stabilizing the excited state of the dansyl group. It has been shown for a

similar chromophore that five molecules of acetonitrile are required to fully stabilize the
excited state and thus shifting kmax to longer wavelengths.10 Upon the onset of a globular

dendritic structure the core of the molecule is perhaps less able to accommodate the

orientation of solvent molecules required to stabilize the excited state, which causes the

blue shift of lnm.

139

Relative Intensity (counts/a)

1.80E+07

 

I G4
1.60E+O7 . + 63
o 62
1.40E+07 s
1.20E+07 2
1.00E+07 2
8.00E+06 .
6.00E+06 2

4.00E+06 2.. h

2.00E+06 2

 

0.00E+00

 

 

 

460 470 480

I T l r I

490 500 510 520 530 540 550
1. (nm)

560

Figure 5.15. Normalized fluorescence emission spectra of dansyl-PAMAM full

generations in acetonitrile.

140

Relative Intensity (counts/s)

 

1.80E+07

9 G45
A (33.5

a 62.5
1.4OE+07 ‘ + G1.5

1.60E+07 2

1.20907 ,
1.00907 —
8.00906 ~
6.00906 -

u
o
e
‘9

4.00E+06 ,

 

2.00E+06 2

 

0.00E+00

 

 

460

480

I I

500 520
1 (nm)

540

560

Figure 5.16. Normalized fluorescence emission spectra of dansyl-PAMAM half

generations in acetonitrile.

141

 

518 1111
;I acetonltllle 3 3

z 3 I s a s
I I I I I I

516 -II-—
1 i 1 i 1

km“ (nm)

1 1 a v o r
r . r . r r
. u r 1 . A
. 1 . , . .
r y r r r r
. . r l . .
_.—... .. .-. .. .6. .-. .. .. .-. .. ”5-- .- .. .-.-.-. ... .. .. .-. .. .. . .4“ -.--. .. .. ..‘
f l I I Z l
. 1 . . r
I I I I I
r . r | r
r r .

 

 

i i 1 i 1 i
0 1 2 3 4 5
Generation

 

Figure 5.17,.7em,x vs. generation for dansyl-cored PAMAM dendrimer in acetonitrile.

142

The transition of the PAMAM didendrons from an extended to a globular

morphology depends on the generation and is independent of the solvent quality. All
solvents tested show a discontinuity in the relationship between km and generation at

dansyl 63.0 to dansyl G4.0. The ability of solvent to hydrogen bond with the dendrimer
does not alter the overall morphology transition of the molecule, since acetonitrile, a poor
hydrogen bond acceptor, and methanol, a good hydrogen bond donor and acceptor, both

demonstrate the onset of a globular dendritic state at generation 4. These results agree
with the previous results of Hawker et al.4 who saw a similar discontinuity in lnm of

poly(benzyl ether) dendrons with a 4-(N,N-dimethylamino)-1-nitrobenzene
solvatochromic probe covalently attached at its focal point. An analogous transition has
also been shown for carboxylate-terminated starburst dendrimers via photoinduced
electron-transfer measurements.20
Conclusion

We have shown the synthesis and characterization of PAMAM di-dendrons up to
generation 5 with a solvatochromic probe at their focal point. Solvatochromic probes are
sensitive to the microenvironments at the core of a PAMAM dendrimer and are capable

of relaying information on the overall morphology of a dendrimer when placed at the
focal point. All solvents tested show a noteworthy change in lnm from dansyl G3.0 to

dansyl G4.0, which signifies the transition from an extended to a globular state. In all

cases, the half generation dendrimers, with methyl ester end groups, show little change in
kmax as the generation and molecular weight increases, implying they have significantly

different morphology than the primary amine terminated full generations. However due

to the polarity differences of the methyl esters and primary amines, the methyl ester

143

groups are less able to stabilize the excited states of the dansyl group, thus explaining the

low values of kmax observed.

144

 

References

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2.

10.

ll.

12.

13.

14.

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Tomalia, D. A. B., H.; Martin, S.; J .; Hall, M.; Kallos, G.; Roeck, J .; Ryder, J .;
Smith, P., Macromolecules 1986, 2466.

Hawker, C. J .; Fréchet, J. M. J ., J. Am. Chem. Soc. 1990, 112, 7638-7647.

Hawker, C. J .; Wooley, K. L.; Frechet, J. M. J ., J. Am. Chem. Soc. 1993, 115,
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Cardona, C. M.; Alvarez, J .; Kaifer, A. B.; McCarley, T. D.; Pandey, S.; Baker, G.
A.; Bonzagni, N. J .; Bright, F. V., J. Am. Chem. Soc. 2000, 122, 6139-6144.

Cardona, C. M.; Wilkes, T.; Ong, W.; Kaifer, A. E.; McCarley, T. D.; Pandey, S.;
Baker, G. A.; Kane, M. N.; Baker, S. N.; Bright, F. V., J. Phys. Chem. B. 2002,
106, 8649—8656.

Pandey, S.; Redden, R. A.; Fletcher, K. A.; Sasaki, D. Y.; Kaifer, A. B.; Baker, G.
A., Chem. Commun. 2004, 1318-1319.

Cardona, C. M.; Jannach, S. H.; Huang, H.; Itojima, Y.; Leblanc, R. M.; Gawley,
R. B.; Baker, G. A.; Brauns, E. E., Helv. Chimi. Act. 2002, 85, 3532-3558.

Wang, B. B.; Zhang, X.; Jia, X. R.; Li, Z. G; Ji, Y.; Yang, L.; Wei, Y., J. Am.
Chem. Soc. 2004, 126, 15180-15194.

Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W., Chem. Rev. 2003, 103, 3899-4031.

Wang, B. B.; Zhang, X.; Jia, X. R.; Li, Z. C.; Yan, J .; Wei, Y., J. Polym. Sci. Part
A: Polym. Chem. 2005, 43, 5512-5519.

Fuchs, S.; Otto, H.; Jehle, S.; Henklein, P.; Schluter, A. D., Chem. Commun.
2005, 1830-1832.

Vogtle, F.; Gestermann, S.; Kauffmann, C.; Ceroni, P.; Vicinelli, V.; De Cola, L.;
Balzani, V.,J. Am. Chem. Soc. 1999, 121, 12161-12166.

Balzani, V.; Ceroni, P.; Gestermann, S.; Kauffmann, C.; Gorka, M.; Vogtle, F.,
Chem. Commun. 2000, 853-854.

Vogtle, F .; Gestermann, S.; Kauffmann, C.; Ceroni, P.; Vicinelli, V.; Balzani, V.,
J. Am. Chem. Soc. 2000, 122, 10398-10404.

Vicinelli, V.; Ceroni, P.; Maestri, M.; Balzani, V.; Gorka, M.; Vogtle, F., J. Am.
Chem. Soc. 2002, 124, 6461-6468.

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18.

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Balzani, V.; Ceroni, P.; Gestermann, S.; Gorka, M.; Kauffmann, C.; Vogtle, F.,

Tetrahedron 2002, 58, 629-637.

Saudan, C.; Balzani, V.; Gorka, M.; Lee, S.; Maestri, M.; Vicinelli, V.; Vogtle, F.,

J. Am. Chem. Soc. 2003, 125, 4424-4425.

Doyle, E. L.; Hunter, C. A.; Phillips, H. C.; Webb, S. J .; Williams, N. H., J. Am.

Chem. Soc. 2003, 125, 4593-4599.

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Macromolecules 1990, 23, 912.

146

 

 

 

CHAPTER SIX: Synthesis and Solution Properties of PEG-PAMAM Linear-
Dendrimer Diblock Copolymers

Introduction

The ability to control the morphology of macromolecules in solution has great
potential for the construction of smart materials, where interest in the design and
synthesis of stimuli-responsive polymers has increased dramatically due to their
promising application in biomedical fields.M In previous work we performed studies on
linear-dendrimer diblock copolymers, where a linear poly(styrene) (PS) chain is attached
to the focal point of a generation 4 (G4) poly(benzyl ether) (PBE) dendrimer (Chapter
3).5 Jeong et al.6 found that the PBE-PS system undergoes a transition in intrinsic
viscosity. At low molecular weights it has an intrinsic viscosity below that of P8 with an
equal molecular weight but an increase in intrinsic viscosity of the hybrid occurs as the
molecular weight of the PS chain increases. This suggests a transition in the structure of
the system due to interactions between the linear polystyrene block and the highly
branched dendritic block. Through a series of small angle neutron scattering experiments
and dynamic light scattering studies on a PBE G4-PS hybrid, we concluded that the
relative molecular weight of the linear chain to the dendrimer block influences the
conformations of the dendrimer block and linear chain, as well as the overall morphology
of the hybrid.5 At low molecular weights of PS, the linear chain is in a more contracted
state than an analogous ‘free’ PS chain, however upon increasing the molecular weight of
the linear chain, the PS chain adopts dimensions similar to a free linear chain.
Concurrently, the dendrimer changes shape from a sphere-like object to a wedge or ‘ice-

cream’ cone.

147

The ability of the dendrimer to change shape is critical for the occurrence of the
single molecular phase transition observed for the PBE-PS system. We wish to apply this
change in morphology to an advanced drug delivery scheme or a medicinal sensing
application, which require the material to be water soluble. The system we chose to
study is a poly(amido amine) (PAMAM) dendrimer with a linear poly(ethy1ene oxide)
(PEO) chain attached to its focal point. We first synthesized an analogous PAMAM
dendron attached to the linear PEO chain to whether it undergoes a size change in
response to its environment, and if so, which generation undergoes the most drastic
change in dimensions (Chapter 4). Small angle neutron scattering (SANS) and dynamic
light scattering (DLS) experiments on generations 3-5 demonstrated that G4 undergoes
the greatest size change and thus this generation number was used to investigate the
morphology of the PEO-PAMAM linear-dendrimer diblocks.

While numerous studies have been conducted on dendrimer conjugates with PEO,
there is little published work on the dilute solution behavior of these molecules. Fréchet
and Gitsov7 found that a PEO chain attached to the focal point of a PBE dendrimer can
shield the hydrophobic dendrimer from a surrounding hydrophilic (methanol/water)
environment when its molecular weight is large enough to allow the system to form
mono-and multimolecular micelles. The formation of these micelles also depends on the
dendrimer generation. When the diblocks were placed in tetrahydrofuran, THF, a good
solvent for the dendrimer block, only monomolecular micelles were formed suggesting
the dendrimer shields the linear chain from the solvent allowing it to occupy the dendritic

cavities.

148

The majority of the literature on linear-dendrimer diblocks concerns forming
multi-molecular self-assembled micelles. Ge and Liu4 recently described the synthesis
and self assembly of poly(benzyl ether)-b-poly(N-isopropylacrylamide) dendrimer-linear
diblock copolymers where micelles form in aqueous solutions and thermoresponsive
conformational changes of the poly(N-isopropylacrylamide) chains located at the micelle
shell cause a sudden collapse of the system at 29-31°C. Fréchet and coworkers8
developed a pH-responsive linear-dendrimer micelle consisting of PEG and either a
poly(lysine) or polyester dendron where hydrophobic groups were attached to the
dendrimer periphery via acid-sensitive acetal linkages. The diblocks were designed to
form stable micelles in aqueous solutions at neutral pH that disassociate into unimers
under mildly acidic conditions, where hydrolysis of the acetal releases the hydrophobic
moiety, i.e. drug.

We are interested in the unimolecular behavior of linear-dendrimer diblocks in
dilute solution and in particular determining the relative location of the linear chain with
respect to the dendrimer block. We hope to determine what environmental factors, if any,
control this morphology to allow these functionalized dendrimers to be used for the
design of unique materials. There are three possible linear-dendrimer diblock
conformational states in solution for a system where the linear and dendrimer blocks are
compatible with each other. Each conformational state pertains to different relative
locations of the two blocks.9 The first state is a knitted coil, where the linear chain
weaves in and out of the dendrimer, thus the dendrimer shields or partially shields the
linear polymer from the surrounding environment. For this to occur, the dendrimer must

have enough free volume to accommodate the linear block. A second hybrid

149

conformation consists of the linear chain wrapping itself around the dendrimer, thus
shielding the dendrimer from the surroundings and forming a unimolecular micelle. This
encapsulated dendrimer state would occur when the linear block is compatible with the
surrounding medium and the dendrimer less compatible, resulting in a core-shell
morphology. A third potential molecular arrangement consists of the linear chain
completely expelled from the cavities within the dendrimer. This could occur if there is
not enough free volume within the dendrimer to contain the entire linear chain and each
component has similar solubility in the solvent.6

The ability to control the conformation of the hybrid has potential for a molecular
machine which would operate as follows (Figure 6.1): A hybrid system where the linear
chain has a drug or fluorescent tag attached via a cleavable linkage shields the drug/tag
from the surrounding environment when the system is in a knitted coil conformation.
The attachment of the dendrimer to a surface triggers a conformational change of the
dendrimer from a sphere to a flattened disk, a pancake, which has been observed by
atomic force microscopy.lo Upon flattening the free volume decreases, thus expelling the
linear chain from the its cavities and exposing the drug/tag to the surrounding

environment.

150

Environmentally
tri ered
conformation change

'n r i x H
from dendrimer

     

Knitted coil: Random coil:
Shielded probe Exposed probe

      

End groups attach 09 Linear chain I Probe
to functionalized if, -“':‘ + ~ gxgeueg from _2 ‘fi
surface ‘ dendrimer {7 '
£2.12
Dendrimer changes shape

Figure 6.1. Cartoon showing the conformational changes of a linear-dendrimer diblock
copolymer.

A probe, which could be a drug or fluorescent tag, is attached to the linear chain.
When the linear chain is inside the cavities of the dendrimer block, the probe is shielded
from the surrounding environment. A conformational change of the system from a
knitted coil to a random coil exposes the probe to the surrounding environment, thus
releasing the drug or producing a fluorescent signal.

151

A variety of strategiesll were used to synthesize linear-dendrimer diblock
copolymers with the most common being coupling a linear chain end to the focal point of

a dendrimer.7‘ '2'”

Alternatively, the dendrimer can be grown divergently from an
appropriately functionalized chain end of the linear block,'5"7 while a third strategy is to
use the focal point of the dendrimer as a macroinitiator for polymerization of the desired
linear block.18 The advantage of the first and third methods is the increased number of
hybrid analogs produced from a given dendrimer, therefore minimizing the number of
difficult dendrimer syntheses required to make a library of compounds. The second
method has the advantage for PAMAM synthesis since the presence of the polymer
simplifies the purification of each generation and we chose to use the divergent process
as this is best suited for the chemical functionality of the PAMAM dendrimer.

We synthesized a series of PEO-PAMAM hybrids with five different molecular
weights, 4.2 kDa, 5.4 kDa, 8.4 kDa, 12.4 kDa and 23.4 kDa, where the molecular weight
includes the PAMAM G4 dendrimer (M=3422 Da) and that of the linear chain. These

molecules were produced in a two step divergent process beginning with an amine
terminated PEO chain.17 We performed DLS experiments on the hybrids and linear PEO
to determine the hydrodynamic radius (Rh) in dilute aqueous solutions and to compare
the scaling behavior.

Experimental

Materials. Poly(ethylene oxide) monomethylether (PEO) (9 kDa and 20 kDa) was
purchased from The Polymer Source, Quebec, Canada. All other chemicals were
purchased from Aldrich Chemical Co., Saint Louis, MO. Ethylenediamine was distilled

under nitrogen prior to use. Methyl acrylate was distilled from KOH to remove the

152

inhibitor. 1H NMR were acquired usina Varian 300 or 500 MHz instrument, as indicated,
with the residual proton signal from the solvent used as the reference peak. NMR spectra
and assigned chemical shifts are displayed in Appendix C.

Synthesis of PEG-NH 2 (2 kDa)

mPEO-OMs (2) 1 (6 g, 3 mmols) was dissolved in 100 mL of dichloromethane and
stirred at 0 °C under nitrogen. Triethylamine (2.10 mL, 15.0 mmol, 1.52 g) was added in
one aliquot, followed by methanesulfonylchloride (1.16 mL, 15 mmol, 1.72 g) as one
aliquot. The reaction mixture was stirred at 0 °C for two hours. Diethyl ether (250 mL)
was added to reaction mixture causing precipitation of the polymer as a white solid.
Filtration and drying gave 5.52 g of 2 as a white solid (92% yield). 1H NMR (CDCl3,
300 MHz) 5 4.34 (m, 2.05 H), 5 3.61 (m, 183.27 H) 5 3.35 (s, 3.02 H), 5 3.06 (s, 3.00 H).
mPEO-phthalimide (3) mPEO-OMs (2.26 g, 1.13 mmol) of was dissolved in DMF (125
mL). Potassium phthalimide (0.63g, 3.40 mmol) was added and the solution was stirred
under nitrogen at 120 °C for three hours. The reaction mixture was cooled to room
temperature and the resulting precipitate was removed by filtration. The DMF solution
was evaporated to dryness under vacuum (100 mTorr), resulting in a light yellow solid.
The solid was dissolved in dichloromethane (150 mL) and after 2 hours the solution was
filtered and the solvent removed in vacuo to provide a white solid. Washing with diethyl
ether (300 mL) followed by filtration gave 2.16 g of 3 (97% yield). (Scheme 6.1) lH
NMR (CDCl3, 300MHz) 5 7.9 (m, 2.0 H), 5 7.25 (m, 2.57 H), 5 3.40 (s, 180 H), 5 3.18 (s,
3.0 H).

mPEO-NH; (4) 3 (2.22 g, 1.10 mmol) was dissolved in 100 mL of ethanol and heated to

reflux. Hydrazine monohydrate (0.53 mL, 11.0 mmol, 0.55 g) was added in one aliquot.

153

After refluxing overnight, the reaction mixture was cooled to room temperature and
filtered to remove insoluble impurities. The solvent was removed in vacuo and the
resulting solid was dissolved in dichloromethane (200 mL). The solids were removed by
filtration and concentration of the filtrate gave a solid residue. Washing with diethylether
(300 mL) gave 2.13 g of 4 as a white solid (97% yield). 1H NMR (CDCI3, 300 MHz) 5:
3.64 (s, 187.18 H), 3.36 (s, 3.00 H), 2.56 (t, 2.55 H).

Synthesis for PEO—NH; (Mw = 750 Da, 5 kDa, 9 kDa, 20 kDa)

The hydroxyl group of 5 kDa, 9 kDa and 20 kDa monomethyl ether PEO was converted
to a primary amine following the procedure used for the system described above. The
synthesis of PEO-NHZ (750 Da) was similar except that precipitation required lowering
temperature of ether solutions to -30 °C for 1-3 hours to obtain crystals. The ether
solution was removed from the freezer and warmed for approximately 20 minutes before
collecting white crystals via vacuum filtration.

mPEO 2 kDa PAMAM G0.5 (5) 4 (2.0 g, 1.0 mmol) was dissolved in 35 mL of
methanol and slowly dripped into uninhibited methyl acrylate (25.8 g, 0.30 mol). The
reaction was stirred at 35 °C under nitrogen for two days. Methanol and unreacted
methyl acrylate was removed by rotary evaporation and the resulting white solid was
washed with diethyl ether (250 mL) to give 2.15 g of 5 (99% yield). 1H NMR (300 MHz,
CDCl3) 5: 3.60 (m, 194H), 3.27 (s, 3H), 2.68 (t, 4H), 2.56 (t, 2H), and 2.35 (t, 4H).
mPEO 2 kDa PAMAM G 1.0 (6) 5 (1.50 g, 0.70 mmol) was dissolved in 30 mL of
methanol and slowly dripped into ethylenediamine (8.20 g, 0.14 mmol). The reaction
was stirred at 55 °C under nitrogen for two days. Methanol and unreacted

ethylenediamine were removed by rotary evaporation, and the resulting white solid was

154

washed with diethyl other (300 mL) to give 1.56 g of 6 (100% yield). 1H NMR (300
MHz, CDC13) 5: 3.63 (m, 185H), 3.38 (s, 3H), 3.31 (t, 4H), 2.85 (m, 8H), 2.65 (t, 2H),
and 2.43 (t, 4H).

General Procedure for Growing Dendrimer. Procedures for formation of subsequent
half and whole generations were analogous to the synthesis of 5 and 6 respectively. Note
that care must to taken to remove all unreacted monomer, as small traces will lead to
impurities. The hybrids prepared from PEO chains with molecular weights of 2 kDa and
higher were purified by precipitation from ether and then soxhlet extraction with ether
when necessary for complete removal of ethylenediamine. The half generation hybrids
synthesized from PEO (M=750 Da) were purified by placing under vacuum (5 mTorr) at
40 °C for 2 days and then precipitating from cold (-30 °C) diethyl ether. The full
generation hybrids synthesized from PEO (M=750 Da) were purified by removal of
methanol and unreacted ethylenediamine under vacuum. To remove residual amounts of
ethylenediamine, the waxy residue was dissolved in octanol (50 mL) and stirred under
vacuum (5 mTorr) at 40 °C for 2 days. The octanol mixture was washed with 50:50
hexanezwater. The water layer was collected and washed three times with hexanes, then
concentrated via rotary evaporation and then dried under vacuum (5 mTorr) at 40 °C to
constant mass. The resulting white wax was dissolved in methanol and placed in a
dialysis bag (MWCO=200 g/mol) for 2 days after which contents remaining in the bag
were concentrated.

mPEO 2 kDa OMS 1H NMR (300 MHz, CDCl3) 5: 4.60 (m, 2H), 3.60 (m, 1820H), 3.30

(s, 3H), and 3.00 (s, 3H).

155

mPEO 2 kDa phth 'H NMR (300 MHz, CDCl3) 5: 7.82 (m, 2H), 7.60 (m, 2H), 3.60
(m, 183H), and 3.35 (s, 3H).

mPEO 2 kDa NH; 1H NMR (300 MHz, CDC13) 5: 3.60 (m, 190H), 3.31 (s, 3H), and
2.80 (t, 2H, J = 4.8 Hz).

mPEO 2 kDa (:05 'H NMR (300 MHz, c003) 5: 3.60 (m, 194H), 3.27 (s, 3H), 2.68
(t, 4H, J = 6.9 Hz), 2.56 (t, 2H, J = 6.3 Hz), and 2.35 (t, 4H, J = 6.9 Hz).

mPEO 2 kDa G1.0 'H NMR (300 MHz, CDCI3) 5: 3.63 (m, 185H), 3.38 (s, 3H), 3.31
(quartet, 4H, J = 6.3 Hz), 2.85 (m, 8H), 2.65 (t, 2H, J = 4.8 Hz), and 2.43 (t, 4H, J = 6.0
Hz).

mPEO 2 kDa 61.5 'H NMR (300 MHz, CDC13) 5: 3.62 (m, 198H), 3.34 (s, 3H), 3.22
(q, 4H, J = 6.0 Hz), 2.86 (t, 4H, J = 6.3 Hz), 2.75 (t, 8H, J = 6.6 Hz), 2.55 (t, 6H, J = 6.6
Hz), and 2.50-2.40 (m, 12H).

mPEO 2 kDa G2.0 1H NMR (300 MHz, d4-methanol) 5: 3.60 (m, 190H), 3.32 (s, 3H),
2.28 (m, 12H), 2.85-2.62 (m, 20H), 2.60 (m, 2H), 2.48 (m, 4H), and 2.40-2.30 (m, 12H).
mPEO 2 kDa G2.5 'H NMR (300 MHz, CDC13) 5: 3.60 (m, 182H), 3.31 (s, 3H), 3.18
(m, 12H), 2.80-2.60 (m, 30H), 2.55 (m, 12H), and 2.40220 (m, 28H).

mPEO 2 kDa G3.0 'H NMR (300 MHz, CDC13) 5: 3.60 (m, 186H), 3.31 (s, 3H), 3.20
(m, 28H), 2.85-2.60 (m, 46H), 2.55 (m, 12H), and 2.40-2.30 (m, 28H).

mPEO 2 kDa G3.5 1H NMR (300 MHz, d4-methanol)5: 3.61 (m, 181H), 3.32 (s, 3H),
3.27 (r, 28H, J = 5.7 Hz), 2.80-2.60 (m, 60H), 2.60-2.50 (m, 28H), 2.44 (t, 32H, J = 6.6
Hz), and 2.30 (t, 28H, J = 6.3 Hz).

mPEO 2 kDa G4.0: lH NMR (300 MHz, CDC13) 5: 3.60 (m, 193H), 3.31 (s, 3H), 3.23

(m, 60H), 2.80-2.60 (m, 92H), 2.55 (m, 28H), and 2.40-2.20 (m, 60H).

156

mPEO 750 Da-PAMAM

mPEO 750 Da OMs: 'H NMR (300 MHz, CDCI3) 5: 4.60 (m, 2H), 3.60 (m, 72H), 3.32
(s, 3H), and 3.01 (s, 3H).

mPEO 750 Da phth 'H NMR (300 MHz, CDClg) 5: 7.82 (m, 2H), 7.60 (m, 2H), 3.60
(m, 69H), and 3.35 (s, 3H).

mPEO 750 Da NH; 'H NMR (300 MHz, CDC13) 5: 3.60 (m, 72H), 3.31 (s, 3H), 3029
(br s, 2H), and 3.78 (t, 3H, J = 3.6 Hz). 5: 3.60 (m, 72H), 3.31 (s, 3H), 3.0-2.9 (br s, 2H),
and 3.78 (t, 3H).

mPEO 750 Da 00.5 'H NMR (500 MHz, cit—methanol) 5: 3.60 (m, 60H), 3.31 (s, 3H),
2.65 (t, 4H, J = 10 Hz), 2.54 (t, 2H, J = 3.6 Hz), and 2.35 (t, 4H, J = 10 Hz).

mPEO 750 Da GLO 'H NMR (500 MHz, CDC13) 5: 3.63 (m, 75H), 3.39 (s, 3H), 3.31
(quartet, 4H), 2.85 (m, 8H), 2.65 (t, 2H), and 2.43 (t, 4H).

mPEO 750 Da G1.5 'H NMR (300 MHz, D20) 5: 3.54 (m, 82H), 3.21 (s, 3H), 3.15
(t, 4H, J = 3.6 Hz), 2.80-2.50 (m, 14H), 2.40 (m, 12H), and 2.30 (t, 4H, J = 7.2 Hz).
mPEO 750 Da 62.0 'H NMR (300 MHz, D20) 5: 3.59 (m, 83H), 3.21 (s, 3H), 3.15
(m, 12H), 2.80-2.50 (m, 22H), 2.45 (t, 4H, J = 6.9 Hz), and 2.30 (m, 12H).

mPEO 750 Da 02.5 ‘H NMR (300 MHz, aid—methanol) 5: 3.59 (m, 98H), 3.31 (s, 3H),
3.20 (t, 12H, J = 6 Hz), 2.80-2.60 (m, 30H), 2.50 (m, 12H), 2.40 (t, 16H, J = 6.6 Hz), and
2.30 (t, 12H, J = 6.3 Hz).

mPEO 750 Da G3.0 'H NMR (300 MHz, D20) 5: 3.60 (m, 78H), 3.20 (s, 3H), 3.15

(m, 28H), 2.80-2.50 (m, 46H), 2.44 (t, 12H, J = 6.6 Hz), and 2.22 (m, 28H).

157

mPEO 750 Da 03.5 lH NMR (300 MHz, rid—methanol) 5: 3.59 (m, 100H), 3.33 (s, 3H),
3.28 (1, 28H, J = 6.3 Hz), 2.80-2.60 (m, 60H), 2.60-2.50 (m, 28H), 2.40 (1, 32H, J = 7.2
Hz), and 2.35 (t, 28H, J = 6.0 Hz).

mPEO 750 Da 04.0 'H NMR (300 MHz, 1320) 5: 3.60 (m, 74H), 3.31 (s, 3H), 3.23
(m, 60H), 2.80-2.60 (m, 92H), 2.55 (1, 28H, J = 6.7 Hz), and 2.40-2.20 (1, 60H, J = 6 Hz).
mPEO s kDa-PAMAM

mPEO 5 kDa OMs 'H NMR (300 MHz, CDCl3) 5: 4.60 (m, 2H), 3.60 (m, 456H), 3.32
(s, 3H), and 3.01 (s, 3.1H).

mPEO 5 kDa phth 'H NMR (300 MHz, 013013) 5: 7.80 (m, 1.9H), 7.60 (m, 2.1H), 3.60
(m, 454.3H), and 3.30 (s, 3H).

mPEO 5 kDa NH2 1H NMR (300 MHz, 013013) 5: 3.60 (m, 457.2H), 3.27 (s, 3H), and
2.79 (1, 2H, J = 4.8 Hz).

mPEO 5 kDa-PAMAM 00.5 1H NMR (300 MHz, 00013) 5: 3.60 (m, 464.1H), 3.27 (s,
3.1H), 2.67 (1, 4.1H, J = 7.2 Hz), 2.54 (1, 2H, J = 7.2 Hz), and 2.33 (1, 4.2H, J = 7.2 Hz).
mPEO 5 kDa-PAMAM 01.0 1H NMR (300 MHz, 00013) 5: 3.63 (m, 451.3H), 3.37 (s,
3H), 3.31 (m, 4.2H), 2.85 (m, 8.1H), 2.64 (1, 2H, J = 4.8 Hz), and 2.41 (t, 4.1H, J = 6.6
Hz).

mPEO 5 kDa-PAMAM 01.5 'H NMR (300 MHz, rid—methanol) 5: 3.62 (m, 458.7H),
3.38 (s, 3H), 3.28 (1, 4H, J = 6 Hz), 2.85 (1, 4H, J = 7.2 Hz), 2.74 (1, 8H, J = 6.9 Hz), 2.56
(1, 6H, J = 6.9 Hz), 2.50 (t, 8H, J = 6.9 Hz), and 2.40 (t, 4H, , J = 7.2 Hz).

mPEO 5 kDa-PAMAM 02.0 'H NMR (300 MHz, 020) 5: 3.60 (m, 449.9H), 3.41
(s, 3H), 3.25 (m, 12H), 2.83-2.60 (m, 20H), 2.57 (m, 2H), 2.52 (m, 4H), and 2.36

(m, 12H).

158

mPEO 5 kDa-PAMAM 02.5 'H NMR (300 MHz, 015013) 5: 3.63 (m, 454.411), 3.39 (s,
3H), 3.25 (m, 12H), 2.85-2.70 (m, 30H), 2.54 (m, 12H), and 2.40-2.30 (m, 28H).

mPEO 5 kDa-PAMAM 03.0 1H NMR (300 MHz, 01301;) 5: 3.60 (m, 459H), 3.31 (s,
3H), 3.20 (m, 28H), 2.85-2.60 (m, 46H), 2.56 (m, 12H), and 2.40-2.30011, 28H).

mPEO 5 kDa-PAMAM 03.5 'H NMR (300 MHz, 114—1115111111161) 5: 3.59 (m, 460H),
3.33 (s, 3H), 3.28 (1, 28H, J = 6.0 Hz), 2.80-2.60 (m, 62H), 2.60-2.50 (m, 28H), 2.43 (1,
32H, J = 6.9 Hz), and 2.35 (1, 28H, J = 6.6 Hz).

mPEO 5 kDa-PAMAM 04.0 1H NMR (300 MHz, 013013) 5: 3.60 (m, 457H), 3.31
(s, 3H), 3.23 (m, 60H), 2.80-2.60 (m, 92H), 2.55 (m, 28H), and 2.40-2.20 (m, 60H).

PEG 9 kDa-PAMAM

mPEO 9 kDa OMS 'H NMR (300 MHz, 01301;) 5: 4.60 (m, 2H), 3.60 (m, 850H), 3.32
(s, 3H), and 3.81 (m, 28H).

mPEO 9 kDa phth 1H NMR (500 MHz, 00013) 5: 7.82 (m, 2H), 7.60 (m, 2H), 3.87
(1, 2H), 3.60 (m, 854H), and 3.35 (s, 3H).

mPEO 9 kDa NH; 1H NMR (500 MHz, 01301;) 1H NMR (300 MHz, 00013) 5: 3.60
(m, 845H), 3.25 (s, 3H), and 2.80 (1, 2H, J = 5.7 Hz).

mPEO 9 kDa-PAMAM 00.5 'H NMR (300 MHz, f—methanol) 5: 3.60 (m, 814H), 3.18
(s, 3H), 2.68 (1, 4H, J = 4.2 Hz), 2.52 (1, 2H, J = 2.1 Hz), and 2.32 (1, 4H, J = 4.2 Hz).
mPEO 9 kDa-PAMAM 01.0 'H NMR (300 MHz, D20) 5: 3.61 (m, mm, 3.30
(s, 3H), 3.21 (1, 4H, J = 5.4 Hz), 2.80 (1, 4 H, J = 5.4 Hz), 2.60 (m, 4H), 2.51 (m, 2H), and
2.41 (m, 4H). '

mPEO 9 kDa-PAMAM 01.5 'H NMR (300 MHz, D20) 5: 3.60 (m, 828H), 3.20

(s, 3H), 3.18 (m, 4H), 2.80-2.50 (m, 18H), and 2.40 (m, 1211).

159

mPEO 9 kDa-PAMAM 02.0 'H NMR (300 MHz, 0001;) 5: 3.60 (m, 1821H), 3.20
(s, 3H), 3.18 (1, 12H, J = 6.3 Hz), 2.80-2.60 (m, 20.3H), 2.56 (1, 2H, J = 6.3 Hz), 2.45
(1, 4H, J = 7.8 Hz), and 2.31 (m, 12H).

mPEO 9 kDa-PAMAM 02.5 1H NMR (300 MHz, 114—1115111111161) 5: 3.60 (m, 820H), 3.31
(s, 3H), 3.18 (1, 12H, J = 6.0 Hz), 2.80-2.60 (m, 28H), 2.52 (1, 12H, J = 6.3 Hz), 2.38
(t, 16H, J = 6.6 Hz), and 2.30 (m, 12H).

mPEO 9 kDa-PAMAM G3.0 1H NMR (300 MHz, f—methanol) 5: 3.61 (m, 808H), 3.40
(s, 3H), 3.21 (m, 28H), 2.80-2.60 (m, 46 H), 2.54—2.42 (m, 12H), and 2.40-2.22 (m, 28H).
mPEO 9 kDa-PAMAM 03.5 'H NMR (300 MHz, 0;0) 5: 3.60 (m, 824H), 3.31
(s, 3H), 3.18 (m, 28H), 2.8-2.6 (m, 60H), 2.40 (m, 28H), 2.39 (1, 32H, J = 6.6 Hz), and
2.29 (m, 28H).

mPEO 9 kDa-PAMAM 04.0 ‘H NMR (500 MHz, rid—methanol) 5: 3.60 (10, mm,
3.31 (s, 3H), 3.23 (m, 60H), 2.80-2.60 (m, 92H), 2.55 (m, 28H), and 2.40-2.20 (m, 60H).
PEO 20 kDa-PAMAM

mPEO 20 kDa-OMS 1H NMR (300 MHz, 0001;) 5: 4.60 (m, 2H), 3.60 (m, 1820H),
3.32 (s, 3H), and 3.81 (m, 28H).

mPEO 20 11011 phth ‘H NMR (300 MHz, 0001;) 5: 7.82 (m, 2H), 7.60 (m, 2H), 3.87
(1, 2H), 3.60 (m, 1820H), and 3.35 (s, 3H).

mPEO 20 kDa-NH; 1H NMR (300 MHz, 0001;) 5: 3.60 (m, 1820H), 3.25 (s, 3H), and
2.80 (1, 2H, J = 5.4 Hz).

mPEO 20 kDa-PAMAM 00.5 'H NMR (300 MHz, 0001;) 5: 3.60 (m, 1824H), 3.21

(s, 3H), 2.70 (t, 4H, J = 6.5 Hz), 2.58 (t, 2H, J = 6.0 Hz), and 2.39 (t, 4H, J = 6.5 Hz).

160

mPEO 20 kDa-PAMAM 01.0 1H NMR (300 MHz, 0;0) 5: 3.65 (m, 1820H), 3.41
(s, 3H), 3.30 (m, 4H), 2.85 (m, 8H), 2.65 (m, 2H), and 2.40 (m, 4H).

mPEO 20 kDa-PAMAM 01.5 1H NMR (300 MHz, 0001;) 5: 3.61 (m, 1819H), 3.41
(s, 3H), 3.30 (m, 4H), 2.80-2.60 (m, 14H), 2.50 (t, 4H J = 5.4 Hz), and 2.40-2.20
(m, 12H).

mPEO 20 kDa-PAMAM G2.0 IH NMR (300 MHz, 114—methanol) 5: 3.56 (m, 1808.7H),
3.35 (s, 3H), 3.18 (m, 12H), 2.80-2.58 (m, 22H), 2.51 (m, 4H), and 2.30 (m, 12H).
mPEO 20 kDa-PAMAM G2.5 lH NMR (300 MHz, CDC13) 5: 3.60 (m, 1821H), 3.40 (s,
3H), 3.25 (m, 12H), 2.82-2.60 (m, 28H), 2.60-2.44 (m, 12H), and 2.44-2.30 (m, 28H).
mPEO 20 kDa-PAMAM 03.0 'H NMR (300 MHz, 114-1115111111161) 5: 3.60 (m, 1802H),
3.40 (s, 3H), 3.21 (m, 28H), 2.80-2.60 (m, 46H), 2.54-2.40 (m, 12H), and 2.40-2.20
(m, 24H).

mPEO 20 kDa-PAMAM G3.5 1H NMR (300 MHz, (114-methanol) 5: 3.60 (m, 1853H),
3.20 (m, 32H), 2.80-2.60 (m, 62H), 2.50-2.40 (m, 28H), 2.38 (t, 32H J = 6.6 Hz), and
2.30 (m, 28H J = 6.0 Hz).

mPEO 20 kDa-PAMAM 04.0 lH NMR (300 MHz, 0001;) 5: 3.60 (m, 1803H), 3.38 (s,
3H), 3.20 (m, 60H), 2.80-2.60 (m, 92H), 2.50 (m, 28H), and 2.40-2.20 (m, 60H).
Dynamic Light Scattering (DLS). Measurements were performed with a Protein
Solutions Dyna Pro-MS/X system with temperature control. All samples were filtered
through a 0le Whatman Anodisc 13 filter as and allowed to equilibrate in the

instrument for 15 minutes at 25 °C before measurements were taken resulting in

calculation of the Rh. The sample is illuminated by a semi-conductor laser with ~ 830 nm

wavelength. The light scattered at an angle of 90° is collected and guided via a fiber

161

optic cable to an actively quenched, solid state Single Photon Counting Module (SPCM),
where the photons are converted to electrical pulses and correlated. Autocorrelation is
used to analyze the time scale of the scattered light intensity fluctuations. The

translational diffusion coefficient (D) of the molecules in the sample cell is determined
from the decay of the intensity autocorrelation data. The Rh of the sample is then derived

from D, using the Stokes-Einstein equation.

_ kBT
67:77R,

 

(6.1)

Here, D is the translational diffusion coefficient, k is Boltzmann’s constant, T is the
absolute temperature, T] is the viscosity of the solvent and Rh is the effective Stokes

radius or the hydrodynamic radius. The uniformity of the sample sizes is determined by a
monomodal curve fit, cumulants, which assumes a single particle size with a Gaussian
distribution. The regularization algorithm makes no assumptions regarding the number
of size populations and can fit a multimodal system. All systems in this study are
monomodal.
Results and Discussion
Synthesis of PEG-PAMAM

PAMAM dendrons were grown divergently from a primary amine terminated
PEO chain to result in linear-dendrimer diblock copolymers that are water soluble
(Scheme 6.1 and 6.2). Each full generation was obtained by exhaustive Michael addition
of the primary amine functionality with methyl acrylate to result in methyl-ester
terminated generation 0.5 (G05) (5). The methyl ester end groups were amidated with

excess ethylenediamine to yield primary amine terminated full generation 1.0 (G10) (6).

162

Most PEG-PAMAM diblocks are white solids. The higher generation PEO 750 Da series
are waxy solids at room temperature. We used lH NMR to confirm end group
functionalization of the PEO chain as well as growth of the dendrimer. For the molecular

weight of PEO used, lH NMR is a reliable end group characterization method.l9

 

/O O OH 5 eq mesyl chloridi /O{/\O>/\/O‘(‘S?/
n 5 eq TEA, CH2C|2 n 6
1 0°C, N2, 2 hours 2
92%

O O
3 tassium hthalimide
2 9999 p g /O€/\O>/\/N
DMF, 120°C, N2, 3 hours n o
3

 

98%

 

97%

10 sq H2NNH; H20 /o</\ >z\/NH2
3 . > 0
ethanol, reflux, overnight n
4

Scheme 6.1. Synthesis of methoxy-amine terminated poly(ethy1ene oxide).

163

O
3008
0 NH q \AO/ o N 0
WWW: 7/V01VW9
n methanol, 35°C, N2 11 2
4 O
5

 

99% yield

 

 

 

/\/NH2
200 69 H2N g /O€AO>/\/N N\/\NH2
methanol, 55°C, N2 n O
100% yield 6 2
o H o
3111.; V11 / /o 1: 11¢ «A /
6 O t O N 0
methanol, 35°C, N; n O
99% yield 7 o o
I 2
H O
methanol, 55°C, N2 0 i H
100°/ ield
8 H

Scheme 6.2. Synthesis of PEG-PAMAM diblock copolymers.

164

Dynamic Light Scattering
We performed DLS experiments on the diblocks and PEG in Milli Q water at

25 °C to obtain the hydrodynamic radius (Rh) from eq 6.1 above. All measurements were

conducted at a concentration of 10 mg/mL as this is the lowest concentration that allowed
a reasonable intensity of scattering. At this concentration we observed a negligible
change in radii with decreasing concentration.

PEO-PAMAM diblock copolymers were dissolved in Milli Q water and filtered
before performing DLS experiments. The theoretical molecular weights of all samples

tested are listed in Table 6.1. The percent mass vs. hydrodynamic radius for PEO 750
Da-G4 has a maximum at an Rh of 1.20 i 0.09 nm (Figure 6.2a), obtained from both the
cummulant and regularization fit (Figure 6.1b). Figures 6.2-6.6 display both the percent
mass vs. Rh and the cummulant and regularization fits to the intensity autocorrelation
function of the hybrids. In all samples both the cummulant and regularization fit resulted
in the same value of Rh indicating a monomodal system. The hydrodynamic radius
increased as the molecular weight of the PEO chain attached to dendrimers’ focal point
increased, Rh values of 1.50 i 0.10 nm, 1.90 i 0.05 nm, 2.60 i 0.10 nm and 4.00 i 0.12

nm were found for PEG 2 kDa-G4, PEG 5 kDa-G4, PEO 9 kDa-G4 and PEG 20 kDa-G4

respectively (Table 6.2).

165

Table 6.1. Sample codes and number average molecular mass (M) of compounds used in
this study“

 

 

 

 

 

 

 

 

M-dendrimer(Da) IM- M-dendrimer(Da) IM-
Sample code poly(ethy1ene oxide) Sample poly(ethy1ene oxide)
(kDa)/M (kDa) code (kDa)/M (kDa)
PEO 750 Da-G4 3422/750/4.2 PEO 2 kDa 0/2.0./2.0
PEO 2 kDa-G4 3422/2.0/5.4 PEO 5 kDa 0/5.0/5..0
PEG 5 kDa-G4 3422/5.0/8.4 PEG 8 kDa 0/8.0/8.0
PEO 9 kDa-G4 3422/9.0/12.4 PEO 20 kDa 0/20.0/20.0
PEO 20 kDa-G4 3422/20.0/23.4 PEO 36 kDa 0/36.0/36.0
G4 3422/0/0

 

 

 

 

a M-dendrimer represents the mass of the dendrimer; M-poly(ethylene oxide) , mass of
the poly(ethy1ene oxide) and M, total mass.

Table 6.2. Hydrodyamic radii (nm) of PEG and PEG-PAMAM samples.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Sample code R1101!!!) Sample code Rh (nm)
PEO 750 01104 1.26 z 0.10 PEG 2 110:: 1.26 1 0.09
PEO 2 kDa-G4 1.50 i 0.10 PEG 5 kDa 1.80 i 0.10
PEG 5 kDa-G4 1.20 i 0.09 PEO 8 kDa 2.30 :t 0.11
PRO 9 kDa-G4 1.90 i 0.05 PEO 20 kDa 4.20 i: 0.10
PEO 20 kDa-G4 2.60 i 0.08 PEO 36 kDa 5.90 i 0.12
G4 1.90 i 0.05
100 2.0
90 .9 _____________________ c 1.9 . - - -PEO750Da-G4b
80 """"""""""""" g 1-3 I — -Cummulant Fit
70 -- ——————————————————— g 1.7 ~ . _ .
3 so » — ................... § 15 1 — Regulanzation Fit
:3 :3 ,_ . - 81501500604- _ _ _ 3 If} i
so ._ 1 ———————————————————— E 1.3
20 ~ - - ——————————————————— § 1.2 i
10 ~ — - ——————————————————— E 1.1 ~
0 4 1.0 1
0 5 10 1 100 10000
8.. (nm) “me (113)

Figure 6.2. Dynamic light scattering results for PEO 750 Da-G4 in Milli Q water.

166

 

 

96M»:

% Mass

3. Percent mass vs. Rh (nm), shows a maximum at 1.20 i 0.10 nm. b. Intensity
autocorrelation for the sample, cummulant and regularization fits vs. time.

100
90

80:
70*

60
50

40~
30*
20*

10

 

 

1..._ - ____________

-4.._-_ ______________

 

 

_—_-___._____..-_._.--__._

~____--__-_._._.______

>__.-_-______-______

 

 

2.0
1 .9
1 .8
1 .7
1 .6

lntenslty Autocorrelation

 

 

- b—--- PEG 2 kDa-G4
------- Curnmulant Fit

r

T

4 — Regularization Fit

 

 

 

1 100 10000 1000000
timeoia)

Figure 6.3. Dynamic light scattering results for PEG 2 kDa-G4 in Milli Q water.

a. Percent mass vs. Rh (nm), shows a maximum at 1.50 ..+. 0.10 nm. b. Intensity
autocorrelation for the sample, cummulant and regularization fits vs. time.

100

80~

70
60
50
4O

20-

10

 

 

 

 

 

 

I a

: PEG 5 kDa G4

0 5 10
8110"“)

2.0

1.7
1.6
1.5

Intensity Autocorrelation

1.1
1.0

1.9 r
1.8 7

1.4 >
1.3 r
1.2 ~

 

- - -PE05kDa-G4 b
— - Cunmdant Fit
t -— Regdarization Fit

 

 

L

1 1 00 1 0000
time (pa)

Figure 6.4. Dynamic light scattering results for PEG 5 kDa-G4 in Milli Q water.

a. Percent mass vs. Rh (1110), shows a maximum at 1.90 i 0.05 nm. b. Intensity
autocorrelation for the sample, cummulant and regularization fits vs. time.

167

100
90
80

707
607
507

% Mass

40
30
2O
10

 

 

 

PEG 9 kDa G4

 

 

I

5
Rh (nrn)

 

10

2.0

1.6

1.4
1.3
1.2
1.1
1.0

Intenelty Autocorrelation

1.9 7
1.8 7
1.7 7

1.5 7

 

- - - PEG 9 kDa-G4 b
— - Cunmulant Fit

- — Regularization Fit

 

 

 

1 100 10000
time (p.11)

Figure 6.5. Dynamic light scattering results for PEG 9 kDa-G4 in Milli Q water.

a. Percent mass vs. Rh (nm), shows a maximum at 2.60 i 0.08 nm. b. Intensity
autocorrelation for the sample, cummulant and regularization fits vs. time.

100

907

80

707
607

50

%Maes

10

 

407
307
207

 

 

 

 

3
PEG 20 kDa G4
5 10
311 (nm)

2.0

1.6

1.3
1.2
1.1
1.0

Intensity Autocorrelation

1.9 7
1.8 7
1.7 7

1.5 7
1.4 7

 

 

 

 

 

- - - PEO 20 kDa-G4 b
— - Cummulart Fit

1 Regularization Fit

1 100 10000

time (118)

Figure 6.6. Dynamic light scattering results for PEO 20 kDa-G4 in Milli Q water.

a. Percent mass vs. Rh (nm), shows a maximum at 4.00 i 0.09 nm. b. Intensity
autocorrelation for the sample, cummulant and regularization fits vs. time.

168

The Rh values of PEO with molecular weight ranging from 2 kDa to 36 kDa were
determined via DLS experiments (Table 6.2). We observed no agglomeration during the
time scale of the experiment. Figure 6.7a displays the percent mass vs. Rh for PEG 2 kDa
with a peak at an Rh value of 1.20 i 0.09 nm, which was obtained from both the
cummulant and regualization fit of the autocorrelation function. PEO chains with
molecular weights of 5 kDa, 8 kDa, 20 kDa and 36 kDa have Rh values of 1.80 i 0.10

nm, 2.30 i 0.11 nm, 4.20 i: 0.10 nm and 5.90 i 0.12 nm respectively (Figures 6.8-6.l l).

 

 

 

 

 

 

 

 

 

 

133 2.0
80 _ 7 a I: 1.9 '- - - 'PEOZKDB b
g 1.8 -
7O "” " "’ g 1.7 _ — -0mmuamFit
,, 60 PEOZkDa 16_ . . .
g g - — Hegdanzation F It
2 5° 15 —
a .
32 40 < 1.4 -' 7 -
30 a? 1.3 1
20 C
3 1.2 7
10 E 1_1 _
° . 1.0 1
0 5 10 1 100 10000
R..(nm) tlme(p.a)

Figure 6.7. Dynamic light scattering results for PEG 2 kDa in Milli Q water.

a. Percent mass vs. Rh (1110), shows a maximum at 1.20 i 0.09 nm. b. Intensity
autocorrelation for the sample, cummulant and regularization fits vs. time.

169

% Mass

%Meee

 

 

 

 

 

 

 

 

 

 

 

100 1.40
90 7 a 5 1,35 . - - 4350511011 0
80 7 z .
70 _ 5 1.30 ~ — -c1mmuan11=11
50 ~ PEG 5- "Da 3 1-25 7 —-Heguarizaiion Fit
50 - g 1.20 ~
40 7 2, 1.15 ~
30 ~ ,
c 1.10 F

20 7 8
10 P E 1.05 “
0 1 1.00 7

0 5 10 1 100 10000

3'10"“) timems)

Figure 6.8. Dynamic light scattering results for PEO 5 kDa in Milli Q water.

a. Percent mass vs. Rh (1110), shows a maximum at 1.80 :l: 0.10 nm. b. Intensity
autocorrelation for the sample, cummulant and regularization fits vs. time.

 

 

 

 

 

 

 

 

 

 

 

100 2.0
90 l a c 1.9 ~ - - -PEO8kDa b
80 g 1.8 — .
70 _ — -Curnmdant Flt
PEOBkDa E ‘7 '
60 7 7 8 1.6 - Regdarization Fit
50 3 15 -
l- : O
40 < 1.4 ~
:0 7 g 1.3 ~
0 b 3 1.2 7
10 ~ 5 1.1
0 r 1.0 _
0 5 10 1 100 10000
R1,(nm) time(us)

Figure 6.9. Dynamic light scattering results for PEG 8 kDa in Milli Q water.

a. Percent mass vs. Rh (nm), shows a maximum at 2.30 i 0.11 nm. b. Intensity
autocorrelation for the sample, cummulant and regularization fits vs. time.

170

°/o Mace

% Mass

 

 

 

 

 

 

 

 

 

 

100 2.0
90 ~ a r: 1.9 l - - -PEOZOkDa b
80 ~ 3 1.8 ~ .
70 1 5 17 - — -CummulantFit
P60 20 kDa g ' . . .
6O 7 - 8 1.6 ~ Regulanzation Fit
0
50 7 g
40 7 ‘5
30 7 E
20 7 §
10 ~ 5
0 1
0 5 10 1 100 10000
R110““) time(us)

Figure 6.10. Dynamic light scattering results for PEO 20 kDa in Milli Q water.

a. Percent mass vs. Rh (nm), shows a maximum at 4.20 i 0.10 nm. b. Intensity
autocorrelation for the sample, cummulant and regularization fits vs. time.

 

 

    

 

 

 

 

 

 

100 2.0
90 7 a 1: 1.9 ~ - - 43503511011 b
80 7 2 18 _
70 - g 1'7 . — -Cummulant Fit
60 7 § 1.6 — Regularization Fit
:3 ‘PE036kD s 1-5 7
7 < 1.4 —
:3 g 1.3 —
7 3 1.2 7
10 E 1.1
o 1 1.0
o 5 10 1 100 10000
BMW“) 0018018)

Figure 6.11. Dynamic light scattering results for PEO 36 kDa in Milli Q water.

a. Percent mass vs. Rh (nm), shows a maximum at 5.90 i 0.12 nm. b. Intensity
autocorrelation for the sample, cummulant and regularization fits vs. time.

171

The scaling behavior of the PEG and PEG-PAMAM diblock copolymers were

compared by plotting Rh vs molecular weight (M) (Figure 6.12). It is clear that the lower

molecular weight hybrids have an Rh value that is smaller than a PEO chain of an

analogous molecular weight. The dimension of 750 kDa-G4, 2 kDa-G4 and 5 kDa-G4
are smaller in solution than a free PEO chain indicating the dendrimer is causing the
molecule to be in a more compact state. Both the 9 kDa-G4 and 20 kDa-G4 hybrids have
molecular dimensions in solution that are similar to a PEO chain with no dendrimer
attached, indicating that increasing of the molecular weight of the linear chain cause the
hybrid to transition from a state that is more compact than PEO to an expanded state

similar to a free PEO chain in solution.

 

 

 

 

7,_ I I I I I If'] T I

6_ I PEG .1
A v PEO-PAMAM 04 '
E 5. ,
5
'D
16
CE 3- .
0
.- I
E I
% 2x10°7 ! z -
9
'2, z
I

i I
1 1 1 1 1 1 1 111 1 LI
2 3 4 56 4 2 3 4
10
M(g/mol)

Figure 6.12. Hydrodynamic radius vs. molecular weight of PEG and PEG-PAMAM
hybrids in Milli Q water at concentrations of 10 mg/mL.

172

These results parallel those found by Jeong et al.6 for the poly(benzyl ether)
(PBE)-poly(styrene) (PS) hybrid where smaller molecular weight hybrids were found to
have a lower intrinsic viscosity than a PS chain of equal molecular weight. However, a
sudden increase in intrinsic viscosity was observed for the hybrids between molecular
weights of 60 kDa and for hybrids greater than 80 kDa, the intrinsic viscosity was equal
to that of linear PS. This suggests that changes in the overall molecular conformation of
the hybrid depend on the relative molecular weight of the two blocks. We performed
neutron scattering experiments on PBE dendrons with a deuterated PS chain attached to
the focal point and via a series of contrast matching experiments and concluded that
increasing the molecular weight of the linear PS chain causes the dendrimer to change
shape with the linear chain changing from a compact to more expanded state.5

These data suggest that the linear chain occupies the free volume of the dendrimer
when the hybrids are in a compact state. To see if there is sufficient room within the
dendrimer for the linear chain, we calculated the free volume of a PAMAM dendron
(G4), analogous in structure to that attached to the PEO chain. DLS experiments of di-

functional PAMAM G4 in Milli Q water (Figure 6.13) displays good cummulant and
regularization fits of the autocorrelation intensity function resulting in an Rh of 1.90 i

0.05 nm.

173

 

 

  

 

 

 

 

 

 

 

 

 

‘00 1.4
gag : a 5 1.4 L- I I .64 b
70 . E 1.3 _ — -CunmdantFit
3 I g 1.3 Regdarization Fit
g 501 §
3 12
o\° 40 7 < 12
so ~ 5
20 g 1.1
10 7 s 1.1
0 1.0 —e
0 5 1° 1 100 10000

R” ("I") time (11:)

Figure 6.13. Dynamic light scattering results for G4 in Milli Q water.

a. Percent mass vs. Rh (nm), shows a maximum at 1.90 :l: 0.05 nm. b. Intensity
autocorrelation for the sample, cummulant and regularization fits vs. time.

Several techniques can be used to calculate the free volume within the G4
dendrimer. The free volume, Vf, is defined through the partition function for a molecule

in the liquid state as”22

1/3 1/3 3
VI =c3{Vm, —V,,C } (6.2)

where c is a numerical factor representing molecular packing and intermolecular
interaction effects, Vmol is molecular volume and V.,; is a characteristic or hard-core
volume. Here we use a value of 2 for c as suggested previously,20 and Vmo] as V1,, which
was calculated from the hydrodynamic radius, Table 6.2, assuming the dendron is shaped
as a sphere. W.,; is taken as the van der Waals volume, Vvdw. The molecular increments

of Edward23 were used to determine the volume contributions of the dendrimers core

174

v,d,, (0) = 63.50 13.3, branching unit, vvdw (BU) = 219.90 .313, and end group, vvdw (E) =

17.60 A3. The total van der Waal’s volume for any generation dendrimer, g, is given by

Vvdw = Vvdw(C) + [28 — lh/vdw(BU) + 28 Vvdw(E) (6-3)

The free volume for a gas is defined as

mo!

Vf = V V1. (6.4)

Both eq 6.2 and 6.3 were used and the smaller of the two resulting values was taken as

the free volume, which came from the gas state equation.
Due to the experimental error of the Rh measurement, the free volume of the G4

dendron ranges from 23 to 28 nm". The number of repeat units of PEO that can ‘fit’
within in this free volume range was found by dividing the free volume of the dendrimer
by the Vvdw of a PEO repeat unit, 0.041 nm,3 resulting in a maximum of 668 repeat units
(a PEO of 29 kDa) that can be accommodated in the cavities of the dendrimer. These
calculations suggest that the dendrimer can contain the largest PEO chains of the hybrids

that are in a collapsed state in addition to the PEO chains of the expanded hybrids (Figure
6.12). However, using Vvdw as the volume of the repeat units underestimates the space

required by the PEO chain and is one explanation of the transition in structure occurring
at a molecular weight below that predicted by free volume calculations. Intramolecular
hydrogen bonding as well as hydrogen bonding with the solvent also could be impacting
the amount of free volume within the dendrimer.

The change in hydrodynamic radii seen in the PEG-PAMAM hybrids suggests
that this hydrophilic system is undergoing a similar single molecular phase transition that

was observed for the hydrophobic PBE-PS system.6 Neutron scattering contrast

175

matching experiments that allow visualization of each block could confirm this transition.
In addition, it would be interesting to investigate the behavior of the of these hybrids as a
function of solvent quality as our scattering experiments on a PAMAM analogous
dendron show that dendron size is very sensitive to the surrounding environment and this
could alter the morphology of the hybrid.
Conclusion

We have synthesized a series of PEG-PAMAM linear-dendrimer diblock

copolymers and investigated their morphology in dilute aqueous solution via a series of
DLS experiments. We measured the Rh values of both hybrids and linear PEO chains

and compared the change in size of the two systems with respect to molecular weight.
The hybrid system is smaller in dimensions than an analogous PEO chain when the

molecular weight of the PEO of the hybrids is 750 Da, 2 kDa or 5k Da. Hybrids with 3
PEG chain of mass greater than 5 kDa have an Rh value within experimental error of a

PEO chain of analogous molecular weight, indicating they are in a more expanded state,
suggesting a random coil conformation. The overall morphology of the PEG-PAMAM
system is sensitive to the molecular weight of the PEO chain, which was also observed
for PBE-PS systems.6 Further SANS studies on deuterated analogs could confirm the
relative location of the two blocks to determine if one or both blocks change
conformation. It appears that the water soluble hybrid system is converting from a knitted

coil to a random coil upon increasing molecular weight of the linear block.

176

 

References

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Glasstone, S.; Laidler, K. J .; Eyring, H., The Theory of Rate Processes. McGraw-
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178

APPENDICIES

179

APPENDIX A

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