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SURFACE INITIATED ATRP OF SUBSTITUTED
STYRENES AND FUNCTIONAL MONOMERS ON FLAT
SURFACES

presented by
Sampa Saha

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SURFACE INITIATED ATRP OF SUBSTITUTED STYRENES AND FUNCTIONAL
MONOMERS ON FLAT SURFACES

By

Sampa Saha

A DISSERTATION
Submitted to
Michigan State University

in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY
Chemistry

2010

ABSTRACT

SURFACE INITIATED ATRP OF SUBSTITUTED STYRENES AND FUNCTIONAL
MONOMERS ON FLAT SURFACES

Surface initiated atom transfer radical polymerization (ATRP) of substituted
styrenes leads to rapid synthesis of uniform and thick substituted polystyrene brushes
(>100 nm in 1 hour) from gold and silicon surfaces. High growth rates were observed for
styrenes substituted with electron withdrawing groups in meta/para positions. The effects
seen in surface and solution polymerizations are similar for styrenes with electron
withdrawing groups, and for electron donors in ortho and para positions. However,
electron donors at meta sites have surprisingly fast growth rates, which may be due to
steric inhibition of termination. The overall surface polymerization rates for substituted

styrenes was analyzed and found to follow the Hammett relation with p = 0.51. The ratio

of kp to kt, is as an indicator of the likelihood that a reaction will reach high degrees of

polymerization before termination.
During surface initiated polymerization, thiols desorb from gold surfaces at low

temperatures (< 60 C’C) and terminate growing polymers during surface initiated ATRP.

Thiol desorption was prevented by forming a cross-linked poly(siloxane) primer layer on
the gold surfaces prior to attaching the initiator layer. These modified surfaces provide

polymer film thicknesses comparable to films grown from silicon surfaces. This strategy

above 100 OC since the film delaminates from the substrate. Usually, difficult monomers,

such as 2-vinyl pyridine, a polyelectrolyte precursor, cannot be grown from thio-initiators
anchored on gold, but the cross-linked initiator enabled growth of thick polymer films.

Polyacrylate brushes with pendent terthiophenes (PTTMM) were successfully
grown from ITO and gold using surface initiated ATRP. Using cyclic voltammetry, the
PTTMM brush was electrochemically cross-linked to form a conjugated polymer network.
The conjugation lengths in the film were short, but were increased via heterocoupling.
These uniformly grafted conducting polymer brushes may find use in photovoltaic
devices.

Click chemistry was used for the post—functionalization of hydrophilic polymer
brushes. The polymer brushes were random copolymers of AZPMA (azidopropyl
methacrylate), a functional monomer with a pendent azo group, and oli go ethylene glycol
methyl ether methacrylate (PEGMA) with varying ethylene oxide chain lengths, which
enabled control over the hydrophilicity and functional group density in the copolymer.
Subsequent post-functionalization of homo and copolymer brushes was demonstrated by
appending an alkyne—modified dye to AZPMA via click reactions. Kinetic studies showed
that the modification of surface-grafted homo/copolymer brushes was fast (> 60%
conversion in 1 min) irrespective of the copolymer composition. In the case of water

soluble high molecular weight alkynes, surface-grafted copolymers with highest amount
of hydrophilic monomer (PEGMA) gave the highest degree of immobilization, which

indicates its potential application in bioconjugation.

To My Beloved Family

iv

ACKNOWLEDGMENTS

I would like to thank my advisor, Professor Gregory L. Baker for being great
mentors during my graduate study at Michigan State University. He has been an
excellent research advisor who really taught me how to think and come up with own
ideas. The Openness to ask questions, propose solutions and the emphasis for critical
thinking were important attributes that made the work environment both fun and
challenging. I express my deep gratitude to all my committee members, Dr. Merlin
Bruening, Dr. GK. Chang and Dr. William Wulff. Professor Merlin Bruening has a
great science attitude and critical thoughts. He is exceptionally good at math. I found
plenty of helpful suggestions from him to achieve my research goal. As my second
reader, I want to thank Professor William Wulff for his great suggestions especially
toward my seminar and a great party with lot of delicious food at his home.

I would like to thank all past and present Baker group members: Bao, Ying, DJ,
Erin, Qin, Gina, Hui, Heyi, Yiding, Wen and Quanxuan. I also enjoyed working with
Bruening group members especially Fei, Weihan, David and Parul. I really enjoyed my
life here with my husband and all of my friends. Special thanks to Debbie Roper for her
kindness to me during my stay in Chemistry Department, especially in my last semester.
She has been always a big help for me.

Though last year I lost my father, I believe his blessings are always with me.
Whatever I am today, it’s all because of my parent’s encouragement. Finally, I would like

to thank my husband who was my second mentor during my graduate study. Last but not

least I would like to thank my sister Dona and brother-in law Apurba for constant

positive support. My beloved family deserve the greatest thank and love.

vi

TABLE OF CONTENTS

Page
List of Tables ................................................................................................................... x
List of Figures .................................................................................................................. xi
List of Schemes ............................................................................................................. xviii
List of Abbreviations ..................................................................................................... xx
Chapter 1. Introduction ............................................................................................... 1
1.1. Polymer brushes —- a brief definition .................................................................. 1
1.2. Structure of polymer brushes ............................................................................. 2
1.3. Synthesis of polymer brushes ~— recent advances ............................................... 5
1.3.1. Polymer Brushes by free radical polymerizations ..................................... 5
1.3.2. Surface-initiated controlled radical Polymerizations ................................. 12
1.3.2.1 Polymer brushes by Atom Transfer Radical Polymerization ............ 12
1.3.2.2 Polymer brushes by Nitroxide Mediated Polymerization .................. 18
1.3.2.3 Polymer brushes by Reversible Radical Addition-Fragmentation
Chain Transfer (RAFT) .................................................................................. 20
1.3.3. Polymer brushes by Cationic Polymerizations .......................................... 22
1.3.4. Polymer brushes by Anionic Polymerizations ........................................... 23
1.3.5. Polymer brushes by Ring Opening Polymerizations ................................. 27
1.3.6. Polymer brushes by Ring Opening Metathesis Polymerizations ............... 29
1.4. Application of Surface Initiated Polymerizations ............................................... 31
1 .5. References ........................................................................................................... 40
Chapter 2. Substituent effect in surface AT RP of polystyrene brushes ................... 46
2.1 . Introduction ............................................................................................................... 46
2.2. Experimental Section ................................................................................................ 51
2.2.1. Material ............................................................................................................ 51
2.2.2. Characterization Methods ................................................................................ 51
2.2.3. Preparation of initiator immobilized Au Substrates ......................................... 52
2.2.4. Preparation of initiator immobilized Si Substrates .......................................... 52
2.2.5. Polymerization of substituted styrene from initiator immobilized Au surfaces
for kinetic Study ...................................................................................... . ......... 53
2.3. Results and Discussions ............................................................................................ 54
2.3.1. Synthesis of substituted styrene monomers ..................................................... 54
2.3.2. Synthesis of substituted styrene brushes from Au surfaces ............................. 56
2.3.3. Kinetic study of polymerization of substituted styrene brushes ...................... 57
2.3.4. Substituent effects on polymerization rate ....................................................... 63
2.3.5. Synthesis of substituted styrene brushes from Si02 surfaces .......................... 71

vii

2.4. Conclusions ............................................................................................................... 73

2.5. References ................................................................................................................. 74
Chapter 3. Surface effects in surface ATRP ................................................................ 77
3.1. Introduction ............................................................................................................... 77
3.2. Experimental Section ................................................................................................ 78
3.2.1. Materials .......................................................................................................... 78
3.2.2. Characterization Methods ................................................................................ 79
3.2.3. Preparation of Initiator-Immobilized Au and SiOg substrates ......................... 80
3.2.4. Surface—Initiated Polymerizations .................................................................... 81
3.3. Results and Discussions ............................................................................................ 81
3.3.1. Surface-Initiated Polymerizations from Au and Si02 substrates .................... 81
3.3.2. Block 00polymer formation on Au and SiOz substrates ................................. 84
3.3.3. Formation of cross-linked initiator and grow polymer there from .................. 90
3.4. Conclusions ............................................................................................................... 103
3.5. References ................................................................................................................. 104
Chapter 4. Conducting Polymer tethered on flat surface via “grafting from”
approach .................................................................................................... 107
4.1. Introduction ............................................................................................................... 107
4.2. Experimental Section ................................................................................................ 108
4.2.1. Materials .......................................................................................................... 108
4.2.2. Characterization Methods ................................................................................ 108
4.2.3. Synthesis of 3-methyl thienyl methacrylate ..................................................... 109
4.2.4. Synthesis of [2, 2’:5’, 2”-terthiophen]~3’-yl methyl methacrylate ................. 110
4.2.5. Synthesis of silane initiator .............................................................................. 112
4.2.6. Preparation of initiator immobilized flat substrates ......................................... 113
4.2.7. Surface initiated polymerization on gold and ITO substrates .......................... 113
4.2.8. Electrochemistry .............................................................................................. 114
4.3. Results and Discussions ............................................................................................ 115
4.3.1. Monomer synthesis .......................................................................................... 115
4.3.2. Silane Initiator Synthesis for ITO substrate ..................................................... 116
4.3.3. Deposition of SAM on ITO surface ................................................................. 117
4.3.4. Synthesis of polymer brush on gold and ITO surfaces .................................... 118
4.3.5. Electrochemical crosslinking ........................................................................... 122
4.3.6. Anodic copolymerization of the grafted polythiophene units and
additional thiophene derivative in solution ~ Heterocoupling .......................................... 131
4.4. Conclusions ............................................................................................................... 135
4.5. References ................................................................................................................. l 36

viii

Chapter 5. Polymerization of azidomethacrylates on gold surface and its

elaboration via click chemistry ..................................................................................... 140
5. l . Introduction ............................................................................................................... 140
5.2. Experimental Section ................................................................................................ 141
5.2.1. Materials .......................................................................................................... 141
5.2.2. Home and Co-polymerization of Azpma, Egma and Pegma from initiators
immobilized on Au and ITO substrates ........................................................................... 142
5.2.3. Click functionalization of the homo and co-polymer brushes ......................... 143
5.2.4. Characterization Methods ................................................................................ 144
5.3. Results and Discussions ............................................................................................ 145
5.3.1. Synthesis of uniform PAzpma brushes from gold surface ............................... 145
5.3.2. Copolymerization of Azpma with Egma and Pegma on gold surface ............. 152
5.3.3. Derivatization of the copolymer brush via click chemistry with a dye ........... 162
5.3.4. Derivatization of the copolymer brush via click chemistry with water soluble
polymer ...................................................................................................................... 173
5.4. Conclusions ............................................................................................................... 179
5.5. References ................................................................................................................. 180

ix

LIST OF TABLES

Table Page

Table 2.1. Propagation and termination rate constants for representative
methacrylates ........................................................................................... 49

Table 2.2. Apparent Rate Coefficients in surface ATRP obtained from model
and Absolute Propagation Rate Constants of Substituted Styrene .......... 66

Figure

Figure 1.1.

Figure 1.2.

Figure 1.3.

Figure 1.4.

Figure 1.5.

Figure 1.6.

Figure 1.7.

LIST OF FIGURES

Graphical representation of a) the ‘grafting to’ technique, and b)
the ‘ grafting from’ technique ..............................................................

a) Schematic showing a thin film comprised of polymer chains in a
random conformation. b) Schematic showing a thin film comprised
of polymer chains in a brush conformation Examples of polymer

systems comprising polymer brushes ...................................................

Time-dependent properties of polymer chains grown by surface-
initiated free radical polymerization of styrene: (a) molecular

weight Mn, (b) grafting densities 5(PS), and (c) polydispersity of

the covalently attached polymers. ...........................................................

Examples of monomers that have been used to synthesize polymer

Page

..... 2

...2

.9

brushes via ATRP .................................................................................... 14

Optical micrographs of patterned surfaces: (left image) 10-um
features in a continuous polymer brush showing regions of
poly(tert—butyl acrylate) (dark) and poly(acrylic acid) (light) and
(right image) interaction of a water droplet with ZOO-pm features
showing an unusual wetting profile and preferential interaction

with poly(acrylic acid) brush domains ..................................................... 33

Top image: PTPAA brushes and bottom image: Cartoon of
inferred structure for CdSe nanocrystal infiltrated polymer brush
photovoltaic device (From bottom to top: ITO-coated glass slide
modified by surface attachment of a bromine end-caped
trichlorosilane self-assembled-monolayer (SAM) (blue squares),
polymer brushes grown from the SAM (red lines), CdSe
nanocrystals infiltrated into the brush network exhibiting some
degree of phase separation in the plane of the film (small black

circles), and caped with an aluminum cathode. ......................................

Top image: PMETAC brushes and bottom image: Change in the
wetting characteristics of PMETAC brushes (height, h ~20 nm)

' after exchanging the two contrasting counterions: TFSI (a) and

polyphosphate (PP) (b). c) Representation of 8 AW as a function of
counter ion (PP and TFSI). The plot depicts the reversible behavior
of PMETAC brushes over repeated cycles of TFSI and PP counter

xi

.35

Figure 1.8.

Figure 2.1.

Figure 2.2.

Figure 2.3.

Figure 2.4.

Figure 2.5.

Figure 2.6.

Figure 2.7.

Figure 3.1a.

ion exchange. On the right the chemical structures of both counter
ions are represented .................................................................................. 37

Schematic description of the procedure. Five acetylene group-
containing compounds: 1-hexyne, 5-hexyn-1—ol, 4—pentanoic acid,
propargyl benzoate, and biotin-PEO-LC-N-pentynoate. ......................... 39

Evolution of the ellipsometric brush thickness with time for the
polymerization of substituted styrenes from gold surface. ...................... 58

Representative examples of reflectance F TIR spectroscopy of gold
surface coated with (a) an immobilized initiator (b) 70 nm
polystyrene brush (0) 55 nm poly (3,5 di-tert butyl styrene) brush
((1) 5 nm poly (2,6 di-methoxy styrene) brush (e) 27 nm poly(a-
methoxy styrene) brush (f) 130 nm poly(3,5 di trifluoromethyl
styrene) brush ........................................................................................... 59

Evolution of the ellipsometric brush thickness with time for the
polymerization of para substituted styrene from gold surface ................. 61

Representative examples of reflectance F TIR spectroscopy of para
substituted styrene grown from gold surface (a) 70 nm polystyrene
brush (b) 10 nm poly(4-methoxy styrene) brush (c) 200 nm poly
(4-trif1uoromethy1 styrene) brush (d) 55 nm poly (4-methyl styrene)
brush (e) 100 nm poly (4-bromo styrene) brush (f) 150nm poly (4-
tert-butyl styrene) brush ........................................................................... 62

A representative example of the best fit curve to Eq 6. Thickness
of poly (4-bromo styrene) film vs time for ATRP on gold wafers. ......... 67

Hammett plots for (kp/kt) in surface ATRP of substituted styrenes

and for absolute kp in conventional polymerization of substituted
styrenes .................................................................................................... 70

Evolution of the ellipsometric brush thickness with time for the
polymerization of 3, 5 ditrifluoromethyl styrene, 4-tert-buty1
styrene, styrene, 4-methy1 styrene and 2-methoxy styrene grown

from SiOz surface .................................................................................... 72
Evolution of the ellipsometric brush thickness with time for the

polymerization of MMA (methyl methacrylate) from gold and
silicon surfaces at 50 OC .......................................................................... 83

xii

Figure 3.1b. Evolution of the ellipsometric brush thickness with time for the
polymerization of styrene firom gold and silicon surfaces at 50 0C ........ 84

Figure 3.2. Reflectance FTIR spectra of gold substrates coated with (a) 97 nm
PtBA brushes grown from the initiator layer; (b) PtBA (97 mm)-
block-PMMA (210 um) copolymer brushes grown from the PtBA
brush surface ............................................................................................ 86

Figure 3.3a. Surface polymerization study of the formation of PtBA-b-PMMA
from gold surface ..................................................................................... 88

Figure 3.3b. Surface polymerization study of the formation of PtBA-b-PMMA
from SiOz surface ................................................................... 89

Figure 3.4a. Surface polymerization study of the formation of PMMA brush
from gold surface immobilized with crosslinked and non-

crosslinked initiator and SiOz surface ..................................................... 93

Figure 3.4b. Surface polymerization study of the formation of PS brush from
gold surface immobilized with crosslinked and non-crosslinked

initiator and SiOz surface.................................. 94

Figure 3.5a. Reflectance FTIR spectra of gold substrates coated with (a)
PMMA brushes grown from a non-crosslinked thio initiator in 1
hour. ......................................................................................................... 95

Figure 3.5b. Reflectance FTIR spectra of gold substrates coated with (0) PS
brushes grown from a crosslinked thio initiator in 8 hours (<1) PS
brushes grown from a crosslinked thio initiator in 8 hours ..................... 96

Figure 3.6a. Temperature dependence study of PMMA brush grown for 1 hour
from various initiators. ............................................................................. 97

Figure 3.6b. Temperature dependence study of PS brush grown for 1 hour from
various initiators ....................................................................................... 98

Figure 3.7. Optical micro graphs of PS brushes grown from gold surfaces
immobilized with crosslinked initiator and viewed through crossed
polarizers: (a) and (b) show a 70 nm thick film grown for 1 hour at

115°C ...................................................................................................... 99

xiii

Figure 3.8. Surface polymerization study of the formation of PVP brush from
various initiators immobilized on flat surface ......................................... 100

Figure 3.9. Reflectance FTIR (right) of PVP brush grown for 8 hours from
gold surface immobilized with (a) crosslinked initiator (b) non-

crosslinked initiator .................................................................................. 101
Figure 3.10. Possible reason of termination of surface-bound radicals on Au ............ 102
Figure 4.1. Evolution of the ellipsometric brush thickness with time for the

polymerization of PMTM and PTTMM from gold and ITO

surfaces .................................................................................................... 121

Figure 4.2. Representative examples of reflectance FTIR spectroscopy of gold
surface grafted with (a) 50 nm PTTMM brush (b) 100 nm PMTM

brush ......................................................................................................... 122

Figure 4.3. Cyclic voltammetry (20 cycles) of ~30 nm PTTMM brush coated
on ITO surface from -25 to 1500 mV at 100 mV/s .................................. 124

Figure 4.4. Scan rate dependency study of PTTMM brushes coated on ITO

surface at scan rates of 20-80 mV/s in a 0.1 M TEAP/CH3CN
electrolyte solution ................................................................................... 125

Figure 4.5. Plot of peak current intensity versus scan rate at the maximum of
the oxidation wave of PTTMM brush coated on ITO surface ................. 126

Figure 4.6. Representative examples of reflectance FTIR spectroscopy of gold
surface coated with (a) 50 nm PTTMM brush (b) electrochemically
crosslinked PTTMM brush ...................................................................... 128

Figure 4.7. UV-vis absorption spectra of PTTMM brush grown on ITO surface
(blue line) and electrochemically cross linked PTTMM (EPTTMM)

brush on ITO surface ............................................................................... 129

Figure 4.8. Topographical AFM images of ITO surface coated with PTTMM
brush and electro polymerized PTTMM brush taken with tapping
mode imaging ................................................................................. i .......... 130

Figure 4.9. Cyclic voltammetry (20 cycles) of ~30 nm PTTMM brush grafted
on ITO surface in CH3CN added with TEAP (0.05 M) in the

presence of EDOT in solution (first and 20th scan) at the scan rate
of 20 mV/s ................................................................................................ 133

xiv

Figure 4.10 UV-vis absorption spectra of PTTMM brush grown on ITO surface
and electrochemically heterocoupled PTTMM brush with EDOT
on ITO surface ......................................................................................... 134

Figure 5.1. Evolution of the ellipsometric brush thickness with time for the
polymerization of Azpma from initiator monolayers on Au
substrates using different catalyst systems .............................................. 148

Figure 5.2. Reflectance FTIR spectra of gold substrates coated with PAzpma
brushes grown for (a) 0.5 h (b) 4 h (c) 6 h and (d) 8 h from the

initiator layer ............................................................................................ 150

Figure 5.3. Topographical AFM image of gold surface coated with 250 nm
PAzpma brush taken with tapping mode imaging ................................... 151

Figure 5.4. Comparison of the ellipsometric brush thickness with time for the
homopolymerization of Azpma and Egma from the initiator
monolayers on Au substrates ................................................................... 153

Figure 5.5. Evolution of the ellipsometric brush thickness with time for the co-
polymerization of Azpma and Egma with various initial feed ratios
from initiator immobilized gold substrates using CuCl / PMDETA

/ DMF system at 50 OC ............................................................................ 154

Figure 5.6. Reflectance FTIR spectra of gold substrates coated with copolymer
brush of Azpma and Egma at different feed ratios .................................. 155

Figure 5.7. Results from FTIR analysis of the copolymer brushes mentoned in
figure 5.6 .................................................................................................. 156

Figure 5.8. Evolution of the ellipsometric brush thickness with time for the co-
polymerization of Azpma and Pegma with various initial feed
ratios from initiator immobilized gold substrates using CuCl /

PMDETA / DMF system at 50 °c .......................................................... 158

Figure 5.9. Reflectance FTIR spectra of gold substrates coated with copolymer
brush of Azpma and Pegma at equimolar (50/50) feed ratio in
different set of polymerization time ......................................................... 159

Figure 5.10. Reflectance FTIR spectra of gold substrates coated with copolymer
brush of Azpma and Pegma at different feed ratios ................................. 160

XV

Figure 5.11.

Figure 5.12.

Figure 5.13.

Figure 5.14.

Figure 5.15.

Figure 5.16.

Figure 5.17.

Figure 5.18.

Figure 5.19.

Figure 5.20.

Figure 5.21.

Results from FTIR analysis of the copolymer brushes mentioned in
Figure 5.10 ............................................................................................... 161

A representative example of kinetics of click reactions with a
fluorescence dye monitored by Uv-Vis spectra of ITO substrates
coated with ~ 180 nm copolymer brushes of Azpma and Pegma at
equimolar (50/50) feed ratios ................................................................... 164

A representative example of kinetics of click reactions with a
fluorescence dye monitored by reflectance FTIR spectra of gold
substrates coated with ~ 200 nm copolymer brushes of Azpma and
Egma at equimolar (50/50) feed ratios ..................................................... 165

Kinetic study of click reactions with a fluorescence dye monitored
by reflectance FT IR spectra of gold substrates coated with ~200 i
30 nm copolymer brushes of Azpma and Egma as well as Azpma
and Pegma at different feed ratios ........................................................... 166

Measurement of molar extinction coefficient of the alkyne
modified fluorescein dye .......................................................................... 169

Click reactions for 5 min with a fluorescence dye monitored by
Uv-Vis spectra of ITO substrates coated with ~ 200 :1: 30 nm
copolymer brushes of Azpma and Pegma at different initial feed
ratios ......................................................................................................... 170

Results obtained from Uv—Vis spectra of the copolymer brushes
mentioned in Figure 5.16. ........................................................................ 171

The dye molecule binding capacity as a function of azide quantity
in the 200 i 30 nm copolymer brush on ITO surface with different

feed ratios. ................................................................................................ 172

A representative fluorescence microscopy image of the clicked
surface and the reference prepared without copper catalyst under
otherwise equivalent conditions ............................................................... 173

Click reactions for 12 hours with a water soluble polymer (alkynyl
mPEG 5000) monitored by reflectance FTIR spectra of gold
substrates coated with ~ 50 :1: 10 nm copolymer brushes of Azpma
and Pegma at different initial feed ratios ................................................. 174

The disappearance of azide via click reaction with alkynyl mPEG
5000 for 12 hours at room temperature as a function of the azide
quantity in the copolymer brush with different feed ratios ...................... 177

xvi

Figure 5.22. Increase in thickness by the click reaction with alkynyl mPEG
5000 for 12 hours at room temperature as a function of azide
content in the copolymer brushes ............................................................ 178

Images in this dissertation are presented in color.

xvii

Scheme

Scheme 1.1.

Scheme 1.2.

Scheme 1.3.

Scheme 1.4.

Scheme 1.5.

Scheme 1.6.

Scheme 1.7.

Scheme 1.8.

Scheme 1.9.

Scheme 1.10.

Scheme 1.11.

Scheme 1.12.

Scheme 1.13.

Scheme 2.1.
Scheme 2.2.

LIST OF SCHEMES

Page
Synthesis of polystyrene brushes on silica and cleavage of the
polymers from the surface ....................................................................... 7
Mechanism of ATRP ............................................................................... 13
Synthesis of PMA-b-PMMA-b-PHEMA triblock copolymer
brushes ..................................................................................................... 15

Polystyrene brushes grown by nitroxide-mediated polymerization ......... 18

Illustration of the intercalation of the quaternary ammonium
alkoxyamine initiator into laponite by cation exchange and the
subsequent formation of ionically bonded Polystyrene chains by
surface-initiated NMP of styrene using a sacrificial alkoxyamine
initiator ..................................................................................................... 19

Polymer brushes grown by RAFT polymerization of MMA ................... 20

General procedure for the preparation of atom transfer addition
modified surfaces and subsequent diblock copolymer brush

formation via RAFT ................................................................................. 21
Surface-initiated cationic polymerization of 2-oxazolines ...................... 23
Surface-initiated anionic polymerization of styrene on gold ................... 24

Immobilization of the DPE initiator followed by polymerization of

the styrene homopolymer to form PS brushes ......................................... 25
Surface-Initiated Anionic Polymerization of Ethylene Oxide from

multi-walled nanotubes (MWNTs) surface ............................................. 27
Surface-initiated ring opening polymerization of lactide ......................... 29

Schematic description of the formation of p(Nb-diMeOH)-b-p(Nb-

COOMe) diblock copolymer brushes on a gold surface .......................... 31
Synthesis of tethered PtBA film on gold surface ..................................... 46
Possibilities of low kt due to steric constraints ........................................ 48

xviii

Scheme 2.3.

Scheme 2.4.
Scheme 2.5.
Scheme 2.6.
Scheme 2.7.

Scheme 3.1.

Scheme 3.2.

Scheme 3.3.

Scheme 4.1.

Scheme 4.2.

Scheme 4.3.

Scheme 4.4.

Scheme 4.5.

Scheme 4.6.

Scheme 4.7.

Scheme 5.1.

General scheme of synthesis of substituted styrene via witti g

reaction ..................................................................................................... 50
Synthesis of substituted styrenes ............................................................. 55
Surface initiated polymerization of styrene derivatives ........................... 56
The initiation, propagation and termination steps of ATRP .................... 64
Energy diagram for different substituents ................................................ 71

Surface initiated polymerization of MMA from silicon and gold

surface ...................................................................................................... 82
Block copolymer formation on flat surface ............................................. 85
Formation of cross linked initiator on gold surface ................................. 91
Synthesis of 3-Methylthienyl Methacrylate (MTM) ................................ 1 15
Synthesis of [2, 2’ :5 ’, 2’ ’-terthiophen]-3’-ylmethyl methacrylate ......... 1 16
Synthesis of trichlorosilane initiator ........................................................ 117
Initiator SAM deposition on ITO surface ................................................ 118
Surface initiated polymerization of MTM from gold surface,

TTMM from gold surface and TTMM from ITO surface ....................... 119
Electrochemical cross linking of PTTMM brush on ITO surface ........... 123
Heterocoupling of PTTMM brush with EDOT on ITO surface .............. 132

Synthesis of random copolymer brushes grafted on gold substrates
and click functionalization ....................................................................... 147

xix

LIST OF ABBREVIATIONS

A
AIBN

AF M
ATR
ATRA

ATRP

Azpma

2-BPB
bpy
CL
CM
CV

CYC LAM

DMF

dNbpy

anbpy

DP

EDOT

Absorbance
2,2’-Azobisisobutyronitrile
Atomic force microscopy
Attenuated total reflectance
Atom transfer radical addition

Atom transfer radical polymerization

Average cross—sectional area of polymer chains

Azidopropyl methacrylate
2—Bromopropionyl bromide

2,2 ’-Bipyn'dine

6—Caprolactone

Chain transfer constant to monomer

Cyclic Voltammetry
1,4,8,l 1-Tetraazacyclotetradecane

N,N-dimethy1formamide
4,4'—Di(5-nonyl)—2,2'-bipyridine
4,4'-Di(n—nonyl)-2,2'-bipyridine

Degree of polymerization
Molar extinction coefficient

3,4 ethylenedioxy thiophene

XX

Egma
EtOAc

F TIR

F

GC

GPC
HEMA
HMTETA

ITO
"p
kpabs

kpapp

kt

ktapp

kact

kdeact

Lactide
LB

LCST

Ethylene glycol methyl ether methacrylate
Ethyl acetate

free energy

Fourier transform infrared

Grafting density

Gas chromatography

Gel permeation chromatography
2-Hydroxyethyl methacrylate

1,1 ,4,7,10,10-Hexamethyltriethylenetetramine

Indium tin oxide

Polymerization rate constant

Absolute propagation rate constant

Apparent polymerization rate constant

Termination rate constant

Apparent termination rate constant

Activation rate constant

Deactivation rate constant

3,6-Dimethy1—l ,4-dioxane-2,5-dione
Langmuir-Blodgett

Lower Critical Solution Temperatures

xxi

MA

Me4Cyclam

Me6TREN

iCP

MMA

mPEG
MS
MPS
MTM
MUD

OTf
PAA
PBA

Methyl acrylate

Methyl

1 ,4,8,1 1 -tetramethyl-1,4,8, I l-tetraazacyclotetradecane

trz's[2-(Dimethylamino)ethyl]amine

Microcontact printing

Methyl methacrylate

Number average molecular weight

methoxy Poly(ethylene glycol)
Mass spectroscopy
Mercaptopropyltrimethoxysilane
3-Methylthienyl Methacrylate

Mercaptoundecanol

Weight average molecular weight

Multiwalled carbon nanotubes

Avogadro’s number

Nitroxide-mediated polymerization
Nuclear magnetic resonance
Norbomene

Triflate

Poly(acrylic acid)

Poly(n-butyl acrylate)

xxii

PCL

PDI

PDMS
PEI
PEG
Pegma
PGMA
Ph
PHEMA
phen
PLA
PLED
PMA
PMEMA
PMMA

PMDETA

PNIPAAM
POEGMA
PPZ

PS

PtBA

PVK

PVP

Poly(a-caprolactone)

Polydispersity index calculated as Mw/Mn

Polydimethylsiloxane
N-propionylethylenimine

Poly(ethylene glycol)

Poly(ethylene glycol methyl ether methacrylate)
Poly(glycidyl methacrylate)

Phenyl

Poly(2-hydroxyethyl methacrylate)
1,10-Phenanthroline

Poly(lactide)

Photo light emitting diode

Poly(methacrylate)
Poly(2-(N-morpholino)-ethyl methacrylate)
Poly(methyl methacrylate)

N ,N,N ',N ',N "-pentamethyldiethylenetriamine
Poly(N-isopropylacrylamide)
Poly(oligoethylene glycol methyl ether methacrylate)
Poly(phosphazenes)

Polystyrene

Poly(tert-butyl acrylate)

Poly(vinyl carbazole)

Poly(4-vinyl pyridine)

xxiii

QR

RAFT
ROMP

ROP

SAM
SEM

SI

tBA
TEAP
TEM
TEMPO
THF
TMEDA
TREN

TTMM

4-VP

Volume fraction
Quenching and re-initiation

Random copolymer

Reversible addition-fragmentation chain transfer
Ring-opening metathesis polymerization

Ring-opening polymerization

Rate of polymerization

Radius of gyration

Hammett constant
Self-assembled monolayer
Scanning electron microscopy
Surface initiated

tert-Butyl acrylate

Tetraethyl ammonium perchlorate
Transmission Electron Microscopy
2,2,6,6-Tetramethylpiperidinyloxy
Tetrahydrofuran
Tetramethylethylenediamine

T ris[2-aminoethyl]amine

[2, 2 ’:5 ’, 2 ’ ’-Terthiophen]-3 ’-ylmethyl methacrylate

Ultraviolet

4-Viny1 pyridine

xxiv

XPS X-ray photoelectron spectroscopy

p Reaction constant

XXV

Chapter 1

Introduction

1.1. Polymer Brushes — a brief definition
The term “polymer brush” refers to an assembly of polymer chains tethered at one
end to a surface and packed sufficiently dense that the polymer chains are forced into an

extended conformation. Polymer brushes can be formed by using either a ‘ grafting to’
technique, where the polymers are absorbed onto a surface,1 or a ‘grafting from’

technique, where the polymers are grown directly from the surface (Figure 1.1).?"3’4 This

study emphasizes the ‘grafting from’ technique due to its ease of synthesis, and the high
density of polymer chains in the resulting film. A number of polymerization methods

have been used to synthesize polymer brushes by the ‘grafting from’ technique, including
free radical polymerization,5 as well as controlled polymerizations such as reversible
addition-fragmentation polymerization (RAFT), 6 nitroxide mediated radical
polymerization (NMP),7 ionic polymerization,8 ring opening metathesis polymerization
(ROMP),9 and atom transfer radical polymerization (ATRP).10 Surface-initiated ATRP is
one of the most important techniques for generating polymer brushes11 from surfaces

10 . . 12
such as gold or s111ca.

a) b)

 

polymer
surface functionality monomer initiator monolayer
l
\
endgroup assemble
functionality onto polymerize
surface
polymer

   

Figure 1.1: Graphical representation of a) the ‘grafting to’ technique, and b) the

‘grafting from’ technique (Reprinted with permission from Gregory L. Whiting, Ph.D.

thesis 2006, University of Cambridge).

1.2. Structure of Polymer Brushes

 

polymer brush

attachment site

Figure 1.2: a) Schematic showing a thin film comprised of polymer chains in a
random conformation. b) Schematic showing a thin film comprised of polymer chains in
a brush conformation (Reprinted with permission from Gregory L. Whiting, Ph. D. thesis

2006, University of Cambridge).

Due to steric interactions, the surface—attached polymer chains in a dense polymer
brush film are expected to be more extended than randomly coiled polymers. Figure 1.2
compares the morphology of polymer chains in a random conformation (spin coated
polymer) and in a brush conformation. A simple energy balance model can be
constructed to compare the deformation of a polymer chain in a brush conformation and a
randomly coiled polymer. Generally, maximization of entropy for polymer chains can be
achieved by adopting a random coil conformation. However, this is not possible for
polymer brushes due to the end—grafted chains and steric interactions from the overlap of

other polymers. The Alexander model takes these two considerations into account for an

end grafted polymer chain.13’14 This model, developed using the Flory approximation
(from random walk experiments), shows that the free energy, f, for a polymer chain in a

good solvent in the brush conformation is given by Equation 1.1.The model assumes that
the concentration of repeating units is constant throughout the polymer brush film, so that
(p = No/h (q): volume fraction of the grafted polymer chains) and that there is a fixed
distance h, where all of the polymer chains terminate. Here, N is the number of repeating
units of diameter a, o is the grafting density, T is temperature, k is the Boltzmann
constant, and v is an excluded volume parameter. Minimizing this expression with respect
to h (df/dh =0), provides Equation 1.2, which indicates that the thickness of the polymer

brush film, in solution, varies linearly with the degree of polymerization.

 

h
2+ thp

~kT
f Na

 

at 311 WIN2 df_ 1’3
ar*""(““~az“ ,2 > “(arwlwlwazl 1'2

This result implies that when the distance between the grafting sites is less than the radius

of gyration, the chains stretch somewhat to minimize this steric energy. This is actually

. . . . 1/2 . 1/2
observable when the unstretched charn dimensron 1ncreases as N (srnce Rg = N a),

the brush thickness increases linearly with respect to N. So, it seems reasonable to accept
that the polymer chains must be deformed somewhat from a completely random
conformation, and the deformation will become more pronounced for higher degrees of

polymerization (N). A similar strategy can be applied to a polymer brush film in a dry

. . . .. . . . 2/3
state. In thrs case it can be shown that the equrlrbnum thrckness 1ncreases as N ,

implying that the polymer chains are also stretched and deformed from a random
configuration in the dry state. This deformation leads to changes in physical properties of

the brush film.
Yamamoto, et al.15 showed that poly(methyl methacrylate) (PMMA) brushes

consistently have higher glass transition temperatures (T g) than spin-coated PMMA films
of similar thickness, and the corresponding brush films consistently have 8 OC higher T gs
at higher thickness. Though the mechanism is obscure, the increased T g must be due to

the stretched nature of the polymer brush chains.14 It should be pointed out that the

authors formed spin-coated films from free polymer synthesized from the same

polymerization as the surface-attached films. Therefore, the two polymer films (brush

and spin-coated) are expected to have similar properties, and the Tg difference can not be

due to differences in chain length, polydispersity or stereochemistry.16 Overall, these
data suggest that polymer brushes exist in a stretched configuration different from that of
a free chain in solution.
1.3. Synthesis of Polymer Brushes - recent advances

There are two distinct pathways to achieve polymer brush architectures, the
“grafting to” and “grafting from” approaches. “Grafting to” utilizes preformed polymers
with an end functionality that interacts or anchors with the substrate surface. This
generally involves a chemical or physical adsorption process. A limitation to this type of
application is that the process is diffusion limited so that as more and more polymer
chains are attached to the surface, the ability for a new polymer chain to diffuse to the
surface of the substrate is greatly hindered (Figure 1.1a). On the other hand, the “grafting
from” or surface initiated polymerization approach places the initiating groups directly on
the surface, eliminating the diffusion problem to a great extent and providing control of
the polymer chain length as well as the grafting density. Locating the initiator on the
surface means that the growing polymer is attached to the surface and the monomer
diffuses to the growing chain end throughout the reaction; 1.6., the growing chain

eventually extends from the substrate (Figure 1.1b). This technique is amenable to a large

. .. . . . . 17
number of surface 1n1t1ators as well as varrous polymenzatrons mechamsms as
discussed below.

1.3.1 Polymer brushes by free radical polymerizations

Surface initiated free radical polymerization was investigated in detail by Minko

. 18
and coworkers using both theoretical and expenmental approaches. They attached azo

and peroxide initiators to solid substrates by either physisorption or chemical
immobilization, and grew polystyrene via a free radical pathway. The kinetics for
polystyrene grown from a silica surface with attached azo—initiators was followed by in
situ ellipsometric measurement of the film growth. The resulting kinetics showed a linear
dependence of the polymerization rate on the concentration of the immobilized initiator,
and an inverse square root dependence on the initiator concentration in bulk, which is in
accord with conventional free radical polymerization. However, their method for
anchoring initiators led to a low grafting density and side reactions. Later, Riihe et a].

developed a one-step initiator anchoring strategy and initiated free radical polymerization

5 . . .
of styrene from surfaces. Scheme 1.1 shows the polymenzauon reactron and the method

used to detach the polymer chains from silica gel. The initiator has three important
functionalities: ( 1) an azo group that generates free radicals upon heating or UV
irradiation, (2) a chlorosilane that allows the initiator to be anchored to the surface
through reaction with silanol groups of the silica substrate, and (3) an ester that can be
hydrolyzed to detach the polymer brushes from the surface. After polymerization of
styrene, the ester bonds that connect the polystyrene to the surface are easily cleaved to
determine the molecular weights of the polymers, which allow comparisons between the
free radical polymerization in solution and from a surface. From the molecular weight
and grafting density, they found that the average distance between tethered polystyrene
chains was 2-3 nm, smaller than the radii of gyration of the corresponding polymer

molecules.

 

5102 surface
1
O
I
+
Q
l
g)
> a:
2:
Z
\\
Z
+5
m
g
o

 

 

 

base

 

8102 Surface
5?
(.0 E
> o
O
01
Z
\\
r
3
m

 

 

 

 

 

 

 

 

 

Styrene
toluene, heat

4V2
8
g Me 0 Me CN ..
'3 Mai—N W‘ b d a
as non on e

M

00-5 e O + polystyrene

* n

p—TsOH

MeOH/Toluene
8 M Me CN e -
N e ,0 1.,
E h-O—S.i_/\/OH Me N—
N ill/1e + O
.9
co

‘ "n

 

 

 

 

 

Scheme 1.1: Synthesis of polystyrene brushes on silica and cleavage of the

polymers from the surface

Prucker and Rtihe also investigated the kinetics and mechanism of surface~

initiated free radical polymerization from a self assembled monolayer of azo initiators

attached to the surface of silica particles.19 The rate of decomposition of the surface-
immobilized initiator was monitored by DSC, as well as by quantifying the amount of
nitrogen generated from the azo component during decomposition. After polymerization,
the polymer chains were detached from the surface and the molecular weights of the de-
grafted polymers and their distribution were studied as a fimction of the reaction

parameters during polymerization. They also irradiated selected areas of surfaces with

UV to pattern th1n polymer layers by surface-1n1t1ated free radrcal polymerization.2
Other substrates have been studied. For example, Velten et al. anchored cation-bearing

peroxides to mica surfaces via ion exchange, and initiated styrene polymerization from

. . . . 21
the 1n1t1ators to obtam surface-bound polymers.

Wittmer and coworkers predicted strong differences between polymer brushes

grown from surfaces and polymers generated in solution.22 They assumed that long
chains are more efficient at adding monomers than short chains because they are more
mobile and more accessible to monomers, and thus polymer brushes formed at the
surface should have a higher polydispersity than the same reaction occurring in solution.
However, the experimental data of Prucker and Riihe for polymers cleaved from surfaces
indicate a small or no effect, with PDls ranging from 1.5 to 2, which is close to the PDI
of free radical polymerizations in solution. Figure 1.3 shows the molecular weights and

polydispersities of the detached polymer brushes.

 

 

 

 

175 ~ .
‘- 1
.:.—' 150 r -
a _ 1
a, 125 ~ -
‘2: r l
37-; 100+ t
2 r 1
75 r 1:1 J

50 9 L A r 1 1 . l 1 1 . l . 1

0 2 4 6 8 1O 12 14

time [h]

Figure 1.3: Time-dependent properties of polymer chains grown by surface-
initiated free radical polymerization of styrene: molecular weight M“. (Reprinted with
permission from Macromolecules 1998, 31, 602-613. Copyright 1998 American

Chemical Society)

 

 

 

 

 

10 r
3" .
as 8"
c» i
a) 6 P
o. .
2 4 ~
3 ,
9:, 2 ’
to .
0 r-
0 2 4 6 8 10 12 14
time[h}

Figure 1.3: Time—dependent properties of polymer chains grown by surface-
initiated free radical polymerization of styrene: Grafting densities 5(PS)
(PS=Polystyrene). (Reprinted with permission from Macromolecules 1998, 31 , 602-613.

Copyright 1998 American Chemical Society)

10

 

2.0~ Cl-
C " D .4
2" _ .
E 1.9
2 b
g 1.81— ~
8 .
8
.22 1'7” 1
'O l.
>~
'5 1.6~ «
0. i
1.5» -

 

 

 

o 2 4 6 8 L10
time[h]

12114

Figure 1.3: Time-dependent properties of polymer chains grown by surface-
initiated free radical polymerization of styrene: Polydispersity of the covalently attached
polymers. (Reprinted with permission from Macromolecules 1998, 31, 602-613.

Copyright 1998 American Chemical Society)

Rtihe and coworkers expanded surface-initiated free radical polymerization to the

preparation of block copolymer brushes, where one block was synthesized by ROP.23 A
PCL (poly(caprolactone)) macroinitiator containing azo groups was physisorbed on a

silicon oxide surface to initiate the radical polymerization of the other monomer to create

24 . .
hydrophobic layers on hydrophilic surfaces. Zhao et al. later appl1edth1s strategy to the

synthesis of amphiphilic Janus silica particles. Azo initiators were anchored to silica

11

particles, and the modified particles were suspended in a mixture of styrene and water.
The particles were adsorbed at the liquid-liquid interface, with one hemisphere of each

particle immersed in an aqueous phase, and the other in a styrene phase. After initiation

at an elevated temperature (90 OC), poly(sodium methacrylate) grew from the hemisphere

immersed in water, and polystyrene chains grew from the particle surface immersed in
styrene. Thermogravimetric analysis and IR spectral data confirmed the grafting of
polymer brushes on the surfaces.

1.3.2 Surface-initiated controlled radical polymerization

Of the various polymerization methods used for polymer brush synthesis,
controlled free radical polymerization (CRP) has attracted much attention due to its
simplicity, wide functional group tolerance, and versatility compared to ionic processes.
CRP provides control over the polymer chain length and length distribution, and also
offers the possibility to design grafted polymer chains with controlled architectures. The
most widely used CRP techniques are ATRP, RAFT, and NMP. All of these techniques
have been used to build up highly dense polymer brushes from surfaces of various
architectures (planar and nanoparticles).
1.3.2.1 Polymer brushes by Atom Transfer Radical Polymerization (ATRP)

ATRP is a versatile technique for the polymerization of various vinyl monomers,
primarily acrylates and methacrylates. Most aspects of ATRP have been

2526,27

reviewed. The mechanism for ATRP is shown schematically in Scheme 1.2. In

ATRP systems, reversible termination is used to reduce the steady state concentration of
growing radicals and suppress bimolecular radical termination reactions. Once a radical

is generated from a dormant initiator, it can either add monomer or deactivate by reacting

l2

with a metal complex (such as Cu(II)) to regenerate the dormant initiator and a metal in a
lower oxidation state. The controlled nature of ATRP results from the equilibrium

strongly favoring the dormant species.

 

activation/ _
deactivation propagation termination
k8 kt
P——x a _______ p .___.___.. 1LT
kd T'
kp
monomer

Scheme 1.2: Mechanism of ATRP
Polymers with low polydispersities are produced by ensuring that the initiation
rate is faster than prOpagation with minimal termination. Since the dormant species is a
growing polymer chain capped with a halogen atom, it is possible for a polymer chain to

act as a macroinitiator, making ATRP a readily applicable technique for producing block

copolymers. The kinetics of ATRP is shown in Equation 1.3.18 In this expression [M],

[13.], [Cu(I)], [Cu(II)X], and [PX] are the concentrations of monomer, active polymer

radicals, copper(l), copper(II) halide, and halide-capped polymer chain respectively. The
rate constants kapp, kp, kact and kdeact refer to the apparent rate constant, the propagation
rate constant, the activation rate constant and the deactivation rate constant respectively
and finally, Kgq is the equilibrium constant. As per this expression, ATRP follows first

order kinetics with respect to monomer concentration ([M]).

13

.. .. . .. [011']
RP - KapptMJ - KprP 11M] - kaquIII—mcuu X] [M]
1.3

Where» K _ kact __ [P'][Cu“X]
eq kdeact [CU Il [PX]

 

 

However, during surface initiated polymerization, the total amount of polymer
synthesized on the surface is very small, and the concentration of monomer in solution
remains essentially constant. This constant monomer concentration should lead to linear

kinetic plots for surface-initiated polymer brush film thickness (proportional to the

polymer molecular weight) with respect to time.28
1.3.2.1 Examples of Polymer Brushes via ATRP

Surface—initiated ATRP has been the most widely employed methodology for the
formation of polymer brushes, and Figure 1.4 shows examples of common monomers

used to synthesize polymer brushes via ATRP.

Figure 1.4: Examples of monomers that have been used to synthesize polymer

brushes via ATRP.

14

Surface initiated ATRP can be applied to nonplanar surfaces such as nanoparticles.
29
In an early example, Huang, et al. used ATRP to grow polyacrylamide brushes from

silica gel, and obtained a ~10 nm thick film after 40 hours at 130 C)C. Husseman et al.7

improved control over the growth of polymer brushes by adding a sacrificial initiator to
decrease the concentration of active chains, and therefore reduced chain termination by
coupling and disproportionation. However, the free initiators produced significant

amount of polymer in solution that must be removed. An alternative method is to add a

. . . 30 . .. .
Cu(II) salt to the polymerrzatron solutron, and shift the equ111br1um more toward
dormant chains. This is a ‘surface confined’ polymerization since no initiator is
explicitly added to the solution, and therefore only small amounts of polymer is generated

in solution.

15

o
s~(CH2)-~o—ti-cH-ar
1‘ CH3

HEMA, CuBr/MeaTREN
CuBr2/2anbpy, DMF/THF, 4o °c

o cH3 1
Au S1-1-*(CH2) o— c-— CH CHz-c Br

CH3 COZCHZCHZOH J

 

\ofi Sn(Oct)2, Toluene. 95 00

 

V

COZCHZCHZOH

 

 

CH2 0 H 0 H
O C‘C‘O‘C‘C‘O H
CH3 CH3 n

Scheme 1.3: Synthesis of PMA-b-PMMA-b-PHEMA triblock copolymer brushes

Similarly, styrene and methyl acrylate were polymerized from an initiator layer of

2-bromoisobutyrate immobilized on silicon wafers. The polymerizations showed a linear

. . . . . . 31 .
increase 1n the polymer thrckness With reactron t1me. Controlled growth was achieved

by the addition of a deactivating Cu(II) species. A combined study of the control of

. . . . . . . 32
1n1t1ator densrty on gold and s111con substrates was carried out by Bao et. al. They

prepared Au and 8102 substrates with various immobilized initiator densities using

systems (size of initiator and diluent were matched) that should lead to a homogeneous

l6

distribution of the initiator on the surface. Variations in the polymerization rate on both
substrates were observed as a function of initiator density and it was found to be
consistent with the decrease in bimolecular termination as well as a decrease in the
number of chains on the surface when the initiator density drops below 10% on Au

surfaces.

A related effect of initiator density was reported by Wu et al.33 In their study, an
initiator density gradient was formed on a silica surface, brushes were synthesized from
the surface, and the thickness was found to vary systematically (as measured by
ellipsometry) at different positions along the substrate. These results show that the
polymer brush thickness decreases slightly with the reduction of the initiator density until
a crossover point, where the polymer film thickness decreases markedly. At the crossover
point the density of polymer is too low to force the polymer chains into a stretched
conformation. As a result, the polymer chains do not interact with one another, and are
present in the ‘mushroom’ regime as randomly coiled chains on the top of the surface.
Since the polymer chains are already spaced far enough apart to not interact with each
other in the mushroom regime, the thickness of the polymer chains does not correlate

with initiator density.

Though room temperature polymerizations are desirable to avoid thermal
polymerization, ATRP has been carried out at elevated temperature to increase the rate of

polymerization. However, thiol on gold SAMs are unstable at temperatures above

34 . . . .
60 CDC. One way to 1ncrease polymerization rates and decrease the react1on temperature,

17

. . . 10, 35 . . . .
rs to use aqueous polymenzatron systems. It rs thought that the hrgh drelectrrc
constant of the polar solvent increases the activity of the ATRP catalyst system.

Another useful method for increasing the polymerization rate is to use ligands

with high stability constants such as Me4Cyclam. Moreover, ATRP catalysts based on
Me4Cyclam can grow 100 nm thick film of poly(tert-butyl acrylate) (PtBA) in 5

. 36 . . . .
minutes. ATRP rs also a useful technrque for the format1on of block copolymers, srnce
the polymer chains can act as macroinitiators for a new monomer. Several examples of
block copolymer brushes via ATRP have been shown. Kim et al. used a simple quench

and re-initiation (QR) approach to grow PMA-b-PMMA-b—PHEMA triblock copolymer
brushes on Au (Scheme 1.3).37 Growing polymer brushes were quenched with a

concentrated CuBrz/ligand solution, preserving the Br atoms at the chain ends for

subsequent re-initiation of the next polymer block. The efficiency of the QR scheme was
found to be superior over a simple solvent washing procedure, which resulted in a higher

loss of active chains. In another example, ABA triblock copolymer brushes of

polystyrene and poly(methyl methacrylate) were synthesized on silica surfaces. 38

Treatment with different solvents reversibly altered the surface morphology leading to
applications in switchable and responsive surfaces. As the synthesis of polymer brushes
via ATRP has become more controlled and an increasingly large number of monomers

have been used, potential applications of this film morphology is now being explored.

18

1.3.2.2 Polymer brushes by Nitroxide Mediated Polymerization (NMP)
The livingness of NMP depends on the reversible capping of the active chain-end

radical with a nitroxide leaving group. Husseman er al. demonstrated that polystyrene (PS)

chains could be grown from the surface by NMP (Scheme 1.4).7 They first attached

alkoxyamine initiators onto the surface and then heated the system to 120 0C to initiate

radical polymerization. During the initiating process, the stable nitroxide radical 2,2,6,6—
tetramethylpiperidinyloxy (TEMPO) is cleaved and reversibly caps the chain-end radicals
to control radical propagation. The addition of free alkoxyamine initiator provided more

control over the molecular weight, but solution polymerization could not be avoided.

CW - iii/glib

Scheme 1.4 Polystyrene brushes grown by nitroxide-mediated polymerization

 

Hawker and coworkers applied NMP to photolithography by patterning polymer brushes

into well-defined hydrophobic and hydrophilic domains (Figure 1.5).39 PtBA brushes
(hydrophobic) were synthesized by surface-initiated NMP and then hydrolyzed to form
hydrophilic poly(acrylic acid) (PAA) brushes. Recently, Konn er al.40 grew polystyrene
chains from the surface of synthetic Laponite clay platelets by nitroxide-mediated
polymerization of styrene using N-tert-butyl—N-[l-diethylphosphono—(2,2-
dimethylprOpyl)] (DEPN) as mediator (Scheme 1.5). A novel water-soluble quaternary

ammonium alkoxyamine was synthesized and intercalated into the clay galleries by

19

cation exchange. Polystyrene (PS) chains with controlled molecular weight and narrow
polydispersities were then grown from the organoclay which displayed a high stability

and dispersibility in organic solvent as well as into polar and nonpolar monomers.

 

Z—
+\

 

DEPN
Styrene, Toluene
Styryl-DEPN,
O
DEPN N+ 110 C, 32h
/ I \

 

 

DEPN= ‘o-N
Eto.) <

EtO’ Ps0

Scheme 1.5: Illustration of the intercalation of the quaternary ammonium alkoxyamine
initiator into laponite by cation exchange and the subsequent formation of ionically
bonded PS chains by surface-initiated NMP of styrene using a sacrificial alkoxyamine

initiator. (Reprinted with permission from Macromolecules 2007, 40, 7464. Copyright

2007 American Chemical Society.)

20

1.3.2.3 Polymer brushes by reversible radical addition-fragmentation chain transfer

R~S 3

CN Ph CN COOMe
a» 8

cu
N=Nzl< 0 34
n Ph
V‘oue

110°C 3

8Y8 ’ n Y3
Ph ©-//

Scheme 1.6 Polymer brushes grown by RAFT polymerization of MMA

 

 

ATRP is arguably the most widely used controlled radical polymerization
technique for the preparation of surface initiated polymer brushes. However, RAFT
offers potential benefits over ATRP in the polymerization of functional monomers.
RAFT is a versatile technique based on a reversible degenerative chain transfer
mechanism in which thiocarbonylthio compounds act as chain transfer agents providing

41 42,43,411

controlled growth of polymer chains (Scheme 1.6). Boyes et a1. modified

surfaces with RAFT chain transfer agents and synthesized a series of homopolymer and

diblock copolymer brushes (Scheme 17).“ They cleverly converted an immobilized
ATRP surface initiator to the RAFT chain transfer agent surface by an atom transfer
addition reaction. RAFT chain transfer agents were anchored to the surface of silicon
wafers, and then homopolymer brushes of PMMA and PS and diblock copolymer brushes

of PMMA-b-PS, PMMA-hPDMAEMA and PS-b-PMA were grown from the substrates.

21

RC“ 1)Piranha solution Me O 3'

Me
OH 0 Br O'“SI“ (CH2T‘O Me
Me HMS Me
2) CI“ SMI" e(Cl“I2)‘1"10 Mee

eToluene
56 °C, 4—6 hrs

S
8.3
.. of .0
Me

Me 3 Me
"— o = o-sr- CH 0 s
O—SeICHZi: Me CuBr, Cu(O) Me( 2Y1: JKQ

N,N',N"-pentamethyldiethylenetriemine,
85 °C, 48 h

 

Monomer 1
Chain transfer agent
2,2'-azobisisobutyronitrile

 

Where R1= H, R2 =

or II
O)=o

Me Me

\ O—SNIe Me(CH2)—IOMSS
Where R1 = CH3, R2 = n)K©

>=O 0' 0:0 Monomer 2

o 0 Chain transfer agent
2,2'-azobisisobutyronitrile

 

 

 

 

Me Me

O—SMI: (CH2)"100

Scheme 1.7: General procedure for the preparation of atom transfer addition

 

modified surfaces and subsequent diblock copolymer brush formation via Reversible

Addition-Fragmentation Chain Transfer Polymerization.

22

1.3.3 Polymer brushes by Cationic Polymerizations

Substantially less work has been reported on the application of cationic

polymerization for the synthesis of polymer brushes from surfaces. Jordan and Ulman46

initiated cationic ring opening polymerization of 2-substituted 2-oxazolines from gold
surfaces, resulting in a 10 nm poly(N-acylethylenimine) brush after 7 days of
polymerization under reflux in chloroform (Scheme 1.8). They also showed that the
polydispersity of the resulting linear polymers were narrow because of the highly living
character of the ring—opening polymerization. This method allows block-copolymers
synthesis by consecutive addition of different 2-substituted monomers. Furthermore,
terminal functionalities are easily introduced by terminating the polymerization with

nucleophiles. They extended this strategy to gold nanoparticles to the preparation of

. . 47 . . 48 .
functional composrtes at the nanometer scale. Zhao and Brittam deposrted SAMs

terminated with cumyl methyl ether moieties on silicon wafer surfaces. Activation with
TiCl4 in the presence of styrene and di-tert-butylpyridine, a proton scavenger, led to 30

nm thick polystyrene brushes in less than an hour. The brush growth was carried out at —

78 0C to suppress chain transfer reactions. The livingness of the polymerization was

confirmed by re-initiating the polymer chains to grow additional polystyrene. Recently,

Patton et (11.49 developed a new class of polymer brushes based on the hybrid inorganic-

organic polymer backbone of poly(phosphazenes) (PPZs). PPZ brushes were synthesized
using surface-bound phosphoranimines as active sites for living cationic surface-initiated

polymerization of chlorophosphoranimines.

23

 

 

 

 

 

OH O
IOZCFS 5‘ Et-{gj
sozcr—‘3 o A
o/ N/ o e N
+ Au ._ \SOZCF3 \__/ OTf
EIOH; (1.; 16 h 1 day; 0 0C -—> reflux
c .-= 1on1nol/l vapor phase
R R
HS S
alk l alk l
csz _<o :4 y
Et 9] e N
n N’AO G N HN/“aIkV' GOTf terminal
, , on \—-alkyl functionality
“—3—.- Et > => 3 E
7 deals. N 1 day: N po'ymer
CHCl3; reflux O ”-1 CHCl3; rt. 0 n-1 SAM
R R

Scheme 1.8: Surface-initiated cationic polymerization of 2-oxazolines.
1.3.4 Polymer brushes by Anionic Polymerizations

The living nature of anionic polymerization has made it an attractive choice for
the synthesis of well defined polymer brushes via the ‘grafting from’ approach. Early

examples involving “grafting to” approaches such as the reaction of lithiated polystyrene
. . . . 50 51 . .

chains With surfaces were first described in the 19705. ’ This usually involved the

reaction of carbanions of living polymers of polystyrene or polyvinylpyridine with

surface bound chlorosilane groups to bind the macromolecule covalently to the silica

surface. Later, Jordan et al.52 initiated anionic polymerization of styrene from gold
substrates. Initially, a bromobiphenyl SAM group was converted to the corresponding

lithium species by metal-halogen exchange with sec-butyllithium, and after addition of

24

styrene, uniform 18 nm thick films grew in 3 days. The initiating efficiency and grafting

density were calculated to be 8% and 3.2-3.6 nmz/chain respectively. Ingall et al.

. . . . . . . 53 . .
polymerized acrylonltnle from 8102 usmg a Similar strategy. A bromine terminated

SAM, formed from 3-bromopropyltrichlorosilane, was lithiated with lithium di—tert-

butylbiphenyl, and subsequent addition of monomer resulted in a 245 nm thick film after

 

 

 

ooomooowooo

 

 

 

Scheme 1.9: Surface-initiated anionic polymerization of styrene on gold

Advincula et al. used n-butyllithium to activate diphenyethylene-terminated

SAMs for the anionic polymerization of styrene54 from gold and silicon substrates. The
polymerization was slow, and reaction times of several days produced thin films (up to
16 nm). The results for silicon surfaces showed large variations in film thickness for
similar reaction conditions. The addition of tetramethylethylenediamine (TMEDA)
produced a thicker film (26 nm). To demonstrate the living nature of the polymerizations,

polystyrene-block—polyisoprene and polybutadiene-block-polystyrene were synthesized

. . . 55 56 57 . .
by the sequential addition of monomers. Later, they ’ ' extended anionic surface-

initiated polymerization to silica and clay nanoparticles. (Scheme 1.10). They confirmed

25

the “living” nature of the polymerization by demonstrating a linear relationship between

monomer concentration and Mn.

CH2
CH “0' ~
CI-SlMez-(CHZ 11

CH2

0 o
H‘O-SlMez-(CH2h-1

Bu
\
O G O
. ? benzene .—
'S'Mez-(CH2111 '

Umml.

©/§
> growing polymer chain

Scheme 1.10: Immobilization of the DPE initiator followed by polymerization of

 

the styrene homopolymer to form PS brushes by SIP.

Baskaran et al.58 used living surface-initiated anionic polymerization (Scheme
1.11) to synthesize poly(ethylene oxide) and polystyrene brushes from the surface of
MWNTs. Using a “grafting from” strategy, MWNTs were covalently functionalized with
4-hydroxyethy1 benzocyclobutene (BCB-EO) and l—benzocylcobutene—l'-phenylethylene
(BCB-PE) through [4 + 2] cycloaddition. Alkoxy anions and alkyllithium anions were
and MWNTs-g-(BCB-PE),, using

generated from MWNTs-g-(BCB-EO),,

26

triphenylmethane and sec-butyllithium for the polymerization of ethylene oxide and
styrene in THF and benzene, respectively. The initiation of ethylene oxide and styrene
from the surface alkoxy and alkyl anions respectively was found to be slow due to

heterogeneous nature of the reaction. But with increasing reaction time, the

polymerization of ethylene oxide from the surface of MWNTs~g-(BCB-EO),, exhibited

linear growth as indicated by the monomer conversion and weight percent of PEO on the
MWNTs. Moreover, this process provided surface grown polymers in high conversion.

Mueller and coworkerssg utilized a similar strategy for the grafting of MWNTs with the

such as poly(e-caprolactone) and

biodegradable and biocompatible polymers

poly(ethylene oxide). In their “grafting from” methodology, an initiator was covalently
attached to the MWNT through a [4 + 2] Diels-Alder cycloaddition reaction. The

resulting polymer—CNT hybrid materials contained a high percentage of polymers (as

high as 98%), as revealed by TGA.

 

   

27

Scheme 1.11: Surface-initiated anionic polymerization of ethylene oxide from

MWNTs-g-(BCB-EO),, surfaces.

Although living anionic polymerization is useful for the synthesis of well-defined
brushes with low polydispersity, it has several disadvantages such as the extreme
sensitivity of anionic polymerization to impurities, which necessitates the use of
specialized glassware and rigorous purification and drying of reagents. Restricted
monomer functionality, long reaction times and low values for final thickness of the
polymer films also hinder the use of this technique for polymer brush growth.

1.3.5 Polymer brushes via surface-initiated Ring-opening Polymerization

Radical surface initiated polymerization is limited to vinyl monomers. However

there is a need for biocompatible and biodegradable polymers such as polylactides and

polylactones for medical applications60. These polyesters are usually synthesized by the

ring opening polymerization of cyclic monomers initiated by organometallic reagents

in a living/controlled fashion, which is the prerequisite for the controlled grafting of
polymers from surfaces. Surface initiated ring opening polymerization is generally
performed after the formation of a hydroxyl- or amine terminated self-assembled
monolayer which initiates the polymerization (Scheme 1.12). The most comprehensive
article on polyester grafting by ring opening polymerization of lactones (s-caprolactam)
was reported by Carrot e1 (11.62 Covalent grafting of polyester chains was achieved by
functionalizing silica surfaces with a trimethoxysilane agent containing. an amine

functionality. Initiation occurred selectively from the surface by activation of amine

groups by an aluminum alkoxide, and the controlled polymerization proceeded via a

28

coordination-insertion mechanism involving the coordination of the carbonyl group of the
a-caprolactam by triethyl aluminum followed by the nucleophilic addition of the amine.

In related work, Choi and Langer formed an oligo(ethylene glycol) terminated SAM on

gold substrate and used tin(II) (2-ethylhexanoate)2 (Sn(Oct)2) to catalyze the ROP of L—

lactide from Au and silicon substrates 63 (Scheme 1.12). Polymerization for 3 days

provided 12 nm thick PLA brushes on Au surfaces, and 70 nm thick films on silicon

surfaces. The PLA brushes were shown to be chiral and crystallized on the surface. Later,

used tin(II) octoate to grow biodegradable, aliphatic

9

Choi and coworkers,

poly(ether-esters) such as poly(p—dioxane) (PPDX) and poly(l,5-dioxepan-2-one) (PDXO)
from an oligo(ethylene glycol)-terminated SAM by the ring opening polymerization.

However the polymerization was not “living” in nature as suggested from the imperfect
. 63 . . .
shapes of the resultant particles. In another study, the Silanol groups of the Silica

particles were used to initiate polymerization. A 140 nm thick polylactide brush was

obtained, which was consistent with thermogravimetric analysis (TGA).

29

”’1. O O O E
i T 0 t )“
XH o o x \n/\o n
,. o

Sn(Oct)2

 

SAMs, X = O or NH

Au recto/V0“ was
9 3 9 3

‘5.

‘é- r
Sl/SIOZ O’IIW \/\NH2 (MeOI3SI\/\/N\/\NH2

\

 

Scheme 1.12: Surface—initiated ring opening polymerization of lactide.

1.3.6 Polymer brushes by Ring-opening metathesis polymerization (ROMP)

ROMP is the transition metal catalyzed ring opening polymerization of strained

cyclic olefins. Via ROMP, Whitesides and co-workers grew a variety of norbomene-

. . . 66 . .
derived polymer brushes from Silicon surfaces. The surface-bound catalytic Sites were

produced by forming a trichlorosilane—derived SAM containing norbomene groups, and

then exposing the SAM to a solution of a Grubbs-type ROMP catalyst. The
polymerization was controlled with rapid initiation, producing 90 nm thick brushes in 30
min. The formation of block copolymer brushes and the use of microprinting to produce
patterned surfaces also were described. In related work, Mingotaud er al. immobilized a
ruthenium catalyst on 200 nm Silica nanoparticles67 by reacting a methathesis catalyst

bearing a hydroxyl group with silica particles modified with acyl chloride functional

groups, and then performing ROMP of norbomene. The authors estimated that 30% of

30

 

the catalyst initiated polymerization. TEM characterization showed a core—Shell

morphology which suggested the presence of the catalyst on the silica surface. Using a

recently invented ruthenium catalyst, [(HgIMes)(3-Brpy)2(C1)2Ru=CHPh], Choi and

coworkers demonstrated that surface-initiated ring opening metathesis polymerization

(SI-ROMP) can be utilized for the formation of diblock copolymer brushes from surfaces

(Scheme 1.13).68 Taking advantage of the highly improved activity of the ruthenium
catalyst and the rapid initiation step of SI-ROMP, they successfully formed thin films of
well-defined diblock copolymers with 5-norbomene-2-endo,3—endo-dimethanol (Nb-
diMeOH) and an endo/exo isomeric mixture (44:56) of norbomene carboxylic acid

methyl ester (Nb-COOMe).

31

 

 

 

 

/ \ .+~C'
N-R1_
— /IuflPh
B Q N
r / /
l
\ OTBS
s/‘Mfi 8' $ sflMeru] OTBS +~
Au
/
1,, .0 1 TBS TB
TBSO/ ~ “ \ores ) (300m> O O S
n
3 2) Ethyl Vinyl ether COOMe
p(Nb-leTBS) p(Nb-diOTBS)- b-p(Nb-COOM0)
“\OH
TBAF __ W0
THF 7 n 3 n v

COOMe

p(Nb-diMeOH)—b—p(Nb—COOMe)
Scheme 1.13: Schematic description of the formation of p(Nb-diMeOH)-b—p(Nb-

COOMe) diblock copolymer brushes on a gold surface.
1.4 Applications of Surface-initiated Polymerization

A significant advantage of polymer brushes compared to other surface
modification methods is their mechanical and chemical stability, accompanied by a high
level of synthetic flexibility towards the introduction of functional groups. This is in
contrast to the physisorbed, non-bound polymer films where chemical modification by
using wet chemistry is difficult to conduct. Additionally, it is now possible to grow
brushes on virtually every surface (flat surfaces, particles or macromolecules), to any
thickness, of every composition, incorporating a multitude of functional groups and

containing series of blocks. More recent applications of polymer brushes include

32

69 . . 70 . . .
nanopattemed surfaces, photochemical deViceS, new adheSive materials,71 protein-

. . 72 . . 73 . 74 1
reSistant biosurfaces, chromatographic deVices, lubricants, polymer surfactants,

. .. l
polymer compatibilizers and many more.

One of the most attractive applications of surface-initiated polymerizations is the
formation of nano-patterned surfaces by soft lithography techniques that combine
microcontact printing (uCP) and graft polymerization. An elegant example is that of
Hawker et al. who combined photolithography with nitroxide-mediated “living” free

radical polymerization to obtain patterned polymer brushes with well-defined

. . 4 .
hydrophobic and hydrophilic domains (Figure 1.5). 9 They extended this concept to

75
synthesize patterned polymer layers by aqueous ATRP.

33

 

 

200 um features Water droplet

 

\

Manual . 1/2000
PAA brush PTBA brush

Figure 1.5: Optical micrographs of patterned surfaces: (left image) IO-pm
features in a continuous polymer brush Showing regions of poly(tert-butyl acrylate)
(dark) and poly(acrylic acid) (light) and (right image) interaction of a water droplet with
ZOO-um features showing an unusual wetting profile and preferential interaction with
poly(acrylic acid) brush domains.(Reprinted with permission from J. Am. Chem. Soc.

2000, 122, 1844-1845. Copyright 2000 American Chemical Society.)

34

 

68
Recently, Huck and coworkers have shown that charge-transporting polymer

brushes (polytriphenyl amine acrylate) can be synthesized from a variety of surfaces (ITO,

Si02 and conducting polymer) relevant for organic electronic device fabrication. These

polymer brush films contain a greater level of ordering at the molecular level and display
higher charge mobility than Spin-coated films of the same polymer, which was attributed
to the controlled polymer brush architecture and morphology. They also demonstrated
substantial uptake of CdSe nanocrystals (with diameter in the range 2.5-2.8 run) into the

polymer brush layers (Figure 1.6) and a photovoltaic quantum efficiencies of up to

50%. 76 In another report, Advincula and coworkers 77 successfully grafted hole-
transporting PVK (poly(vinyl carbazole)) brushes on transparent ITO electrodes. Using
cyclic voltammetry, the PVK brush was electrochemically crosslinked, to form a
conjugated polymer network film. Covalent linkage of PVK led to a direct
electroluminescent PLED device, in which the electroluminecent polymer layer can be

simply solution-cast onto the modified ITO.

35

—0 Br

u\\\\\\i\\\\\\\\\\
O
\ I /
9.)
O

 

 

anode (ITO)

 

Figure 1.6: Top image: PTPAA brushes and bottom image: Cartoon of inferred
structure for CdSe nanocrystal infiltrated polymer brush photovoltaic device (From
bottom to top) ITO-coated glass Slide modified by surface attachment of a bromine end-
caped trichlorosilane self-assembled-monolayer (SAM) (blue squares), polymer brushes
grown from the SAM (red lines), CdSe nanocrystals infiltrated into the brush network
exhibiting some degree of phase separation in the plane of the film (small black circles),

and caped with an aluminum cathode. (Reprinted with permission from Nano Lett. 2005,

5, 1653. Copyright 2005 American Chemical Society.)

36

A more ambitious challenge in surface science is the design of smart surfaces

. . . 78 . .
With dynamrcally controllable properties. Such surfaces have characteristics that can be
changed or tuned in an accurate and predictable manner by using an external stimulus.
Recently, Huck and coworkers have shown that wetting properties of surfaces modified

with cationic polyelectrolyte brushes strongly depend on the nature of the counter ion.

Coordination of polyelectrolyte brushes bearing quaternary ammonium groups (QA+)

With sulfate anions resulted in highly hydrophilic surfaces, 9 whereas, coordination of

Similar brushes with C104 rendered the surface hydrophobic.80

37

e = Counterion

°e° G ’ooeoec 069599000 n

O 3'“ 0 e O

0 e v o e o
o eo :9 e e o e o o G 0

0 G o o o e o o 0

o o 0 e 0 e e o

If +
L- 2...; io—o'um-«s--...K...‘..-.....r.'.';.-...;_u_.., 5‘44; _._ 4y“... ., euurs —N—
' - ”L“ \

(A)

(B)

   

 

PP
o m
"’ 90 ¢m

TFSI

  

 

 

 

105 TFSI (I; g CF
90 i- ‘ _’F3C\ — I’ 3
4" ’/ ‘ ’ \\
75 _ \ [fit ,1 [I f _ 0 N 0
- 1 l I \ l
60 " l I \ -/ \ l \ I . _ O O—
. I . ' I l
9A 45 - \ I t ,‘ l I \ l . 0‘15 ,5/0 _
l I i l I \l _ C.” \O’ \90
30 ' U (I i ' 0:,P P’zo
15 (C) a» I J Y .' 00, ,0\ ,0
PP —0’l:l 5’90

 

Counterion Exchange

Figure 1.7: Top image: PMETAC brushes and bottom image: Change in the

wetting characteristics of PMETAC brushes (height, 11 ~20 nm) after exchanging the two
contrasting counterions: TFSI (a) and polyphosphate (PP) (b). c) Representation of 0 A as

a function of counter ion (PP and TFSI). The plot depicts the reversible behavior of

PMETAC brushes over repeated cycles of TFSI and PP counter ion exchange. On the

38

right the chemical structures of both counter ions are represented. (Reprinted with

permission from Angew. Chem. Int. Ed. 2005, 44, 4578. Copyright 2005 Wiley-VCH.)

Recent research has focused on the Cu(I)-catalyzed, highly specific and efficient

formation of 1,2,3-triazoles via the 1,3-dipolar cycloaddition of azides and terminal

. ,, . 81 . _
alkynes (“click chemistry). This methodology has been used to modify surfaces of

solid metals and cells, because the reaction provides high yields, stereospecificity and
. . . 82 83 . .
proceeds under mild conditions. ’ Click chemistry also has been used for

. . . . . 84 85 . . . .
functionalizmg polymers in solution. ’ Research in nanobiotechnology and biomedical
sciences often involves the manipulation of interfaces between manmade surfaces and
biomolecules (and cells), which generally requires the construction of surfaces that

present chemically active functional groups from non-biofouling supporting materials.

. 86 . . . .
Chor and coworkers used “click” chemistry to couple aZide groups at the termmal of

the non-biofouling polymeric film of poly(oligoethylene glycol methacrylate) with
incoming molecules of interest containing terminal acetylenes (Figure 1.8). As a model
for bioconjugation, biotin was immobilized onto the poly(oligoethylene glycol

methacrylate) film via click chemistry, and biospecific recognition of streptavidin was

demonstrated.

39

n n n
Br \/\OM6 O\/\OMe O\/\OMe
O O O
O O O
O O O

CuBr/Bipy NaN3 Click

———> ———>
OEGMA Alkyne 1

S
.1. J.
ft
HN NH Alkyne 1

H H
MN$0NO$ONNM
O 0

Figure 1.8: Schematic description of the attachment of biotin to polymer brush
via click chemistry [Acetylene group—containing biotin compound: biotin-PEO-LC-N-

pentynoate (1)].

4O

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—_

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Hedrick, J. L. Macromolecules 2000, 33, 597.

70 Whiting, G. L.; Snaith, H. J.; Khodabakhsh, S.; Andreasen, J. W.; Breiby, D. W.;
Nielsen, M. M.; Greenham, N. C.; Friend, R. H.; Huck, W. T. S. Nano Lett. 2006, 6, 573.

71 Raphael, E.; De Gennes, P. G. J. Phys. Chem. 1992, 96, 4002.
72 Amiji, M.; Park, K. J Biomat. Sci. Polym. Ed. 1993, 4, 217.
73 Vanzanten, J. H. Macromolecules 1994, 27, 6797.

74 Joanny, J. F. Langmuir 1992, 8, 989.
75 Vidal, A.; Guyot, A.; Kennedy, J. P. Polym. Bull. 1980, 2, 315.

44

 

 

76 Snaith, H. J.; Whiting, G. L.; Sun, B.; Greenham, N. C.; Huck, W. T. S.; Friend, R. H.

Nano Lett. 2005, 5, 1653.

77 Fulghum, T. M.; Prasad, T.; Advincula, R. C. Macromolecules 2008, 41, 5681.

78 Lahann, J.; Mitragotri, S.; Tran, T. N.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.;
Somorjai, G. A.; Langer, R. Science 2003, 299, 371.

79 Moya, S. E.; Azzaroni, 0.; Farhan, T.; Osborne, V. L.; Huck, W. T. S. Angew. Chem,

Int. Ed. 2005, 44, 4578.
80 Azzaroni, 0.; Moya, S. E.; Farhan, T.; Brown, A. A.; Huck, W. T. S. Macromolecules

2005, 38, 10192.
81 Feldman, A. K.; Colasson, B.; Fokin, V. V. Org. Lett. 2004, 6, 3897.

82 (a) Rostovtsev, V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B. Angew. Chem, Int. Ed.
2002, 41, 2596. (b) Tomoe, C. W.; Christensen, C.; Meldal, M. J Org. Chem. 2002, 67,

3057.
83 Lewis, W. G.; Green, L. G.; Grynszpan, F.; Radic’, Z.; Carlier, P. R.; Taylor, R; Finn,
M. G.; Sharpless, K. B. Angew. Chem, Int. Ed. 2002, 4], 1053.

84 Sumerlin, B. S.; Tsarevsky, N. V.; Louche, G.; Lee, R. Y.; Matyjaszewski, K

Macromolecules 2005, 38, 7540.
85 Gao, H.; Louche, G.; Sumerlin, B. S.; Jahed, N.; Golas, P.; Matyjaszewski, K

Macromolecules 2005, 38, 8979.
86 Lee, B. S.; Lee, J. K.; Kim, W. J.; Jung, Y. H.; Sim, S. J.; Lee, J.; Choi, 1. S.
Biomacromolecules 2007, 8, 744.

45

 

Chapter 2

Substituent effects in surface ATRP of polystyrene brushes

2.]. Introduction

The development of controlled radical polymerizations such as Atom Transfer

. . 1, . .. .
Radical Polymerization (ATRP), 2 Reversrble-Addrtion-Fragmentation Transfer

Polymerization (RAFT)3 and Nitroxide-Mediated Polymerization (NMP),4’5provide

powerful methods for the growth of polymer brushes from surfaces. These

polymerization methods limit radical concentrations during polymerization, minimize

bimolecular termination reactions and provide control over Mn and the polydispersity

(PDI). However, compared to traditional radical polymerization, control comes at the
expense of a substantial reduction in the polymerization rate. Recently, we reported the

remarkably rapid growth of well—defined poly(tert—butyl acrylate) brushes under mild
conditions (50 0C) using ATRP. The key aspect of the polymerization was the use of a
highly active ATRP catalyst [Cu(1)1,4,8,11-tetramethy1-1,4,8,1 1-tetraazacyclotetradecane
(Cu(l)-Me4Cyclam)] which provided 100 nm thick poly(tert-butyl acrylate) (PtBA)
brushes on a flat Au surfaces in just 5 minutes (Scheme 2.1).6 Such polymerization rates

are several orders of magnitude greater than typical controlled polymerizations from

surfaces. The Cu(1)—Me4Cyc1am) system also enabled rapid growth of thick brush layers

from other vinyl monomers such as styrene, vinyl pyridine and methacrylates. Rapid

46

 

growth of brushes could expand the scope and applications for polymer brushes by

greatly reducing the time required for their synthesis.

1..

 

O
o O
Anisole/DMF _
Au —-S-(CH2)--O Br 7 AU —S*(CH2);1-O lCHz—CH‘j-Br
11 CuBr/ Me4Cyclam m
0 O

CuBr2(anbpy)2.5O °C

 

Me4Cyclam = E j anbpy 7'

Scheme 2.1: Synthesis of tethered PtBA film on gold surface 6

The unusually rapid growth of PtBA films compared to other vinyl monomers can be
rationalized as a combination of tBA’s fast propagation rate and reduced bimolecular

coupling due to the steric bulk of the monomer. As shown in Scheme 2.2, sterically

demanding monomers show a high propensity towards head to tail placement during
polymer growth, and the same steric interactions would be expected to hinder termination

by coupling (analogous to forming a head to head linkage). The generality of this

phenomena can be seen in kp (propagation rate constant) and kt (termination rate constant)

. 7,8 .
values for the methacrylate monomers shown in Table 2.1. The first five entries show
data for structurally related methacrylates obtained under similar experimental conditions.
There are two important trends in the data; kt decreases as the size of the ester increases

because of the steric crowding depicted in Scheme 2.2, but a more surprising result is that

in some cases, kp also increases with the size of the ester group. Gilbert suggested that the

47

 

increases in kp may be due to the increased momentum associated with collisions

, . . 9 .
between higher molecular weight specres. Monomers With both high kp and low kt

values, i.e. a high kp / k, ratio, should be the prime candidates for rapid polymerizations

from surfaces. The last two entries show that there are limits to this approach. For

example, kt for trityl methacrylate is <1><10"6 L mol'lsec'l, but the extreme size of the
ester group apparently hinders any enhancement in kp. Ethylene is the least demanding
vinyl monomer and the reported value for kt, 540x10.6 L mol-lsec’ , indicates facile
termination by coupling and disproportionation. When combined with a kp ~242 Lmol'

sec-l, the kp / kt ratio for ethylene is 0.45X10’6, which predicts ethylene as a poor

candidate for rapid growth from surfaces by radical polymerization. The above arguments

suggest that hp and more importantly the hp / kt ratio could be used to identify promising

monomers for the rapid growth of polymer brushes from surfaces.

48

 

 

Scheme 2.2. Steric constraints favor head to tail addition and low kt values.
The polymerization results for PtBA prompted us to examine other bulky
monomers to see if they also would polymerize rapidly and provide thick films. Thick

polystyrene brushes are difficult to grow via surface initiated ATRP (40 nm thick films in

. . 6
1 hour compared to 100 and 200 nm films in 1 hour for MMA or tBA, respectively),

8 . .
consistent with its small kp / kt ratio (4.3 at 60 OC) . We studied substituted styrene

derivatives (Scheme 2.3) to evaluate the effects of steric congestion and electronic effects

on the polymerization rate under the same conditions used for tBA system. These

. . . . . 10,11,12,l3
monomers efficiently polymeriZe via free radical polymerization in solution,

49

but there are no reports of their polymerization via surface-initiated ATRP or other
controlled radical processes.

Table 2.1. Propagation and termination rate constants for representative

 

 

 

 

 

 

 

 

methacrylates8
CH -6 6
H2O 2 kp k,(x10 ) kp/k,(>< 10 )
C) -1 -1 1 1
OR (L mol 560 ) (L mol' sec- )
--CH3 649 25.5 25
“CH2CH3 723 7.35 98
[CH3
——C\H 740 4.52 163
CH3
“(CH2)3CH3 794 10 79
“(CH2)1 1CH3 1011 0.6 1670
9H3
—C'3-CH3 350 14 25
CH3
*C‘@>3 26 0.30 86

 

 

 

 

 

 

From data reported from other types of radical polymeriZations in

14,15,16

solution, we expected that substituents on the styrene ring may affect the

polymerization rate, and enable fast growth of polystyrene brushes from surfaces.

50

Furthermore, we expected that such a study would provide a better understanding of the
mechanism of surface initiated polymerizations (especially the correlation between
surface polymerization rate and monomer structure) as there has been no systematic
investigation of substituent effects in substituted styrenes in surface initiated

polymerizations.

 

Electron withdrawing groups Electron donating groups

a a s
s a 2%

 

 

\
\

6 45k.
ES 6

 

 

 

Scheme 2.3. Substituted styrene monomers polymerized in the study
Herein we report the rapid surface initiated polymerization of substituted styrenes, and

the effect of various substituents on the polymerization kinetics studies. The data are

discussed with regard to the Hammett equation: log (kx/kH) = po
51

2.2. Experimental Section

2.2.1. Materials

Unless otherwise noted, all chemicals were obtained from Aldrich. ll—Mercapto-

l-undecanol (MUD, 97%), 2-bromopropionyl bromide (2-BPB) (97%), anisole (99.7%),

N,N—dimethylformamide (DMF, 99.8%), Cu(I)Br (999999.), Cu(II)Br2 (99.999%),

Me4Cyclam (99%), and 4,4’—dinonyl-2,2’—bipyridine (anbpy. 97%) were used as

received. Triethylamine was vacuum-distilled from calcium hydride. tert~Buty1 acrylate
(tBA) (98%), MMA (99%), styrene (99%), 2-vinylanisole (95%), 4-bromostyrene,
pentafluorostyrene, 4—methylstyrene and 4-tert-butyl styrene were passed through a 10
cm long column of basic alumina followed by distillation to remove inhibitors. 3,5-
Bis(trifluoromethyl)styrene (97%) and 3,5-dimethylstyrene were purified by passing
through a 10 cm long basic alumina column. After purification, the monomers and
solvents were transferred to Schlerik flasks, degassed using three freeze-pump-thaw
cycles, and then transferred into a drybox. Other monomers such as 3,5-di-tert-butyl
styrene, 3,5-dimethoxystyrene, 2,6-dimethoxystyrene, 4-vinylanisole and 4-

(trifluoromethyl)styrene were prepared as previously reported mostly from their aldehyde

. . . . ll
precursors vra the Wittig reaction.

2.2.2. Characterization Methods

Film thicknesses were measured using a rotating analyzer ellipsometer (model M-

44, J. A. Woollam) at an incident angle of 75°. The data were analyzed using WVASE32

software, and thickness and refractive index determinations were performed on at least

three spots on each substrate. The refractive index of the films was assumed to be 1.5 and

52

then fitted with the film thickness. Reflectance FTIR spectroscopy was performed using a

Nicolet Magna-IR 560 spectrometer containing a PIKE grazing angle (80°) attachment.

2.2.3. Preparation of Initiator Immobilized Gold Substrates

Gold-coated Si wafers (200 nm of Au sputtered on 20 nm of Cr on Si(100) wafers)

were UV/O3-cleaned for 15 min before use and then transferred into a Nz-filled glovebag.

Hydroxy-terminated SAMs were formed by immersing the Au-coated substrates in a vial
containing a 1 mM ethanolic solution of MUD for 24 h. After removing the vial from the

glovebag, the substrates were rinsed sequentially with ethanol and water and dried under

a stream of N2. The ellipsometric thickness of the MUD layer was 10-15 A. MUD-coated
substrates were transferred to a drybox filled with N2 and were dipped in a 10 mL

solution of 0.12 M triethylamine in anhydrous THF at 0°C. After 1 min, 10 mL of a

solution of 2—BPB in anhydrous THF (0.1 M) was added dropwise to the solution to form
the immobilized initiator layer. The reaction time was limited to 2-3 min, since thiol—
terminated SAMs could be unstable in the presence of the acyl bromide. After rinsing

with THF, the Au substrates were removed from the drybox, and then rinsed sequentially
with ethyl acetate, ethanol and deionized water (Milli-Q) and dried under a stream of N2.

2.2.4. Preparation of Immobilized Initiators on Si Substrates

UV/O3 cleaned Si wafers with an ellipsometrically determined oxide thickness of

20 A were transferred to a glove bag filled with N2 and immersed in a dry toluene

solution (20 mL) containing 30 11L of [11~(2-bromo-2-methyl)propionyloxy]undecyl

trichlorosilane. After 24 h without stirring, the samples were removed from the solution,

53

placed in fresh toluene and sonicated for 1 min. Following additional rinsing with

toluene, acetone, and ethanol, the substrates were dried under a stream of N2. The

ellipsometric thickness of the initiator layer was ~ 20 A.
2.2.5. Polymerization of Substituted Styrenes from Initiators Immobilized on Au

Substrates - Kinetic Experiments

Note: For best results, solvents, initiators and monomers must be scrupulously

purified and deoxygenated. In a Nz-filled drybox, CuBr (5.7 mg, 0.04 mmol) CuBrz,

(4.5 mg, 0.02 mmol), Me4Cyclam (10.3 mg, 0.04 mmol) and anbpy (16.4 mg, 0.04

mmol) were added to a round—bottom flask containing 20 mL of a degassed solution of

monomer in DMF/anisole (DMF/anisole ~1:1 (vzv), [styrene] ~ 4 M). The mixture was

well-stirred and heated with an oil bath to 50 0C until a transparent, light green solution

formed. The prepared solution was then transferred into a small vial containing an

initiator-modified Au substrate to start the surface-initiated polymerization. After a

predetermined reaction time at 50 0C, the substrate was removed from the vial, washed

with ethyl acetate and THF sequentially, and then dried under a flow of N2 in the drybox.

The same conditions were used for the polymerization of other monomers.

2.3. Results and Discussion

54

2.3.1. Synthesis of Substituted Styrenes
The substituted styrenes used in this investigation are listed in Scheme 2.3. Most
were available from commercial sources, however, 3,5-di-tert-butylstyrene, 3,5-

dimethoxystyrene, 2,6-dimethoxystyrene, 4-vinylanisole and 4-(trifluoromethyl)styrene

. . . 10-13,17 .
were synthesrzed according to literature procedures. The synthetic protocols for

3,5-dimethoxystyrene, 4-vinylanisole and 4-(trifluoromethyl)styrene are outlined below

(Scheme 2.4.).

55

/ G) 6
Base, CH3PPh3Br \

,z X
\x x

x = substituent

/
CH Li, CH P( Ph Br
/O\©/O\ n—BuLi, TMEDA 3 3 )3
DMF,THF 1". 55 °C

 

 

 

 

 

0-10°C,1.5h 45%
72%
tBuCl / A1013 CHB‘2
.40 °C to -100 °C NBS/benzoyl peroxide
2.5h CCI4, reflux,15h
86% 70%
HMTA, water / methanol
reflux, 4 h
68%
/ /0
CH31. PPh3, NaH
THF, 50 °C, 6h
60%

Scheme 2.4: The synthesis of substituted styrenes. (top) 3,5-Di-tert-butylstyrene,
3,5-dimethoxystyrene, 2,6-dimethoxystyrene, 4-vinylanisole and 4-(trifluoromethyl)
styrene were synthesized via the Wittig reaction from commercially available aldehydes.
(middle) Formylation of 1,3-dimethoxystyrene followed by Wittig olefination provided
2,6-dimethoxystyrene in moderate yield. Synthesis of 3,5-di-tert-butylstyrene (bottom)

was prepared via a four step procedure.

2.3.2. Synthesis of Substituted Styrene Brushes via Surface Initiated Polymerization
Scheme 2.5. outlines the synthesis of substituted styrene polymer brushes grown

from gold substrates. The experimental procedure is similar to that described by Bao et al.

for the rapid growth of tert-butyl acrylate from gold surfaces.6 A self assembled

monolayer was prepared on the gold surface using MUD and converted into an initiator
monolayer by reacting its terminal hydroxy group with 2-bromopropionyl bromide, as

described previously. Initiator immobilization was apparent from the appearance of a

carbonyl peak at 1743 cm'1 in the reflectance FTIR spectrum (Figure 2.2a). The purified

substrates were transferred into a dry box, and the polymerization was performed inside

the drybox to avoid contamination from oxygen. The catalyst system was a mixture of

Cu(I)Br/Me4Cyclam and Cu(II)Br2/(anbpy)2 in 1:1 (v/v) solutions of DMF and anisole.

The Cu(II) complex ensures deactivation of active radicals and provided some control

over the rapid polymerization. Polymerizations of substituted styrenes were run at 55 0C

for 8 hours under identical conditions. The substrates were sonicated, and then washed
with ethyl acetate and THF. The IR reflectance spectra of the dried films confirmed
formation of polymer brushes by the appearance of characteristic peaks that correspond

to the substituents on the styrene ring (Figure 2.2).

 

/
\
l \,
’X 0 .
o . H H
ls—(CH2)-o—c-cH-Br an'sme’DME: I'S"(CH2);10 [CH2~CiBr
11 CH3 CuBr l Me4Cyclam \ n
CuBr2(anbpy)2 x= substituent l ,\’

X

Scheme 2.5.: Surface initiated polymerization of styrene derivatives

57

2.3.3. Kinetic study of surface initiated polymerization of substituted styrene from

gold surface.

Figures 2.1 and 2.3 show the evolution of film thickness with time for the
polymerization of substituted styrenes. Figure 2.1 shows data for the surface

polymerization of ortho and meta-substituted monomers while Figure 2.3 shows

comparable data for para-substituted styrenes. The polymerization rates at 55 C>C were

unusually high for the ATRP of 3,5-disubstituted styrenic monomers, providing 100-350

nm thick layers in 4 hours.18 In the early stages of polymerization, the nonlinear
relationship between the film thickness and time suggests a relatively high concentration
of radicals leading to some termination as well as a high polymerization rate. However,
after >2 hours, growth in film thickness with time is roughly linear, indicating that some
level of control was established during the polymerization. Polymerizations of ortho-
substituted monomers were sluggish, likely due to steric effects, and none provided 100

nm thick films in 8 hours.

58

 

 

 

 

300
I
250 -
o
E 200 - '
5 A
8 u
g 150 ~ 0
.C I A
'— o
100 ~
' a
o D
Q a Cl
50 . g +
+
+ + A
0 A 4 i 1 ,
0 2 4 6 8 10

Time (hour)

Figure 2.1: Evolution of the ellipsometric brush thickness with time for the
polymerization of substituted styrenes from gold substrates. 3,5-bis-
(trifluoromethyl)styrene (I), 3,5-dimethoxystyrene (O), 3,5-dimethylstyrene (A), 3,5-di—
tert-butylstyrene (X), styrene (O), 2-(trifluoromethyl)styrene (D), 2-methoxystyrene (+),

2,6-dimethoxystyrene (A).

59

 

t x10

e x 100

x50

A C X100
1

 

b x100
a x 50

LE
iii a? 9

 

 

j l

 

3000 2500 2000 1 500 1 000
Wavenumbers (cm")
Figure 2.2.: Representative reflectance FTIR spectra of gold surfaces coated with
(a) the immobilized initiator (b) 70 nm of polystyrene (c) 55 nm of poly(3,5-di-tert
butylstyrene) (d) 5 nm of poly(2,6-dimethoxystyrene) (e) 27 nm of poly(2-

methoxystyrene) and (f) 130 nm of poly(3,5-bis-(trifluoromethyl)styrene).

60

The IR spectra in Figure 2.2 confirm the growth of polymers from Au surfaces.

Furthermore, the spectral intensities agree well with the kinetic data. Spectrum b shows

the four peaks (overtones and combinations) from 2000-1700 ch characteristic of

mono-substituted benzene rings (styrene). Strong methyl C-H stretching bands at 3000-

2850 cm.1 and the characteristic doublet for tert-butyl groups, a strong peak at 1370 cm.1
and a weaker signal at 1390 cm-1 confirms the growth of 3,5-di tert—butyl styrene in
spectrum 0. The strong, broad, asymmetric (1280-1220 cm'l) and symmetric (1050-1000
cm'l) stretching modes for aryl ethers confirm the attachment of dimethoxy and
monomethoxy styrene (spectra d and e). In spectrum f, a sharp strong peak at 1350 cm”1

(C-F stretching) identifies the CF3 group of poly(3,5-bis-(trifluoromethyl)styrene).

In Figure 2.4, strong peaks between 1280-1220 cm"1 in spectrum b (C-O-C
stretching of an alkyl aryl ether) confirm the growth of 4amethoxystyrene. A sharp strong
peak at 1350 cm'1 (C-F stretching) identifies the CF3 group in poly(4-trifluoromethyl
styrene) (spectrum c). The spectrum of poly(4-methylstyrene) (d) is similar to that of
styrene (a), except for the 2 peaks between 2000-1700 cm'1 (overtones and combinations)
for a 1,4-disubstitutedbenzene ring. The strong, narrow peak seen at ~1000 cm" in
spectrum e is characteristic of aromatic C-Br stretching expected for poly(4-
bromostyrene). In spectrum 0, the characteristic doublet for tert-butyl groups, a strong
peak at 1370 cm" and a weaker signal at 1390 cm'1 confirms the growth of 4—tert-buty1

styrene.

61

 

 

 

 

250
I
200 ~
75‘
5 150 ~ I D
In
(D
a)
.5
'9- 100 P D o
"3 o
'- O
I a Z A
50 - 0
8 A
A
0 A T i T
0 2 4 6 8 10
Time (hour)

Figure 2.3: Evolution of the ellipsometric brush thickness with time for the
polymerization of 4-substituted styrenes from gold substrates. 4-(trifluoromethyl)styrene
(I), 4-tert-butylstyrene (O), 4-bromostyrene (E1), styrene (O), 4-methylstyrene (A), 4-

methoxystyrene (A).

62

 

0.2

 

 

 

 

 

 

l 1

x20
8 x20
5 M
.0
I-
0 x50
(n
.0
<1

x2

 

XW

L l

 

 

3000 2500

2000 1500 1000

Wavenumbers (cm")

Figure 2.4: Representative examples of reflectance FTIR spectra of 4- substituted

styrenes grown from gold substrates (a) 70 nm polystyrene (b) 10 nm poly(4-

methoxystyrene) (c) 200 nm poly(4-(trifluoromethyl)styrene) (d) 55 nm poly(4-

methylstyrene) (e) 100 nm poly(4-bromostyrene) (t) 150 nm poly (4-tert-butylstyrene).

63

2.3.4. Substituent effects on polymerization rate.

The data in Figures 2.1 and 2.3 show that monomers with electron withdrawing
groups polymerize faster than monomers bearing electron donating groups, irrespective
of the substitution pattern on the aromatic ring, (excluding ortho-substituted monomers
where steric effects restrict propagation and termination. We also failed to grow a-methyl
styrene from gold surface (data not shown) due to steric reasons.) Monomers with bulky
electron donating groups (tert~butyl) at meta or para position polymerized faster than
styrene and ortho-substituted monomers. Surprisingly, monomers with electron donating
groups at meta sites polymerized faster than styrene or their ortho/para substituted
analogues, irrespective of their steric demand. The observed film growth rates varied in
the order of m-di—CFg, p-CF3> m-di methoxy> m-dimethyl, m-di-tert—butyl > p-tert-
butyl > p-Br > H > p-methyl > o—methoxy > o-dimethoxy, p-methoxy. The results are

consistent with solution polymerization rate data obtained from conventional radical

polymerizations as well as controlled radical polymerizations such as ATRPI6 and living

radical polymerizations initiated by TEMPO/EPOM’IS

To analyze the surface initiated polymerization rate data, we constructed a
. . . . . 16,19
Hammett plot Similar to the method used to analyze solution polymerizations. A

Hammett plot requires rate constants for the surface polymerization of para-substituted
styrene because, only para-substituted styrene Hammett constants (c) are known. The

propagation and termination rate constants for surface polymerization can be estimated

. . 20
by USlng a model proposed by the Wirth group.

64

The initiation and propagation steps of living radical polymerization have been

well-documented and adopted for ATRP systems as shown in Scheme 2.6.

Initiation:
ka
~Rn-X + (Cu(|)Iigand)X # ~Rn' + (Cu(||)ligand)X2 (1)
d
Propagation:
kp
~Rn' + M ~———-—> ~Rri+1 (2)
Termination:
kt ~
~Rn° “Ra “‘"* in (3)

"Rm
Scheme 2.6: The initiation, propagation and termination steps of ATRP

According to this mechanism, the disappearance of monomer only occurs during

propagation (step 2 of Scheme 2.6).

-9155]- ..- 1
dt .. kp[R][M] ()

If [R-] is constant, the monomer concentration can be reduced according to the

first-order kinetics.

m W2] = kp[R-]t (2)

However, [R-] usually is not constant due to termination, especially in case of surface
polymerization which is clearly visible in Figures 2.1 and 2.3. Assuming only termination

by coupling for surface-initiated ATRP, and ignoring possible contributions from surface

65

 

fouling or side reactions such as radical transfer to solvent, the rate of termination can be

18
expressed as:

d R

is] ._. _ kt[R']2 (3)
[13']

R = 0 <4)

 

Recognizing that monomer-to-initiator ratio is so high that monomer

concentration changes are negligible simplifies the solution of Equation 2.

[M] (5)
— = - k R' 1
[M10 1 pl 1

Substituting Equation 4 into Equation 5 reduces conversion to simple nonlinear

time dependence:

 

[Mic-[M] _ kle'lo‘ (6)
[M10 1 +1R.10ktt

Realizing that the ellipsometric thickness is proportional to [M]0 - [M], the data of

Figure 2.3 were fit to Equation 6. The fits to the data, as shown in Figure 2.5 for 4-

bromostyrene, is reasonable and allows extraction of the apparent rate constants for
propagation (kp app = [M]0 hp [R-]0) and termination (kt app = [R°]0 kt). The data obtained

for different para-substituted styrenes are tabulated in Table 2.2.

66

Table 2.2. Apparent rate coefficients in surface ATRP obtained from the Wirth

. . 8
model and absolute propagation rate constants for substituted styrenes

 

 

 

subsumem “21 kp app (moi-1161) kt app (5'1) kp abs (mol-1Ls-1)8
4-CN 0.66 — - 219
4-CF3 0.54 649 7.23 -
4-Br 0.23 111 1.35 186
4-Cl 0.23 - - 150
4-H 0.00 106 1.82 110
4-Me -0. 17 76 2.03 84
4_CMe3 -0.20 288 6.81 -
4-OMe -0.27 9.0 0.59 71

 

 

 

 

 

 

67

100

 

 

      

 

 

 

75 i-
A
E r
5 2
m R = 0.99513
8 50
C ..
x y - 110.53t l (1 + 1.351)
.2
fi
25

 

 

 

O J 1 r L 1 I r L
O 2 4 6 8
Time (hours)

Figure 2.5: A representative example of the polymerization data for 4-bromo

styrene fit to Equation 6. The polymer was grown from Au substrates at 55 CC.

Using the rate constants from fitting the data to Equation 6, we constructed a

Hammett plot (Figure 2.6) relating the ratio of the apparent rate coefficients (kp app/kt app)

(similar to analyzing product ratios, as applied by several authors to obtain Hammett

22,23,24 . . .
and the Hammett constant, c, for different para substituents. By usmg the

plots)

ratio of the rate constants, we eliminate the radical concentration [R-]0, and in principle,

kp app / kt app should reflect the ratio of absolute rate coefficients. Thus,

68

kp app / kt app = [Mlokp 1R'10/ 1R'10 kt: [M10 kp / kt and

108 (kx/ hr) = 108 [([M10 kp / kt)X / ([Mlo kp / kt)H1 =108 1(kp / kt)x / (kp / kt)H1

where X and H refer to the substituted styrene and styrene respectively. For

conventional radical polymerization of para-substituted styrenes, there is a linear
correlation between log(kp app / kt app) and the Hammett o constants for different
substituents. The value of p for our surface ATRP data (p = 0.51) was similar to that of
conventional radical systems run in solution (p = 0.55, Figure 2.6),25 but much smaller
than the p value reported by Matyjaszewski for the solution ATRP (p = 1.5).16 The

difference between the solution and surface ATRP results is the use of different ordinates.

In solution ATRP, the correlation was established with regard to the apparent propagation
rate constant which comprises [.R'], rather than the ratio of rate coefficients used for

surface ATRP. Thus, the p values for surface ATRP and conventional (solution) radical

polymerizations which also uses absolute rate coefficients as ordinate, are similar.

Based on the qualitative success of the simple Hammett equation similar to the

. . . . . 26 .
solution polymerization V18. free radical pathway, we conclude that these reaction
constants are roughly correlated with Hammett parameters for the para substituents in

surface polymerization with p = 0.51. Such substituent effects are mainly due to an

increase in kp / kt for monomers substituted with electron-withdrawing substituents in
para positions (Table 2.2). However, it does not exclude the possibility of a larger Keq (in

addition to larger kp) for electron withdrawing monomers due to the decrease in the bond

69

dissociation energy (BDE) of the C-X bond by electron-withdrawing substituents and

hence causes the additional enhancement of apparent polymerization rate coefficient with
. . . . . 16 . .
6 (Similar to the solution polymerization) . The energy graph in Scheme 2.7 provrdes a

qualitative picture of the substituent effects on polymerization rates in surface ATRP of

substituted styrenes.

Of the monomers studied, only 4—methoxystyrene failed to grow a significant
surface film. Similar observations were made by Matyzasjewski et al. 16 in their study of
the solution ATRP of 4-methoxystyrene, where they failed to obtain high molecular

weight polymer. They suggested that the active growing species has a cationic nature due

to the presence of strong electron-donating substituent. For this reason, we excluded 4-

methoxystyrene from the Hammett plot.

70

 

..... 10g (kx abs/[CH abs)
10g 1(kp app/kt app)X/(kp app/kt app)H1

____

 

   

 

0.5

04 _ ----y-_-o.55x-o.003 //
' R2=0.91 /
0.3 '

 

 

 

-01 . —y=0.51x-0.04
1?.2 =0.88
-O.2 '/
-0.3 ' ' m '
-0.4 -0.1 0.2 0.5 0.8
0

Figure 2.6: Hammett plots of (kp/kt) in surface-initiated ATRP of substituted
styrenes (O) and .for absolute kp values from conventional polymerizations (Cl) of

. 8
substituted styrenes.

71

 

 

 

4-0.423}, l : —CH2-CH
\ X(H) 1 \
I X X
X(donor) “—7)“— X(acceptor)
—.‘r—'"’ “ s
Energy BDE BDE
BDE ’
" H
x L—" —CH2-C-Bl’
. ’ H
x”, —CH2'C-Bl' I \
__iL_ ,\'
1 \ X(acceptor)
—CH2-C-Br /\,’( (H)
l \
\,
X(donor)
smaller Keq larger Keq

Scheme 2.7: Energy diagram for different substituents16

2.3.5 Synthesis of substituted styrene polymer brushes from SiOz substrates

This polymerization system was broadened to silica substrates as well. As
expected, similar polymerization trends were observed (i.e. monomers with electron
withdrawing substituents polymerize faster than those bearing electron donating
substituents). One interesting observation was that polymerizations initiated from gold

substrates were always slower than those from silica substrates, irrespective of monomer

. 6,27 .
studied (Figure 2.7). This observation, prevrously reported by Bao et al. Will be

explained in detail in Chapter 3.

72

 

500

 

 

 

I
400 ~
A I
E
5 300 -
til
o I
E
g 200
E O
i- I
O
O
100 - 0 A
o
8 A +
g t
O l l J l
0 2 4 6 8 10

Time (hour)
Figure 2.7: Evolution of the ellipsometric brush thickness with time for the
polymerization of 3,5-bis(trifluoromethyl)styrene (I), 4-tert-butylstyrene (O), styrene

(O), 4-methylstyrene (A),and 2-methoxystyrene (+) from silica surfaces.

73

2.4. Conclusions

Surface initiated atom transfer radical polymerization of substituted styrenes
provides uniform and thick brushes (>100 nm in 1 hour) from gold and silicon surfaces.
Styrenes substituted with electron withdrawing groups in meta and para positions
exhibited high growth rates. The substituent effects seen in surface and solution
polymerizations are similar for styrenes with electron withdrawing groups, and for
electron donors in ortho and para positions, but donors at meta sites have surprisingly fast
growth rates, possibly due to steric inhibition of termination. We also showed that the

surface polymerization rate varies with different substituents follow the Hammett relation

with p = 0.51. The ratio of kp to kt, acts as an indicator of the likelihood that a reaction

will reach high degrees of polymerization before termination.

74

 

 

2.5. References

 

3 Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, J.; Jeffery, J.; Le, T. P. T.; Mayadunne,
R. T. A.; Meijs, G. F.; Moad, C. L.; Moad, G.; Rizzardo, E.; Thang, S. H.
Macromolecules 1998, 31, 5559.

4 Couvreur, L.; Lefay, C.; Belleney, J.; Charleux, B.; Guerret, 0.; Magnet, S.
Macromolecules 2003, 36, 8260.

5 Grande, D.; Guerrero, R.; Gnanou, Y. J. Polym. Sci. Polym. Chem. 2005, 43, 519.
6 Bao, Z.; Bruening, M. L; Baker, G. L. J Am. Chem. Soc. 2006, 128, 9056.

7 Odian, G. Principles of Polymerization. 3rd ed.; John Wiley & Sons: New York, 1991.

8 Brandup, J .; Irnmmgut, E. H.; Grulke, E. A. Polymer Handbook. 4th ed.; John Wiley &
Sons: New York, 1999.

10 Pillow, J. N. G.; Halim, M; Lupton, J. M.; Burn, P. L.; Risch, N.; Meyer-Roscher, B.;
Langhals, M. Macromolecules 1999, 32, 5985.

ll Risch, N.; Meyer-Roscher, B.; Langhals, M. Z. Naturforsch., B: Chem. Sci. 1994, 49,
141.

12 Overberger, C. G.; Burns, C. M. J. Polym. Sc, Polym. Chem. 1969, 7, 333.
13 Kawamura, T.; Uryu, T; Matsuzaki, K. Makromol. Chem. 1982, 183, 125.
14 Imoto, M.; Kinoshita, M.; Nishigaki, M. Makromol. Chem. 1965, 86, 217.

15 Kazmaier, P. M.; Daimon, K.; Georges, M. K.; Hamer, G. K. Am. Chem. Soc, Polym.
Prepr. 1996, 3 7, 485.

16 Qiu, Jian ; Matyjaszewski, K. Macromolecules 1997, 30, 5643.
17 Okuma, K.; Sakai, 0.; Shioji, K. Bull. Chem. Soc. Jpn. 2003, 76, 1675.

18 Samadi, A.; Husson, S. M.; Liu, Y.; Luzinov, I.; Kilbey, M. Macromol. Rap. Comm.
2005, 26, 1829.

75

 

19 Coote, M. L.; Davis, T. P. Macromolecules 1999, 32, 4290.

20 Xiao, D. Q.; Wirth, M. J. Macromolecules 2002, 35, 2919.

21 Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165.

22 Watkins, A. L.; Hashiguchi, B. G.; Landis, C. R. Org. Lett. 2008, 10, 4553.
23 Yajima, Tomoko; Okada, Kyoko; Nagano, H. Tetrahedron, 2004, 60, 5683.

24 Galardon, E.; Le Maux, P.; Simonneaux, G. Tetrahedron, 2000, 56, 615.

25 Walling, C.; Briggs, E. R.; Wolfstim, K. 8.; Mayo, F. R. J. Am. Chem. Soc. 1948, 70,

1537.
26 Coote, M. L.; Davis, T. P. Macromolecules 1999, 32, 4290.

27 Bao, Z.; Baker, G. L.; Bruening, M. Macromolecules 2006, 128, 9056.

76

Chapter 3
Surface effects in surface-initiated ATRP

3.1. Introduction

Surface initiated polymerization is a powerful technique for preparing chemically

l,2,3,4,5,6

modified surfaces. Studies aimed at understanding the fundamentals of polymer

growth often use self-assembled monolayers (SAMs) as the initiator layer, because SAMs
provide a closely packed, well-ordered and stable configuration on the surface. Most

SAMs are generated on metal surfaces such as Au, Ag and Cu using thiol chemistry

while alkyl siloxanes anchored to hydroxyl-terminated surfaces, such as $0; A1203 and

other oxides via alkoxy and chlorosilanes. Surface initiated radical polymerization can be
carried out on a broad range of substrates, but most studies have focused on silicon and
gold substrates. Gold surfaces are chemically homogeneous, virtually free of
contamination, easy to clean, applicable to a wide variety of analytical techniques for thin

film characterization. A disadvantage of Au surfaces is that the Au-S bond that links the

9 9

. . . . . 78 . .
initiator layer to the surface 18 labile above 60 OC and hence polymerizations are

typically limited to <60 OC. However, at such temperatures, the low propagation rates for

radical polymerization of important aromatic monomers such as styrene and 4-vinyl

pyridine limit their growth. In addition, Bao et al. observed that film growth rates from

$102 substrates were higher than those grown from Au substrates, regardless of

10
monomer even at temperatures < 60 0C.

77

9a . . . . . . . .
Huang et al. investigated free-radical polymerization of styrene from azo-1n1t1ators

immobilized on flat Au substrates, and showed that free-radical polymerization from Au
is limited by the instability of alkanethiol monolayers. They reported that soluble free
radicals accelerated thiol desorption from Au, and the desorbed alkanethiols appeared to
be efficient chain—transfer reagents that terminate brush growth. To overcome monolayer
instability, they utilized a simple cross-linking procedure to enhance the stability of

SAMs, and made thermal radical polymerization from Au surfaces facile. Later Stephanie
et al.9b restricted thiol desorption at elevated temperatures by coating gold electrodes
with a layer of carboxylic acid functionalized polypyrrole, and in their work on one pot
thermal polymerization from gold nanoparticles, Matyjaszewski et al.“C used a

crosslinked polymer shell to prevent dissociation of linear brushes from gold
nanoparticles at elevated temperatures.

There are other factors that may lead to different polymerization rates on Au and

silica. One difference between Au and SiOz surfaces is the number and density of sites

available on the surface for immobilizing initiator. Studies show that silica surfaces have

a limiting area/chain of 0.6 nmz,“ roughly 40% of the density for a SAM on Au, while

. . 2 . .
typical values for silica range from 0.65 nm2 to 1.54 nmzlcharn. Bao et all investigated

the effect of initiator density on polymerization rates for Au and silica surfaces, and
observed that decreasing the initiator density on silica by 50% decreased the

polymerization rate, but for gold, the rate was nearly unchanged. Additionally, gold is a

78

3,14,15

transition metal capable of quenching radicals,1 which could reduce the number of

active chains, resulting in thin films.

This chapter describes a systematic evaluation of three factors that may be
responsible for the lower film grth rates from gold: 1) differences in the initiator
densities for gold and silica substrates, 2) termination resulting from desorption of thiols

from SAMs on gold surfaces, which is absent in silicon surfaces 3) and radical quenching

’

1 ,1 .
by the gold surface. 5 Based on the work of Bao et al., we suspected that desorption

of thiols from SAMs on gold surfaces at ambient temperatures, followed by chain transfer
of growing polymer chains to thiols was the likely cause of thinner film growth on Au
compared to silica surfaces. We tested this hypothesis by examining polymerizations
from a crosslinked initiator anchored on gold substrates, and by analysis of block
copolymer growth from Au and silica surfaces.
3.2. Experimental Section
3.2.]. Materials

Unless otherwise noted, all chemicals were obtained from Aldrich and stored
under nitrogen. ll-Mercapto-l-undecanol (MUD, 97%), 2-bromo—2-methylpropionyl

bromide (2-BIB, 97%), 3-mercaptoprOpyltrimethoxysilane (MPS), anisole (99.7%), N,N-
dimethylformamide (DMF, 99.8%), Cu(I)Br (99.999%), Cu(II)Br2 (99.999%),
Me4Cyclam (99%), and 4,4’-dinonyl-2,2’-bipyridine (anbe. 97%) were used as

received. (3-Aminopropyl)trimethoxysilane was distilled under vacuum prior to each use.
Triethylamine was distilled from calcium hydride under a nitrogen atmosphere, and

stored under nitrogen. tert-Butyl acrylate (tBA) (98%), MMA (99%) and styrene (99%)

79

were passed through a 10 cm column of activated basic alumina and then distilled from

calcium hydride to remove inhibitors. The disulfide initiator (ll-[(2-bromo-2-

methyl)propionyloxy]undecyldisulfide)7 [(Br-C(CH3)2-COO-(CH2)1[S)2], the trichloro-

silane initiator (11-[(2-bromo-2-methyl)propionyloxylundecyl trichlorosilane” and the
trimethoxysilane initiator [2—bromo—2-methyl-N—(3-trimethoxysilylpropyl)propionamide]
were synthesized using slightly modified versions of literature proceduresl6 Silane
compounds were used and stored under nitrogen. Toluene was distilled under nitrogen in
the presence of sodium/potassium alloy using benzophenone as an indicator. After
purification, the monomers, liquid chemicals and solvents were transferred to Schlenk
flasks, degassed by three freeze-pump-thaw cycles, and then transferred into a drybox.

Silicon wafers and Au-coated wafers (electron-beam evaporation of 200 nm of Au on 20

nm of Ti on Si (100) wafers, or 200 nm of Au sputter-coated on 20 nm of Cr on Si (100)

wafers) were cleaned in a UV/O3 chamber for 30 min prior use.

3.2.2. Characterization Methods
Ellipsometric measurements were obtained with rotating analyzer ellipsometer

(model M-44, J. A. Woollam) using WVASE32 software. The angle of incidence was

750 for all experiments. For the calculation of film thickness, a refractive index of 1.50

was used. Reflectance Fourier Transform Infrared (reflectance FTIR) spectroscopy was

performed using a Nicolet Magna—560 FTIR spectrometer containing a PIKE grazing

angle (80°) attachment. Spectra were typically collected with 128 scans using a MCT

detector. Changes in surface morphology of the polymer film during thermal degradation

80

- 1111111

on gold surface were observed using a Keyence Digital Microscope equipped with a
video camera.

3.2.3. Preparation of immobilized initiators on gold and silicon substrates

Gold—coated wafers were UV/O3-cleaned for 30 min, washed with water and
ethanol, and after transferred into a N2 glovebag, immersed in a 1 mM ethanolic solution

of the disulfide initiator [(Br-C(CH3)2-COO(CH2)11S)2] for 24 h to form the self-

assembled initiator monolayer. The films were then washed with ethanol and dried under

a stream of N2. A crosslinked initiator monolayer was formed by immersing Au-coated

substrates in a vial containing a 2 mM methanolic solution of MPS for 12 h in a glovebag
at room temperature. After deposition, the substrate was rinsed three times with 2 mL of
methanol and dried with nitrogen. The attached silane monolayer was then hydrolyzed at
room temperature with 0.1 M HCl for 15 h to afford a hydroxylated surface. The
modified Au substrate was then treated with a 10 mM solution of trimethoxysilane

initiator [2—bromo-2—methyl-N—(3-trimethoxysilylpropyl)propionamide] in toluene at
55 0C for 12 h under nitrogen. Following the deposition, the substrates were rinsed

repeatedly with toluene and iSOpropanol, and then dried in a stream of nitrogen. The

ellipsometric thickness of the crosslinked initiator layer on gold was 10-15 A.

The trimethoxysilane initiator was immobilized on UV/O3-cleaned silicon wafers

by immersing the substrate in a 10 mM solution of [2-bromo-2-‘methyl-N-(3-

trimethoxysilylpropyl)propionamide] in toluene at 55 0C under nitrogen. After 12 h, the

wafer was rinsed repeatedly with toluene and isopropanol, sonicated in toluene for 1 min

81

and dried in a stream of nitrogen. The ellipsometric thickness of the crosslinked initiator
layer on silicon surface was 10-15 A.

3.2.4. Surface initiated polymerization
In a NZ-filled dry box, CuBr (5.7 mg, 0.04 mmol), CuBr; (4.5 mg, 0.02 mmol),
Me4Cyclam (10.3 mg, 0.04 mmol) and anbpy (16.4 mg, 0.04 mmol) were added to a

round-bottom flask containing 20 mL of a degassed solution of monomer in DMF/anisole

(monomer/DMF/anisole ) 2:1 :1 v:v:v, [monomer]~4.0 M). The mixture was well-stirred

and heated with an oil bath to 50 0C until a transparent, light green solution formed. The

prepared solution was then transferred into a small vial containing an initiator-modified

Au / SiOz substrate to start the surface-initiated polymerization. After a predetermined
reaction time at 50 CC, the substrate was removed from the vial, washed with ethyl

acetate and THF sequentially, and then dried under a stream of N2 in the dry box.

3.3. Results and discussion
3.3.1. Surface initiated polymerization from gold and S102 substrates

Bao et al. had reported that “identical” polymerizations of MMA from Au and

Si02 surfaces gave different polymer film thicknesses, with polymer films grown from

- . . 12
S102 systematically thicker than those grown from Au substrates. To confirm Bao’s

observation, we polymerized MMA and styrene under identical conditions from initiators

anchored on Au and SiOz surfaces (Scheme 3.1), and compared the evolution of the film

thickness with time. Figures 3.1a and 3.1b show the growth of PMMA and polystyrene

82

brushes from flat Au and S102 surfaces. Irrespective of the monomer, Si02 surfaces

yield thicker films and higher apparent polymerization rates than those grown from Au

surfaces under similar conditions. However, the film thickness vs. time profiles are

similar, suggesting that the primary difference between growth from gold and growth

from silica is fewer active chains when brushes are grown from gold surfaces. These data

confirm the observations of Huang et al. and Bao et al., that polymer films grown on Au

substrates are systematically thinner than those polymerize on S102.

 

Au

 

 

 

 

Si

 

b—OH

 

 

P‘-S'-(C1"'12)""O Bf
11

MMA
anisole / DMF

CuBr / Me4Cyclam

CuBr2(anbpy)2
—-———————->

O

55 °C

 

O
MeQ
”0H MeO-lSi-(Csz—N‘ Br
MeO 3 H
Toluene

Overnight , 55 °C

 

31 o o
L"O"Si“((:1‘12)"‘i‘{ i C
I 3 H

O
\

 

 

 

/ O
0. 9H3
”‘O'Si‘WCHZl’N. CH2-C 1—Br
\ 3 H n

 

0
CH3
Au -—s-—- CH -—0 -'
( 2) CH2 C Br
11 A "

O OMe

 

 

 

 

Si

-O-’Si~(CH2)-N\ Br
0 3 H
\

 

 

 

MMA, anisole / DMF
CuBrl Me4Cyclam
CuBr2(anbpy)2

55 °C

A

0 OMe
CH3

HzréT’Br
A n

0 OMe

Scheme 3.1: Surface initiated polymerization of MMA from silicon and gold surfaces

83

 

 

 

 

300
250 ~ 1 i
g 2..- i
(D
8 1
5 150 ~
.2 i
‘5 r
100 — f
' i
50 ~
D
O F I L l 1
0 20 40 60 80 100

Time (min)
Figure 3.13: Evolution of the ellipsometric brush thickness with time for the

polymerization of MMA (methyl methacrylate) from 0 gold and I silicon surfaces at

50 OC. The points are the average of data from two independent runs, and the limits of

the error bars are the measured film thicknesses from the two runs.

84

200

 

 

 

 

150 -
75‘ i
5 i
8
1:100 - i
x
.2 i f
if 5 i

I
50 - § 5
o I l 1 l
0 2 4 6 8 10

Time (hour)
Figure 3.1b: Evolution of the ellipsometric brush thickness with time for the
polymerization of styrene from 0 gold and I silicon surfaces at 50 oC. The points are

the average of data from two independent runs, and the limits of the error bars are the

measured film thicknesses from the two runs.

3.3.2. Block copolymer formation on Au and SiOz substrates
It would be reasonable to expect that the different behaviors seen for films grown

from Au and SiOz substrates would diminish when the growing polymer chains are

distant from the substrate. This hypothesis can be tested by placing the initiator at
increasing distances from the Au surface. One way to form an extended initiator is to use

ATRP to grow polymers from the surface, and then quench the growing chain by adding

85

a large excess of Cu(II)Br2 as shown in Scheme 3.2. The resulting Br-terminated chains

monomer, it was dried under a stream of N2, removed from the dry box, and

characterized by ellipsometry and F TIR spectroscopy. We then returned the substrates to

the drybox and initiated polymerization of MMA for 1 hour from the dormant PtBA

chains. The same process was carried out on a Si02 substrate, which served as a control.

Net growth of PMMA
in 1 hour

   

Br Br Br
Br 3" BrBr tBA/98131361 MMA leatalyst
.. .. SO'Ut'O” lg solution , 1 hour
Initiator attached on gold PtBA brush on gold Block copolymer on surface
or Silicon surface or Silicon surface

(PtBA = Poly(tertbutyl acrylate))

Scheme 3.2: Block copolymer formation on Au and Si02 surfaces

Figure 3.2 shows F TIR spectra of a 97 nm PtBA film and the PtBA-block—PMMA
copolymer after growing 210 nm of PMMA from the initial PtBA film. The spectrum for
the block copolymer is as expected; the intensity of the ester carbonyl increased, and the

relative intensity of the t-butyl doublet decreased compared to the C-H rocking bands,

(135045006114).

86

 

 

 

 

 

0.05

 

Absorbance
’3‘

 

 

 

 

 

 

 

3500 3000 2500 2000 1500 1000
Wavenumbers (cm'1)

 

 

Figure 3.2: Reflectance FTIR spectra of gold substrates coated with (a) 100 nm PtBA
brushes grown from the initiator layer; (b) a PtBA-block-PMMA copolymer brush,

synthesized by growing 210 nm of PMMA from a 100 nm PtBA film.

Figures 3.3a and 3.3b show that PMMA blocks grown for 1 hr from PtBA brushes

were ~200 nm thick, 2-3 times thicker than PMMA films grown directly from gold
surfaces, but comparable to PMMA blocks grown directly from SiOz. These results

suggest that a substantial fraction of the PtBA chains were active after polymerization of

the initial PtBA block, and directly confirm the role of the Au surface in reducing the

87

film thickness when polymers are grown from Au surfaces. For Au substrates, the
thickness of the PMMA layer grown from PtBA films in 1 hour increased with the PtBA
thickness, saturated at ~ 200 nm, and then slowly declined. The decline likely reflects

fewer dormant PtBA chains available to form the PtBA-block-PMMA copolymer. Since

the same effect is not seen for SiOz substrates, the decline can be identified with higher

termination rates for the growth of PtBA on Au, presumably by radical quenching by
electron transfer, or by the loss of growing chains via desorption of thiols from Au
surfaces. Desorbed thiol may act as a chain transfer agent - for polymer brush syntheses,
chain transfer is equivalent to termination. An alternative desorption scenario is that
polymers anchored through Au-S bonds may be mechanically unstable, especially for

highly solvated polymers of high molecular weight.

88

700

600

500

.p.
O
0

Thickness (nm)
(to
O
O

200

100

 

 

 

 

A
A
A
A
.-9-._.

A?" ""0.
e, C1 C1 ."'9

1:1 . o
E] 0
Lo .
O 20 40 60

Time(min)

80

Figure 3.3a: Surface polymerization study of the formation of PtBA-block-PMMA films

on gold surfaces. 1:] PtBA brushes grown from initiators anchored on gold for various

times; A Total film thickness after 1 hour growth of PMMA from PtBA brushes grown

on gold; 0 Net growth of PMMA from PtBA (total film thickness — PtBA brush

thickness) after 1 hour of polymerization (The broken line drawn as a guide to the eye)

0 PMMA grown from initiators anchored on gold for various times, shown for

comparison.

89

 

700

 

 

 

‘ A
600 - ‘
’E‘ 500 -
5 E]
El
fl)
3 400 ~ ‘ E]
C
{a
T: 300 ~
I-
.-""o'-._
2001:” “-0. ....... 2
C] O
100 -
O
O E 1 I l
o 20 . 40 60 80
Time(min)

Figure 3.3b: Surface polymerization study of the formation of PtBA-block-PMMA films

on SiOz surfaces. D PtBA brushes grown from initiators anchored on SiOz surfaces for
various times; A Total film thickness after 1 hour growth of PMMA from PtBA brushes

grown on $0; 0 Net growth of PMMA from PtBA (total film thickness — PtBA brush

thickness) after 1 hour of polymerization (The broken line drawn as a guide to the eye)

9 a PMMA brush grown from initiators anchored on Si02 for various times, shown for

comparison.

90

3.3.3. Formation and polymerization from a cross linked initiator
The previous data do not distinguish between radical quenching and thiol

desorption as the primary factor leading to thinner films growing on Au surfaces

compared to 8102. However, preventing desorption of thiols from the Au surface should

test the validity of the thiol desorption mechanism. To that end, we formed cross—linked

initiators on gold surfaces, similar to those of Holzinger et al., 17 and compared
polymerizations from cross-linked initiator layers to substrates with initiators based on
standard thiols. Scheme 3.3 shows the formation of a cross—linked initiator layer on a
gold surface. The thicknesses of the cross-linked and standard thiol initiator layers were
comparable, and therefore, radical quenching by the gold surface should be comparable
for both initiator layers. Immersion of Au-coated slides in a 2 mM solution of MPS in
methanol formed an MPS monolayer. During hydrolysis, the trimethoxysilane group at
the MPS terminus condensed to form a dense poly(siloxane) network. The siloxane layer
provides lateral stabilization through inter-chain cross-linking and generated a

hydroxylated surface for the subsequent attachment of the trimethoxysilane-ATRP

initiator.

91

Au

 

 

 

 

 

 

 

 

 

 

 

 

 

\
i ' OCH Q
/\/\OFH3 SMSi-08H3SM31'OH
OCH3 AU OCH33 _____3___.. Au E)
OCH overni ht, rt
Methanol flSMSi- OC3H3 g SMSl'OH
overnight,rt J OCH3 /O
V O
MeQ M
MeO- $i- (CH2)N Br
MeO 3H
\ / O Toluene
O O M . o
~S Si-O-Si— CH -N Br overnIght. 55 C
\/\/ ( 2) .
\ 3 H
Au 0 o o
“SWSj-O-Si-(CH2)-N Br
0 3 H
/ \
MMA. anisole / DMF
55 Cl CuBr/Me4Cyclam
CuBr2(anbpy)2
\ / O
O Q
—S\/\/S\i0 Si (CHz)3N.H CH2- (ST—Br
P'S\/\.Si O- Si- (CH)
23“ CH C Br
‘0 o 2 0:3:—
’ \ Aoue

 

 

 

Scheme 3.3: Formation of cross linked initiators on gold surface

We used FTIR spectroscopy and ellipsometry to follow the attachment of the

ATRP initiator to MPS. The reflectance FTIR spectrum of the MPS layer on Au (not

shown here) shows vibrational bands characteristic of MPS (2938 cm.1 for overlapping

CH3 and CH2 bands, 2846 cm-1 for the CH2 symmetric stretch, and 1114 cm.1 for the

Si-O-C stretch). After hydrolysis, the methyl peaks disappeared and the peak at 1114

-1 . .
cm greatly decreased, indicating nearly complete hydrolysrs of the MPS layer. The

ellipsometric thickness of the hydrolyzed MPS was 1.0 nm. The above results are in good

agreement with literature data.

2 c . .
“’18’19’ 0 After attachment of the ATRP Initiator, the film

92

thickness increased to 2.2 nm and amide peaks (1652 and 1548 cm-1) appeared in the

reflectance F TIR spectrum confirming successful attachment of the ATRP-initiator.
Figures 3.4 and 3.5 show data for the surface polymerization of MMA and styrene
from gold and silicon surfaces with the crosslinked initiator. For both MMA and styrene,

films grown from the crosslinked initiator on gold had thicknesses comparable to those

grown on SiOz surfaces. Additionally, the data in Figure 3.5 show increases in the

characteristic IR peaks of the respective polymers consistent with the ellipsometry data.
Therefore, stabilizing the initiator layer via cross-linking efficiently restricts thiol
desorption, and there is no evidence for radical quenching by gold.

To explore the stability of the cross-linked thiol initiator, we ran polymerizations

from room temperature to 115 0C using standard initiators immobilized on Au and Si02

surfaces. The data (Figure 3.6) show that thinner films are formed on Au substrates,

which suggests significant thiol desorption even at low temperatures. In contrast, the

polymerization data for the cross-linked initiator track the data for films grown on SiOz.

Furthermore, the crosslinked initiator enables polymerization of MMA and styrene from

Au surfaces up to ~100 oC, providing 300 nm and 120 nm films grown in 1hr,
respectively. The apparent temperature limit for growth from Au surfaces is ~110 OC.
Inspection of a polystyrene film grown at 115 OC showed large-scale delamination of the
polymer films (Figure 3.7), which is absent in films grown at 90 0C. Kim et al. made

similar observations for poly(hydroxyethyl methacrylate) brushes grown on Au.21

93

 

 

 

 

350
300 -
A250 a f %
E
5
3 200 r
3 i
'5 150 -
E I
" I
100 - §
§
50 » I!
¢
0 I L _L l
O 20 40 60 80 100
Time (min)

Figure 3.43: Surface initiated polymerization of MMA from gold and Si02 surfaces.

0 PMMA grown from a standard (non-crosslinked) initiator on gold; 9 PMMA grown

from a crosslinked thio initiator; Cl PMMA grown from Si02_. The points are the

average of data from three independent runs, and the limits of the error bars are the

measured film thicknesses from the three runs.

94

200

 

 

 

 

150 -
E
5
m ¥
8100 ~
.5 i
2 i r
l- ? §
50 ~ Q 2
0 1 J l l
o 2 4 6 8 10

Time (hour)
Figure 3.4b: Surface initiated polymerization of styrene from gold and SiOz surfaces.

0 Polystyrene grown from a non-crosslinked thio-initiator on gold 0 Polystyrene grown

from a crosslinked thio initiator D Polystyrene grown from Si02_The points are the

average of data from three independent runs, and the limits of the error bars are the

measured film thicknesses from the three runs.

95

 

 

 

 

 

 

 

 

0.05
(a)
23
8
3
(b)
.J’k J

 

3500 3000 2500 2000 1 500 1000
Wavenumbers (cm")

Figure 3.53: Reflectance FTIR spectra of gold substrates with (a) PMMA brushes grown
from a non-crosslinked thio initiator in 1 hour (b) PMMA brushes grown from a

crosslinked thio-initiator in 1 hour

96

 

0.05

Absorbance

 

 

 

 

 

l l l I

3500 3000 2500 2000 1500 1000
Wavenumbers (cm")

Figure 3.5b: Reflectance FTIR spectra of gold substrates with (c) PS grown from a

crosslinked thioinitiator in 8 hours ((1) PS grown from a crosslinked thioinitiator in 8

hours

97

400

 

350 -

II I

 

 

 

Ezso- ¥
3
«woo-é i
C
X
0
3150-
l-

100 - § i i r

i r r

50~

O l l

30 55 80 105

Temperature (°C)

Figure 3.63: Temperature-dependent surface-initiated polymerization of MMA from
various initiators; <> PIVHVIA grown from standard thioinitiators anchored to An surfaces;

0 PMMA grown from the crosslinked thioinitiator shown in Scheme 3.3; and Cl PMMA

grown from SiOz as shown in Scheme 3.3. Each data point indicates the film thickness

after 1 hour of growth. The points are the average of data from two independent runs, and

the limits of the error bars are the measured film thicknesses from the two runs. All

polymerizations were run for 1 hour.

98

160

 

140 2

, I

—L
O
O
r
FOB-i
PB"

Thickness (nm)
CD
0

 

 

 

60 ' 6 §
9 Q 5 §
40 - 8
9 o
20 - °
0 l l l
30 55 80 105 130

Temperature (°C)

Figure 3.6b: Temperature-dependent surface-initiated polymerization of styrene from
various initiators; O polystyrene grown from standard thioinitiators anchored to Au

surfaces; 9 polystyrene grown from the crosslinked thioinitiator shown in Scheme 3.3;

and Cl polystyrene grown from SiOz as shown in Scheme 3.3. Each data point indicates

the film thickness after 1 hour of growth. The points are the average of data from two

independent runs, and the limits of the error bars are the measured film thicknesses from

the two runs. All polymerizations were run for 1 hour.

99

(a) (b)

 
 
 

  
  

Delaminated
polymer

 

1mm

Figure 3.7: Optical micrographs of polystyrene brushes grown from crosslinked

initiators immobilized on gold surfaces: (a) and (b) show a 70 nm thick film grown for 1

hour at 115 oC.

The use of cross-linked initiators to improve polymer film grth fi'om Au
surfaces can be generalized to other monomers such as poly(vinyl pyridine) (PVP).

Despite its potential utility as polyelectrolyte brush, we are unaware of examples of thick

12b .
PVP brushes grown from Au surfaces (20 nm in 2 hours by Bao et al. and 6 nm 1n 5

hours. by Husson et al.22). However, Riihe 23 grew 430 nm thick PVP films from
surface-anchored azo initiators on SiOz in 14 hours. Growing PVP from cross-linked

initiators should provide films on Au with thicknesses that approach those of Riihe.

Polymerization of vinyl pyridine for 8 hours at 50 oC provided 200 nm films from both

Au and Si02 surfaces (Figure 3.8), approximately 10X thicker than previous examples.

The increase in IR intensities, shown in Figure 3.9, is consistent with the ellipsometric

data.

100

250

 

 

 

 

O
200 - C]
5 U
E
5150 ~
(0
(D
o
I:
x
.2 100 —
i5
6
50 -
<> 0 °
0
O l l l l
O 2 4 6 8 10

Time (hours)
Figure 3.8: Surface-initiated polymerization of vinyl pyridine from various initiators at

50 C)C; O poly(vinyl pyridine) grown from a non-crosslinked thio-initiator on gold
9 poly(vinyl pyridine) grown from a crosslinked thio initiator on gold Cl poly(vinyl

pyridine) grown from a SiOz surface

101

 

0.05

 

 

2
2 (a)
2 ML

(b)

 

 

l P I l

3500 3000 2500 2000 1 500 1000
Wavenumbers (cm")

 

 

Figure 3.9: Reflectance FTIR spectra of poly(vinyl pyridine) brushes grown for 8 hours

from gold surfaces (a) using a crosslinked thioinitiator and (b) a standard thioinitiator.

102

growing chain terminated chain
/

non- growing 5
/ \) chain
S E’) ' S .__.._____+ S
I -1” Thermal desorption l Radical induced |

termination

Radical induced
desorption

 

 

 

Effect of free thiol in solution - an analogy

Mo
_.—_._.__.___.>

Chain Transfer Polymer growth

[:ASC] :35:

Figure 3.10. Proposed pathways for terminating surface-bound radicals on Au:
(top) thermal or radical-induced desorption of surface thiol radicals terminate growing
chains, (bottom left) radical termination by reaction with a thiol (analogous to chain-

transfer), which competes with propagation (bottom right).

Our experiments using cross-linked initiators indicate thiol desorption as the main
limitation to growing thick polymer brushes on Au. However. the desorption mechanisms
are unknown. Figure 3.10 depicts some pathways that lead to termination of surface-

bound radicals on Au surfaces. The thermal instability of thiol SAMs on Au may not be

important for low temperature polymerizations (< 60 0C). but likely more important at

high temperatures. Radical—induced desorption of thiol SAMs from growing chains can

103

be important over a broad temperature range. The copper catalyst also may contribute to
thiol desorption. We have observed chain desorption (reduction of the carbonyl peak
height in the FTIR spectra of PMMA brushes) at high copper concentrations. Though the
copper concentration used here is small, the effect may not be negligible since partial
desorption of initiator-containing monolayers from the Au surface would result in a
decreased surface initiator concentration.

3.4. Conclusions

Polymerizations initiated from Au surfaces generally provide thinner polymer

brushes than comparable polymerizations initiated from SiOz surfaces from room

temperature to 100 0C. This disparity can be eliminated by forming a crosslinked

poly(siloxane) primer layer on gold surfaces, which prevents thiol desorption. Using the

crosslinked initiator, polymerizations fi'om Au provide film thicknesses comparable to

polymerizations initiated from SiOz. These results implicate thiols in terminating

growing polymer brushes, and polymer chains with low molecular weights and low film
thickness. These results also exclude radical quenching by gold or the difference in

initiator density between gold and silicon as significant factors. Crosslinked initiators

extend the temperature range for polymerizations initiated from Au surfaces to 100 C)C,

enabling rapid polymerization of monomer less active than methacrylates, such as styrene

and vinyl pyridine. Above 100 0C, we observed macroscopic delamination of polymer

films grown on Au surfaces.

104

References:

 

l Milner, S. T. Science 1991, 251, 905.
2 Huang, X.; Doneski, L. J .; Wirth, M. J. Anal. Chem. 1998, 70, 4023.

3 Bruening, M. L.; Zhou, Y.; Aguilar, G.; Agee, R.; Bergbreiter, D. E.; Crooks, R. M.
Langmuir 1997, 13, 770.

4 Sidorenko, A.; Minko, S.; Schenk-Meuser, K.; Duschner, H.; Stamm, M. Langmuir
1999, 15, 8349.

5 Prucker, 0.; Naumann, C. A.; Riihe, J.; Knoll, W.; Frank, C. W. J. Am. Chem. Soc.
1999, 121, 8766.

6 Edmondson, S.; Osborne, V. L.; Huck, W. T. S. Chem. Soc. Rev. 2004, 33, 14.

7 Ulman, A. Chem. Rev. 1996, 96, 1533.

8 Schlenoff, J. 8.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528.

9 (a) Huang, W.; G, Skanth; Baker, G. L.; Bruening, M. Langmuir 2001, 17, 1731.
(b) Fan, X.; Xia, C.; Fulghum, T.; Park, M.-K.; Locklin, J.; Advincula, R. C. Langmuir
2003, 19, 916. (c) Dong, H.; Zhu, M.; Yoon, J. A.; Gao, H.; Jin, R.; Matyjaszewski, K. J.
Am. Chem. Soc. 2008, 130, 12852. (d) Shan, J .; Nuopponen, M.; Jiang, Hua; K., Esko; T.,
Heikki Macromolecules 2003, 36, 4526.

10 Bao, Z. “Surface-Initiated Polymerizations on Initiator Anchored Substrates:
Synthesis and Characterization of Nanometer Thick Functional Polymer Films.” Ph.D.,
Michigan State University, East Lansing, MI, 2006.

11(a) Fan, X.; Xia, C.; Fulghum, T.; Park, M.-K.; Locklin, J .; Advincula, R. C. Langmuir
2003, I9, 916. (b) Kawai, T.; Saito, K.; Lee, W. J. Chromatogr. B 2003, 790, 131.

12 (a) Bao, Z.; Bruening, M. L.; Baker, G. L. Macromolecules 2006, 39, 5251. (b) Bao,
Z.; Bruening, M. L; Baker, G. L. J. Am. Chem. Soc. 2006, 128, 9056.

13 Henglein, A.; Lilie, J. J. Am. Chem. Soc. 1981, 103, 1059.
14 Sakata, Y.; Imahori, H. J. Phys. Chem. B 2000, 104, 1253.
15 Ganesh, V.; Lakshminarayanan, V. J. Phys. Chem. B 2005, 109, 16372.

16 (a)Yoon, K. R.; Chi, Y. S.; Lee, K.; Lee, Jungkyu K.; Kim, D. J.; Koh, Y.; Joo, S.;
Yun, W. s.; Choi, r. s. J. Mater. Chem. 2003, 13, 2910. (b) Mulvihill, M. J.; Rupert, B,

105

 

L.; He, R.; Hochbaum, A.; Arnold, J .; Yang, P. J. Am. Chem. Soc. 2005, 127, 16040.
17 Holzinger, D.; Liz-Marzan, L. M.; Kickelbick, G. J. Nanosc. Nanotech. 2006, 6, 445.
18 Yan, Y.; Bein, T. J. Phys. Chem. 1992, 96, 9387.

19 Mihailova, B.; Engstrom, V.; Hedlund, J .; Holmgren, A.; Sterte, J. J. Mater. Chem.
1999, 9, 1507.

20 Mintova, S.; Schoeman, B.; Valtchev, V.; Sterte, J .; Mo, S. Y.; Bein, T. Adv. Mater.
1997, 9, 585.

21 Kim, J. 8.; Huang, W. X.; Wang, C. L.; Bruening, M. L.; Baker, G. L. “Bottle Brush
Brushes: Ring-Opening Polymerization of Lactide from Poly(hya’roxyethyl methacrylate)
Surfaces” in Polymer Brushes, Advincula, R. C.; Brittain, W. J .; Caster, K.C.; Riihe, J .,
Eds, Wiley-VCH, Weinheim, Germany, pp. 105-117 (2004).

22 Li, X.; Wei, X. L.; Husson, S. M. Biomacromolecules 2004, 5, 869.

23 Biesalski, M.; Riihe, J. Langmuir 2000, 16, 1943.

106

Chapter 4

Surface Tethered Conducting Polymers via a Grafting-from Approach

4.1. Introduction

Over the past several decades, n-conjugated organic polymer materials have been

. . . . . la . 1b
extensrvely investigated as the active element In sensors, optoelectronic and

. . . 2 . .
semiconducting devrces, and as electroluminescent, photoconductrng, electron-
. . . . 3 .
transporting, hole-transporting, and ron-dopable materials. One of the most wrdely used
materials is poly(thiophene), which has attractive properties such as high electrical

. . . . 4 .
conductrvrty, electrochromrsm and electrolumrnescence. Poly(thlophene) and related

polymers are synthesized by two general methods, electro-polymerization directly from

electrode surfaces, or step-growth methods such as chemical coupling of di-bromo

monomers, or chemical oxidation by FeCl3 and other oxidants.

Electropolymerization conveniently deposits electroactive polymers directly onto
sensor substrates, and permits control over the coating thickness, since the
polymerization proceeds by application of a precisely controlled potential. In spite of

these advantages, poor interfacial adhesion between the conducting polymer and the

5,6,7,8,9,

. . 10 . . . .
electrode rs a major problem. One strategy for Improvmg adhesron rs to form

conducting polymers from polymerizable precursors preadsorbed on a metallic substrate

. 5-7 . .
(gold, nickel, or platinum) via thiol functional groups. An altematrve approach rs to

graft conducting polymers onto metal oxide surfaces, such as indium tin oxide (ITO),9

107

where the key step is-the chemisorption of a preformed conducting polymer or its

precursor.
Recently, conjugated polymer network films were formed on conducting surfaces by
the synthesis of precursor polymers from monomers with pendant electroactive units and

then elaborating the pendant unit by electropolymerization or chemical oxidation.

ll,12,13,14,15,16,17 . . . . .
The resulting polymer films are rntrrnsrc conducting polymer

networks with having both inter- and intramolecular cross-links between the pendant
monomer units.
Surface-initiated ATRP is attractive for modifying surfaces since surface properties

are easily modified by varying the composition of the polymer brush, grafting density,

the degree of polymerization18 and most importantly, delamination of polymer layer
from surface can be eliminated. Recently, Carter et al. reported the successful grafting of

disubstituted polyacetylene brushes grown from modified silicon and quartz surfaces

. . . . . . 19 .
usrng a transition-metal—catalyzed polymerization technrque. Huck and Friend used

surface-initiated ATRP to produce tethered poly(triarylamine acrylate) hole transport

materials for use in photovoltaic devices.20 Compared to devices prepared by solution-
casting methods, devices prepared from tethered polymers showed enhanced conductivity,
which was attributed to control over polymer architecture and morphology. Advincula et
al. grew poly(vinyl carbazole) brushes from ITO by surface initiated free radical

polymerization, controlling the graft density and brush length. In a second step, they

cross-linked the poly(vinyl carbazole) brush electrochemically.21

108

Herein, we report the growth of poly(terthiophene methacrylate) from gold and ITO
substrates using ATRP. Electropolymerization of the pendant terthiophene groups forms
conducting cross-linked polymer network films of oligo— and polythiophene.

4.2. Experimental section
4.2.1. Materials

Unless otherwise noted, all chemicals were obtained from Aldrich. ll-Mercapto-
l-undecanol (MUD, 97%), 2-bromopropionyl bromide (2-BPB) (97%), 3-
thiophenemethanol, thiophene-3-carboxaldehyde, tetrakis(triphenylphosphine) palladium,
thiophene-2-boronic acid, methacryloyl chloride (98%), 3,4-ethy1enedioxythiophene
(EDOT, 97%), NJV-dimethylformamide (DMF, 99.8%), dimethoxyethane (98%), NaBH4,
Cu(I)C1 (99.999%), Cu(I)Br (99.999%) and hexmethyltriethylenetetraamine (HMTETA,
99%) were used as received. Triethylamine and acetonitrile were distilled from calcium

hydride under reduced pressure and under an inert atmosphere. Tetraethylammonium

perchlorate (TEAP) was dried under vacuum at 80 0C for 12 hours and stored in glove

bag. ITO (indium tin oxide) coated glass slides were obtained from Delta technologies.

4.2.2. Characterization Methods

1 l3 . .
H and C NMR analyses were camed out at room temperature on a Varran

UnityPlus-SOO spectrometer at 500 and 300 MHz, respectively, with the chemical shifts
reported in ppm and referenced to signals from residual protons in the solvent. Film

thicknesses were measured using a rotating analyzer ellipsometer (model M-44, J. A.

Woollam) at an incident angle of 75°. The data were analyzed using WVASE32 software,

and thickness and refractive index determinations were performed on at least three spots

109

on each substrate. The refractive index of the films was assumed to be 1.5 and then fitted

with the film thickness. Reflectance FTIR spectroscopy was performed using a Nicolet
Magna-IR 560 spectrometer containing a PIKE grazing angle (80°) attachment. UV—vis
measurements were taken on an Agilent technologies 8453 spectrometer. Atomic force
microscopy (AFM) images were obtained in tapping mode with Multimode AFM and
NanoScope IV software (Digital Instruments, Santa Barbara, CA) at room temperature. A
tapping mode probe (NSClS) with a nominal frequency of 300 kHz was used for all
experiments.
4.2.3. Synthesis of 3—methylthienyl methacrylate (MTM)

3-Thiophenemethanol (5.7 g, 50 mmol), dry triethylamine (7.3 g, 71 mmol), and

CuCl 25 mg were dissolved in 35 mL of dry diethyl ether. A solution of methacryloyl

chloride (5.35 g, 51 mmol) in 35 mL of dry diethyl ether was added slowly at O 0C. The

mixture was stirred for 2 h, and then filtered through a silica gel column to remove
triethylammonium chloride. After solvent evaporation, the residue was stirred overnight

in a 1:1 mixture of methylene chloride and 2 M NaOH. The organic layer was separated,

washed twice with water, and dried over CaClz. After solvent evaporation, the residue
was distilled under vacuum (Vigreux column). bp (1 mmHg): 90 OC. Yield: 70%. 1H
NMR (CDC13): 8 1.9 (s, CH3), 5.1 (s, 0CH2), 5.5 (5, vinyl H), 6.1 (s, vinyl H), 7.06 (m,

ring H), 7.2 (m, ring H).

110

4.2.4. Synthesis of [2, 2':5', 2"-terthiophen]-3’-ylmethyl methacrylate (TTMM)

4.2.4a. Synthesis of 2,5-dibromoformyl-3-thiophene (1): A solution of Brz

(0.562 mL, 20 mmol) in anhydrous CHC13 was added drop wise and under vigorous

stirring, to a solution of thiophene-3—carboxaldehyde (0.500 mL, 5.5 mmol) in anhydrous

CHC13 (1.5 mL) in a 10 mL flask, at 60 CC under inert atmosphere. The reaction mixture

was refluxed for 5 h, and brought to room temperature and then poured into ~10 mL of

water at 0 oC. The organic phase was neutralized with a NazCO3 saturated solution,

dried with MgSO4, filtered, and evaporated to dryness. The residue was chromatographed

on flash silica gel 60 using 90/ 10 hexanes/ethyl acetate as eluant to give a yellowish

orange solid. Crystallization from heptanes affords pale yellowish needlelike crystal in
40% yield. 1H NMR (CDC13): 5 = 9.80 (s, 1H, CHO), 7.34 (s, 1H, H4).

4.2.4b. Synthesis of 3'-formyl-2,2':5',2"-terthiophene (2): A 50 mL 3-neck
flask, equipped with condenser, magnetic stirrer and N2 inlet, was charged with 1,
(0.2086 g, 0.77 mmol), tetrakis(triphenylphosphine) palladium(0) (0.0531g, 0.046 mmol)
and 8 mL of l,2-dimethoxyethane. After stirring for 10 min at room temperature,

thiophene-Z-boronic acid (0.2362 g, 1.84 mmol) was added, and followed immediately

by 5 mL of aqueous 1M NaHCO3 solution. The reaction mixture was refluxed for 4 h
under nitrogen. After cooling to room temperature, the mixture was filtered and the
organic solvent was evaporated under reduced pressure. After removal of the solvent, 10

mL of water were added to the residue, and the mixture was extracted with diethyl ether

(3X50 mL). The combined organic phases were washed with water and with saturated

111

NaCl solution, and dried over MgSO4. After filtration and solvent evaporation, the crude

product was chromatographed on flash silica gel 60, using 90/10 hexanes/ethyl acetate as

the eluant. Removal of the solvent yielded greenish-yellow powdery solid. Yield 75%,
1H NMR (CDC13): 8 10.09 (s, 1H, CHO), 7.57 (s, 1H, H4'), 7.51 (dd, 1H,J== 5.1 Hz,J=
1.2 Hz, H5), 7.32 (dd,1H, J= 3.6 Hz, J=1.2 Hz, H3), 7.30 (dd, 1H, J: 5.1Hz, J=1.2

Hz, H5" ), 7.23 (dd, 1H, J= 3.6 Hz, J: 1.2 Hz,H3"),7.17 (dd,1H,J= 5.1 Hz, J = 3.6
Hz, H4), 7.05 (dd, 1H, J= 5.1 Hz, J== 3.6 Hz, H4”).

4.2.4c. Synthesis of 3'-hydroxymethyl—2,2'5',2"-terthiophene (3): Compound 3
was synthesized by the procedure of Zanardi et al.23in 95% yield. 1H NMR(CDC13): 8
7.33 (dd, 1H, J= 5.1 Hz, J: 1.0 Hz, H5), 7.23 (s, 1H, H4' ), 7.22 (dd, 1H, J= 5.1 Hz, J
=1.0 Hz, H5H ), 7.20 (dd, 1H, J: 3.6 Hz, J== 1.0 Hz, H3), 7.17 (dd, 1H, J: 3.6 Hz, J=
1.0 Hz, H3H ), 7.07 (dd, 1H, J= 3.6 Hz, J: 5.1 Hz, H4), 7.02 (dd, 1H, J= 3.6 Hz, J==

5.1 Hz, H4”), 4.74 (s, 2H,CH2), 1.78 (5 (br), 1H, 0H).

4.2.4d. Synthesis of [2,2':5',2"-terthiophen]-3’-y1methyl methacrylate
(TTMM): Compound 3 (4.5 g), 3.15 mL of dry triethylamine, and CuCl (25 mg) were

dissolved in 50 mL of dry diethyl ether. A solution of freshly distilled methacryloyl

chloride (1.68 mL) in 50 mL of dry diethyl ether was added slowly at O C)C., and the

mixture was stirred for 2 h. The solution was filtered a silica gel column to remove
triethylammonium chloride, and the solvent was evaporated to dryness. The residue was

stirred overnight in a 1:1 mixture of methylene chloride and 2 M NaOH. The organic

112

layer was separated, washed twice with water, and dried over CaClz. After solvent

evaporation, the residue was re-crystallized from diethyl ether to provide TTMM as green

crystals. Yield: 50%. 1H NMR (CDC13): 8 7.37 (dd, 1H, J: 5.5 Hz, J: 1.0 Hz, H5), 7.25
(dd, 1H, J: 5.5 Hz , J= 0.5 Hz, H4’), 7.22 (m, 2H, H5”), 7.20 (dd, 1H, J: 3.5 Hz, J=
1.0 Hz, H3), 7.11 (dd, 1H, J: 4.0 Hz, J: 5.0 Hz, H3”), 7.04 (dd, 1H, J: 4.0 Hz, J= 5.5
Hz, H4), 6.18 (5, vinyl H), 5.62 (5, vinyl H), 5.24 (s, OCHZ), 2.0 (s, CH3). 13c {1H}

NMR (CDC13): 167.41, 136.84, 136.38, 136.15, 134.65, 134.52, 133.32, 128.14, 128.12,
127.0, 126.79, 126.76, 126.29, 125.09, 124.29, 60.49, and 18.61. High resolution MS:
m/z calc. for C17H1402S3+z 346.0156; Found: 346.0150.
4.2.5. Synthesis of the silane initiator: The trichlorosilane initiator was
. . . . 18b . .
synthesrzed followrng prevrously publlshed procedures. 2-Bromorsobutyryl bromide

(1.85 mL, 15.0 mmol), was added dropwise to a stirring solution of allyl alcohol (1.02

mL, 15.0 mmol) and triethylamine (2.51 mi. 18.0 mmol) in dichloromethane (10 mL) at

0 C>C, under nitrogen atmosphere. After 1 hour of stirring the temperature was raised to

room temperature and the reaction mixture was stirred for a further 3 hours. The

precipitate was then removed under reduced pressure and the organic layer was washed

with an aqueous saturated NH4C1 solution, followed by a wash with water. The organic

layer was then dried over anhydrous MgSO4 and the solvent removed in vacuo. The

product was purified by column chromatography (silica) using 9:1 hexane: ethyl acetate

as the eluent. The solvent was then removed under reduced pressure to yield the clear,

113

liquid product prop-2-enyl—2-bromo-2-methyl propionate (1.72 g, 8.31 mmol, 55.4%
yield). 1H NMR (CDC13, 500 MHz): 5 = 1.94 (6H, s), 4.66 (2H, d), 5.27 (1H, d), 5.38
(1H, d), 5.93 (1H, m).

A solution of hexachloroplatinic acid (21 mg, 51 umol) in 1:1 (v/v) ethanol: 1,2-
dimethoxyethane (3.75 mL) was added dropwise to a solution of pr0p-2-enyl-2-bromo-2-
methyl propionate (0.97 g, 4.7 mmol) in trichlorosilane (15 mL, 0.15 mol) under nitrogen
atmosphere. The reaction was stirred (in the dark) for 18 hours. Toluene (5.0 mL) was
then added and unreacted trichlorosilane was removed under reduced pressure.

Dichloromethane (20 mL) was added and then removed in vacuo to remove all remaining

trichlorosilane. The resulting product, 2-bromo-2-methyl-propionic acid 3-
trichlorosilanyl—propyl ester was used without further purificatioan NMR (CDCl3, 300
MHz): 8 = 1.50 (2H, m), 1.93 (8H, m), 4.20 (2H, m).

4.2.6. Preparation of initiator immobilized flat substrates: ITO slides (~1 cmz)
were first sonicated in acetone (10 min), followed by sonication in IPA (10 min). All

substrates were cleaned using UV/O3 chamber for 1 hour. ITO substrates were then

placed in a ~l mM solution of the trichlorosilane initiator, containing ~10 mM

triethylamine in toluene. Substrates were left, covered, in the solution, at room

temperature, for 18 hours under N2 atmosphere. The substrates were then removed and,

successively: washed with toluene, sonicated for 1 minute in toluene, washed with
acetone, washed with ethanol, and dried under a stream of nitrogen. The surfaces were

stored under nitrogen until further use.

114

4.2.7. Surface initiated polymerization on gold and ITO substrates: In a N2-

filled drybox, 0.7 mg of CuBr and 5.55 mg of HMTETA were added to a round-bottom
flask containing 1 mL of a degassed solution of TTMM monomer (500 mg) in DMF

(DMF : monomer ~1 :1 (vzv). The mixture was well-stirred and heated with an oil bath to
50 0C until a transparent, green solution formed. The prepared solution was then
transferred into a small vial containing an initiator-modified Au substrate to start the

surface-initiated polymerization. After a set reaction time at 50 0C, the substrate was

removed from the vial, washed with THF and isopropanol sequentially, and then was

dried under a flow of N2. The similar polymerization condition. has been applied for

growing MTM from surfaces.
4.2.8. Electrochemistry: All electrochemical modifications were performed
using a CHI650a computerized potentiostat (CH Instruments, Inc., Austin, TX).The

electrochemical experiments, i.e., CV (cyclic voltammetry) were carried out in

acetonitrile containing tetraethyl ammonium perchlorate (TEAP~5><102 M) as a

conducting salt. For heterocoupling experiments, the solution contained 10‘2 M of EDOT

along with TEAP. All experiments were carried out in a glovebox under an inert and dry
atmosphere at room temperature. In a three electrode system, the working electrode (gold
or ITO-coated glass substrate) was placed between the reference electrode (Pt wire) and
the counter-electrode (graphite). All potentials were measured in this work with respect
to a Pt wire used as a quasi-reference electrode. For this reason, the potentials may not be

directly compared.

115

4.3. Results and Discussion:

4.3.1. Monomer synthesis: As shown in Schemes 4.1 and 4.2, the synthesis of 3-

methylthienyl methacrylate (MTM) and the precursors to 3’-methylterthienyl

methacrylate (TTMM), i.e. compounds 1, 2 and 3, were synthesized by modified versions

of the procedures of Yagci22 and Seeber23et al. TTMM was obtained in high purity by
the reaction of 3 with methacryloyl chloride in dry diethyl ether, similar to the synthesis
of monomer MTM. Methacrylate monomers were chosen for their facile polymerization

via ATRP, while the polymerizable thiophene and terthiophene groups have shown good

. . . 24,25,26
conducting pr0pert1es In related polymers.

 

HO
O
Et3N, Dry Ether o
/ \ + O >
8 Cl 0 cc. 2 h / \
70% 3

Scheme 4.1: Synthesis of 3-methylthienyl methacrylate (MTM)

116

 

KP! Biz, CHCI3 KP!
/ \ j» Br /\ Br

 

 

S RT,24h S
70%
O
H
Pd PPh ,NaHCO
¢( 314 3 U + / \
600/0 8 B(OH)2 Br 8 Br
85‘V NaBH4, RT
0
45 min

 

Et3N, Dry Ether
+ O F

(3' RT, 24 h
78%

 

   

Scheme 4.2: Synthesis of [2,2':5',2"-terthiophen]-3'-ylmethyl methacrylate (TTMM)

4.3.2. Synthesis of silane initiators for ITO substrates
Typically, we use a ll-carbon long alkyl chain in trichlorosilane initiators to ensure a
well-defined SAM on ITO. However, we used a short alkyl chain since the longer chain

may act as a resistive element and inhibit the polymerization of the pendent thiophene

methacrylares. The initiator was synthesized following the method of Huck et al.20

(Scheme 4.3), esterification of 2-bromoisobutyryl bromide with allyl alcohol, followed
by hydrosilation of the intermediate ester to give the trichlorosilane initiator. The

trichlorosilane functionality was chosen because trifunctional organosilanes are more

. . . 27
reactive towards surfaces than thelr monosubstltuted analogues. These SAMs are

117

expected to be stable due to formation of polysiloxane networks bound to the substrate

28,29
surface For gold surfaces, we used a 11-carbon long ATRP initiator as shorter chain

initiators failed to form uniform self-assembled monolayers on gold surfaces.
Br

0 El N 2 h
WOH 4' Br/lksr S730? WOW
0

O

l
H
Br

01‘
Cl 0

Scheme 4.3: Synthesis of the trichlorosilane initiator

Cl
Pt(0)l C'xs'i/Cl 100%

4.3.3. Deposition of SAMs on ITO surface

The formation of trichlorosilane-based SAMs on silica surface is well known.28

However, generating a well defined SAM on an ITO surface was expected to be more

difficult because of the high surface roughness of ITO and the low coverage of hydroxyl
groups.30 Nevertheless, some methods have produced SAMs on ITO. Examples include
microcontact printing,31 a one-hour soak in a 1 mM solution of the chlorosilane at room
temperature,32 and refluxing a solution of trimethoxysilane in toluene over ITO for 7
days.30 For the latter case, Markovich, et al. achieved about 90% coverage of surface
sites. Considering the higher reactivity of a trichlorosilane compared with a
. . . ' 20
trimethoxysilane, the reaction was carried out srmllar to the method of Huck et al., at

room temperature for 18 hours to avoid polymerization (Scheme 4.4). In addition,

triethylamine was added18b to drive the reaction to completion. While this SAM

118

deposition method may not provide complete surface coverage, estimates of initiator

efficiency suggest that ~10% of surface bound initiating molecules initiate a polymer

brush,33 and therefore having less than full surface coverage should not be a significant

factor in the polymer brush synthesis. Typically, a full ITO wafer was completely

. . . . 2 . .
denvatlzed, stored under nrtrogen, and fractured lnto (~1 X 2 cm ) pleces Just before use.

The surfaces were stable and polymerizations carried out with l-year old initiator

monolayers gave similar film thicknesses when used in similar polymerization conditions.

 

 

 

Br
Cl\ ,OH O
-OH Cl—Si—(CH2)—-—-O *O-Si—(CH2)—-O Br
01’ 3 o \ 3
lTO ;> ITO O
Toluene, RT,18 h / O
—OH "O‘Sl—(CH2)"‘O
\OH 3 Br

 

 

 

 

 

 

Scheme 4.4: Initiator SAM deposition on ITO surfaces

4.3.4. Synthesis of polymer brushes from gold and ITO surfaces

The syntheses of polyMTM and polyTTMM brushes from gold and ITO surfaces
are shown in Scheme 4.5. Films on gold surfaces were characterized using ellipsometry,
but for ITO surfaces, tapping mode AFM was used to measure PolyTTMM film
thicknesses since the film absorbs light, which makes ellipsometric measurement

ambiguous.

119

 

 

 

 

 

 

 

 

 

ITO

 

O

r--O

 

 

0

Au L—S—(CH2)1—1-O

Br

OH

Br

AU F's—(CH2)1T'O

O

—o-$i-(CH2)-o

3

Br

0

=2

 

CuBr I PMDETA
DMF , 55 °C

‘2.

O

 

/ \
\ /
Wig
—>

CuBr/ PMDETA
DMF , 55 °C

CuBrl HMTETA
DMF, 90 °C

ll

 

Au

 

 

 

 

 

 

 

 

 

 

 

H—o
lTo—o-jSI—(CH )o (
——-o 28

0
9H3
I_...S_ __ _
(CH21110 (Cszzgr
O O
[l \i
S

 

O

 

Scheme 4.5: Surface initiated polymerization of MTM from gold surface (top),

TTMM from gold surface (middle) and TTMM from ITO surface (bottom).

120

PMTM brushes were grown from gold surfaces at 55 0C in anhydrous DMF using

a CuBr/HMTETA catalyst. This system was previously shown to be compatible with the

synthesis of methacrylate monomers via ATRP,34 and also proved successful for the
synthesis of polyMTM brushes. The polyMTM brush was typically synthesized from a
concentrated polymerization solution (1 g/mL), which allowed access to a greater range of
film thicknesses. Similar solutions were used for the synthesis of polyTMM brushes.

The kinetic plots for the synthesis of both brushes (Figure 4.1) show the
characteristics of significant termination during ATRP;35the polymer chains initially
grow rapidly, and as termination consumes growing chain ends, the plots plateau.
Complete termination occurs more quickly for the polyTTMM systems than than for
polyMTM. High radical concentrations result in increased probability of radical
combination and termination of growing polymer chains. Usually, increasing the reaction

temperature provides thicker films, but when the polymerization temperature was

increased from 55 0C to 90 0C, the growth rate for polyTTMM brushes grown on ITO

surfaces did not increase.

No CuBr2 was explicitly added to the polymerizations, but Cu(II) should be

present as a consequence of of CuBr with the initiator. For polyTTMM brushes, early

termination may be related to the stronger complexing ability of terthiophene units with

Cu(II) than for monothiophenes, as evidenced by Rajesh et al.36 Thus, Cu(II) generated
during initiating of polymerization is sequestered by the pendent terthiophene of TTMM,

leading to a high concentration of radicals and almost no deactivation.

121

The polyTTMM film thicknesses were intentionally kept low to ensure their
crosslinking by electrochemical methods. In thicker films, electrochemical-based cross-
linking may be hindered by low permeation rates of anions or monomer through the
polyTTMM matrix. PolyMTM and polyTTMM brushes grown from gold surfaces were

characterized by FTIR. The spectra for polyMTM and polyTTMM show the expected

bands for the carbonyl peak at 1733 cm-1 and sp2 C-H stretching at 3100 cm.1 expected

for thiOphene rings (Figure 4.2).

 

 

 

 

250
O
8
E O
5150 r
to
(o
“c’
3:, 100 2).
.c
l-
O
50 1- Q [j I:
A
gm
0 I _L A P _1
0 5 10 15 20 25 30

Time (hours)

Figure 4.1: Evolution of the ellipsometric brush thickness with time for the
polymerization of polyMTM and polyTTMM from gold and ITO surfaces; a polyMTM
brush grown on gold surface (9), a polyMTM brush grown on ITO surface (0), a
polyTTMM brush grown on gold surface ([3), a polyTTMM brush grown on ITO surface

(A).

122

 

 

 

 

 

 

 

 

 

 

 

 

0.02 (a)

Absorbance

(b)

 

 

 

 

I I I l

3500 3000 2500 2000 1500 1000
Wavenumbers (cm'l)

 

Figure 4.2: Representative examples of reflectance FTIR spectroscopy of gold
surface grafted with (a) 50 nm polyTTMM brush (b) 100 nm polyMTM brush.
4.3.5. Electrochemical crosslinking

PolyTTMM films (~30 nm) on ITO and gold substrates were placed in a 0.1 M
tetraethylammonium perchlorate (TEAP) solution in acetonitrile under an inert
atmosphere to avoid overoxidation, and then crosslinked electrochemically (Scheme 4.6).
The substrates served as the working electrode, a graphite electrode as the counter

electrode and a Pt wire as the reference electrode. The substrate was scanned at 100 mV/s

123

 

 

O

 

 

 

 

Electrochemical
l—O . . CH
._ . Polymerlzatlon 0. 9H3 . 3
lTO—O-SI- CH O C B ._ L. - -_
o’ ( 2):3 (CH2 imr _ fl ITO 00,81 (CH2)3O CHzC—tCHzCigBr
O O O O

 

 

 

   

Scheme 4.6: Electrochemical cross linking of polyTTMM brush on ITO surfaces
from -25 to +1500 mV for up to 20 cycles in order to cross-link the polyTTMM film
(Figure 4.3). The direct electrochemical polymerization of the polyMTM brush was not

achievable under our conditions, but consistent with previous reports of the low reactivity

of thiophenes substituted with esters in the 3-position.26 PolyTTMM’s higher reactivity

. . 26 .
stems from the extended n-structure of the terthienyl morety which lowers the

oxidation potential of polyTTMM brush relative to polyMTM. In addition, the terthienyl

unit is less sterically challenged with respect to coupling in the 2- and 5”-positions,

compared to the 2- and 5-positions of a single thienyl unit.

A well-defined oxidation peak starting at ~1.1 V/Pt (Figure 4.3, first scan) was
observed for the polyTTMM brush by cyclic voltammetry. Because films of grafted poly
(alkylacrylate), e.g., PEA (poly(ethyl acrylate)), do not react upon anodic polarization,
this peak can only be attributed to the oxidation of the aromatic thiophene rings. The
reduction peak of the accordingly formed polyTTMM two-component films is observed
at ~ 0.63 V/Pt during the reverse scan. These observations confirm that the thienyl rings
attached to the conducting substrate (ITO / gold) as a result of the surface initiated
polymerization via ATRP of the parent acrylates (TTMM), remain available to

electrochemical polymerization and form polymers with a more extended 7: electron

124

2.50E-04

 

 

 

 

 

 

 

2.00E-04 )-
2: 1505-04 ~
5
S.
5- 1005-04 -
2
5' 5.00E—05 -

0.00E+00 -

-5.00E-05 e . 1 .

-0.5 0 05 1 1.5 2

'E(v)

Figure 4.3: Cyclic voltammetry (20 cycles) of ~30 nm polyTTMM brush coated

on ITO surface from -25 to 1500 mV at 100 mV/s.

conjugation. The voltammetric scans were repeated (20 times) (Figure 4.3, 20th scan) or

less if the polymerization peak was no longer observable. On the last scan (20th scan), the
oxidation peak appears at a lower anodic potential (E~0.75 V/Pt), and corresponds to a
37,38,39,40,4l

more extended conjugated system than the monomeric terthiophenes. The

de-doping peak remains visible. These redox potentials are higher than values reported
. . . . . . 42 .
for terthiophene reductlon under the srmrlar condrtrons but, they are comparable to the

data reported by Advincula et al.21 for copolymers of carbazole and terthiophene

+ +
containing methacrylate (EC = 0.6 V/Ag/Ag and Ea = 1.08 V/Ag/Ag ). The reduced

125

mobility of a thiophene attached to polymethacrylate chains might explain the formation
of polythiophenes with shorter conjugation lengths. Scan rate dependence studies (Figure

4.4) of the cross-linked polyTTMM at scan rates of 20-80 mV/s in a 0.1 M
TEAP/CH3CN electrolyte solution (potentials are reported relative to Pt as a quasi
reference electrode) revealed linear behavior (Figure 4.5) similar to data of Sotzing et
al.39 for electropolymerized poly(terthiophene) films. This indicates a surface-confined

reaction as the sole phenomenon.

5.00E-05

 

Scan rate

decreasing
4.00E-05 2

3.00E-05 -

 

2.00E-05 -

1.00E-05 r

Current (Amp)

 

0.00E+OO r

 

-1.00E-05 -

 

 

 

 

-2.00E-05 1 ' ' ‘
.05 0 0.5 1 1.5 2
E (v)

Figure 4.4. Scan rate dependency study of polyTTMM brushes coated on ITO

surface at scan rates of 20-80 mV/s in a 0.1 M TEAP/CH3CN electrolyte solution.

126

 

 

 

 

 

3.5 -
’5.
E a -
Z‘
17, 2.5 -
C
.9
.E 2 -
72'.
t 1.5 -
3
0
2 1 -
Q)
a
0.5 -
0 r r r
10 3O 50 7O 90

Sean rate (mv/sec)

Figure 4.5: Plot of peak current intensity versus scan rate at the maximum of the
oxidation wave of polyTTMM brush coated on ITO surface.
Examination of the films by UV-vis before and after the electro-polymerization

showed a small spectral difference arising from the formation of conjugated

oligothiophene species (Figure 4.7). The Amax for the polyTTMM brush before

. . 39 .
crosslinking was ~ 360 nm (characteristic vibronrc pattern of terthiophene), and shifted

to ~ 370 nm after electro polymerization. This pattern is comparable to the sexithiophene

. 43 .
based structure investigated prevrously. IR spectroscopy also suppports terthiophene

coupling. The spectrum of polyTTMM (Figure 4.6) shows bands at 3100, 1450 and 790

cm'] due to OH stretching, wagging and C-H out-of-plane deformation modes from the

thiophene rings respectively. After crosslinking, these bands decreased dramatically. A

127

-l . .
new band at 1620 cm , due to the conjugated throphene, appears in the spectrum (Figure

4b). The persistence of the carbonyl stretching mode at 1710 cm.1 in both spectra

confirms that the polymer backbone is apparently unchanged. These results are consistent
with a network of conducting polythiophene chains of short conjugation length, perhaps
dimers, grown from pristine polyTTMM.

The morphology of the pregrafted polyTTMM films may change upon oxidation
of the terthiophene units. Films before and after oxidation were investigated by tapping
mode AFM. The surface shows an irregular morphology (Figure 4.8), compared to the
smooth films before oxidation. We suspect that anodic oxidation rigidified the
polythiophene segments as well as the polyacrylate chains, which resulted in a more
heterogeneous surface dominated by grains of various sizes. However, the surface

roughness did not change significantly after oxidation.

128

 

 

 

 

0.01
(b)
d)
0
C
N
D
L-
O
(I)
.0
<
(a)
3250 2750 2250 1750 1250 750

Wavenumbers (cm")

Figure 4.6: Representative examples of reflectance FTIR spectroscopy of gold

surface coated with (a) 50 nm polyTTMM brush (b) electrochemically crosslinked

polyTTMM brush.

129

 

 
 
     

 

 

 

 

 

0.05
PTTMM brush on ITO surface
or ‘/
o
c
In
a
o ElectrOpolymerized PTTMM brush on ITO
3 surface
42
300 400 500 600 700 800

Wavelength(nm)

Figure 4.7: UV-vis absorption spectra of polyTTMM brush grown on ITO

surface (blue line) and electrochemically cross linked polyTTMM (EPTTMM) brush on

ITO surface (red line).

130

5.00

 

 

100.0 nm
50.0 nm
0.0
2.50 nm
' 0.0
0.0 2.50 5.00 pm
5'00 50.0 nm
25.0 nm
0.0 nm
2.50
, 0.0
0.0 2.50 5.00 mm

Figure 4.8: Topographical AF M images of ITO surface coated with polyTTMM
brush (upper image) and electro polymerized polyTTMM brush (bottom image) taken
with tapping mode imaging. Upper image is a 5 X 5 pm survey scan with high surface
coverage and an rrns roughness of 3.8 nm, as calculated with Nanoscope IV software.

The bottom image is a 5 X 5 am scan with an rms value of 2.9 nm.

131

4.3.6. Anodic copolymerization of the grafted polythiophene units and additional
thiophene derivative in solution - heterocoupling

The data for the electrochemical oxidation of polyTTMM suggest that
terthiophene cross-linking was inefficient. To increase the conjugation length of
polyTTMM films, the potentiostatic oxidation of polyTTMM can be performed in the

presence of thiophene derivatives such as 3,4 ethylenedioxythiophene (EDOT) (Scheme

4.7). 44 ’46 According to Zotti and coworkers,42 EDOT is the best choice for

heterocoupling terthiophene monolayers as well as polyterthiophene films.

Electropolymerization involves the formation of carbon-carbon bonds by reaction

. . 45,47 . . . . .
between two radical cations, and the prrmary reqursrte for heterocoupling IS the

generation of radical cations on the electrode surface (for the polyTTMM brush) and in
the solution close to the electrode (for the monomer). Thus, the heterocoupling is favored
by a small difference in oxidation potential between the surface and the solution
components.

The second factor for the coupling process is the reactivity of radical cations

toward coupling. Zotti et al.42 claim the second factor is predominant as the oxidation
potential of polyterthiophene films (0.5 V/ Ag/Ag+) is much lower than the oxidation

potential of EDOT (1.16 V/ Ag/Ag+). They surmised that the strong positive shift of

polyterthiophene oxidation moves it close to EDOT oxidation which should kinetically
favor the copolymerization.
Figure 4.9 shows the voltarnmogram recorded for the oxidation of a polyTTMM

film in the presence of EDOT. The polymerization peak starts at ~l.3 V/Pt, nearly the

132

same potential as the oxidation of the polyTTMM film in the absence of EDOT.
Repeated cycling (20 times) of the polyTTMM film between -0.5 V and 1.5 V, while

immersed in a dilute solution of EDOT, resulted in the deposition of a polymer film and

a concomitant growth of a redox wave in the cyclic voltammogram (Figure 4.9, 20th

scan). With each scan, more material deposited onto the electrode, resulting in an
increase in the anodic and cathodic peak currents associated with the polymer. The
polymerization peak (~1.1V) of the as-prepared conducting polymer occurs at lower
cathodic potentials compared to polyTTMM in the absence of EDOT, which is consistent
with the formation of chains with longer conjugated length. The UV-vis spectra of a
polyTTMM brush before and after heterocoupling with EDOT (Figure 4.10) shows
broadening of the absorption band and a shift of oscillator strength to longer wavelengths,
consistent with increased conjugation. However, we cannot exclude the possibility of a
small amount of EDOT homopolymer adsorbed on the ITO surface after extensive

washing.

Electrochemical

 

 

O 0 CH3 crosslinking H—0 0 CH
L _‘ -_ . . H , 3
|T0__OO.SI (CH2)30 (CHzeigr ——> lroeo-si—(cnzgo (CHZClFBr

r—\
00 To 00

 

 

 

 

 

 

   

Scheme 4.7: Heterocoupling of polyTTMM brush with EDOT on an ITO surface

133

5.00E—03

 

4.00E-03 r
3.00E-03 r
2.00E-03 )-
1.00E-03 *-
0.00E+00 -

E \
-1.00E-03

th
20 scan

Current (amp)

 

 

 

 

-2.00E-03 H 4 ‘ ‘ '
-1 05 0 0.5 1 1.5 2
E (v)
Figure 4.9: Cyclic voltammetry (20 cycles) of ~30 nm polyTTMM brush grafted

 

on ITO surface in C H3CN added with TEAP (0.05 M) in the presence of EDOT in

solution (first (blue line) and 20th scan (red line)) at the scan rate of 20 mV/s

134

 

  
      

 

 

 

 

 

 

0.05
PTTMM brush on ITO surface

a:

u

:

en

E

O Electrochemically heterocoupled PTTMM brush
3 on ITO surface

<

300 400 500 600 700 800

Wavelength(nm)

Figure 4.10: UV-vis absorption spectra of polyTTMM brush grown on ITO

surface (blue line) and electrochemically heterocoupled polyTTMM brush with EDOT on

ITO surface (red line).

135

4.4. Conclusions

Poly(terthiophene methacrylate) brushes were successfully grown from ITO and
gold electrodes using surface initiated ATRP. The resulting films were homogeneous
with smooth domains and features. The polyTTMM brush were electrochemically
oxidized by cyclic voltammetry, to form a crosslinked polymer network with short
segments of conjugated poly(terthiophene) segements. Including EDOT during
oxidation resulted in heterocoupled EDOT and polyTTMM with increased
conjugation lengths. Uniformly grafted conducting polymer brush may be useful in

photovoltaic devices.

136

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W.; Kehlenbeck, M.; Koblitz, J. J. Micromech. Microeng. 2006, 16, 2259.

30 Markovich, 1.; Mandler, D. J. Electroanal. Chem. 2001, 500, 453.

31 Koide, Y.; Such, M. W.; Basu, R.; Evmenenko, G.; Cui, J.; Dutta, P.; Hersam, M. C.;
Marks, T. J. Langmuir 2003, 19, 86.

32 Lee, J.; Jung, B. J.; Lee, J. 1.; Chu, H. Y.; Do, L. M.; Shim, H. K. J. Mater. Chem.
2002, 12, 3494.

138

 

33 Jones, D. M.; Brown, A. A.; Huck, W. T. S. Langmuir 2002, 18, 1265.
34 Patten, T. F.; Matyjaszewski, K. Adv. Mater. 1998, 10, 901.

35 Huang, W. X.; Kim, J. B.; Bruening, M. L.; Baker, G. L. Macromolecules 2002, 35,
1175.

36 Mahajan, R. K.; Sood, P. Int. J. Electrochem. Sci, 2007, 2, 832.

37 Skompska, M.; Peter, L. M. J. Electroanal. Chem. 1995, 383, 43.

38 (a) Taranekar, P.; Baba, A.; Fulghum, T. M.; Advincula, R. Macromolecules 2005, 38,
3679.. (b) Taranekar, P.; Fulghum, T.; Baba, A.; Patton, D.; Advincula, R. C.
Langmuir 2007, 23, 908.

39 Jang, S. Y.; Sotzing, G. A.; Marquez, M. Macromolecules 2002, 35, 7293.

40 Fulghum, T.; Karim, S. M. A.; Baba, A.; Taranekar, P.; Nakai, T.; Masuda, T.;
Advincula, R. C. Macromolecules 2006, 39, 1467.

41 Taranekar, P.; Fulghum, T.; Baba, A.; Patton, D.; Advincula, R. Langmuir 2007, 23,
908.

42 Zotti, G.; Zecchin, S.; Vercelli, B.; Berlin, A.; Grimoldi, S.; Groenendaal, L.;
Bertoncello, R.; Natali, M. Chem. Mat. 2005, 1 7, 3681.

43 F ichou, D.; Horowitz, G.; Xu, 2.; Gamier, F. Synth. Met. 1992, 48, 167.

44 Labaye, D. E.; Jerome, C.; Geskin, V. M.; Louette, P.; Lazzaroni, R.; Martinot, L.;
Jerome, R. Langmuir 2002, 18, 5222-5230.

45 (a) Waltman, R. J.; Bargon, J. Tetrahedron 1984, 40, 3963. (b) Waltman, R. J.;
Bargon, J. Can. J. Chem. 1986, 64, 76. (c) Andrieux, C. P.; Audebert, P.; Hapiot, P.;
Saveant, J. M. J. Phys. Chem. 1991, 95, 10158. (d) Audebert, P.; Catel, J. M.;
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Duchenet, V.; Guyard, L.; Hapiot, P.; LeCoustumer, J. Synth. Met. 1999, 101, 642.

139

Chapter 5

Polymerization of azidomethacrylates on gold surface and its elaboration via click
chemistry
5.1. Introduction
Atom Transfer Radical Polymerization (ATRP), one of the most widely
deveIOped controlled radical polymerization (CRP) techniques, is extensively used to

prepare polymer materials with predetermined molecular weights, narrow

polydispersities, and advanced architectures.l’2’3’4 ATRP is performed under mild

conditions, and is applicable to a wide range of monomers and solvents. Recently, the
copper-catalyzed Huisgen 1,3-dipolar cycloaddition, one of several chemistries termed as

“click reactions”, has drawn attention due to their high efficiency, tolerance to various
. . . . . . . . 5,6,7 . . .

functional groups, technical srmplrcrty and high specrficrty. Combrnlng click

reactions with ATRP is a versatile strategy for the synthesis of highly functionalized

polymers.8 Generally, polymer functionalization strategies are limited to end-modified
and side-modified polymers. In the first approach, an ATRP initiator containing an azide
or alkyne moiety is prepared and used to mediate the polymerization of various
monomers. The resulting polymers contain terminal alkynyl or azido functionalities,
which can be used in click reactions with functional azides or alkynes, respectively.

Using this approach, Tsarevsky and Haddleton et al. separately synthesized functional

telechelic polymersg’10 In another approach, a polymer with pendant alkyne or azido

groups are first synthesized by ATRP, and subsequently functionalized via click reactions.

Recently, Matyjaszewski et al. reported the modification of poly(3-azid0pr0pyl

140

1 . . . . . . l
methacrylate) 1 (Side-chain functionalization) and linear polystyrene 2 (end

functionalization ) by click chemistry.

Surface modification with synthetic polymers, especially polymer brushes, is of

. . . . . . . . . 13
great interest for 1ndustr1al and biological applications as well as in academic research.

Strategies have been developed for the functionalization of surfaces with polymer

14,15,16,17

brushes to immobilize biomolecules, and to realize “smart surfaces” with

. . . 8,1
responsrve and adaptive pr0pert1es.1 9 The recent development of ATRP has proved to

be a versatile tool for modifying surfaces with a variety of functional polymers with

grafted polymer chains ranging from low to high graft densities. Surface-based click

20,21,22,23

chemistry is attracting increasing attention, especially for attaching

biomolecules. Bein and coworkers presented a novel approach for the covalent

modification of mesoporous silica surfaces (modified with azide monolayer) with intact

enzymes (derivatized with an alkyne) via click chemistry.24 They reported a high density
of covalently bound enzyme while retaining enzyme activity and the absence of leaching.
Recently, Choi and coworkers demonstrated that the click chemistry could be used to
modify the termini of polymeric nanobrushes (synthesized via ATRP) by replacing the
bromine at the end of the polymer chain with azide groups.

To expand the versatility of surface-initiated ATRP we report post polymerization
modification of polymer brushes by combining surface-initiated ATRP and click
reactions. Azidopropyl methacrylate (AZPMA) was polymerized from initiators anchored
on gold and indium tin oxide substrates, and via post-polymerization functionalization,

dyes and water soluble polymers were attached to the pendent azides via click chemistry.

141

 

The AZPMA homopolymer has a high density of azides, and modification of the
homopolymer may create a sterically congested system that reduces the efficiency of the
click reaction. More importantly, to be suitable for click chemistry in the aqueous
environments required for the surface attachment of water soluble molecules (e.g.
biological molecules) AZPMA must be copolymerized with hydrophilic monomers.
Random copolymer brushes are especially attractive since they should exhibit a uniform
distribution of functional groups as well uniform swellingzs’26 Moreover, if both of the
comonomers are functional, then polymer brushes can be created with orthogonal
functionalities. In this chapter, we describe the ATRP of AZPMA (hydrophobic
functional monomer) with ethylene glycol methyl ether methacrylate (EGMA,
hydrophilic) and polyethylene glycol methyl ether methacrylate (PEGMA). The lengths
of the ethylene oxide chain in EGMA and PEGMA are different, which permits tuning of
the hydrophilicity of the AZPMA copolymers. Click chemistry was used to modify
poly(AZPMA) and AZPMA copolymers with an alkynylated organic dye and a water
soluble polymer, polyethylene glycol monomethyl ether.

5.2. Experimental Section
5.2.1. Materials

Unless otherwise noted, all chemicals were obtained from Aldrich. Fluorescein,
polyethylene glycol monomethyl ether (mPEG, Mn = 5000 g/mol), sodium hydride,

propargyl bromide (80% solution in toluene), ll-mercapto-l-undecanol (MUD, 97%), 2-

bromopropionyl bromide (2-BPB, 97%), anisole (99.7%), N,N-dimethylformamide

(DMF, 99.8%), Cu(I)Br (99.999%), Cu(II)Br2 (99.999%), Me4Cyclam (99%) 4,4’-

142

dinonyl-2,2’-bipyridyl (anbpy, 97%) and pentarnethyldiethylene triamine (PMDETA,

97%) were used as received. 2,2’-Bipyridine (bpy, 99%) was recrystallized from hexanes

and sublimed prior to use. Triethylamine was distilled from calcium hydride under an

argon atmosphere at reduced pressure. EGMA (ethylene glycol methyl ether methacrylate,

99%) and PEGMA (poly(oligoethylene glycol methyl ether methacrylate), MW ~300,

98%) were passed through a 10 cm column of basic alumina to remove inhibitors.

Azidopropyl methacrylate (AZPMA) was synthesized using the two step procedure

reported in the literature.11 After purification, monomers and solvents were transferred to
Schlenk flasks, de-gassed using three freeze-pump-thaw cycles and then transferred into a
drybox. Alkynylated mPEG (Mw ~5000) was synthesis by a published procedure.” The
alkynylated fluorescein methyl ester was synthesized over two steps starting from

. . . 28 . . . . . . .
fluorescein usrng the method of HVilsted et al. The process of immobilizmg initiators

on gold and ITO substrates was described in Chapter 4.

5.2.2. Homo and Copolymerization of AZPMA, EGMA and PEGMA from initiators

immobilized on An and ITO substrates
The procedure for the polymerization of AZPMA is described. In a Nz-filled

drybox, CuBr (6 mg, 0.04 mmol), CuBrz (5 mg, 0.02 mmol), Me4Cyclam (10 mg, 0.04

mmol), and anbpy (16 mg, 0.04 mmol) were added to a round bottom flask containing a

20 mL solution of monomer in DMF/anisole (AZPMA/DMF/anisole = 2:1:1 v:v:v,

[AZPMA] = 4.0 M). The well-stirred mixture was heated in an oil bath at 50 0C until a

143

transparent light green solution formed. The solution was then transferred into small vials

containing initiator-modified Au or ITO substrates to start the surface-initiated

ol erization. After a set reaction time at 50 0C, the substrate was removed from the
P ym

vial, washed sequentially with ethyl acetate and THF, and then dried under a flow of N2

in a drybox. The same conditions were used for homo polymerization of EGMA ([EGMA]
= 4 M). The same procedure was used for homo and copolymerizations of the monomers
using copper catalysts with PMDETA and bpy. In all cases, the ratio of
[monomer]:[Cu(I)]:[Cu(II)]:[ligand] was 300:1:0.l:1.1 and the monomer:solvent was 1:1
(v/v). For copolyrnerizations, the total monomer concentration was 4 M, and the mole

ratio of the co monomers was varied.
5.2.3. Click functionalization of homo and copolymer brushes

Click chemistry was performed in a drybox by placing gold (or ITO) substrates
coated with AZPMA homopolymer or copolymer brushes in a DMF solution (5 mL) of

alkynylated fluorescein (97 mg, 0.25 mmol), CuBr (9 mg, 0.0625 mmol) and PMDETA

(l3 uL, 63 umol) at 50 c)C. After a set reaction time, the substrate was thoroughly

washed with DMF and THF, sonicated for l min in THF, and again rinsed with THF to
remove unreacted dye. The rinsed films were then dried in a stream of nitrogen. For

modification of polymer brushes by aqueous click chemistry, substrates were transferred

inside a glove bag filled with N2 and placed in degassed solutions (5 mL) of alkynylated

mPEG (0.252 g), CuBr (1.8 mg), and bpy (4 mg) in Milli-Q water (18.2 M!) cm). The

reaction was run for 12 hours at room temperature, and then the substrates were rinsed

with deionized water and dried in a stream of N2.

144

5.2.4. Characterization Methods

13 . .
1H and C NMR analyses were carried out at room temperature on a Varian

UnityPlus—SOO spectrometer at 500 and 300 MHz, respectively, with the chemical shifts
reported in ppm and referenced to signals from residual protons in the solvent. Film

thicknesses were measured using a rotating analyzer ellipsometer (model M-44; J. A.

Woollam) at an incident angle of 75°. The data were analyzed using WVASE32 software,

and thickness and refractive index determinations were performed on at least three spots
on each substrate. The refractive index of the films was assumed to be 1.5 and then fitted

with the film thickness. Reflectance FTIR spectroscopy was performed using a Nicolet

Magma—IR 560 spectrometer containing a PIKE grazing angle (80°) attachment. UV-vis

measurements were taken on Perkin Elmer lambda 400 spectrometer. Atomic force
microscopy (AFM) images were obtained in tapping mode with Multimode AFM and
NanoScope IV software (Digital Instruments, Santa Barbara, CA) at room temperature. A
tapping mode probe (NSCIS) with a nominal frequency of 300 kHz was used for all
experiments. The confocal system was the Olympus Flroiew FVlOOO Laser Scanning
Confocal Microscope (Tokyo, Japan). The images were collected using the 20><
UPlanFLN (NAO.5) objectives with a 3x optical zoom. The fluorescence was excited
using the 488 nm laser line of the Ar gas laser. The emission was collected using a 535-

565 nm band-pass filter.

145

5.3. Results and Discussion
5.3.1 Synthesis of uniform ponAZPMA brushes from gold surfaces

Scheme 5.1 shows the synthetic route to polymethacrylate copolymer brushes
with pendent alkynes and their modification by click chemistry. Initially, we examined
the kinetics for the surface initiated homopolymerization of azidopropyl methacrylate
(AZPMA) using several catalyst systems (Figure 5.1). Usually, well-controlled ATRPs
maintain a low concentration of active radicals to minimize termination, indicated by a
linear relationship between film thickness and time, and provide control over the

molecular mass and polydispersities. Typically, the rate of ATRP is low, but the use of
Me4Cyclam/anbpy as ligands for Cu catalyst systems yields unusually rapid film
growth and high film thicknesses. However, Figure 5.1 shows a decline in film growth

rate with time for this system, and suggests significant termination as the consequence of

a relatively high radical concentration that also leads to rapid polymerization.

Compared to the Me4Cyclam/anbpy catalyst systems, using PMDET A as the

Cu ligand provides more controlled polymerizations, as evidenced by a nearly linear
increase in thickness with time for the first 4 hours of polymerization (after an rapid rate
following initiation). At longer times, the polymerization rate slows and the film
thickness reaches 260-300 nm after 12 hours (data not shown here). In prior research,
solution polymerization of AZPMA using PMDETA/CuCl and bpy/CuBr provided

significant control over the molecular weight distribution and retention of the azide

functionality.1 1’29 Liu and coworkers found that the PMDETA/CuCl system gave higher

monomer conversion and lower polydispersities than catalysts with 2,2'-bipyridyl or

146

Me6TREN as the ligands.29 It has been well-established that the combination of a CuCl

catalyst and a bromo-capped initiator results in halogen exchange and increases the

relative rate of initiation compared to propagation, and results in a controlled radical

’

polymerization with a high initiating efficiency.

147

 

0

Au F__S(_’CHZ);?JJVKBr

 

 

O OCHZCHZCH2N3
CuCl, PMDETA AZPMA
DMF, 55 °c

O(CH2CH20)BCHZCHZOCH3

p = 0: EGMA
p ~ 4: PEGMA

 

V
o H39 H39
(— CHQCHCHz-C‘>Br
Au r-S CH 0 1-m
{hi 0 0 m Ofl<o

-* ’3? H

 

 

n

 

 

p = O: poly(AZPMA-co-EGMA) p N3
p ~ 4: poly(AZPMAa-co—PEGMA) I
9
Click reaction CH3

CuBr, PMDETA or bpy, H—CEC-R
DMF or water

 

 

All l—S(‘CH2&O

 

 

 

 

11 0 0 R =
- i KL
\Qsip “(l/8’1?
N=N
9
CH3

Scheme 5.1. Synthesis of polyAZPMA and AZPMA copolymer brushes grafted on gold

substrates and their functionalization by click chemistry.

148

 

350

 

 

 

300r +
+
+
A250r
2 + .
‘JZOOr
en
2
§150r
.:
I- o
100. ‘
’ A
+
50* o ‘ <> 0
o 0 O A A
O A I I I l
O 1 2 3 4 5

Time (hours)

Figure 5.]. Evolution of the ellipsometric brush thickness with time for the

polymerization of AZPMA from initiator monolayers on Au substrates using

CuBr/Me4Cyclam/anbpy)/DMF at 50 °C (+), CuCl/PMDETA/DMF at 50 °C (0),
CuCl/PMDETA/CuBrz/DMF at 50 0C (O), CuCl/bpy/CuBrz/isopropanol at 50 0C (A)

and CuCl/bpy/CuBrz/isopropanol at RT (A). Here, each point represents a different film.

149

However, surface polymerizations of AZPMA using bpy as the catalyst ligand yielded
~lOO nm thick polyAZPMA films after 4 hrs of polymerization at 50 C’C, compared to
~200 nm thick brushes obtained under similar conditions using PMDETA as the ligand.
As proposed by Matyjaszewski, 32 multidentate ligands such as Me4Cyclam and

PMDETA may complex Cu(II) species more efficiently than bpy, shifting the equilibrium
toward the Cu(II) complex and providing a higher radical concentration and faster

polymerizations than those catalyzed by Cu-bpy complexes.
Previous studies demonstrated that Me4Cyclam and PMDETA Cu complexes are
. . . 33,34 . .
highly active catalysts for solution ATRP. To control the polymerizations, we added

10% CuBr; to the CuCl/PMDETA catalyst system and observed sluggish, but controlled

growth of thick polyAZPMA brushes. As shown in Figure 5.1, the CuCU/PMDETA
catalyst system was superior to the other catalyst systems that we tested, and it provides
reasonable growth rates with minimal termination. This system was used exclusively in

subsequent experiments.

The successful growth of polyAZPMA brushes was apparent in the reflectance

FTIR spectra of the film by the appearance of strong carbonyl and azide bands at 1740

and 2150 cm-1, respectively (Figure 5.2). The growth in film thickness matches the

changes in the intensity of the IR bands. In addition, a topographical AF M (height) image
of a 250 nm thick polyAZPMA brush was smooth and uniform with an nns roughness

< 2 nm (< 1% of the film thickness).

150

 

0.1

Absorbance
:5,

 

 

 

 

 

l l l I

3500 3000 2500 2000 1500 1000

Wavenumbers (cm'1)

 

Figure 5.2. Reflectance FTIR spectra of gold substrates coated with polyAZPMA

brushes grown for (a) 0.5 h (b) 4 h (c) 6 h and (d) 8 h from the initiator layer. A UV/O3

cleaned gold slide was used as a background.

151

 

Figure 5.3: Topographical AFM image (tapping mode) of a gold surface coated with a
250 nm polyAZPMA brush. The image is a 5 X 5 um survey scan with high surface
coverage and an rms (root mean square) roughness of 1.9 nm, as calculated by the

Nanoscope IV software.

152

5.3.2 Copolymerization of AZPMA with EGMA and PEGMA from gold surfaces
The conditions used for the homopolymerization of AZPMA were applied to the
homopolymerization of EGMA from gold surfaces. Kinetic studies showed that the

CuCl/PMDETA system also provided good control over EGMA polymerizations; the

CuBr/Me4Cyclam/anpr system gave higher growth rates but with less control (Figure

5.4). With good control and comparable polymerization rates for AZPMA and EGMA,

the CuBr/Me4Cyclam/anpr system was used for the copolymerization studies.

Copolyrnerizations were run at various comonomer feed ratios and characterized
by ellipsometry and reflectance FTIR (Figures 5 .5 and 5 .6 respectively). The copolymer
composition was calculated from the reflectance FTIR data by integrating the
characteristic azide and carbonyl peaks from AZPMA and the carbonyl peak from
EGMA. The copolymer composition correlates well with the initial feed ratios of the
comonomers (Figure 5.7), consistent with a random copolymer brush. Moreover,
copolymers and homopolymer films synthesized at various feed ratios and characterized
at identical polymerization times had similar ellipsometric thicknesses, suggesting that

the reactivities of AZPMA and EGMA are comparable and consistent with a random

. . 25,26
copolymerization.

153

 

350

 

 

 

300 r- i]
A 250 r l
E i
5
a, 200 - * ¥
0)
3
g 150 L D '3
r? i I
100 t“ {
E i
50 § i
O I I I J¥
0 1 2 3 4 5

Time (hours)

Figure 5.4. The evolution of the ellipsometric brush thickness with time for the

homopolymerization of AZPMA and EGMA from initiator monolayers on Au substrates

using CuBr/Me4Cyclam/anbpy/DMF as the catalyst at 50 oC (IAZPMA, DEGMA)

and CuC /PMDETA/DMF at 50 °C (OAZPMA, OEGMA). Here, each point represents

the average of three independent runs, and the error bars correspond to the standard

deviation.

154

 

250

 

 

 

8
200 *- A
0 I
A
’5‘ D
5150 ~ '
a :
_g 100 - B
.c
I- o o
o
50 E
o
0 l I I i
O 1 2 3 4 5
Time(hours)

Figure 5.5. Evolution of the ellipsometric brush thickness with time for polyAZPMA and
poly(AZPMA-co-EGMA) at various initial feed ratios from initiator monolayers

immobilized on gold substrates using CuCl/PMDETA/DMF as the catalyst system at

50 °C. OAZPMA/EGMA (100/0), AAZPMA/EGMA (75/25), IAZPMA/EGMA

(SO/50), DAZPMA/EGMA (25/75), OAZPMA/EGMA (0/100). Here, each point

represents a different film.

155

 

 

 

 

 

 

>6)

 

 

 

3500 3000 2500 2000 1500 1000
Wavenumbers (cm'1)

Figure 5.6. Reflectance FTIR spectra of polyAZPMA and poly(AZPMA-co-EGMA)
brushes grown from gold substrates at different feed ratios. (a) AZPMA/EGMA (0/ 100)
(b) AZPMA/EGMA (75/25) (c) AZPMA/EGMA (SO/50) (d) AZPMA/EGMA (25/75) (e)

AZPMA/EGMA (100/0).

156

 

 

 
   

y = 0.97x + 0.03
R2 = 0.99

 

9.0.0.0999
w‘hU'ICDVGDCD—k

.09
6N

  

 

 

L _J I

   

L

0 0.2 0.4 0.6 0.8 1

  

Mole fraction of Azpma in copolymer
0

Mole fraction of Azpma in feed

Figure 5.7. FTIR analysis of the copolymer data in Figure 5.6. The ratio of the
comonomers in the copolymer was determined by integrating the azide and carbonyl

peaks and then normalized by the carbonyl peak area.

157

We also copolymerized AZPMA and PEGMA. The ethylene oxide side chain of
PEGMA (average molecular weight ~ 300) is significantly longer compared to EGMA,
and copolymer brushes with PEGMA are more hydrophilic. Because of the large
difference in side chain length in AZPMA and PEGMA, we carried out a similar
copolymerization study to assess the randomness of copolymerizations. We first used
reflectance FTIR to analyze several gold surfaces coated with poly(AZPMA-co-PEGMA)
([AZPMA] = [PEGMA] in the feed) corresponding to different polymerization times
from 5 min to 4 h. Figure 5.8 shows that the mole fraction of AZPMA, determined from
the peak ratios of the azide and carbonyl bands, was 0.50 i 0.02 for samples from 90 nm
to 250 nm. We then synthesized poly(AZPMA-co-PEGMA) brushes at different feed
ratios and characterized them by ellipsometry and FTIR (Figures 5.9 and 5.10). Figure
5.9 shows that the polymerization profiles of PEGMA and AZPMA are similar, but
copolymerizations were faster than homopolyrnerizations. This behavior suggests that
copolymerization favors alternation, i.e. the growing chains preferentially add the
comonomer. However, analysis of the FTIR data for the copolymer brushes (Figure 5.11)
indicates that the copolymer compositions match the feed ratios, as expected for a
random copolymerization. Returning to the copolymerization data for poly(EGMA-co-
AZPMA) in Figure 5.5, there may be similar rate enhancements for copolymerizations,
but not as significant as poly(PEGMA-co-AZPMA). However, the IR data indicate that
irrespective of their side chain length, EGMA and PEGMA form random copolymer

brushes with AZPMA.

158

 

0.2

 

 

 

 

 

 

Absorbance
> E

 

 

 

 

 

 

L I I _l

3500 3000 2500 2000 1500 1000
Wavenumbers (cm'1)

 

Figure 5.8. Reflectance FTIR spectra of gold substrates coated with poly(AZPMA-co-

PEGMA) brushes at 50 0C using an equimolar feed ratio at different polymerization

times (a) 5 min (b) 30 min (0) 1 h (d) 2 h and (e) 4 h. The polymerization system was

CuCI/PMDETA/DMF.

159

 

 

 

 

300
250 e 9
A
A I
’5‘ 200 e A x
I:
V X
3’» o
g 150 - D
g A O O
I— 100 a 0
Cl 9
50 2) O
O
O L 4 _L 4
o 1 2 3 4 5

Time (hours)

Figure 5.9. Evolution of the ellipsometric thickness with time at 50 0C for polyAZPMA

and poly(AZPMA-co-PEGMA) brushes with time grown from initiators immobilized on

gold substrates at various initial feed ratios, using the CuCl/PMDETA/DMF system.

OAZPMA/PEGMA (100/0), AAZPMA/PEGMA (75/25), xAZPMA/PEGMA (SO/50),

DAZPMA/PEGMA (25/75), OAZPMA/PEGMA (0/100). Each point represents a

different film.

160

 

0.2

 

 

 

 

 

Absorbance
> 9’:

 

 

 

 

 

 

_L 4 l A

3500 3000 2500 2000 1500 1000
Wavenumbers (cm'1)

 

Figure 5.10. Reflectance FTIR spectra of gold substrates coated with polyAZPMA,
poly(AZPMA-co-PEGMA) and polyPEGMA brushes grown at different feed ratios. (a)
AZPMA/PEGMA (lOO/O) (b) AZPMA/PEGMA (75/25) (c) AZPMA/PEGMA (SO/50) (d)

AZPMA/PEGMA (25/75) (e) AZPMA/PEGMA (0/1 00). The polymerization system was

CuCl/PMDETA/DMF at 50 °C.

161

 

  

0-8 F y = 0.94x + 0.03
R2 = 0.99

 

 

 

Mole fraction of Azpma in copolymer

 

O ‘7 I i I _I_
O 0.2 0.4 0.6 0.8 1

Mole fraction of Azpma in feed

Figure 5.11. FTIR analysis of the copolymer brush data of Figure 5.10. The copolymer
composition was determined by integration of the azide and carbonyl peaks and

normalized by the carbonyl peak area.

162

5.3.3. Derivatization of copolymer brushes with dyes via click chemistry

Surface grafted polyAZPMA and AZPMA copolymers are excellent substrates for
the synthesis of modified polymer brushes via click chemistry. The number of reactive
azide groups can be controlled by the c0polymer composition, which is simplified by the
apparent random incorporation of PEGMA and EGMA comonomers into the brush.
C0polymers are particularly useful since they reduce the high local concentration of

azides within the polymer film which may reduce the efficiency of click functionalization
compared to reactions in dilute solutions of polyAZPMA. 11 To improve click efficiency,
increase polymer hydrophilicity, and reduce the functional group density, we synthesized
copolymer brushes with the hydrophilic comonomers, PEGMA and EGMA. To assess

the click reaction rate for grafted copolymer brushes, we appended a fluorescent dye,

modified with a terminal alkyne, to AZPMA homopolymers and c0polymers in DMF

using CuBr/PMDETA as the catalyst at 50 OC. Both gold and ITO surfaces were used as

substrates; gold for analysis by reflectance FTIR, and ITO for UV-vis spectrosc0py.
Figure 5.12 shows representative UV-vis spectra of ~200 nm thick poly(AZPMA-co-
PEGMA) (SO/50) brushes grown from ITO surfaces for different reaction times. The
click reaction was almost complete within 5 min, as the absorbance marginally increased
from 5 min to 1 hour.

To quantify the click reaction kinetics, we used reflectance FTIR to monitor the

disappearance of azide peak at ~ 2100 cm-1 and the appearance of aromatic C=C

. -l . . .
stretching at ~16OO cm during the first hour span of the reaction, where desorption

observed was negligible. The kinetic study (Figure 5.13 and Figure 5.14) confirmed that

163

the click reaction is fast, with > 60% of the azides reacted within 1 min. The rate was
nearly independent of the brush architecture, with copolymers having fewer azides
reacting slightly faster than polyAZPMA. There are several reasons for the rapid click
reaction: (1) the local concentration of azide groups in a brush is much higher than

soluble polyAZPMA, as observed by Li et al. in the case of azide grafted on silica

nanoparticles;35 (2) while the azide is immobilized on a polymer brush, the dye molecule

is small and readily diffuses through the brush; (3) the azide group forms triazoles, which

bind and stabilize Cu(I)36’37and hence enhances the local concentration of copper

catalyst; and (4) CuBr/PMDETA is a highly efficient click catalyst.11

Interestingly, the click reaction never went to completion when the substrate was
polyAZPMA brushes or copolymer brushes with >50% AZPMA. This may reflect
reduced diffusion of the alkynes through the brush architecture, especially near the
substrate surface. The effect seems to be limited to azides near the surface, since the
reaction rates are similar for homopolymers, and copolymers with varying ethylene oxide

chain lengths. The rapid click reaction may be assisted by the DMF solvent, since DMF

at 50 0C is a good solvent for homopolymer and copolymer brushes, enabling the dye

molecule and catalysts to diffuse through the brush.

An important observation is that prolonged soaking the brush-coated substrate in
the click solution results in brush desorption. After 24 h of reaction, the dye absorbance
decreased instead of increasing. A similar phenomenon was observed via reflectance
FTIR spectroscopy where the intensities of the characteristics peaks of thick films (>150

nm) decreased over time.

164

 

Absorbance

 

 

 

 

 

 

 

 

0.2 ~
0.1 L
O J L L I
300 400 500 600 700 800

Wavelength (nm)

Figure 5.12. UV-vis spectra of ITO substrates coated with ~l80 rim of poly(AZPMA-
co-PEGMA) (50/50) showing the evolution of the click reaction between the copolymer

and a fluorescent dye . Blue line: 5 min, pink line: 1 h, red line: 4 h and green line: 24 h.

165

 

 

0.05

 

 

I
i
[I l

 

 

 

 

Absorbance
l l

g E
E g

 

 

l

 

 

 

I _l L I

3500 3000 2500 20001 1 500 1 000
Wavenumbers (cm‘ )

 

Figure 5.13. Reflectance FTIR spectra of gold substrates coated with ~200 nm
copolymer brushes of poly(AZPMA-co-PEGMA) (SO/50) showing the evolution of the
click reaction between the copolymer and a fluorescent dye. (a) 1 h (b) 30 min (0) 15 min

(d) 5 min (e) l min (f) 0 min.

166

 

 

 

 

100 CI 3}
[:1
Di * * ‘
O X
80 L 2 8 g
ifi R
A
A X
0
23 50 -
C
.9
2
°>’
O
U
20 r
O 1 I_I I I l i l I 1 I l I I
O 15 30 45 60 75

Time (min)

Figure 5.14. Kinetics of the click reaction between a fluorescent dye and ~200 d: 30 nm
films of poly(AZPMA), poly(AZPMA-co-EGMA) and poly(AZPMA-co-PEGMA)
grown from gold substrates. The data were extracted from reflectance FTIR spectra

analogous to those shown in Figure 5.13. <>AZPMA/EGMA (lOO/O), —

AZPMA/EGMA (75/25), OAZPMA/EGMA (SO/50), DAZPMA/EGMA (25/75),

xAZPMA/PEGMA (75/25), AAZPMA/PEGMA (SO/50), +AZPMA/PEGMA (25/75).

Here, each point represents a different film.

167

We used the absorbance values at Amax to quantify the covalently bound dye in

the copolymer brushes. Alternative methods such as measuring the change in molecular
weight or using ellipsometry to monitor changes in film thickness after the click reaction
were less precise. UV-vis spectra of the dye were obtained in air and in various solvents,
and since we found no changes in the spectra, we assumed that the molar extinction
coefficients for the dye in solution and on surfaces is identical. Rearranging the Lambert-
Beer’s law, A = a b c; where A = absorbance, a = molar extinction coefficient, b = path
length, and c = concentration of the solution to A = (c/a) a b c; where, a = area of the
solution exposed to the light, provides A = s F; where F = the grafting density,

(moles/area). To calculate the grafiing density (F), we need to know the surface

absorbance at Amax and molar extinction coefficient of the dye. The molar extinction
coefficient of the dye (a = 52227 cmZ/mol) was calculated from the absorbance values at

kmax in THF at 4 different concentrations (Figure 5.15). We estimated the dye molecule

grafting density (F) from c and the data from Figures 5.16, 5.17 and 5.18. The dye
absorbance tracks the azide content in poly(AZPMA-co- PEGMA), until it reaches its
maxima for the homopolymer (100% azide).

Figure 5.19 shows fluorescence micrOSCOpy images from ITO coated with ~180
nm of poly(AZPMA-co-EGMA) (1:1) after a 5 min click reaction with the alkynylated
dye, and a control surface, treated identically but without the copper catalyst. The
fluorescence of the clicked surface confirms that the dye has been bound to the surface,

while the lack of fluorescence for the control surface confirms that physical adsorption of

. . 28
the fluorescent reactant can be disregarded, as observed by Hvrlsted and coworkers.

168

 

 

9.0.0 7‘?
A 03 oo _. m 4:-

Absorbance at peak maximum
O
to

 

 

 

 

0

0.0E+00 5.0E-06 1.0E—05 1.5E-05 2.0E-05 2.5E-05
Concentration (mol/L)

Figure 5.15. Measurement of the molar extinction coefficient of the alkyne modified

fluorescein dye.

169

 

 

 

0.1 f

 

Absorbance

 

 

 

 

 

 

 

I 4 4 J I l 1 I l

300 400 500 600 700 800

 

Wavelength (nm)

Figure 5.16. UV-vis spectra of ~ 200 i 30 nm thick polyAZPMA and poly(AZMPA-co-
PEGMA), and polyPEGMA brushes grown from ITO substrates after a 5 min click
reaction with a alkynylated fluorescent dye. Green line: before clicking to AZPMA
(100/0), blue line: after clicking to AZPMA (25/75), magenta line: after clicking to
AZPMA/PEGMA (SO/50), black line: after clicking to AZPMA/ PEGMA (75/25), red

line: after clicking to AZPMA/ PEGMA (100/0).

170

 

 

.0 .0 .0
h 0) 00
l I l

O

Absorbance at peak maxima
O
to

 

 

f

I I I

25 50 75 100

O

% of Azpma in the copolymer brush

Figure 5.17. Absorbance at kmax (463 nm) obtained from the UV-vis spectra in Figure

5.15.

171

 

 

Bound dye molecule (pg/cmz)

 

 

O L I I I
O 25 50 75 100

 

°/o of Azpma in the copolymer brush

Figure 5.18. Dye molecule binding as a function of the per cent AZPMA in a 200 i 30
nm poly(AZMPA-co-PEGMA) brushes on ITO surface. The dye molecule binding was
determined by dividing the UV-vis spectral peak maxima of the different copolymer

brushes (Figure 5.17) with the molar extinction coefficient obtained from Figure 5 .15.

172

  

20 um

20 um

   

Figure 5.19. Representative fluorescence microscopy images of the clicked surface (left
image) and the control (right), prepared without the copper catalyst but under otherwise
equivalent conditions. The images were recorded using equal lighting and camera
settings.
5.3.4 Derivatization of copolymer brushes via click chemistry with a water soluble
polymer

The previous data show that dye molecules in DMF show comparable reactivity
in click reactions with homopolymer and copolymer substrates. However, tethering
biomolecules to brushes requires an aqueous environment, and therefore we synthesized a
water soluble copolymer brush to test whether the click reaction would be successful in
aqueous environments. For this experiment. alkynylated mPEG 5000, a water soluble
polymer, was used for the click reactions. Figure 5.20 shows reflectance F TIR spectra for
~50 nm thick films of polyAZPMA and poly(AZPMA-co-PEGMA) grown from gold
surfaces, before and after click reactions with an alkynylated mPEG 5000 in water at
room temperature for 12 hours. Compared to click reactions with the alkynylated dye, the
reaction of the alkynylated mPEG polymer was slow. likely due to slower diffusion of the

higher molecular weight of the polymer (5000 g/mol).

173

 

 

 

 

 

 

 

Absorbance
LED,

 

 

 

I I L I

3500 3000 2500 2000 1500 1000

 

Wavenumbers (cm'1)

Figure 5.20. Click reactions for 12 hours with a water soluble polymer (alkynylated
mPEG 5000) monitored by reflectance FTIR spectra of gold substrates coated with ~ 50
i 10 nm of poly(AZPMA) and poly(AZPMA-co-PEGMA) brushes (a) after clicking to

AZPMA/PEGMA (100/0), (b) before clicking to AZPMA/PEGMA (100/0), (c) after

174

clicking to AZPMA/PEGMA (75/25), (d) before clicking to AZPMA/PEGMA (75/25), (c)
after clicking to AZPMA/PEGMA (50/50), (f) before clicking to AZPMA/PEGMA.
(SO/50), (g) after clicking to AZPMA/PEGMA (25/75), (h) before clicking to

AZPMA/PEGMA (25/75).

Other reasons for the slow click reaction may include (1) the hydrophobicity of

azide-containing polymer which could collapse and restrict access to the azide, (2) the

lower reaction temperature (ambient temperature vs. 50 0C, and (3) the water soluble

catalyst, CuBr/bpy, may be less efficient than the CuBr/PMDETA complex.38
Integrating the azide peaks at ~2100 cm-1 in FTIR spectra before and after the

click reactions (Figure 5 .20) reveals that click conversion increases as the PEGMA mole
fraction in the polymer brush increases and becomes more hydrophilic (Figure 5.21). In

addition, the changes in the intensities of the C-0 stretching bands at 1100 and 1350, and

OH stretching at 2900 cm.1 are consistent with the azide peak analysis. Moreover, the

thickness of the most hydrophilic brush, poly(AZPMA-co-PEGMA) (AZPMA/PEGMA
= 25/75) nearly doubled, from 47 nm to 92 nm, after the click reaction. The thickness of
the poly(AZPMA) brush was almost unchanged afier the click reaction (< 10% change)
under identical conditions (Figure 5.22).

The expansion of clicked poly(AZPMA-co-PEGMA) (25/75) film, and the IR
analyses confirm that the click reaction occurs in the bulk film, while poly(AZPMA)
films react primarily on the surface. Figures 21 and 22 also show that the click
conversion and copolymer compositions, inferred from the change in film thickness,

(Figure 5.22), are consistent for all polymers tested. Also, the more hydrophobic

175

poly(AZPMA—co-EGMA) films (shorter ethylene oxide chain, p=0, see Scheme 1) was
marginally better than poly(AZPMA) films in terms the water-based click reactions (data
not shown here). Therefore, the longer ethylene oxide chains in poly(AZPMA-co-
PEGMA) films (p ~4, see Scheme 1) make the polymers sufficiently hydrophilic to effect

immobilization of water soluble polymers by the click reaction.

176

 

 

70

6C

4 (1‘
A0\0v coumhw>coo

 

 

 

 

 

70
O
60 r-
A5O -
o\° o
.540 ‘
2 o
“3’30 r
O
O
20 r
10 L o
O L l l l I
0 25 50 75 100 125

°/o of azide in copolymer

Figure 5.21. The disappearance of azide the via click reaction with alkynylated mPEG
5000 for 12 hours at room temperature as a function of the molar per cent azide in the
COPOIymer brush for different feed ratios (see Figure 5.20). The conversion was
calculated from FTIR spectra by integrating the azide peak before and after the click

reaction.

177

 

 

100
90 .
8O . "
70 -
60 ~ .
50 -

 

% increase in thickness

10L I

 

 

 

O i L I l l

0 25 50 75 100 125
% of azide in copolymer

Figure 5.22. Increase in thickness after the click reaction with alkynylated mPEG 5000
for 12 hours at room temperature as a function of the molar per cent azide in the

copolymer brush.

178

Interestingly, FTIR spectroscopy detected significant chain desorption while
immobilizing alkynylated mPEG onto thick c0polymer brushes (> 150 nm), and the click
reaction was reasonably fast (significant conversion within 30 min). However, no
desorption (no change of carbonyl peak height) occurred for thin films (~ 50 nm) even

after 12 h of reaction. The severe desorption for poly(AZPMA—co-PEGMA) brushes, may

be due to the long water-soluble side chains39 and extreme steric crowding of the thick
film. However, this needs to be investigated in detail.
5.4. Conclusions

Azidopropyl methacrylate (AZPMA) a functional monomer with a pendent
clickable moiety, was grown from gold and ITO surfaces using surface-initiated ATRP.
AZPMA enables post-polymerization modification of polymer brushes via the 1,3-dipolar
cycloaddition of azides with acetylenes, and copolymerization with PEGMA and EGMA,
provides control over the hydrophilicity and fimctional density of AZPMA polymers.
Post-polymerization modification of homo and copolymer brushes was demonstrated by
reacting with an alkyne-modified dye via click reactions. Kinetic studies showed that

click reactions on surface-grafted homo/copolymer brushes were fast when run in DMF

at 50 0C (> 60% conversion in 1 min) irrespective of the copolymer composition. The

outcome of aqueous click reactions depends on the composition of the polymer brush.
Using a alkynylated polyethylene glycol methyl ether (mPEG, Mn ~5000), the most
hydrophilic c0polymers, i.e. those with the highest fraction of PEGMA, immobilized the
most mPEG, as evidence by the increase of the dry film thickness. Aqueous click
reactions offer an alternative to ester or amide formation for the covalent immobilization

of biomacromolecules.

179

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182

 

 

 

 

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