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"(wt-i- .1:. ii 1:, .- 5... at}. :1). :13‘9. .. .2; Lynn... \Zfiflu‘n nah“... : , n;M.91<.;iP THEF- 'l 300; This is to certify that the dissertation entitled SURFACE-INITIATED POLYMERIZATIONS ON INITIATOR ANCHORED SUBSTRATES: SYNTHESIS AND CHARACTERIZATION OF NANOMETER THICK FUNCTIONAL POLYMER FILMS presented by ZHIYI BAO has been accepted towards fulfillment of the requirements for the Doctoral degree in Chemistry mafia: ’b'llajor Professor’s Signature 31/1707 2006 Date MSU is an Affirmative Action/Equal Opportunity Institution LIBRARY Whiga: . State University _.:_ -.-.— ---0-.-.-l-l-l-l-I-O-I-I-l-O-Q-I-I-.. PLACE lN RETURN BOX to remove this checkout from your record. TO AVOID FINES return on or before date due. MAY BE RECALLED with earlier due date if requested. DATE DUE DATE DUE DATE DUE 2/05 p:/ClRC/DateDue.indd-p.1 SURFACE-INITIATED POLYMERIZATIONS ON INITIATOR ANCHORED SUBSTRATES: SYNTHESIS AND CHARACTERIZATION OF NANOMETER THICK FUNCTIONAL POLYMER FILMS By Zhiyi Bao A Dissertation Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Chemistry 2006 ABSTRACT SURFACE—INITIATED LIVING POLYMERIZATIONS ON IN ITIATOR ANCHORED SUBSTRATES: SYNTHESIS AND CHARACTERIZATION OF NANOMETER THICK FUNCTIONAL POLYMER FILMS By Zhiyi Bao We describe the surface-initiated ring-opening polymerization (ROP) of lactide from poly(2-hydroxyethyl methacrylate) (PHEMA) brushes anchored to Au substrates. The resulting comb polymers have a “bottle brush” architecture. During hydrolytic degradation of PLA in pH 7.4 buffer at 55 °C, large, highly symmetric domains (~50-100 um) unexpectedly formed. The purpose of the research described in this chapter was to devise a model that describes their formation. Control experiments during degradation study link high lactide polymerization temperature to the formation of the defects. A likely mechanism is the scission of Au-S bonds at high temperatures, causing defects that swell when placed in the buffer solution. We demonstrated enhanced control over polymer brushes through variation of the areal density of the immobilized initiators used for their growth. Reaction of mercaptoundecanol monolayers on Au with both an acyl bromide initiator and a structurally similar acyl bromide diluent yields monolayers whose composition reflects the ratio of the acyl bromides in solution. Similarly, derivatization of SiOz with an initiator and a diluent monochlorosilane also affords control over initiator density. The thickness of polymer films grown from these modified substrates drop dramatically when the fractional coverage of the surface by initiator decreases below 10% of a monolayer because the area per polymer chain increases. However, reduced termination at low initiator coverage results in substantial increases in initiator efficiency as measured by film grth rates normalized by the fractional coverage of the surface by initiator. Variation of chain density also affords control over film swelling. PHEMA films prepared with 0.1% initiator densities swell 20-fold more in water than films grown from monolayers containing only initiators. Such control should prove valuable in the use of brushes for immobilization of active, accessible biomacromolecules such as single- stranded DNA or antibodies. We report the remarkably rapid synthesis of polymer brushes under mild conditions (50 °C) using surface-initiated polymerization. The use of the highly active atom transfer radical polymerization catalyst Cu(I)1,4,8,1l-tetramethyl-l,4,8,11- tetraazacyclotetradecane allows synthesis of 100 nm thick poly(tert-butyl acrylate) brushes from initiator-modified Au surfaces in just 5 minutes. Using the same catalyst, polymerization of hydroxyethyl methacrylate and methyl methacrylate yielded 100 nm thick films in 10 and 60 minutes, respectively. Such polymerization rates are an order of magnitude greater than those for traditional free-radical polymerizations initiated from surfaces. It is important to note that though these rapid polymerizations from surfaces are not “living”, they retain some features of controlled radical polymerizations such as the ability to form block copolymer brushes. Such rapid polymerization from a surface will be very important in potential applications of polymer brushes as skin layers in separation membranes and as substrate coatings for probe immobilization in gene and protein chips. To My Family iv ACKNOWLEDGMENTS First, I would like to thank my advisor, Professor Gregory L, Baker for his great help during my graduate study at Michigan State University. He has been an excellent research advisor as well as a mentor for me. The experience of studying and working under his guidance in these five years has been invaluable to me, and will accompany me in the future as I pursue my goals. I express my deep appreciation to all my committee members. Professor Merlin Bruening has a great science attitude and critical thoughts. I developed good research abilities with his help. As my second reader, Professor Babak Borhan was always ready to help me during my graduate study, as well as teaching me good organic and bioorganic chemistry. I also want to thank Professor William Wulff for his great suggestions toward my seminar and delicious homemade soup, both of which provide me the nutrition. I would like to thank all Baker group members: I B, Ying, Feng, Ping, Lesile, Fadi, quei, DJ, John, Erin, Qin, and Sampa. I also enjoyed working with J inhua, Lei, Wenxi, and Matt. I really enjoyed my life here with all of my fi'iends. Special thanks to Lisa Dillingham for her kindness to me during my stay in Chemistry Department. She has been always a big help for me. Finally, I want to thank my family: my beautifiil wife Gaoming, my father J itao, my mother Liuru, and my brother Zhifeng, they are the most important people in my life, and always give me positive support whenever I need them. They deserve my biggest thanks and love. TABLE OF CONTENTS Page List of Schemes ................................................................................................................ xi List of Tables ................................................................................................................... xiii List of Figures .................................................................................................................. xiv List of Abbreviations ....................................................................................................... xx Chapter 1. Introduction ................................................................................................ 1 I. Polymer Brushes ........................................................................................................ I I-1. Their Definition and Physical Properties .......................................................... I I-2. Polymer Brushes on Various Substrates and their Applications ....................... 3 L3. Polymer Brushes on Flat Surfaces ..................................................................... 5 L4. Polymer Brushes on Particles and Non-planar Surfaces ................................... 7 LS. Polymer Brushes Tethered on Macromolecules ................................................ 10 H. Surface-initiated Polymerizations .............................................................................. ll H-l. Preparation of Polymer Brushes ........................................................................ 11 11-2. Surface-initiated Cationic Polymerizations ....................................................... 13 11-3. Surface-initiated Anionic Polymerizations ....................................................... 15 11-4. Surface-initiated Ring-opening Polymerization ................................................ 17 11-5. Surface-initiated Free Radical Polymerizations ................................................ 20 11-6. Surface-initiated Controlled Radical Polymerization ....................................... 24 III. Atom Transfer Radical Polymerization (ATRP) ...................................................... 28 III-1. Introducation .................................................................................................... 28 III-2. Mechanism and Kinetics .................................................................................. 30 111-3. Monomers, Catalyst and Initiator Systems ...................................................... 32 111-4. Ligands, Solvents and Other Factors ............................................................... 36 vi III-5. Surface-initiated ATRP .................................................................................... 40 IV. References ................................................................................................................. 48 Chapter 2. Controlled Growth and Degradation of PLA Films Grafted to PHEMA Brushes on Au Substrates ....................................................................... 56 I. Introduction ............................................................................................................... 56 H. Experimental Section ............................................................................................... 59 11-1. Material ............................................................................................................. 59 H-2. Characterization Methods ................................................................................. 60 11-3. Synthesis of PHEMA Brushes from Au Substrates .......................................... 61 11-4. Synthesis of PHEMA Brushes from Au Substrate by Surface-Initiated ATRP for Kinetic Study ................................................................................... 61 II-5. Ring-opening Polymerization of Lactide from PHEMA Surfaces for Kinetic Study ................................................................................................... 62 II-6. Hydrolytic Degradation of PLA Films .............................................................. 63 II-7. Observation of PHEMA-g-PLA F ilrns During Hydrolytic Degradation ........... 63 IH. Synthesis of PHEMA-g-PLA .................................................................................. 64 IV. Kinetics of Polymerization of HEMA from Au Surfaces ........................................ 67 V. Kinetics of Lactide Polymerization from PHEMA Surfaces .................................. 69 VI. Defect structures in PHEMA-g-PLA films ............................................................. 71 VII. Conclusions ............................................................................................................ 77 VIII. References .............................................................................................................. 78 Chapter 3. Control of the Density of Polymer Brushes in Surface-Initiated ATRP from Au Surfaces ...................................................................................... 80 I. Introduction .............................................................................................................. 80 II. Experimental Section ............................................................................................... 84 vii H-l. Materials ............................................................................................................ 84 II-2. Characterization Methods ................................................................................. 84 II-3. Synthesis of 2-Methylpropionyl Bromide (2-MPB) (2) .................................... 85 II-4. Determination of the Relative Reactivity of 2-Bromopropionyl Bromide (2-BPB, 1) and 2-MPB with Alcohols ............................................................. 86 H-S. Preparation of Initiator-Immobilized Au Substrates ......................................... 87 II-6. Surface-Initiated Polymerizations of MMA ...................................................... 87 II-7. Surface-Initiated Polymerization of HEMA ...................................................... 88 H8 Surface-initiated ATRP of GMA ...................................................................... 89 11-9. Detachment of Polymer Brushes from Au Substrate Surfaces .......................... 89 HI. Preparation of Au Substrates with Controlled Initiator Densities ........................... 90 IV. Polymerization of MMA from Diluted Initiators Anchored to Au Substrates ....... 95 V. Polymerization of HEMA from Diluted Initiators Anchored to Au substrates ....... 102 VI. Aqueous Swelling of PHEMA Films Grown from Au Substrates ......................... 103 VH. Polymerization of GMA from Diluted Initiators Anchored to Au Substrates ........ 105 VIII. Conclusions ............................................................................................................ 107 IX. References ............................................................................................................... 108 Chapter 4. Control of the Density of Polymer Brushes in Surface-Initiated ATRP from Silicon Substrates ............................................................................ 111 I. Introduction .............................................................................................................. 111 11. Experimental Section ............................................................................................... 114 H-l. Materials ............................................................................................................ 114 II-2. Characterization Methods ................................................................................. 114 II-3. Synthesis of(11-(2-Bromo-2-methyl)propionyloxy)-undecyldimethylchloro silane (1) and (11-(2,2-Dimethyl)propionyloxy) -undecyldimethylchlorosilane (2) .................................................................... 1 15 viii II-4. Determination of the Relative Reactivity of a Silane Initiator and Diluent with Alcohols ................................................................................................... 116 II-S. Preparation of Initiator-Immobilized SiOz Substrates ...................................... 116 II-6. Surface-Initiated Polymerizations of MMA ...................................................... 117 II-7. Surface-Initiated Polymerization of HEMA ...................................................... 117 1]]. Preparation of SiOz substrates with Controlled Initiator Densities ......................... 118 Polymerization of HEMA from Diluted Initiators Anchored on SiOz Substrates ..121 V. Polymerization of MMA from Diluted Initiators Anchored on SiOz Substrates ....125 VI. Controlling Initiator Densities by Using Trimethylsilyl Chloride as the Diluent ...127 VII. Conclusions ............................................................................................................ 131 VIII. References .............................................................................................................. 132 Chapter 5. Rapid Growth of Polymer Brushes from Immobilized Initiators ......... 134 I. Introduction .............................................................................................................. 134 11. Experimental Section ............................................................................................... 136 II-l. Materials ............................................................................................................ 136 II-2. Preparation of Immobilized Initiators on Gold Substrates ................................ 136 II-3. Preparation of Immobilized Initiators on Si Substrates .................................... 137 II-4. Polymerization of tBA, MMA, Styrene and 4-VP from Initiators Immobilized on Au and Si Substrates .............................................................. 138 II-S. Polymerization of HEMA from Initiators Immobilized on Gold Substrates ....138 II-6. Characterization Methods ................................................................................. 139 III. Synthesis of PAA Brushes from Au Substrate Surface ........................................... 140 IV. Kinetic Study of Rapid Polymerization of tBA from Immobilized Initiators ......... 143 V. Other Relevant Factors in the Rapid tBA Polymerization System .......................... 149 VI. Rapid Synthesis of Various Polymer Brushes from Au and SiOz Substrates ......... 154 ix VH. Conclusions ............................................................................................................. 160 VIII. References ............................................................................................................... 161 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 1.14. Scheme 1.15. Scheme 1.16. LIST OF SCHEMES Page Surface-initiated cationic polymerization of 2-oxazolines ...................... 14 Surface-initiated anionic polymerization of styrene on gold ................... 15 Surface-initiated anionic polymerization of acrylonitrile on SiOz ........... 16 Surface-initiated ring opening polymerization of lactide ......................... 18 Surface-initiated ring opening metathesis polymerization of functionalized norbomenes ...................................................................... 19 Synthesis of polystyrene brushes on silica and cleavage of the polymers from the surface ........................................................................ 21 Polystyrene brushes and copolymer brushes grown by nitroxide- mediated polymerization (NMP) ............................................................. 25 Polymer brushes grown by RAFT polymerization. (a) PMMA, (b) PS ............................................................................................................. 27 Radical generation in conventional and atom transfer radical polymerization ......................................................................................... 29 An example of an atom transfer radical addition ..................................... 30 Transition-Metal-Catalyzed ATRP .......................................................... 31 Various monomers polymerized by ATRP .............................................. 33 Various initiators used in ATRP .............................................................. 35 Ligands used for copper-mediated ATRP ................................................ 37 ATRP initiator attachment to Au and SiOz surfaces ................................ 40 Schematic illustration of ATRP initiator immobilization on Si by the LB technique ...................................................................................... 42 xi Scheme 1.17. Synthesis of PMA-b-PMMA-b-PHEMA triblock copolymer brushes ..................................................................................................... 44 Scheme 1.18. ATRP of MMA from multi-walled carbon nanotubes (MWNT) ............ 45 Scheme 1.19. Surface patterning by microcontact printing, surface-initiated ATRP, and etching ................................................................................... 47 Scheme 2.1. Synthetic pathway for PHEMA-g—PLA ................................................... 58 Scheme 3.1. Surface-initiated ATRP of MMA from diluted-initiator monolayers on Au substrates ....................................................................................... 91 Scheme 4.1. Surface-initiated ATRP of HEMA from diluted-initiator monolayer on SiOz substrates .................................................................................... 119 Scheme 4.2. Surface-initiated ATRP of HEMA from diluted-initiator monolayers on Si02 substrates using trimethylsilyl chloride ................... 128 Scheme 5.1. Preparation of PAA brushes via surface-initiated polymerization of tBA and subsequent hydrolysis ................................................................ 141 xii Table Table 3.1. Table 3.2. Table 3.3. Table 3.4. Table 4.1. Table 4.2. LIST OF TABLES Page MMA polymerization rate from Au substrates at various initiator densites ..................................................................................................... 96 GPC Characterization of PMMA brushes grown from diluted- initiators anchored to Au substrates (brushes were detached from the surface before characterization) ......................................................... 98 HEMA polymerization rate from Au substrates at various initiator densites ..................................................................................................... 102 Aqueous swelling of PHEMA brushes on Au substrates ......................... 104 HEMA polymerization rate from SiOz substrates at various initiator densities ...................................................................................... 123 HEMA polymerization rates from silicon substrates using trimethylsilyl chloride (3) and 2 as diluents ............................................. 129 xiii Figure Figure 1.1. Figure 1.2. Figure 1.3. Figure 1.4. Figure 1.5. Figure 1.6. Figure 1.7. Figure 2.1. ' Figure 2.2. LIST OF FIGURES Page Schematic illustration of the conformation of polymers end- attached to a surface ................................................................................. 2 Examples of polymer systems comprising polymer brushes ................... 3 Examples of polymer brushes tethered from various substrates, such as flat wafers, particles, colloids and polymers ............................... 4 Reversible response of triblock-copolymer brushes to different solvents .................................................................................................... 7 Tapping mode AF M image of a molecular brush cast onto a mica substrate ................................................................................................... 10 Time-dependent properties of polymer chains grown by surface- initiated free radical polymerization of styrene: (a) molecular weight M", (b) grafting densities 5(PS), and (c) polydispersity of the covalently attached polymers ............................................................. 23 Optical micrographs of patterned surfaces ............................................... 26 Optical micrographs of PHEMA (177 nm)-g-PLA (353 nm) showing the evolution in surface morphology during the drying of water-swollen films .................................................................................. 59 Reflectance FTIR spectra of (a) the initiator layer anchored to the Au surface, (b) a 170 mn PHEMA polymerized from the initiator layer, (c) a PHEMA (170 nm)-g-PLA (85 mm) film formed after 4 h of lactide polymerization, (d) a PHEMA (170 nm)-g-PLA (226 nm) film formed after 24 h ....................................................................... 66 xiv Figure 2.3. Figure 2.4. Figure 2.5. Figure 2.6. Figure 2.7. Figure 2.8. Evolution of the film thickness with polymerization time during the surface initiated ATRP of HEMA from initiators anchored on Au ............................................................................................................. 68 The evolution of thickness with polymerization time for the ROP of PLA from PHEMA using: I, 174 :1: 2 nm thick PHEMA substrates, :1, 49 i 1 nm thick PHEMA substrates .................................. 7O Reflectance FTIR spectra of (a) PHEMA (153 nm)-g-PLA(362 run) before degradation, (b) PHEMA-g-PLA films after 6 h hydrolytic degradation, (c) PHEMA-g-PLA films after 10 days hydrolytic degradation, ((1) PHEMA-g-PLA films after 15 days hydrolytic degradation ............................................................................................... 72 Surface images of PHEMA (153 nm)-g-PLA (362 nm) obtained by Optical microscopy during hydrolytic degradation in pH 7 .4 buffer at 55 °C: (a) before degradation, (b) after 6 hrs of degradation of PLA, (c) after 18 hrs of degradation of PLA, ((1) after 42 days of degradation of PLA .................................................................................. 73 Surface images of PHEMA (153 nm)-g-PLA (362 nm) obtained by optical microscopy after 6 h of hydrolytic degradation in pH 7.4 buffer at 55 °C: (a) water-covered substrate, (b)-(c) substrates drying in the air, ((1) completely dry substrate ......................................... 74 Surface images of PHEMA-g-PLA obtained by optical microscopy after 6 h of hydrolytic degradation in pH 7.4 buffer at 55 °C: (a) PHEMA (168 nm)-g-PLA (65 nm), polymerization of PLA at 70 °C; (b) PHEMA (188 nm)-g-PLA (336 nm), polymerization of PLA at 80 °C; (0) PHEMA ( 153 nm)-g-PLA (362 nm), polymerization of PLA at 90 °C; (d) PHEMA (183 nm)-g-PLA (367 nm), polymerization of PLA at 100 °C. ........................................... 76 XV Figure 2.9. Figure 3.1. Figure 3.2. Figure 3.3. Figure 3.4 Figure 3.5. Figure 3.6 Figure 3.7. Figure 3.8 Surface images of PHEMA-g-PLA obtained by optical microscopy after 6 h of hydrolytic degradation in pH 7.4 buffer at 55 °C: (a) PHEMA (183 nm)-g-PLA (520 nm), polymerization of PLA at 90 °C; (b) PHEMA (153 nm)-g-PLA (362 nm), polymerization of PLA at 90 °C ............................................................................................ 77 Results from 1H NMR analysis of the reaction of 2-BPB and 2- MPB with hexadecanol ............................................................................ 92 Reflectance F TIR spectra of diluted-initiator monolayers on gold substrates prepared from mixtures of 1 and 2. (a) 100% 1, (b) 50% 1, (c) 25% l, (d) 1% 1 .............................................................................. 93 Evolution of the ellipsometric brush thickness with time for the polymerization of MMA from diluted-initiator monolayers on Au substrates at 28 °C .................................................................................... 94 Polymerization rate and normalized polymerization rate in surface- initiated polymerization of MMA from diluted-initiator monolayers on Au Substrates 97 The initiating efficiency in ATRP of MMA from surfaces with various initiator densities (data calculated from GPC results) ................. 99 Evolution of the ellipsometric brush thickness with time for the polymerization of HEMA from diluted-initiator monolayers on Au substrates at 28 °C ................................................................... 100 Polymerization rate and normalized polymerization rate in surface- initiated polymerization of HEMA from diluted-initiator monolayers on Au substrates. .................................................................. 101 Aqueous swelling of brushes prepared by polymerization of HEMA from diluted-initiator monolayers on Au Substrates. .................. 105 xvi Figure 3.9. Figure 4.1. Figure 4.2. Figure 4.3. Figure 4.4. Figure 4.5. Figure 4.6. Figure 5.1. Plot of ellipsometric polymer brush thickness vs polymerization time for polymerization of GMA at 28 °C on Au substrates anchored with mixed initiators ................................................................. 106 Possible products of the reaction of alkylchlorosilane with SiOz surfaces .................................................................................................... 113 Evolution of the ellipsometric brush thickness with time for the polymerization of HEMA from 100% initiator monolayers on SiOz substrates at 28 °C .................................................................................... 120 Evolution of the ellipsometric brush thickness with time for the polymerization of HEMA from diluted-initiator monolayers on SiOz substrates at 28 °C ........................................................................... 122 Polymerization rate and normalized polymerization rate in surface- initiated polymerization of HEMA from diluted-initiator monolayers on SiOz substrates ................................................................. 124 Evolution of the ellipsometric brush thickness with time for the polymerization of MMA fiom diluted-initiator monolayers on SiOz at 28 °C .................................................................................................... 126 Evolution of the ellipsometric brush thickness with time for the polymerization of HEMA from diluted-initiator monolayers (with two different diluents) on Si02 at 28 °C .................................................. 130 Reflectance FTIR spectra of gold substrates coated with (a) an immobilized initiator layer; (b) 150 nm PtBA brushes grown from the initiator layer; (c) 60 nm PAA brushes prepared by a 10-min hydrolysis of the PtBA film in a 150 mM solution of CH3SO3H in CH2C12; and (d) PAA brushes after immersion in a pH 10 buffer solution for 10 min and rinsing with ethanol ........................................... 142 xvii Figure 5.2. Evolution of the ellipsometric brush thickness with time for the polymerization of tBA from initiator monolayers on Au substrates at 50 °C .................................................................................................... 144 Figure 5.3. Evolution of the ellipsometric brush thickness with time for the polymerization of tBA from initiator monolayers on Au substrates using three different ligand systems ......................................................... 146 Figure 5.4. Evolution of the ellipsometric brush thickness with time for the polymerization of tBA from initiator monolayers on Au substrates using Me4Cyclam and anbpy as the Cu(II) source ............. I ................... 147 Figure 5.5. Reflectance FTIR spectra of gold substrates coated with (a) 97 nm PtBA brushes grown from the initiator layer; (b) PtBA (97 nm)- block-PMMA (210 nm) copolymer brushes grown fi'om the PtBA brush layer ................................................................................................ 148 Figure 5.6. Evolution of the ellipsometric brush thickness with time for the polymerization of tBA from initiator monolayers on Au substrates using different solvent systems ................................................................ 150 Figure 5.7. Evolution of the ellipsometric brush thickness with time for the polymerization of tBA from initiator monolayers on Au substrates using the same catalyst system at three different temperatures ................ 151 Figure 5.8. Evolution of the ellipsometric brush thickness with time for the polymerization of tBA from initiator monolayers on Au substrates at different monomer concentrations ....................................................... 152 Figure 5.9 Evolution of the ellipsometric brush thickness with time for the polymerization of tBA from initiator monolayers on Au substrates at 50 °C at various Cu(D/Cu(II) ratios ..................................................... 153 Figure 5.10. Evolution of the ellipsometric brush thickness with time for the polymerization of tert-butylacrylate (tBA, I), methyl methacrylate xviii Figure 5.11. Figure 5.12. Figure 5.13. (MMA, Cl), styrene (o), 4-vinyl pyridine (4-VP, 0) from initiator monolayers on Au substrates ................................................................... 155 Evolution Of the ellipsometric brush thickness with time for the polymerization of HEMA from initiator monolayers on Au substrates using two different catalyst systems ........................................ 156 Reflectance FTIR spectra of gold substrates modified with polymer brushes grown from immobilized initiators for 1 h. (a) 50 nm polystyrene; (b) 100 nm poly(4-viny1 pyridine), after polymerization for 24 h; (c) 120 nm poly(methyl methacrylate); and (d) 160 nm poly(2-hydroxy ethyl methacrylate). A UV/O3 cleaned gold slide was used as a background .......................................... 157 Evolution of the ellipsometric brush thickness with time for the polymerization of tert-butylacrylate (tBA, I), methyl methacrylate (MMA, Cl), styrene (0), 4-vinyl pyridine (4-VP, 0) from initiator monolayers on Au substrates ................................................................... 159 xix AIBN AF M ATR ATRA ATRP Av 2-BPB 2-MPB BPMA BPMOA bpy CL CYCLAM DETA depy DMF dNbpy anbpy DP EtOAc FTIR GC GMA GPC HEMA HMTETA kp k: kact kdeact lactide LB LCST MA Me Me4Cyclam Me6TREN uCP MMA Mn MS MUD LIST OF ABBREVIATIONS 2,2’-Azobisisobutyronitrile Atomic force microscopy Attenuated total reflectance Atom transfer radical addition Atom transfer radical polymerization Average cross-sectional area of polymer chains 2-Bromopropionyl bromide 2-Methylpropionyl bromide N,N-bis(2-pyridylmethyl)amine N,N-bis(2-pyridylmethyl)octylamine 2,2’-Bipyridine 6-Caprolactone 1 ,4,8,1 l-Tetraazacyclotetradecane Diethylenetriamine 4,4'-Diheptyl-2,2'-bipyridine N,N-dimethylformamide 4,4'-Di(5-nonyl)-2,2'-bipyridine 4,4'-Di(n-nonyl)-2,2'-bipyridine Degree of polymerization Ethyl acetate Fourier transform infrared Gas chromatography Glycidyl methacrylate Gel permeation chromatography 2-Hydroxyethyl methacrylate 1 ,1 ,4,7,1 0,1 O-Hexamethyltriethylenetetramine Polymerization rate constant Termination rate constant Activation rate constant Deactivation rate constant 3,6-Dimethyl-1 ,4-dioxane-2,5-dione Langmuir-Blodgett Lower Critical Solution Temperatures Methyl acrylate Methyl 1 ,4,8,1 1 -tetramethyl-1 ,4,8,1 l-tetraazacyclotetradecane tris[2-(Dimethy1amino)ethyl]amine Microcontact printing Methyl methacrylate Number average molecular weight Mass spectroscopy Mercaptoundecanol XX Mw MWNT NA NMP NMR OTf PAA PBA PCL PDI PDMS PEI PEG PGMA Ph PHEMA phen PLA PMA PMEMA PMMA PMDETA PNIPAAM PS PtBA PTFE PVP QR RAFT ROMP ROP RP SAM SEM t tBA TEMPO TETA TI-IF TMEDA TMSCl tthy TPMA tPY TREN Weight average molecular weight Multiwalled carbon nanotubes Avogadro’s number Nitroxide-mediated polymerization Nuclear magnetic resonance Triflate Poly(acrylic acid) Poly(n-butyl acrylate) Poly(a-caprolactone) Polydispersity index calculated as Mw/Mn Polydimethylsiloxane N-propionylethylenimine Poly(ethylene glycol) Poly(glycidyl methacrylate) Phenyl Poly(2-hydroxyethyl methacrylate) 1 , 1 O-Phenanthroline Poly(lactide) Poly(methacrylate) Poly(2-(N-morpholino)-ethy1 methacrylate) Poly(methyl methacrylate) N,N,N',N',N"-pentamethyldiethylenetriamine Poly(N-isopropylacrylarnide) Polystyrene Poly(tert-butyl acrylate) Poly(tetrafluroethylene) Poly(4-vinyl pyridine) Quenching and re-initiation Reversible addition-fragmentation chain transfer Ring-opening metathesis polymerization Ring-opening polymerization Rate of polymerization Self-assembled monolayer Scanning electron microscopy Film thickness tert-Butyl acrylate 2,2,6,6-Tetramethylpiperidinyloxy Triethylenetetramine Tetrahydrofuran Tetramethylethylenediamine Trimethylchlorosilane 4,4’ ,4”- T ris(5-nonyl)-2,2’ :6’ ,2”-terpyridine T ris[(2-pyridyl)methyl]amine 2,2’:6’,2”-Terpyridine T ris[2-arninoethyl] amine xxi 4-VP XPS Ultraviolet 4-Vinyl pyridine X-ray photoelectron spectroscopy Density xxii Chapter 1. Introduction I. Polymer Brushes [-1. Their Definition and Physical Properties As a defined by Milner, a polymer brush refers to an assembly of polymer chains tethered by one end to a surface or an interface.1 The physical properties of polymer brushes largely depend on the chain length, grafting density, backbone flexibility, and excluded volume. Alexander2 and de Gennes3 first developed the scaling theories for polymers irreversibly attached by one end to a surface. Three different regimes were identified that depend on the graft density and the distance between neighboring surface- attached polymer molecules (Figure 1.1). The simplest case is when the distance between chains is larger than the characteristic size of the polymers. The polymers do not overlap, and in this case, the conformation of the polymer depends on the interaction between the polymer segments and the surface. Strong interactions lead to polymer segments adsorbing to the surface and a “pancake” conformation for the tethered polymer. Non- absorbing polymers are described as having a mushroom conformation, where the chain segments avoid contact with the surface leading to a coiled polymer tethered through a short, terminal polymer segment. A high chain density leads to substantial inter and intramolecular segrnent-segrnent interactions, forcing the polymers to extend from the surface. This conformation is termed a “polymer brush”. In polymer brushes, the polymer chains may be tethered to the surface of a solid substrate, at an interface between two liquids,_between a liquid and air, or between melts or solutions of homopolymers. Anchoring polymer chains to surfaces or interfaces can be reversible or irreversible. All of the polymer brush systems shown in Figure 1.2 have a 5 configuration different from that of a free chain in solution." 4’ Figure 1.1. Schematic illustration of the conformation of polymers end-attached to a surface: (a) “pancakes”; (b) “mushrooms”; (c) “brushes”. ’ I A adsorbed dIbIOCk polymer micelle dIbIOCk copolymer copolymers melt - O ' ’ l V - t I - A graft copolymers at block copolymers at end-grafted fluid-fluid interfaces fluid-fluid interfaces polymers Figure 1.2. Examples of polymer systems comprising polymer brushes. (Redrawn with permission from Prog. Polym. Sci. 2000, 25, 677-710. Copyright 2000 Wiley-VCH.) I-2. Polymer Brushes on Various Substrates and their Applications Polymer brushes may be useful in a broad range of applications because their mechanical and chemical robustness can be augmented by inclusion of a variety of functional groups and nanostructures. In the early 19505, grafting polymer chains onto colloidal particles was found to be an effective strategy for preventing flocculation.“9 In these “steric stabilized” systems, the attached polymer chains prefer the suspension solvent to the colloid particle surface, and their extension into the solvent stabilizes colloids by their resistance to chain-chain overlap. The repulsive force between particles ultimately arises from the high osmotic pressure inside the brushes. More recent applications of polymer brushes include new adhesive materials,10 protein-resistant biosurfaces,ll chromatographic devices,12 lubricants,l3 polymer surfactants1 and polymer compatibilizers.l Some polymer brushes have Lower Critical Solution Temperatures (LCSTs) near room temperature and exhibit different wetting properties above and below the LCST.14 Polymer brushes covalently tethered on porous membranes can act as pH sensitive, photosensitive, and redox sensitive chemical gates.”l7 Suter et a1. prepared polystyrene brushes on high surface area mica for the fabrication of organic-inorganic 18,19 hybrid nanocomposites, and patterned thin organic films have been investigated for 0 controlled cell growth,21 biomimetic material applications in microelectrics,2 fabrication,22 as microreaction vessels and for drug delivery.” The broad spectrum of polymer brush applications also stems from their compatibility with various substrates, including flat surfaces, particles or macromolecules (Figure 1.3). O O polymer brushes on polymer brushes on polymer brushes on flat surface non-planar surface macromolecules Figure 1.3. Examples of polymer brushes tethered from various substrates, such as flat wafers, particles, colloids and polymers. 1-3. Polymer Brushes on Flat Surfaces Polymer brushes on flat wafers and surfaces are the most extensively investigated systems, partly due to well-developed surface characterization methods such as ellipsometry, contact angle measurements, x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), and fourier transform infrared (FTIR), but also from potential applications in advanced responsive materials, nanoprinting, and biotechnology. 24-26 The responsive nature of some polymer brushes upon changes in solvent, 27’ 28 also lead to altered film morphologies, thickness, ionic strength,27 or temperature and/or barrier properties. A number of these stimuli-responsive or “smart” brushes have been synthesized and characterized. Block copolymer brushes grown from flat substrates have been widely studied because their conformation changes with changes in their environment. Brittain et a1. grew ABA triblock copolymer brushes from flat silicon substrates where the physicochemical properties of the middle block are different from the end blocks.26 As shown in Figure 1.4, the film should adopt an extended brush configuration when exposed to a good solvent for all the blocks, but when exposed to a good solvent for only the middle, the brush will fold to minimize exposure of the terminal A block to solvent. Experimentally, poly(methyl acrylate)-b-polystyrene (PMA-b-PS) brushes synthesized from a planar silicon surface show ~20° change in the advancing water contact angle when exposed to dichloromethane and cyclohexane. “Smart” polymer brushes can also be prepared by attaching poly(N-isopropylacrylamide) (PNIPAAM) brushes, which exhibit an LCST of ~ 32-33 °C. Below the LCST, the polymer is an extended brush and the surface is hydrophilic due to intermolecular H- bonding between PNIPAAM chains and water. Above the LCST, the water is expelled, and intermolecular hydrogen bonding renders the collapsed polymer structure hydrophobic. By tethering these polymer brushes to a well-defined rough surface, Jiang et al. created polymer fihns that switch between superhydrophobicity (contact angle ~ 150°) and superhydrophilicity (contact angle ~0°) over a ~10 °C change in temperature.29 Similar temperature responsive behavior for other systems, may prove to be important in thermally responsive drug-delivery vehicles and temperature-controlled gates or switches.” 3‘ Polymer brushes grown from flat surfaces also are useful materials for many biological applications such as substrates for protein microarrays, which allow high throughput studies of protein-protein specific interactions. However, many biological systems tend to physically adsorb onto solid substrates without specific receptor- recognition interactions (nonspecific adsorption), and the background noise associated with nonspecific adsorption typically reduces the efficiency of protein microarrays. Poly(ethylene glycol) (PEG) is often used for biological applications because it is nontoxic, non-immunogenic, and resists protein and cell adhesion. Chilkoti et al. polymerized PEG-containing acrylate monomers from a flat gold surface and found that the polymer brushes successfully resisted protein and cell adhesion.32 Andruzzi et al. also produced PEG-containing polymer brushes that resisted protein and cell adsorption.33 good solvent for middle block SE 8 goodsolvent /T///7///7////// for all blocks f///7/////////// Figure 1.4. Reversible response of triblock-copolymer brushes to different solvents. Polymer brushes anchored to flat surfaces are compatible with a number of patterning and microfabrication strategies. Various techniques such as standard photolithography,34 ultraviolet litrhography,35 microcontact printing (uCP),36’ 37 nanoshaving,25 electron-beam lithography,38 and dip-pen lithography"9 were used to produce micro to nano-sized features in polymer brushes attached to surfaces. Microcontact printing” 37 and ultraviolet light lithography” generally enable rapid patterning of large surface areas (up to a few cmz). Serial processes such as electron beam lithography produced features as small as 70 nm with reasonable throughput.38 AFM- based techniques that use the AF M tip to remove and backfill areas (nanoshaving)25 or deposit molecular components (dip-pen lithography)39 can in principal produce submicrometer features with site-specific control over features. 1-4. Polymer Brushes on Particles and Non-planar Surfaces A variety of non-planar surfaces have also been used as substrates for polymer 42-46 brushes, including gold,4°’ 4' silica, alumina,47 clay,48 latex,49 dextran,so magnetic particles,51 and carbon nanotubes.”57 Nearly any substrate may be suitable as long as the surface can be functionalized and it is compatible with the method used to tether the brush. Most modifications of non-planar substrates are motivated by the need to alter the interactions of these materials with the surrounding environment. Substantial work has been devoted to non-planar surfaces in chromatographic supports, nanoparticles and nanotubes. In separations research, the emphasis has been functionalizing materials to improve selectivity while nanotube and particle research emphasizes dispersion of these materials in solvents, encapsulating particles in a polymer film to protect them from chemical or mechanical damage, or preparing well-defined nanostructures. Surface modification is widely used to improve the selectivity of separation process.“' 58' 59 Buchmeiser et al.“’ 60 used ring opening metathesis polymerization (ROMP) to grow functionalized norbomenes from silica surfaces for the chromatographic separation of phenols, anilines, lutidines, and hydroxyquinolines. Preparing supports by coating polymer solutions onto silica tends to clog pores and reduce surface area. Anchoring polymer brushes to surfaces avoids clogging and provided improved separations. They also used reverse-phase chromatography to separate biomolecules using copolymers of 2-norbomene and 1,4,4a,5,8,8a-hexaydro-1,4,5,8-exo,endo- dimethano-naphthalene grafted onto borosilicate monoliths.59 Separation of the various proteins was dependent on the physicochemical properties and microstructure of the grafted monoliths. There also has been substantial interest in polymer brushes tethered to particles that range from micrometer to nanometer in size. As noted earlier, research on polymer brushes attached to particles has mainly focused on increasing their solubility or dispersibility in solutions. As an example, carbon black has wide applications as a reinforcing agent for rubbers, pigments for coatings, inks and toners. However, small carbon blacks (10-75 nm) easily fuse into aggregates that range from 50-500 nm in size. Grafting polymers onto the surface of carbon black provides steric stabilization against flocculation and greatly improves their dispersal in resins and solvents.“ 62 Other efforts have been devoted to the synthesis of novel materials. Blomberg et a1. created hollow nanospheres by attaching poly (styrene-co-vinylbenzocyclobutene) or poly (styrene-co- maleic anhydride) brushes on 600 nm size silica nanoparticles.63 Both polymers are cross-linkable at high temperatures, and after cross-linking the polymer brushes, the silica core was etched away with HF leaving a hollow polymer capsule. The protection or gradual release of an inner substance such as drugs and dyes was suggested as an application for nanocapsules. Single walled and multi-walled carbon nanotubes have the potential to be used in a variety of applications (molecular wires, sensors, and composite materials) due to their extraordinary mechanical and electrical properties?“ 55 However, the poor solubility in most solvent hinders their processability.54 Tethering polymer brushes to carbon nanotubes renders them soluble in common solvents with minimal changes in the nanotube structure. Qin et a1. successfully attached PS brushes to carbon nanotubes, and observed that in addition to good solubility in common organic solvents, the initial carbon nanotube bundles tend to separate into small bundles or even individual tubes during the attachment process.52 Kong et a1. synthesized poly(methyl methacrylate) (PMMA)-b-poly(2-hydroxyethyl methacrylate) (PHEMA) polymer brushes from carbon nanotubes, and the solubility of the resulting nanotubes was dependent on solvent, with good solvents for PHEMA providing effective solvation.53 Figure 1.5. Tapping mode AFM image of a molecular brush cast onto a mica substrate possessing a main chain DP = 400 and poly(n-butyl acrylate) side chains of DP = 30. (Reprinted with permission from Macromolecules. 2001, 34, 8354-8360. Copyright 2001 American Chemical Society.) [-5. Polymer Brushes Tethered on Macromolecules Densely grafted linear or dendritic copolymers are described as “molecular brushes.” Due to the high local concentration of tethered chains, the grafted chains must extend away from the polymeric backbone to minimize steric crowding effects. Initially, the attention to molecular brushes was largely driven by their possible visualization by AFM. When densely grafted brushes are deposited on surfaces, favorable interactions between the graft chains and the surface cause the chains to spread on the surface, enabling AFM imaging with single chain resolution. Shown in Figure 1.5 is such an example, where Matyjaszewski et al. used AFM to image a molecular brush with poly(n- butyl acrylate) (PBA) grafted side chains that had been deposited on mica.“ 65 These materials were recently applied as templates for the directed mineralization of inorganic nanocrystals66 and for the formation of high-aspect-ratio nanowires.67 II. Surface-initiated Polymerizations II-l. Preparation of Polymer Brushes There are two general ways to prepare polymer brushes: physisorption and covalent attachment. While physisorption processes use multiple weak polymer-surface interactions to anchor the polymer to the substrate, both the “grafting to” and “grafting from” (surface-initiated polymerization) approaches connect the polymer to the surface through strong covalent bonds. Physisorption is a reversible process for tethering of polymer chains to a solid surface, usually achieved by the self-assembly of polymeric surfactants or end- functionalized polymers on the surface.68 Homopolymers, block copolymers and graft copolymers have been successfully tethered onto substrate surfaces via physisorption.”72 The resulting polymer brushes are often thermally and solvolytically labile due to the weak van der Walls forces or hydrogen bonding interactions, and exposure to a good solvent or more strongly adsorbing species may cause desorption. ll In a “grafting to” approach, pre-forrned end-functionalized polymer molecules react with an appropriate substrate to form polymer brushes. This approach is now more widely used than physisorption for the preparation of polymer brushes since the covalent bond formed between surface and polymer chain makes the polymer brushes more robust. The substrate surface is usually modified to accorrunodate the functional groups at the terminus of the polymer chain by modifying the surface with coupling agents or SAMs. 1.73’ 74 synthesized a series of thiol-terminated polystyrenes with a low PDI, Koutos et a and these polystyrene chains were end-grafted to a gold surface via formation of gold- thiolate bonds. Using hydrosilation, Yang et a1. successfully prepared poly(methylhydrosiloxane) brushes on silicon surfaces that had been modified with vinyl- terminated SAMs.75 In most “grafting to” cases, only a small amount of polymer can be immobilized onto the surface since polymer chains must diffuse through the existing polymer film to reach the reactive sites on the surface. As the film thickness increase, this steric barrier becomes more severe. Thus, polymer brushes obtained by the “grafting to” approach generally have a low grafting density and low film thickness. Comparing to physisorption and the “grafting to” method, the “grafting from” approach can provide polymer brushes with a high grafting density. “Grafting from” is accomplished by covalently anchoring initiators on a surface, and activating the initiators to start the polymerization. Recent progress in polymer synthesis techniques makes it possible to produce polymer chains with controllable lengths. Polymerization methods used to synthesize polymer brushes include cationic, anionic, ring opening, free radical and controlled radical polymerization. The following sections will emphasize the 12 synthesis of polymer brushes using these various surface-initiated polymerization methods. “-2. Surface-initiated Cationic Polymerizations In the early 19805, Vidal et al.76’77 used surface-initiated cationic polymerization to graft polyisobutylene to a silica surface. 2-(Chloromethylphenyl)ethyldimethyl chlorosilane was anchored to the silica surface, and the reaction of diethylaluminum chloride with the immobilized initiator produced carbocationic species which initiated the polymerization of isobutylene. Jordan and Ulrnan78 reported performed surface-initiated cationic polymerization of N—propionylethylenimine (PEI) on gold surfaces. (Scheme 1.1) They first formed a hydroxy-terminated SAM on gold surfaces and then exposed the monolayer to a stream of trifluoromethanesulfonic anhydride vapor to convert the hydroxy groups to triflates. After polymerization of PEI for 7 days under reflux, ellipsometry measurements indicated formation of a 10 nm polymer brush, which they confirmed by external reflection FTIR and contact angle measurements. 13 Scheme 1.1. Surface-initiated cationic polymerization of 2-oxazolines. (Redrawn with permission from J. Am. Chem. Soc. 1998, 120, 243-247. Copyright 1998 American Chemical Society.) OH OH HO HO SOZCFg O I B Et—(Oj /802CF3 O A N \ N 0 f9 sochs \ / 0T EtOH; r..;t 16 h 1 day; 0 DC _, rem; = 10 umol/l vapor phase R R HS al I alk l 0sz ka Y N’ o lat—<6] Fa'ky' "Le NH) terminal n \_J 90" HN\_ functionalin ’1 ‘ alkyl ————> > E1 N 1 day 7 polymer 7 days; 1 _’ CHCI3; reflux "' CHC'3' "t' SAM R Recently, Zhao and Brittain79 successfully synthesized PS brushes via surface- initiated cationic polymerization. 2-(4-Trichlorosilylphenyl)-2-methoxy-d3-propane was immobilized on a silica substrate and addition of TiCl4 initiated polymerization of a 34 nm thick brush. The initiator efficiency estimated by FTIR-ATR (attenuated total reflectance) was ~ 7%, and additional initiator was consumed by termination reactions. 14 Because cationic polymerizations are ionic reactions, factors such as solvent polarity and Lewis base additives will influence the brush thickness. 11-3. Surface-initiated Anionic Polymerizations Anionic polymerization, the most widely used living polymerization technique, has also been adapted to the synthesis of polymer brushes via the “grafting from” approach. Jordan et al. used anionic polymerization to synthesize polystyrene brushes on gold substrates.80 As shown in Scheme 1.2, a SAM of 4’-bromo-4-mercaptobiphenyl on gold reacted with sec-BuLi to form a monolayer of'biphenyllithium. Addition of styrene initiated the polymerization, eventually forming 18 nm thick PS brushes. Based on the ellipsometric data from in situ swelling experiments, the grafting density was calculated to be 32-36 an/chains. The initiating efficiency was estimated to be ~ 8%. Scheme 1.2. Surface-initiated anionic polymerization of styrene on gold 0 oé a» 000 MOODODO S I J8 Au I Au rpm 15 Scheme 1.3. Surface-initiated anionic polymerization of acrylonitrile on SiOz OHOHOHOHOH I I l I I Br(CH2)3SiCI3 -3HC| Li Li Br Br ‘ Li /Sll\ /Sl|\ SI SI OOOOO III II \ _O- / 2‘ / 2. MeOH l6 Qingye et al. also polymerized styrene from clay surfaces using anionic polymerization.“ A linear relationship was found between the monomer concentration and the Mn of the cleaved polymers, which is consistent with a living anionic polymerization mechanism. Ingall et a1. polymerized acrylonitrile from SiOz using a similar strategy.82 A SAM formed from 3-bromopropyltrichlorosilane was lithiated with lithium di-tert-butylbiphenyl, and subsequent addition of monomer to the system initiated the anionic polymerization (Scheme 1.3). Polymerization for 8 days yielded tethered poly(acrylonitrile) films with thicknesses up to 245 nm. 114. Surface-initiated Ring-opening Polymerization Surface-initiated ring-opening polymerization (ROP) is an attractive route for coating surfaces with thin layers of polycaprolactone, polylactide and other polymers. Husseman and coworkers prepared a SAM terminated with di(ethylene glycol) moieties,36 and using the pendent OH groups for initiation, they carried out the aluminum alkoxide catalyzed ROP of e—caprolactone (CL). They obtained 70 nm thick PCL brushes after a few hours at room temperature. Terminating the SAM with di(ethylene glycol) gave more reproducible polymer brush growth and better long-term stability than simple long chain alcohol SAMs. In related work, Choi and Langer formed an oligo(ethylene glycol) terminated SAM on gold, and used tin(II) (2-ethy1hexanoate); (Sn(Oct)2) to catalyze the ROP of L- lactide from Au and silicon substrates83 (Scheme 1.4). Poly(lactic acid) (PLA) is an important biodegradable polymer used in medical applications, and PLA brushes present a possible route to well-defined surfaces with controlled release properties. 17 Polymerization for 3 days at 40 °C provided PLA brushes up to 12 nm thick, and 70 nm thick PLA brushes were obtained from silicon surfaces after polymerization for 3 days at 80 °C. The PLA brushes were reported to be chiral and crystallized on the surface. Scheme 1.4. Surface—initiated ring Opening polymerization of lactide. (Redrawn with permission from Macromolecules 2001, 34, 5361-5363. Copyright 2001 American Chemical Society.) 0 i I/ X“ o o aid/Kr III/\O n Sn(Oct)2 SAMs, X = Oor NH Au i’SAfl/‘(OAIOH HSwO/YOH 9 3 9 3 1 Wv’. Q, 'i H Si/Si02 03,5; N\/\NH2 (MeolssiWNwNH 2 \ 2 It has been suggested that coating conductive substrates with well-defined polymer brushes such as functionalized norbomenes can be useful in the production of polymer electronic devices. These strained cyclic monomers are usually polymerized by ROMP. As shown in Scheme 1.5, various norbomene-derived polymer brushes were 18 grown from silicon surfaces by Whitesides and co-workers.84 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. Addition of the monomer initiated a rapid, but controlled polymerization, producing 90 nm thick brushes in 30 min. The formation of block copolymer brushes and the use of microprinting to produce patterned surfaces also was described. Scheme 1.5. Surface-initiated ring opening metathesis polymerization of functionalized norbomenes. (Redrawn with permission from Macromolecules 2000, 33, 2793-2795. Copyright 2000 American Chemical Society.) “‘7'" “7”" 3' 37% 9% A? 9% [R] OH ———> O- Si O— Si ‘3:- %- [RU] “7“” 9 /[R] 3’0 Si 0 Cl PC” Q IE Ii 0 O ;Ru‘x"‘H SiCl3 / Si(OEt)3 MN-Cm Cl/ PCY3 0 1 2 3 4 l9 Poly(norbomene) brushes were also grown from silicon substrate by Grubbs and co-workers using surface-initiated ROMP85 using an alternative initiator attachment scheme. A direct Si-C bond to the surface was used to anchor the initiator instead of the Si-O bond formed via condensation of chlorosilanes. The polymer brushes grown from the initiatiators was very thick (up to 5.5 pm). II-S. Surface-initiated Free Radical Polymerization In a typical surface initiated radical polymerization, the immobilization of the radical initiators usually involves a series of steps. In an early example Boven et a1. initiated radical polymerization of methyl methacrylate (MMA) chains fiom immobilized azo initiators.86 Sugawara and Matsuda used a similar strategy to graft PS on poly(vinyl alcohol) film, and poly(acrylamide) on poly(ethylene terephthalate) film.87 The reactive azo initiators were attached onto the surface by coating the substrate with partially derivatized poly(allylamine). Minko et a1. studied surface initiated radical polymerization using both theoretical and experimental approaches.”93 In their experimental work, azo or peroxide initiators were attached on solid substrates by either physisorption or chemical immobilization. Initiator anchoring comprised of priming surfaces with 3-glycidyloxypropyl- trimethoxysilane followed by the reaction with 4,4’-azobis(4-cyanovaleric acid). The surface-initiated radical polymerization was followed by in situ ellipsometric measurements of the amount of grafted chains. The resulting kinetics showed a linear dependence of the polymerization rate on the surface concentration of the initiator and an 20 Scheme 1.6. Synthesis of polystyrene brushes on silica and cleavage of the polymers from the surface CN Me O MeMe OH + Cl—Sli—/\/ M N¢N\}/ e Me 0 CN 1 base CN II”3 0 MeMe O—Si—/\/ «NT l Me Me O N J toluene/styrene 4:“ 3 (D N 9 (D heat, -N2 Me CN 0 Me 0 Me nonbonded polystyrene n p-TsOH MeOH/toluene CN Me OH + Me/O o—s'I—/\/ 0 Me Me inverse square root dependence on the initiator concentration in solution, which is consistent with conventional free radical polymerization. However, their method of anchoring initiator led to low initiator densities and side reactions. 21 To circumvent this problem, Rithe et a1. developed a one step initiator anchoring strategy to initiate flee radical polymerization flom surface.45'94'96 As shown in Scheme 1.6, their initiator system includes three important components: (1) an azo group that produces flee radicals upon heating or irradiation by UV (ultraviolet), (2) a chlorosilane acting as the linker between the initiator and substrate surface, and (3) an ester that can be hydrolyzed to detach the polymer brushes flom the surface. The initiator was self- assembled on the surface, and following flee radical polymerization of styrene or other monomers, the ester bonds that connected the polymer brushes to the surface were cleaved. The molecular weights of the detached polymers were determined and compared to the flee polymer formed in solution polymerization. The density of chains on the surface was calculated based on the molecular weight and the mass of the grafted polymers. They found that the average distance between tethered PS chains was 2-3 nm, smaller than the radii of gyration of the corresponding polymer molecules. Wittrner et a1. predicted significant differences between polymer brushes grown flom surfaces and polymers generated in solution during the flee radical polymerization.97 They suggested that polymer brushes formed flom the surface should have a higher polydispersity (PDI) than those formed in the solution. Due to their high mobility, long chains should be easily accessible to monomers and thus more efficient at adding monomers compared to short ones. The PDIs of the detached polymer brushes prepared by Prucker and Riihe ranged flom 1.5 to 2, close to the PDI expected for flee radical polymerizations in solution. Consequently, they concluded that surface immobilization does not cause excessive broadening of the molecular weight distribution. 22 Figure 1.6 shows the molecular weights and polydispersities of the detached polymer brushes. (a) 175 » « .1— 150 r 1 L. g * 1 no, 125{ 1 O E; 100 ~ 1 2 » I 75 ~ - 50 gr .......... 0 2 4 6 8 10 12 14 time[h] (b) (C) 10 L - I . O O-l 2.0- u a ‘ l .9‘ U * O ‘7) 6 ~ E 1.81- 1 n. > E l 1 a 4 - .1, 1.7L J - » a i . E: 2 - 4 ‘8 1.6: - Q 0 1.5r 0C2TL§I 6448‘10‘12T14 o 2 4 6 8101214 time [h] time [h] Figure 1.6. Time-dependent properties of polymer chains grown by surface-initiated flee radical polymerization of styrene: (a) molecular weight Mn, (b) grafting densities 8(PS), and (c) polydispersity of the covalently attached polymers. Reprinted with permission from Macromolecules 1998, 31, 602-613. Copyright 1998 American Chemical Society. 23 Rfihe and coworkers extended their surface-initiated flee radical polymerization strategy to the preparation of block copolymer brushes, where one block was synthesized by ROP.‘A PCL macroinitiator containing azo groups was physisorbed on a silicon oxide surface to initiate the radical polymerization of the other monomer. This simple physisorbed macroinitiator system allows the creation of hydrophobic layers on hydrophilic surfaces. “-6. Surface-initiated Controlled Radical Polymerization Compared to conventional flee radical polymerization, controlled radical polymerizations such as atom transfer radical polymerization (ATRP), nitroxide- mediated polymerization (NMP), and reversible addition-flagrnentation chain transfer (RAFT) provide several advantages for brush synthesis, principally the simple preparation of block COpolymer brushes. The living character of these polymerization systems could provide a better control of molecular weight and polydispersity. The living character of NMP depends on the reversible capping of the active chain-end radical with a nitroxide leaving group. As shown in Scheme 1.7 (a), Husseman et al. described the first example of NMP applied to the synthesis of polymer brushes.98 They first attached alkoxyamine initiators onto the surface and then heated the system to 120 °C to initiate radical polymerization. The stable nitroxide radical 2,2,6,6- tetrarnethylpiperidinyloxy (TEMPO), cleaves during the initiating process and reversibly caps the chain-end radicals to control radical propagation. The addition of flee alkoxyamine initiator provides better control over the molecular weight, but induces polymerization in solution. Later, Hawker et a1.63 formed crosslinked, hollow 24 nanoparticles by using NMP (Scheme 1.7 (b)) to prepare random copolymer brushes of styrene and 4-vinylbenzocyclobutene anchored to silicon nanoparticles. The benzocyclobutenes are sites that readily cross-linked upon heating to 220 °C to form polymer-coated nanoparticles. Using hydrofluoric acid, the silica core was removed to give hollow cross-linked polymer spheres, which can be used for drug delivery. Scheme 1.7. Polystyrene brushes and copolymer brushes grown by nitroxide-mediated polymerization (NMP). (a) Polystyrene, (b) random copolymer, leading to cross-linked hollow nanoparticles. (a) 35:) CH - awe? CW (b) W94 - nangg'acrficles ph>—_< @‘J/ 1) Thermal _. . O 0‘N>_< crosslinking n Ph ii) Remove 0 O Silica (HF) . hollow nanoparticle 25 Hawker and coworkers combined photolithography with NMP to yield patterned polymer brushes with well-defined hydrophobic and hydrophilic domains (Figure 1.7).99 Poly(tert-butyl acrylate) (PtBA) brushes (hydrophobic) was synthesized by surface- initiated NMP and their hydrolysis formed poly(acrylic acid) (PAA) brushes, which is hydrophilic. (a) (b) . _ rxuzoae PAA brush PTBA brush Figure 1.7. Optical micrographs of patterned surfaces: (a) IO-um features in a continuous polymer brush showing regions of poly(tert-butyl acrylate) (dark) and poly(acrylic acid) (light) and (b) 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 flom J. Am. Chem. Soc. 2000, 122, 1844-1845. Copyright 2000 American Chemical Society. 26 Scheme 1.8. Polymer brushes grown by RAFT polymerization. (a) PMMA, (b) PS R—S S COOMe Ph (3) —l—N CN = ‘j—l/H‘S S \ CN ‘11.... 0 CN " 1]: YKOMe 110°C S S RAFT is another important technique for controlled radical polymerization. Chain growth is initiated using a conventional radical initiator such as 2,2’- azobisisobutyronitrile (AIBN), but propagation is mediated by a dithioester chain transfer agent that reversibly adds to chain ends to provide the polymerization its living character. As shown in Scheme 1.8, Brittain et al. synthesized PMMA, poly(N,N- dimethylacrylamide), and PS brushes flom silica surfaces using surface-initiated RAFT polymerization.loo The initiator was anchored via formation of SAM monolayer that contains an azo initiator or a dithiobenzoate group. Although RAFT polymerization is relatively slow compared to techniques such as ATRP and NMP, it is highly living, supported by the easy re-initiation of the polymer chains. 27 III. Atom Transfer Radical Polymerization III-1. Introduction Due to its compatibility with many monomers, flee radical polymerization is widely used to prepare commercial polymers. With knowledge of the kinetic steps associated with initiation, propagation and termination, it is relatively easy to synthesize polymers of predictable molecular weight. However, the inevitable radical coupling and disproportionation reactions that occur during flee radical polymerization prevent the synthesis of polymers with narrow molecular weight distributions, block copolymers, and more complicated polymer architectures. The past decade has seen the development of several schemes for controlled radical polymerization methods such as NMP, RAFT and ATRP. Since its independent development by Swamoto101 and Matyjaszewski,102 ATRP has become one of the most studied methods for controlled radical polymerization. Scheme 1.9 compares radical formation in ATRP with conventional formation of radicals by photochemical or thermal decomposition of initiator precursors. The principal difference between ATRP and conventional flee radical polymerizations is the reversible nature of radical formation in ATRP. Strategies for controlled polymerization exploit reversible radical formation to maintain a low radical concentration during polymerization, thus minimizing the bimolecular coupling and disproportionation reactions that are responsible for termination of the kinetic chain in radical polymerizations. In ATRP, the mechanism used to control radical concentration is a reversible one-electron reduction that transfers a halide flom an initiator to a catalyst, leaving a radical capable of adding monomer in a step identical to that of conventional polymerizations. Setting the equilibrium to favor dormant initiator (oxidized) over the 28 active state (reduced, radical) minimizes, but does not eliminate termination reactions. Therefore, ATRP is not a true living polymerization and is often described as “controlled”. In living polymerizations, there are no termination reaction and the chain ends continue to grow as long as monomer is added to the polymerization. Scheme 1.9 Radical generation in conventional and atom transfer radical polymerization i) Conventional Free Radical Polymerization H H H 3 a: 3 heat or UV ? 3 Nae—c—N=N—c‘:-CEN > 2 Nae—c- + Nzl CH3 CH3 CH3 AIBN ii) Atom Transfer Radical Polymerization i? ”3 i? 9'3 CHacHzo—c—c—Br + Cu(l)Br/2bpy —~ CH3CH20—c—c- + Cu(II)Br2/2bpy CH3 CH3 Ethyl 2-bromoisobutyrate ATRP has its roots in the Kharasch addition reaction.103 ’ '04 As shown in Scheme 1.10, atom transfer radical addition (ATRA) corresponds to one cycle of ATRP, involving reduction of an organohalide by a metal complex to form a carbon centered radical, addition of one monomer, and transfer of the halide to the organic radical to form the addition product and regenerate the catalyst. 29 Scheme 1.10 An example of an atom transfer radical addition Cu(l)C| + cc:4 --‘ Cu(ll)Cl2 + ~ccua °CCI3 + Hzc=CH—x —> Hzc—éH—x 0013 cu —c':H—x + Cu(Il)Cl2 —;—___—~ Hztf—CH—X + Cu(|)Cl ecu ecu III-2. Mechanism and Kinetics Scheme 1.11 shows a simple scheme that captures the salient features of ATRP. The principle difference between ATRP and ATRA is the repeated reversible activation of a substrate that allows the growth of a polymer chain. Matyjaszewski in particular has carried out detailed studies to understand the kinetics of ATRP. The initial step in ATRP is the reduction of initiator to form the radical species R-, which can either add a monomer with a rate constant kp, terminate, or deactivate with a rate constant kdeact. In ATRP, kdeact is normally greater than km to minimize termination. As the radical concentration builds, an equilibrium is set up between R- and the dormant initiator RX. From detailed studies of ATRP for styrene,‘05 MA,‘06 and MMAm’ ‘08 under homogeneous conditions, Matyjaszewski estimated the steady state concentration of radicals in ATRP to be as low as 10'7 to 10'8 M. Assuming rapid establishment of equilibrium and excluding termination effects leads to the rate law shown in equation 1 (for a Cu(I) catalyzed polymerization), and the evolution of the polydispersity (MW/Mn) as a function of conversion in equation 2. As shown in equation 1, the rate of 30 polymerization is inversely proportional to the deactivator (Cu(II)). The equilibrium constant keq is equal to the kact/kdeact, which is also proportional to the polymerization rate. Scheme 1.11 Transition-Metal-Catalyzed ATRP kact R- + x— Mtn—Y /Ligand kdeact U x k ‘x p ~, k x t monomer termination R—X + Mtn—Y I Rp =kp jag—[M][1]O [Cu 1]] eq.1 kdeact [XCu ] k 1 MW =1+[ p[ 10 H 134] eq.2 Mn kdeact[XCu 1 p The first-order kinetics with respect to monomer, initiator, and catalyst (Cu(I)) concentration have been confirmed experimentally. Using ATRP of styrene at 110 °C (1 mol % initiator and catalyst), Matyjaszewski estimated the time intervals, I, for various steps in the polymerization.109 At 30 % conversion, the chain end is activated to a radical every 22 sec, and then is deactivated to its dormant species after 0.018 msec. Since the deactivation rate is seven times faster than propagation (r = 12 msec), one 31 monomer adds to a polymer chain in seven activation/deactivation cycles (2.5 min). Under these conditions, it would take 4 hours to grow a polymer with a degree of polymerization of 100. ATRP can provide well-defined polymers with low-polydispersities (1.05 < MW/Mn < 1.5). As shown in equation 2, the low polydispersities are due to a high deactivation rate and low radical concentration. If km >> kdeact, the polymerization resembles a conventional redox-initiated radical polymerization, resulting in high polymerization rates and high polydispersities. Adding Cu(II) species favors deactivation and decreased radical concentration and termination. Controlled polymerization is achieved at the cost of a slower polymerization rate, but ATRP provides access to polymer structures that cannot be prepared by other methods. III-3. Monomers, Catalysts and Initiating Systems Because it is a radical polymerization, ATRP is compatible with a broad range of monomers including styrenes, acrylates, methacrylates, acrylamides, and acrylonitrile (Scheme 1.12). Each monomer has a unique atom transfer equilibrium constant for its active and dormant species, which is mainly dependent on the structure of the monomer. Whether a monomer performs well in ATRP depends on the structure of the dormant chain end. Removal of a halide from the chain end must be facile, reversible, and lead to a stabilized radical. Methacrylates are particularly well-suited to ATRP and polymerize at 50 °C; styrenes require temperatures near 100 °C. Two major classes of monomers not yet been successfully polymerized by ATRP, acidic monomers and the alkyl substituted olefins. The former fail because they react with the N-containing ligands 32 of most Cu-based systems, while the latter have a very low radical stability and are difficult to polymerize by any radical mechanism. Scheme 1.12. Various monomers polymerized by ATRP \ \ \ \ \ tag: —— —— o l \ —\CEN _>=o N N/ _N\ {Br (:Fé =C>=O =o>=O :0):0 i0 i0 i0 3.. <3 x?” ‘20 J: =10N0)(u\ gk’ 0 Sfihfle3 A variety of metal catalysts have been successfully used in ATRP, including llO.l|l . 1 ' copper, iron, 05 ”2 ruthenium,'°"113 nickel,”4’ ”5 and rhodium complexes.116 The metal center of a good catalyst must have two readily accessible oxidation states separated by one electron and have a reasonable affinity toward the halide in the initiator. In addition, strong ligand binding is helpful as it ensures a stable catalyst, a requirement ofien satisfied by the use of chelating ligands. The most popular ATRP catalyst systems are copper halide complexes having bipyridine or related ligands, which are particularly effective catalysts for polymerization of (meth)acrylate monomers. A variety of initiators, including halogenated alkanes, benzylic halides, a- bromoesters, a-haloketones and sulfonyl halides (Scheme 1.13) have been used 33 successfully in ATRP. A good initiator system provides faster initiation than propagation. The halogen atom plays a very important role in the radical generation step. Bromine and chlorine are most commonly used halides, with brominated initiators more reactive that chlorinated initiators. (The C-Cl bond energy in benzyl chloride is~ 284.7 kJ/mol vs. 213.5 kJ/mol for benzyl bromide.) Because of their instability, iodide initiators are seldom used. The functional groups a to the halide also play a very important role, and stabilize radicals in the order of CN > C(=O)R > C(=O)OR > Ph > C1 > Me. As these groups are commonly found in monomers that undergo radical polymerization, a common strategy is to match the reactivity of the initiator to the dormant polymer chain by choosing an initiator whose structure mimics that of the monomer. Usually the initiator, catalyst, and deactivator have the same type of halogen atom in the initiator and in the metal salt. However, Matyj aszewski found that a mixed initiator system of R-Br/CuCl provides fast initiation, but better control than R-Br/CuBr.117 The use of CuCl presumably increased the deactivation rate due to the formation of the stronger alkyl-chloride bond. 34 Scheme 1.13 Various initiators used in ATRP i) halogenated alkanes and benzylic halides I—x R—$l—Cl X CF, R = H, CH3 R = H, Ph x = Br, Cl COQCH3 X = Bl’, Cl ii) a-bromoesters o o o R H Br /\o/l$< Era/ngoa O\/\ 3' B CH3 R: H, CH3 Mira % Br \/\ Br O\/\D/fi\l/BIr H 0 iii) sulfonyl chlorides r r i R O R—S—CI R S—Cl S—CI if a 0" .. O R = CCI3, CH3 R = OCH3, CH3, F, COZH R = Cl, OCH3 R = H, N(CH3)2 E—Cl COR fi_ _C| /< > K©§Ci 35 III-4. Ligands, Solvents and Other Factors Ligands. The metal ligand in ATRP plays two important roles, to render the transition-metal salt soluble in organic, and in some cases, aqueous media, and to adjust the redox potential of the complex so that the metal center has the appropriate reactivity to carry out the activation/deactivation steps at the heart of the ATRP reaction. The most 118 common ligands include nitrogen chelates for Cu-based catalysts, and phosphorus 9 1 118 20 ligands for transition metals including rhenium,” ruthenium,lo iron, rhodium,l 114 1, nicke and palladium.121 Ligands based on metal coordination to oxygen and sulfiir have also been described, but are used infrequently. The extensive use of Cu-catalyzed ATRP has motivated extensive research on the reactivity of nitrogen-based Cu complexes. In particular, Matyjaszewski et a1. tested a large number Cu catalysts having multidentate nitrogen ligands (Scheme 1.14) and developed structure—activity relationships for the catalysts. In general, the activity of nitrogen-based ligands in ATRP decreases with the number of coordinating sites (N4 > N3 > N2 >> N1) and with the number of linking C-atoms (C2 > C3 >> C4). It also decreases in the order alkyl amine z pyridine > alkyl imine >> aryl imine > aryl amine. The activity of bridged and cyclic systems is normally higher than for linear analogs. These results were interpreted in terms of catalyst activity decreasing as the ligand stabilizes the Cu(I) state of the catalyst and favors deactivation. 36 Scheme 1.14. Ligands used for copper-mediated ATRP i) bidentate ligands R R _ ’— — _ _ \ \/—\/ bPY dNbpy, when R = 5—nonyl phen TMEDA anbpy, when R = n-nonyl dHDDY. when R = n-heptyl 2\ z/ ii) tridentate ligands '3 |\ N |\ /N N/ BPMA, when R = H tpy, when R = H DETA, when R = H BPMOA, when R = n-noctyl tthy, when R = 5-nonyl PMDETA, when R = methyl R 1 Rs (\N/fi IR R’N N‘R iii) tetradentate ligands R R R \N N/ \ \ N N R\fl/ (— 3 I N’ \ EN N] _ _ N _ \ R /N N\ R \ I R/ V R R R TETA, when R = H TREN. when R = H TPMA CYCLAM. when R = H HMTETA, when R = methyl MesTREN, when R = methyl Me4Cyclam. when R = methyl Bipyridine (bpy) is probably the most commonly used ligand in ATRP,‘°2’ 122 but the poor solubility of the Cu(II) complex limits use to relatively polar solvents and monomers. The Cu(II) complexes of alkyl-substituted bipyridines such as 4,4'-dihepty1- 37 2,2'-bipyridine (depy) and 4,4’-di(5-nonyl)-2,2'-bipyridine (dNbpy) are more soluble and provide better control over ATRP in nonpolar solventsws’ ‘23 The use of catalysts with multidentate amine ligands such as N,N’,N’,N",N"-pentamethyldiethylenetriamine (PMDETA), l,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA), and tris[2- (dimethylamino)ethyl]amine (MeéTREN) provides faster polymerization rates than Cu- bpy complexesm’ ‘25 MeéTREN—CuBr and structurally related complexes have lower redox potentials than copper-bpy complexes.126 Compared to bpy, Me6TREN increases the polymerization rate by shifting the dormant species/radical equilibrium to favor a higher radical concentration. Obviously, the increased radical concentration also favors increased termination and there is some loss of control compared to the bpy system. The higher activity of Me6TREN enables activation at lower temperatures. MA was successfully polymerized at ambient temperatures when Me6TREN was used as the ligand in ATRP,125 while the corresponding dNbpy-based complex required polymerization at 90 °C.106 ATRP of MMA using PMDETA as the ligand significantly deviates from the first order kinetics.‘24 Solvents. ATRP can be run in bulk, in solution, or even in a heterogeneous system (emulsion and suspension polymerization). Solvents are commonly used since the catalyst must be soluble to effect the deactivation step in ATRP. Catalyst solubility is a common problem with Cu-based ATRP since the Cu(II) complexes often have lower solubilities and precipitate from nonpolar solvents, leading to loss of control over the polymerization.127 The ATRP of n-butyl acrylate in benzene was poorly controlled with a PDI of 2.4, but switching to the more polar ethylene carbonate provided good control over molecular weight and a low PDI.128 In addition, some polymers such as 38 polyacrylonitrile, are poorly soluble in their monomer. In general, solvents compatible with radical polymerization and the catalyst are acceptable for ATRP, and those used include benzene, toluene, anisole, diphenyl ether, ethyl acetate, acetone, dimethyl formamide, ethylene carbonate, alcohol, water, carbon dioxide, and many others. ATRP in aqueous media is particularly attractive. Armes reported the accelerated ATRP of methoxy-capped oligo(ethylene glycol) methacrylate in water using a Cu(I)/bpy '29 The fast polymerization rate was ascribed to the high polarity of water, WhiCh catalyst. promoted the formation of [Cu(bpy)2]+ a very active mononuclear catalyst. Similar results were reported by Huck and Bruening and Baker in the aqueous ATRP of MMA and 2- hydroxyethyl methaCrylate (HEMA).3 " 130 Temperature. In general, the rate of polymerization in ATRP increases with increasing temperature because both the radical propagation rate constant and the atom transfer equilibrium constant increase with the temperature However, chain transfer and other side reactions may also occur more at elevated temperatures. Additives. Because of its tolerance to a variety of functional groups, additives are normally not required. However, in some cases, additives proved essential for a successful ATRP. For example, polymerization rates for styrene and MMA were greatly accelerated when a small amount of copper(0) was added to the ATRP systems.l3 1‘ '32 Also, Lewis acids (such as aluminum alkoxides) are required for the controlled polymerization of MMA catalyzed by RuClz-(PPh3)3 and similar complexes.”"’133 39 III-5. Surface-initiated ATRP Because of its wide compatibility with various fimctionalized monomers, and its controlled nature, surface initiated ATRP has become the most common “grafting fi'om” approach for surface modification. Of the “grafling from” methods described earlier, surface initiated anionic and cationic polymerizations require rigorously dry conditions; other controlled radical polymerization methods such as NMP, and RAFT either require more complex initiator attachment steps or relatively high temperatures. The synthesis of thiol and silane initiators for surface-initiated ATRP and their attachment to Au and silicon substrates is straight forward, as shown in Scheme 1.15. Scheme 1.15. ATRP initiator attachment to Au and Si02 surfaces —OH -OH Au SiOz —-0H LOH OH Br 0 ’ o H 9' He HS-[CHZ o CI—Si—[CH2+——o é' 11 11 THF V toluene —OH __ O H 0\ >—<~Br Au —-S~[CH2‘]-—O SiOZ—O—Si-{CHzi—O _ O/ 11 11 —OH 40 Fukuda and coworkers published the first example of surface initiated ATRP in 1998.134 Using the Langmuir-Blodgett (LB) technique, they attached a well-ordered monolayer with aryl sulfononyl chloride head groups to a surface. Immersing the substrate in monomer and adding CuCl initiated the ATRP of MMA from the sulfonyl chlorides (Scheme 1.16). The polymerization was not well controlled, but the addition of free initiator to the polymerization solution increased the Cu(II) concentration, the deactivation rate, and control of the polymerization. Concurrently, PMMA formed in solution and was characterized by conventional methods. However, the formation of free polymer requires extensive washing to remove physically adsorbed free polymer. By adding a Cu(II) complex instead of free initiator as a deactivator, Matyjaszewski et a1. achieved controlled polymerization of PS, PMA, and PMA.135 The linear relationship between the thickness of polymer brush and polymerization time confirmed controlled polymerization. 41 Scheme 1.16. Schematic illustration of ATRP initiator immobilization on Si by the LB technique. (Redrawn with permission from Macromolecules 1998, 31, 5934-5936. Copyright 1998 American Chemical Society.) Cl 802 spread hydrolysis = ———> ——-> Si on the water surface Si CH2 H3CO/I \OCH HO/I \ H (:3H2 H300 3 HO O Si Hagoc/é \OCH3 3 monolayer formation compression I I I I I I I I ‘5 condesation water LB through deposition . graft polymn c: fixation l I ____. ............ . > - water Of MMA 42 The first diblock copolymer brushes, PS-b-PMMA, were reported by Zhao and Brittain using sequential carbocationic polymerization and ATRP.136 Like Fukuda‘s example, the addition of free initiator during ATRP was necessary to ensure a sufficient concentration of deactivating Cu(II) species, otherwise the polymerization was not controlled. Matyj aszewski et al. synthesized several block copolymer brushes such as PS- b-PMA, PS-b-PtBA, and PS-b-PAA by sequential ATRP.135 For example, a 10 nm PS film was re-initiated to form a PS-b-PMA block polymer film. However, a significant fraction of active chain-ends were either buried in the polymer brush or lost via termination during grth of the PS block, since the initiator efficiency for the grthh of the second block was reduced. Growth of the 90 nm PMA took much longer (20 hrs) than expected. Kim et al. used a simple but effective quenching and re-initiation (QR) approach to grow PMA-b-PMMA-b-PHEMA triblock copolymer brushes on Au (Scheme 1.17).137 Polymerization was effectively stopped by quenching a growing polymer brush with a concentrated CuBrz/ligand solution, preserving the Br atoms at the chain ends for subsequent re-initiation of the next polymer block. It was found that the efficiency of the QR scheme was better than a simple solvent washing procedure, which resulted in a higher loss of active chains. 43 Scheme 1.17. Synthesis of PMA-b-PMMA-b-PHEMA triblock copolymer brushes 0 A S-(CH2)1-1—O-&-C'3H-Br CH3 i) initiation in MA ii) quenching o H I Au S-(CH2)1TO-&-CH CHz—c Br CH3 002cm A i) re-initiation in MMA ii) quenching r 9H3 A R CH2-? CH2“? Br 00201-13 L COZCH3 It m l l i) re-initiation in HEMA ii) quenching CH3 1 I (In-I3 1 CHz-CI: GHQ—9 Bf C020H3 m L COzCHzCHzOHJ n PMA PMMA PHEMA = —s—(CH2),,—o-&':-9H— CH3 44 Surface initiated ATRP has also been applied to the synthesis of polymer brushes from non-planar substrates such as nanoparticles, carbon nanotubes, and polymer supports. Huang and Wirth synthesized polyacrylamide brushes from porous silica gel by surface initiated ATRP and used them for the separation of proteins by size exclusion.46 Armes and coworkers synthesized poly(2-(N-morpholino)-ethyl methacrylate) (PMEMA) from a silane initiator on the silica particles.138 The PMEMA-silica particles began aggregating at the LCST of PMEMA and re-dispersed upon cooling. Yan and coworkers53 initiated ATRP from multiwalled carbon nanotubes (MWNT) as shown in Scheme 1.18. To attach the ATRP initiator, the MWNT was treated sequentially with HNO3, and 8002, and finally ethylene glycol to produce a hydroxyl-covered surface. The ATRP initiators was readily anchored to the surface by reaction with or- bromoisobutyl bromide. ATRP of MMA provided a PMMA covered MWNT, and sequential polymerization of MMA and HEMA yielded carbon nanotubes coated with arnphiphilic PMMA-b-PHEMA polymer brushes. Scheme 1.18. ATRP of MMA from multi-walled carbon nanotubes (MWNT). O 0 Br r“ m n 1) HNQS, soc|2 Ao/VofiBr VILONOM; 2) HOCHZCHZOH o 4) CuBr/PMDETA o 0 OCHa MMA, 60 °C Br 3’ w *Nfier Aver o u 0 u o o OCH3 o MWNT MWNT-Br MWNT-PMMA 45 Genzer et a1. recently used a technique they called mechanically assisted polymer assembly to produce polymer brushes of polyacrylamide on a cross-linked polydimethylsiloxane (PDMS) surface.139 After stretching the PDMS substrate and generating silanol (Si-OH) groups on the surface by exposure to UV/O3, they attached a trichlorosilane ATRP initiator onto the surface from the vapor phase. The substrate was kept stretched until the poly(acrylamide) brushes were formed by ATRP at 130 °C. They then released the strain, allowing the PDMS substrate to return to its former size. One advantage of this approach is that the brush grafting density can be controlled by altering the stretching extent of the PDMS substrate. Bontempo et a1. synthesized a variety of polymer brushes from polystyrene microspheres using surface initiated ATRP in aqueous media.140 Guerrini and coworkers grew poly(2-hydroxyethyl acrylate) and other polymer brushes from cross-linked poly(styrene-co-2-(2-bromopropionyloxy)) latex particles to form particles with a hydrophobic core and a hydrophilic shell.49 Surface initiated ATRP can also be applied to form patterned polymer brushes by microprinting as other “grafting from” methods. Shah and coworkers reported the use of surface-initiated ATRP to amplify patterned initiator layers on gold films.37 PMMA, PHEMA, PtBA, and poly(dimethylaminoethyl methacrylate) were grown from spatially patterned initiators and then the pattern was transferred into the substrates by using the brushes as barriers to wet chemical etching of gold (Scheme 1.19). 46 Scheme 1.19. Surface patterning by microcontact printing, surface-initiated ATRP, and etching. 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Mater. 2002, 14, (17), 1239-1241. 55 Chapter 2 Controlled Growth and Degradation of PLA Films Grafted to PHEMA Brushes on Au Substrates 1. Introduction Polylactide (PLA) is enviromnentally degradable and its precursor, lactic acid, is available, from renewable resources such as corn, sugar and starch. Because of its favorable properties and its degradability, PLA was recently introduced as a commodity polymer for fibers and packaging materials. The biodegradability and biocompatibility of PLA have led to significant applications in medicine such as surgical sutures, tissue scaffolds, and bone screws. In addition to bulk PLA, thin films of PLA are especially important for applications in drug delivery systems1 and as drug-eluting coatings on medical devices.”4 The thickness of a coating and its degradation profile are important factors that must be considered in designing materials for controlled drug release. Therefore, precise control over the thickness of the PLA layer is indispensable. Thin films can be coated on surfaces by a variety of methods including solvent casting, spin-coating, and spray deposition. Physically adsorbed layers are often thermally unstable due to weak interactions between the polymer and the solid substrate and lack resistance to solvents and friction, while covalently attached films should offer improved thermal stability and mechanical properties. Surface initiated polymerizations should provide robust polymer films since the polymers are anchored to surfaces through covalent bonds. There are a few examples where the ring opening polymerization (ROP) of lactides’ 6 and related monomers was used to grow aliphatic polyesters from surfaces. 56 Poly(s-carprolactone) (PCL) was grown from polymer7 and gold surfaces8 by ROP. Langer and Choi used Sn(2-ethylhexanoate)2 to catalyze the surface-initiated polymerization of L-lactide from gold and silicon oxide surfaces, coating each surface with a biocompatible and biodegradable poly(L-lactide) film,9 while Hedrick and co- workers reported the Sn(OTf)2-catalyzed ROP of lactide from gold.10 In these two examples, lactide polymerization was initiated from —OH or —NH2 groups at the termini of long chain alkyl thiolates on gold or silica surfaces. Kim et al. reported the growth of lactide from poly(Z-hydroxyethyl methacrylate) (PHEMA) chains anchored to Au substrates. This approach is analogous to several syntheses of “molecular brushes” where ROP was initiated from solution-phase PHEMA. Kim described the surface- initiated ATRP of 2-hydroxyethyl methacrylate (HEMA) from Au substrates, followed by the controlled ROP of lactide from the —OH groups of PHEMA brushes.ll The resulting comb polymers have a “bottle brush” architecture as shown in Scheme 2.1. The kinetics of polymer growth, derived from IR and ellipsometric analyses, show the ROP is well-controlled, and spectroscopic analyses of the hydrolytic degradation of the PHEMA/PLA comb polymer in pH 7.4 buffer at 55 °C were unremarkable. However, visual inspection of the polymer film revealed development of an unusual pattern of large, symmetric defects, some of which had regular geometric shapes (Figure 2.1).12 The defects eventually were associated with bubble-like defects that formed in the first few hours of degradation, well before IR could detect any loss of the polylactide component. To understand the formation of these unusual features, we tested how changes in experimental parameters affect the formation of the domains. We found that the 57 temperature used to carry out the ROP of lactide largely determines the size and distribution of these features. Scheme 2.]. Synthetic pathway for PHEMA-g—PLA O S-(CH2)11—O-C-CH-Br CH3 HEMA, CuBr/MesTREN CuBr2/2anbpy, DMF/T HF, 40°C 9 9H3 1 AU S-(CH2)11—O-C‘C':H CHz—(I: n or CH3 COZCHZCHZOH J, Kb: Sn(Oct)2, Toluene, 95°C V T _ 9H3 q CHz—CH3 CHz—(E Br C02CH2CH20H (3:0 I _I-m _ O -m CHZOl-ll or]: o c-c-o-c-c-o H 58 Figure 2.1. Optical micrographs of PHEMA (177 nm)-g-PLA (353 nm) showing the evolution in surface morphology during the drying of water-swollen films, the films were aged for 6 h in pH 7.4 buffer at 55 °C. (Kim, J. B. Surface-Initiated Living Polymerizations on Gold: Synthesis and Characterization of Nanometer Thick Polymer Films. Ph.D., Michigan State University, East Lansing, MI, 2002.) II. Experimental Section II-l . Materials Triethylamine (Aldrich, 99.5%) was distilled from calcium hydride under an argon atmosphere at reduced pressure. 2,2’-Bipyridine (bpy) (Aldrich, 99%) was recrystallized from hexane and then sublimed. 3,6-Dimethyl-l,4-dioxane-2,5-dione 59 (lactide) (Aldrich) was recystallized twice from 8:2 (vzv) EtOAc/hexane and then sublimed. HEMA (Aldrich, 97%) was passed through a 10 cm column of basic alumina to remove inhibitors. Tris[2-(dimethylamino)ethyl]amine (Me6TREN) was synthesized by a literature procedure.l3 THF (Aldrich, HPLC grade) and acetonitrile (Aldrich, 99.8%) for polymerizations were passed through an activated basic alumina column and filtered through 0.2 pm PTFE syringe filters. Toluene (Aldrich, anhydrous) was distilled from sodium benzophenone ketyl. Alter purification, HEMA, solvents and all required liquid chemicals were transferred to Schlenk flasks, de-gassed using three freeze-pump-thaw cycles and then transferred into a drybox. ll-Mercapto-l-undecanol (MUD) (Aldrich, 97%), 2-bromopropionyl bromide (2-BPB) (Aldrich, 97%), 4,4’-dinonyl-2,2’-bipyridyl (anbpy) (Aldrich, 97%), Cu(I)Br (Aldrich, 99.999%), Cu(I)C1 (Aldrich, 99.999%), Cu(II)Br2 (Aldrich, 99.999%), and methanol (Aldrich, 99.93%) were used as received. II-2. Characterization Methods Film thicknesses were obtained with a rotating analyzer ellipsometer (model M- 44; J. A. Woollam) at an incident angle of 75°. Thickness measurements were taken at least three spots on each substrate. Reflectance FTIR spectroscopy was performed using a Nicolet Magna-IR 560 spectrometer containing a PIKE grazing angle (80°) attachment. Changes in surface roughness of the polymer films during the hydrolytic degradation of PHEMA-g-PLA were observed using a Nikon OptiphotZ-POL polarizing optical microscope equipped with a video camera. 60 lI-3. Synthesis of PHEMA Brushes from An Substrates PHEMA brushes were synthesized by surface-initiated ATRP from initiator- anchored Au substrates using a diluted catalyst system to minimize contamination from the copper catalyst. To prepare monomer solutions, Cu(I)Br (4.5 mg), Cu(II)Br2 (1.6 mg), and MC6TREN (8.9 mg) were dissolved in 5 mL of CH3CN2THF (5:1, v/v) in a drybox. HEMA (15 mL, 0.123 mol) was added and the solution was diluted to 35 mL with the cosolvent. The concentrations of each component were: Cu(I)Br/MeéTREN (0.9 mM), Cu(II)Br2/Me6TREN (0.2 mM), and HEMA (3.5 M). The initiator-immobilized substrates were immersed in the monomer solution and polymerized for 5 h at 40 °C. After polymerization, the samples were removed from the monomer solution and immersed in anhydrous DMF to remove catalyst and residual monomer. The sample surface was rinsed with EtOAc, EtOH, and de-ionized water, and then dried under a flow of N2. The PHEMA films were characterized by ellipsometry and surface reflectance FTIR. “-4. Synthesis of PHEMA Brushes from Au by Surface-Initiated ATRP - Kinetics Experiments PHEMA brushes on Au substrates were synthesized using a previously described procedure.'4 For polymerization in methanol, 0.63 g (4.0 mmol) of bipyridine was added to 20 mL of a solution of monomer (HEMA/methanol, 1:1 v:v) in a Schlenk flask. The mixture was stirred until homogeneous, and then was degassed using three freeze-pump- thaw cycles. CuBr (0.26 g, 1.8 mmol) and CuBr; (0.04 g, 0.18 mmol) were added quickly into the flask under Ar, and this mixture was sonicated for one minute and transferred into a glove bag filled with N2. The concentrations of each component were: CuBr (90 61 mM), CuBr2 (9.0 mM), bpy (0.20 M), and HEMA (3.5 M). For polymerization in water, 0.61 g (3.9 mmol) of bipyridine was added to 20 mL of a solution of monomer (HEMA/H2O, 1:1 v:v) in a Schlenk flask. The mixture was stirred until homogeneous, and then was degassed using three freeze-pump-thaw cycles. CuCl (0.14 g, 1.4 mmol) and CuBr2 (0.09 g, 0.40 mmol) were added quickly into the flask under Ar, and this mixture was sonicated for one minute and transferred into a glove bag filled with N2. The concentrations of each component were: CuCl (70 mM), CuBr2 (20 mM), bpy (0.20 M), and HEMA (3.5 M). After stirring the catalyst mixture for an hour in the glove bag, the solution was poured into a second vial containing an initiator-covered Au substrate. The polymerization was allowed to proceed at room temperature for a set reaction time of 0.5- 8 h, and then the vial was removed from the glove bag. The substrate was removed from the vial, washed sequentially with water, ethyl acetate, ethanol, and water, dried under a stream of N2 and characterized by ellipsometry and reflectance FTIR. 11-5. Ring-opening Polymerization of Lactide from PHEMA - Kinetic Study The ring-opening polymerization of lactide from the PHEMA followed the procedure of Kim.11 A saturated solution of lactide was prepared in a drybox by stirring an excess of lactide in toluene (120 mL) at room temperature for one hour. The homogeneous solution was decanted from undissolved lactide, and the concentration of the lactide solution was determined by gravimetry to be ~0.13 M. Sn(2-ethylhexanoate)2 catalyst (0.0844 g) was added to the lactide solution (100 mL) to give a catalyst concentration of ~2.1 mM. To perform ring opening polymerization of lactide in a N2- filled drybox, Au substrates coated with PHEMA brushes were immersed in the lactide 62 (0.13 M) and Sn(2-ethylhexanoate)2 (2.1 mM) solution. Silanized glass vials (15 mL) with silicon rubber caps were used as reaction vessels to avoid polymerization from surface silanol groups. The vials were suspended in an oil bath from copper wires, and a hot plate was used to heat the bath to 90 °C. Vials were removed from the oil bath at various intervals, and the samples were removed from the vial and were sequentially immersed in a series of vials containing toluene, ethyl acetate, ethanol, and de-ionized water to clean the surface. The surface was dried under a flow of N2 and characterized by reflectance FT IR and ellipsometry. II-6. Hydrolytic Degradation of PLA Films To study the hydrolytic degradation of PHEMA-g—PLA on gold, substrates were immersed in phosphate buffer solution (pH 7.40) at 55 i 0.l °C. At predetermined times, the sample was removed from the buffer, washed thoroughly with de-ionized water, and dried under a flow of N2. The sample was characterized by surface reflectance FTIR and optical microscopy, and then the sample was returned to the proper buffer solution. 11-7. Observation of PHEMA-g-PLA Films During Hydrolytic Degradation Changes in surface roughness of the polymer films during the hydrolytic degradation of PHEMA-g-PLA were observed using a Nikon OptiphotZ-POL polarizing optical microscope equipped with a video camera. Digital images of the dried polymer surface and changes in the surface morphology during the drying process were captured as graphic files. 63 111. Synthesis of PHEMA-g-PLA Scheme 2.1 outlines the synthesis of PHEMA-g-PLA polymer brushes tethered to silicon wafers coated with a 200 nm layer of sputtered Au. The detailed experimental procedure is similar to that of Kim and is described in the experimental section. Using this approach, Kim et al. reported the grth of controlled polymerization of lactide from PHEMA brushes,11 resulting in increases in film thickness as high as 450 nm. A self- assembled monolayer was prepared on the Au surface using mercaptoundecanol (MUD) and converted into an initiator monolayer by reaction 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 FT IR spectrum (Figure 2.2, a). Polymerizations of HEMA from the initiator-anchored surfaces were run in a dry box to avoid contamination from oxygen. The catalyst system was a mixture of Cu(I)Br/MeGTREN (0.3 mol% based on monomer) and Cu(II)Br2/2 equivalents of anbpy in a 5:1 (v/v) solution of acetonitrile and THF. The Cu(II) complex (40 mol%, relative to Cu(I)), ensures the deactivation of active radicals and provides some control over the polymerization. Polymerizations of HEMA were run at 40 °C for 5 h. The formation of PHEMA was apparent from the appearance of a carbonyl peak at 1733 cm'1 and a broad hydroxy peak at 3200-3600 cm'1 in the reflectance FTIR spectrum (Figure 2.2, b). Using the hydroxy groups of PHEMA side chains as initiators, rac-lactide was polymerized in toluene at 90 °C using Sn(Z-ethylhexanoate)2 as the catalyst. The IR data in Figure 2.2 show the growth of PLA from PHEMA. The initial PHEMA spectrum (b) showed a single carbonyl peak at 1733 cm'1 from PHEMA, but after 4 h of lactide 64 polymerization, the carbonyl peak broadened (Figure 2.2, c) and shifted to higher wave numbers. Eventually, the PLA carbonyl peak dominated the spectrum and only a single peak at 1767 cm’1 was observed (Figure 2.2, (1). Parallel growth in the methyl stretching peak at 2993 cm’1 and a decline of the hydroxy peak at 3200-3600 cm'1 also confirms PLA formation. Since H2O could be the competing initiator during polymerization of lactide, we tried to exclude H2O by working in a dry box, using dry solvents, and silanizing reaction vials. 65 0.1 absorbance O X30 4000 3500 3000 2500 2000 1500 1000 wavenumber (cm'1) Figure 2.2. Reflectance FT IR spectra of (a) the initiator layer anchored to the Au surface, (b) a 170 nm PHEMA polymerized from the initiator layer, (0) a PHEMA (170 nm)-g- PLA (85 nm) film formed after 4 h of lactide polymerization, (d) a PHEMA (170 nm)-g- PLA (226 nm) film formed after 24 h. 66 IV. Kinetics of Polymerization of HEMA from Au Surfaces To test the effects that different film compositions and thickness have on the formation of defects during the degradation of PHEMA-g-PLA films, we measured the film growth rates for PHEMA and PHEMA-g—PLA. ATRP is described as a “controlled” polymerization because the irreversible termination reactions that consume radicals are suppressed. In absence of termination, one should observe a linear growth in the thickness of polymer films during surface-initiated ATRP. We measured the film growth rate for the ATRP of HEMA from the initiator-anchored gold substrates by immersing the substrates in a solution of monomer and catalyst. A mixture of monomer (HEMA), catalyst (CuBr), ligand (bpy), deactivator (CuBr2), and solvent (methanol) was stirred until a homogeneous dark brown solution formed. At fixed times ranging from 30 min to 48 h, the substrates were removed from the solution, washed with THF, dried and characterized by ellipsometry and FTIR. The kinetic data (Figure 2.3, I) showed a steady but nonlinear growth in film thickness with polymerization time. We investigated two refinements to the polymerization protocol to achieve more control over the polymerization. Matyjaszewski and co-workers reported that mixed halide initiation systems provide better control of ATRP because C-Cl bonds are more stable than C-Br bonds,15 and Armes reported that ATRP of hydrophilic monomers can greatly accelerated in aqueous media.” '7 Thus, we altered the catalyst system to CuCl/CuBr2 (30 mol %) and used water as the solvent for ATRP of HEMA. As shown in Figure 2.3, we observed a faster polymerization rate, 144 nm of HEMA in 8 h using water as the solvent vs. 58 nm in 8 h in methanol, but no significant improvement in the linearity of the thickness vs. time relationship. 67 200 I 175 - E 150 - I D g 125 ~ ' LIJ I D O- 100 — ‘5 u (I) g; 75 - U .E [:1 E 50 —- Cl 25 F, 0 l 1 l 1 1 o 10 20 30 40 50 60 polymerization time (h) Figure 2.3. Evolution of the film thickness with polymerization time during the surface initiated ATRP of HEMA from initiators anchored on Au: I, using H2O as the solvent and CuCl/CuBr2 as the catalyst; 1:], using methanol as the solvent and CuBr/CuBr2 as the catalyst. 68 V. Kinetics of Lactide Polymerization from PHEMA Surfaces Two groups of PHEMA substrates were used to define the kinetics of the ROP of lactide. One group of PHEMA films had ellipsometric thicknesses of ~174 nm, while the other substrates were thinner, ~49 nm. The films were immersed in a 90 °C solution of rac-lactide in toluene (prepared as a saturated solution at room temperature) and the ROP initiator, Sn(2-ethylhexanoate)2. At various times, films were removed from the solution, rinsed with THF, and dried under a stream of nitrogen. The growth of the PLA film was followed by monitoring the change in the film thickness with time. Plotted in Figure 2.4 are the net increases in the thickness of the PLA layer, calculated by subtracting the thickness of the PHEMA fihn from the total film thickness. The ROP of lactide follows a “coordination-insertion" mechanism,l8 and the data of Figure 2.4 should show a linear increase in film thickness with polymerization time. Kim et al. reported linearity for comparable polymerizations through 8 hours, with the thickness saturating at longer times.11 The data of Figure 2.4 have more scatter but show similar trends, a fast early growth that tails at longer times, and a faster film grth rate from thicker PHEMA fihrrs. The structure of PHEMA-g—PLA should be viewed as a graft copolymer, with at least some comb-like character. 69 250 :- 200 r ’é‘ I I 5 j 150 — a “5 ‘8 8 100 - fi I a 1:1 1:1 5 50 - D I O a l l l l 0 4 8 12 16 20 polymerization time (h) Figure 2.4. The evolution of thickness with polymerization time for the ROP of PLA from PHEMA using: I, 174 i 2 nm thick PHEMA substrates, :1, 49 d: 1 nm thick PHEMA substrates. The ellipsometric thicknesses were measures at three different spots on a sample and the error bars are smaller than the symbols. 7O VI. Defect structures in PHEMA-g-PLA films Kim investigated the hydrolytic degradation of PHEMA-g-PLA in phosphate buffer (pH 7.40) at 55 °C. During degradation, the sample surfaces quickly became rough, and reliable ellipsometric thicknesses could not be measured. When observed under an optical microscope, the surface was initially covered with highly symmetrical surface domains that continued to evolve with further degradation. The goal of the research described in this chapter was to understand the formation of these regularly patterned surface domains The surface-grafted PHEMA-g-PLA films prepared in this study were immersed in the buffer solution and were removed at intervals and characterized by FTIR. The spectra, shown in Figure 2.5, show data from a representative polymer brush, PHEMA (153 nm)-g- PLA (362 nm). During hydrolytic degradation, the methyl stretching peak at 2993 cm'1 and the carbonyl peak at 1767 cm'1 decreased, while the hydroxy peak at 3200- 3600 cm'l increased as expected for the hydrolytic loss of the PLA graft chains. As shown in Figure 2.6, PHEMA (153 nm)-g-PLA (362 nm) films are initially smooth before degradation (5a), but after 6 h of degradation, the dried surface was covered with numerous gear-shaped domains (5b). With further degradation, the domains evolved into a “dendrimer-like” pattern (5c), and eventually a flower-like motif separated by mottled areas (5d). These results are similar to those reported by Kim, although the details of the defect patterns differ somewhat. To confirm that these features form via the same pathway seen by Kim, samples were pulled from the buffer solution and immediately observed by optical microscopy. As shown in Figure 2.7, the surface of a sample that had been degraded for 6 h was 71 decorated with blister-like domains that collapsed as the surface dried, eventually forming gear-like domains. These data confirm the same general features seen by Kim, blister formation followed by collapse of the blister and formation of regular domain structures. | 0.1 0' absorbance l l l I 4000 3500 3000 2500 2009 1500 1000 wavenumbers (cm‘ ) Figure 2.5. Reflectance FTIR spectra of (a) PHEMA (153 nm)-g-PLA(362 nm) before degradation, (b) PHEMA-g-PLA films after 6 h hydrolytic degradation, (c) PHEMA-g- PLA films after 10 days hydrolytic degradation, ((1) PHEMA-g-PLA films after 15 days hydrolytic degradation. 72 (a) Start: 0 hr (b) 6 hr Figure 2.6. Surface images of PHEMA (153 nm)-g-PLA (362 nm) obtained by optical microscopy during hydrolytic degradation in pH 7.4 buffer at 55 °C: (a) before degradation, (b) after 6 hrs of degradation of PLA, (c) after 18 hrs of degradation of PLA, (d) after 42 days of degradation of PLA. 73 (a) W (b) drying Figure 2.7. Surface images of PHEMA (153 nm)-g-PLA (362 nm) obtained by optical microscopy after 6 h of hydrolytic degradation in pH 7.4 buffer at 55 °C: (a) water- covered substrate, (b)-(c) substrates drying in air, (d) completely dry substrate. 74 We examined the effects of film composition and the temperature used to add the PLA grafts to the PHEM brushes. The data clearly show that the polymerization temperature significantly affects defect formation. When the lactide polymerization was run at 70 °C, the substrates failed to develop defects, (Figure 2.8), a few formed at 80°, and the number increased with temperature. Polymerizations at 110 °C (not shown in Figure 2.7) resulted in complete delamination of the PLA-g-PLA film from the substrate. The images in Figure 2.9 further implicate temperature as the cause of domain formation. Both films were prepared at 90 °C, but the lactide polymerization lasted 26 h for the film shown in panel a, and 12 h for the film in panel b. The longer exposure of a to high temperatures led to a morphology similar to the sample shown in Figure 2.8d which was prepared at 100 °C. These observations suggest that surface domain formation is related to the Au-S bonds that link the film to the substrate. It is well known that Au-S bonds are not stable above 60 °C.”'21 The temperature used for most lactide polymerizations was >70 °C, but if some of the Au-S bonds are cleaved, polymer chains will not desorb from the surface since the PHEMA fihn is partially cross-linked. However, chains that desorbed and fail to reform Au-S bonds may act as latent defects. During degradation, we believe water diffuses to sites on the gold surface where the film has detached, leading to localized blistering fiom swelling, irreversibly stretching the polymer film. With removal of the water, the blisters collapse from the center, forming a circular domain, with a ridge at the outer edge of the blister. As the film continues to dry, the ridge is under compression and eventually buckles to give the characteristic geometric patterns seen in some of the dried films. The results in Figure 2.8 generally show an increase in the number of domains 75 (defects) with the polymerization temperature, as expected for increased scission of Au-S bonds at higher temperature, leading to more sites for swelling by water. (a) (b) Figure 2.8. Surface images of PHEMA-g-PLA obtained by optical microscopy after 6 h of hydrolytic degradation in pH 7.4 buffer at 55 °C: (a) PHEMA (168 nm)-g-PLA (65 nm), polymerization of PLA at 70 °C; (b) PHEMA (188 nm)-g-PLA (336 nm), polymerization of PLA at 80 °C; (0) PHEMA (153 nm)-g-PLA (362 nm), polymerization of PLA at 90 °C; ((1) PHEMA (183 nm)-g-PLA (367 nm), polymerization of PLA at 100 °C. 76 (a) (b) Figure 2.9. Surface images of PHEMA-g-PLA obtained by optical microscopy after 6 h of hydrolytic degradation in pH 7.4 buffer at 55 °C: (a) PHEMA (183 nm)-g-PLA (520 nm), polymerization of PLA at 90 °C; (b) PHEMA (153 nm)-g-PLA (362 nm), polymerization of PLA at 90 °C. VII. Conclusions Nanometer thick films of PHEMA-g—PLA were synthesized by the sequential ATRP of HEMA and the ROP of rac-lactide initiated fi'om the hydroxy groups of PHEMA. The degradation of the PLA grafts in pH 7.4 buffer at 55 °C produces interesting and unexpected defect structures in the surface films. Control experiments link formation of the defects to lactide polymerization temperatures (> 70 °C). A likely mechanism is the scission of Au-S bonds at high temperatures, causing defects to swell when placed in the buffer solution. 77 VIII. 10. 11. 12. 13. 14. References Jain, R. A., Biomaterials 2000, 21 , (23), 2475-2490. Jeong, B.; Kibbey, M. R.; Bimbaum, J. C.; Won, Y. Y.; Gutowska, A., Macromolecules 2000, 33, (22), 8317-8322. Middleton, J. C.; Tipton, A. J ., Biomaterials 2000, 21 , (23), 2335-2346. Schmidmaier, G.; Wildemann, B.; Stemberger, A.; Haas, N. P.; Raschke, M., J. Biomed. Mater. Res. 2001, 58, (4), 449-455. Kim, Y.; Jnaneshwara, G. K.; Verkade, J. G., Inorg. Chem. 2003, 42, (5), 1437- 1447. Sodergard, A.; Stolt, M., Prog. Polym. Sci. 2002, 27, (6), 1123-1163. Lahann, J .; Langer, R., Macromol. Rapid Commun. 2001, 22, (12), 968-971. Husemann, M.; Mecerreyes, D.; Hawker, C. J.; Hedrick, J. L.; Shah, R.; Abbott, N. L., Angew. Chem-Int. Edit. 1999, 38, (5), 647-649. Choi, I. S.; Langer, R., Macromolecules 2001, 34, (16), 5361-5363. Moller, M.; Nederberg, F.; Lim, L. S.; Kange, R.; Hawker, C. J .; Hedrick, J. L.; Gu, Y. D.; Shah, R.; Abbott, N. L., J. Polym. Sci. Pol. Chem. 2001, 39, (20), 3529-3538. 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(hydroxyethyl methacrylate) Surfaces. In Polymer Brushes, Advincula, R. C.; Brittain, W.; Caster, K.; Riihe, J ., Eds. Wiley-VCH: 2004; pp 105-117. Kim, J. B. Surface-Initiated Living Polymerizations on Gold: Synthesis and Characterization of Nanometer Thick Polymer Films. Ph.D., Michigan State University, East Lansing, MI, 2002. Ciampoli.M; Nardi, N., Inorg. Chem. 1966, 5, (1), 41-44. Huang, W. X.; Kim, J. B.; Bruening, M. L.; Baker, G. L., Macromolecules 2002, 35, (4), 1175-1179. 78 15. 16. 17. 18. 19. 20. 21. Matyjaszewski, K.; Shipp, D. A.; Wang, J. L.; Grimaud, T.; Patten, T. E., Macromolecules 1998, 31 , (20), 6836-6840. Robinson, K. L.; Khan, M. A.; Banez, M. V. D.; Wang, X. S.; Armes, S. P., Macromolecules 2001, 34, (10), 3155-3158. Wang, X. S.; Lascelles, S. F.; Jackson, R. A.; Armes, S. P., Chem. Commun. 1999, (18),1817-1818. Stridsberg, K.; Ryner, M.; Albertsson, A. C., Macromolecules 2000, 33, (8), 2862-2869. Kim, J. B.; Bruening, M. L.; Baker, G. L., J. Am. Chem. Soc. 2000, 122, (31), 7616-7617. Schlenoff, J. H; Li, M.; Ly, H., J. Am. Chem. Soc. 1995, 117, (50), 12528-12536. Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J .; Whitesides, G. M.; Nuzzo, R. G., J. Am. Chem. Soc. 1989, 111, (1), 321-335. 79 Chapter 3 Control of the Density of Polymer Brushes in Surface-Initiated ATRP from Au Surfaces I. Introduction The growth of dense arrays of polymers from solid substrates represents a new and exciting approach to the modification of surfaces."10 Recent advances in the growth of polymer brushes on a variety of substrates enable experiments that address the firndamental questions related to polymer brush conformations on surfaces and their response to external stimuli.”13 In addition, the development of diverse methods for polymer brush synthesis suggests important applications of these materials as protective coatings that exploit the high density of chains on the surface, as environmentally responsive surfaces derived from phase changes in block copolymers triggered by ”“7 and more recently, as fimctional coatings-18’ 19 Am0ng the changes in solvent quality, many procedures for preparing polymer brushes, atom transfer radical polymerization (ATRP) from immobilized initiators is especially attractive for its control over the molecular weight of the grafted polymers, tolerance to water and impurities, compatibility with a variety of functionalized monomers, as well as the option of carrying out polymerizations at relatively low temperatures. Moreover, because it is a controlled technique, ATRP is capable of producing thick surface-grafted polymers, binary polymers, and block copolymersz’ 3’ 5' '0' 20‘” Along with several other methods for producing polymer brushes, ATRP yields dense polymer films, which is important for the use of these materials as anticorrosion 26-28 29, 30 coatings, etch masks, and lithographic coatings.” 31'” However in some 80 applications, such as attachment of accessible biomacromolecules to gene or protein chips, open films are desirable. If brushes are to be used to increase the sensitivity of sensors based on immobilized molecules, the entire brush should be available during both probe-molecule attachment and sensing. As an example,27 poly(Z-hydroxyethyl methacrylate) (PHEMA) brushes are accessible to small molecules such as perfluorooctanoyl chloride, and reaction of PHEMA with this molecule occurs in near- quantitative yield. However, when we used PHEMA brushes to initiate the ring opening polymerization of lactide to give a polymer brush having a bottle brush architecture,34 the degree of polymerization for lactide was only ~6. This suggests that lactide polymerization is sterically limited and could be improved by using PHEMA brushes with lower areal densities of polymer chains. More importantly, covalent immobilization of proteins to modified PHEMA seems to occur only at the film surface, so open films will be required for depositing more than a monolayer of biomacromolecules. This work aims at developing methods for reducing and controlling the density of polymer brushes grown from a surface using ATRP. Such control requires a technique for decreasing the areal density of active, immobilized initiators, and two basic strategies can be used for this purpose. In the first, either the number of available initiators (e. g. control of initiator concentration) or the length of time the initiator solution is in contact with the surface is used to limit the density of immobilized initiator. This strategy is difficult to apply because it requires either precise knowledge of the kinetics of the attachment reaction or fine control of a very low concentration of initiator molecules. Luzinov et al. employed the reaction of carboxylic acids with epoxides along with vapor-phase dosing of the carboxylic acid to control the amount of initiator anchored to glycidyl methacrylate 81 on SiO2.35 Bohn et a1. recently reported an electropolymerization approach to gradients of poly(acrylic acid) and poly(acrylamide) on surfaces.36 A related strategy is to chemically activate or deactivate sites on surfaces using photochemical or scanning probe techniques,” 37 but these methods generally are limited to flat surfaces. Analogs of this approach are photochemical or thermally-initiated free radical polymerization from azo or cholorosulfonyl38 initiators anchored on surfaces, where the number of chains initiated is related to the quantum yield and half-life of the initiator. However, these methods should yield brushes with a high polydispersity. A second strategy for controlling initiator density is to fully functionalize a surface with a mixture of the initiator and an inert analog. Assuming both molecules have the same reactivity for the surface, it should be possible to generate an arbitrary concentration of active initiator homogeneously diluted in a matrix of inactive molecules. Advantages of this strategy include insensitivity to the kinetics of the anchoring step, the ability to cover surfaces of arbitrary size and shape, and generalization to surfaces ranging from inorganic oxides to natural materials such as cellulose. Huck et al. described the co-deposition of the ATRP initiator mercaptoundecyl a-bromoisobutyrate and undecanethiol on Au.3'9 They found that initiation of the ATRP of methyl methacrylate from these surfaces gave film thicknesses that were proportional to the fraction of initiator in the self assembled monolayer (SAM), implying constant initiator efficiency. This result is at odds with the expectation that bimolecular coupling should decrease with dilution of the initiator and lead to an increase in initiator efficiency. (Several studies suggest that only one in 10 initiators in a 100% initiator monolayer lead 2, 24, 35 t0 polymerization. ) One possible explanation for this finding of constant initiator 82 efficiency is that the two thiols phase separated to give islands of pure initiator embedded in undecanethiol. Such segregation of thiols in mixed self-assembled monolayers is well precedented.40’ 41 Within each island, initiation efficiency would be constant, and dilution of initiators would simply decrease the number or size of islands. Similar phase separation effects were reported by Ejaz et a1. during the co-deposition of a triethoxysilane terminated initiator (2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane) and n-octadecyltrimethoxysilane, an inert diluent.2| This chapter describes methods for controlling the density of surface-initiated polymer brushes to create arbitrarily dense arrays of polymers on Au surfaces. To overcome “island effects” caused by phase separation in SAMs of thiols on Au, we first transform the Au surface to an alcohol-terminated monolayer using mercaptoundecanol. Treating this surface with mixtures of a-bromopropionyl bromide and a-methylpropionyl bromide yields active initiators dispersed in a matrix of inactive a-methylpropionate esters. Because the resultant initiator and diluent molecules differ only in the replacement of a bromo group by a methyl group, little phase separation is expected. Polymerization from Au surfaces yields films whose thickness depends greatly on initiator density when the fraction of initiator in the monolayer drops to <10% so initiation efficiency is high. Moreover, control of the density of PHEMA brushes allows swelling in water that ranges from 85 — 2000%. 83 11. Experimental Section II-l. Materials ll-Mercapto-l-undecanol (MUD) (Aldrich, 97%), 2-bromopropionyl bromide (2- BPB, 1) (Aldrich, 97%), Cu(I)Br (Aldrich, 99.999%), Cu(I)C1 (Aldrich, 99.999%), Cu(II)Br2 (Aldrich, 99.999%), phosphorus tribromide (Aldrich, 99%), and l-hexadecanol (Aldrich, 99%) were used as received. 2,2’-Bipyridine (bpy) (Aldrich, 99%) was recrystallized from hexane and then sublimed. Triethylamine (Aldrich, 99.5%) was distilled from calcium hydride under an argon atmosphere at reduced pressure. Methyl methacrylate (MMA) (Aldrich 99%), glycidyl methacrylate (GMA) (Aldrich, 97%) and 2-hydroxyethyl methacrylate (HEMA) (Aldrich, 98%) were passed through a 10 cm column of basic alumina to remove inhibitors. After purification, the monomers, solvents and all required liquid chemicals were transferred to Schlenk flasks, de-gassed using three freeze-pump-thaw cycles and then transferred into a drybox. 11-2. Characterization Methods Film thicknesses were measured using a rotating analyzer ellipsometer (model M- 44; J. A. Woollarn) at an incident angle of 75° using 44 wavelengths of light between 414.0 nm and 736.1 nm. Thickness measurements were taken on at least three spots on each substrate. For films with thicknesses greater than 40 nm, both thickness and refractive index were calculated, while the refractive index of thinner fihns was usually assumed to be 1.5. (Assuming refractive indices of 1.45 or 1.6 result in thickness changes of only ~10 %). For swelling measurements, Au wafers modified with PHEMA 84 brushes were placed in a trapezoidal cell containing glass windows aligned perpendicular to the light beam. After measuring the ellipsometric parameters of the film/substrate in the air-filled cell, the cell was filled with deionized water. Two min later, ellipsometric measurements were performed on the swollen films immersed in water. The optical constants of water were obtained from literature data,42 and the refractive indices determined for the swollen films reflected the approximate volume fractions of water (refractive index around 1.333) and polymer (refractive index of 1.5) in the film. For example, swollen films suggesting a composition of ~95% water had refractive indices around 1.334, while fihns containing 50% water had refractive indices around 1.416. Reflectance FTIR spectroscopy was performed using a Nicolet Magna-IR 560 spectrometer containing a PHi’ ‘91- ? ‘1’” ‘1’ ‘1’”? //////////// covalent attachment H3C ls \ Cl monochlorosilane self-assembly < horizontal polymerization 05555 HO OH 0 HOHOH 7‘777‘77i77777 Figure 4.1. CH3 CH3 CH3 £5.55 ‘Si\ HO- Si Si- OH O/ \O O OH O OH OH 0. m,o\°’ EEO‘ covalent attachment vertical polymerization CH3 OH OH OH OH OH \Si dichlorosilane Cl’ \ Cl Cl /\/\/\/S,i~c1 CI trichlorosilane i covalent attachment vertical polymerization CH3 CH3 CH3 CH3 CH3 —q CH3 [Si‘o/ \ ’Si-O’ o 0. HO‘Si Ho- Si Si- 0H \0'33 0 \O O 0H0 OH O OH OH 757777777771 Possible reaction products of alkylchlorosilanes with 8102 surfaces. (Reprinted with permission fi‘om Langmuir 2000, 16, 7268-7274. Copyright 2000 American Chemical Society.) 113 11. Experimental Section lI-l. Materials Cu(I)Br (Aldrich, 99.999%), Cu(I)Cl (Aldrich, 99.999%), Cu(II)Br2 (Aldrich, 99.999%), chlorodimethylsilane (Aldrich, 98%), hydrogen hexachloroplatinate(IV) hydrate (Aldrich, 99.9%), and l-hexadecanol (Aldrich, 99%) were used as received. 2,2’- Bipyridine (bpy) (Aldrich, 99%) was recrystallized from hexane and then sublimed. Triethylamine (Aldrich, 99.5%) was distilled from calcium hydride under an argon atmosphere at reduced pressure. Methyl methacrylate (MMA) (Aldrich 99%) and 2- hydroxyethyl methacrylate (HEMA) (Aldrich, 98%) were passed through a 10 cm column of basic alumina to remove inhibitors. After purification, the monomers, solvents and all required liquid chemicals were transferred to Schlenk flasks, de-gassed using three freeze-pump-thaw cycles and then transferred into a drybox. “-2. Characterization Methods Film thicknesses were measured using a rotating analyzer ellipsometer (model M- 44; J. A. Woollam) at an incident angle of 75° using 44 wavelengths of light between 414.0 nm and 736.1 nm. Thickness measurements were taken on at least three spots on each substrate. For films with thicknesses greater than 40 run, both thickness and refractive index were calculated, while the refractive index of thinner films was usually assumed to be 1.5. (Assuming refractive indices of 1.45 or 1.6 result in thickness changes of only ~10 %). Unless otherwise specified, routine 1H NMR (500 MHz) and '3 C NMR (125 MHz) spectra were carried out in CDCl3 using a Varian UnityPlus-SOO 114 spectrometer with the residual proton signals from the solvent used as the chemical shift standard. Mass Spectral Analyses were carried out on a VG Trio-1 Benchtop GC-MS. II-3. Synthesis of (1l-(2-Bromo-Z-methyl)propionyloxy)-undecyldimethylchloro silane (1) and (11-(2,2-Dimethyl)propionyloxy)-undecyldimethylchlorosilane (2) 10-Undecen-l-yl-2-bromo-2-methylpropionate25 (14.4 g, 45.1 mmol) and 49 mL of dimethylchlorosilane (451 mmol) were added to an oven-dried dry flask. The hydrogen hexachloroplatinate (IV) catalyst was then added (48 mg), and the mixture was stirred at room temperature overnight. The solution was then diluted in toluene and quickly filtered through a 5 cm plug of activated carbon to remove the catalyst. Removal of the solvent under reduced pressure gave 5.80 g of 1 as a colorless oil (31.1 %), which was stored in a drybox at 0 °C until used. lH-NMR 8 4.14 (t, 2H, CH2), 1.90 (s, 6H, CH3), , 1.65 (m, 2H, CH2), 1.37-1.24 (m, 16H, CH2), 60.79 (t, 2H, CH2), 0.37 (s, 6H, CH3). l3C- NMR 8 171.69 (C=O), 66.11 (CH2), 55.95 (C), 32.92 (CH2), 29.54 (CH2), 29.47 (CH2), 29.42 (CH2), 29.20 (CH2), 29.13 (CH2), 28.30 (CH3), 25.75 (CH2), 22.93 (CH2), 18.94 (CH2), 1.64 (CH3). A similar procedure was used to prepare (2) in 68.1 % yield. lH-NMR 8 4.02 (t, 2H, CH2), 1.59 (m, 2H, CH2), 1.37-1.24 (m, 16H, CH2), 1.17 (s, 9H, CH3), 0.79 (t, 2H, CH2), 0.37 (s, 6H, CH3). 13C-NMR 3 178.63 (C=O), 64.43 (CH2), 38.70 (C), 32.94 (CH2), 29.53 (CH2), 29.47 (CH2), 29.45 (CH2), 29.21 (CH2), 28.59 (CH2), 27.19 (CH3), 25.88 (CH2), 22.95 (CH2), 18.96 (CH2), 1.641(CH3). 115 II-4. Determination of the Relative Reactivity of a Silane Initiator and Diluent with Alcohols Using a literature procedure for the synthesis of the esters as a guide,26 an equimolar mixture of initiator l and diluent 2 (4 mmol) was added by syringe to a well- stirred solution of hexadecanol (0.847 g, 3.5 mmol) in 15 mL CH2C12 at room temperature under N2. After 24 h, the reaction was quenched by adding 100 mL of saturated aqueous mm, and the mixture was extracted with diethyl ether (2 x 100 mL). The combined organic layers were dried over magnesium sulfate and the solvent was removed in vacuo. The ratio of two esters in the product was determined by integrating the corresponding methyl resonances in the 1H NMR spectrum. II-5. Preparation of Initiator-Immobilized SiOz Substrates UV/O3 cleaned Si wafers with an ellipsometrically determined oxide thickness of 16 A were transferred to a dry box filled with N2 and immersed in a toluene solution (20 mL) containing triethylamine (150 uL) and 30 uL of a mixture of initiator 1 and either diluent 2 or trimethylchlorosilane (TMSCl). (Initiator layers give thicker polymer brushes when they are prepared in solutions containing triethylamine.) After 48 h without stirring, the samples were removed from the solution, placed in fresh toluene and sonicated for 1 minute. 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 ~10 A. 116 II-6. Surface-Initiated Polymerizations of MMA Polymerizations of MMA were carried using a procedure developed by Huck and coworkers as a guide.27 In an Nz-filled drybox, MMA (10 g, 100 mmol), bpy (312 mg, 2.0 mmol) and CuBr (143 mg, 1.0 mmol) were added to a 30 mL scintillation vial containing well-stirred MeOI-I (8 mL). After removing the vial from the drybox, deionized water (2 mL) was added to the mixture with a syringe, and this vial along with a second vial containing an initiator-modified Au or SiOz substrate were transferred into a glove bag filled with N2. The deionized water was not degassed and some reaction of Cu(I) with 02 may yield Cu(II) to help control the polymerization. After stirring the catalyst mixture for an hour, the solution was poured into the vial containing the initiator- modified substrate to initiate polymerization. Following a 05-8 h reaction time, the vial was removed from the glove bag and the substrate was washed sequentially with water, ethyl acetate, ethanol and water and dried under a stream of N2. II-7. Surface-Initiated Polymerization of HEMA The polymerization of HEMA was based on a previously described procedure.28 In a Schlenk flask, 244 mg (1.56 mmol) of bpy was added to 20 mL of an aqueous solution of monomer (HEMA/H20, 1:1 v:v). The mixture was stirred until homogeneous, and then was degassed using three freeze-pump-thaw cycles. CuCl (55 mg, 0.55 mmol) and CuBr2 (36 mg, 0.16 mmol) were added quickly into the flask under Ar, and this mixture was sonicated for one minute and transferred into a glove bag filled with N2. After stirring the catalyst mixture for an hour in the glove bag, the solution was poured into a second vial containing an initiator-covered Au or SiOz substrate. The 117 polymerization was allowed to proceed at room temperature for a set reaction time of 0.5- 8 h, and then the vial was removed from the glove bag. The substrate was removed from the vial, washed sequentially with water, ethyl acetate, ethanol, and water and dried under a stream of N2. 111. Preparation of SiOz substrates with Controlled Initiator Densities In the case of SiOz substrates, we controlled initiator density through silanization with mixtures of the two monochlorosilanes shown in Scheme 4.1. We utilized monochloroalkysilanes rather than trichlorosilanes because polymerization of tricholoralkylsilanes in the presence of trace amounts of water gives rise to a number of possible surface structures (Figure 4.1).24 The monochloroalkylsilanes, 1 and 2, are structurally similar, but one is an a-bromoester capable of initiating ATRP, while the second is inert. Reactions of l and 2 with hydroxy groups in solution show that their reactivities are indistinguishable, and based on their similar sizes and shapes, we expect that their reactivities toward hydroxy-terminated surfaces should also be identical. Thus, the composition of a silane monolayer should accurately reflect the molar ratio of 1 and 2 in the solution used to modify the surface. Since the reactivity of monochlorosilanes is lower than trichlorosilanes, we added triethylamine during the initiator anchoring step ensure a dense initiator monolayer. For comparison, we prepared initiator layers with and without adding triethylamine and polymerized HEMA from both substrates under the same conditions. As shown in Figure 4.2, the grafted PHEMA brush grown from the initiator layers formed using added 118 triethylamine was ~ 20% thicker than from the initiator layer prepared without triethylamine. Subsequently, all initiator anchoring experiments used triethylamine Scheme 4.1. Surface-initiated ATRP of HEMA from diluted-initiator monolayers on SiOz substrates 0 OH I HBr 1 i—OH cu—sli—{Cth—o 8102—0” —-0H 0 —OH | 2‘? Cl—Sli—[Cth—O 2 —OH mixture of 1 & 2 toluene >—-
30—fold on going from 100% to 1% initiator, but in this case the polymerization rate decreased monotonically as a function of initiator packing density (Figure 4.4 and Table 4.1). An important difference between Au and Si surfaces is the number and density of sites available on the surface for binding initiator. Studies of anchoring octadecyldimethylchlorosilane to a variety of silica surfaces29 indicate a limiting area/molecule of 0.6 nm2, with typical values of ~0.65 nm2 2. Since this represents an areal density roughly 40% of that for a or ~1.54 chains/nm SAM on Au, the same initiator/diluent ratio applied to Au and SiOz will yield a larger average distance between initiators on SiOz. When the data are normalized on the basis of area/initiator, the polymerization rates from Au and SiOz are similar when the initiator is diluted to <10%, using the Au surface as the reference. More concentrated initiator layers show distinctly different behavior for SiOz and Au surfaces. HEMA polymerization rates from $10; increase with initiator concentration, but the rates from Au are nearly constant. We also found that surface initiated HEMA polymerization from Au is much slower than polymerization from SiOz surfaces using the same conditions. The reason for the difference is unclear, but could be related to surface effects at the early stages of polymerization, such as radical quenching by the metallic Au surface. 121 350 300 r 250 + T 200 ~ [I] 150 . 100 J " ii 50 " / 1' 0 _1 I; -_—-_-—_‘.. ' 1 + 2 T PHEMA film thickness (nm) n f F l 4 6 8 10 polymerization time (h) Figure 4.3. Evolution of the ellipsometric brush thickness with time for the polymerization of HEMA (2-hydroxyethyl methacrylate) from diluted-initiator monolayers on SiOz substrates at 28 °C. (I, 100% 3; 1:1, 50% 3; A, 5% 3; A 1% 3). Polymerization conditions: [CuCl] = 27.5 mM, [CuBr2] = 8.0 mM, [2,2’-bipyridine] = 78 mM, in 20 mL of a 1:1 (v:v) mixture of HEMA and H20 ([HEMA] = 4M). 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. The lines are a least square fit to the 0-4 h data, constrained to intersect the origin. 122 Table 4.1. HEMA polymerization rate from SiOz substrates at various initiator densitiesa initiator 1n1t1ator b density‘ .PHEMA film growth normalized composrtion 2 th1ckness at 8 d e 0 (nm /per rate (nm/h) rate (nm/h) (/o 1) . h (nm) cha1n) 100 0.6 213 36.2 i 2.2 36.2 i 2.2 50 1.2 150 27.7 i 1.4 55 :t 3 10 6 23 4.9 d: 0.5 49 d: 5 5 12 16 3.7i0.4 74:1:8 1 60 5.7 1.3:t0.1 130:1:12 a. the conditions for the polymerization are described in Figure 4.3. b. based on the ratio of 1 and 2 used in the anchoring step. 0. assuming full coverage = 0.65 an/site.29 d. defined as the slope of the line defined by least square fits to the 0-4 h data in Figure 4.3. The errors are the standard deviations derived from the linear fits to the data. e. defined as the film growth rate divided by fractional coverage of the surface by initiator. 123 150 - 40 g — 35 120 -_ ~ 30 C A E 'P s 5 -— 25 g 93 I .92 90 -. —~ 20 *- 8 E E' 1% 15 E w e E or o 60 -_ —— 10 E c m E D 1— 30 q i l 1 1 0 l T T 1 00 80 60 4O 20 0 % initiator Figure 4.4. Film growth rate (I) and normalized polymerization rate (:1) (observed film growth rate/fractional coverage of the surface by initiator) in surface-initiated polymerization of HEMA (2-hydroxyethyl methacrylate) from diluted-initiator monolayers on SiOz substrates. The polymerization conditions are described in Figure 4.3. The error bars are derived from the standard deviations calculated from the linear fits to the data in Figure 5. Film growth rates and normalized rates appear in Table 4.1. 124 V. Polymerization of MMA from Diluted Initiators Anchored on SiOz Substrates Similar to the results described in Chapter 3 for Au surfaces, the rate of polymerization of MMA on SiOz decreased as a function of initiator packing density (>20-fold from 100% tol% initiator). Like the Au-based system, these data clearly show that decreasing the initiator density below a threshold value results in dramatic increases in the normalized polymerization rate. However, the decrease in growth rate with time (nonlinearity in Figure 4.5) suggests that there is significantly more termination with this system than on Au. Since the same pattern is seen for HEMA and MMA polymerizations, we speculate that the difference in the polymerization rates at high initiator concentrations on SiOz and Au is related to the decreased chain density on SiOz surfaces. 125 80 70+ ' ! III E 60—— 8 g 50 2* I E x .9 5 40+ E : El “- 30 ~~ . <1: 2 E 204 E! 1.-- 4 I 2 A A ‘3 o O O 0 l i i l 0 2 4 6 8 10 polymerization time (h) Figure 4.5. Evolution of the ellipsometric brush thickness with time for the polymerization of MMA from diluted-initiator monolayers on $10; at 28 °C (I, 100% 1; 1:1, 50% 1; A, 10% 1; A, 5% l; e, 1% 1). Polymerization conditions: [MMA] = 5 M, [CuBr] = 0.05 M, [bpy] = 0.1 M, in 20 mL of 4:1 (v:v) MeOH/HZO. 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. 126 VI. Controlling Initiator Densities by Using Trimethylsilyl Chloride as the Diluent A reasonable question is whether the sizes and shapes of initiator and diluent need to be matched for effective control of initiator densities on SiOz. Unlike SAMs on Au, the reaction of a chlorosilane with silanols on SiOz substrates forms covalent Si-O bonds and phase separation on SiOz surfaces is very slow.30 To test for size effects in the anchoring process we prepared two 50% initiator layers, one using diluent 2 and a second where trimethylsilyl chloride substituted for 2. (Scheme 4.2) Trimethylsilyl chloride should have the same chemical reactivity as 2, and its cross-sectional area at SiOz should also be similar. Polymerizations were carried out from 50% initiator layers, one diluted with trimethylsilyl chloride and the other with 2. Polymerizations from substrates with 100% initiator were run concurrently with each 50%-initiator substrate, enabling direct comparison of the growth rates from the two initiator dilution schemes. As shown in Table 4.2 and Figure 4.6, substrates where the initiators were diluted to 50% with 2 have HEMA polymerization rates ~2/3 of those from 100% 1. In contrast, polymerization rates decreased to 1/6th of the 100% control when trimethylsilyl chloride was used as the diluent, and we conclude that the chain length on the diluent does matter. (The polymerization rates for the two 100% control samples differed by less than 10%). We think that initially, both initiator and diluent have equal access to the surface and deposit homogeneously. However, as the surface is increasingly covered, initiators screen adjacent silanols and the shorter trimethylsilyl chloride competes more efficiently than 1 for surface silanols, resulting in lower than expected initiator densities and film growth rates. Thus, simply using the same 127 functional group for anchoring initiator and diluent cannot guarantee homogeneous dilution of initiators on SiOz and other substrates. Scheme 4.2. Surface-initiated ATRP of HEMA from diluted-initiator monolayers on SiOz substrates using trimethylsilyl chloride 0 "—OH I %Br CI—Si—[CHzi—o 1 -—OH I 11 Sioz—OH —OH I -OH cu—ss— 3 OH I mixture of 1& 3 toluene _ O OH I >—-
Au —s-(CH2),,—o AU 8 (CH2)11_O r CuBr/Me4Cydam [CHZ—CH If:r CuBr2(anbpy)2 O O MeSO3HICH2Cl2 OH ’ AU —S-(CH2)11-—O CH2_CH‘+B O Hrn Hydrolysis of a 150 nm thick film of PtBA using 150 mM methanesulfonic acid in 10 mL CH2Cl2 for 10 min yielded a 60 nm PAA film.17 The formation of PAA was apparent from a broad carboxylic acid peak at 3000-3500 cm'1 and disappearance of the tert-butyl ester peaks (Figure 5.1, spectrum c). To prove essentially quantitative conversion of the tert-butyl ester to the corresponding acid, we treated the film with a pH 10 sodium diphosphate solution followed by rinsing with ethanol. The resulting FTIR spectrum showed the loss of the OH band at 3000-3500 cm‘1 and the disappearance of the acid carbonyl peak at 1740 cm'l, as well as the growth of characteristic carboxylate peaks at 1610 cm'1 and 1450 cm'1 (Figure 5.1, spectrum (1). The disappearance of the acid carbonyl peak upon deprotonation did not reveal an underlying ester carbonyl peak, confirming complete hydrolysis. 141 0.1 absorbance >. .A— a AAA x50 W — J l J_ L 1 4000 3500 3000 2500 2000 1 500 1000 wavenumbers(cm") Figure 5.1. Reflectance FTIR spectra of gold substrates coated with (a) an immobilized initiator layer; (b) 150 nm PtBA brushes grown from the initiator layer; (c) 60 nm PAA brushes prepared by a 10 min hydrolysis of the PtBA fihn in a 150 mM solution of CH3SO3H in CH2C12; and (d) PAA brushes after immersion in a pH 10 buffer solution for 10 min and rinsing with ethanol. A UV/O3 cleaned gold slide was used as a background. 142 IV. Kinetic Study of Rapid Polymerization of tBA from Immobilized Initiators Figure 5.2 shows the evolution of fihn thickness with time for polymerization of tBA The high polymerization rate and thicknesses for PtBA are unusual for ATRP systems, which generally provide control over molecular weight and polydispersity by maintaining a low concentration of active (radical) chain ends. The nonlinear relationship between film thickness and time for PtBA (Figure 5.2) suggests that in this case a relatively high concentration of radicals leads to both termination and a high polymerization rate, especially early in the polymerization (see inset to Figure 5.2). However, the loss of some control in this polymerization system is more than compensated by the possibility of growing thick films in a few minutes. A 90 nm PtBA film was synthesized from a Au substrate at room temperature in 1 h. The polymer was detached from the Au substrate using iodine and after isolation, its molecular weight was measured by GPC. The number average of molecular weight was 265,000 daltons with a polydispersity of 1.64. While such measurements on small amounts of sample involve significant uncertainty, the data are consistent with a rapid polymerization with some control (polydispersity <2). 143 ED 280 r g 240 — i g 200 - n ., E 160 «5 i i m 120 - . I CD 2 120 4 ' x 80 ~ 0 I E 80 - - “‘" D 40 «- 40 0 p 0 2 4 8 10 0 I I T I I 1 0 10 20 30 40 50 60 polymerization time (min) Figure 5.2. Evolution of the ellipsometric brush thickness with time for the polymerization of tBA from initiator monolayers on Au substrates at 50 °C. The polymerizations were carried out using a mixture of CuBr/Me4Cyclam (2 mM) and CuBr2(anbpy)2 (1 mM) in 20 mL of a 2: 1:1 (v:vzv) tBA/DMF/anisole solution ([tBA] = 3.5 M). The filled squares show the average of three independent runs, and the error bars correspond to the standard deviation. The inset shows data from 0-10 min. The open squares are data from the polymerization of tBA from initiators anchored on SiO2 using the same polymerization conditions. 144 Polymerizations using the structurally related MCfiTREN and HMTETA ligands under the same conditions were not accelerated and yielded <20 nm films in 60 minutes (Figure 5.3). While the Me4Cyclam/CuBr system seems to be unique, other combinations of ligands and polymerization conditions also may yield ultra-fast polymerizations. To examine to what extent the polymerization exhibits features of a controlled polymerization, we used CuBr2(Me4Cyclam)2 as the Cu(II) source. Prior research showed that the use of Me4Cyclam/CuBr for the solution ATRP of dimethylacrylamide'g’ ‘9 and 2-vinyl-4,4-dimethyl-5-oxazolone20 provides marginal control over the molecular weight, most likely due to an inefficient back reaction (R- + CuX2 —> RX + CuX) and hence, insufficient deactivation of chain ends.21 When CuBr2(Me4Cyclam)2 was used as the Cu(II) source in tBA polymerizations, the initial polymerization rate increased as expected for an uncontrolled polymerization, and ultimately yielded thinner films due to a rapid decrease in polymerization rate after initiation (Figure 5.4). Using CuBr2(anbpy)2, the initial polymerization rate is slower, but the polymerization yields thicker films, indicating some level of control in polymerizations using CuBr2(anbpy)2. 145 200 El 160 ~ ’53 :J 5 E 120 D E.‘ 1:.— O 8 a) 80 EZI C e E E3 40 - ' E21 I l.- ' O t A I I I A 0 4 8 12 16 20 24 polymerization time (h) Figure 5.3. Evolution of the ellipsometric brush thickness with time for the polymerization of tBA from initiator monolayers on Au substrates using three different ligand systems: [CuBr] = 2 mM, [CuBr2] = 1 mM, [ligand] = 6 M, in 20 mL of a 2:1:1 (v:vzv) tBA/DMF/Anisole solution ([tBA] = 3.5 M); El, Me4Cyclam; I, Me6TREN; A, HMTETA. Polymerizations were performed in a N2-filled drybox at 50 °C. 146 250 I 200 2 E c: 5 g 150 1 B E “5 Cl 3 g 100i I x C] .9 .c 50 1:. 0 F 1 1 l l I O 10 20 30 40 50 60 polymerization time (min) Figure 5.4. Evolution of the ellipsometric brush thickness with time for the polymerization of tBA from initiator monolayers on Au substrates using Me4Cyclam and anbpy as the Cu(II) source. I, [CuBr/Me4Cyclam] = 2 mM, [CuBr2(anbpy)2] = 1 mM, in 20 mL of a 2:121 (v:vzv) tBA/DMF/Anisole solution ([tBA] = 3.5 M); E], [CuBr/Me4Cyclam] = 2 mM, [CuBr2(Me4Cyclam)2] = 1 mM, in 20 mL of a 2:121 (v:vzv) tBA/DMF/Anisole solution ([tBA] = 3.5 M). The polymerizations were performed in a N2-filled drybox at 50 °C. 147 (105 absorbance A 4000 3500 3000 2500 2000 1500 1000 wavenumbers (cm'1) # Figure 5.5. Reflectance FTIR spectra of gold substrates coated with (a) 97 nm PtBA brushes grown from the initiator layer; (b) PtBA (97 nm)-block-PMMA (210 nm) copolymer brushes grown from the PtBA brush layer. A UV/O3 cleaned gold slide was used as a background. A more rigorous test was the successful formation of block copolymers. A 97 nm PtBA film (Figure 5.5, a) was grown in 5 minutes, rinsed with solvent and dried under N2. The sample was removed from the dry box, and characterized by ellipsometry and 148 FTIR spectroscopy. After 24 hours, the substrate was returned to the dry box, and a PMMA film with a thickness of 210 nm (Figure 5.5, b) was grown fiom the PtBA film in 1 h using the Me4Cyclam-based catalyst. Formation of the block was confirmed by an increase in the carbonyl peak in FTIR spectra. Interestingly, the PMMA block is comparable in thickness to a PMMA film grown directly from surface anchored initiators, suggesting that a substantial fraction of the chains were active after polymerization of the initial PtBA block. V. Other Relevant Factors in the Rapid tBA Polymerization System We examined other experimental parameters to better understand the scope of polymerization conditions. Polymerizations were run in different solvents Figure 5.6; no significant differences were found which eliminated anisole as a significant contributor to the high polymerization rate. Polymerization of PtBA at at 30 °C (Figure 5.7) generated a 50 nm film in just 5 minutes, suggesting the possibility of running polymerizations at subambient temperatures. The effects of monomer concentration (Figure 5.8) were also examined. When the monomer concentration decreased by one half, the film thickness decreased to 1/3 to 1/4 of its original value. Varying the ratio of Cu(I)/Cu(II) (Figure 5.9) suggests some element of control in these polymerizations. Using CuBr2(anbpy)2 as the Cu(II) source and the metric of fihn thickness at 60 minutes, the thickest films were obtained using a 2:1 Cu(I)/Cu(II) ratio, suggesting that the added Cu(II) provides some control over the polymerization. 149 250 I PtBA-DMF/Anisole ! DPtBA-DMF 200 - A c: E 5 g 150 - B E “5 8 C x .9 5 50 5 0 I T I 1 I I 0 10 20 30 40 50 60 polymerization time (nin) Figure 5.6. Evolution of the ellipsometric brush thickness with time for the polymerization of tBA from initiator monolayers on Au substrates using different solvent systems. Catalyst system: [CuBr/Me4Cyclam] = 2 mM, [CuBr2(anbpy)2] = 1 mM, in 20 mL I, 22121 (v:v:v) tBA/DMF/Anisole solution ; El, 2:2 (v:v) tBA/DMF solution ([tBA] = 3.5 M). Polymerization was performed in a N2-filled drybox at 50 °C. 150 120 100" CI E 5 80~ . < %‘3 s... 60" 0 I 8 QC) [:1 x 40- O '5: I C] 20* [j- 0"3 T I I I r O 1 2 3 4 5 6 polymerization time (min) Figure 5.7. Evolution of the ellipsometric brush thickness with time for the polymerization of tBA from initiator monolayers on Au substrates using the same catalyst system at three different temperatures: [CuBr/Me4Cyclam] = 2 mM, [CuBr2(anbpy)2] = 1 mM, in 20 mL of a 2:1:1 (v:v:v) tBA/DMF/Anisole solution ([tBA] = 3.5 M). C1, 50 °C; 0, 40 °C; I, 30 °C. Polymerization was performed in a N2-filled drybox at 50 °C. 151 140 D 120 — ’E‘ 100 - 5 g 804 D “5 8 60— Q) C ‘6 g 401 ‘ 20 - ‘ I I 0 I I I I f o 5 10 15 20 25 30 polymerization time (min) Figure 5.8. Evolution of the ellipsometric brush thickness with time for the polymerization of tBA from initiator monolayers on Au substrates at different monomer concentrations. [CuBr/Me4Cyclam] = 2 mM, [CuBr2(anbpy)2] = 1 mM, in 20 mL 1:1 (v:v) DMF/Anisole solution (13, [tBA] = 3.5 M; A, [tBA] = 1.8 M; I, [tBA] = 0.9 M). Polymerizations were performed in a N2-filled drybox at 50 °C. 152 350 + 300 - I E 250 4 ELI If C1 C0 200 - -- I 9: a o 150 q) .. 8 I E El .9 100 ~ 5 50 I: A A £1 0 I I I I I I 0 20 40 60 80 100 120 polymerization time (min) Figure 5.9. Evolution of the ellipsometric brush thickness with time for the polymerization of tBA from initiator monolayers on Au substrates at 50 °C at various Cu(I)/Cu(II) ratios, in 20 mL of a 2:1:1 (v:v:v) tBA/DMF/anisole solution ([tBA] = 3.5 M). O, [CuBr/Me4Cyclam] = 2 mM, [CuBr2(anbpy)2] = 0.67 mM; I, [CuBr/Me4Cyclam] = 2 mM, [CuBr2(anbpy)2] = 1 mM; Cl, [CuBr/Me4Cyclam] = 2 mM, [CuBr2(anbpy)2] = 1.34 mM,. 153 VI. Rapid Synthesis of Various Polymer Brushes from Au and Si02 Substrates The unusually high grth rates for PtBA films prompted us to examine other monomers to see if they too could be polymerized rapidly to provide thick films. As shown in Figure 5.10, polymerization rates for styrene, methyl methacrylate and vinyl pyridine were slower than for tBA, and the limiting film thicknesses were also lower. However, the polymerization rates were still significantly higher than those described to date. For example, Huck et al. reported the growth of 35 nm thick PMMA films from Au in 2 h,22 whereas the Me4Cyclam/CuBr system gave a 100 nm thick PMMA film in just 1 hour. Husson et al. grew a 40 nm thick polystyrene film in 25 h from a silicon substrate at 50 °C,23 compared to a 30 nm film from Au in just 1 h at 50 °C with the Me4Cyclam/CuBr system. Despite the potential utility of poly(4-vinyl pyridine) (PVP) brushes, we are unaware of only one example of the polymerization of 4-vinyl pyridine from a surface, Rilhe’s grth of 430 nm thick films from surface anchored azo initiators in 14 h.24 However, Husson et al. reported very slow growth rates for poly(2-vinyl pyridine), (6 nm in 5 h).25 In the case of PHEMA (Figure 5.11), polymerization occurred in aqueous solution, and the initial rate of polymerization was approximately 70 times faster than when using a bipyridine catalyst under similar conditions. Thus, PHEMA provides a second polymer system that can be rapidly polymerized and readily derivatized to control functionality. The reflectance FTIR spectra of polystyrene, poly(4- vinyl pyridine), PMMA, and PHEMA polymer brushes grown from immobilized initiators are shown in Figure 5.12. 154 350 300 r N 01 O I N O O I i I C] 100 "II . $ ‘. 0 v Q l O 1 Q l l 0 20 40 60 80 100 120 polymerization time (min) thickness of polymer brush (nm) or c> E] Figure 5.10. Evolution of the ellipsometric brush thickness with time for the polymerization of tert-butylacrylate (tBA, I, the error bar is the standard deviation from results of 3 independent runs), methyl methacrylate (MMA, El), styrene (o), 4-vinyl pyridine (4-VP, o) from initiator monolayers on Au substrates. Polymerization conditions: [CuBr/Me4Cyclam] = 2 mM, [CuBr2(anbpy)2] = 1 mM, in a 20 mL solution of 2:1:1 (v:v:v) monomer/DMF/anisole ([tBA] = 3.5 M; [MMA] = 4 M; [styrene] = 4.6 M; [4-VP] = 4.5 M). Polymerization was performed in a N2-filled glovebox at 50 °C. 155 200 I I I I A160 — E 5 < 2 120 ~ UJ I 9. I- O c 8 80 CD [:1 c 8 g 40 ~ D El El 0 1 l l J 0 50 100 150 200 250 polymerization time (min) Figure 5.11. Evolution of the ellipsometric brush thickness with time for the polymerization of HEMA from initiator monolayers on Au substrates using two different catalyst systems: I, polymerization conditions: [CuBr/Me4Cyclam] = 2 mM, [CuBr2(anbpy)2] = 1 mM, in 20 mL of a 5:1:4 (v:v:v) HEMA/DMF/H2O solution ([HEMA] = 4M). D, Polymerization conditions: [CuCl] = 27.5 mM, [CuBr2] = 8.0 mM, [bpy] = 78 mM, in 20 mL of a 1:1 (v:v) mixture of HEMA and H20 ([HEMA] = 4M). Polymerization was performed in a N2-filled glovebag at 28 °C. 156 x5 b x 10 a VJV“ 4000 3500 3000 2500 2000 1 500 1 000 wavenumbers(cm'1 ) absorbance O 1 E ; Figure 5.12. Reflectance FTIR spectra of gold substrates modified with polymer brushes grown from immobilized initiators for l h. (a) 50 nm polystyrene; (b) 100 nm poly(4- vinyl pyridine), after polymerization for 24 h; (c) 120 nm poly(methyl methacrylate); and (d) 160 nm poly(2-hydroxy ethyl methacrylate). A UV/O3 cleaned gold slide was used as a background. 157 Comparison of the polymerization rates of tBA, MMA, styrene, and 4—vinyl pyridine in DMF/anisole suggests that the unusually rapid growth of PtBA films stems from a combination of tBA’s fast propagation rate and reduced bimolecular coupling due to the steric bulk of the monomer. Neglecting chain transfer processes, the degree of polymerization realized in a chain-growth polymerization is defined by the relative probabilities of a growing chain by adding a monomer or terminating. Since most syntheses of polymer brushes from surfaces are carried out at high monomer concentrations that are approximately constant during the polymerization, the ratio of kp to k,, the rate constants for propagation and termination, serves as an indicator of the likelihood that a reaction will reach high degrees of polymerization before termination. Moreover, preliminary data show that tBA polymerizations show a strong dependence on monomer concentration (Figure 5.8). The order of the limiting film thicknesses shown in Figure 5.11 is consistent with the relative values of kp / k, for the monomers.26 As can be anticipated from the data presented in earlier chapters, this fast polymerization system can be transferred from Au to silicon substrates (Figure 5.13). We observed rapid polymerization tBA, MMA, styrene, and 4-vinyl pyridine. Polymerization of 4-vinyl pyridine was much faster from silicon than from Au, which may be related to the difference in polymerization rates seen for Au and SiO2 as described in Chapter 4. 158 350 1 300 - ' I tBA Cl MMA O styrene O 4-VP 250 ~ 200- I 150 I 100 — DO A thickness of polymer brush (nm) 63 0 g 1 1 0 60 120 180 240 polymerization time (min) Figure 5.13. Evolution of the ellipsometric brush thickness with time for the polymerization of tert-butylacrylate (tBA, I), methyl methacrylate (MMA, El), styrene (0), 4-vinyl pyridine (4-VP, O) from initiator monolayers on SiO2 substrates. Polymerization conditions: [CuBr/Me4Cyclam] = 2 mM, [CuBr2(anbpy)2] = 1 mM, in a 20 mL solution of 2:1:1 (v:v:v) monomer/DMF/anisole ([tBA] = 3.5 M; [MMA] = 4 M; [styrene] = 4.6 M; [4-VP] = 4.5 M). Polymerization was performed in a N2-filled glovebox at 50 °C. 159 VII. Conclusions The use of the Me4Cyclam/CuBr catalyst system allows rapid polymerization of a variety of monomers. 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