ANIONIC SYNTHESIS OF IN-CHAIN AND CHAIN-END FUNCTIONALIZED
POLYMERS
A Dissertation
Presented to
The Graduate Faculty of The University of Akron
In Partial Fulfillment
of the Requirements for the Degree
Doctor of Philosophy
Sumana Roy Chowdhury
December, 2006
ANIONIC SYNTHESIS OF IN-CHAIN AND CHAIN-END FUNCTIONALIZED
POLYMERS
Sumana Roy Chowdhury
Dissertation
Approved: Accepted:
______Advisor Department Chair Dr. Roderic P. Quirk Dr. Mark D. Foster
______Committee Member Dean of College Dr. Judit E. Puskas Dr. Frank N. Kelley
______Committee Member Dean of Graduate School Dr. Frank N. Kelley Dr. George R. Newkome
______Committee Member Date Dr. Frank W. Harris
______Committee Member Dr. Chrys Wesdemiotis
ii
ABSTRACT
The objective of this work was to anionically synthesize well-defined polymers
having functional groups either at the chain-end or along the polymer chain. General
functionalization methods (GFM) were used for synthesizing both kinds of polymers.
Chain-end functionalized polymers were synthesized by terminating the anionically
synthesized, living polymer chains using chlorodimethylsilane. Hydrosilation reactions were then done between the silyl-hydride groups at the chain-ends and the double bonds of commercially available substituted alkenes. This produced a range of well-defined polymers having the desired functional groups at the chain-ends. In- chain functionalized polymers were synthesized by anionically polymerizing a silyl- hydride functionalized styrene monomer: (4-vinylphenyl)dimethysilane.
Polymerizations were done at room temperature in hydrocarbon solvents to produce well-defined polymers. Functional groups were then introduced into the polymer chains by use of hydrosilation reactions done post-polymerization. The functionalized polymers produced were characterized using SEC, 1H and 13C NMR, FTIR, MALDI
TOF mass spectrometry and DSC.
The monomer reactivity ratios in the copolymerization of styrene with (4-
vinylphenyl)dimethylsilane were also measured. A series of copolymerizaions was
done with different molar ratios of styrene(S) and (4-vinylphenyl)dimethylsilane(Si).
iii Three different methods were used to determine the values of the monomer reactivity
ratios : Fineman-Ross, Kelen-Tudos and Error-In-Variable (EVM) methods. The
average values of the two monomer reactivity ratios obtained were: rSi = 0.16 and rS =
1.74. From these values it was observed that in the copolymerization of styrene with
(4-vinylphenyl)dimethylsilane, the second monomer was preferentially incorporated
into the copolymer chain. Also, rSirS = 0.27, which shows that the copolymer has a
tendency to have an alternating structure.
Amino acid-functionalized polymers (biohybrids) were synthesized by using a simple and efficient, three-step method. The first step was to make a copolymer of styrene with (4-vinylphenyl)dimethylsilane, followed by introduction of amine functional groups into the polymer chain, using a hydrosilation reaction between
the silyl-hydride units in the copolymer chain and the double bond of allyl amine.
The third step was a condensation reaction between these amine functional groups on
the copolymer chain and the carboxyl group on N-carbobenzyloxy-phenylalanine
(a protected amino acid). Although this method has been used to incorporate a
particular amino acid onto the polymer chain, it maybe possible to extend this
procedure to introduce virtually any amino acid or peptide group into the polymer
chain.
Finally a thermoplastic elastomer (TPE) was synthesized using the monomer (4-
vinylphenyl)dimethylsilane. The first block of this TPE was a copolymer block of
styrene with (4-vinylphenyl)dimethylsilane, followed by a polyisoprene block and
finally another copolymer block of styrene and (4-vinylphenyl)dimethylsilane. This
polymer was characterized using SEC, 1H and 13C NMR, FTIR, DSC, DMTA, TEM
iv and tensile testing. It was seen to exhibit properties similar to those of a regular styrene-diene-styrene TPE. However, the silyl-hydride units introduced into this polymer chain can be easily converted to different functional groups using hydrosilation reactions. Introduction of such functional groups would be helpful in tailoring the properties of the TPE.
v
ACKNOWLEDGEMENTS
I am deeply grateful to my advisor, Professor Roderic P. Quirk, for his continued support and encouragement during the past few years. I am also very grateful to all my group members (past and present) for their help and also for making my stay here more enjoyable.
I also want to thank my parents and my husband, Jayanta, for their support during my stay here. Without their help and encouragement I would not have been able to reach here.
vi
TABLE OF CONTENTS
Page
LIST OF FIGURES……………………………………………………………...... xii
LIST OF TABLES………………………………………………………………….xvi
LIST OF SCHEMES……………………………………………………………….xvii
CHAPTER
I. INTRODUCTION…………………….…………………………………….…....1
II. HISTORICAL BACKGROUND………………………………………….….....4
2.1 Living Anionic Polymerization…………………………….……………...... 4
2.1.1 General Aspects………………………………………………….……4
2.1.2 Monomers……………………………………………………….…….7
2.1.3 Solvents…………………………………………………………….....8
2.1.4 Initiators………………………………………………………….……9
2.1.5 Lewis Base Effects………………………………………………...... 12
2.1.6 Initiation Reactions………………………………………………...... 13
2.1.7 Propagation Reactions…………………………………………….....15
2.1.8 Stereochemistry of diene polymerization……….…………………...16
2.2 Copolymerization…………………………………………………………..18
2.2.1 Block Copolymers…………………………………………………...20
2.2.1.1 Three-step sequential monomer addition……………….…..20
vii
2.2.1.2 Two-step monomer sequential addition and coupling………..21
2.2.1.3 Difunctional initiation and two-step monomer addition……...21
2.2.2 Determination of monomer reactivity ratios………………………….22
2.2.2.1 Fineman-Ross Method……………………………………….22
2.2.2.2 Kelen-Tudos Method………………………………………....23
2.2.2.3 Error-In-Variable (EVM) Method……………………………24
2.3 Functionalized Polymers…………………………………………………….24
2.3.1 Chain-end functionalized polymers…………….…………………….25
2.3.1.1 Hydrosilation Reactions…………….………………………..27
2.3.1.2 A General Functionalization Method by combination of anionic polymerization and hydrosilation reaction………….29
2.3.2 In-chain functionalized polymers………………………….…………30
III. EXPERIMENTAL…..…………………………………………………………34
3.1 Purification of Solvents, Monomers and Reagents…………………….…...36
3.1.1 General……………………………..…………………………………36
3.1.2 Hydrocarbon Solvents……………………………..………………….36
3.1.3 Ethers…………………………………………..……………………..37
3.1.4 Alcohols………………………………………………..……………..38
3.1.5 Styrene…………………………………………………..……………38
3.1.6 Isoprene…………………………………………………..…………...39
3.1.7 Chlorodimethylsilane……………………………………...………….39
3.1.8 Allyl functional compounds…………………………………………..39
3.2 Synthesis of (4-vinylphenyl)dimethylsilane……………………………...…39
viii
3.3 Introduction and modification of functional groups post- polymerization………………………………………………………………40
3.3.1 Hydrosilation Reactions………………………………………………40
3.4 Polymerization Techniques………………………………………………….42
3.5 Polymerization Studies……………………………………...………………44
3.5.1 Polystyrene and Poly(4-vinylphenyl)dimethylsilane…….…………..44
3.5.2 Poly(styrene-co-4-(vinylphenyl)dimethylsilane)…………….………46
3.5.3 Polyisoprene………………………………………………….………48
3.6 Synthesis of functional polymers………………………….………….…….48
3.6.1 Silyl-hydride chain-end functional polystyrene………………….…..48
3.6.2 Synthesis of cyanide-end functionalized polystyrene………………..49
3.6.3 Synthesis of ethyl ether, chain-end functionalized polymer……….…49
3.6.4 Synthesis of acetate, chain-end functionalized polymer……………...50
3.6.5 Synthesis of epoxide, in-chain functionalized polymer………………50
3.6.6 Synthesis of diol in-chain functionalized polymer…………………...51
3.6.7 Synthesis of perfluoroalkyl in-chain functionalized polymer………..51
3.6.8 Synthesis of polymer-peptide biohybrid……………………………..52
3.6.8.1 Synthesis of in-chain amine functionalized copolymer……..52
3.6.8.2 Synthesis of the biohybrid…………………………….……..52
3.6.9 Synthesis of the thermoplastic elastomer (TPE) using (4-vinylphenyl)dimethylsilane………………………………...... 53
3.7 Polymer Characterization………………..………………………………….54
3.7.1 Molecular Weight and Molecular Weight Distribution……..……….54
ix 3.7.1.1 Size Exclusion Chromatography (SEC)…..…………………54
3.7.1.2 Matrix Assisted Laser Desorption-Ionization Time of Flight (MALDI TOF) Mass Spectroscopy………….…….55
3.7.2 Nuclear Magnetic Resonance (NMR) Spectroscopy………….….…..55
3.7.3 Fourier Transform Infrared Spectroscopy (FT-IR)…………………...56
3.7.4 Thermal Properties of Copolymers…………………………………...56
3.7.4.1 Differential Scanning Calorimetry (DSC)…………………...56
3.7.4.2 Dynamic Mechanical Thermal Analysis (DMTA)……….….56
3.7.4.3 Thermogravimetric Analysis (TGA)………………………...57
3.7.5 Thin Layer Chromatography (TLC)………………………..………...57
3.7.6 Column Chromatography……………………………………………..57 . 3.7.7 Tensile Testing…………………………………………………….….57
3.7.8 Transmission Electron Microscopy (TEM)……………………….….58
3.7.9 Contact Angle Measurements………………………………………...58
3.7.10 Ellipsometry Measurements………………………………………....58
3.7.11 Compression Molding……………………………………………….59
IV. RESULTS AND DISCUSSION………………………………………….……60
4.1 Synthesis of chain-end functionalized polystyrene……………………….…60
4.1.1 Synthesis of cyanide-functionalized polystyrene…………………...... 60
4.1.2 Synthesis of ethyl-ether terminated polystyrene………………….…..74
4.1.3 Synthesis of acetate-functionalized polystyrene……………………...83
4.2 Synthesis of in-chain functionalized polystyrenes……………………….….90
4.2.1 Synthesis of in-chain epoxide and hydroxyl functionalized polymers……………………………………………………………..100
x
4.2.1.1 Synthesis of epoxide-functionalized polystyrene…………..102
4.2.1.2 Synthesis of hydroxyl-functionalized polystyrene…………..107
4.2.2 Synthesis of perfluoroalkyl-functionalized polymer………………..109
4.3 Determination of the monomer reactivity ratios in the anionic copolymerization of styrene with (4-vinylphenyl)dimethylsilane…………116
4.4 Synthesis of a polymer biohybrid using a combination of anionic polymerization and hydrosilation reactions……………………………..…125
4.4.1 Preparation of the Copolymer of styrene with (4-vinylphenyl)- dimethylsilane……………………………………………..………...127
4.4.2 Preparation of Amine-Functionalized Copoymer by Hydrosilation of the Copolymer of styrene with (4-vinylphenyl)- dimethylsilane……………………………..………………………...132
4.4.3 Preparation of the (N-carbobenzyloxy)-L-phenylalanine- Functionalized Copolymer by a Condensation Reaction of the Amine-functionalized Copolymer with (N-carbo- benzyloxy)-L-phenylalanine………………………………………...137
4.5 Synthesis of a thermoplastic elastomer using (4-vinylphenyl)- dimethylsilane…………………………………………………………..….145
V. SUMMARY…………………………………….………………………….….160
REFERENCES………...……………………………………………………….…..164
xi
LIST OF FIGURES
Figure Page
2.1 Synthesis of the naphthalene radical anion ………………………………….10
2.2 Synthesis of a styrene dianionic initiator using the naphthalene radical anion………………………………………………………………….10
2.3 Synthesis of dilithium initiator from 1,3-bis(1-phenylethylene)- benzene……………………………….……………………………………...12
2.4 Chalk-Harrod and modified Chalk-Harrod mechanism……………………...28
2.5 Synthesis of in-chain functionalized polymer using the protection- deprotection strategy………………………………………………………....31
3.1 Schematic of a high vacuum line used for anionic polymerization……….....35
3.2 Reactor for anionic polymerization……….………………………………....42
4.1 SEC chromatogram of the silyl hydride-terminated polymer………………..62
4.2 1H NMR spectrum of the silyl hydride-terminated polystyrene……………..63
4.3 13C NMR of the silyl hydride-functionalized polystyrene…………………...65
4.4 MALDI TOF spectrum of the silyl hydride-terminated polymer……………66
4.5 FTIR spectrum of the silyl hydride-terminated polymer………………….....67
4.6 Cyanide-terminated polystyrene……………………………………………..67
4.7 SEC chromatogram of the cyanide-terminated polystyrene…………………69
4.8 1H NMR spectrum of the cyanide-terminated polystyrene…………………..70
4.9 DEPT-135 13C NMR spectrum of the cyanide terminated polymer………....71
4.10 13C NMR spectrum of the cyanide-terminated polystyrene……………...….72
xii
4.11 MALDI TOF mass spectrum for the cyanide-terminated polystyrene…...…73
4.12 FTIR spectrum of the cyanide-terminated polymer…………………………74
4.13 SEC chromatogram for the ethyl ether-terminated polymer……………...... 76
4.14 1H NMR spectrum of the ethyl ether-terminated polymer…………………..77
4.15 13C NMR spectrum of the ethyl ether-terminated polymer…………………79
4.16 FTIR spectrum of the ethyl ether-terminated polymer……………………...80
4.17 MALDI TOF mass spectrum of the ethyl ether-terminated polymer……….82
4.18 SEC chromatogram of the acetate-functionalized polymer…………………84
4.19 1H NMR spectra of the acetate-functionalized polymer at different attenuations…………………………………………………………………85
4.20 13C NMR spectrum of the acetate-functionalized polymer…………………86
4.21 FTIR spectrum of the acetate-functionalized polymer……………………...87
4.22 MALDI TOF mass spectrum of the acetate-functionalized polymer……….89
4.23 1H NMR spectrum of (4-vinylphenyl)dimethylsilane…………………….…92
4.24 13C NMR spectrum for (4-vinylphenyl)dimethylsilane……………………..93
4.25 SEC chromatograms of poly(4-vinylphenyl)dimethylsilanes……….………94
4.26 1H NMR spectrum of poly(4-vinylphenyl)dimethylsilane (Mn = 33000 g/mol)……………………………………...……………….....95
4.27 13C NMR spectrum of poly(4-vinylphenyl)dimethylsilane…………………96
4.28 FTIR spectrum of poly(4-vinylphenyl)dimethylsilane……………………..97
4.29 DSC of poly(4-vinylphenyl)dimethylsilane………………………………...97
4.30 MALDI TOF mass spectrum of poly(4-vinylphenyl)dimethylsilane……....99
4.31 SEC chromatogram of the epoxide-functionalized polymer (Mn = 50000 g/mol, PDI = 1.04)………….……………………………….103
xiii
4.32 1H NMR spectrum of the epoxide-functionalized polymer……………….104
4.33 13C NMR spectrum of the epoxide-functionalized polymer………………104
4.34 FTIR spectrum of the epoxide-functionalized polymer…………………..106
4.35 DSC of the epoxide-functionalized polymer……………………………..106
4.36 1H NMR spectrum of the hydroxyl-functionalized polymer……………..107
4.37 13C NMR spectrum of the hydroxyl-functionalized polymer…………….108
4.38 FTIR spectrum of the hydroxyl-functionalized polymer…………………109
4.39 SEC chromatogram for the perfluoroalkyl-functionalized polymer (Mn = 72000 g/mol, PDI = 1.08)………………………………………….111
4.40 1H NMR spectrum of the perfluoroalkyl-functionalized polymer………..111
4.41 19F NMR spectrum of the fluorinated polymer (upper spectrum : perfluroalkyl-functionalized polymer; lower spectrum : starting material)………………………………..……...112
4.42 FTIR spectrum of the perfluoroalkyl-functionalized polymer……………113
4.43 DSC of the fluorinated polymer…………………………………………..115
4.44 1H NMR spectra for the series of copolymers synthesized with different mol/ mol ratios of styrene and (4-vinylphenyl)dimethylsilane………………………………………...…119
4.45 Plot of styrene mol% in the feed versus the styrene mol% in the copolymer………………………………………………………………...121
4.46 Fineman-Ross Plot…………………………………………………….….123
4.47 Kelen-Tudos Plot…………………………………………………………123
4.48 EVM Plot for the copolymers…………………………………………….124
4.49 SEC chromatogram of the copolymer……………………………….……129
4.50 1H NMR spectrum for the copolymer of styrene with (4-vinylphenyl)dimethylsilane at different attenuations………………....131
xiv
4.51 FTIR spectrum for the copolymer of styrene with (4-vinylphenyl)- dimethylsilane…………………………………………………………….132
4.52 13C NMR spectrum for the copolymer of styrene with (4-vinylphenyl)-dimethylsilane…………………………………………..133
4.53 1H NMR for the amine-functionalized copolymer……………………….135
4.54 Amine-functionalized copolymer………………………………………...135
4.55 FTIR for the amine-functionalized copolymer…………………………...136
4.56 13C NMR for the amine-functionalized copolymer………………………138
4.57 SEC chromatogram of the polymer biohybrid……………………………140
4.58 Amino acid-functionalized copolymer…………………………………...140
4.59 FTIR for the amino acid-functionalized copolymer……………………...141
4.60 1H NMR for the amino acid-functionalized copolymer at different attenuations……………………………………………………………....143
4.61 1H NMR spectrum of N-cbz-Phe…………………………………………144
4.62 13C NMR spectrum of N-cbz-Phe………………………………………..144
4.63 13C NMR for the amino acid-functionalized copolymer………………….146
4.64 SEC of the thermoplastic elastomer………………………………………152
4.65 1H NMR spectrum of the TPE……………………………………………153
4.66 13C NMR spectrum of the TPE……………………………………….…..154
4.67 DSC of the thermoplastic elastomer (TPE)……………………………....155
4.68 Measurement of the Young’s Modulus of the TPE……………………....156
4.69 Stress versus strain curve of the thermoplastic elastomer……………...... 158
4.70 DMTA measurements of the TPE………………………………………..159
4.71 TEM image of the thermoplastic elastomer………………………………160
xv
LIST OF TABLES
Table Page
4.1 Contact angle values for standard polystyrene and the fluorinated polymer…………………………………………………………..114
4.2 Refractive Index values for a polystyrene standard and the fluorinated polymer…………………………………………………………..115
4.3 Integration data for each copolymer………………………………………….120
4.4 Values of the monomer feed ratios and the instantaneous copolymer compositions…………………………………………………….121
4.4 Calculated parameters in the Fineman-Ross Method……………………….122
4.5 Calculated parametes for the Kelen-Tudos Method………………………...122
4.6 Values of r1(styrene) and r2(SiH) obtained by all three methods…………………125
xvi
LIST OF SCHEMES
Scheme Page
2.1 Winstein Spectrum…………………………………………………….…….…..8
2.2 Kinetic equation for the rate of propagation ([PSLi]0 is the dissociated species which is the reactive species for addition of monomer)…………...…...15
2.3 Isomerization of the poly(isoprenyl)lithium chain-ends………….………….…17
2.4 Kinetic equations for copolymerization of two monomers….……………….…18
2.5 Use of functionalized chlorosilane to introduce functional groups at the polymer chain ends………...... …………………………….….………………..26
2.6 Use of 1,1-diphenylethylene derivatives to introduce functional groups on a polymer chain……...……….…………………….……………………...... 27
2.7 Addition of a Si-H group across a carbon-carbon double bond……….…….….27
2.8 Anti-Markovnikov addition of the Si-H group in a hydrosilation reaction….…29
2.9 Introduction of an epoxide group at a polymer chain-end using hydrosilation……………...……………………….…...... 29
2.10 General functionalization method (GFM) for synthesis of chain-end functionalized polymers………………………..………….…..………………..30
2.11 General Functionalization Method developed for synthesis of in-chain functionalized polymers………………………………………………………..33
4.1 Cyanide chain-end functionalization of polystyrene…...……………….……...61
4.2 Ethyl ether-terminated polystyrene…….……………..…………….…………..75
4.3 Synthesis of poly(styrene-b-4-vinylphenyldimethylsilane)….....………………91
4.4 Synthesis of (4-vinylphenyl)dimethylsilane….………………………………...92
xvii 4.5 Polymerization of (4-vinylphenyl)dimethylsilane in a hydrocarbon solvent at room temperature………………………………….………………....94
4.6 Synthesis of epoxide-functionalized polystyrene.…...…………………...…...102
4.7 Synthesis of hydroxyl-functionalized polystyrene…………………….………102
4.8 Synthesis of perfluoroalkyl-functionalized polymer……………..…………...110
4.9 Functionalization of the copolymer of styrene with (4-vinylphenyl)- dimethylsilane…………………………………………….…………………...118
4.10 Synthesis of the polymer biohybrid...………………………………………...128
4.11 Anionic copolymerization of styrene with (4-vinylphenyl)- dimethylsilane…………...... 128
4.12 Amine functionalization of the copolymer using hydrosilation………...……134
4.13 Synthesis of the polymer biohybrid…………...……………………………...139
4.14 Hydrosilation of the polydiene block of poly(styrene-b-diene).………...……148
4.15 Formation of side-loops due to hydrosilation of polybutadiene…...…………149
4.16 Synthesis of the thermoplastic elastomer……………………………………..150
xviii
CHAPTER I
INTRODUCTION
Living anionic polymerization is a powerful technique to synthesize well- defined in-chain and chain-end functionalized polymers. Introduction of functional groups on a polymer chain results in modification of many properties of the polymer, such as thermal properties, polarity/solubility, adhesion, etc. Alkyllithium-initiated polymerizations of styrene and diene monomers can produce well-defined, functionalized polymers. Living polymerizations produce stable, anionic polymer
chain-ends, which can be reacted with a variety of electrophilic agents to introduce
functional groups at the chain-ends. However, this method seldom produces polymers
with 100 % chain-end functionalization. Also, it is necessary to protect most
functional groups in order to avoid side reactions of active functional groups with the
living carbanionic polymer chain-ends. Another commonly used method to synthesize
chain-end functionalized polymers using anionic polymerization techniques is to use
a functionalized initiator. Although this produces 100 % chain-end functionalized
polymers, there is still a need to protect the active functional groups to avoid side reactions with the living polymer chain-ends. One commonly used method to synthesize functionalized polymers is the reaction of the living polymer chains with substituted 1,1-diphenylethylenes. However, this method too has the same
1 disadvantage as described above i.e. the need to protect most functional groups.
Anionic in-chain functionalized polymers are usually synthesized by polymerizing substituted monomers. In this case too, it is necessary to protect the active functional groups prior to polymerization and to deprotect them post-polymerization. Also, for each functionalized polymer to be synthesized, it is necessary to synthesize the functionalized monomer.
So, there was a need to develop a general functionalization method (GFM) to synthesize in-chain and chain-end functionalized polymers. Chain-end functionalized polymers were synthesized by anionically synthesizing well-defined polymer chains and then terminating the living polymers using chlorodimethylsilane. This produced silyl-hydride terminated polymers, which could then be easily converted to a variety of different functional groups using hydrosilation reactions. This eliminates the need to protect the functional groups since the functional groups were introduced into the polymer chain post-polymerization,i.e. in the absence of carbanionic chain-ends.
Also, the availability of a wide variety of substituted styrenes makes it easy to introduce different functional groups at the chain-ends using hydrosilation reactions.
In-chain functionalized polymers were made by anionically polymerizing the monomer, (4-vinylphenyl)dimethylsilane in hydrocarbon solvents at room temperature. The silyl-hydride groups in this polymer chain could then be converted into different functional groups by use of hydrosilation reactions. This method also allows synthesis of a variety of in-chain functionalized polymers due to the easy availability of functionalized alkenes. Since functional groups were introduced on the
2 polymer chain post-polymerization, again there was no need to protect these groups.
Anionic synthesis of only a single functionalized polymer (silyl-hydride) was required and all other functional groups were introduced into the polymer chain by use of simple hydrosilation reactions done post-polymerization.
It was necessary to study the anionic copolymerization of styrene with (4- vinylphenyl)dimethylsilane, since this would provide control of the number and distribution of silyl-hydride units that could be incorporated in the copolymer chain.
Copolymerization studies were done and the monomer reactivity ratios of the two monomers were determined. Using this information a copolymer was synthesized and the silyl-hydride units on it were further functionalized with an amino acid, in an attempt to make a polymer biohybrid. The method used is a very versatile technique which can be used to introduce virtually any amino acid or peptide unit into the copolymer chain.
Anionic polymerization can be used to synthesize well-defined thermoplastic
elastomers (TPE) which are commercially useful materials. However, there is no known easy method to introduce functional groups into the styrene segment of these anionically synthesized TPEs. Introduction of functional groups would result in modification of many properties of these materials depending on need. By using the monomer (4-vinylphenyl)dimethylsilane to synthesize the TPE it becomes easy to incorporate different functional groups on the polymer chain by using hydrosilation reactions. A TPE was synthesized the first block of which was a copolymer of styrene with (4-vinylphenyl)dimethylsilane, followed by a polyisoprene block and finally another block of the copolymer of styrene and (4-vinylphenyl)dimethylsilane.
3
CHAPTER II
HISTORICAL BACKGROUND
2.1 Living Anionic Polymerization
2.1.1 General Aspects
A living polymerization is defined as one that proceeds in the absence of chain transfer and termination reactions1-3. Well-defined polymers with low degrees of
compositional heterogeneity can be prepared by this method. There are several major
variables that affect the polymer properties and their control by anionic
polymerization is described in this section. A high molecular weight is the key feature
that differentiates polymers from low molecular weight compounds. The molecular
weight of a polymer synthesized by living polymerization techniques can be controlled by the stoichiometry of the reaction and also the degree of conversion. The
number average molecular weight for a polymer after complete conversion using a monofunctional initiator is shown below.
Mn = grams of monomer/moles of initiator.
For the case of a difunctional initiator the molecular weight is calculated according to
the equation : Mn = grams of monomer/(1/2 moles of initiator). In the case when
the conversion is not 100 %, the following equation is used :
4
Mn = grams of monomer consumed/moles of initiator.
A living polymerization makes it possible to synthesize polymers with narrow
molecular weight distributions. This is usually achieved by use of an active initiating
species, such that the rate of initiation is faster than or competitive with the rate of
4 propagation, i.e. Ri ≥ Rp . An active initiator ensures that all polymer chains are initiated at approximately the same time and so they also grow for approximately the same time period, giving rise to polymers of similar length. This is how a narrow molecular weight distribution is obtained.
One of the unique features of a living polymerization is that after the monomer
has been consumed all of the polymer chains retain their active chain ends. Further
addition of monomer results in continued polymerization. Using this property, it is
possible to synthesize controlled block copolymers. Also, since all the polymer chains
retain their active centers after consumption of all of the monomer, it is possible to
introduce suitable functional groups at the end of the polymer chain by use of the
suitable terminating agents. Although the efficiency of these reactions is often not
100 %, it still provides a methodology for synthesis of chain-end functionalized
polymers5,6. P- + X-Y (electrophilic agent) Æ P-X + Y-
Extending the concept for synthesizing chain-end functionalized polymers, it is
also possible to make star polymers by use of a suitable linking agent7-9. A suitable n-
functional linking agent can be added to a system of living polymer chains giving rise
to a n-armed star with controlled arm molecular weight and narrow molecular weight
distribution.
5
In order to decide if a polymerization is living or not, there are several
prescribed criteria10. The first criterion established by Szwarc and coworkers11 is that
the polymerization should proceed until all of the monomer has been consumed and further addition of monomer should result in continuation of the polymerization. This criteria is one of the oldest and best methods for determining if a polymerization is living or not. According to another criterion, a plot of the molecular weight versus the conversion of the polymer should be a straight line in the case of a living polymerization. However, it has been observed by Penczek and coworkers12 that this
criteria is not a rigorous test for chain termination reactions, although it indicates the
absence of any chain transfer reactions. Chain transfer reactions can also be detected
by the criterion that the number of polymer chains should remain constant with
conversion because any chain transfer would result in an increase in the number of
polymer chains. One important criterion to determine the livingness of a
polymerization is the dependence of the number average molecular weight on the
stoichiometry and the degree of monomer conversion of the reaction.
It is important to note that a narrow polydispersity is not a necessary or sufficient
condition to consider a polymerization to be a living polymerization13. If the rate of
initiation is slow compared to the rate of propagation, then a broad polydispersity
may be observed. However such a polymerization may still be living. It is also
possible to obtain narrow molecular weight distributions in systems which are clearly
not living 14. To illustrate this behavior, an experiment was conducted where samples were prepared by mixing different monodisperse polymers together. The
6
resulting mixture was analyzed by SEC and it exhibited a narrow polydispersity.10
Livingness of a polymerization can also be determined by the synthesis of block copolymers and chain-end functionalized polymers, as already described above. So, living polymerizations can be used to synthesize polymers with control over many parameters, such as molecular weight, molecular weight distribution, copolymer composition, molecular architecture and chain-end functionality.
2.1.2 Monomers
There are two classes of monomers that can be polymerized anionically: vinyl/diene type monomers, where polymerization takes place at the double bond; and cyclic monomers, in which case the polymerization proceeds by ring-opening. For the anionic polymerization of vinyl monomers, generally monomers with electron- withdrawing groups are reactive. These substituents are capable of stabilizing the negatively charged carbanion, as shown below.
- - R + CH2=C(XY) Æ RCH2C (XY)
It is also necessary that these substituents be stable in the presence of the reactive
chain-ends. If the substituents contain active hydrogen atoms which are susceptible to
reaction with the carbanionic chain-ends, it is necessary to protect these functional
groups with suitable protecting groups15. In the absence of suitable protecting groups
for the substituents, chain transfer or chain termination reactions may occur, which
would result in loss of control over the polymerization. There exists a range of
monomers that can be polymerized anionically without any transfer or termination
reactions. Some examples are styrenes, dienes, epoxides, cyclic siloxanes and
7 lactones. Monomers containing polar functionalities like acrylonitriles16 and
cyanoacrylates17 cannot undergo controlled anionic polymerization due to side
reactions of these functional groups with the active initiating species.
The reactivity of monomers in anionic polymerization depends upon the pKa of the conjugate acids of their propagating anions. The stability of the corresponding carbanionic chain ends and choice of a suitable initiator are also important things to be considered. Usually, a proper initiator for any monomer is one that has a pKa value for it’s conjugate acid that is similar to the propagating carbanionic chain ends.4
2.1.3 Solvents
Choice of a proper solvent is a very important issue for the controlled anionic
synthesis of polymers. Polymerization of styrene and diene monomers can be done in either a hydrocarbon solvent, like benzene, or in a polar solvent, e. g. tetrahydrofuran
(THF). In a hydrocarbon solvent, both aggregated [(RMt)n] and unaggregated (RMt)
species can exist. On the other hand, in a polar solvent, in addition to the aggregated
and unaggregated species, free ions (R+ + Mt-), contact ion pairs (R+,Mt-) and solvent-
separated ion pairs (R+//Mt-) can also exist, as shown in Scheme 2.1 (Winstein
spectrum)18,19. - + R-Mt+ _ + R + Mt (RMt)n n RMt R // Mt
Scheme 2.1. Winstein spectrum.
Each of the species in the Winstein spectrum can participate as propagating species in anionic polymerization. The major intermediate that exists depends on the structure of the carbanion, the nature of the solvent, the temperature and also the counterion 8
present. Another important aspect of the use of polar solvents is that there is a
possibility of chain transfer reactions of the active initiator or the propagating
species with the solvent during styrene and diene polymerizations, producing side
reactions. Therefore, it is advisable to carry out polymerizations in polar solvents at
low temperatures. In contrast, there are usually no side reactions at room temperature in hydrocarbon solvents (except for toluene)19. However, since the carbanionic chain
ends remain aggregated in hydrocarbon solvents, addition of a small amount of Lewis
base helps to dissociate the aggregates and makes the reaction more efficient, since
the C-Li bond is more active in the unaggregated species compared to the
aggregates19. But if a large amount of a polar solvent is added to the polymerization of a diene taking place in a hydrocarbon solvent, polymers with high amounts of 1,2- microstructure are produced20. It is generally desirable to synthesize polydienes having a high 1,4-microstructure, because this produces elastomers and thermoplastic elastomers having better material properties as described in more detail later.
Aromatic hydrocarbon solvents generally reduce the degree of aggregation of the
carbanionic chain-ends compared to aliphatic solvents like cyclohexane 21.
2.1.4 Initiators
In 1911, Matthews and Strange22 polymerized isoprene using metallic sodium as the initiator. Metallic sodium is known to initiate the polymerization of styrene and
diene monomers by first forming a diene radical-anion that dimerizes to form a
dianion and then monomers add to these dianions to form oligomers. This initiating
reaction using alkali metals is a heterogenous initiation which does not provide good
9 control over the polymerization. Nowadays the use of soluble aromatic radical anions as initiators of living anionic polymerizations has become a popular practice (Figure
2.1)3. According to studies by Szwarc and coworkers3, addition of sodium to
naphthalene in THF produces a stable radical anion of naphthalene. The oxidation-
reduction reaction of the naphthalene with sodium is reversible upon the removal of
THF. This leads to the problem that this kind of initiation makes it necessary to use
THF as the solvent which affects the diene microstructure. The naphthalene radical anion can react with monomers like styrene to give the radical anion of the styrene
monomer (Figure 2.2). These monomer radical anions can then dimerize rapidly
giving rise to dianions, thus resulting in homogeneous initiation of the polymerization.
THF Naph-. Mt+ Naph + Mt
Figure 2.1. Synthesis of the naphthalene radical anion.
-. Mt+
Naph-. Mt+ + + Naph
-. Mt+ - + 2 Mt+ - Mt
Figure 2.2. Synthesis of a styrene dianionic initiator using the naphthalene radical anion.
10 A variety of alkyllithium initiators are commercially available in hydrocarbon solvents. The relative reactivity of these alkyllithium-intiators with respect to styrene and diene monomers is dependent on their degree of aggregation. Usually less
aggregated initiators are more active initiating species. The order of reactivity of
some commonly used alkyllithium initiators for the anionic polymerization of styrene
is shown below.23
menthyllithium > sec-BuLi > i-PrLi > i-BuLi > n-BuLi > t-BuLi.
sec-BuLi is a commonly used initiator in the anionic polymerization of styrene and
diene monomers, since it effects rapid rates of initiation compared to propagation, resulting in narrow molecular weights. Another common class of initiators are difunctional initiators which are used for the two-step synthesis of triblock copolymers. Aromatic radical anions are good difunctional initiators. However, to
synthesize these initiators it is necessary to use polar solvents, like THF, which
produce polydienes having high 1,2-microstructure20 (as mentioned above). Also
when THF is used as the solvent there exists an equilibrium between contact ion
pairs, solvent separated ion pairs and free ions (Winstein spectrum), each of which
species can propagate with a different rate constant making the molecular weight
distribution broader. One of the unique features of using organolithium initiators in
hydrocarbon solvents is their ability to produce polydienes with high 1,4-
microstructures. This results in better material properties of the thermoplastic
elastomers containing these polydiene units. In order to eliminate this problem of
polydiene microstructure, a very popular hydrocarbon-soluble, dilithium initiator was
synthesized from 1,3-bis-(1-phenylethylene)benzene24. The addition reaction of sec-
11
BuLi to this compound to produce the dilithium initiator is shown in Figure 2.3. This initiator is very active and produces polymers with narrow molecular weight distributions and controlled molecular weights. Also since the reactions are done in hydrocarbon solvents at room temperature, the desired high 1,4-polydiene microstructure is formed.
Li BuCH2 BuLi +
BuLi
Li BuCH2 Li BuCH2
Figure 2.3. Synthesis of dilithium initiator from 1,3-bis(1-phenylethylene)- benzene.
2.1.5 Lewis Base Effects
Addition of Lewis bases and alkali metal alkoxides can affect both the initiation and propagation reactions in alkyllithium-initiated anionic polymerizations in hydrocarbon solvents. Addition of small amounts (comparable to the amount of the initiator) of Lewis bases, like ethers and amines, results in dramatic increases in the rates of initiation compared to the rates of propagation in the polymerization of diene
12 and styrene monomers25-28. There is an increase in the rate of propagation too but it is less compared to that of the initiation. This effect of Lewis bases is believed to be due to their ability to dissociate aggregates of the initiating and propagating species, which are the predominant species in hydrocarbon solvents. However, it has been observed that increasing the amount of Lewis base added to the system beyond a certain amount causes the rates of initiation and propagation to decrease. It is expected that at low concentrations of the Lewis base the unassociated species would complex with the base29 making the C-Li bond more polar and thus increasing it’s reactivity. But, when the concentration of the base becomes too high, the monomer has to compete with the base for complexation with the vacant sites on lithium at the carbanionic chain ends.
2.1.6 Initiation Reactions
It has been possible to extensively study the initiation reactions of alkyllithium compounds with styrene and diene monomers in hydrocarbon solvents30,31.
It has been observed that rates of initiation are much faster in polar solvents; however, in these solvents there may be side reactions and also the diene microstructure is affected. The rate of initiation for n-BuLi-initiated polymerization of a styrene monomer in benzene is shown below,
1/6 Ri = kiKd[BuLi] [M] where Kd and ki can be defined from the following two equations :
13 Kd 6 RLi (RLi)6
ki RCH CHLi(C H ) RLi + CH2=CH(C6H5) 2 6 5
Bywater and Worsfold 21 observed that usually the fractional order of the initiator concentration in the rate equation was the reciprocal of the degree of aggregation of the initiator. So, in keeping with the equation shown above and for the case of reactions initiated by n-BuLi, the fractional order of the initiating species in the rate equation is 0.17, which is the reciprocal of the degree of aggregation of n-BuLi which exists as hexamers. Also, for initiation reactions with sec-BuLi, the fractional kinetic order of the initiator in the rate equation was observed to be 0.25, which is the reciprocal of the degree of aggregation of the initiator units in this case. These observations led to the conclusion that it was the dissociated species that was responsible for the initiation rather than the aggregates. However, it was observed that for aliphatic solvents like cyclohexane the inverse order of the degree of association with respect to the fractional kinetic order in initiator concentration was not observed, i.e., a first order dependence on initiator concentration was observed. Also the rate of initiation exhibited a sigmoidal behavior. Based on these observations it was proposed that in aliphatic solvents the active initiating species is not the dissociated species21. Initiation would occur by addition of the monomer to the aggregate of the organolithium species as shown in the following equation. A rapid inter-aggregate exchange is known to take place which results in a mixture of initiating and propagating species at short reaction times which further complicates the kinetics :
14
(RLi)n + M Æ [(RLi)n-1(RMLi)]
Another important observation is that the initiation is more rapid in aromatic solvents
compared to aliphatic solvents due to the greater capability of aromatic solvents to
dissociate aggregates of the initiating species19.
2.1.7 Propagation Reactions
Poly(styryl)lithium chains are associated into dimers in hydrocarbon solvents. The kinetic order of the propagation reactions are independent of the nature of the hydrocarbon solvent (aliphatic or aromatic), which is different from the situation observed for the initiation reactions19. The active propagating species is
expected to be the dissociated species since the rate of propagation has a one half
order dependence on the concentration of poly(styryl)lithium chains. This is shown
below in Scheme 2.2, where, kp is the propagation rate constant and KD is the
dissociation constant for the dimer - unimer equilibrium.
Rp = -d[S]/dt = kp[PSLi]0[S]
1/2 1/2 = kp(Kd/2) [PSLi] [S]
1/2 = kobs[PSLi] [S]
Scheme 2.2. Kinetic equations for the rate of propagation ([PSLi]0 is the dissociated species which is the reactive species for addition of monomer).
The propagation kinetics of styrenes and dienes in hydrocarbon solvents exhibit a first
order dependence on the monomer concentration. However, there is confusion
regarding the state of aggregation of the diene chain ends in hydrocarbon
solvents32,33. Assuming that the propagating species are the unassociated chain ends,
15 there has been reported dimeric and tetrameric degrees of aggregation of the chain-
ends21.
2.1.8 Stereochemistry of diene polymerization
One of the unique features of polydienes synthesized anionically in hydrocarbon solvents with lithium as the counterion is that they exhibit a high 1,4-microstructure.
When isoprene is polymerized in benzene at room temperature with lithium as counterion, 70% 1,4-cis-, 24% 1,4-trans- and 6% 3,4-microstructures are observed34.
Use of an aliphatic solvent, like cyclohexane, results in an increase in the amount of cis-1,4-microstructure. However the total amount of 1,4 microstructure (cis + trans) always remains constant. Polymerization in hydrocarbon solvents at room temperature with lithium as counterion usually gives a high cis-1,4-microstructure. To explain this behavior, Gerbert and coworkers35 suggested that the initially formed cis
isomer of the poly(dienyl)lithium chain ends could isomerize to the trans isomer and
that the rate of this isomerization is competitive with the rate of monomer addition to
the chain end. This isomerization is shown in Scheme 2.3. In addition, model studies
carried out by Worsfold and Bywater36 showed that the rate of monomer addition to
the cis-poly(isoprenyl)lithium chain ends occurs almost 8 times faster than monomer
addition to the trans-poly(isoprenyl)lithium chain ends (kp,cis/kp,trans = 8). This ratio
for poly(butadienyl)lithium chain ends is about 2 36. These studies also determined
that at equilibrium about 70% of the poly(isoprenyl)lithium chain ends have a trans-
microstructure and 30% have a cis-microstructure. For poly(butadienyl)lithium the
relative concentrations are 40% of cis- and 60% of trans- chain ends. The lower cis-
1,4 microstructure in polybutadiene compared to polyisoprene is due to the lower rate
16
of monomer addition to cis-isomer compared to the trans-isomer in polybutadiene
when compared to polyisoprene. However, one thing to be kept in mind is that in
these calculations the aggregation of the chain ends have been neglected.
cis kp ------cis* + D Æ ------cis,cis* k1 ↑↓k -1 trans k p ------trans* + D Æ ------trans,cis*
Scheme 2.3. Isomerization of the poly(isoprenyl)lithium chain ends.
The microstructure of polydienes is known to be very sensitive to the nature of the
solvent used. In presence of polar solvents with lithium as counterion, the high 1,4-
microstructure is no longer obtained and high amounts of 1,2-polybutadiene and
3,4-polyisoprene are obtained19. As the size of the counterion increases, there is a
greater tendency for formation of polydienes with a high 1,2-microstructure in non-
polar and in polar solvents. In presence of polar solvents versus hydrocarbon solvents,
the polydienyl chain ends become less aggregated. Apart from that, a polar solvent
solvates a smaller counterion to a greater extent. So, when lithium is present as the
counterion, solvent-separated, ion-pairs exist which result in delocalization of the negative charge from the alpha carbon atom. This results in more charge on the gamma carbon atom and less on the alpha carbon atom. On the other hand, in hydrocarbon solvents the negative charge is more localized on the alpha carbon atom giving rise to high 1,4-microstructures. In conclusion, the stereochemistry of polydienes appear to be strongly dependent upon the nature of the solvent, counterion and the presence of polar additives19.
17
2.2 Copolymerization
Random copolymerization of two different monomers would give a polymer having properties intermediate between the properties of the polymers obtained from both of these monomers. However, in many copolymerizations random incorporation of the monomer units does not take place. Usually there occurs a preferential incorporation of one monomer with respect to the other. In the case of living polymerizations, which proceed in the absence of any transfer or termination reactions, this results in a polymer having compositional heterogeneity along the chain. There are four kinetic equations that define the copolymerization of two monomers, assuming that the rates depend only on the nature of the chain-end unit
(Scheme 2.4). k11 ------M1 + M1 Æ ------M1-M1 k12 ------M1 + M2 Æ ------M1-M2
k21 ------M2 + M1 Æ ------M2-M1 k22 ------M2 + M2 Æ ------M2-M2 Scheme 2.4. Kinetic equations for copolymerization of two monomers.
Using the equations shown above and making a steady state assumption
- - (d[P1 ]/dt = d[P2 ]/dt = 0), the instantaneous copolymerization equation can be
19 obtained as shown below . Here r1 = k11/k12 and r2 = k22/k21 are the monomer
reactivity ratios.
d[M1]/d[M2] = [M1](r1[M1] + [M2])/[M2](r2[M2] + [M1])
Depending on the monomer reactivity ratios obtained from the equation shown above,
it is possible to predict the copolymerization behavior of the monomers. For example,
18
if r1r2 = 0, an alternating copolymer will be formed. A value of r1r2 =1
gives a random copolymer and if r1r2 >> 1, a block copolymer structure is obtained.
The monomer reactivity ratios for the anionic polymerization of styrene and
diene monomers depend on the nature of the solvents in which the polymerizations
are done, the counterions and also on the nature of the substituents present on the monomers. In anionic polymerizations done in hydrocarbon solvents, the general trend observed is that a styrene monomer prefers to add to another styrene monomer having an electron withdrawing group (ρ > 0) on it rather than to itself (r1 < 1 and r2 >
1)19. On the other hand, styrene prefers to add to itself rather than to a styrene having an electron donating group (ρ < 0) on it19. The alkyllithium-initiated copolymerization
of styrene with diene monomers in hydrocarbon solvents exhibits an unexpected
behavior. It has been observed, in general, that the rate of homopolymerization for a
37 styrene monomer is greater than for a diene monomer (kss > kDD) . But when a
styrene and a diene monomer are copolymerized, there is observed a preferential
incorporation of the diene monomer into the copolymer and the styrene monomer is
introduced only after all the diene monomer has been depleted. This produces a
polymer with a tapered structure. This behavior can be explained from the crossover
kinetics of these two monomers. It has been observed that for the copolymerizations
in hydrocarbon solvents the crossover rates follow the order as shown: kSD >> kSS >
37,38 kDD > kDS (D = diene; S = styrene). From this order it is easy to see why there is a
preferential incorporation of the less reactive diene monomer into the copolymer.
This copolymerization behavior of styrenes and dienes shows that the
19 homopolymerization rates cannot be used to predict the copolymerization behavior.
However, it has been observed that by changing the solvent from a hydrocarbon to a
polar solvent there is a preferential incorporation of styrene into the copolymer,
giving a polymer which still has a tapered structure but has styrene incorporated first
and then the diene monomer. This leads to one obvious inference, that addition of
small amounts of Lewis bases to a copolymerization of styrene and diene in
hydrocarbon solvents should result in a random copolymer, by increasing the rate of
styrene incorporation initially into the copolymer. However, the amount of Lewis
base should be kept low in order to obtain a high amount of 1,4-diene microstructure.
2.2.1 Block Copolymers19
Anionic polymerization provides a powerful method for the synthesis of well-
defined block copolymers. The synthesis of block copolymers by this method is a
consequence of the living nature of anionic chain-ends. This makes monomer purity very important. Also, crossover of a carbanionic chain end formed from one monomer to another monomer occurs only if the carbanion of the second monomer has approximately the same stability or is more stable than the first one. So crossover can take place only from a monomer whose conjugate acid has a higher pKa to those
monomers whose conjugate acids have a similar or lower value of pKa. There are three methods that can be used for the synthesis of triblock copolymers.
2.2.1.1 Three-step, sequential monomer addition19
This method involves use of a monofunctional initiator to initiate the
polymerization of the first monomer, e.g. styrene. After the polymerization of the
first block is completed, the second monomer is added. It is important that the second
20 monomer has a very high level of purity and also that it’s conjugate acid should
have a pKa similar to or lower than the first monomer in order to result in formation
of a more stable carbanionic chain end. Crossover from styrene to a diene monomer
occurs very rapidly (Ri > Rp). This gives a second block with narrow polydispersity.
The third block added is usually a styrene monomer again; but, since the rate of
crossover from a poly(dienyl)lithium chain end to a poly(styryl)lithium chain end is
slow, the final polymer would have a broad polydispersity (Ri < Rp). Addition of
small amounts of Lewis bases promotes crossover by dissociating the aggregates and
making the polydispersity narrower.
2.2.1.2 Two-step, sequential monomer addition and coupling19
This method involves use of a monofunctional initiator to initiate the
polymerization of first the styrene block and then an efficient crossover to form the dienyllithium chain ends. However the amount of diene monomer added is only half the amount required to form the center block. A difunctional electrophilic linking agent can then be added to the system containing living dienyllithium chain ends.
This method eliminates the problem of crossover to the third monomer and also reduces the introduction of impurities by eliminating the addition of a third monomer
into the polymerization system. This method also increases the versatility with respect
to the nature of the middle block. However, this method requires an efficient coupling
reaction and a high control over the stoichiometry of the reaction.
2.2.1.3 Difunctional Initiation and two-step monomer addition19
Use of a difunctional initiator results in one of the most versatile methods for
synthesis of triblock copolymers. Use of a hydrocarbon-soluble, dilithium initiator,
21 e.g. the adduct of two sec-BuLi units with 1,3-bis(1-phenylethenyl)benzene
(DDPE)24, once again eliminates the need for the third monomer addition step. This
method also provides more versatility regarding the nature of the end blocks. Also,
use of a hydrocarbon-soluble, dilithium initiator produces a diene center block with a
high 1,4- microstructure.
2.2.2 Determination of monomer reactivity ratios
The copolymerization equation can be used along with experimentally obtained
data to determine the values of the monomer reactivity ratios39. It is necessary to
carry out a series of experiments with different concentrations of the two monomers.
The copolymerization equation can be used only at low conversions, since at higher
conversions the copolymer composition and monomer feed composition tends to
drift. Sometimes the integrated form of the copolymerization equation may also be
used40, but the calculations in this case are more cumbersome. A number of different
methods use the differential form of the copolymerization equation to extract the
monomer reactivity ratios. However, many of these methods may not give reliable
values due to experimental errors and some bias in error treatment40.
2.2.2.1 Fineman-Ross Method41
The copolymerization equation can be rearranged to obtain the equation shown
below, by using f = dM1/dM2 and F = M1/M2.
f = F (1+ r1x)/(r2 + x) (Equation for Fineman-Ross method)
The above equation can be linearized as shown by the equation below (Fineman-
Ross): y = r1x – r2 ,
y = F(f-1)/f and x = F2/f
22
Graphically plotting this equation using the experimentally obtained values gives a straight line, the slope of which gives r1 and the intercept gives r2. However, one problem of using this method to determine r1 and r2 is that the experimental data are not equally distributed along the straight line and the data points obtained for low monomer concentrations have the greatest effect on the slope of the line40.
2.2.2.2 Kelen-Tudos Method42
This method is used to eliminate the drawbacks of the Fineman-Ross method.
The copolymerization equation is once again written in a linear form.
η = r1ξ – r2(1-ξ)/α (Equation for Kelen-Tudos method)
Two new parameters η and ξ were introduced into the equation for the Kelen-Tudos method to give the equation below :
η = G/(α + F) and ξ = F/(α + F); where, G = x(y-1)/y and F = x2/y.
Also, here α (> 0) is an arbitrary constant which can be used to optimize the distribution of the data along the straight line. The value of α is given as shown below
α = √ FminFmax
The values of η and ξ can be obtained by use of the experimental data and these values can be plotted to obtain a straight line. This line can be extrapolated to ξ=0 and
ξ=1 to give two intercepts, –r2/α and r1. This method distributes the experimental data uniformly along the straight line in the interval (0,1). However, the results obtained by this method may also have some error due to linearization of the copolymerization equation40.
23 2.2.2.3 Error-in-Variable-Model (EVM) Method 43-45
A non-linear method was proposed by Behnken43, in order to eliminate the error
in estimation of the true monomer reactivity ratios. This non-linear approach was
further extended and the EVM model was developed, which correctly takes into
account the error in all of the variables45. This method takes into account the error in
both the independent and dependent variables (monomer feed and copolymer
compositions, respectively). By doing this, it gives rise to a joint-confidence region.
The resulting plot gives the value of the two monomer reactivity ratios in the
copolymerization. The EVM method involves complicated calculations; however,
the values of the monomer reactivity ratios can be easily determined by incorporating
the experimentally obtained data into a computer program that has been developed for
44 this purpose .
2.3 Functionalized Polymers
Since many anionic polymerizations proceed in the absence of any chain transfer or termination reactions, they can produce well-defined polymers with the desired
structure and architecture. Use of anionic polymerization techniques, especially
alkyllithium-initiated polymerizations of styrene and diene monomers, for the
synthesis of well-defined in-chain and chain-end functionalized polymers has been of
46-60 interest of late . A large number of chain-end and in-chain functionalized
polymers have been synthesized anionically, but unfortunately these polymers have
not been completely characterized in most cases. This makes it difficult to develop a
general procedure for the synthesis of functionalized polymers with the desired
structure and architecture. Study of functionalized polymers is of much interest since
24
by introducing different functional groups into or at the end of the polymer chain it is
possible to tailor many properties of the polymer molecules, e. g. mechanical
properties, polarity, adhesion, etc.
2.3.1 Chain-End Functionalized Polymers
Many anionic polymerizations proceed in the absence of any chain
transfer or chain termination reactions. This can result in formation of polymers
having narrow molecular weight distributions and controlled molecular weights. In
the alkyllithium-initiated polymerizations of styrene and diene monomers after all the
monomer in the system is consumed, there are living carbanionic chain ends present.
Reaction of these stable chain ends with suitable electrophilic agents results in end-
61-71 functional polymers . However, these methods almost never produce 100 % chain-
end functionalized polymers. Also, for many of the polymers synthesized by these
methods there may be a chance of some side reactions of the functional groups with
the living carbanionic chain ends. In these cases suitable protecting groups are needed
for the functional groups. Another common method of producing chain-end
functionalized polymers is to use a functionalized initiator where in many cases the
functional group may need to be protected. Although a variety of chain-end
functionalized polymers have been synthesized, unfortunately all of these polymers
have not been characterized completely. This makes it important to develop different,
general, functionalization techniques that can produce end-functionalized polymers efficiently. DeSimone and coworkers72 developed a method based on the efficient
reaction between living carbanionic chain ends and silyl halides. This reaction occurs
25 in the absence of any competing side reactions. The strategy followed was to first react the chlorosilanes with any commercially available alkene, having a functional group X on it, by use of a platinum-catalyzed hydrosilation reaction. This functionalized chlorosilane was then allowed to react with the living polymeric chain ends, resulting in chain-end functional polymers. This is shown in Scheme 2.5.
X Pt Catalyst (CH3)2SiCl(CH2)3X (CH3)SiHCl +
P-Si(CH ) (CH ) X P-Li + (CH3)2SiCl(CH2)3X 3 2 2 3
Scheme 2.5. Use of functionalized chlorosilane to introduce functional groups at the polymer chain-ends.
The problem with this method is the susceptibility of silyl halide compounds to moisture, which makes handling these materials problematic73. Another commonly used method for synthesis of chain end functional polymers is by use of substituted 1,1-diphenylethylenes. Reactions of living polymer chain ends occurs with functionalized 1,1-diphenylethylene derivatives quantitatively and with an efficient rate of crossover74-79. One unique feature about this functionalization method is that it does not terminate the polymerization, instead it produces another carbanionic species which can initiate the polymerization of some other added monomer unit. A description for this technique is shown in Scheme 2.6.
26 C6H5X C6H5X
P-CH2C-Li P-Li + CH2=C
C6H5X C6H5X
Scheme 2.6 Use of 1,1-diphenylethylene derivatives to introduce functional groups on a polymer chain.
2.3.1.1 Hydrosilation Reactions
Hydrosilation reactions are usually defined as the addition of the silyl hydride group across a carbon carbon double bond giving rise to new Si-C and C-H bonds80.
An example is shown in Scheme 2.7. These reactions can be effected using many catalysts, but the most commonly used and efficient catalysts are transition metal catalysts80-83.
+ Si H Si
H Scheme 2.7 Addition of a Si-H group across a carbon-carbon double bond.
One of the unique features of hydrosilation reactions is that they proceed under conditions under which most functional groups are stable. For homogeneous transition metal catalyzed systems, two mechanisms have been proposed, the Chalk-
Harrod mechanism and the modified Chalk-Harrod mechanism as shown in Figure
2.4 84,85
27 H-SiR3
Oxidative addition
H M
SiR3 Chalk-Harrod M R'-CH2-CH2-SiR3 SiR3 CH2=CHR' H Reductive Elimination M
CH CH 2 2 R' Modified Chalk-Harrod H H SiR 3 M M SiR3 CH2 CH CH2=CH-R'
R'
Figure 2.4. Chalk-Harrod and modified Chalk-Harrod mechanism.
A number of different transition metals can be used to catalyze hydrosilation reactions and their order of activity is Pt > Rh > Ir = Ru > Os > Pd86,87. The most commonly used platinum-based hydrosilation catalysts80 are Speier’s catalyst
86,87 (H2PtCl6) and Karstedt’s catalyst (Pt{[Me2(vinyl)Si]2O}1.5) . Another important feature of platinum catalyzed hydrosilation reactions is that they produce exclusively the anti-Markovnikov addition products as shown in Scheme 2.880.
28 Karstedt's PS-SiH + CH2=CH-R PS-SiCH2CH2R Catalyst Scheme 2.8 Anti-Markovnikov addition of the Si-H group in a hydrosilation reaction.
2.3.1.2 A General Functionalization Method by combination of anionic polymerization and hydrosilation reactions90
It has already been described that living anionic polymerization provides a useful technique to prepare well-defined, chain-end functionalized polymers. Also, as discussed in the previous section, transition metal-catalyzed hydrosilation reactions proceed very efficiently in the absence of any side reactions with a variety of functional groups. Recently Riffle and coworkers88 combined both these methods together and synthesized chain end functionalized polymers. The method followed by them is shown in Scheme 2.9.
[H] (CH3)2SiHCl PBd-Li PEB-Li PEBSi(CH3)2H
O Karstedt's catalyst PEBSi(CH3)2H PEB O O O Scheme 2.9 Introduction of an epoxide group at a polymer chain-end using hydrosilation.
Using a similar method Loos and Müller89 synthesized maltoheptaose-block- polystyrene by use of a hydrosilation reaction between silyl hydride functionalized polystyrene and trieicosacetyl-N-allylmaltoheptonamide. The functional groups were protected prior to the reaction and deprotected after the hydrosilation. Following 29 this strategy, Quirk and Kim90 developed a general functionalization method (GFM) for the synthesis of chain end functionalized polymers. The method followed is shown in Scheme 2.10. This method allows the synthesis of a variety of chain-end functionalized polymers in the absence of any need to protect the functional groups, since most groups are stable to hydrosilation reaction conditions. Also the easy availability of a wide variety of substituted alkenes increases the versatility of this method.
P-Li + (CH3)2SiHCl P-Si(CH3)2H + LiCl
X P-Si(CH ) (CH ) X P-Si(CH3)2H + Karstedt Catalyst 3 2 2 3
Scheme 2.10 General Functionalization Method (GFM) for synthesis of chain-end functionalized polymers.
2.3.2 In-chain Functionalized Polymers
In-chain functionalized polymers are polymers with a functional group on every monomer unit along the polymer chain. The obvious way to synthesize these polymers is by use of functionalized monomers. However, functional groups having acidic hydrogens in them would participate in chain transfer or termination reactions during the course of anionic polymerizations. One possibility is to protect these functional groups with suitable protecting groups, that are stable during the polymerization. These groups can then be removed easily post-polymerization91. In the situation where the functional groups are stable in presence of the active polymerization initiator and the living polymer chain ends, the monomers can be easily polymerized in absence of any protecting groups. A variety of such styrene
30 functional monomers have been polymerized anionically 91,92-121. In the nineteen nineties, Nakahama and coworkers91 synthesized a large number of such styrenic in- chain functional polymers, having functional groups that needed to be protected in order to produce controlled polymers anionically. They developed a strategy for making these polymers, as shown in Figure 2.5. NH2 N(SiMe3)2
H H H H n/2 n/2 n/2 n/2 1. THF, -78C MtNaph + n 1. HCl
2.CH3OH 2. base
N(SiMe3)2 N(SiMe ) 3 2 NH2
Figure 2.5. Synthesis of in-chain functionalized polymers using the protection- deprotection strategy.
This strategy was based on protecting the functional groups prior to polymerization, followed by anionic polymerization in THF at -78 ºC, and finally deprotection of the protecting groups post-polymerization. This procedure has the obvious disadvantage of having two additional steps: one, the introduction of a protecting group and the other, a deprotection step post-polymerization. Also, all of the polymerizations were done in THF at -78 ºC. This is a condition that is difficult to maintain and it is always preferable to carry out anionic polymerizations at room temperature in hydrocarbon solvents, whenever possible. This led to attempts to develop a more general method
31 for the synthesis of well-defined, in-chain functionalized polymers. The methodology was an extension from the general functionalization method (GFM) developed for chain-end functional polymers that has been described in the previous section. This method once again combines anionic polymerization with hydrosilation chemistry to develop a general procedure for synthesis of controlled in-chain functional polymers.
The proposed strategy is shown in Scheme 2.11. The silyl hydride functionalized styrene monomer, (4-vinylphenyl)dimethylsilane, was not commercially available and had to be synthesized. An attempt was made to polymerize this monomer anionically, using sec-BuLi as initiator, in a hydrocarbon solvent at room temperature. Assuming that this produces a controlled polymer with silyl hydride units on every monomer unit along the polymer chain, the next step was to do a hydrosilation reaction between the silyl hydride groups on the polymer chain and the double bond of any commercially available compound with a functional group at the other end. This would produce polymers with different functional groups in the polymer chain. Since the functional groups are introduced post-polymerization there is no need to protect them. Thus, this procedure eliminates two steps, i.e. protection of the functional groups before polymerization and their deprotection post- polymerization. Also, the unique feature of this method is that it only requires the synthesis of a single functional polymer, poly(4-vinylphenyl)dimethylsilane, and all other functional groups can be introduced into the polymer chain by doing simple hydrosilation reactions post-polymerization. Of course, as mentioned earlier, these reactions were done in hydrocarbon solvents at room temperature and not in THF at
–78 ºC.
32 H sec-Bu n
X
H3CCHSi 3 H Pt (0) (CH ) X sec-Bu 2 3 1.Benzene sec-BuLi + n n R. T. 2. CH3OH
Y
H H3CCHSi 3 H3C Si CH3 sec-Bu n H H Pt(0)
H3C Si CH3
(CH2)3Y
Scheme 2.11 General Functionalization Method developed for synthesis of in-chain functionalized polymers.
33
CHAPTER III
EXPERIMENTAL
Living anionic polymerization takes place in the absence of any chain termination or chain transfer reactions. High vacuum techniques using sealed glass reactors are excellent for exclusion of reactive impurities, e.g. water, air or carbon dioxide, that may cause transfer or termination reactions. All solvents and reagents used for anionic polymerization need to be purified thoroughly and carefully. These purifications were done using standard high vacuum techniques, as described by
Morton and Fetters122. Solvents and reagents were distilled on a high vacuum line,
Figure 3.1. The vacuum line consisted of two horizontal Pyrex glass manifolds, primary (D) and secondary (E). These were separated by Teflon® Rotoflo® stopcocks. The following were attached in series to the primary manifold: a vacuum pump (A) to create the initial vacuum, followed by a silicone oil diffusion pump (B) which was used to create high vacuum conditions (10-5 –10-6 mm Hg) and finally a liquid nitrogen trap (C) to condense any volatile impurities. A Tesla coil was used to check whether a high level of vacuum (< 10-3 mm of Hg) was produced in the vacuum line123. In case some reactive impurities were present in the vacuum line, the
Tesla coil would produce a purple electric discharge and some sound.
34
The transfer and handling of air- and moisture-sensitive compounds were done in a Vacuum Atmospheres dry-box (Model He-193), in which an inert atmosphere was maintained by recirculation of extra pure nitrogen. The atmosphere of the dry box was tested for traces of oxygen and moisture before and after each use with a titanium complex as indicator124.
Figure 3.1 Schematic of a high vacuum line used for anionic polymerization.
A Schlenck line124 was used for most of the synthesis work done, i.e. synthesis of the monomer (Grignard reaction), hydrosilation reactions and the drying of some products. The Schlenck line consisted of a glass manifold with two way stopcocks to enable creating either a vacuum or an inert atmosphere in the manifold. The inert gas used was nitrogen which was dried by passing through a catalyst tower filled with a mixture of alumina and deoxo catalysts (Alfa Aesar, De-Ox). The pressure of the nitrogen in the line was monitored by an oil-bubbler attached at the end of the
Schlenck line. A vacuum could alternatively be created in the manifold by connecting it to a mechanical pump via a trap cooled by liquid nitrogen which helped condense moisture and volatile solvents.
35
3.1 Purification of Solvents, Monomers and Reagents
3.1.1 General
The following compounds were used without any further purification:
sec-Butyllithum (Lithium Division, FMC and Chemetall Foote Corp ca. 1.5 M in cyclohexane).
n-Butyllithium (Lithium Division, FMC ca. 1.5 M in cyclohexane).
2,6-Di-tert-butyl-4-methylphenol (BHT) (Aldrich, > 99 %).
Calcium Hydride (Alfa, 95 %).
Dibutylmagnesium (Lithium Division, FMC, ca. 15 wt % in heptanes).
Sodium dispersion (Alfa, 50 wt % in paraffin oil).
Dicyclohexylcarbodiimide (DCC) (Aldrich, 99 %).
4-Dimethylaminopyridine (DMAP) (Aldrich, > 99 %).
(N-Carbobenzyloxy)-L-phenylalanine (N-Cbz-Phe) (Aldrich, 99 %).
Bismuth (III) Chloride (Fisher, 98 %).
4-Bromostyrene (Aldrich, 98 %).
1,2-Dibromoethane (Aldrich, 99 %).
Magnesium turnings (Aldrich, 95 %).
Karstedt’s catalyst (Gelest, 2.1-2.4 % Pt concentration in xylene)
Osmium tetroxide (2 wt % solution in water, Aldrich, 98 %).
3.1.2 Hydrocarbon Solvents
All hydrocarbon solvents used, i.e. benzene and cyclohexane (Fisher, reagent grade), were purified using similar methods122. The solvent was stirred over sulfuric acid for a week followed by washing with a saturated aqueous solution of sodium
36 bicarbonate in a separating funnel. The remaining moisture in the benzene was removed by stirring it over anhydrous magnesium sulfate (Fisher Scientific, 99.5 %) for 12 h. This solvent was filtered into a 1000-mL, round-bottomed flask containing powdered calcium hydride and was put onto the high vacuum line. The solvent was allowed to stir for 12 h on the high vacuum line. Then it was frozen-degassed-thawed three times and distilled onto a sodium dispersion in another 1000-mL, round- bottomed flask. It was once again stirred overnight and freeze-degassed-thawed 2-3 times after that. Shiny pieces of metallic sodium could be observed. A 1000-mL, round-bottomed storage flask equipped with a Rotoflo® stopcock was then put onto the vacuum line. Under a positive flow of nitrogen, sec-butyllithium was added to the flask, followed by distillation of several mL of styrene monomer into it. An orange color was observed indicating the presence of living poly(styryl)lithium anion. The solvent which was stirred overnight over the sodium dispersion was then distilled into this storage flask. After melting the frozen solvent in the storage flask with a warm water bath, the flask was taken off the vacuum line and the solvent was stored in it until used.
3.1.3 Ethers
All ethers, including tetrahydrofuran (THF) (Fisher Scientific, Certified ACS) and diethyl ether (Fisher Scientific, Certified ACS) were purified in the same manner.
The ether was first stirred over powdered calcium hydride overnight in a 1000-mL, round-bottomed flask. It was then frozen-degassed-thawed three times prior to distilling it into a flask containing a sodium dispersion, over which it was stirred
37 overnight once again. If the sodium pieces did not remain shiny, the solvent was distilled once again over another fresh sodium dispersion. The solvent was freeze- thawed-degassed 2-3 times and when the sodium pieces appeared to be shiny it was distilled into a 1000-mL flask equipped with a Rotoflo® stopcock, into which 0.25 g of benzophenone and 0.5 g of sodium metal had been introduced. The solvent attained a dark-green color due to formation of the sodium-benzophenone complex.
This indicated that the ether was dry and free from impurities.
3.1.4 Alcohols
Methanol (Fisher, reagent grade) used to terminate the living polymer chains was degassed on a vacuum line prior to distillation into ampoules equipped with breakseals. The ampoules were cooled with liquid nitrogen to condense the methanol and then the ampoules were sealed using a hand torch.
3.1.5 Styrene
Styrene (Fisher, certified grade) was stirred overnight in a 250-mL flask over finely powdered calcium hydride. It was then frozen-degassed-thawed 2-3 times and distilled into a storage flask equipped with a Rotoflo® stopcock into which dibutyl magnesium was added. A light yellow color appeared in one hour indicating completion of the purging operation. The purified monomer was then taken off the vacuum line and stored in a refrigerator. It was distilled via a short path into ampoules prior to use.
3.1.6 Isoprene
Isoprene (Goodyear Chemical, 99%) was stirred over finely ground calcium hydride on a vacuum line for 24 hours with periodic degassing. The monomer was
38 then distilled onto neat n-butyllithium and stirred for 30 minutes at 0 ºC. This step was repeated once again and a light yellow color was observed. The monomer was then distilled into ampoules equipped with breakseals and were heat-sealed with a flame. These ampoules were stored at a temperature of -20 ºC.
3.1.7 Chlorodimethylsilane
Chlorodimethylsilane (Aldrich, 98%) was purified on the vacuum line by stirring for one day over finely powdered calcium hydride with periodic degassing. It was then distilled into ampoules equipped with breakseals on the vacuum line. These ampoules were removed from the vacuum line by sealing with a hand torch.
3.1.8 Allyl functional compounds
Allyl glycidyl ether (Aldrich, 98 %) , 3,3,4,4,5,5,6,6,8,8,8-tridecafluoro-1- octene (Aldrich, 99 %), allyl cyanide (Aldrich, 98 %) and allylamine (Aldrich, 98 %) were all purified in the same manner. The compounds were stirred over calcium hydride overnight, followed by freeze-degassing-thawing 2-3 times on the vacuum line and were then distilled into round-bottomed flasks equipped with Rotoflo® stopcocks, containing activated molecular sieves. Samples of these compounds were removed by use of a gas-tight syringe.
3.2 Synthesis of (4-vinylphenyl)dimethylsilane 125
A solution of 4-bromostyrene (0.14 mol, 20 g) in dry diethyl ether was added dropwise to magnesium (0.18 mol, 4.5 g) in the presence of a catalytic amount of 1,2-dibromoethane. The reaction was exothermic in nature and caused the diethyl ether to reflux. The system was stirred at room temperature under a nitrogen atmosphere for one hour. This solution was then slowly added to chlorodimethyl-
39 silane (0.2 mol, 25 mL, d = 0.76 g/mol) and the reaction mixture was stirred at room temperature overnight under a nitrogen atmosphere. The precipitated salt was then removed by filtration and the crude product obtained was purified first by passing through a column of silica gel in hexane as solvent, followed by distillation under high vacuum (< 10-3 mm of Hg) (bp = 215 ºC; bplit = 217 ºC125). The product was obtained in a 80-90 % yield. 1H NMR 125: δ 4.2; 4.3lit (Si-H) and 0.3; 0.32lit ppm
13 125 -1 lit [Si-(CH3)2]. C NMR: δ -0.2 ppm [Si-(CH3)2]. FT-IR : Si-H (2116 cm ; 2114 cm-1).
3.3 Introduction and modification of functional groups post-polymerization
3.3.1 Hydrosilation Reactions90
Hydrosilation reactions were effected using a platinum based catalyst,
Karstedt’s catalyst. The silane functionalized polymers (0.015 mmol SiH) were dissolved in dried benzene inside the dry box. The necessary amount (1.5 equivalents) of the allyl functional compound was then added to the solution of the polymer. The system was taken out of the dry-box and a few drops of the Karstedt’s catalyst was added at room temperature under a nitrogen atmosphere. In most cases the solution turned yellow almost immediately. The system was stirred at room temperature for periods ranging from 2 days to a week, depending on the nature of the polymer and the functional group to be introduced, i. e. depending on whether the functional group contained active hydrogens.
A specific example of introduction of a functional group at the polymer chain end using hydrosilation reactions is as follows. The ω-silyl-hydride chain end
40 functionalized polystyrene [Mn (obs) = 3100 g/mol, PDI =1.03, 5 g] was dissolved in dry benzene. Allyl cyanide (2.5 mmol; 0.05 mL; d = 0.83 g/mol, MW = 67 g/mol) was added to the system at room temperature. Karstedt’s catalyst (2-3 drops) was added under a nitrogen atmosphere and the system was stirred at room temperature for one week. The reaction was monitored using thin layer chromatography (TLC).
The polymer was isolated by precipitation into an excess of methanol. After drying in a vacuum oven for one week the polymer was obtained in 96 % yield, 4.8 g.
A specific example of an in-chain functionalized polymer that was synthesized is the epoxide-functionalized poly(4-vinylphenyl)dimethylsilane. The detailed procedure used for the hydrosilation reaction is as follows. The poly(4-vinylphenyl)- dimethylsilane (2.0 g, Mn = 33000 g/mol, PDI = 1.03) was dissolved in dry benzene in the dry-box. It was stirred for 30 minutes at room temperature and then allyl glycidyl epoxide (0.02 mol, 1.9 mL, d = 0.9 g/mol, MW = 114 g/mol) was added to the system. The reaction was then taken out of the dry box and under a flow of nitrogen, 2-3 drops of Karstedt’s catalyst was added by use of a gas-tight syringe. The reaction was stirred at room temperature under a nitrogen atmosphere for two days.
The progress of the reaction was monitored by use of 1H NMR. The polymer was isolated by precipitation into a large excess of methanol followed by drying in the vacuum oven for one week. The product was obtained in 95 % yield [1.9 g, Mn(obs) =
50000 g/mol, Mn(calc) = 56500 g/mol, PDI = 1.04].
3.4 Polymerization Techniques
The polymerizations were carried out in sealed glass reactors (Figure 3.2), under high vacuum produced by attaching the reactors to a high vacuum line, Figure
41 3.1. The glass reactors were made by glass blowing and they were annealed in an oven to a temperature of 800 ºF, followed by slow cooling to room temperature.
The reactors were then attached to the vacuum line by glass blowing. The vacuum in the reactor was tested by use of a Tesla coil.
Figure 3.2. Reactor for anionic polymerization.
The initiator, sec-BuLi, was used as received after double titration with allyl bromide. The Gilman double titration method126 was used. This method is used to determine the concentrations of the total base (for both carbanionic and non- carbanionic lithium compounds) and the free base (for only the non-carbanionic lithium). For the total base titration, 4 mL of the initiator, sec-BuLi, was injected into three dried, crimp-sealed bottles inside of the dry box. Each of these bottles also contained 15 mL of dried cyclohexane. These bottles were then taken out of the dry- 42 box and the initiator was quenched with 20 mL of deionized water. These solutions were then titrated with 1.00 N HCl and the end-point was determined using 2-3 drops of phenolphthalein as the indicator. The titration of the free base was also carried out in the similar manner. However, after taking the three bottles out of the dry-box, 4 mL of allyl bromide was added to all of the solutions. This resulted in immediate precipitation of lithium bromide. Following this the solutions were quenched by addition of 20 mL of deionized water. The solutions were titrated with 0.1 N HCl with phenolphthalein as indicator. The concentration of the active carbanionic species was obtained by subtracting the free base concentration from the total base concentration. Values between 1.45 M and 1.55 M were obtained. This initiator was introduced into the polymerization reactors using clean, dry, gas-tight syringes through the injection arm (A), Figure 3.2. Both the initiator bottle and the reactor were first flushed with pure nitrogen gas. The needle attached to the syringe was then introduced into the initiator bottle through the rubber septum and nitrogen was pulled out in it and expelled. Then the needle was again forced through the rubber septum on the initiator bottle and the desired amount of initiator was pulled out. The initiator was injected into the reactor, which was then cooled to -78 ºC using a dry ice/isopropyl alcohol bath, under a positive flow of nitrogen. The injection arm was then sealed using a hand torch. The desired amount of solvent (to make a 10 % solution of the monomer) was distilled into the reactor on the vacuum line. The reactor was removed from the vacuum line by heat-sealing the arm (B), Figure 3.2, by use of a hand torch.
43 Many of the reagents used were introduced into ampoules equipped with breakseals either by distillation or in the dry box. These ampoules were attached to the reactor using a hand torch, as shown in Figure 3.2. Iron hammers, enclosed by glass were introduced adjacent to the breakseals and were held in place by another external iron bar. An external horseshoe magnet was used to move the hammer to crush the breakseal when required. The polymerization was terminated by crushing the breakseal of the ampoule containing the terminating agent (methanol in most cases). The reactor was then opened and the polymer was precipitated into a large excess of methanol. It was dried in a vacuum oven for a week.
3.5 Polymerization Studies
The reactor shown in Figure 3.2 was used to synthesize polystyrene, poly(4- vinylphenyl)dimethylsilane, polyisoprene and their copolymers in hydrocarbon solvents using sec-butyllithium as an initiator
3.5.1 Polystyrene and Poly(4-vinylphenyl)dimethylsilane
The polymerization method for both monomers was the same. The polymerization apparatus is shown in Figure 3.2. For the polymerization of polystyrene, the reactor was joined onto the vacuum line using a hand torch and it was evacuated to 10-6 mm Hg. The reactor was then cooled with a dry-ice/isopropyl alcohol bath and the required amount of the initiator (2.5 mmol; 1.67 mL; 1.494 M), sec-butyllithium, was introduced under a positive flow of nitrogen through the injection arm (A). The arm (A) was sealed and the desired amount of benzene
(approximately 50 mL) was distilled into the reactor through (B) to give approximately a 10 vol % solution of monomer. The reactor was then removed from
44 the vacuum line by sealing (B). The breakseal of the ampoule containing the purified styrene monomer was broken and the monomer was introduced into the reactor (0.05 mol; 5 g; 5.5 mL; d = 0.906 g/mL). A characteristic orange color, indicating the formation of living polymer chains was observed. The reaction was allowed to continue overnight in a temperature-controlled bath at a temperature of 25 ºC. The polymerization was terminated with degassed methanol by breaking the breakseal of the attached ampoule. The polymer was precipitated into a large excess of methanol and dried in a vacuum oven for one week. The polymer was obtained in a 98 % yield
(4.9 g; Mn = 2200 g/mol; PDI = 1.04; Mn (calc) = 2000 g/mol).
Poly(4-vinylphenyl)dimethylsilane was also polymerized using a reactor similar to the one shown in Figure 3.2. The reactor was joined onto the vacuum line using a hand torch and was subsequently evacuated to 10-6 mm Hg. Following that, the reactor was cooled to a temperature of -78 ºC using a dry-ice/isopropyl alcohol bath.
Under a positive flow of pure nitrogen gas, sec-BuLi (0.3 mmol; 0.2 mL; 1.494 M) was introduced into the reactor through the injection port (A). The injection port (A) was sealed using a hand torch and then about 50 mL of benzene was then distilled into the reactor. The reactor was subsequently removed from the vacuum line by heat- sealing the arm (B). The breakseal of the ampoule containing the monomer, (4- vinylphenyl)dimethylsilane, was then broken and the required amount of the monomer was introduced into the reactor (9 g; 10 mL; d = 0.9 g/mL). The polymerization was allowed to continue at 18 ºC for 12 h after which the breakseal of the ampoule containing methanol was smashed to terminate the polymerization
45 reaction. The product was isolated by precipitation into methanol, followed by drying for one week in a vacuum oven. The polymer was obtained in a 94 % yield [8.46 g;
Mn = 33000g/mol; PDI = 1.03; Mn(calc) = 30000 g/mol].
3.5.2 Poly(styrene-co-4-(vinylphenyl)dimethylsilane)
The copolymerization of styrene with (4-vinylphenyl)dimethylsilane was done in a reactor similar to the one shown in Figure 3.2. The purified monomers [styrene and 4-(vinylphenyl)dimethylsilane] were stored in flasks equipped with Rotoflo® stopcocks. Different mol/mol ratios of the two monomers were required for this study
[90/10, 80/20, 65/35, 50/50, 30/70 of styrene/(4-vinylphenyl)dimethylsilane]. These were taken into the dry-box and the desired ratio (mol/mol) of the two monomers was measured out, using a gas-tight syringe, into an ampoule with a Rotoflo® stopcock attached to it. This ampoule was taken out of the dry box and attached to the vacuum line from which it was sealed off. For the copolymer with a 90/10 molar ratio, the required amounts of styrene (1.8 g; 0.017 mol; 2 mL; 0.906 g/mL) and (4-vinyl- phenyl)dimethylsilane (0.31 g; 1.88 mmol; 0.35 mL; 0.9 g/mL) were mixed in the ampoule. This ampoule was then attached to the reactor and the reactor was subsequently attached to the vacuum line through the arm (B). The reactor was cooled to -78 ºC using a dry-ice/isopropyl alcohol bath. sec-BuLi (0.7 mmol; 0.71 mL; 1.494
M) was added into the reactor using a gas-tight syringe through the injection port at
(A) under a positive flow of nitrogen. The injection arm was heat-sealed and then cyclohexane (70 mL) was distilled into the reactor. After this the reactor was removed from the vacuum line by sealing the arm (B) and the breakseal of the ampoule containing the monomer mixture was smashed. An orange color was immediately
46 observed. In the case where monomer reactivity ratios were to be determined the reactions had to be terminated at very short times (< 10 % conversion; 120 seconds;
20 ºC). In these cases, the breakseal of the ampoule containing the methanol was smashed immediately to produce an oligomer with some units of styrene and some units of (4-vinylphenyl)dimethylsilane. The polymer was obtained by precipitation in methanol and drying for long times in the vacuum oven. The yield of the polymer obtained was 0.17 g (8 % conversion).
A copolymer of styrene with (4-vinylphenyl)dimethylsilane [Mn (calc) = 8500 g/mol] was also synthesized using the same procedure as described above. The two monomers were once again mixed in a 90/10 molar ratio [styrene: 0.04 mol, 4.53 g,
5 mL, d = 0.906 g/mL; (4-vinylphenyl)dimethylsilane: 4.4 mmol, 0.8 g, 0.88 mL, d =
0.9 g/mL]. The initiator, sec-BuLi (0.6 mmol, 0.4 mL, 1.494 M), was introduced through the arm (A) of the reactor, after cooling the reactor with a dry-ice/isopropyl alcohol bath. The arm (A) was then heat-sealed and benzene (70 mL) was distilled into the reactor, after which the reactor was removed from the vacuum line by sealing the arm (B). The monomer mixture was introduced into the reactor by smashing the breakseal of the ampoule containing the monomers, immediately producing an orange colored solution. The polymerization was allowed to go to completion (12 h, R. T.).
The copolymer was isolated using the standard procedure and was obtained in 95% yield [5 g; Mn = 8600 g/mol; PDI = 1.06; Mn (calc) = 8500 g/mol].
47 3.5.3 Polyisoprene
The polymerization of isoprene was similar to that of styrene, except that an isoprene ampoule (12 g, 17. 25 mL, d = 0.69 g/mL) was used in place of a styrene ampoule. The initiator, sec-BuLi (6 mmol, 3.85 mL, 1.494 M) was introduced into the reactor through the injection arm followed by distillation of about 300 mL of benzene. The reactor was removed from the vacuum line using a hand torch and then the breakseal of the ampoule containing the isoprene monomer was smashed.
Immediately a pale yellowish color appeared indicating the presence of the living poly(isoprenyl)lithium chain ends. The polymerization was done at 50 ºC for 8 h. The polymerization was then terminated by smashing the breakseal of the ampoule containing degassed methanol. The polymer was isolated by precipitation into a large excess of methanol containing 0.5% BHT (2,4-Di-tert-butyl-4-methylphenol) by weight of polymer. After drying in a vacuum oven for one week the polymer was obtained in 97% yield [16.7 g, Mn = 2100 g/mol, Mn (calc) = 2000 g/mol)].
3.6 Synthesis of functionalized polymers
3.6.1 Silyl-hydride, chain-end functionalized polystyrene (PS-SiH)
The styrene monomer (22.65 g, 25 mL, d = 0.906 g/mL) was polymerized in the usual manner as described above, using sec-BuLi (7.6 mmol, 5.1 mL, 1.494 M) as the initiator in 250 mL of benzene. The living polymer chains were terminated by addition of the desired amount of chlorodimethylsilane (0.015 mol, 1.8 mL, d = 0.87 g/mL, MW = 94 g/mol) instead of methanol, from an ampoule attached to the reactor.
The silyl-hydride, end-functionalized polymer (PS-SiH) [Mn = 3100 g/mol; PDI =
1.03; Mn (calc) = 3000 g/mol] was then isolated by precipitation into a large excess of
48 methanol. The polymer was dried in a vacuum oven for one week as usual and was obtained in 98 % yield (22.2 g).
3.6.2 Synthesis of the cyanide-end functionalized polystyrene
The silyl-hydride, chain-end functionalized polymer [Mn (obs) = 3100 g/mol,
PDI =1.03, 5 g, 2.65 mmol] was dissolved in 50 mL of dry benzene. Allyl cyanide (2.5 mmol, 0.2 mL, d = 0.83 g/mol, MW = 67 g/mol) was added to the system at room temperature in the dry-box. The entire system was then taken out of the dry-box and
Karstedt’s catalyst (2-3 drops) was added by a gas-tight syringe under a nitrogen atmosphere. The system was stirred at room temperature for one week. The progress of the reaction was monitored using thin layer chromatography (TLC). After completion of the reaction the polymer was isolated by precipitation into an excess of methanol. After drying in a vacuum oven for one week the polymer was obtained in 96 % yield, 4.8 g (Mn
= 3200 g/mol, PDI = 1.04). The polymer obtained had traces of catalyst in it, due to which it exhibited a brown color. The catalyst was removed by dissolving the polymer in dried benzene and stirring vigorously for 24 h in the presence of a small amount of carbon black. The polymer could then be obtained in a pure (colorless) form by filtering this solution through a celite bed.
3.6.3 Synthesis of the ethyl ether, chain-end functionalized polymer
The same procedure as described above was used. The functionalizing agent used in this case was allyl ethyl ether (2.65 mmol, 0.3 mL, d = 0.76 g/mL, MW = 86 g/mol), which was reacted for 24 h at 25 ºC with the silyl hydride functionalized polystyrene (PS-SiH) (5 g, 1.6 mmol, Mn = 3100 g/mol, PDI = 1.03) in 50 mL of dry
49 benzene in the presence of 2-3 drops of Karstedt’s catalyst. The reaction was monitored by use of thin layer chromatography (TLC). The functionalized polymer
(Mn = 3200 g/mol, PDI =1.03) was isolated in 97 % yield, 4.8 g. The catalyst in the polymer was removed using the same procedure as described in section 3.6.2.
3.6.4 Synthesis of the acetate, chain-end functionalized polymer
A similar procedure was used as above. The silyl hydride functionalized polymer
(PS-SiH) (5g, 1.6 mmol, Mn = 3100 g/mol, PDI = 1.03) was dissolved in 50 mL of dried benzene. To this solution allyl acetate (2.65 mmol, MW = 100 g/mol, d = 0.93 g/mL, 0.28 mL) was added under a nitrogen atmosphere, followed by addition of 2-3 drops of
Karstedt’s catalyst. The system was stirred at room temperature for 3 days and the progress of the reaction was followed by use of thin layer chromatography (TLC). The functionalized polymer (Mn = 3200 g/mol, PDI = 1.05) was obtained in 95 % yield, 4.7 g.
The catalyst in the polymer was removed using the same procedure as described in section 3.6.2.
3.6.5 Synthesis of epoxide, in-chain functionalized polymer
Poly(4-vinylphenyl)dimethylsilane (0.013 mol of SiH, 2.0 g, Mn = 33000 g/mol,
PDI = 1.03) was dissolved in dry benzene in the dry-box. It was stirred for 30 minutes at room temperature and then allyl glycidyl epoxide (0.02 mol, 2.5 mL, d = 0.9 g/mol, MW = 114 g/mol) was added to the system. The reaction was then taken out of the dry-box and under a flow of nitrogen 2-3 drops of Karstedt’s catalyst was added by use of a gas-tight syringe. The reaction was stirred at room temperature under a nitrogen atmosphere for two days. The reaction was monitored by 1H NMR by removing samples from the reaction mixture. The polymer was isolated by
50 precipitation into a large excess of methanol followed by drying in the vacuum oven for one week. The product was obtained in 95 % yield (1.9 g, Mn = 50000 g/mol, PDI
= 1.04). The catalyst in the polymer was removed once again using the procedure described in section 3.6.2.
3.6.6 Synthesis of diol in-chain functionalized polymer 127
Catalytic hydrolysis of the epoxide functionality introduced into the polymer chain was carried out with bismuth (III) chloride in THF. The epoxide functionalized polymer (3.3 mmol of epoxide groups, 0.5 g, Mn = 50000 g/mol, PDI = 1.04) was dissolved in 50 mL of THF and 5 mL of water. Bismuth chloride (Fisher, 98 %)
(0.004 mmol, 0.57 mg) was added to this system and it was heated under reflux for
4 h. Then the THF was evaporated using a rotary evaporator and the remaining aqueous part was saturated with brine and extracted with diethyl ether. The organic solvent was removed using a rotary evaporator and the polymer obtained was dried in a vacuum oven for one week. The product was obtained in 95 % yield, 0.48 g.
3.6.7 Synthesis of perfluoroalkyl, in-chain functionalized polymer
Poly(4-vinylphenyl)dimethylsilane (0.013 mol of SiH, 2 g, Mn = 33000 g/mol,
PDI = 1.03) was hydrosilated in the same way as described in section 3.6.4. The functionalizing agent used in this case was 1H, 1H, 2H-perfluoro-1-octene (0.018 mol, 5 mL, d = 1.3 g/mL, MW = 346 g/mol). The reaction was stirred at room temperature under a nitrogen atmosphere for 24 h. The reaction was monitored by use of 1H NMR by removing samples from the reaction mixture. The polymer obtained in
95 % yield (1.9 g, Mn = 72000 g/mol, PDI = 1.08) was insoluble in benzene and fell
51 out of solution. The remaining polymer was obtained by precipitation into an excess of methanol, followed by drying in the vacuum oven for one week.
3.6.8 Synthesis of the polymer-peptide biohybrid
3.6.8.1 Synthesis of the in-chain amine functionalized copolymer90
The copolymer of styrene and (4-vinylphenyl)dimethylsilane [S/SiH = 90/10] was synthesized as described in section 3.3.2. This polymer (1.3 mmol of SiH, 2 g,
Mn = 8600 g/mol, PDI = 1.06) was dissolved in 50 mL of dry benzene inside the dry- box followed by addition of allylamine (4 mmol, 0.3 mL, d = 0.761 g/mL, MW = 57 g/mol) to this solution. The system was stirred for 30 minutes before taking out of the dry-box. Outside, 2-3 drops of Karstedt’s catalyst was added to the reaction mixture under a nitrogen atmosphere. The reaction was stirred at room temperature for one week. The reaction was continuously monitored by use of 1H NMR by removing samples from the reaction mixture. The polymer was then isolated using the standard procedure and was obtained in 95 % yield, 1.9 g.
3.6.8.2 Synthesis of the biohybrid128
The (N-carbobenzyloxy)-L-phenylalanine (0.05 mmol, 0.16 g, MW = 297 g/mol) was dissolved in dichloromethane (10 mL) at room temperature under a nitrogen atmosphere. DMAP (4-dimethylaminopyridine) (0.06 g) and the amine functionalized copolymer (0.06 mmol, 0.5 g) were added to the system, followed by addition of DCC (dicyclohexylcarbodiimide) (0.2 g). The mixture was stirred at room temperature overnight. The polymer was precipitated into methanol and dried in a vacuum oven for one week. The product was obtained in 92 % yield (0.46 g, 9200 g/mol, PDI = 1.08).
52
3.6.9 Synthesis of a thermoplastic elastomer (TPE) using (4-vinylphenyl)- dimethylsilane
A triblock copolymer was synthesized using a copolymer of styrene and (4- vinylphenyl)dimethylsilane as the first block, polyisoprene as the middle block and the copolymer as the third block. The copolymerization was done in a reactor similar to the one shown in Figure 3.2. Three different ampoules were used containing the monomers corresponding to the three blocks of the triblock copolymer to be synthesized. The ampoules corresponding to the first and the third block contained a mixture of styrene (0.04 mol, 4.53 g, 5 mL, d = 0.906 g/mol) and (4-vinylphenyl)- dimethylsilane (0.45 mmol, 0.8 g, 0.88 mL, d = 0.9 g/mol) taken in a 90:10 molar ratio.
The middle block of the triblock copolymer was formed from isoprene (0.6 mol, 45 mL, d = 0.69 g/ mL). The expected number average molecular weight ratio of the three blocks of the copolymer was 12/76/12. An ampoule containing 3-4 equivalents of THF (2.9 mmol, 0.21 g, 0.3 mL, d = 0.7 g/ mL) was also attached to the reactor. The polymerization was initiated as usual using sec-BuLi (0.45 mmol, 0.32 mL, 1.494 M) as the initiator and was done in benzene (500 mL). The first block was polymerized by smashing the break-seal of one of the ampoules containing the mixture of styrene and (4- vinylphenyl)-dimethylsilane. An orange color was immediately observed, indicating formation of the living chain ends. The polymerization was allowed to continue for 12 h at 25 ºC after which the breakseal of the ampoule containing isoprene was smashed which caused the orange color of the solution to immediately disappear giving rise to a light yellow colored solution. The temperature of the system was raised to 50 ºC and the polymerization was allowed to continue at this temperature for 8 h. After this period the
53 reaction system was allowed to cool to room temperature after which the ampoule containing THF was smashed. The final block was then polymerized by breaking the breakseal of the second ampoule containing the 90/10 mol/mol mixture of styrene and (4- vinylphenyl)-dimethylsilane. The polymerization was allowed to continue for another 12 h at room temperature after which the polymer was isolated in 93 % yield, 39 g, by precipitation into a large excess of methanol to which 0.5 % by weight of BHT was added to prevent oxidation of the polymer (Mn = 130000, PDI = 1.08). Aliquots were taken out from the polymerization system after polymerization of each of the first two blocks as well.
3.7 Polymer Characterization
3.7.1 Molecular Weight and Molecular Weight Distribution
Molecular weights were determined by size exclusion chromatography (SEC), matrix assisted laser desorption time of flight (MALDI TOF) mass spectroscopy and
1H NMR.
3.7.1.1 Size Exclusion Chromatography (SEC)
Molecular weights and molecular weight distributions were determined by size exclusion chromatography (SEC) using a Waters TM 150-C plus chromatograph with a Viscotek model 301 triple detector system. The detector system consisted of a differential refractometer (Waters 410), a differential viscometer (Viscotek 100) and a laser light scattering detector (Wyatt Technology, DAWN EOS, λ= 670 nm) equilibrated at 30 ºC. Chromatographic separations were effected with a six Phenogel column set (two 500, 1000, two 104 and 105 Å) at a flow rate of 1.0 mL/min in tetrahydrofuran (THF). The values of molecular weights and molecular weight
54 distributions were calculated using universal calibration129 after calibration with polystyrene standards (Polymer Laboratories).
3.7.1.2 Matrix Assisted Laser Desorption-Ionization Time of Flight (MALDI TOF) Mass Spectroscopy
MALDI TOF mass spectra were recorded on a Bruker Reflex-III
TOF mass spectrometer (Bruker Daltonics, Billerica, MA). The instrument was equipped with an LSI model VSL-337ND pulsed 337 nm nitrogen laser (3 nm pulse width), a single-staged pulsed ion extraction source, and a two-stage gridless reflector. Solutions of dithranol (20 mg/mL) (Fluka, 1,8,9-anthracenetriol, 99%), polymer sample (10 mg/mL), and silver or sodium trifluoroacetate (10 mg/mL)
(Aldrich, 98%) were prepared in THF (Aldrich, 99.9%). These solutions were mixed in the ratio of matrix/cationizing agent/polymer (10:1:2), and 0.5 μL of the mixture was applied to the MALDI sample target and allowed to dry. Mass spectra were measured in the linear and reflector modes, and the mass scale was calibrated externally using the peaks of a polystyrene standard at the molecular weight under consideration. These spectra were obtained in the laboratories of Professor Chrys
Wesdemiotis in the Department of Chemistry at the University of Akron.
3.7.2 Nuclear Magnetic Resonance (NMR) Spectroscopy
Both 1H and 13C NMR spectra were obtained by using a Varian Mercury-300 spectrometer. Polymer concentrations of 5 % and 15 % (wt/wt), respectively, in
CDCl3 were used. The chemical shifts were referenced to the chemical shift of deuterated chloroform, i.e. 7.27 ppm for the 1H NMR (200 MHz) and 77.23 ppm for the 13C NMR (75 MHz)130. The peaks in the 1H NMR were integrated to obtain the
55 relative amounts of the protons of each type.
3.7.3 Fourier Transform Infrared Spectroscopy (FT-IR)
The infrared (FT-IR) spectra of the polymers were obtained on a Beckman
Instruments FT-2100 FTIR spectrometer with the help of Mr. Jon Page. Polymer films were cast on a potassium bromide (KBr) pellet by dissolving the polymers in
THF and then evaporating the solvents to obtain a thin film.
3.7.5 Thermal Properties of Copolymers
3.7.5.1 Differential Scanning Calorimetry (DSC)
The glass transition temperatures of the polymers were measured by differential scanning calorimetry on a DuPont Instruments DSC 2910 at a heating rate of 10 ºC/min up to 160 ºC using a nitrogen flow rate of 40 μL/s. A second heating scan was used to determine the glass transition temperature of the polymer. DSC results were analyzed using the DuPont software.
3.7.5.2 Dynamic Mechanical Thermal Analysis (DMTA)-samplesiz
Mr. Bob Seiple at the University of Akron Applied Research Laboratory kindly conducted the DMTA analyses on the desired polymers by use of a
Rheomatrix Mark II dynamic mechanical thermal analyzer. Dog-bone shaped samples were prepared by using a die (ASTM 638, type V) having an overall width of 9.53 mm and total length of 63.5 mm. The moduli were determined by heating the samples from –90 ºC to 100 ºC at a heating rate of 10 ºC/min and were analyzed using
Rheomatrix software.
56
3.7.5.3 Thermogravimetric Analysis (TGA)
A DuPont 9900 thermogravimetric analyzer was used to perform TGA on the desired polymers. A heating rate of 20 ºC/min was used and the analyses were performed using a 40 μL/s flow rate of nitrogen. TGA results were analyzed using
DuPont software.
3.7.6 Thin Layer Chromatography (TLC)
Thin layer chromatographic (TLC) analyses were performed on the chain-end functionalized polymers by spotting solutions of these polymers on flexible silica plates (Selecto Scientific, Silica Gel 60, F-254 with fluorescent indicator) and then developing them. In most cases toluene or a mixture of toluene/hexane was used as the eluent.
3.7.7 Column Chromatography 131
Silica gel (EM Science, Silica Gel 60) with particle size of 0.040-0.063 nm (230-
400 mesh) was used to pack the column and hexane was the solvent used to separate the (4-vinylphenyl)dimethylsilane from the 4-bromostyrene present in the system.
3.7.7 Tensile Testing
All measurements and sample preparations were kindly carried out by the
University of Akron Applied Research Laboratory. Samples for tensile testing were prepared by first making a film (15 x 5 x 0.05 cm) of the thermoplastic elastomer (5 g) by using a heated press (10 tons) at a temperature of 95 ºC. The compressed film was then cooled to room temperature under pressure (20 tons). Dog-bone shaped samples were cut out of this film by using a die (ASTM 638, type V) having a total
57 length of 63.5 mm and a total width of 9.53 mm. Three such samples were pulled apart in an Instron, Model 5567, at a crosshead speed of 20 inch/min in order to determine the maximum strength that the material could withstand.
3.7.8 Transmission Electron Microscopy (TEM)
TEM experiments were carried out with a model JEM-1200EXII Japan
Electron Optics Laboratoty (JEOL) at an accelerating voltage of 120 kV at several magnifications (59000x – 97000x) in the microscopy laboratory at the zuniversity opf
Akron, with the help of Dr. Bojie Wang. Thin slices for TEM observation were obtained using a model Ultracut S/FC S Leica Reichert cryomicrotome at -120 ºC.
These films were placed on a 400 mesh copper grid. The films were stained with the vapor of OsO4 (Aldrich, 98 %) (2 wt % water solution) overnight. The staining was done in a hood, by putting the copper grids on a wire scaffolding which rested on top of a small jar containing the OsO4. This jar was put inside a larger jar, which was covered with a lid.
3.7.9 Contact Angle Measurements
Contact angle measurements were carried out in Professor W. J. Brittain’s laboratories with the help of Ying Wang, on a Rame Hart NRL-100 goniometer equipped with a tilting base mounted on a vibrationless table. Advancing and receding contact angles were measured of a 10 µL drop using the tilting stage method.
58
3.7.10 Ellipsometry Measurements
Ellipsometry measurements were carried out in Professor W. J. Brittain’s laboratories with the help of Ying Wang, on a Gaertner Model L116C ellipsometer with a He-Ne laser (λ = 632.8 nm) and a fixed angle of incidence at 70º.
3.7.11 Compression Molding
Compression molded films were prepared at the University of Akron Applied
Research Laboratory with the help of Mr. Bob Seiple. About 5 g of the material was put into a 15 x 5 x 0.05 cm metal frame in between two metal plates. These plates were preheated for 30 minutes to a temperature of 95 ºC. After this the polymer in between these plates was compressed by first increasing the load on the plates to 10 tons. The load was then increased to 15 tons and was held there for 10 minutes after which the load was further increased to 25 tons where it was held for an additional 20 minutes. The compressed polymer between the plates was then cooled under pressure for 30 minutes to give a translucent film.
59
CHAPTER IV
RESULTS AND DISCUSSION
4.1 Synthesis of chain-end functionalized polystyrene
4.1.1 Synthesis of cyanide-functionalized polystyrene
The general functionalization method (GFM) developed by Quirk and Kim90 was used to synthesize well-defined polymers with quantitative cyanide chain-end functionalization. To our knowledge there is no simple, versatile method to synthesize cyanide-terminated polymers anionically. The cyanide functionality is interesting to study due to its highly polar nature and also since this group can be easily converted to a variety of other functional groups, e. g., carboxyl, amine, ester etc., by use of simple chemical reactions132.
The monomers, 4-cyanostyrene and 4-α-methylcyanostyrene, have been anionically polymerized previously at -78 ºC in THF114, 133. This produced well-defined polymers with the cyanide group in each repeating unit along the polymer chain. Nakahama and coworkers134 synthesized a cyanide- terminated polystyrene by polymerizing styrene anionically at -78 ºC in THF and then reacting the living carbanionic polymers with a variety of alkyl halides containing conjugated 1,3-dienyl groups, e.g., 6-bromo-3- methylene-1-hexene and 6-iodo-3-methylene-1-hexene. These conjugated dienyl groups could then be converted to a cyanide functionality by use of a Diels-Alder
60 reaction carried out between the conjugated dienes at the polymer chain ends and the double bond on allyl or vinyl cyanide133. Herein the synthesis of ω-chain end cyanide functionalized polystyrenes was attempted using the procedure shown in Scheme 4.1.
Li sec-Bu sec-BuLi + n Benzene/ R.T n
CH3
Li Si H sec-Bu sec-Bu
n (CH3)2SiHCl (1.2 equiv) n CH3
(1 equiv)
CH3 CH3
Si H Si CH CH CH CN sec-Bu sec-Bu 2 2 2 CN n CH3 n CH3
(1 equiv) (1.5 equiv) Karstedt's catalyst (2-3 drops)
Scheme 4.1 Cyanide chain-end functionalization of polystyrene.
Polymerization of the styrene monomer was initiated using sec-BuLi in benzene at room temperature. To the system containing the living poly(styryl)lithium chains, chlorodimethylsilane was added as the terminating agent. This produced polystyrene chains with a silyl-hydride functionality at the polymer chain end. The SEC chromatogram of this polymer is shown in Figure 4.1. The molecular weight distribution for this polymer is narrow and unimodal and there is good agreement between Mn (calc) and Mn (obs), indicating a controlled polymerization. The silyl hydride-terminated 61 polymer was also characterized by 1H NMR as shown in Figure 4.2. The peak at δ 3.9 ppm is due the proton bonded to the silicon atom. The peak at δ 0.32 ppm is due to the six methyl protons on the carbon atoms bonded to the silicon atom. This peak
Mn(calc) =3000 g/mol Mn(obs) = 3100 g/mol PDI = 1.03
(R. I.)
Figure 4.1 SEC chromatogram of the silyl hydride-terminated polymer.
62 Figure 4.2 1H NMR spectrum of silyl hydride-terminated polystyrene.
was integrated with respect to the peak due to the six protons in the sec-butyl initiator unit. A 1:1 integration ratio was observed.
Figure 4.3 shows the 13C NMR spectrum of the silyl-hydride polymer. This spectrum is similar the spectrum for polystyrene, except for the additional peak at δ –0.2 ppm, which is due to the methyl carbons adjacent to the silicon atom. It is also noteworthy that there is no peak at δ 33.6 ppm, showing that there are no non-functionalized polystyrene chain ends present135.
The MALDI-TOF mass spectrum of this polymer is shown in Figure 4.4. This spectrum shows a single distribution, indicating formation of a single product and
63 absence of any side reaction products. The observed monoisotopic masses of the 15-mer and the 16-mer (as shown in the expanded region of the spectrum) exhibit good agreement with the calculated monoisotopic masses. The peak at m/z = 1700.1
+ corresponds to C4H9-(C8H8)15-SiH(CH3)2.Na ; the calculated monoisotopic mass is 57.07
+ (C4H9) + 15 x 104.06 [(C8H8)15] + 59.03 [SiH(CH3)2] + 22.99 (Na ) = 1699.99 Da. The distance between any two oligomer units in a mass spectrum should give the mass of the repeating unit of the polymer. In this case the difference in m/z between the monoisotopic
15-mer (1700.0) and the monoisotopic16-mer (1804.1) is equal to the molecular weight of a styrene unit (104.1). Sodium trifluoroacetate was used as the cationizing agent as opposed to the more commonly used silver trifluoroacetate. This is because it was observed that the silyl-hydride unit could be easily oxidized in presence of silver as the cationizing agent90.
64
No non-functional polystyrene
Figure 4.3 13C NMR of the silyl hydride-functionalized polystyrene.
65
1700.0 1804.1
104.1
Figure 4.4 MALDI TOF mass spectrum of the silyl hydride-terminated polymer.
66
Figure 4.5 shows the FTIR spectrum of this polymer. It shows at peak at 2116 cm-1 corresponding to the Si-H bond.
2116 cm-1
Figure 4.5 FTIR spectrum of the silyl hydride-terminated polymer.
(a) CH 3 (b) (d)
Si (c) CN sec-Bu n (e) CH3 (a)
Figure 4.6 Cyanide-terminated polystyrene.
67
The silyl hydride-terminated polymer was then reacted with allyl cyanide, which was dried by stirring over calcium hydride. The polymer was dissolved in dry benzene at room temperature under a nitrogen atmosphere. Dried allyl cyanide (1.5 equiv) was then added to this solution and the system was allowed to stir for 30 minutes.
Following this 2-3 drops of the Karstedt’s catalyst was added to this solution and the reaction was stirred under nitrogen for five days. The progress of the reaction was monitored by use of thin layer chromatography (TLC), using toluene as the eluent. The silyl-hydride functionalized polystyrene moved to the top of the TLC plate, while the cyanide chain-end functionalized polymer moved only a little bit and remained close to the bottom of the TLC plate. Completion of the hydrosilation reaction was detected by observation of a single spot on the TLC plate. After the reaction was completed, the functionalized polymer was isolated by precipitating it in methanol and then it was dried.
The SEC chromatogram of this polymer was narrow and unimodal as shown in Figure
4.7. There is a small bump in the SEC as observed in Figure 4.7 below, which was not present in the silyl hydride-functionalized polymer (see Figure 4.1). One possible explanation for the appearance of this peak is formation of a siloxane dimer in the presence of moisture and the platinum-based catalyst, as shown : P-SiH + H2O + P-SiH
Æ P-Si-O-Si-P73. This kind of dimer formation has been observed before while using
90 chlorosilane derivatives, i.e., P-Si-Cl + H2O + P-Si-Cl Æ P-Si-O-Si-P . However, the
Si-O-Si bond is known to exhibit a strong absorption in FTIR at 1050 cm-1 130 and no such absorption was observed in the FTIR spectrum (Figure 4.12) of this polymer.
68 Mn (calc) = 3168 g/mol Mn (obs) = 3200 g/mol PDI = 1.04
R. I.
Figure 4.7 SEC chromatogram of the cyanide-terminated polystyrene.
The 1H NMR spectrum (Figure 4.8) of this polymer shows that the original resonance at δ 3.9 ppm from silyl hydride proton (see Figure 4.3) has disappeared. A new resonance was observed at δ 0.7 ppm [(b) in Figure 4.2] and this corresponds to the two protons belonging to the methylene group adjacent to the silicon atom130. In the expanded region of this spectrum, the integration ratio of the peak due to these two methylene protons with respect to the peak due to the six initiator protons (δ 0.9 ppm) is shown. An exact integration ratio of 1:3 is observed as expected.
69
Figure 4.8 1H NMR spectrum of the cyanide-terminated polystyrene.
70
Figure 4.9 DEPT-135 13C NMR spectrum of the cyanide-terminated polymer.
In a DEPT 13C NMR spectrum the carbon atoms attached to an even number of protons exhibit a negative resonance (down) and the carbon atoms attached to an odd number of protons exhibit a positive phase (up). The DEPT-135 13C NMR spectrum of the cyanide terminated polymer is shown in Figure 4.9. This spectrum shows distinct negative signals at 15.2 ppm (Si-CH2-), 20.8 ppm (Si-CH2-CH2-) and 21.3 ppm (Si-CH2-
CH2-CH2-) arising from the three different methylene units between the silicon atom and the cyanide group at the chain end. It is interesting to note that no negative signal was observed at δ 33.6 ppm corresponding to the benzylic carbon in non-functionalized polystyrene. The carbon atom in the cyanide functional group (δ 120.5 ppm)130, was unfortunately not observed in this spectrum. This is as expected since this carbon atom in the cyanide functional group is not attached to any protons.
71
Figure 4.10 13C NMR spectrum of the cyanide-terminated polystyrene.
Figure 4.10 shows the 13C NMR spectrum of the polymer. The peaks in this spectrum were assigned with the help of the 13C DEPT spectrum (Figure 4.9) and ChemDraw.
Several additional peaks were observed in this spectrum, compared to the 13C NMR spectrum obtained for the silyl hydride terminated polymer. The peak at δ 15.2 ppm is due to the methylene carbon adjacent to the silicon atom. The other two methylene carbons appear very close to each other, δ 20.8 ppm and δ 21.3 ppm, respectively, so it is difficult to distinguish between them in the 13C NMR spectrum. The small peak at δ
120.5 ppm is due to the carbon atom in the cyanide functional group130.
72 The MALDI TOF mass spectrum for this polymer is shown in Figure 4.11. A single monomodal distribution was obtained, indicating formation of a single product.
The cationizing agent used in this case was silver trifluoroacetate. The peak corresponding to the cyanide-functionalized 21-mer appears at m/z = 2475.1. The calculated monoisotopic for this peak is {57.07 (C4H9) + 21 x 104.06 [(C8H8)21] + 126.09
+ [Si(CH3)2(CH2)3CN] + 106.9 (Ag )} = 2475.32 Da.
2475.1 2579.1
104.0
Figure 4.11 MALDI TOF mass spectrum for the cyanide-terminated polystyrene. 73 The FTIR spectrum of this polymer (Figure 4.12) shows no peak at 2116 cm-1 consistent with complete conversion of the Si-H bond to Si(CH3)2-CH2-. This spectrum shows a new peak at 2350 cm-1 corresponding to the stretching of the C-N bond130. In conclusion, the cyanide-terminated polymer was successfully synthesized and characterized completely showing formation of a single product and quantitative chain- end functionalization.
2350 cm-1
Figure 4.12 FTIR spectrum of the cyanide-terminated polymer.
4.1.2 Synthesis of ethyl ether-terminated polystyrene
The ethyl ether-terminated polystyrene was synthesized by using the general chain-end functionalization method developed. The structure of this polymer is shown in
Scheme 4.2.
74 (a) CH3 (d) (b) (e) Si O CH3 (f) sec-Bu (c) n CH3 (a)
Scheme 4.2 Ethyl ether-functionalized polystyrene.
To a system consisting of living poly(styryl)lithium chain ends (1 equiv), chloro- dimethylsilane (1.5 equiv) was added as the terminating agent. This produced polymer chains with silyl hydride groups at the chain-ends [Mn = 3100 g/mol; Mn(calc) = 3000 g/mol; PDI =1.03]. Post-polymerization hydrosilation reactions were done between the silyl hydride groups at the polymer chain ends and the double bond of allyl ether
(purified as described in section 3.1.8) which is a commercially available compound.
Thin layer chromatography (TLC) was used to monitor the progress of the reaction. A
50 : 50 mixture of toluene and cyclohexane was used as the eluent. The silyl hydride- functionalized polymer moved just below the solvent front and the functionalized polymer moved only a little below that (the two spots were separated from each other).
Completion of the reaction was indicated by observation of a single spot in the TLC. The chain-end functionalized polymer obtained was then characterized using 1H NMR, 13C
NMR, SEC, FTIR and MALDI TOF mass spectrometry. The SEC chromatogram of this functionalized polymer is shown in Figure 4.13. It shows that the polymer obtained has a narrow and unimodal molecular weight distribution. The 1H NMR spectrum is shown in
75 Figure 4.14. The spectrum exhibits a peak at δ 0.8 ppm, corresponding to the six methyl protons in the initiator segment. There are two peaks at δ 1.2 ppm and δ 1.4 ppm which are due to the methyl protons labeled (f) in Scheme 4.2 and the methylene protons labeled (c), respectively130. The peak labeled (b) appears at 0.6 ppm130. The two peaks at
δ 3.37 and δ 3.41 are due to the methylene protons labeled (d) and (e)130. The peaks (d) and (e) corresponding to four protons were integrated with respect to the six methyl protons in the initiator fragment. The integration ratio obtained [1.6 (obs), 1.5 (expected)] is consistent with quantitative incorporation of the functional group at the polymer chain end. All of the peak assignments were further confirmed using ChemDraw software.
Mn (calc) = 3173 g/mol Mn (obs) = 3200 g/mol PDI = 1.03
R. I.
Figure 4.13 SEC chromatogram for the ethyl ether-functionalized polymer.
76
Figure 4.14 1H NMR spectrum of the ethyl ether-functionalized polymer.
77 Figure 4.15 shows the 13C NMR spectrum of this functionalized polymer. The peaks in this spectrum were assigned with the help of the 13C NMR spectrum of the starting material from the Aldrich catalog and ChemDraw. The peak at δ –0.4 ppm is from the two methyl carbon atoms (a) bonded to the silicon atom at the chain end. The peak at δ 10.1 ppm is for the carbon atom labeled (b), the peaks labeled (c) and (f) appear at δ 26.3 ppm and δ 17.2 ppm, respectively. The two peaks at δ 67.1 ppm and δ 77.5 ppm correspond to the peaks (e) and (d), respectively.
The 13C NMR spectrum also does not show any peak at δ 33.6 ppm, showing the absence of any non-functional polystyrene. The FTIR spectrum of the chain-end functionalized polymer is shown in Figure 4.16. It exhibits no peak at 2116 cm-1, as expected, consistent with complete conversion of the Si-H bond. The characteristic absorption exhibited by aliphatic ether appears at 1150 cm-1 130 and can be observed in the
FTIR spectrum of this polymer.
78
Figure 4.15 13C NMR spectrum of the ethyl ether-functionalized polymer.
79 1150 cm-1
Figure 4.16 FTIR spectrum of the ethyl ether-functionalized polymer.
MALDI TOF mass spectrometric analysis was done on this compound and the results are shown in Figure 4.17. Silver trifluoroacetate was used as the cationizing agent in presence of a dithranol matrix. A single distribution was obtained, indicating formation of a single product and the absence of any side reactions. The expanded part of the spectrum between the 20-mer and the 21-mer is shown in the inset. Both oligomer peaks show good agreement between the observed and calculated monoisotopic masses; for the
20-mer, the calculated monoisotopic mass is {57.07 (C4H9) + 20 x 104.06 [(C8H8)20] +
+ 145 [Si(CH3)2(CH2)3OCH2CH3] + 106.9 (Ag )} = 2390.17 Da; (m/z) (obs) = 2390.1.
The calculated monoisotopic mass for the 21-mer is {57.07 (C4H9) + 21 x 104.06
80 + [(C8H8)21] + 145 [Si(CH3)2(CH2)3OCH2CH3] + 106.9 (Ag )} = 2494.23; (m/z) (obs) =
2494.3. The distance between the two oligomer units gives the mass of the repeat unit of this polymer, i.e. styrene. There are no peaks in the region between the two oligomer peaks which is consistent with the absence of any non-functional polystyrene and also quantitative chain end functionalization.
81
2494.3
2390.2
104.1
Figure 4.17 MALDI TOF mass spectrum of the ethyl ether-functionalized polymer.
82 4.1.3 Synthesis of acetate-functionalized polystyrene
The acetate-functionalized polymer was synthesized by use of the general chain-end functionalization method that has been developed. To a system consisting of living poly(styryl)lithium chain ends (1 equiv), chlorodimethylsilane (1.5 equiv) was added as the terminating agent. This produced polymer chains with silyl hydride groups at the chain-ends [Mn = 3100 g/mol; Mn(calc) = 3000 g/mol; PDI =1.03]. Post-polymerization hydrosilation reactions were done between the silyl hydride groups at the polymer chain ends and the double bond of allyl acetate (purified as described in section 3.1.8) which is a commercially available compound. Progress of the reaction was monitored by use of
1H NMR and the completion of the reaction was confirmed by the disappearance of the the peak due to the Si-H bond at 4.2 ppm. The acetate-functionalized polymer was then characterized using SEC, 1H NMR, 13C NMR and MALDI TOF mass spectrometry. The
SEC chromatogram of this polymer is shown in Figure 4.18. The chromatogram exhibits a single, unimodal distribution, indicating absence of any side-reaction products.
83 Mn (obs) = 3200 g/mol PDI = 1.05
Intensity (R. I.)
Volume (mL)
Figure 4.18 SEC chromatogram of the acetate-functionalized polymer.
The 1H NMR spectrum of this polymer is shown in Figure 4.19. This spectrum shows a new peak at 4.6 ppm, which corresponds to the protons labeled (e)130. This peak which arises from three protons was integrated with respect to the six protons in the initiator fragment and a 1:2 integration ratio was obtained, as expected. The peaks labeled b, c and d at 1.3, 1.6 and 2.25 ppm respectively, cannot be observed in this spectrum since the peaks corresponding to the polymer backbone overlap with these peaks. All the peaks were identified and labeled by use of ChemDraw software.
84
(a) CH3 (b) (d) Si (e) O CH3 sec-Bu (c) n CH3 (a) O
Figure 4.19 1H NMR spectra of the acetate-functionalized polymer at different attenuations. 85 Figure 4.20 shows the 13C NMR spectrum of the acetate-functionalized polymer.
The peaks at 9.1, 19.1, 37.0, 161.5 and 50.8 ppm correspond to the protons labeled b, c, d, e and f respectively. All peaks were labeled by use of suitable references130 and
ChemDraw software. Also 13C NMR of the starting material (allyl acetate), as obtained from the Aldrich catalog helped in the assignment of some peaks.
(a) CH3 (b) (d) Si (e) O CH3 sec-Bu (c) n CH3 (a) O
Figure 4.20 13C NMR spectrum of the acetate-functionalized polymer.
86 FTIR spectrum of the acetate-functionalized polymer is shown in Figure 4.21. The peak at 2116 cm-1, corresponding to the Si-H bond was not observed, indicating complete conversion of the SiH bond. A new resonance was observed at 1650 cm-1, corresponding to the asymmetric stretch of the C-O bond in the acetate group130. The resonance at 1350 cm-1 corresponds to a weak symmetric stretch of the C-O bond in the acetate moiety130.
This indicates introduction of the acetate functional group at the polymer chain-end.
Figure 4.21 FTIR spectrum of the acetate-functionalized polymer.
The MALDI TOF mass spectrum of this acetate-functionalized polymer is shown in
Figure 4.22. Silver trifluoroacetate was use as the cationizing agent in presence of a dithranol matrix. Two different distributions can be observed in this spectrum. The expanded region between the 17-mer and the 18-mer is shown in the inset in Figure 4.22.
87 There is observed a good agreement between the observed and calculated monoisotopic masses for both oligomer units in the main distribution. The calculated monoisotopic mass for the peak observed at m/z = 2092.9 Da is {57.07 (C4H9) + 17 x 104.06 [(C8H8)17]
+ + 160 [Si(CH3)2(CH2)3COOCH3] + 106.9 (Ag )} = 2092.99 Da. The calculated monoisotopic mass for the 18-mer is {57.07 (C4H9) + 18 x 104.06 [(C8H8)18] + 160
+ [Si(CH3)2(CH2)3COOCH3] + 106.9 (Ag )} = 2197. 05 Da; m/z (obs) = 2197 Da. The second distribution observed at m/z = 2154.2 Da, may correspond to some product obtained due to fragmentation during characterization, since formation of any side- reaction product was not observed from 1H and 13C NMR results. One possibility is formation of C4H9-(C8H8)18-Si(CH3)2COOCH3, which has a calculated monoisotopic mass in good agreement with the observed value of m/z = 2154.9 Da for this peak; {57.07
+ (C4H9) + 18 x 104.06 [(C8H8)17] + 118 [Si(CH3)2COOCH3] + 106.9 (Ag )} = 2155.05 Da.
The distance between the two oligomer units (17-mer and 18-mer) gives the mass of the repeat unit of the polymer, i.e. styrene [Δm/z (obs) = 104.1 Da] . The small peak observed at m/z = 2112.0 Da corresponds to the silyl hydride terminated polymer, where the silyl hydride group has been hydrolyzed in the presence of the silver trifluoroacetate cationizing agent, i.e., C4H9-(C8H8)18-Si(CH3)2OH. The calculated monoisotopic mass is
+ {57.07 (C4H9) + 104.06 x 18 [(C4H9)18] + 18 x 75.027 [Si(CH3)2OH] + 106.9 (Ag )} =
2112.05 Da.
88 2197.0
2092.9
2112.0 2154.9
104.1
Figure 4.22 MALDI TOF mass spectrum of the acetate-functionalized polymer.
89 4.2 Synthesis of in-chain functionalized polystyrenes
The general methods that have been used to prepare in-chain functionalized polystyrenes have involved the polymerization of functionalized monomers43-60.
However, functional groups having acidic hydrogen atoms would participate in chain- transfer and chain-termination reactions leading to the loss of control over the polymerization. So, in such cases it was necessary to use a suitable protecting group for the functional group in order to avoid side-reactions91. Also, most of the polymerizations have been done in THF at -78 ºC which is a condition of limited practical utility. Herein a general functionalization method has been developed to synthesize in-chain functionalized polymers by using the procedure shown in Scheme 2.11. In this method the monomer used was a silyl-hydride functionalized styrene monomer, (4-vinylphenyl)- dimethylsilane. This monomer has been synthesized and polymerized in the past.
Nakahama and coworkers125 polymerized (4-vinylphenyl)dimethylsilane using potassium naphthalenide as the initiator in THF at -78ºC . The polymer obtained had a narrow molecular weight distribution and exhibited good agreement between observed and calculated molecular weights. However, Nakahama and coworkers did not convert the silyl hydride groups to any other functional groups. Pearce and coworkers136 have reported the anionic synthesis of a block copolymer of styrene with (4-vinyl- phenyl)dimethylsilane using sec-BuLi as the initiator in THF at -78 ºC. They converted the silyl hydride groups on the polymer chain to hydroxysilyl groups by reaction with dimethyloxirane. However, this is the only functional group transformation that they did
(Scheme 4.3). Although it was possible to synthesize this polymer anionically at low temperature in THF, the challenge was to do the polymerization in a hydrocarbon solvent
90 at room temperature and see if a controlled polymerization could occur. The pKa of the silyl-hydride group is 41.2137, which is close to that of the living polystyrene chain ends
(41-43)19. This gives rise to a possibility of side reactions at the silyl hydride groups and subsequent loss of control over the polymerization.
The monomer, (4-vinylphenyl)dimethylsilane, was not commercially available and so it was synthesized starting from 4-bromostyrene, which was reacted with Mg in dry ether to give the Grignard reagent as shown in Scheme 4.4. This Grignard reagent was reacted with chlorodimethylsilane to obtain (4-vinylphenyl)dimethylsilane.
m b H sec-Bu n m H3C Si CH3 Li THF, -78 sec-BuLi + n sec-Bu n H
H3CCHSi 3
b H H3CCH3 b H H sec-Bu n m sec-Bu n m
OO
0 C, Acetone
H CCHSi 3 3 H3CCHSi 3 H OH
Scheme 4.3 Synthesis of poly(styrene-b-4-vinylphenyldimethylsilane).
91
(CH ) SiHCl Mg / dry ether 3 2
dry ether ( R.T ) R.T H C Si CH Br 3 3 MgBr Scheme 4.4 Synthesis of (4-vinylphenyl)dimethylsilane. H
Figure 4.23 1H NMR spectrum of (4-vinylphenyl)dimethylsilane.
92 (b) (a) (e) (c) (c)
(d) (d) (f)
(g) H3CCHSi 3 (g)
H
f c
a
(e) (b/d)
Figure 4.24 13C NMR spectrum for (4-vinylphenyl)dimethylsilane.
The monomer was characterized by 1H and 13C NMR spectroscopy (Figures 4.23 and
4.24, respectively). In Figure 4.23 the various peaks in the spectrum have been labeled corresponding to different protons in the monomer125. The 13C NMR spectrum (Figure
4.24) shows a peak at δ -0.2 ppm (g) corresponding to the two methyl carbon atoms bonded to the silicon atom125. All other carbon atoms in the monomer have been labelled as shown in Figure 4.23125. This monomer was then polymerized using sec-BuLi as initiator in benzene at room temperature (Scheme 4.5).
93
sec-Bu H 1. Benzene (R.T) n sec- BuLi + n 2. CH3OH H C Si CH 3 3 H C Si CH 3 3 H H Scheme 4.5 Polymerization of (4-vinylphenyl)dimethylsilane in a hydrocarbon solvent at room temperature.
Polymers of high and low molecular weights were synthesized and their SECs are shown in Figure 4.25. The SECs obtained for polymers of both high and low molecular weights are narrow and unimodal and exhibit good agreement between Mn (obs) and Mn
(calc).
Mn(obs) = 3200 g/mol Mn(calc) = 3000 g/mol PDI = 1.05 Mn(obs) = 33000 g/mol Mn(calc) = 30000 g/mol PDI = 1.03
Figure 4.25 SEC chromatograms of poly(4-vinylphenyl)dimethylsilanes.
This indicates that the polymerization of (4-vinylphenyl)dimethysilane, using sec-BuLi as the initiator in benzene at room temperature, proceeds in the absence of any side 94 reactions. This is an encouraging result since the pKa of the silyl hydride group (41.2) is close to that of the living polymer chain ends (41-43) and there could have been some side reaction such as chain transfer to monomer during the course of the polymerization.
1 The H NMR spectrum of the polymer with Mn = 33000 g/mol is shown in Figure 4.26.
The peak at 4.4 ppm is from the silyl-hydride proton. This peak was integrated with respect to the peak arising from the four aromatic protons in each unit of the polymer chain (δ 5.9-7.1 ppm). A 1:4 integration ratio was observed as expected indicating absence of any side reactions of the polymer chain ends with the silyl-hydride units on the monomer units. The 13C NMR spectrum (Figure 4.27) of this polymer shows a peak at -0.2 ppm (a) which arises due to the two methyl carbon atoms bonded to the silicon atom in each monomer unit. The peak due to the carbon atom labeled (b), appears at 145 ppm. The peaks due to the carbon atoms labeled (c) and (d) appear at 127.1 ppm and
135.4 ppm respectively. The peak at 130.1 ppm is due to the carbon atom labeled (e).
Figure 4.26 1H NMR spectrum of poly(4-vinylphenyl)dimethylsilane (Mn = 33000 g/mol). 95
H sec-Bu n (b)
(c) (c)
(d) (d) (e)
(a) H3C Si CH3 (a)
H
Figure 4.27 13C NMR spectrum of poly(4-vinylphenyl)dimethylsilane.
FTIR is a powerful technique which was used to detect the presence of the Si-H bond in a compound. In Figure 4.28 the FTIR spectrum of poly(4-vinylphenyl)dimethylsilane is shown. The intense absorption peak at 2116 cm-1 arises from the Si-H bonds present in each repeating unit along the polymer chain. DSC analysis was done on this polymer and, as can be seen from Figure 4.29, it exhibits a glass-transition temperature of 90 ºC. The glass transition temperature of polystyrene (Mn = 30000 g/mol) is 100 ºC (as observed from measurements).
96
Figure 4.28 FTIR spectrum of poly(4-vinylphenyl)dimethylsilane.
Heat Flow (W/g)
Figure 4.29 DSC of poly(4-vinylphenyl)dimethylsilane.
The MALDI TOF mass spectrum (Figure 4.30) of the polymer shows an unimodal distribution, indicating formation of a single product. Taking a closer look at a portion of this spectrum, as shown in the inset, it can be seen that the observed and calculated 97 monoisotopic masses agree well for both oligomer units. The peak at m/z = 1377.1 Da
+ is assigned to the 8-mer, C4H9-[C8H7SiH(CH3)2]8H.Na ; the calculated monoisotopic
+ mass is 57.07 (C4H9) + 8 x 162 [C8H7SiH(CH3)2]8 + 1.00 (H) + 22.99 (Na ) = 1377.06
Da. In the MALDI TOF mass spectrum for a polymer the distance between any two consecutive monoisotopic oligomer units should correspond to the mass of the repeat unit of the polymer. The calculated monoisotopic mass for the 9-mer is 57.07 (C4H9) + 9 x
+ [C8H7SiH(CH3)2]9 + 1.00 (H) + 22.99 (Na ) = 1539.06 Da. The difference in monoisotopic mass between the two oligomer units in the inset has a value of Δ(m/z) =
162, which is close to the expected mass of the repeat unit of this polymer (162.02).
Thus, the polymer, poly(4-vinylphenyl)dimethylsilane, was anionically synthesized and characterized by SEC, 1H and 13C NMR, FTIR, MALDI TOF mass spectrometry and
DSC measurements. From these characterization results it was concluded that the monomer (4-vinylphenyl)dimethylsilane could be anionically polymerized in a controlled manner using sec-BuLi as initiator in benzene at room temperature.
98 1539.1 1377.1
162
m/z Figure 4.30 MALDI TOF mass spectrum of poly(4-vinylphenyl)dimethylsilane.
99
4.2.1 Synthesis of in-chain epoxide and hydroxyl functionalized polymers
Introduction of epoxide and hydroxyl functional groups on the polymer chain would be useful to modify many properties of the resulting polymer such as polarity, glass transition temperature and solubility. It has been possible to anionically synthesize polymers with an epoxide group at the end of the polymer chain. Nakahama and coworkers138 developed an efficient method to synthesize such polymers by reacting the living polymer chain-ends with (2-bromoethyl)oxirane. However, these reactions were done in THF at -78 ºC and so have limited practical utility. Quirk and Zhuo139 synthesized epoxide-terminated polystyrenes and polybutadienes by reacting the living polymer chain-ends with epichlorohydrin in benzene at room temperature. Also, Riffle and coworkers88 anionically synthesized epoxide-terminated polymers, by first introducing a silyl-hydride unit at the end of the living polymer chains. The polymer was then hydrogenated and finally the epoxide unit was introduced by using a hydrosilation reaction between the silyl-hydride groups at the chain-ends and the double bond of allyl glycidyl ether. However, there is no known method to anionically synthesize in-chain, epoxide-functionalized polystyrenes.
In-chain, hydroxyl-functionalized polymers have been anionically synthesized by the polymerization of hydroxyl-functionalized styrenes91, in which the hydroxyl groups have been suitably protected91. This makes it necessary to deprotect these functional groups post-polymerization. Also, these reactions were done in THF at -78 ºC91. Herein, a simple procedure has been used to anionically synthesize in-chain, epoxide- and hydroxyl-functionalized polystyrenes. The methods used are shown in Schemes 4.6 and
100 4.7, respectively. These polymerizations were done in hydrocarbon solvents at room temperature as opposed to THF at -78 ºC. Also, there was no need to protect the functional groups prior to the polymerization since both functional groups were introduced into the polymer chain post-polymerization. The epoxide group was introduced by use of a hydrosilation reaction between the silyl-hydride groups on the polymer chain and the double bond of allyl glycidyl ether, which is commercially available. The hydroxyl group were produced by simple hydrolysis of the epoxide moiety. Schemes 4.6 and 4.7 provide a useful and practical procedure for synthesizing in-chain epoxide- and hydroxyl-functionalized polymers.
101 H sec-Bu n H sec-Bu n Karstedt's catalyst/R. T.
1. Benzene, R. T. O
O 2. CH3OH
H CCHSi H3CCHSi 3 3 3 H3CCHSi 3 H (CH2)3OCH2 H O
Scheme 4.6 Synthesis of epoxide-functionalized polystyrene.
H sec-Bu sec-Bu H n n
THF/ H2O, BiCl3, reflux
H3CCHSi 3 H3CCHSi 3
(CH2)3OCH2 (CH2)3OCH2CH(OH)CH2OH
O Scheme 4.7 Synthesis of the hydroxyl-functionalized polystyrene.
4.2.1.1 Synthesis of epoxide-functionalized polystyrene
The silyl-hydride groups on poly(4-vinylphenyl)dimethylsilane [Mn = 33000 g/mol,
PDI = 1.03] were reacted with the double bonds of allyl glycidyl ether (purified as described in section 3.1.8) by a hydrosilation reaction. Hydrosilation reactions were done under mild reaction conditions and the epoxide functional group was stable under these conditions, eliminating the need to protect this functional group. The SEC chromatogram of the epoxide-functionalized polymer is narrow and unimodal as shown in Figure 4.31. 102
R. I.
Figure 4.31 SEC chromatogram of the epoxide-functionalized polymer [Mn (obs) = 50000 g/mol, Mn (calc) = 56500 g/mol, PDI = 1.04].
The 1H NMR spectrum of this polymer (Figure 4.32) shows that there is no peak at δ 4.2 ppm corresponding to the silyl-hydride proton. The various peaks which arise from protons in the functional group introduced on the polymer chain have been labeled. The peak labeled (a) arises from the six methyl protons bonded to the silicon atom on each monomer unit. This peak could be integrated with respect to the peaks labeled d, e, f and g (δ 2.6-3.9 ppm)88, which arise from the seven protons in the ether and epoxide functional groups introduced on the polymer chain. The integration ratio has a value
(1.18) as expected (1.17) showing quantitative incorporation of the functional group on the polymer chain. The peak labeled b overlapped with the peak corresponding to the six methyl protons in the initiator (sec-BuLi). So, this peak could not be integrated with respect to the peak labeled (a). 103 sec-Bu H n
(a) (a) H3CCHSi 3 (f) (g) (a) CH2CH2CH2OCH2 O (b) (c) (d) (e) d, e, f, g (c) (b)
Figure 4.32 1H NMR spectrum of the epoxide-functionalized polymer.
sec-Bu H n
(a) (a) H3CCHSi 3 (f) (g) CH2CH2CH2OCH2 O (b) (c) (d) (e)
(c) (b) (a) d, e f
Figure 4.33 13C NMR spectrum of the epoxide-functionalized polymer.
104
Figure 4.33 shows the 13C NMR spectrum of this polymer. The peak labeled (a) at
δ -0.2 ppm is from the two methyl carbons bonded to the silicon atom on the monomer units. The peak labeled (b) at δ 11.2 ppm is from the methylene carbon α to the silicon atom. The peak (c) at δ 23.1 ppm is due to the methylene proton in the β position with respect to the silicon atom88,90. The peaks at δ 73.5 ppm and δ 76.7 ppm correspond to the peaks labeled (d) and (e)88,90. The peak labeled (f) at δ 53.2 ppm is from one carbon atom in the epoxide unit, the other carbon atom labeled (g) would be expected to appear at δ 42.5 ppm and cannot be distinguished in this spectrum88,90. All the peaks assignments in the 1H and 13C NMR spectra were further confirmed using ChemDraw software.
The FTIR spectrum (Figure 4.34) of this polymer shows no peak at 2116 cm-1 showing that all the silyl hydride units have been converted to Si-CH2 groups, which
-1 appear at 1255 cm . The DSC in Figure 4.35 shows that the Tg of the polymer has increased to 132 ºC (for the epoxide-functionalized polymer) from 90 ºC observed for the silyl hydride-polymer. Introduction of the epoxide group into the polymer chain is expected to cause an increase in free volume and thus result in a decrease in the Tg of the polymer. The increase in the Tg may be due to the dipole-dipole interactions between the epoxide groups introduced onto the polymer chain.
105 2116 cm-1
1255 cm-1
Figure 4.34 FTIR spectrum of the epoxide-functionalized polymer.
Figure 4.35 DSC of epoxide-functionalized polymer.
106
4.2.1.2 Synthesis of the hydroxyl-functionalized polystyrene
The epoxide moiety introduced on the polymer chain (see Scheme 4.6) was
127 hydrolyzed using BiCl3 as catalyst . The reaction is shown in Scheme 4.7. Hydrolysis of the epoxide ring produces a polymer having a diol group on each monomer unit. This polymer was highly polar in nature and was readily soluble (0.1 g/1 mL) in polar solvents such as methanol, ethanol and isopropanol. The 1H NMR spectrum of this polymer is shown in Figure 4.36.
sec-Bu H n d, f, g
c b a (a) (a) H3CCHSi 3
CH2CH2CH2OCH2CH(OH)CH2OH e (b) (c) (d) (e) (f) (g)
Figure 4.36 1H NMR spectrum of the hydroxyl-functionalized polymer.
The different peaks in this spectrum have been labeled. The bump at around δ 2.9 ppm arises from the –OH group in the polymer chain. Addition of a small amount of D2O in the NMR tube caused this peak to disappear, confirming that this peak is due to hydroxyl groups in the polymer chain. The peaks labeled d, e, f, and g correspond to protons present in the functional group that has been introduced on the polymer chain88. The peaks d, f and g, which correspond to five protons were integrated with respect to the 107 peak (a) which corresponds to the six methyl protons adjacent to the silicon atom in each monomer unit. The integration ratio obtained (1.2) is as expected (1.2). Figure 4.37 shows the 13C NMR spectrum of this polymer. The peaks labeled d (77.5 ppm), e (74.3 ppm), f (73.8 ppm)and g (73.1 ppm) correspond to carbon atoms in the functional group introduced onto every monomer unit88. The peak labeled a, corresponds to the two methyl protons bonded to the silicon atom and peaks b and c are from the two methylene carbons introduced adjacent to the silicon atom. The peak at δ 138 ppm corresponds to the ipso carbon atom in each repeating unit of the polymer chain. From the 1H and 13C
NMR spectra it can be concluded that there are no residual epoxide groups in the polymer chain indicating complete hydrolysis of the epoxide groups.
sec-Bu H n
(a) (a) H3CCHSi 3
CH2CH2CH2OCH2CH(OH)CH2OH (b) (c) (d) (e) (f) (g)
c b a
d, e, f, g
Figure 4.37 13C NMR spectrum of the hydroxyl-functionalized polymer.
108
Figure 4.38 FTIR spectrum of the hydroxyl-functionalized polymer.
The FTIR spectrum of this polymer (Figure 4.38) shows a broad peak at 3500 cm-1 which shows the presence of an O-H bond. Also, there is no peak at 2116 cm-1 in this spectrum showing the absence of the Si-H bond. The C-O stretching vibrations of the epoxide group usually appear at 1250 cm-1 130. Since the resonance from the polymer backbone appear in this region too, it is difficult to distinguish from the FTIR spectrum whether the epoxide groups have completely disappeared or not.
4.2.2 Synthesis of perfluoroalkyl-functionalized polymer
Fluorinated polymers are known to exhibit many interesting properties, such as improved chemical resistance, lower refractive index and higher contact angle values132.
Fluorinated polymers have been synthesized in the past by DeSimone and coworkers140.
109 Perfluoroalkyl groups were introduced onto chlorodimethylsilane by use of platinum catalyzed hydrosilation reactions between chlorodimethylsilane and commercially available perfluoroalkyl-substituted alkenes. This fluorinated compound was then reacted with anionically synthesized living polymer chains and chain-end perfluoroalkyl- functionalized polymers could be obtained. However, there is no known general method to synthesize in-chain functionalized polymers. The general functionalization method developed above was used to introduce perfluoroalkyl groups onto poly(4-vinylphenyl)- dimethylsilane, using hydrosilation reactions done post-polymerization. The reaction scheme used is shown below (Scheme 4.8). The SEC chromatogram (Figure 4.39) of the polymer is narrow and unimodal showing the absence of any side reactions during the hydrosilation reaction.
H H sec-Bu n sec-Bu n
Karstedt's catalyst 12 h, 250C, Benzene
(CF2)5CF3 (1.5 equiv) H3C Si CH3 H3C Si CH3
H (CF2)5CF3
Scheme 4.8 Synthesis of perfluoroalkyl-functionalized polymer.
110 R. I.
Figure 4.39 SEC chromatogram for the perfluoroalkyl-functionalized polymer. [Mn (obs) = 72000 g/mol, Mn (calc) = 77000 g/mol, PDI = 1.08].
The 1H NMR spectrum (Figure 4.40) of this polymer shows the absence of the silyl- hydride peak at 4.2 ppm. There are two new peaks which are labeled b and c. These two peaks correspond to protons in the two methylene groups adjacent to the silicon atom.
sec-Bu H n
(a) H3C Si CH3 (a)
(b) (c) (CF2)5CF3 c b a
Figure 4.40 1H NMR spectrum of the perfluoroalkyl-functionalized polymer.
111
Since these peaks are not well separated from the peaks from the polystyrene chain, it was difficult to integrate them for quantitative analysis. 19F NMR analysis was done and the spectrum for the functionalized polymer is shown in the upper spectrum in
141 Figure 4.41. C6F6 was used as the standard (δ -163 ppm) . The lower spectrum is from the starting material. Both of these spectra are similar to each other with the same number of peaks at almost the same positions. The peak observed at -114.67 ppm corresponds to
141 the two fluorine atoms shown Si(CH3)2(CH2)2CF2- . The four characteristic peaks corresponding to the other four difluoromethylene groups are observed at -122.36,
-123.63, -123.39 and -126.95 ppm, respectively141. The peak at -181.91 ppm is from the trifluoromethyl group141.
sec-Bu H n
-114.674 -122.36,-123.63,-123.39,-126.95 C6H6 -181.91
H3C Si CH3
(CF2)5CF3
-114.44 -122.24,-123.48,-124.23,-126.74 C6H6 -181.57
Figure 4.41 19F NMR spectrum of the perfluoroalkyl-functionalized polymer and corresponding monomer. (upper spectrum : perfluoroalkyl-functionalized polymer; lower spectrum : starting material).
112 120
100
80
60 No SiH -1 2116 cm Transmittance (%) 40
20 C-F bonds
0 3900 3400 2900 2400 1900 1400 900 400 Wavelength Figure 4.42 FTIR spectrum of the perfluoroalkyl-functionalized polymer.
Figure 4.42 shows the FTIR spectrum of the fluorinated polymer. There is no peak observed at 2116 cm-1 showing the absence of the Si-H bond. The significant broadening in the region between 600-1300 cm-1 is due to the C-F bonds in the perfluoroalkyl groups141 that have been introduced on the polymer chain.
Two interesting characteristics exhibited by fluorinated polymers are their values of contact angle and refractive index141. Samples for measuring the contact angle were prepared by making a 4 wt % solution in THF of the desired polymer (Mn = 72000 g/mol) and spin coating this solution onto a silicon wafer. The thicknesses of the films obtained were measured by using ellipsometry measurements. Contact angle measurements were done on the fluorinated polymer and a standard polystyrene sample, which had a similar molecular weight compared to that of the functionalized polymer. The polystyrene sample was used for the purpose of reference. The values of advancing and receding contact angles obtained for both polymers are shown in Table 4.1. The values show that
113 introduction of fluorine groups onto the polymer chain causes a considerable increase in values of both the advancing and receding contact angles. The contact angle value obtained for the fluorinated polymer was compared with the reported values for another fluorinated polymer, poly[2,3,5,6-tetrafluoro-4-(2,2,3,3,3-pentafluoropropoxy)styrene].
This polymer exhibited a value of 115 for the advancing contact angle and a value of 105 for the receding contact angle141.
Table 4.1 Contact angle values for standard polystyrene and the fluorinated polymer.
Θ (advancing) Θ (receding) PS a 90.5 [95141] 82 [80141]
PSF b 114 101
a PS : Polystyrene (TSK standard) (Mn = 70000 g/mol, PDI = 1.05). b PSF : perfluoroalkyl-functionalized polystyrene (Mn = 72000 g/mol).
Fluorinated polymers are expected to exhibit lower values of refractive indices. The refractive index of the fluorinated polymer (PSF) and a standard polystyrene sample were measured by using the same spin-coated films. The values obtained are shown in Table
4.2. These values can be compared with the values obtained for a perfluoroalkyl substituted polystyrene (n-C3F7 = perfluoroalkyl chain) . The refractive index value reported for this polymer is 1.50142. The values obtained for the perfluoroalkyl- functionalized polymer synthesized (PSF) is much lower than this value since the degree of fluorination of this polymer is higher.
114
Table 4.2 Refractive Index values for a polystyrene standard and the fluorinated polymer.
R.I
PS a 1.57 [1.58]142
PSF b 1.39
a PS : Polystyrene (TSK standard) (Mn = 70,000 g/mol, PDI = 1.05) b PSF : perfluoroalkyl-functionalized polymer (Mn = 72000 g/mol)
DSC was done on the fluorinated polymer to measure it’s Tg (Figure 4.43). The glass transition temperature of this polymer was 55 ºC, which is lower than the value for poly(4-vinylphenyl)dimethylsilane (90 ºC). The decrease in the glass transition temperature could be a result of introduction of long side-chains on the polymer. This would be expected to increase the free volume in the polymer, resulting in a reduction of the glass transition temperature. The broad transition observed in the DSC was believed to be due to some side-chain ordering in the perfluoroalkyl groups introduced on the monomer units along the polymer chain143.
115
Figure 4.43 DSC of the fluorinated polymer.
4.3 Determination of monomer reactivity ratios in the anionic copolymerization of styrene with (4-vinylphenyl)dimethylsilane
Copolymerization of monomers enables the variation and control of polymer properties. Random introduction of monomers into the polymer chain would result in a polymer with intermediate properties between the two homopolymers.144 However, more often in anionic polymerization one monomer is preferentially incorporated into the polymer chain initially, and the second monomer is incorporated once the first is depleted19. The copolymerization behavior is determined by the product of the monomer reactivity ratios of the two monomers. If r1r2 = 0, an alternating copolymer is obtained, while r1r2 = 1 gives a random copolymer and a value of r1r2 > 1 gives a blocky copolymer.144 In the case of anionic polymerization, the electron withdrawing or delocalizing abilities of the substituents on the vinyl monomers can be used to predict relative monomer reactivities. As a general rule, electron-donating substituents decrease
116 the monomer reactivity and carbanion stability while the electron-withdrawing groups would be expected to be more reactive towards carbanion addition since they stabilize the
19 carbanion relative to an unsubstituted benzyl carbanion. The Si(CH3)2H group exhibits an electron withdrawing effect (σ = 1.58 145) due to the presence of energetically available vacant d-orbitals in the silicon atom82. This causes this group to have a greater stabilizing effect on the benzylic carbanion when compared to an unsubstituted styrene unit. The monomer reactivity ratios can be measured by use of the copolymerization equation , which links the feed and copolymer compositions144. It involves fitting the data obtained by analyzing the compositions of copolymers formed from several different concentrations of monomers. Several methods for extracting the monomer reactivity ratios from differential forms of the copolymer equation are widely used.
Anionic copolymerizations of styrene and (4-vinylphenyl)dimethylsilane were carried out with sec-BuLi as initiator in cyclohexane. By studying the copolymerization behavior of styrene with (4-vinylphenyl)dimethylsilane, it is possible to control the number of (4-vinylphenyl)dimethylsilane units introduced into the copolymer and subsequently the number of functional groups introduced into the copolymer, as shown in
Scheme 4.9. The Finemann-Ross, Kelen-Tudos and EVM methods were used for the determination of the monomer reactivity ratios. For the purpose of accuracy, it was attempted to control the polymerizations to under 10 % conversion, so that the change in concentrations of monomers could be kept at a minimum. For all copolymerizations that were carried out the degree of conversion was between 5-10 %. The polymerizations were quenched within 120 seconds of the initiation reaction. The breakseal of the ampoule containing methanol was smashed to introduce the terminating agent into the
117 reaction mixture. The polymers thus obtained were isolated by precipitation into methanol. Since these polymers had very low molecular weights, there was a possibility of some polymer dissolving in methanol. This remaining polymer was recovered by removing the methanol in a rotary evaporator. The expected Mn (calc) for all the copolymers at complete conversion was 3000 g/mol. The 1 H NMR spectra of copolymers obtained for different molar ratios of styrene and (4-vinylphenyl)dimethylsilane are shown in Figure 4.44.
Scheme 4.9 Functionalization of the copolymer of styrene with (4-vinylphenyl)- dimethylsilane.
118 [S]/[Si]
Figure 4.44 1H NMR spectra for the series of copolymers synthesized with different mol/ mol ratios of styrene and (4-vinylphenyl)dimethylsilane
The aromatic protons were integrated with respect to the proton bonded to the Si atom in order to determine the mol % styrene in the initial polymer. The calculations done for the
90/10 (S/Si) copolymer are as shown: area of 5 styrene protons + 4 aromatic SiH = 93.69; area of SiH = 8.31.
So, area of 5 styrene protons = 60.45; area of one styrene proton = 12.09.
(area of one styrene proton) / (area of one aromatic SiH proton) = 12.09/8.31 = 1.45/1.
The styrene mol % for the initial copolymer was calculated for the copolymer synthesized for an initial feed composition of S / SiH = 90/10, as shown : composition of styrene in the copolymer, [sty] = 1.45; composition of SiH in the copolymer, [SiH] = 1. This corresponds to a copolymer with 59.2 % styrene and 40.8 % 119 (4-vinylphenyl)-dimethysilane at complete conversion. The ratio of the two monomer units incorporated into the initial copolymer (8 % conversion) was lower than the initial feed composition (90/10), due to obvious reasons. Table 4.3 shows the integration data obtained for each copolymer .
Table 4.3 Integration data for each copolymer
Integration values for the Integration values for the SiH
aromatic protons in the protons in the copolymer
copolymer
90/10 93.69 8.31
80/20 80.47 9.53
65/35 90.81 13.24
50/50 86.24 12.76
30/70 89.96 16.64
So, styrene mol % = [sty] / {[sty] + [SiH]} = [1.45/ (1.45 + 1)] x 100 = 59.2 %.
Figure 4.45 shows a plot of mol % styrene in the initial polymer versus the mol % styrene in the feed. The shape of the curve suggests a preferential incorporation of (4- vinylphenyl)dimethylsilane relative to styrene in the copolymer. Table 4.4 shows the values of the monomer feed ratios, the instantaneous copolymer compositions and the styrene mol % in the initial copolymer for each experiment.
120 Table 4.4 Values of the monomer feed ratios and instantaneous copolymer compositions.
a F (monomer b f Styrene mol % feed ratio) (instantaneous in initial copolymer copolymer composition) 90/10 9 1.45 59.2 80/20 4 1.09 52.2 65/35 1.8 0.54 35.1 50/50 1 0.45 31.0 30/70 0.43 0.21 17.3 a F = monomer feed ratio (ratio of the compositions of the two monomers in the feed) b f = instantaneous copolymer composition (ratio of the compositions of the two monomers in the initial copolymer formed).
70
60
50
40
30
20
10
styrene mol % in initialstyrene copolymer 0 020406080100 styrene mol % in feed
Figure 4.45 Plot of mol % styrene in the feed versus the styrene mol % in the copolymer.
The Fineman-Ross plot41 is shown in Figure 4.46 and it gives reactivity ratios of
0.17 ± 0.01 for styrene and 1.9 ± 0.23 for (4-vinylphenyl)dimethylsilane. The values of all the parameters calculated in the Fineman-Ross method are shown in Table 4.5.
121
Table 4.5 Calculated parameters in the Fineman-Ross Method.
Styrene (monomer SiH (monomer F(M /M ) f(dM /dM ) F(f-1)/f (y) F2/f (x) feed ratio) feed ratio) 1 2 1 2 30 70 0.43 0.21 -1.62 0.88 50 50 1 0.45 -1.22 2.22 65 35 1.8 0.54 -1.5 6 80 20 4 1.09 0.33 14.7 90 10 9 1.45 4.83 37.5
Figure 4.48 was used to determine the monomer reactivity ratios obtained by using the
Kelen-Tudos method42. The different parameters calculated in the Kelen-Tudos method are described in Table 4.6 below.
Table 4.5 Calculated parameters for the Kelen-Tudos Method.
x y G F η Ε 9 1.45 4.83 37.5 0.112 0.87 4 1.09 0.33 14.7 0.016 0.72 1.8 0.54 -1.5 6 -0.12 0.51 1 0.45 -1.22 2.22 -0.15 0.28 0.43 0.21 -1.62 0.88 -0.24 0.133 2 x = M1/M2 (feed); y = m1/m2 (copolymer); G = x(y-1)/y; F = x /y; η = G/(α+F); 42 1/2 1/2 ε = F/(α+F) , α = (FminFmax) = (37.5 x 0.88) = 5.75.
This method also gives values of monomer reactivity ratios close to those obtained by the Fineman-Ross method. The values of r1 and r2 can be obtained by extrapolating the plot in Figure 4.47. By this method the reactivity ratio for styrene is
0.16 ± 0.01 and for (4-vinylphenyl)dimethylsilane is 1.75 ± 0.018.
122 6 5 y = 0.1756x - 1.9067 R2 = 0.9893 4
3 2
1 F(f-1)/f 0 0 10203040 -1 -2 -3 (F x F)/f
Figure 4.46 Fineman-Ross plot.
0.15
0.1 y = 0.4501x - 0.3026 R2 = 0.9555 0.05
0 η 0 0.2 0.4 0.6 0.8 1 -0.05
-0.1
-0.15
-0.2
-0.25
-0.3
ε
Figure 4.47 Kelen-Tudos plot.
123 In order to eliminate the error estimation in determination of the true values of the monomer reactivity ratios, a nonlinear method was initially proposed by Behnken43. An extension of this non-linear approach results in the error-in-variable method (EVM). This method correctly takes into account the error in both the independent and dependent variables i.e., the monomer feed and copolymer compositions, respectively. This method minimizes the weighted sum of squares of the distance from the observed point to the estimated value. Figure 4.48 shows the values of monomer reactivity ratios obtained using the EVM model. This plot can be obtained by introducing the required parameters
[values of r1 and r2 as obtained from other methods; x and y (as shown in Table 4.5)] into
44 a computer program . The values of r1 and r2 obtained by this method (r1= 0.16 and r2= 1.8) are also in agreement with the values obtained from the other two methods (see
Table 4.7).
Figure 4.48 EVM plot for the copolymers. 124
Table 4.7 Values of r1(styrene) and r2(SiH) obtained by all three methods.
r1(sty) (avg) r2(SiH) (avg)
F-R Method 0.17 1.9
K-T Method 0.16 1.75
EVM Method 0.16 1.8
It can be concluded that the value of the monomer reactivity ratio of (4-vinylphenyl)- dimethylsilane, 1.8 (average value) is much higher than the value for styrene, 0.16
(average value). The product of the two monomer reactivity ratios is less than 1, i.e. 0.29.
The usual interpretation of these reactivity ratios would suggest that a growing anionic styryl chain-end much prefers to react with a (4-vinylphenyl)dimethylsilane molecule rather than with another styrene molecule, while a (4-vinylphenyl)dimethylsilane chain- end also prefers to add a (4-vinylphenyl)dimethylsilane molecule. Thus monomer reactivity ratio results indicate that this copolymerization would tend towards an alternating structure with preferential incorporation of (4-vinylphenyl)dimethylsilane.
125
4.4 Synthesis of a polymer biohybrid using a combination of anionic polymerization and hydrosilation reactions
The challenge of synthesizing polymer-peptide hybrids presents an interesting and powerful opportunity for combining the properties of each class of material.146 Recently, several advances have been made in peptide chemistry, polymer chemistry and nanoscience which emphasize the need for the development of some simple and versatile methodologies for synthesis of polymer biohybrids.147 It has been observed that some peptide sequences act as ligands that can facilitate transfer across cell membranes.148 These peptide sequences can be combined with a polymer, which acts as a delivery vehicle for transporting drugs or molecular probes across cell membranes.149
Another popular method of producing complex nanostructured materials is the self- assembly of block copolymers.150 Nguyen and coworkers151 have developed a sophisticated method for synthesis of DNA-block copolymer conjugates by use of post- polymerization modification of ROMP polymers and block copolymers with DNA, to give hybrid materials with many interesting properties. Development of solid-phase techniques also allows the coupling between biological and synthetic macromolecules. In these methods free-radical polymerization is conducted from initiating sites located on the chain termini of peptides which have been loaded onto a solid support.152-155
Given the growing interest in the development of new synthetic methods for the preparation of well-defined polymers with in-chain functional groups using living polymerizations,146 the use of the anionic polymerization method as a template for synthesis of polymer-peptide hybrids offers several distinct advantages. The methodology of living anionic polymerizations, especially alkyllithium initiated polymerizations of
126 styrene and diene monomers, is suitable for development of polymers with controlled molecular weights, narrow polydispersities and well-defined structures.156-158 In the past
Nakahama and coworkers91 have developed a strategy for synthesizing in-chain functionalized polymers by using living anionic polymerization to polymerize a series of suitably protected functionalized monomers, followed by post-polymerization deprotection of the protecting groups. Despite the potential of this methodology, one problem is that introduction of each functional group requires polymerization of a different monomer. Also, there are additional steps of protecting and deprotecting these functional groups prior to and post-polymerization, respectively. So, optimized reaction conditions must be found for each new functional polymer.
Herein, the first step in the synthesis of the polymer biohybrid, i.e. the (N- carbobenzyloxy)-L-phenylalanine-functionalized copolymer, was done by first synthesizing a copolymer of styrene with (4-vinylphenyl)dimethylsilane by anionic copolymerization with sec-BuLi as initiator in a hydrocarbon solvent at room temperature, followed by use of a hydrosilation reaction post-polymerization to introduce an amine functionality into the copolymer chain159. The final step was a condensation reaction between this amine functionality and the carboxyl moiety on the amino-acid, (N- carbobenzyloxy)-L-phenylalanine (Scheme 4.10). This is a simple and facile method for development of well-defined polymer-biohybrids, which although demonstrated for (N- carbobenzyloxy)L-phenylalanine, can potentially be extended to any peptide sequence.
127 Introduction of the amino acid DCC P-NH2 P-NH-CO-Phe-N-cbz DMAP, N-cbz-Phe, CH2Cl2
N=C=N DCC (Dicyclohexylcarbodiimide)
H3C CH3 N
DMAP (4-dimethyl, amino pyridine)
N
O
O NH
COOH N-cbz-Phe (N-carbobenzyloxy phenylalanine)
61
Scheme 4.10 Synthesis of the polymer biohybrid.
4.4.1 Preparation of the Copolymer of Styrene with (4-vinylphenyl)dimethylsilane
The sec-BuLi initiated copolymerization of styrene (S) with (4-vinylphenyl)- dimethylsilane (Si) (90/10, mol/mol) was effected in benzene at room temperature. The copolymer obtained was expected to exhibit a tendency towards an alternating structure, from our previous studies159 (Scheme 4.11).
co H sec-Bu nm 1. C6H 6 sec-BuLi + n + m 2. CH3OH
H 3C Si CH3
H C Si CH H 3 3
H Scheme 4.11 Anionic copolymerization of styrene with (4-vinylphenyl)dimethylsilane.
128 The SEC chromatogram (Figure 4.49) for the silane-functionalized copolymer [Mn
(obs) =8600 g/mol, Mn (calc) = 8500 g/mol, Mw/Mn=1.06] exhibited a narrow, monomodal distribution. The copolymer was obtained in 97 % yield. The bump in the
SEC may correspond to hydrolysis of the silyl hydride group in the polymer chain in
73 presence of the platinum catalyst (P-SiH + P-SiH + H2O + Pt (catalyst) Æ PSi-O-SiP ).
However, further characterization results of this polymer (FTIR) do not show any evidence of formation of a hydrolyzed product (1050 cm-1, Si-O-Si)73.
Figure 4.49 SEC chromatogram of the copolymer.
The 1H NMR spectrum (Figure 4.50) of this copolymer showed characteristic resonance for the silane proton at δ 4.4 ppm and for the methyl groups bonded to the silicon atom at δ 0.32 ppm. Integration of the silane proton (1 H) with respect to the aromatic protons (4 Hs) followed by subsequent calculations gave a value for the mol/mol ratio of styrene to (4-vinylphenyl)dimethylsilane in the copolymer of 9 : 1 (9 : 1, expected). Integration of the silane proton relative to the six protons on the methyl
129 carbons bonded to the silicon atom corresponded to an integration ratio of 1: 5.9, as expected (1 : 6). The calculations are as follows :
Area of 5 styrene protons + 4 aromatic SiH = 91.14; area of SiH = 1.85. This means, area of 5 styrene protons = 83.74 and one styrene proton = 83.74/5 = 16.7.
So, (one styrene proton) / SiH = 16.7/1.85 = 9/1.
FTIR spectroscopic analysis was found to be useful tool for studying the presence of the silyl hydride group. The silyl hydride group exhibits a strong absorption band at 2114 cm-1 corresponding to a Si-H stretching vibration, along with a band at 880 cm-1 due to rocking absorption of the Si-CH3 bond (Figure 4.51).
130
Figure 4.50 1H NMR spectra for the copolymer of styrene with (4-vinylphenyl)- dimethylsilane at different attenuations.
The 13C NMR spectrum (Figure 4.52) showed a distinct resonance for the methyl carbons adjacent to the silicon atom at δ -0.2 ppm. All of the other peaks in this spectrum are similar to the peaks in the 13C NMR spectrum for poly(4-vinylphenyl)dimethylsilane 131
(Figure 4.22). Thin layer chromatographic (TLC) analysis was carried out on this polymer by using toluene as the eluent. The polymer appeared as a single spot which moved slightly below the solvent front due to its non-polar nature.
Figure 4.51 FTIR spectrum for the copolymer of styrene with (4-vinylphenyl)- dimethylsilane.
132
Figure 4.52 13C NMR spectrum for the copolymer of styrene with (4-vinylphenyl)- dimethylsilane.
On the basis of 1H NMR, 13C NMR, FTIR and TLC analyses it was concluded that the copolymerization of styrene with (4-vinylphenyl)dimethylsilane was successfully carried out in hydrocarbon solvents at room temperature resulting in almost quantitative yield of the silyl hydride-functionalized copolymer.
4.4.2 Preparation of Amine-Functionalized Copolymer by Hydrosilation of the Copolymer of Styrene with (4-vinylphenyl)dimethylsilane
The second step in the synthesis of the polymer biohybrid was the introduction of an amine functionality in the polymer chain by a platinum-catalyzed hydrosilation reaction using Karstedt’s catalyst between the silyl hydride groups on the (4-vinylphenyl)- dimethylsilane units incorporated into the copolymer with allylamine (1.2 equivalents) in benzene at room temperature. The reaction was stirred under a nitrogen atmosphere for one week and was monitored from time to time using 1H NMR. (Scheme 4.12).
133 co co H H sec-Bu sec-Bu nm nm
C 6H 6, Pt(0)
NH2
H 3C Si CH3 H 3C Si CH3
H n/m = 90/10
H 2N Scheme 4.12 Amine functionalization of the copolymer using hydrosilation.
The amine-functionalized polymer was isolated in 95% yield. The characteristic resonance for the silane proton at δ 4.4 ppm was absent in the 1H NMR spectrum (Figure
4.53, next page), indicating almost complete conversion of the Si-H group to a Si-CH2 group as a consequence of the hydrosilation reaction. A new resonance at δ 2.7 ppm (c)
(Figure 4.54) appeared corresponding to the methylene protons on the carbon adjacent to the amine group (γ protons, with respect to the silicon atom)90. The resonances due to the
α (a) and β (b) protons with respect to the silicon atom are expected to appear at δ 0.62 ppm and 1.6 ppm respectively90, but they cannot be observed due to overlapping resonances from the initiator and the polymer backbone, respectively. Integration of the methylene protons at δ 2.7 ppm with respect to the methyl protons on the carbon bonded to the silicon atom at δ 0.32 ppm yields a value of 3.2:1 (expected value 3:1), indicating functionalization of 96 % of the silyl hydride functional groups in the copolymer with the amine groups of allylamine. The integration of the methylene protons [Si(CH3)2-CH2] with respect to the aromatic protons followed by subsequent calculations yields a value of mol/mol ratio of styrene/(silane-functional monomer units) in the copolymer of 9:1 , which is close to the value which was obtained for the copolymer of styrene with (4- vinylphenyl)dimethylsilane. 134
Figure 4.53 1H NMR for the amine-functionalized copolymer.
H co m sec-Bu n
H3CCHSi 3
(a) (b) (c)
H2N
Figure 4.54 Amine-functionalized copolymer.
The calculations are as follows :
Area of 5 styrene protons + 4 aromatic SiH = 66.25. Area of Si(CH3)2CH2 = 2.65. This means that the area of SiH = 2.65/2 = 1.33. So, the area of 5 styrene protons + 4 x 1.33 =
66.25. Therefore, the area of styrene = 12.18 (1 H), and the area of SiCH2 (1 H) = 1.33.
So, styrene (1 H) / SiH (CH2, 1 H) = 12.18/1.33 = 9.1/1. This also corresponds to almost
135 100 % functionalization of the the silyl hydride units. The FTIR spectrum (Figure 4.55) of the copolymer shows no absorption due to the Si-H stretching vibration at
2114 cm-1, thus indicating an almost quantitative hydrosilation reaction. A new
-1 absorption band appears at 1056 cm due to the C-N stretching vibration of the –CH2-
130 NH2 bond . Resonance bands due to the asymmetric and symmetric stretching of the N-
H bond of the primary amine appear at 3432 and 3306 cm-1, respectively135.
2114 cm1
Figure 4.55 FTIR for the amine-functionalized copolymer.
In the 13C NMR spectrum (Figure 4.56) of the amine-functionalized copolymer the resonance at δ -0.2 ppm is due to the methyl carbons adjacent to silicon. Additional resonances, which were absent in the initial copolymer, appear at δ 13.7 ppm and 23.0 ppm, corresponding to the carbon atoms in the α and β positions with respect to the 136 silicon atom, respectively90. The resonance due to the γ carbon with respect to the silicon atom is expected to appear at 45.5 ppm but it cannot be observed due to overlapping resonances from the polymer chain90. TLC was done by using toluene as the eluent once again. This shows a single spot which has moved only a little bit from the bottom of the chromatographic plate. This is due to the polar nature of the amine functionality introduced on the polymer chain. However, this does not give us any conclusive evidence about the quantitative introduction of amine groups at the polymer chain-ends, since TLC cannot show partial functionalization.
Thus, the results from 1H NMR, 13C NMR, FTIR and TLC analysis shows that the silyl hydride units on the copolymer undergo efficient hydrosilation reactions with the double bond of the allylamine in presence of Karstedt’s catalyst in benzene at room temperature, giving almost quantitative yields of the functional copolymer.
137
Figure 4.56 13C NMR for the amine-functionalized copolymer.
4.4.3 Preparation of the (N-carbobenzyloxy)-L-phenylalanine-Functionalized Copolymer by a Condensation Reaction of the Amine-Functionalized Copolymer with (N-carbobenzyloxy)-L-phenylalanine
The final step in the synthesis of the polymer biohybrid involved a condensation reaction between the amine functionality in the polymer chain with the free carboxyl group on the (N-carbobenzyloxy)-L-phenylalanine, giving rise to an amide linkage
(Scheme 4.13). For the condensation reaction, the (N-carbobenzyloxy)-L-phenylalanine
(N-cbz-Phe) was dissolved in dichloromethane, followed by addition of 4- dimethylaminopyridine (DMAP), the amine-functional copolymer and dicyclohexyl- carbodiimide (DCC) at room temperature under a nitrogen atmosphere followed by stirring overnight.
138
co H co sec-Bu H nm sec-Bu nm
DMAP/DCC
N-cbz-Phe, CH2Cl2
H3C Si CH3 H3C Si CH3 O n/m = 90/10
O NH NH
H2N
O
Scheme 4.13 Synthesis of the polymer bio-hybrid.
The polymer was isolated in 92 % yield. The SEC of this polymer is shown in
Figure 4.57. It shows that the distribution is narrow and unimodal. The structure of the amino acid-functionalized copolymer is shown in Figure 4.58. The FTIR spectrum
(Figure 4.59) exhibits a new resonance band at 1707 cm-1 assigned to the stretching vibration of the C=O bond of the amide130, and at 3431 cm-1 assigned to the stretching vibration of the N-H bond of the amide130. The resonance bands due to the bending vibration of the N-H bond of the amide were expected to appear at around 1500 cm-1 130. This cannot be distinguished due to overlapping absorptions from bonds in the parent polymer.
139 Mn (obs) = 9230 g/mol PDI = 1.08 Intensity (R. I.)
Figure 4.57 SEC chromatogram of the polymer bio-hybrid.
H co sec-Bu n m
H 3C (a) CH3 (b) Si (c) (d) (g) HN O NHCOO (e)
(f)
Figure 4.58 Amino acid-functionalized copolymer.
140
Figure 4.59 FTIR for the amino acid-functionalized copolymer.
The 1H NMR spectrum (Figure 4.60) of the resulting polymer exhibits a characteristic resonance at δ 0.32 ppm (b) due to the methyl protons on the carbon bonded to the silicon atom. The resonance at δ 2.8 ppm (d) is due to the methylene protons on the carbon in the
γ position with respect to the silicon atom130. Three new resonances appear at δ 3.05 (f),
3.6 (e) and 4.85 ppm (g) due to protons in the (N-carbobenzyloxy)-L-phenylalanine moiety introduced onto the polymer chain (see Figure 4.58). These peaks could be assigned by comparison with the 1H NMR spectrum of the starting material (N-cbz-Phe)
(Figure 4.61). Integrating the methylene protons on the carbon in the γ position (d) (δ 2.8 ppm) relative to the silicon atom with respect to the resonance which arises due to the methylene protons on the carbon bonded to the phenyl ring at 4.85 ppm (g), an
141 integration ratio of 1.12:1 is obtained, which is only slightly higher than the 1:1 ratio to be expected in case of 100 % efficiency of the condensation reaction. This ratio (1.12 :
1), corresponds to 90 % conversion of the amine group to an amide linkage. The integration of the protons at (d, δ 2.85 ppm) with respect to the aromatic protons (δ 6.9-
7.2 ppm) followed by subsequent calculations yields a value of mol/mol ratio of styrene/(silane-functional monomer units) in the copolymer of 8.8:1.
The calculations are as follows :
Area of 5 styrene protons + 4 aromatic SiH = 94.74. Area of SiCH2 = 3.93/2 = 1.96 (1 H).
So, area of 5 styrene protons = 86.88 and the area corresponding to one styrene proton
= 17.3 (1 H). So, Styrene (1 H) / SiCH2 (1 H) = 17.3/1.96 = 8.8.
142
Figure 4.60 1H NMR spectra for the amino acid-functionalized copolymer at different attenuations.
143 (c) O (a) (b)
O N H OH O
(a) (b) (c)
Figure 4.61 1H NMR spectrum of N-cbz-Phe.
(d) (a) O (c) (d) O (b) N H OH O
(a) (c) (e) (d) (b)
Figure 4.62 13C NMR spectrum of N-cbz-Phe.
144
In the 13C NMR spectrum (Figure 4.63) of the copolymer resonances at δ
0.2 (a), 13.9 (b) and 22.8 ppm (c) correspond to the methyl carbons adjacent to silicon and the α and β methylene carbons with respect to silicon, respectively130. The new resonance that appears at δ 174.7 ppm is assigned to the carbonyl carbon (h) in the newly formed amide linkage as a result of the condensation reaction130. Other resonances appear at δ 56.9 (g), 68.0 (e), 128.5 (i) and 157.5 (h) ppm, which are characteristic resonances for the (N-carbobenzyl-oxy)-L-phenylalanine moiety (Figure 4.62). All the peaks in the
1H and 13C NMR spectra of the starting material were assigned by use of ChemDraw software and also from the Aldrich catalog. The sample was analyzed by TLC using toluene as eluent. It exhibited a single spot near the bottom of the TLC plate.
However it was not possible to distinguish between the amine-functionalized polymer and the amino acid-functionalized polymer. Use of mixtures of toluene and cyclohexane as the eluent also did not provide a good separation between these two compounds in
TLC analyses. So conclusive evidence about the conversion of the amine functionality to an amide linkage could not be obtained on the basis of the TLC analyses.
145
H sec-Bu co n m
H3C CH3 (a) (b) Si (c) (d) (g) HN (h) (i)
O NHCOO (e)
(f)
(a) (h)(i) (e) (g) (f,c) (b)
84
Figure 4.63 13C NMR for the amino acid-functionalized copolymer.
It can be concluded on the basis of 1H NMR, 13C NMR, FTIR and TLC analyses that an efficient condensation reaction took place between the amine group on the copolymer and carboxyl group of the (N-carbobenzyloxy)-L-phenylalanine.
4.5 Synthesis of a thermoplastic elastomer using (4-vinylphenyl)dimethylsilane
Thermoplastic elastomers (TPE) are named for the combination of properties that they exhibit. These materials have long-range, reversible extensibility analogous to crosslinked rubbery materials and can be processed like thermoplastic materials160. One
146 of the most important types of TPEs used industrially is the class of anionically synthesized polystyrene-b-polydiene-b-polystyrene triblock copolymers. Copolymers are polymers which are obtained by polymerizing two different monomer units144.
Depending upon the method used to polymerize these monomers, the resulting copolymer may have the monomer units randomly distributed along the polymer chain or arranged in the form of blocks. Simple block copolymers are known to exhibit some properties corresponding to each type of homopolymer. However, these polymers also exhibit some unique properties due to combination of different polymer segments. For a styrene-diene- styrene type of triblock copolymer, the soft (diene) segment is in the middle and the two hard (styrene) segments are at the ends of the polymer chain. These triblock copolymers exhibit phase separation because of the incompatibility of the two types of blocks. The polystyrene blocks associate into domains and the different domains are linked to each other by the long rubbery polydiene units. Since these materials possess two different phases they also have two glass transition temperatures. Below their Tg the polystyrene domains act as physical crosslinks between the rubbery polydiene domains, giving the material good tensile strength without the need for chemical crosslinking or vulcanization. Above the Tg of the polystyrene domains, these materials lose their strength since the glassy domains at the end of the polydiene chains soften and can no longer behave as rigid crosslinks. Although anionically synthesized thermoplastic elastomers are very useful materials commercially, it has been observed that there is no known simple method to incorporate functional groups into the polystyrene domains of these polymers. The ability to introduce different functional groups into the polystyrene domain would help in modifying many properties of these materials, depending on the
147 nature of the functional groups. The proposed methodology was to make a TPE where the polystyrene blocks were replaced by a copolymer of styrene with (4-vinylphenyl)- dimethylsilane. The silyl hydride groups on this TPE could then be converted to different functional groups by using hydrosilation reactions to modify the properties of the polymer. Rempel and coworkers161 have introduced functional groups onto a polystyrene- polydiene block copolymer by using hydrosilation reactions. However, hydrosilation was done between the double bonds on the diene block of the copolymer and the silyl hydride unit present on a hydrosiloxane moiety. This resulted in introduction of the functional groups into the polydiene block of the copolymer instead of into the polystyrene block.
The reaction scheme used is shown in Scheme 4.14 below.
R CH CH=CHCH CH CH H 2 2 y 2 z x
CH CH 3 3
RhCl(PPh3)3 H Si-O-Si CH3 20hrs (Ar) CH3 CH3
R CH CH=CHCH CH CH CH CH H x 2 2 2 2 y z' z-z'
H3CCHSi 3
O
CH H3C Si 3
CH3
Scheme 4.14 Hydrosilation of the polydiene block of poly(styrene-b-butadiene).
148 It is noteworthy that all of the silyl hydride groups were added across the double
bonds in the diene units with a 1,2-microstructure (as observed from 1H and 13C NMR
evidence) 160. There was no addition across double bonds in the diene units having a 1,4-
microstructure. Another interesting observation by Baum and coworkers162 show that
when hydrosilation reactions were done between the double bonds on polybutadiene and
the silyl-hydride units on a difunctional hydrosiloxane molecule, there was formation of
side loops as a result of intramolecular reactions. The formation of these side-loops is
shown in Scheme 4.15.
HSiMe -(OSiMe ) -OSiMe H + 2 2 n 2
R h C l ( P P h3 )3 3 - 5 d a y s ( toluene) r e f l u x H M e2 S i O (OSiMe2 )3 (OSiMe2 )n
SiMe 2 OSiMe S i M e2 2
m m
Scheme 4.15 Formation of side-loops due to hydrosilation of polybutadiene.
Herein, a new TPE was synthesized with in-chain functional groups in the hard
(styrenic) phase. The thermoplastic elastomer was prepared using the sequential
monomer addition approach. The three step reaction is shown in Scheme 4.16. The first
block of the TPE was a copolymer of styrene and (4-vinylphenyl)dimethylsilane taken in
149 Li co m Benzene/ RT sec-Bu sec-BuLi + n + m n
H3C Si CH3 H3C Si CH3
H H
b co Li m co m Benzene/ 50 C sec-Bu n Li H3C sec-Bu n x2 x1 H3C CH3 x
H3C Si CH3 H3C Si CH3 H H
b co m sec-Bu n Li + n + m H3C x2 x1
CH3
H3C Si CH3 H3C Si CH3 H H 1. Benzene/THF 2. CH3OH
b co m co sec-Bu n H H3C b x2 n m x1
CH3
H3C Si CH3
H
H3C Si CH3
H
Scheme 4.16 Synthesis of the thermoplastic elastomer (TPE).
150 a 90/10 mol/mol ratio. The polymerization was initiated using sec-BuLi as the initiator and was done in benzene at room temperature. After polymerization of the first block was completed, the desired amount of isoprene was added to the system containing the living polymer chain ends. The polymerization was allowed to proceed at 50 ºC. To this polymer system, once again a mixture of styrene and (4-vinylphenyl)dimethylsilane
(90/10 mol/mol) was added. However, before addition of this mixture, 3-4 equivalents of
THF was added to the polymerization system. Addition of THF helps to dissociate the aggregates, making the crossover from a polyisoprene chain end to a polystyrene chain end more efficient19. The weight ratios of the three blocks of the TPE was expected to be 12/76/12. The SEC of the final polymer obtained is shown in Figure 4.64.
In the 1H NMR spectrum (Figure 4.65) of this polymer, the different peaks corresponding to protons in the polymer have been labeled. The small peak at δ 4.4 ppm is from the proton in the Si-H group. This peak is so small since the number of styrene units with Si-H groups on them is very small. The peak labeled (a) corresponds to the six methyl protons adjacent to the silicon atom. The peak labeled (b) is from one proton in the polyisoprene unit with a 1,4-microstructure19. The peak labeled (c) is from two protons in the polyisoprene unit having a 3,4-microstructure19. These two peaks were integrated with respect to each other and from the integration values it was observed that about 90% of the polyisoprene block had a 1,4-microstructure and the remaining portion exhibits a 3,4-microstructure. High amounts of 1,4-microstructure in the polydiene unit of a TPE is desirable since it results in a material with better elastomeric properties.
151
Mn (calc) = 100,000 g/mol Mn (obs) = 130,000 g/mol PDI = 1.08
R. I.
Figure 4.64 SEC of the thermoplastic elastomer.
The better properties arise due to the lower Tg of a polydiene segment with high 1,4- microstructure19. The peak labeled (a) is from the six methyl protons bonded to the silicon atom in the styrenic copolymer blocks. This peak (a) was integrated with respect to the peak labeled (c), which is due to the proton in the 3,4 part of the polyisoprene block. The calculated number average molecular weight of the styrenic copolymer blocks is 24,000 g/mol. Since 90 % of this copolymer is comprised of styrene units (104 g/mol), the number of styrene units in the copolymer blocks is [(24,000 x 90)/100]/104 = 205 styrene units. The remaining 10 % of the styrenic copolymer blocks is made of (4- vinylphenyl)dimethylsilane (162 g/mol) units. So, the number of (4-vinylphenyl)- dimethylsilane units is the copolymer is [(24,000 x 10)/100]/162 = 14 units. So, the
152 expected number of methyl protons bonded to the silicon atom in the TPE is 14 x 6 = 84
Hs. The calculated number average molecular weight for the polyisoprene block is
76,000 g/mol and 10 % of it has a 3,4 microstructure as was observed from the 1H NMR spectrum (Figure 4.65). Since the mass of one repeating unit of polyisoprene is 69 g/mol, the number of isoprene units with a 3,4-microstructure are [(76,000 x 10)/100]/69 = 101.
So, the number of (c) protons expected in the TPE are 101. The expected ratio of (a) to
(c) is 0.83. The ratio of (a) to (c) obtained from the 1H NMR spectrum of the copolymer is 12.27/15.43 = 0.8. This corresponds to 96 % conversion.
(cis + trans) (g) (g) co (b) (e) sec-Bu m n H (e) co CH3 (e) x1 m (d) (f) x2 n
H3C (d) (c) H C Si CH 3 3 (a) (f)
H H3C Si CH3 (a) H (d)
(b) (c) SiH
(a)
69
Figure 4.65 1H NMR spectrum of the TPE.
153 The 13C NMR spectrum of the TPE is shown in Figure 4.66 and the various peaks have been labelled19. The large number of peaks in this spectrum is due to the diverse microstructure of the polyisoprene block.
(b, cis-1,4) (c, trans-1,4) (h, cis-1,4) (i, trans-1,4)
co sec-Bu m n H co CH3 x1 m x2 n
(f, cis-1,4) H3C (g, trans-1,4) (j, 3,4) H3C Si CH3 (a)
H H3C Si CH3 (a)
(k, 3,4) H
(d, cis-1,4) (b) (e, trans-1,4) (l, 3,4) (c) (d)
(f, h, g, i) (k) (j) (l) (e) (a)
70
Figure 4.66 13C NMR spectrum of the TPE.
From the 13C NMR spectrum (Figure 4.66) of this polymer it can be seen that the polyisoprene unit exhibits two microstructures, i.e. 1,4 and 3,4. The carbon atoms in the part of the polydiene with a 1,4 microstructure were labeled as follows: δ 28.5 (b), 33.2
(c), 26.7 (d), 18.3 (e), 131.8 (f/g) and 124.3 (h/i) ppm19. The peaks in the 3,4 part of the polydiene were labeled as shown: δ 20.8 (j), 113.3 (k) and 148.6 (l) ppm19. Also part of the double bonds in the polyisoprene segments have a cis configuration and the others have a trans configuration. 154 DSC measurements were done on this material to measure the glass transition temperatures of the two blocks in this polymer. The DSC is shown in Figure 4.67 and as expected it exhibits two different transitions. The transition at -57 ºC (-60 to -70 ºC)19 corresponds to the polyisoprene block and the transition at 97 ºC is due to the poly(styrene-co-(4-vinylphenyl)dimethylsilane) block. The glass-transition temperature was taken to be the point at the middle of the transition. The glass-transition temperature that was obtained for poly(4-vinylphenyl)dimethylsilane (Mn = 3300 g/mol) was 90 ºC.
Temperature
Figure 4.67 DSC of the thermoplastic elastomer (TPE).
The Young’s modulus of an elastomeric material is obtained from the slope of the straight line obtained by plotting the initial stress versus strain of that material at 400 % elongation). The stress versus strain curve of this TPE is shown in Figure 4.68. The
Young’s modulus can be measured from the slope of the linear portion at the beginning
155 of the curve. The value obtained is 1.84 x 105 Pa, which is in good agreement with the value of the Young’s modulus of a regular (with unfunctionalized styrene units)
TPE (1.9 x 105 Pa)160, 163,164.
Figure 4.68 Measurement of the Young’s Modulus of the TPE.
The tensile strength of this material was measured by doing tensile tests on the TPE.
The styrene-diene-styrene block copolymers are known to exhibit a stress-strain behavior similar to that of vulcanized elastomers. The stress-strain behavior of TPEs are only dependent on the composition (20-40 wt % block polystyrene content) and independent of the molecular weight when the polystyrene blocks are in the useful molecular weight range (10,000-20,000 g/mol). In this composition range (20-40 wt % block styrene) the material exhibits tensile strengths of 15-32 MPa160. The first step was to make films of the polymer by use of compression molding. Since this material behaves like a pressure
156 sensitive adhesive so it was necessary to use pressure release molds in order to prevent the material from sticking to the mold used for compression molding. Compression molding was done by putting a weighed amount of the polymer (approximately 5 gms) in a 15 x 5 x 0.05 cm metal frame between two metal plates. These plates were preheated for half an hour to 95 ºC. After this period the polymer was compressed by increasing the load on the two plates. The film obtained was translucent and did not appear to have any air bubbles or air channels which could result in erroneous values of tensile strength of the material. Dog-bone shaped samples were cut out of this film using a die of the desired size. These samples were pulled apart in an Instron at a rate of 20 in/min and the maximum stress that the material could withstand was determined. Three different measurements were done and similar values of tensile strength were obtained in all cases.
A typical stress versus strain curve obtained is shown in Figure 4.69. It is seen that the material can withstand a maximum stress of 15 MPa (2174 psi) at about 1000 % elongation.
157
Figure 4.69 Stress versus strain curve of the thermoplastic elastomer.
DMTA (Dynamic mechanical testing analysis) is a technique where a sinusoidally varying stress is applied to a material. DMTA measurements were done on this material and the results obtained are rich in information. Curves for the storage modulus (E’), the loss modulus (E’’) and tan δ were obtained (Figure 4.70). From these curves the glass transition temperatures of the two blocks of the polymer could be obtained. The values are in good agreement with the values obtained from DSC measurements (Tg,1 = -61 ºC and Tg,2 = 100 ºC).
158 E’ tan δ
E”
E’ (Pa) Tan δ E” (Pa)
Figure 4.70 DMTA measurements of the TPE.
Transmission Electron Microscopy (TEM) was done on the polymer in order to study the morphology of this TPE. Styrene-diene block copolymers are known to be capable of forming primarily five different types of morphologies: spheres, hexagonally packed cylinders, lamellae, hexagonally perforated layers and bicontinuous gyroids162-164. The morphology obtained depends upon three parameters: the volume fraction of each polymer segment, the Flory interaction parameter and the molecular weight. The TEM image obtained for the TPE is shown in Figure 4.71. The prepared sample was stained with osmium tetroxide in order to differentiate between the polystyrene and the polyisoprene domains. The osmium tetroxide was expected to stain the polyisoprene domains thus making them appear darker. However surprising results were obtained in the TEM image. Darker spherical domains (corresponding to polystyrene) were seen to be dispersed in a lighter matrix (polyisoprene). These results can be explained since the silyl hydride units in the polystyrene domain of this TPE can easily be oxidized by
159 a suitable oxidizing agent73, which is osmium tetroxide in this case. This oxidation of the polystyrene domains can explain the reverse staining of the two different blocks of the
TPE. The TEM image shows a spherical morphology which is what is expected based on the volume fraction of the two different polymer domains.
Figure 4.71 TEM image of the thermoplastic elastomer.
From the above results it can be concluded that it is possible to prepare a thermoplastic elastomer by using a copolymer of styrene and (4-vinylphenyl)- dimethylsilane, which exhibits properties similar to that of a thermoplastic elastomer having unmodified styrene units. The TPE that we have synthesized, (with silyl hydride functionalized styrene units) can be easily modified by introduction of different functional groups onto the styrenic blocks, by use of hydrosilation reactions, thus giving materials with desired properties. 160
CHAPTER V
SUMMARY
Chain-end and in-chain functionalized polymers were synthesized using alkyl- lithium initiated, living anionic polymerizations followed by hydrosilation reactions.
Chain-end functionalized polymers were synthesized by a simple two-step procedure, where the living poly(styryl)lithium chains were terminated with chlorodimethyl- silane to produce silyl-hydride terminated polymer chains. These silyl-hydride functional groups were further converted to different functional groups (cyanide, allyl ether) by use of hydrosilation reactions between the Si-H groups at the chain-ends and the double bonds of commercially available compounds (allyl cyanide, allyl ether). The reactions were catalyzed using 2-3 drops of a platinum-based catalyst,
Karstedt’s catalyst. Since the hydrosilation reactions proceed under mild reaction conditions, the functional groups were stable during the reaction.
A new living anionic polymerization method was developed to synthesize well- defined in-chain functionalized polymers. The monomer, (4-vinylphenyl)dimethyl- silane was synthesized and polymerized using sec-BuLi as the initiator in a hydrocarbon solvent at room temperature. The polymerization was observed to proceed in a controlled manner and polymers of high (Mn = 30000 g/mol) and low
(Mn = 3000 g/mol) molecular weights were synthesized and characterized. This
161 polymer, poly(4-vinylphenyl)dimethylsilane has a silyl-hydride group in every monomer unit in the polymer chain. Hydrosilation reactions were done between these silyl-hydride groups on the polymer chain and the double bonds present on commercially available compounds, to incorporate different functional groups on the polymer chain. There is no known simple method to synthesize well-defined, anionic polymers with epoxide functional groups in the polymer chain. However, a simple hydrosilation reaction done between the Si-H groups in poly(4-vinylphenyl)dimethyl- silane and the double bond of allyl glycidyl ether, gave a well-defined polymer with epoxide-functional groups in the polymer chain. These epoxide groups were further hydrolyzed to produce a polymer with a diol-functionality on every monomer unit.
The resultant polymeric product was soluble in many solvents, such as, methanol, ethanol and isopropanol. Due to the interesting properties (chemical resistance, contact angle, refractive index) exhibited by fluorinated polymers, a perfluoralkyl group was introduced on poly(4-vinylphenyl)dimethylsilane, by using hydrosilation reactions. The functionalized polymer obtained was characterized and it exhibited many interesting properties characteristic of fluorinated compounds.
Experiments were done to study the copolymerization behavior of styrene with
(4-vinylphenyl)dimethylsilane by synthesizing a series of copolymers at different mol/mol ratios of the two monomers (90/10, 80/20, 50/50, 30/70). Monomer reactivity ratios were determined by using three different methods, Fineman-Ross,
Kelen-Tudos and Error-In-Variable (EVM) method. All three methods produced values of monomer reactivity ratios in good agreement with each other [rS (avg) =
162 1.74, rSi (avg) = 0.16]. From these values it could be concluded that the monomer, (4- vinylphenyl)dimethylsilane is preferentially incorporated into the polymer chain with respect to the styrene monomer. The product of the two monomer reactivity ratios approached zero, rather being close to unity (rSrSi = 0.27). This indicates that the system exhibits a tendency towards an alternating copolymerization as opposed to a random structure.
Amino acid-functionalized polymers (biohybrids) were synthesized by first making a copolymer of styrene with (4-vinylphenyl)dimethylsilane (90/10, mol/mol) followed by a hydrosilation reaction between the silyl-hydride groups on the copolymer chain and the double bond of allylamine. The amine-functional groups on the copolymer were then reacted with the free carboxyl group on N-carbobenzyloxy- phenylalanine (N-cbz-Phe), by use of a simple condensation reaction. This produced a polymer with the amino acid (N-cbz-Phe) group on it. So, a simple, efficient three step synthesis could be used to make polymer biohybrids. The versatility of this technique arises from the fact that it can be extended to incorporate virtually any amino acid or peptide sequence into the copolymer chain. Thus, this methodology provides a technique to prepare a variety of amino acid- and peptide- functionalized, well-defined, polymers by a combination of living anionic polymerization, hydrosilation and a simple condensation reaction.
Anionically synthesized thermoplastic elastomers (TPE) are very commercially useful, but there is no simple method to introduce functional groups into the styrene blocks of these polymers. So, a TPE was synthesized using the monomer (4- vinylphenyl)dimethylsilane. The silyl-hydride units in the resultant polymer can then
163 be easily converted to different functional groups by using simple hydrosilation chemistry. The first block of this TPE is a copolymer of styrene with (4-vinylphenyl)- dimethylsilane taken in a 90/10 mol/mol ratio.This is followed by a block of polyisoprene and finally another block of the copolymer of styrene with (4- vinylphenyl)dimethylsilane. The polymer obtained was characterized and by 1H and
13C NMR. Tensile testing, DMTA measurements and DSC were done on this polymer to determine it’s properties. It was observed that this polymer exhibits properties similar to a regular thermoplastic elastomer. TEM was done on thin films of this TPE, which were obtained by cryo-ultra-microtoming. It was observed that the styrenic blocks of this copolymer exist as spherical domains in a polyisoprene matrix. This morphology is expected from the molar ratio of the polystyrene to the polyisoprene segments in the polymer. So, it was possible to anionically synthesize a thermoplastic elastomer containing segments of (4-vinylphenyl)dimethysilane, which can easily be coverted to other functional groups to help in modification of many properties of the
TPE.
164
REFERENCES
1. Szwarc, M. Nature, 1956, 178, 1168.
2. Szwarc, M.; Levy, M.; Milkovich, R. J. Am. Chem. Soc. 1956, 78, 2656.
3. Szwarc, M. Carbanions, Living Polymers and Electron Transfer Processes; Interscience: New York, 1968.
4. Rempp, P.; Franta, E.; Herz, J.-E. Adv. Polym. Sci. 1988, 86, 145.
5. Kennedy, J. P.; Ivan, B. Designed Polymers by Carbocationic Macromolecular Engineering: Theory and Practice; Hanser Publishers: Munich, 1992.
6. Hadjidichristidis, N.; Pitsikalis, M.; Pispas, S.; Iatrou, H. Chem. Rev. 2001, 101, 3747.
7. Bauer, B. J.; Fetters, L. J. Rubber Chem. Tech. 1978, 51, 406.
8. Bywater, S. Adv. Polym. Sci. 1979, 30, 89.
9. Roovers, J. In Encyclopedia of Polymer Science and Engineering, 2nd ed.; Kroschwitz, J. I., Ed.; Wiley: New York, 1987, Vol. 2, p 478.
10. Quirk, R. P.; Lee, B. Polym. Int. 1992, 27, 359.
11. Henderson, J. F.; Swarcz, M. J. Polym. Sci., Macromol. Rev. 1968, 3, 317.
12. Penczek, S.; Kubisa, P.; Szymanski, R. Makromol. Chem., Rapid Commun. 1991, 12, 77.
13. Hsieh, H. L.; McKinney, O. F. Polym. Lett. 1966, 4, 843.
14. VerStrate, G.; Cozewith, C.; Ju, S. Macromolecules 1988, 21, 3360.
15. Nakahama, S.; Hirao, A. Prog. Polym. Sci. 1990, 15, 299.
165 16. Zilkha, A.; Feit, B.–A.; Frankel, M. J. Polym. Sci. 1961, 49, 231.
17. Eromosele, I. C.; Pepper, D. C.; Ryan, B. Makromol. Chem. 1989, 190, 1613.
18. Winstein, W.; Clippinger, E.; Fainberg, A. H.; Robinson, G. C. J. Am. Chem. Soc. 1954, 76, 2597.
19. Hsieh, H. L.; Quirk, R. P. In Anionic Polymerization: Principles and Practical Applications; Marcel Dekker: New York, 1996.
20. Milner, R.; Young, R. N.; Luxton, A. R. Polymer 1983, 24, 543.
21. Bywater, S.; Worsfold, D. J. J. Organometal. Chem. 1967, 10, 1.
22. Mathews, F. E.; Strange, E. H. British Patent, 1910, 24, 790.
23. Bywater, S. In Encyclopedia of Polymer Science and Engineering, 2nd ed.; Kroshwitz, J. I., Ed.; Wiley-Interscience: New York, 1985; Vol 2, p 1.
24. Quirk, R. P.; Ma, J.–J. Polym. Int. 1991, 24, 197.
25. Bywater, S.; Worsfold, D. J. Can. J. Chem. 1962, 40, 1564.
26. Alexander, I. A.; Bywater, S. J. Polym. Sci., A1, 1968, 6, 3407.
27. Worsfold, D. J.; Bywater, S. J. Chem. Soc. 1960, 5234.
28. Fetters, L. J.; Morton, M. Macromol. Syn. 1972, 4, 77.
29. Graham, G. D.; Marynick, D. S.; Lipscomb, W. N. J. Am. Chem. Soc. 1980, 102, 4572.
30. Bywater, S. In Comprehensive Chemical Kinetics, Non-Radical Polymerization; Bamford, C. H. and Tipper, C. F. H., Eds.; Elsevier: New York, 1976, p 1.
31. Hsieh, H. L.; Glaze, W. H. Rubber Chem. Tech. 1970, 43, 22.
32. Finnegan, R. A.; Kutta, H. W. J. Org. Chem. 1965, 30, 4138.
33. Glaze, W. H.; Lin, J.; Felton, E. G. J. Org. Chem. 1965, 30, 1258.
34. Morton, M.; Rupert, J. R. In Initiation of Polymerization Bailey, F. E. Jr., Ed.; ACS Symposium Series 212, American Chemical Society: Washington D. C., 1983, p 284.
166 35. Gerbert, W.; Hinz, J.; Sinn, H. Makromol. Chem. 1971, 144, 97.
36. Worsfold, D. J.; Bywater, S. Macromolecules 1978, 11, 582.
37. Ohlinger, R.; Bandermann, F. Makromol. Chem. 1980, 181, 1935.
38. Morton, M. Anionic Polymerization, Principles and Practice; Academic Press, Inc.: New York, 1983.
39. Mayo, F. R.; Walling, C. Chem. Rev. 1950, 46, 191.
40. Zhou, J. –P. Ph.D Dissertation, The University of Akron, 2001.
41. Fineman, M.; Ross, S. D. J. Polym. Sci. 1950, 5, 259.
42. Tudos, S.; Kelen, T. J. Macromol. Sci. Chem. 1981, A16 1283.
43. Behnken, D. W. J. Polym. Sci. Part A 1964, 2, 645.
44. Polic, L.; Duever, T. A.; Pendilis, A. J. Polym. Sci.: Part A: Polym. Chem. 1997, 36, 813.
45. Patino-Leal, H.; Reilly, P. M.; O’Driscoll, K. F. J. Polym. Sci., Polym. Lett. 1980, 18, 219.
46. Telechelic Polymers: Synthesis and Applications; E. J. Goethals, Ed.; CRC Press: Boca Raton, Florida, 1980.
47. Schulz, D. N.; Sanda, J. C.; Willoughby, B. C. In Anionic Polymerization Kinetics Mechanisms and Synthesis; J. E. McGrath, Ed.; ACS Symposium Series No. 166: American Chemical Society, Washington D. C., 1981; p 427.
48. Reactive Oligomers; Harris, F. W. and Spinelli, H. J., Eds.; ACS Symposium Series No. 282: American Chemical Society, Washington, D. C., 1985.
49. Rempp, P. F.; Franta, E. Adv. Polym. Sci. 1984, 58, 1.
50. Brosse, J. –C.; Derouet, D.; Epaillard, F.; Soutif, J. –C.; Legeay, G.; Dusek, K. Adv. Polym. Sci. 1987, 81, 167.
51. Akelah, A. J. Mat. Sci. 1986, 21, 2977.
52. Paulus, G.; Jerome, R.; Teyssie, P. Brit. Polym. J. 1987, 19, 361.
167
53. Functional Polymer, Bergbreiter, D. E. and Martin, C. R., Eds.; Plenum Press: New York, 1989.
54. Quirk, R. P.; Yin, J.; Guo, S.–H.; Hu, X.–W.; Summers, G.; Kim, J.; Zhu, L.; Schock, L. E. Makromol. Chem., Macromol. Symp. 1990, 32, 47.
55. Quirk, R. P. In Comprehensive Polymer Science; First Supplement, S. L. Agarwal and S. Russo, Eds.; Pergamon Press, Elmsford: New York, 1992, p 83.
56. Quirk, R. P.; Yin, J.; Guo, S. –H.; Hu, X. –W.; Summers, G.; Kim, J.; Zhu, L. F.; Ma, J. –J.; Takizawa, T.; Lynch, T. Rubber Chem. Tech. 1991, 64, 648.
57. Gnanou, Y. Ind. J. Technol. 1993, 31, 317.
58. Goethals, E. J. In Ring-Opening Polymerization. Mechanisms, Catalysis, Structure Utility; Brunelle, D. J., Ed.; Hanser: New York, 1993, p 295.
59. Quirk, R. P.; Kim, J. In Ring-Opening Polymerization. Mechanisms, Catalysis, Structure Utility,Brunelle, D. J.; Ed.; Hanser: New York, 1993, p 263.
60. Mays, J. W.; Hadjichristidis, N. Polym. Bull. 1989, 22, 471.
61. Richards, D. H.; Eastmond, G. C.; Stewart, M. J. In Telechelic Polymers: Synthesis and Applications; Goethals, E. J., Ed.; CRC Press: Boca Raton, Florida, 1989, p 33.
62. Fontainille, M. In Comprehensive Polymer Science, Chain Polymerization I; Eastmond, G. C.; Ledwith, A.; Russo, S.; Sigwalt, P., Eds.; Pergamon Press: Elmsford, New York, 1989; Vol. 3, p 425.
63. Fetters, L. J. J. Polym. Sci., Macromol. Rev. 1967, 2, 71.
64. Morton, M.; Fetters, L. J. J. Polym. Sci., Part C. 1969, 26, 1.
65. Bywater, S. Prog. Polym. Sci. 1974, 4, 27.
66. Wakefield, B. J. In Organolithium Methods; Academic Press: San Diego, California, 1988.
67. Brandsma, L.; Verkuijsse, H. D. In Preparative Polar Organometallic Chemistry; Springer-Verlag: Berlin, 1987; Vol. 1.
68. Bates, R. B.; Ogle, C. A. Carbanion Chemistry; Springer-Verlag: Berlin, 1983.
168
69. Wakefield, B. J. The Chemistry of Organolithium Compounds; Pergamon Press Elmsford, New York, 1974.
70. Wardell, J. L. In The Chemistry of the Metal-Carbon Bond; F. R. Hartley, Ed.; Wiley: New York, 1987; Vol. 4, p 1.
71. Wakefield, B. J. In Comprehensive Organometallic Chemistry; G. Wilkinson, Ed.; Pergamon: Oxford, 1982; Chapter 44.
72. Hunt, M. O.; Belu, A. M.; Linton, R. W.; Desimone, J. M. Macromolecules 1993, 26, 4854.
73. Kim, H. Ph. D. Dissertation, The University of Akron, 2005.
74. Szwarc, M. Adv. Polym. Sci. 1983, 49, 1.
75. Laita, Z,; Szwarc, M. Macromolecules 1969, 2, 412.
76. Busson, R.; van Beylen, M. Macromolecules 1977, 10, 1320.
77. Ziegler, K.; Gellert, H. G.; Justus Lieb. Ann. Chem. 1950, 567, 179.
78. Köbrich, G.; Stober, I. Chem. Ber. 1970, 103, 2744.
79. Wittig, G.; Scholpokf, U. Chem. Ber. 1954, 87, 1318.
80. Marcienec, B. In Comprehensive Handbook on Hydrosilylation; Pergamon Press: Oxford, 1992.
81. Ojima, I. In The Chemistry of Organic Silicon Compounds; Patai, S.; Pappoport, Z., Eds.; John Wiley and Sons: New York, 1989.
82. Marcienec, B. Silicon Chemistry 2002, 1, 155.
83. Brunner, H. Angew. Chem. Int. Ed. Engl. 2004, 43, 2749.
84. Chalk, A. J.; Harrod, J. F. J. Am. Chem. Soc. 1965, 87, 16.
85. Schroeder, M. A.; Wrighton, M. S. J. Organomet. Chem. 1977, 128, 345.
86. Lewis, L. N.; Lewis, N. J. Am. Chem. Soc. 1986, 108, 7228.
87. Lewis, L. N.; Lewis, N. Chem. Mater. 1989, 1, 106.
169
88. Jayaraman, R. B.; Facinelli, J. V.; Riffle, J. S.; George, S. E. J. Polym. Sci., Part A: Polym. Chem.1996, 34, 1543.
89. Loos, K.; Müller, A. H. E. Biomacromolecules 2002, 3(2), 368.
90. Quirk, R. P.; Kim, H. Macromolecules 2005, 38(19), 7895.
91. Nakahama, S.; Hirao, A. Prog. Polym. Sci. 1990, 15, 299.
92. Mays, J. W.; Hadjichristidis, N. Polym. Bull. 1989, 22, 471.
93. Quirk, R. P.; Sarkis, M. T.; Meier, D. J. In Advances in Elastomers and Rubber Elasticity; Lal, J. and Mark, J. E., Eds.; Plenum Press: New York, 1986, p 143.
94. Chen, J. C.; Fetters, L. J. Polym. Eng. Sci. 1987, 27, 1300.
95. Conlon, D. A.; Crivello, J. V.; Lee, J. L.; O’Brien, M. J. Macromolecules 1989, 22, 509.
96. Yuki, H.; Okamoto, Y.; Kuwae, Y.; Hatada, K. J. Polym. Sci.. A1 1969, 7, 1933.
97. Geerts, J.; Van Beylen, M.; Smets, G. J. Polym. Sci., A1 1969, 7, 2859.
98. Se, K.; Kijima, M.; Fujimoto, T. Polym. J. 1988, 20, 791.
99. Shima, M.; Ogawa, E.; Konishi, K. Makromol. Chem. 1976, 177, 241.
100. Ogawa, E.; Shima, M. Polym. J. 1986, 20, 791.
101. Konigsberg, I.; Jagur-Grondinski, J. J. Polym. Sci. Polym. Chem. Ed. 1983, 21, 2535.
102. Konigsberg, I.; Jagur-Grondinski, J. J. Polym. Sci. Polym. Chem. Ed. 1983, 21, 2649.
103. Narita, T.; Hagiwara, T.; Hamana, H.; Irie, H.; Sugiyama, H. Polym. J. 1987, 19, 985.
104. Chaumont, P.; Beinert, G.; Herz, J.–E.; Rempp, P. Makromol. Chem. 1982, 183, 1181.
105. Hatada, K.; Okamoto, Y.; Kitayama, T.; Sasaki, S. Polym. Bull. 1983, 9, 228.
170 106. Okay, O.; Funke, W. Macromolecules 1990, 23, 2623.
107. Nagasaki, Y.; Ito, H.; Tsuruta,T. Makromol. Chem. 1986, 187, 23.
108. Kajiwara, K.; Suzuki, H.; Inagaki, H.; Maeda, M.; Tsuruta, T. Makromol. Chem. 1986, 187, 2257.
109. Nagasaki, Y.; Tamura, Y.; Tsuruta, T. Makromol. Chem., Rapid Commun. 1988, 9, 31.
110. Nakagawa, H.; Matsushita, Y. Tsuge, S. Polymer 1987, 28, 1512.
111. Nasirova, R. M.; Murav’eva, L. S.; Mushina, E. A.; Krentsel, B. A. Russ. Chem. Rev. 1979, 48, 692.
112. Worsfold, D. J. Macromolecules 1970, 3, 514.
113. Young, R. N.; Fetters, L. J. Macromolecules 1978, 11, 899.
114. Ishizone, T.; Hirao, A.; Nakahama, S. Macromolecules 1991, 24, 625.
115. Ishizone, T.; Wakabayashi, S.; Hirao, A.; Nakahama, S. Macromolecules 1991, 24, 5015.
116. Whicher, S. J.; Brash, J. L. J. Appl. Polym. Sci. 1985, 30, 2297.
117. Whicher, S. J.; Brash, J. L. J. Polym. Sci., Polym. Chem. Ed. 1981, 19, 1995.
118. Kase, T.; Imahori, M.; Kazama, T.; Isono, Y.; Fujimoto, T. Macromolecules 1991, 24, 1714.
119. Ishizone, T.; Hirao, A.; Nakahama, S. Macromolecules 1989, 22, 2895.
120. Yamakazi, N.; Nakahama, S.; Hirao, A.; Goto, J.; Shiraishi, Y.; Martinez, F.; Phung, H. M. J. Macromol. Sci. Chem. 1981, 16, 1129.
121. Hirao, A.; Shiraishi, Y.; Martinez, F.; Phung, H. M.; Nakahama. S.; Yamakazi, N. Makromol. Chem. 1983, 184, 961.
122. Morton. M.; Fetters, L. J. Rubber Chem. Tech. 1975, 48, 359.
123. Sekutowski, D. G.; Stucky, G. D. J. Chem. Ed. 1976, 53, 110.
124. Shriver, D. F.; Drezdon, M. A. In The Manipulation of Air-Sensitive Compounds, Eds.; Wiley: New York, 1986.
171
125. Hirao, A.; Hatayama, T.; Nakahama, S. Macromolecules 1987, 20, 1505.
126. Gilman, H.; Cartledge, F. K. J. Organomet. Chem. 1964, 2, 447.
127. Baltork, M. I.; Tangestaninejad, S.; Aliyan, S.; Mirkhani, V. Synth. Commun. 2000, 30(13), 2365.
128. Bernard, H. J.; Sears, M. L.; Corcoran, R. C.; Carron, K. T. Anal. Chem. 1994, 66, 1572.
129. Grubisic, Z.; Rempp, P.; Benoit, H. J. Polym. Sci. 1967, B2, 753.
130. Still, W. C.; Kahn, M.; Mitra, A. J. Org. Chem. 1978, 43, 2923.
131. Patai, S. In The Chemistry of the functional groups, Eds.; Wiley: New York, 1964.
132. Ishizone, T.; Okazawa, Y.; Ohnuma, K.; Hirao, A.; Nakahama, S. Macromolecules 1997, 30, 757.
133. Katsuhiko, T.; Tomoo, S.; Yashuhiro, H.; Hirao, A.; Nakahama, S. Polym. Prep. (Am. Chem. Soc., Polym. Chem) 1996, 37(2), 658.
134. Quirk, R. P.; Ma, J.-J. J. Polym. Sci., Part A: Polym. Chem. 1988, 26, 2031.
135. Silverstein, R. M.; Webster, F. X. In Spectroscopic Identification of Organic Compounds, 6th ed.; Wiley: New York, 1998.
136. Han, Y.; Pearce, E. M.; Kwei, T. K. Macromolecules 2000, 33, 1321.
137. Bordwell, F. G.; Algrim, D. J. J. Am. Chem. Soc. 1988, 110, 2964.
138. Takenaka, K.; Hirao, A.; Nakahama, S. Polym. Int. 1995, 37, 291.
139. Quirk, R. P.; Zhuo, Q. Macromolecules 1997, 30, 1531.
140. Hunt, M. O.; Belu, A. M.; Linton, R. W.; DeSimone, J. M. Macromolecules 1993, 26, 4854.
141. Zhou, Z.; He, Y.; Weng, Z.; Qu, Y.; Zhao, C. J. Fluorine Chem. 1996, 79, 1.
142. Borkar, S.; Jankova, K.; Siesler, H. W.; Hvilsted, S. Macromolecules 2004, 37, 788.
172
143. Odian, G., Principles of Polymerization, 3rd ed., Wiley, New York, 1991.
144. Watanabe, H.; Ohsawa, N.; Sawai, M.; Fukasawa, Y.; Matsumoto, H. J. Organomet. Chem.. 1975, 93(2), 173.
147. Hawker, C. J.; Wooley, K. L. Science 2005, 19, 1200.
148. Becker, M. L.; Liu, J.; Wooley, K. L. Chem. Commun. 2003, 180.
149. Schwarze, S. R.; Ho, A.; Vocero-Akbani, A. Science, 1995, 285, 1569.
150. Fujimoto, K.; Iwasaki, C.; Arai, C.; Kuwako, M; Yasugi, E. Biomacromolecules, 2001, 1, 515.
151. Klok, A.,-K.; Lecommandoux, S. Adv. Mater. 2001, 13, 1217.
152. Watson, K. J.; Park, S. J.; Im, J.; Nquyen, S. T.; Mirkin, C. A. J. Am. Soc. 2001, 123, 5592.
153. Becker, M. L.; Liu, J.; Wooley, K. L. Chem. Commun. 2003, 180.
154. Becker, M. L.; Liu, J.; Wooley, K. L. Biomacromolecules, 2005, 6, 220.
155. Becker, M. L.; Remsen, E. E.; Pan, D.; Wooley, K. L. Bioconjugate Chem. 2004, 15, 699.
156. Mei, Y.; Beers, K. L.; Byrd, H. C. M.; VanderHart, D. L.; Washburn, N. R. J. Am. Chem. Soc. 2004, 126, 3472.
157. Young, R. N.; Quirk, R. P.; Fetters, L. J. Adv. Polym. Sci. 1984, 56, 1.
158. Rempp, P.; Franta, E.; Herz, J. –E. Adv. Polym. Sci. 1988, 86, 145.
159. Szwarc, M. Adv. Polym. Sci. 1983, 49, 1.
160. Quirk, R. P.; Roy Chowdhury, S. Polym. Prep .(Am. Chem. Soc., Div. Polym. Chem.) 2005, 46(2), 585.
161. Guo, X.; Farwaha, R.; Rempel, G. L. Macromolecules 1990, 24, 5047.
162. Baum, K.; Baum, J. C.; Ho, T. J. Am. Chem. Soc. 1990, 8, 2993.
163. Molau, G. E. In Block Polymers; Agarwal, S. L., Ed.; Plenum Press: New York- London, 1970, Ch 1.
173
164. Bates, F. S.; Glenn, F. H. Ann. Rev. Phys. Chem. 1990, 41, 525.
165. Holden, G. In Thermoplastic Elastomers; Holden, G., Kricheldorf, H. R., Quirk, R. P., Eds., Hanser Publishers, Munich, 2004.
174