Poly(amide-graft-acrylate) interfacial compounds

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Title:
Poly(amide-graft-acrylate) interfacial compounds
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xiv, 182 leaves : ill. ; 29 cm.
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English
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Zamora, Michael Perez, 1970-
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Materials Science and Engineering thesis, Ph.D   ( lcsh )
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bibliography   ( marcgt )
non-fiction   ( marcgt )

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Thesis:
Thesis (Ph.D.)--University of Florida, 1997.
Bibliography:
Includes bibliographical references (leaves 173-180).
Statement of Responsibility:
by Michael Perez Zamora.
General Note:
Typescript.
General Note:
Vita.

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University of Florida
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oclc - 38854827
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POLY(AMIDE-GRAFT-ACRYLATE) INTERFACIAL COMPOUNDS


By

MICHAEL PEREZ ZAMORA

























A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


1997







































This dissertation is dedicated to all those who supported me
emotionally, physically, and financially throughout my
seemingly endless years of education; especially my wife
Karen, my parents Pablo and Sylvia, and my grandmother
Bella; and to the loving memory of Oscar Zamora and Manuel
Leon.

















ACKNOWLEG[IE[ITS


I cannot express enough gratitude and appreciation to

my advisor and doctoral committee chairman, Dr. Anthony

Brennan, for his years of teaching, guidance and support in

areas that extended well beyond my academic endeavors. I

would also like to express sincere thanks to the members of

my supervisory committee for their advice and teaching: Dr.

Jim Adair, Dr. Chris Batich, Dr. Eugene Goldberg, and Dr.

Ken Wagener.

I consider this experience substantially enriched by

the support of my colleagues. Whether there was

collaboration, debate or simply encouragement, it would have

been endlessly more difficult without the following: Dr.

Michael Antonell, Dr. Drew Amery, Craig Habeger, Jeff

Kerchner, Dr. James Marotta, Jeremy Mehlem, Dr. Rodrigo

Orefice, Luxsamee Plangsmangas, Mark Schwarz, and Dr. Chris

Widenhouse.

Special thanks are extended to Arthur Gavrin for his

assistance with NMR spectroscopy and for the lively

discussions in his short time here, and Ananth Naman and Dr.

Rob Chodelka for their unwavering support and friendship. I

would be remiss not to mention Jesse Arnold and Tom Miller,

my colleagues of five years with whom I have grown, matured,











and learned. I will forever be appreciative of our

continuous discussions, debates, and collaboration as well

as their undying support.
















TABLE OF CONTENTS


ACKNOWLEDGMENTS ......................................... iii

LIST OF TABLES ......................................... viii

LIST OF FIGURES .......................................... ix

ABSTRACT ................................................ xiii

1. INTRODUCTION ................ ................................ 1
1.1. Poly(amide-g-acrylate) Graft Copolymers ............ 1
1.1.1. Macromonomers ................................... 1
1.1.2. Addition-Condensation Graft Copolymers ........ 3
1.1.3. Applications .................................... 4
1.2. Offsetting Polymerization Shrinkage in Dental
Resins through the Incorporation of Maleic
Anhydride ............................................ 5

2. BACKGROUND ............................................. 7
2.1. Graft Copolymers ..................................... 8
2.1.1. Synthetic Routes to Graft Copolymers .......... 8
2.1.1.1. Anionically polymerized macromonomers..... 9
2.1.1.2. Macromonomers through chain transfer
functionalization ................................. 9
2.1.1.3. Modeling chain transfer reaction ......... 15
2.1.2. Polyamide Addition-Condensation Multiphase
Copolymers ......................................... 19
2.1.2.1. Poly(amide-b-olefin) block copolymers.... 19
2.1.2.2. Poly(amide-g-olefin) graft copolymers.... 28
2.2. Polymerization Shrinkage in Dental Composites..... 35
2.2.1. History of Dental Composites ................. 36
2.2.2. Reduction of Polymerization Shrinkage in
Dental Resins ..................................... 39
2.2.3. Use of Anhydrides in Dental Applications ..... 41
2.2.4. Filler Modification in the Reduction of
Overall Composite Shrinkage....................... 42

3. MATERIALS AND METHODS ................................... 44
3.1. Materials .......................................... 44
3.1.1. Macromonomer Reactants ...................... 44
3.1.2. Graft Copolymer Reactants .................... 45
3.1.3. Dental Monomers ............................... 45
3.2. Methods ........................................... 46










3.2.1. Synthesis and Characterization of
Macromonomers....................................... 46
3.2.2. Synthesis and Characterization of Graft
Copolymers ........................................... 49
3.2.3. Synthesis and Characterization of Anhydride
Modified Dental Resins............................ 52

4. RESULTS AND DISCUSSION ....... ...................... .... 55
4.1. Synthesis and Characterization of Amino Acid-
terminated Poly(acrylate) Macromonomers using
Chain Transfer Chemistry ........................... 55
4.1.1. Determination of an Appropriate Solvent
System............................................. 59
4.1.2. Preliminary Studies of the Effectiveness of
Cysteine as a Chain Transfer Agent in the
Polymerization of Butyl Acrylate................... 61
4.1.3. Determination of Chain Transfer Constant of
Cysteine in the Synthesis of Amino Acid-
terminated Poly(butyl acrylate) Macromonomers..... 75
4.1.4. Cysteine Chain Transfer in the Synthesis of
Amino Acid-terminated Poly(methyl
methacrylate:octafluoropentyl methacrylate)
Macromonomers..................................... 88
4.2. Polyamide-g-poly(acrylate) graft copolymers
from Amino Acid-terminated Macromonomers ............ 99
4.2.1. Synthesis of Poly(amide-g-butyl acrylate) .... 99
4.2.2. Characterization of Graft Copolymers ........ 104
4.2.3. Blends of Graft Copolymers with Nylon 6 ..... 139
4.3. Offsetting Polymerization Shrinkage in Dental
Resins through the Incorporation of Maleic
Anhydride .......................................... 144
4.3.1. Copolymer Compositions ..................... 146
4.3.2. Copolymer Characterization .................. 149

5. SUMMARY AND CONCLUSIONS ............................... 158
5.1. Chain Transfer Functionalization of
Poly(acrylates) and Poly(methacrylates) ............ 161
5.1.1. Conclusions for Preliminary Evaluation of
Cysteine Chain Transfer Agent .................... 161
5.1.2 Conclusions for Synthesis and
Characterization of Amino Acid-terminated
Poly(butyl acrylate) ............................ 162
5.1.3 Conclusions for Synthesis and
Characterization of Amino Acid-terminated
Poly(MMA-co-OFPMA) ................................ 162
5.2. Graft Copolymerizations of Macromonomers with
Polyamide Precursors ................................ 163
5.2.1. Conclusions for Synthesis of Poly(amide-g-
acrylate) Graft Copolymers. ...................... 164










5.2.2. Conclusions for Characterization of
Poly(amide-g-acrylate) Graft Copolymers.......... 165
5.2.3. Conclusions for Mechanical Properties of
Nylon 6/Graft Copolymer Blends.................... 166
5.3. Offsetting Polymerization Shrinkage in
Poly(dimethacrylate) Dental Resins ................. 166
5.3.1. Conclusions for Characterization of Maleic
Anhydride-containing Dental Resins................ 167
5.3.2. Conclusions for Synthesis and
Characterization of Anhydride Copolymer with
PEMA ............................................. 168

6. FUTURE WORK ............ . .................... .. .. ... 169
6.1. Macromonomers and Graft Copolymers................ 169
6.1.1. Macromonomer Work .......................... 169
6.1.2. Graft Copolymers ............................. 170
6.2. Anhydride-containing Dental Resins................ 171

LIST OF REFERENCES ....................................... 173

BIOGRAPHICAL SKETCH ...................................... 181













LIST OF TABLES


TABLE page


4.1 Solvent determination for monomer and chain
transfer agent ....................... .................. 60

4.2 Percent functionalization versus molar mass for
poly(butyl acrylate) macromonomers.. ................. 87

4.3 Molar mass values of fluoroacrylate copolymers from
GPC .. ................................................ 91

4.4 Percent weight loss from Soxhiet extraction for
polyamide graft copolymers ......................... 105

4.5 Chemical composition of purified graft copolymers from
elemental analysis and NMR ......................... 137

4.6 Inherent viscosities of polyamide graft copolymers. 138

4.7 Tensile properties of nylon 6 blends ............... 140

4.8 Experimental matrix of dental monomer compositions. 148

4.9 PEMA-maleic anhydride monomer compositions. ........ 149

4.10 EWC of maleic anhydride dental resins. ............. 150

4.11 Residual weight gain, anhydride incorporation and post
polymerization expansion of maleic anhydride dental
resins ............................................... 151

4.12 Glass transition temperatures and composition of PEMA-
maleic anhydride copolymers ........................ 155

4.13 Molar mass averages from GPC for PEMA-anhydride
copolymers ........................................... .157


viii



















LIST OF FIGURES


Figure page


1.1 Schematic illustration of general graft copolymer
structure ........................................... 1

2.1. Mechanism of functionalization using chain transfer
agents ............................................. 11

2.2. Schematic illustration of utilized chain transfer
agents and macromonomers thereof. ................... 16

2.3. Mechanism of block copolymer formation through
sequential polymerizations of vinyl monomer and
isocyanates .. ........................................ 21

2.4. Reaction schematic for macroinitiator formation and
subsequent anionic block polymerization of caprolactam.
. . . . . . .. . . . . . . . . . . . . . . . . . . .. 2 3

2.5 Reaction schematic of block copolymerization initiated
by nitrosated polyamide macroinitiators. ............ 25

2.6 Reaction schematic of AIBN containing polyamide and
block copolymer thereof ............................ 27

2.7 Reaction schematic of in situ graft copolymer formation
from glycidyl methacrylate copolymers. .............. 31

2.8 Reaction schematic of in situ graft copolymer formation
from maleic anhydride modified polyolefins copolymers..
. . . . . . . . . . . . .. . . . . . . . . . . . .. 33

2.9 Mechanism of ring opening polymerization of spiro
orthocarbonate monomers ............................ 40

4.1. Mechanism of functionalization using chain transfer
reactions .. .......................................... 57

4.2 Schematic of ideal amino acid functionalization during
polymerization of butyl acrylate. ................... 62












4.3 GPC results of preliminary butyl acrylate
polymerizations ... .................................... 64

4.4 DSC trace of side product .. ......................... 66

4.5 FTIR spectra of side product of cysteine modified
P (BA ) .............................................. 67

4.6 Comparison of FTIR spectra of side product with butyl
acrylate monomer..................................... 68

4.7 Reaction pathway of cysteine with acrylates. ........ 69

4.8 Structure and elemental analysis of side product. ... 70

4.9 Side reaction preventing complete chain transfer. ... 73

4.10 Effect of acidification on chain transfer
functionalization of poly(butyl acrylate). .......... 76

4.11 GPC results of polymerizations of butyl acrylate
varying cysteine concentrations. .................... 80

4.12 Mayo plot for chain transfer constant determination for
cysteine:butyl acrylate system ...................... 81

4.13 FTIR spectra of poly(butyl acrylate) and cysteine end-
capped p(BA) macromonomer. Macromonomer is the
1.2kg/mol p(BA) synthesized using 1000:64:1 butyl
acrylate:cysteine:AIBN mole ratio .................... 83

4.14 'H-NMR spectra of neat poly(butyl acrylate) ......... 84

4.15 'H-NMR spectra of 1.2kg/mol cysteine modified
poly(butyl acrylate) ................................ 85

4.16 13C-NMR spectra of 1.2kg/mol cysteine modified
poly(butyl acrylate) ................................ 86

4.17 Chemical structure of fluoroacrylate copolymer. ..... 90

4.18 GPC result of chain transfer polymerizations of
fluoroacrylate copolymers .. ......................... 91

4.19 Comparison of effectiveness of chain transfer for p(BA)
and fluoroacrylate copolymer ........................ 93

4.20 1H-NMR spectra of '1MA-OFPMA macromonomer ............ 95












4.21 Mayo plot for chain transfer constant determination for
cysteine:MMA-co-OFPMA system ........................ 96

4.22 FTIR spectra of MMA-OFPMA copolymers.. ............... 98

4.23 Amide-acrylate graft copolymer structure. .......... 100

4.24 Triphenyl phosphite driven amide formation. ........ 102

4.25 Synthesized graft copolymer compositions. .......... 103

4.26 Molar mass distributions of dissolved polymer in THF
extractant solution of 33PABA-g-66BA (refractive index
detector) .... ....................................... 107

4.27 UV spectra of dissolved polymer in THF extractant
solution of 33PABA-g-66BA from GPC-UV.. ............. 108

4.28 Molar mass distributions of dissolved polymer in THF
extractant solution of 33PhDAA-g-66BA (refractive index
detector) ............................................ 110

4.29 UV spectra of dissolved polymer in THF extractant
solution of 33PhDAA-g-66BA from GPC-UV.. ............ 112

4.30 Molar mass distributions of dissolved polymer
in THF extractant solution of 10PhDAA-g-90BA
(refractive index detector) ....................... 113

4.31 UV spectra of dissolved polymer in THF extractant
solution of 10PhDAA-g-90BA from GPC-UV.. ............ 114

4.32 Molar mass distributions of dissolved polymer in THF
extractant solution of 33PhDAA-g-66FA (refractive index
detector) .... ....................................... 116

4.33 UV spectra of dissolved polymer in THF extractant
solution of 33PhDAA-g-66FA from GPC-UV.. ............ 117

4.34 Molar mass distributions of dissolved polymer in THF
extractant solution of 33PhDAA-g-66UBA (refractive
index detector) ..................................... .119

4.35 UV spectra of dissolved polymer in THF extractant
solution of 33PhDAA-g-66UBA from GPC-UV.. ........... 120

4.36 Transmission FTIR spectra of 66PABA-g-33BAx graft
copolymer .... ........................................ 122

4.37 Transmission FTIR spectra of 33PABA-g-66BAx graft
copolymer .... ........................................ 123













4.38 Transmission FTIR spectra of 33PhDAA-g-66BAx graft
copolymer .... ......................................... 125
4.39 Transmission FTIR spectra of 10PhDAA-g-90BAx graft
copolymer .... ......................................... 126

4.40 Transmission FTIR spectra of poly(butyl acrylate)
grafted polyamides ... ............................... 128

4.41 Transmission FTIR spectra of 10PhDAA-g-90BAx graft
copolymer .... ......................................... 129

4.42 'H-NMR spectra of 66PABA-g-33BAx.. .................. 131

4.43 'H-NMR spectra of 33PABA-g-66BAx.. .................. 132

4.44 'H-NMR spectra of PhDAA homopolyamide.. ............. 133

4.45 1H-NMR spectra of 33PhDAA-g-66BAx.. ................. 134

4.46 1H-NMR spectra of 10PhDAA-g-90BAx.. ................. 135

4.47 1H-NMR spectra of 33PhDAA-g-66FAx.. ................. 136

4.48 TG/DTA analysis of 33PhDAA-g-66BA graft copolymer and
PhDAA homopolyamide ................................. 142

4.49 DSC analysis of 33PhDAA-g-66BA graft copolymer and
PhDAA homopolyamide ................................. 143

4.50 Chemical structures of methacrylate and anhydride
monomers for dental applications.. .................. 147

4.51 TG/DTA of anhydride modified dental resin .......... 153

4.52 FTIR spectra of poly(PEMA) and poly(60PEMA-co-40maleic
anhydride) .......................................... 154










Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy


POLY(AMIDE-GRAFT-ACRYLATE) INTERFACIAL COMPOUNDS

By

Michael Perez Zamora

December, 1997

Chairman: Dr. Anthony B. Brennan
Major Department: Materials Science and Engineering


Graft copolymers with segments of dissimilar

chemistries have been shown to be useful in a variety of

applications as surfactants, compatibilizers, impact

modifiers, and surface modifiers. The most common route to

well defined graft copolymers is through the use of

macromonomers, polymers containing a reactive functionality

and thus capable of further polymerization. However, the

majority of the studies thus far have focused on the

synthesis of macromonomers capable of reacting with vinyl

monomers to form graft copolymers.

This study focused on the synthesis of macromonomers

capable of participating in condensation polymerizations. A

chain transfer functionalization method was utilized.

Cysteine was evaluated as a chain transfer agent for the

synthesis of amino acid functionalized poly(acrylate) and

poly(methacrylate) macromonomers. Low molar mass,

functionalized macromonomers were produced. These

macromonomers were proven to be capable of reacting with


xiii











amide precursors to form poly(amide-g-acrylate) graft

copolymers.

Macromonomers and graft copolymers were characterized

by gel permeation chromatography (GPC), Fourier transform

infrared spectroscopy (FTIR), nuclear magnetic resonance

(NMR) spectroscopy, elemental analysis (EA), inductively

coupled plasma (ICP), and differential scanning calorimetry

(DSC).

The second part of this research involved

poly(dimethacrylate) dental restorative materials.

Volumetric shrinkage during the cure of these resins results

in a poor interface between the resin and the remaining

tooth structure, limiting the lifetime of these materials.

Cyclic anhydrides were incorporated into common monomer

compositions used in dental applications. Volume expansion

from the ring opening hydrolysis of these anhydrides was

shown to be feasible.

The modified dental resins were characterized by

swelling, extraction and ultraviolet spectroscopy (UV), and

density measurements. Linear polymers designed to model the

crosslinked dental resins were characterized by FTIR, GPC,

and DSC.


xiv
















CHAPTER 1
INTRODUCTION


1.1. Poly(amide-g-acrylate) Graft Copolymers


Graft copolymers are macromolecules composed of

chemically dissimilar segments in a branched architecture

(Figure 1.1). They have been studied and utilized in a

variety of applications because of their ability to combine

the properties of their individual segments.



A A A A


Figure 1.1 Schematic illustration of general graft
copolymer structure.


1.1.1.Macromonomers


The most prevalent synthetic route to well defined

graft copolymers is through the use of macromonomers, low











molar mass polymers containing a polymerizable end group. A

review of the literature has shown that current studies,

including those within this laboratory, have concentrated on

synthesizing macromonomers which contain a residual

unsaturation at one end. Therefore, they are generally

polymerizable only with addition type monomers through a

free radical mechanism. Because the macromonomers

themselves are generally synthesized through either anionic

or free radical polymerization in the presence of a

functionalizing agent, the resulting graft copolymers which

can be synthesized through this method are generally limited

to addition-addition graft copolymers.

It was the objective of the first part of this study to

synthesize condensation polymerizable macromonomers,

specifically amino acid terminated macromonomers, capable of

reacting with amino acids in the synthesis of polyamide

graft copolymers. An amino acid functionality is preferred

over other end groups such as diacids or diamines due to the

inherent stoichiometry that it provides. This stoichiometry

is required in the condensation graft reaction to insure the

highest degree of polymerization possible.

The approach taken in this study involves the free

radical polymerization of acrylate and methacrylate monomers

in the presence of a functional chain transfer agent.

Mercaptans, compounds containing a sulfur-hydrogen bond, are

commonly used in chain transfer reactions. In fact,

mercaptans are commonly used to control molecular weight in











commercial polymerization reactors. Cysteine, a naturally

occurring amino acid, contains the sulfhydryl group required

for mercaptan chain transfer reactions. The addition of

cysteine, if effective as a chain transfer agent, would

result in an amino acid functionality.


l.l.2.Addition-Condensation Graft Copolymers


There are few reports of well defined polyamide graft

copolymers with addition polymers such as poly(acrylates) or

poly(methacrylates). Most studies of amide graft

copolymerizations with addition polymers involve either the

in situ formation of graft copolymers in polymer blends or

radiation induced surface graft techniques. Although

effective for their intended applications, neither method

produces a well defined graft copolymer that can be isolated

and studied.

The objective of the second part of this research was

to evaluate the ability of the previously synthesized

macromonomers to participate in a condensation

polymerization of polyamide precursors. If successful, the

resulting structure would be a poly(amide-g-acrylate) graft

copolymer.

The properties of the synthesized macromonomers and

graft copolymers were characterized by a variety of

techniques including GPC, FTIR spectroscopy, NMR











spectroscopy, UV spectroscopy, elemental analysis, ICP, and

DSC.


1.1.3.Applications


One of the most common applications of graft copolymers

is their use as compatibilizers. The appropriate graft

copolymer has been shown to migrate to the interface between

two dissimilar materials, reducing interfacial tension and

increasing the bonding at the interface.

The initial motivation for the study of polyamide-

polyacrylate graft copolymers came from the field of

dentistry. The new class of dental restorative composites,

better known as 'fillings' consist of a crosslinked

dimethacrylate matrix surrounding glass particles. The

failure of dental restorative composites, and poor

performance as compared to amalgam restorations, is

generally attributed to a poor interface. Although the

glass-resin interface has been studied extensively, the

source of failure is usually the interface between the

composite restoration and the remaining tooth structure.

Two of the main sources of this poor interface are

1) poor bonding between the exposed tooth structure,

composed of hydrophilic proteinaceous dentin tubules

and the hydrophobic dimethacrylate composite and

2) the polymerization shrinkage during composite cure

causing the restoration to pull away from the remaining











tooth structure. This leads to marginal leakage, the

infiltration of saliva and bacteria under the

restoration, which can lead to the secondary caries.

The macromonomer and graft copolymer work was targeted to

determine the ability to synthesize a copolymer capable of

interacting with both the hydrophobic methacrylate and

hydrophilic dentin structure at the tooth-restoration

interface. Although the synthesized amide-acrylate graft

copolymers were not tested in such a system and their

aromatic structures may not make them suitable for this

application, the feasibility of synthesizing the desired

structures has been evaluated.


1.2. Offsetting Polymerization Shrinkage in Dental Resins
through the Incorporation of Maleic Anhydride


The last part of this research addresses the

polymerization shrinkage problem in these dental resins.

This shrinkage is inherent to the free radical

polymerization of the multifunctional methacrylate resins

used in the dental composites. The volumetric shrinkage is

due to the reduction in molar volume, or spacing between

monomer units, that occurs when vinyl compounds are

polymerized. Polymerization shrinkage creates both a weak

interface between the tooth structure and the restoration as

well as residual stresses within the composite structure,

leading to premature failure in the restoration.











There are many research programs focused on different

chemical structures and processes that will reduce

polymerization shrinkage. Although various methods and

different monomers have been studied to alleviate this

problem, no solution to date has been discovered that

eliminates shrinkage without significantly altering the cure

and mechanical properties of the resin.

The goal of this study was to demonstrate that we can

offset the polymerization shrinkage of common dimethacrylate

resins without significantly changing the comonomer

structures through the copolymerization with maleic

anhydride. When maleic anhydride (MA) is ring opened by

hydrolysis to maleic acid, there is a corresponding

theoretical 10% increase in molar volume.

The properties of the anhydride-methacrylate

copolymers, including the ability of the anhydride to offset

the polymerization shrinkage, were characterized by

swelling, extraction and ultraviolet spectroscopy (UV), and

density measurements, FTIR, GPC, and DSC.
















CHAPTER 2
BACKGROUND



The first section of this chapter addresses how our

study of amino acid-terminated poly(acrylate) macromonomers

and poly(amide-g-acrylate) graft copolymers fits within the

massive amount of literature on the synthesis and

characterization of multiphase copolymers. An overview of

graft copolymers is given, followed by a description of the

methodology and limitations of common synthetic routes as

they relate to synthesizing addition-condensation graft

copolymers. This is followed by an analysis of other

studies concentrating specifically on polyamide graft and

block copolymers.

As was mentioned previously, the initial motivation for

amide-acrylate graft copolymers comes from the field of

dental materials. Specifically, the failure of dental

restorative materials at the tooth-resin interface is of

concern. It was mentioned that one of the main causes of

this failure is the cure shrinkage of these resins during

application, causing the resin to pull away from the tooth

surface. The second part of this chapter addresses the

problem of polymerization shrinkage in dental resins, and











analyzes the approaches described in the literature to

alleviate this problem.


2.1. Graft Copolymers


Graft and block multiphase copolymers have received an

increasing amount of attention over the past twenty years.

They have been shown to possess desirable combinations of

physical and chemical properties that allow them to be

useful in a variety of applications (Nos77, Pei86, Nat88,

Tan90, Zha90, Pei94) including blend compatibilizers,

surfactants, impact modifiers, and surface modifiers.

All of the applications mentioned take advantage of the

individual properties of chemically dissimilar segments.

Most of these depend on the ability of these copolymers to

act at a particular interface. For example, these

copolymers can be added to a blend of two immiscible

homopolymers that have affinities for the individual

segments of the multiphase copolymer. The copolymer

migrates to the interface between the two phases (Shu90),

reducing the interfacial tension (Gai8O, Ana89) and thus

enhancing the strength of the interface (Fay82, Bro89,

Cre89).


2.1.l.Synthetic Routes to Graft Copolymers


One of the most widely used methods of synthesizing

multiphase copolymers is through sequential anionic block











copolymerization (Has83). But the number of monomers that

can be polymerized anionically is limited due to unwanted

side reactions (Has83). The stringent polymerization

conditions required for the anionic polymerization of

certain monomers (Nos77) also limits its applicability.

These limitations severely reduce the applicability of this

route to a wide variety of combinations of block segments.

In order to expand the possible combinations of monomers

used as the two phases, graft copolymerizations have been

investigated (Mei73).

2.1.1.l.Anionically polymerized macromonomers

One of the most prevalent and reliable synthetic routes

to graft copolymers is through the use of macromonomers

(Cor84, Rem84b, Muh87, Mei90, Pei94). Macromonomers are end

functional macromolecules capable of further polymerization

(Rem84a). They are generally synthesized through the

anionic polymerization of a monomer followed by reaction of

the living anion with an end capping agent such as

methacryloyl chloride, producing a methacrylate or vinyl

terminated macromonomer (Mas82, Sch82, Ham84, Sch84a,

Sch84b, Cam85, Cam86, Gna87, Gna88). Once again,

application of this synthetic procedure is limited in scope

due to monomer restrictions in anionic polymerizations.

2.1.1.2.Macromonomers through chain transfer
functionalization

A more versatile route to macromonomers involves the

free radical polymerization of a monomer in the presence of











a functional chain transfer agent (Ito77). Functionalized

mercaptans are commonly used. Mercaptans are compounds

containing a sulfhydryl, -SH, functional group. They are

commonly used to control molecular weight in commercial

polymerization reactors (Ros82).

The mechanism by which functionalization can occur is

depicted in Figure 2.1. Steps 1 and 2 are typical process

of free radical initiation. When exposed to heat, the AIBN

breaks down into free radicals and nitrogen gas is evolved.

The AIBN radicals can thus initiate the polymerization of

vinyl compounds. If there is no chain transfer agent

present, the polymerization continues until termination by

disproportionation or combination occur. In the presence of

a mercaptan, termination can occur through chain transfer.

The hydrogen from the sulfhydryl group of the mercaptan

is readily extractable. A propagating polymer chain can

thus react with the mercaptan (Step 3), terminating

propagation and leaving a sulfur radical on the mercaptan.

If the concentration of mercaptan is high, the mercaptan

itself can react with the AIBN radical (Step 4), also giving

a sulfur radical. The resulting sulfur radical can then

initiate the free radical polymerization of a vinyl monomer

(Step 5).

If the mercaptan contains hydroxyl or carboxylic acid

functional groups (R'), the initiating sulfur radical

introduces functionality to one end of the macromolecule.

The growing functionalized polymer radical can again react










AIBN


C -N=N --CH A
I CH3 -=N H3
CH 3 H 3


2 CH 3-- +
CH3


C N monomer polymerization
2 CH3-C. +3= ---- -----1 . . .
CH3 R


3 V...... + H:S-R'


C N mercaptan
4 CH3-- N + H:S-R'
CH3

5 R'S.- + -
R

6 R' + H:S-R'


...----------a ..-.- + RS .


Q NN
--- CH 3- H +
CH3


R'S.


- R' +


Figure 2.1 Mechanism of functionalization using chain
transfer agents.











with the mercaptan (Step 6) yielding a terminated

functionalized chain and another molecule of sulfur radical

which can react with more monomer (Step 5) to form a

reaction loop. The effectiveness of functionalization is

dependent on the chain transfer constant of the mercaptan as

well as the relative concentrations of mercaptan, monomer,

and free radical initiator (Tsu91). The AIBN concentration

is kept extremely low relative to the chain transfer agent

to minimize the number of chains initiated by the AIBN. Any

chains initiated by AIBN will be non-functionalized (see

Steps 2 and 3).

The advantage of this chain transfer method is that it

can be used with a wide variety of vinyl monomer systems.

Macromonomers composed of any monomer which can be

polymerized free radically should be able to be synthesized

using this method. Also, macromonomers which themselves are

random copolymers also become feasible.

The resulting functionalized macromolecule can be

utilized in two different ways. As with the anionic

macromonomers, the resulting hydroxyl or carboxylic acid

monofunctionality can be converted to a methacrylate or

vinyl functionality for further polymerization with vinyl

monomers (Alb86, Che91, Tsu91). Chen and Jones (Che91) have

synthesized hydroxyl functionalized polyacrylates using

mercaptoethanol as the chain transfer agent. Similarly,

Albrecht and Wunderlich (Alb86) have synthesized hydroxyl

terminated PMMA with Mw of 6500- 23000 g/mole. Both studies











have indicated that high levels of functionalization are

realized. The hydroxyl group in each case was converted to

a vinyl functionality through reaction with isocyanatoethyl

methacrylate. Tsukahara et al. (Tsu91) have synthesized

carboxylic terminated PMMA using mercaptoacetic acid as the

chain transfer agent. The acid functionality was reacted

with glycidyl methacrylate to produce methacrylate end

capped PMMA.

These routes produce methacrylate terminated polymers

which can be subsequently polymerized with a vinyl monomer

similarly to the anionically polymerized macromonomer. In

order to synthesize graft copolymers, these macromonomers

are dissolved in a solution containing a different vinyl

monomer as well as a free radical initiator. Graft

copolymers of a wide range of monomers can be synthesized in

this fashion (Che91). But this macromonomer chemistry has

generated graft copolymers mostly limited to vinyl-vinyl

type systems (Nai92).

The functionalized amine, hydroxyl or carboxylic acid

terminated macromonomer can also be used without the vinyl

functionalization by direct coupling with other condensable

terminated macromonomers to form block copolymers (Imi84).

For example, amine-terminated poly(methacrylates) and

polystyrene have been synthesized using mercaptoethyl

ammonium chloride as the chain transfer agent (Imi84,

DeB73). Block copolymers of these macromolecules with

carboxylic acid terminated polymers were synthesized by a











coupling reaction. However, due to the monofunctionality of

the macromonomer, coupling produced only very low molar mass

A-B type block copolymers.

Another interesting use of chain transfer agents for

graft copolymerization is one developed by Moraes et al.

(Mor96). They modified an ethylene-vinyl acetate (EVA)

copolymer by hydrolysis and subsequent esterification with

mercaptoacetic acid. This produced a sulfhydryl containing

polymer backbone. In essence, this is a polymeric chain

transfer agent. Methyl methacrylate (Mor96) and styrene

(Bar96) were polymerized in the presence of the mercapto-

modified EVA to give poly(EVA-g-methyl methacrylate) and

poly(EVA-g-styrene) graft copolymers.

Again, the described macromonomers and graft copolymers

have been mostly limited to addition-addition copolymers.

There are only a few cases in which addition type

macromonomers have been graft copolymerized with

condensation type monomers (Yam8l, Chu82, Chu84, Chu88a).

In order for the macromonomers to be capable of undergoing

condensation reactions in the production of graft

copolymers, they must be difunctional. This does not imply

that each end of the polymer is functionalized. Instead, one

end of the polymer contains a difunctional reactive group.

Only two research groups have reported the synthesis of

difunctional macromonomers using chain transfer agents.

Nair (Nai92) has demonstrated the free radical chain

transfer polymerization of styrene and various acrylates in











the presence of mercaptosuccinic acid. Dicarboxylic acid

terminated polymers were synthesized in a range of molecular

weights from 1 to 10 kg/mole and were shown to be highly

functionalized.

Yamashita et al. (Yam81, Chu82, Chu84, Chu88a) carried

out brief studies on the preparation of dicarboxylic acid as

well as dihydroxyl functional macromonomers of various

methacrylate monomers. They also employed mercaptosuccinic

acid as well as thioglycerol in the macromonomer

preparation. The structures and resulting macromonomer

structures for each of the chain transfer agents discussed

are illustrated in Figure 2.2.

The work by Yamashita is the most closely related to

our synthetic approach: the evaluation of cysteine as a

chain transfer agent in the synthesis of amino acid

terminated macromonomers. Our survey of the literature

found no mention of amino acid terminated macromonomers.

2.1.1.3.Modeling chain transfer reaction

The first step in the use of chain transfer agents is

the determination of the chain transfer constant. Knowledge

of the chain transfer constant allows us to predict such

critical variables as molar mass and functionality (Nai92).

The chain transfer constant can be determined using the Mayo

model (Ros82, Nai92). The variation of the degree of

polymerization for a free radical chain transfer

polymerization is given by














Monofunctional Chain Transfer Agents


CH3
2 -


0
HS-CH2-Q-OH

HS-CH2-CH2-OH --


CH
AW W......W..OOH
reactive methacrylate
--------OH -


HS-CH2-CH2-NH2:HCI N ------NH2


Difunctional Chain Transfer Agents


OH
HO-CH2-CH2-CH2-OH .. .
SH OH

0 0 COOH
HO-QC-CH2-CH2-C-OH s ...-----.---.-
H ..OOH



Figure 2.2 Schematic illustration of utilized chain
transfer agents and macromonomers thereof.











I k, FR, [S
I k Y [](2.1)
P k ~ Is]
DP,, [ +Ci


where Cs = Chain transfer constant = ktransfer/kp

[S]= Chain transfer agent concentration
[M]= Monomer concentration
DPn= Degree of polymerization
kt,kp,Rp= rate constant for termination,

propagation, and the rate of polymerization
respectively.


The degree of polymerization in the absence of chain
transfer to transfer agent, DPno, can be described by
equation 2.2 where


1 (k, RP
___j k 2 2'f (2.2)



Substituting this value in equation 2.1 gives us the Mayo
equation (2.3) for prediction of the chain transfer constant
where


1 1 [S]
+Cs[ (2.3)
DP,, DP,,o [M]


This equation is valid only when the initiator
concentration is low. By synthesizing a series of polymers
with different ratios of chain transfer agent to monomer, a










Mayo plot can be used to determine the Cs. Knowledge of the

Cs for a particular system enables one to adjust the

reactant concentrations in order to target a specific molar

mass polymer.

The Mayo model can also be used to predict the

functionality of the polymer obtained. If we multiply both

sides of equation 2.3 by DPn, we get


DP,, [ IS]
1= +DPCs (2.4)
DP,,o [M



where the two terms on the right represent the fraction of

unfunctionalized and functionalized chains. If the value of

Cs and therefore the rate of chain transfer is high,

termination occurs primarily by chain transfer and high

rates of functionalization are expected. The extent of

functionalization also increases with increasing mercaptan

content. If a lower concentration of chain transfer agent

is used or if a lower value of Cs is observed, the

probability of termination through other methods such as

disproportionation or combination increases. As other

termination mechanisms become more prevalent, the extent of

functionalization decreases.

It is important to note the limitations of the Mayo

model. This model is valid only under certain assumptions

(Ath77, Nai92). The first of these is that chain transfer

occurs exclusively to the chain transfer agent. In











practice, however, some chain transfer to solvent and

initiator is generally observed. Also, these values, as in

the case of copolymer reactivity ratios, are valid at

instantaneous conditions. In other words, low conversions

are desired in order to limit the composition drift between

the monomer and chain transfer agent. With these

assumptions in mind, determined values of Cs and predicted

functionalities are only estimates or theoretical

predictions assuming ideal conditions.


2.1.2.Polyamide Addition-Condensation Multiphase Copolymers


A variety of polyamide containing graft and block

copolymers are described in the literature. Our specific

interest lies in the ability to graft or block copolymerize

polyamides with addition polymers such as methacrylates and

other olefinic monomers. Several approaches have been taken

in the synthesis of these structures. The benefits and

limitations of each approach are described herein.

2.1.2.l.Poly(amide-b-olefin) block copolymers

Anionic block copolymerizations. The first evidence in

the literature of block or graft copolymerizations of vinyl

monomers with polyamide were found in the patent literature

(Fur63, Bak65, God69). The synthetic approach taken in

these investigation involved the sequential anionic

polymerization of a vinyl monomer and an isocyanate.

Specifically, Godfrey (God69) showed that anionically











polymerized 'living' polystyrene, polyisoprene, and

poly(methyl methacrylate) could initiate the polymerization

of butyl isocyanate. The resulting product can be thought

of as a N-butyl nylon 1-b-olefin block copolymer. The

mechanism of block copolymerization is illustrated in Figure

2.3. High molar mass block copolymers were formed with

polydispersities ranging from 1.2 to 1.6. Although this

reaction was successful, the choice of polyamide in this

synthesis is limited due to the isocyanate precursors. All

block copolymers involving isocyanates will form nylon 1

type polyamides. Also, the choice of olefinic monomer is

also limited to the previously mentioned limitations of

anionic polymerizations.

Macroinitiators for lactam polymerization. The

majority of the literature concerning polyamide block

copolymers involves the anionic copolymerization of

caprolactam (Pet79, Ste82, Bor88, Mou93, YnM94), the

precursor to nylon 6. The first step in these

copolymerizations is the synthesis of polymeric

macroinitiators from the desired vinyl monomer. The end

group must be suitable for the initiation of the anionic

polymerization of caprolactam. The most recent example

involves the block copolymerization of amine-terminated

butadiene nitrile copolymer (ATBN) with caprolactam (YnM94).

The amine group is reacted with terephthaloyl biscaprolactam

to form a polymeric activator. Upon addition to a













Anionic 'living' polymer
Bu--(CH-CH-)-CH2-CH:-Li
R R


BirxLi


isocyanate
0
&=N 1


Amide-Vinyl
block copolymer


Bu--(CH2-CH),-CH2-CH:-Li +
R ~ R


Figure 2.3 Mechanism of block copolymer formation through
sequential polymerizations of vinyl monomer and
isocyanates.











caprolactam solution containing additional initiator, A-B-A

type block copolymers are formed with a ATBN center block.

The molar masses of the resulting copolymers were not

evaluated. Studies of the block copolymers concentrated on

their microstructure and mechanical properties.

Similar block copolymerizations were run using ester

terminated polystyrene and isocyanate terminated

polybutadiene (Pet79) and isocyanate terminated

polyisobutylene (Won82). Both end groups can react with

caprolactam to form a macroinitiator for the polymerization

of nylon 6. A schematic of this macroinitiator formation

and block copolymerization is illustrated in Figure 2.4.

Spectroscopic evidence of A-B block copolymer structure is

shown. However, significanthomopolymer formation, i.e., >

30%, was observed due to coupling reactions between two

functionalized macroinitiator molecules.

Although some success has been demonstrated using the

macroinitiator method, several limitations exist. One

limitation of this approach is that this mechanism is

restricted to the polymerization of lactam based polyamides.

More importantly, Stehlicek (Ste77) and Hergenrother (Her74)

have reported that synthetic approaches that employ the

anionic polymerization of lactams activated by

macroinitiators are handicapped by the occurrence of side

reactions that may yield insoluble, crosslinked product.














Hydroxyl terminated Hexamethylene diisocyanate
polymer


0 0
+ (=N-.(CH2-)6_N4=


Isocyanate terminated
polymer


0 0
1 c- _)CH2_H-CH.0-.9-NH-(CH2)6-N =J
A


0 0
2 -CH2-CHxO--!-NH-(CH2)6--N = +



Caprolactam terminated
o polymer o o
3 -("CH2--CH -C-NH-(CH2)6-NH---N-2
v (^2-)


Caprolactam
o
NH-- --



Nylon 6 block copolymer
o ^^o
0 NaHi
+ NH-(!! 4H2-CH);(NH-(CH2)5-(!!-y
(H2") A


Figure 2.4 Reaction schematic for macroinitiator formation
and subsequent anionic block polymerization of
caprolactam.


1--fCH2--CH-)OH
l{x











Functional polyamide macroinitiators. The synthetic

routes for block copolymerization previously described

utilize anionic polymerization methods. One interesting

twist on the macroinitiator method is to synthesize

polyamide macroinitiators (Cra80, Cra82, Den89). These

polyamides contain functional groups which, under

stimulation from light or heat, can dissociate into

macroradicals. Thus, these macroinitiators are capable of

initiating the free radical polymerization of vinyl

monomers.

Two distinct routes have been reported. The first

(Cra80, Cra82) involves the modification of aliphatic

polyamides such as nylon 6 and nylon 6,10. The reaction

schematic for polyamide modification and subsequent block

copolymerization is illustrated in Figure 2.5. The

secondary amines in the polyamides can be nitrosated using a

variety of nitrosating agents including nitrous acid and

dinitrogen trioxide. The resulting N-nitrosoamines can

rearrange to form a diazo linkage. Diazo compounds are well

known as free radical initiators (Ros82). The polyamide can

then, under exposure to heat (Cra8O) or light (Cra82),

decompose into macroradicals and initiate the polymerization

of olefinic monomers. Block copolymers of nylon 6 and nylon

6,10 with MMA, styrene, vinyl acetate and styrene-

acrylonitrile have been synthesized by this method.

The second route to polyamide macroinitiators (Den89)

also involves the introduction of dissociative azo linkages.



















Aliphatic polyamide
o o
H20
-,-NH m-... -NH-- + N203 --



I

0 0
2 N-N(NO) -N(NO)




C) 0


Nitrosated polyamide
0 o
-w-N(NO)..-.m'-m -N(NO)-


Diazo ester modified polyamide
o o
--)-N=N .m.A..... --O-N=N--




Polyamide macroinitiators


0 0
3 C-O-N =N---c-0-N=N- -... "-
-N2, -CO2


Olefinic
monomer


A-B and A-B-A


amide-olefm block copolymer
Nylon Olefin Olefin Nylon Olefin
================= q- :::::::::::::::::::::::::


Figure 2.5


Reaction schematic of block copolymerization
initiated by nitrosated polyamide
macroinitiators.











The difference is that the azo linkages are introduced

during the polyamide synthesis. Denizligil showed that

dinitriles can react with formaldehyde in the presence of

strong acids to form a polyamide. The reaction schematic is

illustrated in Figure 2.6.

Specifically, AIBN was copolymerized with formaldehyde

in the presence of sulfuric acid. AIBN is of interest

because it has both dinitrile functionality as well as a

labile azo functionality which can decompose to free

radicals under heat. The resulting polyamide was used to

initiate the polymerization of MMA and styrene in a

DMSO/methylene chloride solvent system.

Although block copolymers were formed by both processes

outline above, no account was given as to the extent of

degradation of the polyamide. These polyamide

macroinitiators function only through their ability to

degrade. That is, block copolymerization occurs through the

chain scission of the polyamide. The extent of chain

scission, especially in the AIBN based polyamide which has a

very low molar mass between azo groups, could severely deter

application of this process.

Coupling of low molar mass reactive polymers. The

synthesis of block copolymers from the coupling reaction of

prepolymerized polyamide and polyolefin with reactive end

groups has also been investigated (Mas79, Ima84, Kim92).

Synthetically, this is the least complicated method of block

copolymer synthesis. For example, Imanishi has reacted




















Azo functional polyamide
AIBN
CH3 CH3 0 0 CH3 CH,
1 1 11 H2SO4 1t I 1 1H
N cC--N=N-C-C=N + HCH g -- NH2-C--C-N=N-I-- -NH-CH2
CH3 CH3 CH3 CH3


0 CH3 CH3,
II! I t 1|
2 {-NH2--C-N =N -C--N-_CH24 __A
CH3 CH3 -N2



Olefinic
monomer


3 Polyamide macroinitiator


Polyamide macroinitiator






A-B-A
block copolymer
Olefin Amide Olefin
ANMMWVMMWWWiW


Figure 2.6 Reaction schematic of AIBN containing polyamide
and block copolymer thereof.











amine terminated polystyrene with terminally haloacetylated

polyamides to produce a polystyrene-polyamide block

copolymer. Although some block copolymer is formed using

this type of coupling reaction, this method is characterized

by the highest level of homopolymer contamination and

product heterogeneity.

2.1.2.2.Poly(amide-g-olefin) graft copolymers

The majority of the literature on amide-olefin graft

copolymers in concerned with one of two topics- the grafting

of vinyl monomers onto polyamide substrates through high

energy processes or the formation of in situ graft

copolymers at the interface of polymer blends.

Grafting onto polyamide substrates. Various methods

have been used to graft olefinic polymer chains off the

backbone of prepolymerized polyamides. The method most

often and most recently employed is through the use of high

energy processes such as UV irradiation (Bog93, You95) ,

severe oxidation/peroxidation (Ela92, Buc96a, Buc96b),

gamma irradiation (Mue93, Mis96), and plasma grafting

(You95, Lee97).

In all of these cases, grafting is surface directed.

Polyamide fibers, films, and membranes are subjected to some

form of radiation or oxidation. High energy processes are

used in order to form radical or ions at the polyamide

surface. These radicals can initiate the polymerization of

vinyl monomers to form a grafted surface.










The majority of these studies involve the grafting of

hydrophilic monomers such as acrylic acid and acrylamide

onto polyamide surfaces. One exception is the study by

Elangovan (Ela92) which has shown the grafting of PMMA onto

wool fibers through the oxidation and subsequent redox

initiation of MMA. This was done to improve the acid and

alkali resistance of the fibers. Various other applications

for these types of graft copolymers have been targeted

including the production of pH responsive membranes (You95,

Lee97, Mis96), antibacterial fibers (Buc96a, Buc96b), and

new media for affinity chromatography (Mue93).

These processes are useful for their intended

application, i.e., surface directed grafting. Well defined,

isolatable graft copolymers are not synthesized by this

method. In fact, radiation grafting is usually accompanied

by significant crosslinking at the substrate surface as well

as in the graft copolymer layer (Arn97).

In situ graft copolymers. As mentioned previously,

graft and block copolymers can be added as a compatibilizer

to immiscible polymer blends in order to improve the overall

blend mechanical properties. To this effect, the greatest

concentration of research in the synthesis of amide-olefin

graft copolymers is in the area of melt compatibilization.

This method may also be the most industrially applicable due

to the simplicity of the process as compared to the more

elaborate and labor intensive block copolymerization

methods.










Graft copolymers can be formed in situ, during the melt

blending of polyamides and polyolefins, if the polyolefin is

functionalized with a reactive group. Specific examples

include the modification of polyolefins with maleic

anhydride (Maj92, Osh92, Mod93, WuC93, Maj94a, Maj94b,

Gon95a, Gon95b, Gon95c, Sea95), glycidyl methacrylate

(Chi96) and oxazoline (Bec96).

Glycidyl methacrylate and oxazoline have been copolymerized

with styrene. These copolymers have been blended with

polystyrene and nylon 6 to compatibilize the blend. The

randomly dispersed oxazoline and epoxide functionalities

within the styrene copolymer can react, during melt

processing, with amine end groups from the polyamide. This

results in a polyamide-g-polystyrene at the blend interface.

Both studies (Chi96, Bec96) show reduced phase size in the

blend as well as improved mechanical properties. A schematic

of this reaction is illustrated in Figure 2.7.

The most widespread use of the in situ technique for

polyamides graft copolymers involves maleic anhydride

modified polyolefins. Immiscible polyamide blends with

polyethylene, polypropylene, and S-B-S block copolymers have

been compatibilized using maleic anhydride (Maj92, Osh92,

Gon95a, Gon95b). In general, polyolefins such as

polyethylene can be modified by grafting of maleic anhydride

in the presence of free radical initiators (Sea95). The

resulting anhydride functionality on the polyolefin can

react with polyamide end groups at high temperatures during



















Poly(styrene-co-glycidyl methacrylate) Nylon 6
CH3 0
+ NH2-jCH2)5-4C-NH---)
(H H +x Iy extrusion
=o + polystyrene







Poly(styrene-graft-nylon 6) + unreacted nylon 6 + polystyrene


CH3
+CH2-CHt-CH2-Q*
&==o

6H2

CH-OH
CH2
H



Nylon 6


Figure 2.7


compatibilized blend


PS

Polystyrene


Nylon 6


Nylon 6


Reaction schematic of in situ graft copolymer
formation from glycidyl methacrylate copolymers.










extrusion of the blend. A schematic of this reaction is

illustrated in Figure 2.8.

The synthesis and application of in situ polyamide

graft copolymers has been shown to be extremely effective at

improving the phase dispersion within a variety of polymer

blends. However, these graft copolymers have not been

isolated and studied separate from the blend. This is due

in part to the crosslinking that can occur during the graft

reaction (Sea95) due to reaction of both ends of some

polyamide chains.

Macromonomer approach to amide-olefin graft copolymers.

Previous work in the area of a macromonomer approach to

polyamide graft copolymer synthesis is of particular

interest. Free radical routes such as chain transfer

functionalization offer more flexibility in monomer

selection than anionic macromonomer synthesis. However, as

mentioned previously, most of the previous work using either

macromonomer method has been directed at the synthesis of

vinyl terminated polyolefins and thus addition-addition

graft copolymers.

In order to synthesize graft copolymers from polyolefin

macromonomers, the macromonomer must contain a difunctional,

condensable end group. For the synthesis of polyamide graft

copolymers, these functional groups must be composed of acid

or amine functionalities. Only Yamashita et al. (Yam8l,

Chu82, Chu84, Chu88a) have reported the graft













Maleated high density
polyethylene (HDPE)


Nylon 6
V A
+ NH2-jCH2)5-C-NH-)-
Y extrusion
+ HDPE


2 Poly(ethylene-graft-nylon 6) + unreacted nylon 6 + HDPE


compatibilized blend


HDPE

HDPE


Nylon 6


Nylon 6


Figure 2.8 Reaction schematic of in situ graft copolymer
formation from maleic anhydride modified
polyolefins.


-(-CH2-CH-)t-CH2-CH-""--MW
CH-CH2
I \
0=7 =o
NH OH



SNylon 6











copolymerizations of such macromonomers, dicarboxylic acid

terminated poly(methacrylates), with polyamide precursors.

They employed mercaptosuccinic acid (Figure 2.2) as a

chain transfer agent in the preparation of PMMA and

poly(hydroxyethyl methacrylate) macromonomers.

Macromonomers were copolymerized with aromatic diamines and

diacids to form graft copolymers. One limitation of this

approach is the stoichiometry. The degree of polymerization

in condensation reactions is controlled primarily by

conversion and stoichiometry as defined by Caruthers'

equation (Ros82):


l+r
DP, (2.5)



where r is the stoichiometric ratio of reactive functional

groups. This equation is valid under the assumption of

complete conversion.

Determining the relative concentrations of amine and

carboxylic acid groups is complicated by the introduction of

macromonomers. The amount of dicarboxylic acid added in the

reaction by Yamashita must be reduced to account for the

acid end groups on the polymer and determining the exact

amount of dicarboxylic acid becomes complicated.

Macromonomers with built in stoichiometry, that is, with one

amine and one carboxylic acid group, would be preferred.

The precise stoichiometry of amino acid terminated











macromonomers becomes especially beneficial either for

grafting with amino acids or for maximizing of graft

copolymer molar mass.

Also, only high Tg poly(methacrylate) difunctional

macromonomers were synthesized. There is no evidence in the

literature of these type of graft copolymers containing a

low Tg rubbery phase as investigated in this study.

An interesting twist on the macromonomer approach to

amide-olefin graft copolymers was developed by Izawa et al.

(Iza93). They synthesized vinyl functionalized polyamide

macromonomers which could be free radically polymerized with

vinyl monomers. A polycondensation of aromatic amino acids

was run in the presence of methacrylic acid and p-

carboxystyrene chain terminators. Complete consumption of

the macromonomers during the free radical graft

copolymerization with MMA revealed almost complete

functionalization in the macromonomers. Although this

method resulted in vinyl functionalized chains, the molar

mass of these macromonomers is about 600g/mol. This low

molar mass can be explained by the stoichiometric imbalance

caused by addition of the end capping agent.


2.2. Polymerization Shrinkage in Dental Composites


Polymer based dental composites are replacing amalgam

as the material of choice for dental restorations. The

major drive toward the use of polymer based systems is based











on the aesthetics of the restoration. Composites compare

favorably with silver amalgam in this aspect. However, the

acceptance of composites is hampered by certain property

limitations. The evolution of developments in these

materials is described below.


2.2.l.History of Dental Composites


The evolution of dental composites represents a logical

sequence of developments based upon current technologies

well known to the non dental community. The first acrylic

filling materials were used at the time when polymer science

was a young immature science whose growth was largely due to

World War II and the need for synthetic rubber and a non

breakable canopy for fighter planes. The early polymer

based restoratives, based on methyl methacrylate monomer,

exhibited large volumetric shrinkage, low mechanical

strength, a high propensity for staining, high wear rates,

marginal leakage and inflammatory tissue responses. None of

the first generation materials had sufficient strength or

adhesion to tooth structures to withstand the rigorous

forces of oral function.

The next generation of restorative materials were

composites. The composites were based upon the

incorporation of glass particles into the resin. The glass

particles increased the mechanical strength and abrasion

resistance and reduced total volumetric shrinkage simply by











reducing the content of resin in the restoration. In

addition to the development of the composite materials, a

new reactive methacrylate type monomer was developed by

Bowen (Bow62). Bowen's studies with epoxide resins led to a

pivotal combination of the mechanical properties of the

epoxy resin with the fast reacting methacrylate resin in the

form of BisGMA. The first BisGM.A systems introduced were

polymerized by the chemical process wherein benzoyl peroxide

is combined with a tertiary amine to form free radicals at

room temperature. Later numerous variations of the light

cured systems were introduced to the spectrum of dental

materials including both UV and visible light activated

materials. The UV light was scattered by the filler

particles in the composites and thus the depth of cure was

limited. The visible light activated restorations could

achieve a greater depth of cure and thus have become the

main system.

In addition to the change in cure mechanism, numerous

examples of modifications to the BisGMA structure and

synthesis of other reactive methacrylate monomers (Lee89,

Kaw89, Joh89, Ven93) have been reported. Most of the

modifications involve either elimination of the hydroxyl

group or modifications through esterifications or

substitution with urethane groups. There are slight

improvements in both wet and dry properties as a result of

modifications to BisGMA, however usually the differences are











not significant and many manufacturers still rely on the

BisGMA monomer for their dental composites.

The failure of these dental composites, and poor

lifetime performance as compared to amalgam restorations, is

generally attributed to a poor interfacial properties

(Sod91). Although the glass-resin interface has been

studied extensively, the failure is usually attributed to

the interface between the composite restoration and the

remaining tooth structure.

Two of the main sources of this poor interface are as

follows:

1) the polymerization shrinkage during composite cure

causing the restoration to pull away from the remaining

tooth structure (Bau82).

2) poor bonding between the exposed tooth structure,

composed of hydrophilic proteinacebus dentin tubules,

and the hydrophobic dimethacrylate composite (Sod91).

The volumetric shrinkage is due to the reduction in

molar volume that occurs as vinyl monomers move from Vander

Waals distances to covalent bond distances during

polymerization. Volumetric shrinkage leads to poor marginal

adaptation to the tooth structure which causes marginal

leakage and recurrence of caries (Bra86). Also, excessive

stresses are generated in the restoration which create

failures in both the remaining tooth structure and or the

restoration depending upon the geometry of the restoration

(Dav91). Shrinkage in current BisGMA based composite ranges











from 1 to 6 volume % (Sul93). Variation may be attributed

more to different glass loadings and varying levels of

conversions than to any major resin development.


2.2.2.Reduction of Polymerization Shrinkage in Dental Resins


There are many research programs focused on different

chemical structures and processes that will reduce

polymerization shrinkage (Bra92, Bye92, Sta91, Liu90). The

main thrust in dentistry has been the spiro orthocarbonate

(SOC) based systems which are the result of early pioneering

work by Bailey (Bai72). The spiro orthocarbonate reaction

involves a cationic dual ring opening mechanism which

increases the molar volume of the polymer compared to that

of the monomer. A general reaction schematic of spiro

orthocarbonate polymerization is shown in Figure 2.9.

Although zero shrinkage resins can be produced, deficiencies

with this ring opening system include the slow cure

kinetics, the inability to reduce shrinkage under non-ideal

conditions and cost of the reactive monomer (Bra92, Bye92).

Typically, dental restorations can be cured within a few

minutes whereas the spiro orthocarbonates are very slow

reacting.

These systems do provide some insight as to a logical

step in the evolution of dental restoratives. The ring

opening polymerization kinetics may be too slow, however,












SOC monomer

R-9 0: R



o
R R

R R


ationic initiator
+


propagation
R -o 0 R
R-XD-R


Poly(SOQC
o
0

x
R


Figure 2.9 Mechanism of ring opening polymerization of
spiro orthocarbonate monomers.











the ring opening mechanism does provide a net increase in

molar volume.

The objective of this study is to determine if the

combination of the rapid kinetics of the methacrylate resin

with the ring opening of anhydride structures can be used to

minimize the polymerization shrinkage. Specifically, cyclic

anhydride functionality in the form of maleic anhydride was

be incorporated into the dental restorative based upon

propoxylated BisGMA resin.


2.2.3.Use of Anhydrides in Dental Applications


Anhydrides have received some attention in dental

applications. However, the anhydride is used as part of a

bonding agent and generally the ring opened form is

generally present at the time of application.

Peutzfeldt and Asmussen (Peu91) have shown that the

addition of maleic anhydride to dentin bonding agents can

increase the mechanical properties by nearly 300% when

combined with a secondary amine containing monomer such as

urethane dimethacrylate. Their studies demonstrate the

ability of the maleic anhydride to increase mechanical

properties, however, they fail to isolate the ring opening

mechanism. By mixing maleic anhydride and other anhydrides

with the primary amine or a hydroxyl containing monomer such

as hydroxyethyl methacrylate (HEMA), either an amide linkage

or an ester linkage is formed. Thus, the ring opening










occurs prior to the polymerization reaction and hence has no

influence on the volumetric shrinkage.

Another example of the use of the anhydride structure

in dental restorative materials is 4-META or 4-

methacryloxyethyltrimellitate. Normally this reactive

component is supplied in the dicarboxylic acid form and thus

has no influence on polymerization shrinkage. It is however

very effective in promoting adhesion to the enamel structure

as well as numerous other substrates (Nak80). The

dicarboxylic acid structure of the 4-META monomer enhances

the wetting or spread of the resin onto the tooth structure

by lowering the surface energy of the exposed structure.


2.2.4.Filler Modification in the Reduction of Overall
Composite Shrinkage


Methods involving reactions of the composite

reinforcing phase have been evaluated as a possible method

of offsetting polymerization shrinkage in dental composites.

Liu et al. (Liu90) have used ammonia modified

montmorillonite (MMT) as the reinforcing phase in BisGMA

composites. At temperatures between 45 and 80C, gaseous

ammonia is released. The gas remains trapped within the

reinforcing phase and causes dilation of the montmorillonite

particles.

Liu et al. have shown that polymerization shrinkage can

be completely offset using this method in cold cure systems.

However, this process has not been extended to directly







43



placed dental restorations. One possible reason is that the

1NIT functions due to the heat rise within the composite

system that elevates the composite temperature. The heat

rise in experimental systems using larger volumes of resin

may be greater than the heat rise seen in actual dental

restorations.
















CHAPTER 3
MATERIALS AND METHODS


3.1. Materials



3.1.1.Macromonomer Reactants


Monomers used in the macromonomer synthesis included

butyl acrylate, methyl methacrylate, and octafluoropentyl

methacrylate. The monomers were obtained from Aldrich

Chemical Co. All were purified by fractional vacuum

distillation (l-2mm Hg) and the middle 80% was collected.

All monomers were stored under dry nitrogen over molecular

sieves.

The functionalizing agent used for the chain transfer

polymerization was cysteine. Cysteine was also obtained

from Aldrich and was used as received. Extreme care was

taken to keep the cysteine stored under a dry nitrogen purge

in order to prevent oxidation to cystine, the disulfide

product of the oxidation.

The solvents in the macromonomer synthesis, HPLC grade

tetrahydrofuran (Fisher), ACS grade ethanol (Fisher) and

UltrapureTM water were used as received. HCl was also used

in the reaction. It was obtained from Adlrich as a ION HC1

solution and diluted as required. Azobisisobutyronitrile











(AIBN) initiator was obtained from Kodak and purified by

recrystallization from ethanol.


3.1.2.Graft Copolymer Reactants


Catalysts for the condensation polymerization,

triphenyl phosphite and LiCl, were obtained from Aldrich and

use without further purification. Care was taken to keep

both of these hygroscopic chemical dry. They were stored

under dry nitrogen and only opened within a drybox.

Pyridine and N-methyl pyrrolidone were used as solvents

in the graft copolymerization of the macromonomers with

polyamide precursors. Anhydrous pyridine was purchased from

Aldrich and kept continuously under dry nitrogen. Peptide

synthesis grade N-methyl pyrrolidone (NMP) was obtained from

Fisher, purified by distillation, and stored over molecular

sieves.

The amide precursors used in this study, p-aminobenzoic

acid (ABA), adipic acid (AA), and p-phenylenediamine (PhD),

were obtained from Aldrich. ABA was used without further

purification. AA was recrystallized from ethanol-water and

PhD was recrystallized from ether.


3.1.3.Dental Monomers


Monomers used in the modification of dental resins

included propoxylated Bisphenol A glycidyl methacrylate

(pBisGMA), triethyleneglycol dimethacrylate (TEGDMA), 2-










phenylethyl methacrylate (PEMA), and maleic anhydride (MA)

All methacrylate monomers were obtained from Polysciences

Inc. Maleic anhydride in the form of briquettes were

obtained from Aldrich.

TEGDMA and pBisGMA were purified by passing acetone

solutions through Aldrich inhibitor removal columns,

followed by evaporation at reduced pressure to remove the

acetone. PEMA was fractionally vacuum distilled and maleic

anhydride was recrystallized from benzene. AIBN initiator

was purified by recrystallization from ethanol.


3.2. Methods



3.2.1.Synthesis and Characterization of Macromonomers


Synthetic procedure. Monomer concentrations in the

polymerizations were maintained constant at 16wt.%. AIBN

concentrations also remained constant at 0.1 mole % of the

monomer concentration. Cysteine levels were varied in order

to determine its affect on the polymerization of acrylates

and methacrylates.

In a typical polymerization, cysteine was dissolved in

the prescribed amount of 1ON HC1 in a 200ml roundbottom

flask equipped with a magnetic stirrer. Water and THF were

then added in concentrations yielding a 50g solution of

96.5/3/0.5 ratio, by weight, THF/water/HCl. 10g of monomer

were added and the desired AIBN concentration was then










dissolved in the reaction mixture. A reflux condenser was

attached to the flask. The reaction setup was then placed in

a glycerin bath at 65C and run for 6 hours under constant

stirring. The isolation and purification of the various

macromonomers synthesized is described in the corresponding

results section 4.1.2, 4.1.3, and 4.1.4.

Characterization of macromonomers. The molar mass

distributions of the synthesized macromonomers were

characterized by GPC using a Waters HPLC system including a

Waters 600 Fluid Delivery Systems, a Waters 717 Autosampler,

and a Waters 410 Differential Refractometer detector. Four

Phenomenex crosslinked polystyrene columns with pore sizes

of 105, 104, 500, and 100A were used in series. The flow

rate was 0.4ml/min. Sample concentrations were

approximately 0.25% in HPLC grade THF. The injection volume

was 50ptl. All molar mass values were calculated using a

polystyrene calibration curve. Anionically polymerized

polystyrene standards were obtained from Polymer

Laboratories.

Transmission FTIR spectra of macromonomers were

collected using a Nicolet 20SX spectrometer. 128 scans were

collected for each sample at a resolution of 4cm-. All

liquids were run between KRS-5 crystals. Solids were run in

transmission with KBr.

NMR spectroscopy was performed using a 300MHz Gemini.

The solvent used for macromonomer characterization was










deuterated chloroform. TMS was used as an internal

standard. 64 acquisitions were collected for each sample.

Elemental analysis for determination of C, H and N

content was run on an Eager 200.

ICP was run in order to determine the functionalization

efficiency of the macromonomer synthesis. A Perkin Elmer

Plasma 40 Emission Spectrometer was used and a wavelength of

180.73nm was monitored. This wavelength was used to

determine the sulfur content of the polymer. A sulfur

standard was obtained from Fisher and diluted using

volumetric flasks.

Sample preparation for ICP involved making a 0.5 wt.%

solution of the macromonomer in a 96/4 mixture, by weight,

of water/triton X. Triton X was used as a surfactant in

order to stabilize the emulsion of the poly(butyl acrylate)

in water. Only the liquid, low Tg, macromonomers could be

analyzed by this method. Stable emulsion of high Tg

methacrylate copolymers could not be obtained.

Sulfur content was determined in ppm in solution.

Using the value of sulfur concentration in combination with

the molar mass values we can calculate the extent of

functionalization. The following is an example of one of

these calculations. One mole of 2.6kg/mol poly(butyl

acrylate), from GPC analysis, in which every chain is end

capped by one mercaptan chain transfer agent residue will

contain one mole of sulfur or 32g. Therefore, 32/2600 or

1.23 wt.%. If the prepared solution contains 0.5 wt.%










macromonomer, the solution should contain 0.005*0.0123=

62ppm of sulfur. Again, if every chain were functionalized,

we should measure a sulfur concentration of 62ppm. Instead

a concentration of 46ppm was measured. From this we

estimate that 46/62 or 75% of all chains contain one sulfur

molecule or 75% are functionalized.

DSC analysis was performed using a Seiko DSC 220

interfaced with a Seiko 6500H Rheostation. The analysis was

performed at heating rate of 10C/min under a continuous

flow, 10Oml/min, of dry nitrogen gas. On average, 10 mg

samples were analyzed in crimped aluminum pans versus an

inert sapphire reference.


3.2.2.Synthesis and Characterization of Graft Copolymers


Synthetic procedure. Stoichiometric molar

concentrations of amine and carboxylic acid groups were used

with a total amount of reactants equal to 5mmol. Triphenyl

phosphite was added at a 1:1 mole ratio of TPP:carboxylic

acid groups. The amount of LiCl added was kept constant

throughout as was the type and amount of solvent used.

In a typical polymerization, 1.37g of 2.6 kg/mol p(BA)

macromonomer (0.53 mmol), 0.241g (2.24 mmol) p-

phenylenediamine, 0.326g (2.24 mmol) adipic acid, 1.55g TPP

(Smmol), and 0.09g LiCl were dissolved in 30ml of an 80/20

NMP/pyridine solution in a 100ml flask. All mass readings

and component mixing was performed in a drybox. The










reaction mixture was then heated at 100C for 4 hours. The

resulting polymer, a tacky light brown solid, was obtained

almost quantitatively by precipitation in an excess of 50/50

water/methanol nonsolvent, filtered, washed with methanol

and dried overnight under vacuum at 40C.

Characterization of graft copolymers. Purification of

the graft copolymers was done by Soxhlet extraction using

HPLC grade THF. This was done in order to remove any

homopolymer which may result from unfunctionalized

macromonomers. Samples ranging from 0.8-l.Og were extracted

to constant weight using a Whatman cellulose extraction

thimble.

The extractant solutions were diluted to appropriate

concentrations for GPC analysis. The level of dilution

required was dependent on the amount of material extracted.

GPC was run with simultaneous detection using the

differential refractometer described previously as well as a

Waters 996 Photodiode Array UV detector. This detector

allows one to get a full UV scan at each elution time. This

affords the ability to determine structural differences

between UV absorbing fractions within the solution. All

other testing conditions were identical to those for

characterization of macromonomers.

Transmission FTIR spectra of the graft copolymers were

collected using KBr with collection parameters equivalent to

those described previously.










Elemental analysis and DSC methods were identical to

those used in macromonomer characterization.

The only difference in the NMR spectroscopy from that

of the macromonomer was the solvent employed. Deuterated

sulfuric acid was used as the solvent and the solvent peak

from residual undeuterated acid was used as an internal

standard.

The synthesized graft copolymers were blended with

commercial extrusion grade Nylon 6 from BASF and with blends

of Nylon 6 with 65kg/mol poly(butyl acrylate) synthesized in

this laboratory. Initial attempts were made to blend the

polymers by dissolution in dichloroacteic acid followed by

coprecipitation in methanol. After vigorously drying the

resulting powders, films were compression molded at 230C.

This method was abandoned when the pure nylon 6 prepared in

this manner showed severe embrittlement. Either residual

acid caused degradation or the dissolution-precipitation

step removed a stabilizer.

The materials were then mixed in the solid state. In

order to get the most homogeneous mixture of graft copolymer

with Nylon 6 and graft copolymer with Nylon 6/Poly(butyl

acrylate), the samples were mixed at cryogenic temperatures.

A SPEX 6200 Freezer Mill was used at maximum impact

frequency with the sample immersed in liquid nitrogen. All

samples were milled for 8 minutes. The resulting powders

were homogeneous in appearance. All blends were compression

molded at 230C between Teflon coated polyimide films and











allowed to cool in the mold. The resulting films were

approximately 0.2mm thick.

Tensile properties of the films were measured using an

Instron 1122 equipped with an 890 Newton load cell at

ambient conditions. Five samples were tested for each

material according to ASTM D638M.


3.2.3.Synthesis and Characterization of Anhydride Modified
Dental Resins


Synthesis and sample preparation. An experimental

matrix was prepared composed of varying anhydride, pBisGMA,

and TEGDMA concentrations. Dental resin samples were

prepared by dissolving the maleic anhydride in the dental

methacrylate monomer compositions. 0.4 wt.% AIBN initiator

was added to the solution. The resulting viscous solutions

were transferred to a 2mm thick mold. The mold consisted of

two glass plates lined with Teflon coated polyimide film

and a fluoropolymer elastomeric tubing to keep the solution

in the mold. The solutions were cured under a dry nitrogen

atmosphere at 75C for 12 hours followed by a postcure at

160C for 2 hours.

Linear copolymers of maleic anhydride with phenylethyl

methacrylate were synthesized in bulk. The desired

anhydride amount was dissolved in PEMA in a 15ml glass test

tube and 0.4 wt.% AIBN was added. The solution was purged

with dry nitrogen, sealed, and polymerized at 75C for 4

hours. The resulting polymer was isolated by dissolution in











chloroform and precipitation in ether. Samples were filterd

and dried overnight under vacuum at 40C.

Characterization of anhydride copolymers. Equilibrium

water content was measured using Ultrapure water. Samples

were approximately 2mm x 5mm x 10mm. Samples were placed in

an incubator at 37C until a constant weight was reached.

The swelling solution was then changed, adding fresh

Ultrapure water and weight again monitored to insure all

unreacted anhydride has been extracted. On average

equilibrium was reached after 1-2 weeks.

All weights were measured using a Denver Instruments A-

200DS with readings taken to the fifth decimal place.

Density measurements were taken using a Mettler 33360

Density determination apparatus in combination with the

Denver Instruments scale. The Archimedes' method was used

and measurements ere taken at 25C using Ultrapure water.

All samples were re-weighed after testing to insure that the

time scale of these measurements were insufficient to allow

the samples to absorb water.

UV absorption spectroscopy was run on the extractant

solutions in order to monitor the extraction of maleic

anhydride. Actually, any extracted anhydride would be

extracted as maleic acid due to hydrolysis. A calibration

curve for absorbance versus concentration was made using

standards. Standards were prepared in the expected

concentration range by hydrolyzing and dissolving maleic

anhydride in Ultrapure water. A wavelength of 274nm was











used. Thus, by monitoring the absorbance at 274nm of the

extractant solutions, concentrations of maleic acid could be

determined. Knowing the volume of water used in the

swelling experiments, we can calculate the mass of maleic

anhydride extracted. Assuming all free anhydride was

extracted, the anhydride not extracted is assumed to be

incorporated.

GPC, FTIR, and DSC of the linear maleic anhydride

copolymers were run with parameters as described in the

characterization of the polyamide graft copolymers.
















CHAPTER 4
RESULTS AND DISCUSSION


4.1. Synthesis and Characterization of Amino Acid-terminated
Poly(acrylate) Macromonomers using Chain Transfer
Chemistry


The most prevalent synthetic route to well defined

graft copolymers is through the use of macromonomers, low

molar mass polymers containing a polymerizable end group. A

review of the literature has shown that current studies,

including those within this laboratory, have concentrated on

synthesizing macromonomers which are polymerizable through a

vinyl functionality. That is, they are polymerizable only

with addition type monomers through a free radical

mechanism. Because the macromonomers themselves are

generally synthesized through either anionic or free radical

polymerization in the presence of a functionalizing agent,

the resulting graft copolymers which can be synthesized

through this method are generally limited to addition-

addition graft copolymers such as poly(styrene-graft-

acrylate), poly(acrylate-graft-methacrylate), etc...

It was the objective of this study to synthesize

condensation polymerizable macromonomers, specifically amino

acid terminated macromonomers, capable of reacting with

amine and carboxylic acid groups in the synthesis of











polyamide graft copolymers. An amino acid functionality is

preferred over other end groups such as diacids or diamines

due to the inherent stoichiometry that it provides. This

stoichiometry is required in the condensation graft reaction

to insure the highest degree of polymerization possible.

The approach taken in this study involves the free

radical polymerization of a vinyl monomer in the presence of

a functional chain transfer agent (Ito77). Mercaptans,

compounds containing a sulfur-hydrogen bond, are commonly

used in chain transfer reactions. In fact, mercaptans are

commonly used to control molecular weight in commercial

polymerization reactors (Ros82).

The mechanism by which functionalization can occur is

depicted in Figure 4.1. Steps 1 and 2 are typical processes

of free radical initiation. When exposed to heat, the AIBN

breaks down into free radicals and nitrogen gas is evolved.

The AIBN radicals can thus initiate the polymerization of

vinyl compounds. If there is no chain transfer agent

present, the polymerization continues until termination by

disproportionation or combination occur. In the presence of

a mercaptan or other chain transfer agent, termination can

occur through chain transfer. The hydrogen from the

sulfhydryl group of the mercaptan is readily extractable. A

propagating polymer chain can thus react with the mercaptan

(Step 3), terminating propagation and leaving a sulfur

radical on the mercaptan. If the concentration of mercaptan











AIBN
C, -N C=, N
I CH3-,-N=N -CH3 3
CH 3 CH 3


__C=N
2 CH3- +
CH 3


monomer polymerization
i ---


3 o---- ... + H:S-R'


CN mercaptan
4 CH 3 + H:S-R'
CH 3

5 R'S.- + -- B


6 R' -- + H:S-R'


-------- I- + R'S


--- CH3- H + R'S.
CH 3


Figure 4.1. Mechanism of functionalization using chain
transfer reactions.


CH3


---IN R' ---------------+ R'S.










is high, the mercaptan itself can react with the AIBN

radical (Step 4), also giving a sulfur radical. The

resulting sulfur radical can then initiate the free radical

polymerization of a vinyl monomer (Step 5).

If the mercaptan contains hydroxyl or carboxylic acid

functional groups (R'), the initiating sulfur radical

introduces functionality to one end of the macromolecule.

The growing functionalized polymer radical can again react

with the mercaptan (Step 6) yielding a terminated

functionalized chain and another molecule of sulfur radical

which can react with more monomer (Step 5) to form a

reaction loop. The effectiveness of functionalization is

dependent on the chain transfer constant of the mercaptan as

well as the relative concentrations of mercaptan and free

radical initiator (Tsu91). The AIBN concentration is kept

extremely low relative to the chain transfer agent to

minimize the number of chains initiated by the AIBN. Any

chains initiated by AIBN will be non-functionalized (see

Steps 2 and 3).

Cysteine, a naturally occurring amino acid containing a

sulfhydryl functional group, was evaluated as a chain

transfer agent in the polymerization of acrylates and

methacrylates. If cysteine were to act as an effective

chain transfer agent, it would provide the desired amino

acid functionality.

Specifically, poly(butyl acrylate) and poly(methyl

methacrylate-co-octafluoropentyl methacrylate) macromonomers











were synthesized in the presence of cysteine. Poly(butyl

acrylate) was chosen because it has a very low glass

transition temperature, -54C (Bra89), and therefore any

graft copolymers containing it may be used as rubber

modifiers. The fluoroacrylate copolymer was chosen because,

due to the low surface energy of fluoropolymers in general,

graft copolymers could be utilized as surface modifiers.


4.1.1.Determination of an Appropriate Solvent System


A common solvent for both the monomer, either butyl

acrylate or the fluoroacrylate-MMA mixture, and the cysteine

chain transfer agent must be identified in order to insure a

homogeneous solution during polymerization. Cysteine is a

crystalline powder insoluble in common organic solvents.

Solubility tests in the approximate concentrations required

for synthesis of a 3kg/mole macromonomers were performed.

As is shown in Table 4.1, at the appropriate concentrations,

cysteine is insoluble in some common organic solvents which

are suitable for the polymerization of butyl acrylate. Also

butyl acrylate is completely immiscible with water, quickly

separating into two layers.










TABLE 4.1 Solvent determination for monomer and chain
transfer agent


_____Toluene THF DMF Ethanol n-Butanol H20

Cysteine i i i i i s


Butyl s s s s s i
acrylate__________________________________
Concentration of cysteine= 0.03g/5g solvent. Concentration of monomer=
lg/5g solvent. i= insoluble, s=soluble.


Because of the strong H-bonding interactions within the

amino acid, it appeared to be necessary to add H20 to

disrupt crystalline structure. Cysteine was then pre-

dissolved in water at high concentrations prior to the

addition of THF. This method was successful in keeping

cysteine dissolved in a THF/water mixture. One of two

things generally occurred upon the addition of butyl

acrylate. Either the concentration of water was too high to

allow the butyl acrylate to dissolve, or the concentration

was too low to prevent the precipitation of cysteine upon

the addition of the acrylate monomer. It was determined,

after much trial and error, that ethanol could be added in

low concentrations in order to stabilize the

THF/water/cysteine/butyl acrylate solution. The ethanol was

effective in preventing the butyl acrylate from forming a

second phase. The composition of the solvent system used

was a 80/10/10 ratio, by weight, of THF/EtOH/H20. The

monomer concentration was 15 wt.%.










4.1.2.Preliminary Studies of the Effectiveness of Cysteine
as a Chain Transfer Agent in the Polymerization of Butyl
Acrylate


The synthesis of poly(butyl acrylate) in the presence

of cysteine was carried out using the solvent system

described above. Figure 4.2 depicts the desired amino acid

functionality of the macromonomer. The

monomer:cysteine:AIBN molar ratio used was 1000:30:1. The

AIBN concentration must be kept low in order to minimize the

number of chains initiated by AIBN. As stated previously,

any chains initiated by the AIBN initiator and not the chain

transfer agent will be in effect 'dead' chains. That is,

they will lack the desired amino acid functionality. The

polymerization was run under nitrogen at 65C for 7 hours.

A control polymerization was also run under the identical

conditions in the absence of the cysteine chain transfer

agent. The resulting polymers were isolated by rotary

evaporation under vacuum at 40C. Due its low glass

transition temperature, poly(butyl acrylate) is virtually

impossible to isolate by precipitation in a non-solvent.

The reaction product of the control reaction was a clear,

extremely tacky, viscous material with a yellowish haze.

The cysteine modified product was very similar with the

exception of the presence of a white precipitate dispersed

within the poly(butyl acrylate). This precipitate could be

separated from the polymer by dissolving the poly(butyl













butyl acrylate

CH2=CH
-o +


8H2
&H2
&H2
&3H


cysteine
0
NH2-CH---OH


IH
0.2 wt % AIBN
65C, 7 hrs.
THF: EtOH :H20


0
NH2Z-CH---OH
&2




Poly(butyl acrylate)


Figure 4.2 Schematic of ideal amino acid functionalization
during polymerization of butyl acrylate.










acrylate) in THF. The precipitate was insoluble in THF and

could therefore be collected by filtration.

Gel permeation chromatography of p(BA). Molar mass

averages and distributions were measured using GPC in order

to determine the effectiveness of chain transfer. Chain

transfer should significantly reduce the molar mass of the

resulting polymer. If cysteine were to have a chain

transfer constant equivalent to other commonly studied

mercaptans, it should drastically decrease the molar mass of

p(BA) when compared to a neat polymerizaion. For example,

similar concentrations of mercaptoethanol chain transfer

agent have been used in this laboratory in the synthesis of

hydroxyl terminated poly(styrene-acrylonitrile). Using the

Mayo equation described in section 2.1.1.3, we can calculate

that mercaptoethanol (chain transfer constant=l.1 (Zam95))

would yield an oligomer with a molar mass of @4kg/mol.

The molar mass distributions of the control and

cysteine modified P(BA) are shown in Figure 4.3. The number

average molar mass (Mn) for the control sample was 63kg/mol,

with a polydispersity index of 2.9. The product of the

cysteine modified polymerization has an Mn= 19kg/mol. The

molar mass was reduced, but not as much as would be expected

if efficient chain transfer occurred. Also, the cysteine

modified P(BA) has a much broader distribution, with a

polydispersity index of 4.8.

Analysis of GPC data. In order to properly explain

these results, the presence of the side product of the


















/


I:


Log MW



Figure 4.3 GPC results of preliminary butyl acrylate
polymerizations.


- -- -neat Poly(butyl acrylate)
Mn=63Kg/mol Mw=186Kg/mol
PDI=2.9
............. PBA-cysteine pH=6.7
Mn=19Kg/mol Mw=91Kg/mol
PDI=4.8


I
1











cysteine reaction must be explained. The side product, with

a melting point of @ 196C, was determined to be crystalline

by DTA (Figure 4.4). The FTIR spectra of this product,

shown in Figure 4.5, shows the presence of a ester carbonyl

at 1740cm-1. The broad absorption from 3400 to 2400cm-1

suggests the presence of a carboxylic acid functionality.

Figure 4.6 shows the same spectra of the side product,

focusing on the area from 1800 to 1500cm-1, as compared to

butyl acrylate monomer. The side product does not have the

sharp absorbance at 1640cm-1 associated with the vinyl

functionality in the butyl acrylate monomer. There is a

broader absorption centered around 1580cm-1 which is

representative of an amine functional group.

Upon investigation of the literature for possible

explanations of this side reaction of cysteine, a study by

Friedman et al. (Fri65) was found in which they investigated

possible blocking agents for sulfhydryl groups in proteins.

They showed that acrylates, in aqueous conditions, can react

with cysteine through the sulfhydryl group. The S:H

functional group can ionize under basic conditions into a

sulfur anion according to their reaction pathway (Figure

4.7). The sulfur anion then attacks the vinyl group of the

acrylate. The resulting carbanion is immediately capped by

a proton in the aqueous solution.









2-


0-


-2-


-4-


-6-


-8-


-10-


-12


Tm=196C


i I I
0 50 100


I I I I I
150 200 250 300 350 400
Temperature (C)


Figure 4.4 DTA trace of side product.










1.0



0.8



0.6-



0.4



0.2



0.0
4000 3000 2000 1000
Wavenumbers (cm-1)


Figure 4.5


FTIR spectra of side product of cysteine
modified P(BA).











Side product of cysteine reaction
Samine


1.0



0.8



0.6



0.4


0.0
1800 1700 1600 1500
Wavenumbers (cm-1)


Figure 4.6 Comparison of FTIR spectra of side product with
butyl acrylate monomer.


ester


Butyl acrylate
monomer


C=C
I












RSH +H20 RS- + H30+

RS- + CH2=CH-COOR RS-CH2-CH-COOR

RS-CH2-CH-COOR + H30+ RS-CH2-CH2-COOR + H20

Figure 4.7 Reaction pathway of cysteine with acrylates.



When butyl acrylate is used, the product formed is

S-carbobutoxyethylcysteine (Figure 4.8). According to

Friedman, this compound has a melting point of 194-195C,

which is in good correlation to the Tm of the side product,

i.e., 196C. Elemental analysis of the side product

confirmed the correct chemical formula for S-

carbobutoxyethylcysteine (Figure 4.8).

In order to minimize or eliminate this side reaction,

we must understand why it is occurring. In an aqueous

solution, the ionization of the sulfhydryl group is an

equilibrium reaction. It is the anionic form of cysteine

which can react with butyl acrylate to form the side

product. This equilibrium reaction has a pKa associated

with it and therefore the relative concentrations of ionized

to unionized species are influenced by the pH of the

solution as governed by the Henderson-Hasselbach equation,

(Is l
pH = pKa + log( ) (4.1)


where [S-] is the concentration of sulfur anion and [SH] is

the concentration of the unionized sulfhydryl group. The pKa







S-Carbobutoxyethylcysteine

NFH-CH--OH

6H2
6H2

6=o
6
ICH2)3
CH3b

Calculated wt.% for Co10H19N04S:
C, 48.19; H, 7.63; N, 5.62
Found:
C, 48.47; H, 7.65; N, 5.51

Figure 4.8 Structure and elemental analysis of side
product.











of the sulfhydryl group of cysteine is 8.3 (Voe90). In

order to determine the relative concentration of ionized

cysteine, the pH of the THF/EtOH/H20/butyl acrylate/cysteine

solution was then measured. The pH of the solution, 6.7, is

below the pKa. Using equation 4.1, we can then determine

that approximately 3% of the cysteine at a pH of 6.7 is

present in the ionized form and therefore 3% is capable of

reacting with butyl acrylate in the side reaction.

If the effective concentration of chain transfer agent

were only being reduced by 3%, we would expect that there

would be sufficient unionized active cysteine to

significantly affect the molar mass of the poly(butyl

acrylate). Again, in these chain transfer polymerizations,

the molar mass is governed by the chain transfer constant as

well as the relative concentrations of monomer, chain

transfer agent, and free radical initiator. A 5% decrease in

cysteine concentration would indicate that instead of a

1000:32:1 monomer:cysteine:AIBN molar ratio, we would be

working with a 1000:31:1 molar ratio which should still

significantly decrease the molar mass of the poly(butyl

acrylate). One would expect a 5kg/mole instead of 4kg/mole

macromonomer.

The final mixture of products in this reaction depends

upon the relative rates of cysteine consumption in the chain

transfer reaction and the side reaction. Friedman has shown

that the reaction between the sulfur anion and acrylates

occurs almost instantaneously (Fri65). On the other hand,











the polymerization of butyl acrylate occurs over a period of

hours. The ionized cysteine is thus consumed at a higher

rate than unionized cysteine. The desire for equilibrium in

the ionization reaction (Figure 4.9) drives the reaction

further to the right which leads to further ionization of

cysteine and thus formation of more side product. Over

time, the unionized cysteine concentration decreases until

there is no cysteine present to cause chain transfer.

The unexpected GPC results shown previously in Figure

4.3 can now then be explained. At the early stages of the

polymerization of poly(butyl acrylate), unionized cysteine

is present at relatively high concentrations and low molar

mass macromonomer is being produced. As time goes on, the

cysteine concentration relative to butyl acrylate is

decreasing leading to less termination through chain

transfer and therefore larger molar masses. By the end of

the reaction, all of the cysteine has been consumed by the

side reaction and high molar mass poly(butyl acrylate) is

being formed. This would explain the extremely broad molar

mass distribution observed in the GPC analysis. More

pertinent than the actual molar mass values is that in the

absence of cysteine, initiation will come mainly from the

AIBN and termination will occur by disproportionation

(Bra89), leading to high molar mass unfunctionalized chains.

Prevention of undesired side reaction. It is clear

that in order to achieve complete chain transfer and







0 0
II pKa=8.3 II
NH2-CH-C-OH NH2-CH-C-OH
CH2 CH2
SH I
SH b:-.


Sbutyl acrylate

Chain transfer
functionalization


0
II
NH2-CH-C-OH

i:H c
Jc;H
C=0
(CH2)3
tH3


+ H+


y butyl acrylate


3-Carbobutoxyethyl-
:ysteine


Figure 4.9 Side reaction preventing complete chain
transfer.











therefore produce highly functionalized macromonomers, side

reactions must be reduced or eliminated. In order to

prevent the formation of S-carbobutoxyethylcysteine, we must

prevent the ionization of cysteine. By acidifying the

polymerization solution, the initial concentration of sulfur

anion can be virtually eliminated. For example, again using

equation 4.1, if we were to acidify the polymerization

solution to a pH of 1, the concentration of ionized cysteine

would be reduced from 5% to 50 parts per billion. Although

this ionized cysteine will still form the byproduct, the

effective concentration of unionized cysteine will remain

the dominant species.

A new reaction was run under similar conditions to the

previous cysteine modified polymerization with the addition

of hydrochloric acid. The addition of HCl facilitated the

dissolution of cysteine. In fact, a homogeneous

polymerization solution could be achieved in a solvent

system containing 96.5/3/0.5 weight ratio THF/water/HCl.

Because the concentration of water could be significantly

reduced, the addition of ethanol was not necessary to keep

the butyl acrylate monomer in solution. The pH of the

reaction mixture was reduced to -0.76 and the reaction again

was run for 7 hours at 65C. At this pH, only lppb of

cysteine is present in the ionized form.

The resulting solution was neutralized with pyridine.

Pyridine hydrochloride immediately began to precipitate and

was filtered out. The poly(butyl acrylate) solution was











evaporated to dryness under vacuum at 40C. The product

yield was 52% of theoretical. Again a yellowish viscous

liquid was isolated. The most significant visual difference

between this product and those of the previous reaction was

the much lower viscosity, evidence of lower molar mass

polymer. Also there was no evidence of the formation of the

S-carbobutoxyethylcysteine.

GPC was run on the isolated product. Figure 4.10 shows

the effect of acidification on the molar mass distribution

of poly(butyl acrylate). The concentration of cysteine was

identical to that in the previous reaction. Yet, the molar

mass and polydispersity of the poly(butyl acrylate) was

significantly reduced, i.e., with an Mn of 2.6kg/mol and a

polydispersity index of 2.3. The addition of HC1 allowed

cysteine to participate in the chain transfer reaction. As

was stated previously, if cysteine were to behave as

effectively as other mercaptan chain transfer agents, a

molar mass of 4kg/mol would be expected.


4.1.3.Determination of Chain Transfer Constant of Cysteine
in the Synthesis of Amino Acid- terminated Poly(butyl
acrylate) Macromonomers


The first step in the use of chain transfer agents is

the determination of the chain transfer constant. Knowledge

of the chain transfer constant allows us to predict critical

variables such as molar mass and functionality (Nai92). The

chain transfer constant can be determined using the Mayo
















- neat Poly(butyl acrylate)
Mn=63Kg/mol Mw=186Kg/mol
PDI=2.9
PBA-cysteine pH=6.7
Mn=19Kg/mol Mw=91Kg/mol
PDI=4.8
PBA-cysteine:HCI pH=-0.76
Mn=2.6Kg/mol Mw=6.1Kg/mol
1 ___ P D I= 2 .3_ _ _



*-/ \\

) I \ ""
(C, I..
C
I ,/ \ \... .


0 I II
7 6 5 4 3 2
Log MW


Figure 4.10 Effect of acidification on chain transfer
functionalization of poly(butyl acrylate).










model (Ros82). The variation of the degree of polymerization

in a free radical chain transfer polymerization is given by


1Pr k __Y +Cs [S] (4.2)




where Cs = Chain transfer constant

[S]= Chain transfer agent concentration

[M]= Monomer concentration

DPn= Degree of polymerization

kt,kp,Rp= rate constant for termination,

propagation, and the rate of polymerization

respectively.


The degree of polymerization in the absence of chain

transfer, DPno, can be described by equation 4.3 where




DP ~o [[2 (4.3)



Substituting this value in equation 4.2 gives us the Mayo

equation (4.4) for prediction of the chain transfer constant

where


1 1 [S]
+Cs[- (4.4)
DPn DPno ]










This equation is valid only when the initiator

concentration is low. By synthesizing a series of polymers

with different ratios of chain transfer agent to monomer, a

Mayo plot (Tsu91) can be used to determine Cs to prove that

cysteine is an effective chain transfer agent.

The Mayo model can also be used to predict the

functionality of the polymer obtained. If we multiply both

sides of equation 4.4 by DPn, we get


~DP,, IS]
SDP,, +DP,,Cs[] (4.5)
DPp10 [M]

where the two terms on the right represent the fraction of

unfunctionalized and functionalized chains.

Synthesis. Butyl acrylate:cysteine:AIBN mole ratios of

1000:64:1, 1000:32:1, 1000:16:1, 1000:0:1, were polymerized

in a THF/H20/HC1 solvent system identical to that previously

described in section 4.1.2. The only difference was in the

purification. After isolation, the polymers were extracted

with UltrapureTM water in order to remove any traces of

unreacted cysteine or AIBN.

GPC results. Molar mass distributions are shown in

Figure 4.11. The Mn systematically decreases as the

relative concentration of chain transfer agent is increased.

The Mn of the neat poly(butyl acrylate) was 65kg/mol with a

polydispersity index is 3.0. The Mn at the highest

concentration of cysteine was 1.3kg/mol. The










polydispersities of the reactions in which cysteine is

present are all significantly lower, i.e., around 2.0.

A Mayo plot of the GPC data is depicted in Figure 4.12.

Referring back to equation 4.4, we can plot the relative

concentration of chain transfer agent to monomer, [S]/[M],

versus the inverse of the number average degree on

polymerization. After performing a regression on the data,

the slope of the line is the chain transfer constant, Cs.

The chain transfer constant



Cs= r/kP (4.6)




where ktr is the rate constant for chain transfer. A larger

chain transfer constant indicates increasing termination by

chain transfer and thus the effectiveness of the chain

transfer agent in participating in the polymerization. The

chain transfer constant calculated for the butyl

acrylate:cysteine system was 1.49. Reported Cs values for

other mercaptans are generally between 0.8 and 1.2. As

stated previously, we have studied mercaptoethanol as a

chain transfer agent and found a Cs of 1.1. Thus, cysteine

is an extremely effective chain transfer agent in the

polymerization of poly(butyl acrylate).

Spectroscopic characterization. FTIR was run in order

to observe any differences in structure between the neat

poly(butyl acrylate) and the product of the cysteine














40 1000:64 BA:cyste
1000:3264 BA:cyste
-.o.-1000:32 BA:cyste
...... 1000:16 BA:cyste
-~- neat PBA
30


20
E

10




7 6 5 4 3 2
log (MW)


Figure 4.11 GPC results of polymerizations of butyl
acrylate varying cysteine concentrations.












12

10 1/DPn= 1/DPno+ Cs[S]/[M]

8 Cs=1.49
V r2 0.99 I
0
T- 6
x
a 4
ST = 65
2 THF/H20/HCI
[AIBN] = 0.2 wt %
0 S : Cysteine


0 1 2 3 4 5 6 7

[S]/[M] x 102


Figure 4.12 Mayo plot for chain transfer constant
determination for cysteine:butyl acrylate
system.










modified polymerizations. Spectroscopic determination and

quantification of amino acid functional groups is

complicated by the low end group concentration relative to

p(BA).

The only difference in the FTIR spectra of the lowest

molar mass poly(butyl acrylate) macromonomer (1.2kg/mol) and

the neat poly(butyl acrylate) (Figure 4.13). The only

significant difference between the two spectra was the low

intensity broad absorption from 3400-2400cm'-. This was

attributed the carboxylic acid from the cysteine end group.

NMR spectroscopy was used in an attempt to better prove

the presence of the amino acid end-group. A comparison of

the 'H-NMR spectra of neat poly(butyl acrylate) (Fig 4.14)

and the 1.2kg/mol macromonomer (Fig 4.15) reveals the

presence of two very small, broad peaks around 6 2.8 and 3.3

for the macromonomer. The chemical shift values are equal

to the predicted shifts of the methylene and methine protons

from the cysteine end-group (Sil91). Again, the peaks are

very low in intensity due to the low end group

concentration. The 13C-NMR spectra of the low molar mass

macromonomer is shown in Figure 4.16. All peaks can be

assigned to that of poly(butyl acrylate) with the exception

of a small peak at 6 31.8 which is assigned to the C-S-R

carbon.

TCP was used to determine the sulfur concentration in

the p(BA). As described in section 3.2.1, this value in













1.0 -



0.8



0.6



0.4 -"


Poly(butyl acrylate)


0.2:i1f
0.2 Amino acid
Terminated
P(BA)
0.0-
4000 3000 2000 1000
Wavenumbers (cm-1)


Figure 4.13


FTIR spectra of poly(butyl acrylate) and
cysteine end-capped p(BA) macromonomer.
Macromonomer is the 1.2kg/mol p(BA)
synthesized using 1000:64:1 butyl
acrylate:cysteine:AIBN mole ratio.







f e
4CH2-CH-)- a TMS
I x
C-O
I
0
I
CH2 d
I

CH2 b
OH3 a
C
b
d




4e3 2 1 ppm

4 3 2 1 0 PPM1


1H-NMR spectra of neat poly(butyl acrylate).


Figure 4.14








TMS


OH
I
U- 9 f e
hC -CH2-S-CH2-CH-)-
h x
NH2 C-0O

0

CH2d

CH2C

CH2 b
IH3 a
CH3a



d




egA

h g A /


I I I tI I I I l II I I I I


4


1 0 ppm


Figure 4.15 'H-NMR spectra of 1.2kg/mol cysteine modidied
poly(butyl acrylate).





180 160 140 120 100 80


)DCI3


h _g f
CH-CH2--SCH2-CH4-
I x
NH2 e C-0O
I
0
I
d CH2

C CH2
b CH2


a CH3





e

,t, li lu.,., .L AL IL JI ,I d ,iu h. ,. ,Ll d. q i ,, I. .. ..


SHI BM n I lr llvll I' iff~all l i .llVifJlmir I' I ,' I I r 1, jl
^ "'n 'A6 1 .'1 1T[ j -


60 40 20


Figure 4.16 13C-NMR spectra of 1.2kg/mol cysteine modified
poly(butyl acrylate).


OH
0=I
O=u


a






TMS


0 ppm


lj.l Jl .l .l. _l. t .1


h

f

HL