The stereochemistry of group transfer polymerization of methyl, diphenylmethyl, and triphenylmethyl methacrylates

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Title:
The stereochemistry of group transfer polymerization of methyl, diphenylmethyl, and triphenylmethyl methacrylates
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vii, 125 leaves : ill. ; 28 cm.
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English
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Banerjee, Krishna G., 1959-
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Methyl methacrylate   ( lcsh )
Stereochemistry   ( lcsh )
Polymerization   ( lcsh )
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bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )

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Thesis:
Thesis (Ph. D.)--University of Florida, 1988.
Bibliography:
Includes bibliographical references.
Statement of Responsibility:
by Krishna G. Banerjee.
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Typescript.
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Vita.

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THE STEREOCHEMISTRY OF GROUP TRANSFER
POLYMERIZATION OF METHYL, DIPHENYLMETHYL, AND
TRIPHENYLMETHYL METHACRYLATES








By

KRISHNA G. BANERJEE


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

1988


EU- F, UBFIUA aIF






















This dissertation is dedicated to my spiritual master, Om Visnupada
Hridayananda dasa Goswami Acaryadeva, (ardent disciple of His Divine
Grace A. C. Bhaktivedanta Swami Prabhupada) who taught me how to
dedicate one's so called assets in this world in the loving service of the
Almighty.













ACKNOWLEDGEMENTS


I am gratefully indebted to the members of my supervisory committee,
Dr. George B. Butler, Dr. John F. Helling, Dr. John Eyler, and Dr. Charles
Beaty. Special thanks and deep appreciation are due to Dr. Thieo E. Hogen-
Esch for his invaluable guidance, moral support, encouragement, patience,
and friendship. Especially valuable was his encouragement during the most
difficult times of the research project, when nothing but failures stared me in
the face.
Thanks are also due to the glassblowers Dick Mosier and Rudy
Strohschein for their fine glassblowing and ever-joking mood; to Mrs. Lorraine
Williams, the polymer floor secretary, for her kindness and motherly affection;
to Dr. Ken Wagener, for his encouragement; and to the other members of the
"polymer floor" for their association and encouragement. Last, but not the
least, I would like to acknowledge the encouragement and support of my
parents, without whom none of this work would have been possible.











TABLE OF CONTENTS



ACKNOWLEDGEMENTS ............................................................................. iii

ABSTRACT ........................................... ....................................................... vi

CHAPTER

I INTRODUCTION ........................................................................... 1

Background ................................................................................... 1
Objectives ...................................................................................... 16

II EXPERIMENTAL ...................................................................... ... 21

Catalyst Syntheses ..................................................................... 22
Synthesis and Purification of Initiators ........................................24
Monomer Syntheses and Purification .........................................35

Methyl Methacrylate (MMA) .................................. ............. 35
Silver Methacrylate ................................................................35
Triphenylmethyl (Trityl) Methacrylate........................................37
Diphenylmethyl Methacrylate..............................................40

Polymerization Reactions ............................................................41

Group Transfer Polymerization of Methyl Methacrylate .......41
Methylation of Chain End of PMMA prepared by GTP.........45
Polymer Isolation for PMMA prepared by GTP.................... 45
Group-Transfer Polymerization of Diphenylmethyl and
Triphenylmethyl Methacrylate........................................... 47
Anionic Polymerization of MMA.................................... ..47
Anionic Polymerization of TrMA..................................... ...49

Polymer Hydrolysis ...................................................................... 52

Poly TrMa ........................................................................... 52
PDMA........................................................................................52

Diazomethane Methylation .......................................... .......... 53
Titration of Alkyl Lithium Solutions.................................. .....53
Instrumentation .............................................................................. 55






CHAPTER age

Gas Chromatography .......................................................... 55
Preparative Liquid Chromatography .....................................56
NMR Spectroscopy ............................................................... 57
Size Exclusion Chromatography (SEC) ...............................58

III GROUP TRANSFER POLYMERIZATION OF METHYL
METHACRYLATE ...................................................................... 59

Background ...................................... ........................................ 59
Stereochemical Kinetics: 13C NMR Analysis of PMMA
Terminated with Labelled End Groups..................................67

Side Reactions in Group Transfer Polymerization....................82

IV GROUP TRANSFER POLYMERIZATION OF DIPHENYL-
METHYL AND TRIPHENYLMETHYL METHACRYLATES..87

Background ...................................... ..............................................87
Group Transfer Polymerization of TrMA......................................88

Methylation Attempts and the Possibility of a Dissociative
Mechanism for the GTP of TrMA.......................................94
Stereochemistry of GTP of TrMA.......................................... 101

Group Transfer Polymerization of DMA................................. 114

Experimental Conditions and SEC Results........................ 114
Stereochemistry of GTP of DMA............................................. 115

REFERENCES ................................................................................................ 120

BIOGRAPHICAL SKETCH............................................................................. 125












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


THE STEREOCHEMISTRY OF GROUP TRANSFER
POLYMERIZATION OF METHYL, DIPHENYLMETHYL, AND
TRIPHENYLMETHYL METHACRYLATES

By

Krishna G. Banerjee

August 1988

Chairman: Thieo E. Hogen-Esch
Major Department: Chemistry


The stereochemistry of the chain-end of PMMA prepared by group
transfer polymerization (GTP) and terminated with 13CH31 in the presence of
tris(dimethylamino)sulfonium difluorotrimethyl siliconate (TASSiMe3F2) was

determined by 13C NMR and compared with the tacticity of the chain. The
propagation statistics reveal consistency with a Bernoullian process for the
entire temperature range studied (-960C to 45oC), confirming previous reports
based on main chain triad tacticity data alone. The results indicate that the E
and Z stereoisomers demonstrated for these systems propagate with identical
stereochemistry and also confirm a previous suggestion that a comparison of
the stereochemistry of the end group and the main chain is a valuable new
method for analyzing the statistics of vinyl polymers.








The GTP of diphenylmethyl methacrylate (DMA) and triphenylmethyl
methacrylate (TrMA) at various temperatures using various nucleophilic
catalysts is also described. The strong fluoride ion donor catalysts appear to
be the most effective in polymerizing TrMA but are required in much higher
concentrations than for methyl methacrylate (MMA) polymerizations. The GTP
of both DMA and TrMA are affected by side reactions competing with initiation,
resulting in partial destruction of the silyl ketene acetal initiator. Molecular
weight control is poor but the molecular weight distributions remain
moderately narrow (1 < Mw/Mn < 2). Attempts to methylate the chain-end of

poly (TrMA) prepared by GTP were unsuccessful.
In contrast to the radical and anionic polymerizations of TrMA, the
stereoselectivity of GTP of TrMA increases with increasing temperature with a
higher isotactic content at room temperature. A sigmoidal curve is obtained
when the tactic content of poly(TrMA) is plotted as In (kr/km) versus 1/T. The

possibility is discussed of the polymerization proceeding through two active
species such an enolate and a silyl ketene acetal which are interconverting
rapidly during the polymerization. On the other hand, the tacticity results for
the GTP of DMA are similar to those of anionic polymerization in that the
syndiotactic content decreases and the heterotactic content increases with
increasing temperature.











CHAPTER I

INTRODUCTION


Background
Group transfer polymerization (GTP) is a new technique for the
polymerization of acrylic monomers discovered by the scientists at Dupont.1-4
This technique gives "living" polymers (polymers that are capable of further
increase in molecular weight upon the addition of additional monomer) and
can be carried out at room temperature or above, in contrast to the
corresponding anionic polymerization that only works well at low
temperatures (below -50oC). Although the method works best for
methacrylates, other monomers such as acrylates, acrylonitrile, maleimides
and vinyl ketones can also be polymerized. As block and graft copolymers
have found increasing use as elastomers, compatibilizers, adhesives and
components of high performance finishes, there has been a a great emphasis
on new synthetic methods to prepare well-characterized blocks with functional
end groups which could serve as building blocks for copolymers of
predetermined architecture. Group transfer polymerization appears to
overcome some of the major disadvantages of other types of polymerization
for the preparation of well defined functional blocks. For example, although
anionic polymerization of styrene and butadiene has been carried out
commercially, the anionic polymerization of methacrylates is uneconomical
due to the low temperatures (< -50oC) required to maintain "living" conditions.
Condensation polymerization leads to polymers with broad molecular weight
distribution and hydrolytically unstable backbone linkages although they are





2

very useful polymers of great commercial importance. Due to chain transfer
and termination reactions, and the resulting inability to rigorously control the
MW distribution, free radical polymerization is also unsuitable. Group transfer
polymerization however offers a great deal of practical advantages not offered
by any other methods, namely operability over a broad temperature range, a
wide choice of solvents, good molecular weight control, the ability to
functionalize the polymer ends and the ability to prepare block and random
copolymers. In addition, although group transfer polymerization requires
highly purified reagents and scrupulously dry conditions, a rigorously oxygen-
free atmosphere is not required.
Group transfer polymerization is an example of Michael addition of silyl
ketene acetal to an alpha,B- unsaturated carbonyl compound. It is a first
application of such chemistry to polymer formation by sequential additions.
Figure 1-1 illustrates the polymerization of methyl methacrylate (MMA)
with dimethylketene methyl trimethylsilyl acetal, 1, as initiator. The
trimethylsilyl group is transferred from the initiator and the growing end to the
incoming monomer (hence the name, "group transfer polymerization"). A
catalyst is required for the polymerization to proceed and these can be
classified into two types, the anion catalysts1-3 and the Lewis acid
catalysts.1, 4
Of the anion type, tris(dimethylamino)sulfonium bifluoride (TASHF2)

has given the best overall results and has been used most extensively.
Several other catalysts of both types used for GTP are listed in Table 1-1.









Me OMe


Me OSiMe3


CO2Me


MeO OSiMe3
TASHF2

Me
Me Me
CO2Me


TAS = [N(CH3)2]3S+


Me

CO2Me


Me OMe
COgMe Me Me Me
I FCH I O
Me PMMA. CH2-CHCO2Me --- Me --(CH2C-- OSiMe

Me + CO2Me CO2Me
M3SIOR


Fig. 1-1. Scheme illustrating group transfer polymerization of MMA.









Table 1-1. Various Anion and Lewis Acid Catalysts for GTP


Nucleophilic Catalysts
(Activate Initiator)


TASSiMe3F2
TASHF2
TASCN
TASN3
RCO2
H(RCO2)2


Lewis Acid Catalysts
(Activate Monomer)


ZnBr2
Znl2
ZnCI2
(i-Bu2AI)20
i-Bu2AICI
Et2AICI


__









The mechanism of anion catalyzed GTP5, 6 is rather complex and is far
from being completely worked out and established, in detail. However,
several experiments by the Dupont scientists strongly indicate an associative

silyl transfer mechanism rather than a dissociative (i.e., through enolate
anions) one, at least for the bifluoride catalyzed GTP of MMA. The proposed
mechanism is illustrated in Fig. 1-2. In the first step, the nucleophilic catalyst
activates the initiator by coordination to silicon, to generate a
pentacoordinated silicon species 2 (active species). The activated initiator
and monomer a were originally proposed to form a hypervalent silicon
intermediate 4_(hexacoordinated) and new C-C and Si-O bonds are formed,
cleaving the old Si-O bond. More recently however, 4 has been considered to
be unlikely as an intermediate, both by the Dupont group7 and by others
working in the GTP field.8 Instead, it has been recognized that monomer
addition may well take place in two steps, as illustrated in Figure 1-2.To test
the involvement of the hypothesized silicon species 2, a stable
pentacoordinate siliconate Z (equation 1-1) was synthesized5, 6 as a model
compound by treatment of the silane 5 with the lithium enolate of methyl
isobutyrate, i.



3 3 L,6
< CF3 CH3


(1-1)












+ Nu-


Rmo


R i R
Si
R'LACt,


.OMe


R= CH3

Nu- = nucleophilic catalyst


Nu- +


0


Nu

Si- R
R* k (


0 OMe


Nu
R., O
Si- R
RI
0


OMe


MeO'


R

R =w-. Si.-w R
o


Fig. 1-2. Proposed mechanistic scheme for bifluoride catalyzed group
transfer polymerization of MMA.


MeO


OMe









It was hypothesized that a species such as 2 should react with MMA without
added nucleophilic catalysts. Compound Z indeed reacted with MMA at room
temperature, without added catalyst to give PMMA of reasonably narrow
molecular weight distribution. Although the ligands of 2 and Z are admittedly
different, this result strongly supports (along with other corroborative
evidence) the involvement of a pentacoordinate siliconate species in GTP.
Additional evidence for the associative mechanism comes from detailed
labelling studies5, 6 (see chapter IV). In bifluoride catalyzed GTP, there is no
exchange of the silyl group on the chain end with added trialkylsilyl fluoride.
Therefore, if the function of the catalyst is to generate a small amount of
enolate anion for anionic polymerization, these anions should be recapped by
silyl fluoride, which they are not (Fig. 1-3). In addition, in a double labelling
experiment,5, 6 it was shown that exchange does not occur between living
poly(butyl methacrylate) chain ends and living poly(methyl methacrylate)
chain ends. These results rules out the dissociative route depicted in Fig. 1-4
where at any one time there would be only trace amounts of PMMA enolate
ions present.



MeC COSiR3 + Nu- Me2C CO SiNu
2 + NR3SiNu


Me Me


I R'3SiF
PMMA-C-=COSiR'3 --- PMMA-C CO
I I
OMe OMe

Fig. 1-3. Scheme illustrating the attempted recapping of enolate ions in
GTP of MMA.









\ OSiMe3 0O

O Me + Nu 7 OMe /OMe




PMMA O


> OMe



PMMA O PMMA OSiMe3 PMMA OSiMe3

e OMe < OMe

PMMA 0

OMe

Fig. 1-4. Dissociative scheme showing recapping of PMMA enolate ions by
PMMA silyl ketene acetal.



In the case of Lewis acid catalyzed GTP,1, 4 the monomer is believed to be
activated by coordination of the Lewis acid to the carbonyl oxygen, for
Michael addition of the silyl ketene acetal to MMA.
Polymerization of MMA by GTP is rapid and exothermic. Because
initiators and living polymer sites are very water-sensitive, equipment and
reagents must be scrupulously dry. The monomer to initiator ratio determines
the molecular weight which may be varied over a wide range (1000 < M >
105). Only trace amounts of catalyst (0.1-1.0 mole % of initiator for anion







catalysts and 10% for Lewis acid catalysts) are required for MMA
polymerizations. Monodisperse PMMA with molecular weight as high as
100,000 has been claimed by Dupont using highly purified solvents and
reagents. Tetrahydrofuran (THF), toluene, and acetonitrile are typical solvents
for nucleophile anionn) catalyzed GTP and toluene and halocarbons for Lewis
acid catalyzed polymerization.

PMMA prepared by GTP with the anion catalysts at ambient
temperature contains approximately a 55:45 ratio of syndiotactic and
heterotactic sequences respectively, with no measurable isotactic
components in all solvents examined. The syndiotactic content increases
from 50% at 600C to 75% at -950C for anion catalyzed GTP in THF as the
temperature is lowered, with a final syndiotactic: heterotactic ratio of 3:1.
Lewis acid catalysis of GTP of MMA generally provides a much more
syndiotactic PMMA than do anion catalysts, but detailed temperature
dependence of tacticity studies have not been reported for this system. The
tacticity of PMMA prepared by GTP appears to be independent of solvent but
is dependent on the reaction temperature and the nature of the catalyst (i.e.,
anionic vs. Lewis acid). This is in sharp contrast to the anionic polymerization
of MMA initiated by alkyllithium reagents, where the tacticity of PMMA is
dependent on the polarity of the solvent media. Thus for the anionic
polymerization of vinyl monomers of the structure CH2=C(R)C(Y)=X (X,Y = O

or N, R = H or alkyl) isotactic PMMA is obtained in the presence of Li or Mg
initiators in non-polar solvents (e.g. toluene), while polar solvents (e.g., THF)
give predominantly syndiotactic polymers.9-12
In many cases, the silyl ketene acetal is stable and may be used for
further reactions or for preparing block polymers by changing to a second
vinyl monomer.1, 3 Figure 1-5 illustrates the formation of a triblock polymer
from MMA, n-butyl methacrylate and allyl methacrylate.








OMe PMMA OMe
+ MMA -
> OSiMe3 OSiMe3

n-BUTYL METHACRYLATE



PMMA-PBMA-PAMA OMe PMMA-PBMA OBu
57% 32% 11% < __
/ \ ALLYL METHACRYLATE \
OSiMe3 OSiMe3

Fig. 1-5. Scheme illustrating the synthesis of a triblock copolymer by GTP


The figure also illustrates the point that methacrylates bearing free radical

sensitive groups can be introduced into the polymer chains by GTP. A

polymer with 11% allyl methacrylate would form a gel if prepared by free

radical initiation. Polymers with thermally sensitive functionality such as

glycidyl groups can also be prepared, but polymerization temperatures must

be maintained at OoC or lower. When mixtures of various methacrylate

monomers are used, random copolymers form.

End functionalized polyacrylates can be obtained by employing
properly designed initiators.1, 3 The corresponding living polymers can be

coupled readily to telechelic polymers. Thus, use of a (Fig. 1-6) as an initiator

for polymerization of MMA or ethyl acrylate (EA) gave polymers with protected

terminal functional groups. The existence of ia or 92 in solution was

demonstrated by reaction with a proton source (e.g. methanol) or with a

suitable alkylating agent (e.g. benzyl bromide) in the presence of

stoichiometric amounts of a strong fluoride ion source (such as
TASSi(CH3)3F2) to give 10 or 11 (Figure 1-7).









v OCH2CH20SiMe3
+ (n+1) H2C= C--COg2R
OSiMe3

OCH R'
8 I I
C-C-(CH2-C -)n OR
Me3SiOCH2CH20 I C
CH3 CO2R
R' OSiMe3

9a R= R=CH3
9b R = C2H5, R'= H


Fig. 1-6. Scheme showing the synthesis of telechelic polymers by GTP.




Deprotection of 1Q and 11 with n-Bu4NF (TBAF) gave the corresponding

polymers with 100% hydroxyl groups in one terminal position. The

functionalized polymers may be readily distinguished from the non-

functionalized species by high performance liquid chromatography (HPLC).

Size exclusion chromatography (SEC) analyses showed that the polymers
had narrow molecular weight distribution (Mw/Mn = 1.0-1.3).

When the alkylation reaction was carried out with 1,4-bis (bromomethyl)

benzene, the intermediate 9a gave after deprotection, alpha, omega -

dihydroxypoly(methyl methacrylate) 12 in a quantitative yield (Figure 1-8). The

coupling reaction was carried out in the presence of one equivalent (with
respect to the initiator) fluoride ion (TASSiMe3F2). The extent of coupling was

determined by GPC, HPLC and hydroxyl group analysis.









CH3 R'

C-C-(CH2-C --CH2 X
Me3SiOCH2CH20 I I
CH3 CO2R

11 X'= H, R = R'= CH3




1. BrCH2-OX' 2. TASSiMe3F2 (1 equivalent)


MeOH or Br2


CH3 R'
O\ F
C -C(CH2-C X
Me3SiOCH2CH20 OI I
CH3 CO2R


10aX H, R R'=CH3
10bX=Br, R=R'=CH3
10cX=H, R= C2Hs,R=H


Figure 1-7. Alkylation, halogenation and protonation of living PMMA by GTP.








9a

1. BrCH2-QCH2Br

S2. TASSiMe3F2 (1 equivalent)



SCH R' CH3 CH3
OC-[C H2CCH CH2 CH 2F CHCC-CH
Me3SiO(CH2)20 OH n CH OCH2CH2OH
CO2R CO2R



n-Bu4NF





O CH3 R' CH3 CH, 0
C-C-(CH2-C -CH, CH2.' CH) -C.
HOCH2CH20 0I In f ni OCH2CH2OH
CH3 CO2R CH3 CH3
12



Fig. 1-8. Coupling of two PMMA chains with a difunctional coupling agent.


Similarly, alpha, omega-dicarboxylpoly(methyl methacrylate) was
prepared by initiating the polymerization with 1,1'-bis (trimethylsiloxy)-2-
methylpropene-1 (13) and hydrolyzing the end group of the resulting polymer
(Figure 1-9.).

A control sample was removed before the addition of the coupling
agent. Comparison of the control with the coupled product enables one to
determine the extent of coupling. Thus the monofunctional polymer (control)










+ CO2aMe
OSiMe3 H2C
13

S CH3 CH3 CH3 OSiMe3
0C C---(CH2--C CH2- C=C
Me3SiO' I
Me CH3 CH3 OMe
14





H CH3 CH3 CH3 CH3 O
C-C-(CH C-rCH, CH-C-CH -CO
HO CH3 COPCH Ca CH3

15
1. BrCH2-- CH2Br
2. H3Ct

14

SH3C


0 CH3 CH3

-I I
HO II
S CH3 C02CH3 16

Fig. 1-9. Synthesis of alpha, omega- dicarboxy-PMMA.



had Mn = 2100, Mw = 2700, while the alpha, omega-difunctionalized coupled

product 16 had Mn = 4200 and Mw = 5600, indicating quantitative coupling.

The reaction of silyl enol ethers with tertiary alkyl halides and the direct

coupling of silyl ketene acetals with titanium tetrachloride (TiCI4) are

known.13, 14 Telechelic polymers were prepared taking advantage of these

reactions. Using a mixture of Br2 and TiCl4 as the coupling agent, living

polymers 9a and J14were reacted independently (Figure 1-10), to yield the

coupled polymers 192a and 19 b.









Star polymers can be synthesized by generating polyfunctional

initiators in situ by the Michael addition reaction of polyfunctional monomers

with silicon reagents (Fig. 1-11).1 The alternative approach would involve

initiation of GTP with polyfunctional initiators, which presents the problem of

synthesis of such large, reactive, initiators.


Br2-TiCI,
9a 2 4 -
Or CH2C2. 0C
14


- CH, CH3
S I I
C-C-H,-CH cn---Br +
CH3 CO2CH3

17a R = CH2CH2OSiMe3
17b R SiMe3

SCH3 CH3 CH3 OTiCI3
C.C---+CH2 C CH2-C =C
R CH3 CH3 OMe

18a R= CH2CH2OSiMe3
8b R = SiMe3


-BrTICI3 H30


CH3
C
COACH3
2


19a R=CH2CHO2a-
19b R=H


Fig. 1-10. Coupling of PMMA chains with bromine/TiCl4.









- CH2OOC 3

20













EA = Ethyl acrylate


> OSiMe3 (i-BuAI)20
+ 3)== -------
+ OMe
-1


4CO2Et


CO2 Me


-__ 3


Fig. 1-11. Synthesis of star polymers by GTP.


When one equivalent of a n-functional monomer is allowed to react with

n-equivalents of ketene silyl acetal initiator (1) in the presence of a Lewis acid

catalyst (such as dialkylaluminum chloride or dialkylaluminum oxide) followed

by the addition of excess monofunctional monomer, cross linking does not

occur, but, instead, star polymers are formed. Thus the reaction of 1 with a

0.33 molar equivalent of trimethylolpropane triacrylate 20 at -780C followed

by ten molar equivalents of EA gave a quantitative yield of soluble star

polymer containing no residual unsaturation with Mn = 2190, Mw = 3040, and

D = 1.39 (theoretical Mn = 3300) (Fig. 1-11).

Similarly, treatment of 1 at -78oC with 0.25 molar equivalent of

pentaerythritol tetraacrylate, 21, in the presence of diisobutylaluminum oxide

followed by ten equivalents of ethyl acrylate gave a quantitative yield of








soluble polymer with Mn = 2400, Mw = 2970, and polydispersity 1.24

(theoretical Mn = 4752)(equation 1- 2)





C C 0H OOC> 1 C(poly EA)4
E 4 2. EA

21 (1-2)




Objectives
One of our original objectives was to investigate the effect of alkyl or aryl

substitution on Si in the initiator molecule on the tacticity of PMMA prepared

by GTP. We also wanted to investigate what substituent size could be

tolerated on silicon to yield PMMA by GTP. In this regard, the effect of chiral

silicon in the initiator molecule on the stereochemistry of the prochiral carbon

was of special interest.


Chiral silicon


PMMA OSIR1 R2 R3

CHa3 OMe

Prochiral carbon



We were invested in exploring the possibility for chirality transfer on the

prochiral carbon ensuing in possible tacticity control. Hathaway and

Paquette15 tested the concept of chiral transfer with the following reaction:










'OCH3 BF.Et2 OCH3
CH3.,I Si + Ph H CH2=CH-CH2----C
I I' OPh
CH2CH=CH2 OCH3 H h



22 23 24


(1-3)


Using the allylsilane 22 and the dimethyl acetal, 2a he found about 5%
enantiomeric excess of 24. Thus there was clear evidence for the transfer of

chirality from silicon to carbon. We were interested in investigating the degree
of specificity for chirality transfer in the GTP process and how this may be
optimized by Si substitution. Pentacoordinate species such as 2. formed by
complexation of the catalyst with the initiator may racemize through
pseudorotation16-23 (an intramolecular ligand exchange process). If the rate
of pseudorotation is slow enough for measurement (and therefore slow on the

polymerization time scale) then it may be possible to monitor the racemization
by optical rotation. Racemization by pseudorotation would diminish chirality

transfer to prochiral carbon. We wanted to carry out some exploratory work in
this area by first trying to synthesize racemic and then optically active (1-
naphthyl)phenylmethylsilyl enolate of the GTP initiator, 2& (equation 1-4)
Initially we wanted to prepare the racemic compound. Preparation of the silyl
chloride, 25, involved four steps following Sommer's procedure.24












CI- Si- Ph

Me
25



However, we could not synthesise it by the usual method (equation 1-4).


Me2CHCO2Me


1. N(iPr)2Li
2. (a-Np)(P)(Me)SiCI
2. (a-Np)(Ph)(Me)SiCl


Me O-Si-Ph
Me

Me OMe


(1-4)


We did not try further to synthesize it, thinking the project highly risky for
various reasons. It is to be noted that the Dupont group, in trying to test the
intermediacy of a hexacoordinate structure 4 (which would require retention of
configuration at silicon during the transfer step as the silyl group has been
shown to remain invariant in the bifluoride catalyzed GTP process)
synthesized both diastereomers of silacyclopentane initiators (27a and 2ZL).6







Me

Me
Me O- Si


27a cis
Me OMe
Me OMe 27b trans




But these cyclic silyl ketene acetals underwent rapid pseudorotation
(stereomutation) at silicon under GTP reaction conditions. Thus, it is likely that
even if the synthesis of the target compound 26 were possible, the process of
pseudorotation would have made the optical rotation studies difficult, if the
pseudorotation process were too fast to measure, as it now seems likely.
Initially, when work on GTP began in this laboratory, Dupont was still
engaged in fundamental research in this area. As it turned out, some of our
areas of investigation were explored by them and the results published while
we were still actively engaged in that area. For example, in order to explore
the effects of alkyl (or aryl) substitution in Si on PMMA tacticity, we synthesized
a variety of silyl ketene acetals to be used as initiators for this purpose (see
experimental section), and experienced some difficulty in polymerizing MMA
using some of them under conditions of very low catalyst level with respect to
the initiator. However, before we could proceed further we learned that much
of what we were about to do was already investigated by Dupont and going to
be published.1 We also found out in this connection that substitution on
silicon in the initiator essentially had no effect on the tacticity of PMMA
prepared by GTP. This information was never published but was obtained by
private communication.25








We therefore shifted our attention to other areas of investigation in GTP,
such as the trapping of PMMA living ends with labelled groups, in order to
obtain information on the chain end stereochemistry. Comparison of the
stereochemistry of the main chain with that of the chain end has been recently
shown to be an independent method for analysing the propagation statistics in
the polymerization of vinyl monomers.26 Thus, we wanted to extend the
application of the end-group method (so far used only for anionic
polymerization in these labs) to GTP, to elucidate the propagation statistics of
PMMA prepared by GTP. Another area of investigation has been to examine
the GTP of monomers other than MMA, and the tacticity of the corresponding
polymers. Thus the GTP of diphenylmethyl methacrylate (DMA) and the
unusual monomer triphenylmethyl methacrylate (TrMA) have been
investigated in detail and the tacticities of the polymers compared to those
prepared by anionic and radical methods. In this regard, we have obtained
some evidence that the associative mechanism proposed for the GTP of MMA
by the Dupont group may not be true for other systems, such as TrMA.











CHAPTER II

EXPERIMENTAL

General
Much of the work involving carbanions was carried out under high
vacuum (10-6mm Hg). The vacuum was generated by the combination of a
mechanical pump (Welch Duo-Seal Vacuum Pump) and a mercury diffusion
pump. All stopcocks and ground glass joints were lubricated with Dow
Corning high vacuum silicone grease.
All glassware was constructed from pyrex using a natural gas/oxygen
torch. The glassware used in the vacuum line work was treated in the
following manner. First, an apparatus was rinsed successively with 5% HF,
water, and acetone. Then it was dried at 110oC in the drying oven. Prior to all
reactions on the vacuum line, further drying was carried out on the vacuum
line by flame degassing which involved heating an evacuated apparatus with
a torch.


Solvents
Tetrahydrofuran (THF) was dried by first refluxing over Na/K
(approximately 1:2 ratio) for 24 hours. About two liters of THF was then
distilled at atmospheric pressure. After flushing the distillate flask for several
minutes with argon, fresh Na and K metals were added to it (1:2 ratio of
sodium to potassium) along with 0.5 g benzophenone. The flask was then
attached to the vacuum line and degassed. The solvent turned purple within a







half hour indicating the presence of benzophenone dianion which indicated
the absence of water and oxygen.
Unless stated otherwise, all other solvents used were purified by first
stirring with CaH2 for several hours, followed by distillation from CaH2.


Catalyst Syntheses

Tris(dimethylamino)sulfonium bifluoride (TASHF2)
This was prepared1 3 from commercially available TASSiMe3F2 (Me-
TAS-SiMe3F2; Aldrich, technical grade, containing about 10% TASHF2).

2 (NMe2)3S+ SiMe3F2- + H20 -------->2 (NMe2)3S+ HF2- +(SiMe3)20 (2-1)


To the TASSiMe3F2 (5.0 g; 16 mmol) was added 0.14 mL(7.78 mmol) of water
dissolved in 6 ml of acetonitrile (distilled from CaH2). Two layers formed. The

volatiles were pumped off and to the residue were added 3ml of acetonitrile
first and then 30 ml of THF (CH3CN/THF = 1/10 ratio). Precipitation or

crystallization occurred immediately. Filtration under argon (in a dry box)
followed by drying with high vacuum gave 3.1 g (95% yield) of TASHF2 in
crystalline form. The 13C NMR (CD3CN, 25MHz) showed only one

absorbance at 37.3 ppm for the carbon corresponding to the TAS moiety.
The absence of any Si-CH3 signal indicated quantitative conversion of the
starting material (TASSiMe3F2). In the proton NMR, the protons of the TAS

moiety appeared at 3.05 ppm.
N-tetrabutylammonium acetate (NBu40Ac; Alfa) and N-
tetrabutylammonium fluoride (NBu4F; Aldrich; 1.0M solution in THF,

containing less than 5 wt% water) were used without further purification.








Tris(diethylamino)sulfonium trimethyldifluorosiliconate (Et-TASF)
This was prepared according to a patent procedure27. SF4

(Matheson) was first condensed into a graduated 10 mL cylinder under
vacuum at -780C. Some of it (1.4 ml; 0.0252 mole) of this was then distilled
through the vacuum line into the reaction flask. Diethyl ether (previously
distilled once from CaH2) was distilled again from CaH2 in vacuo into the

reaction flask. Then N,N,-Diethyltrimethylsilylamine (TMSDEA, 97%, Aldrich,
10.98 g, 75.6 mmole) was added by means of a syringe to the reaction vessel
under argon at -780C. The reaction mixture was stirred for five days (warming
up to room temp). Two liquid layers formed, the bottom layer being dark
brown. The top layer most probably was SiMe3F (a by-product of the

reaction):

3 Et2NSiMe3 + SF4---->(Et2N)3S+ Me3F2Si- + 2 SiMe3F (2-2)


The ether and trimethylfluorosilane (TMSF) were removed by evaporation in
vacuo. According to the patent procedure, one is erroneously led to believe
that at this point, that the (Et2N)3S+ Me3F2Si- will appear as light grey

crystals. However, this was not the case, thus proving the general statement
that "patents are made to claim, and not to reveal information". Even private
communication with Dupont scientists revealed that they themselves were
unable to obtain results according to the exact wordings of the patent
procedure. A dark brown oil was the final product, which even after
continuous evacuation at high vacuum (10-6 torr) for more than 24 hours did
not yield a solid. The reaction apparatus was then transferred under argon to
a dry box for preparation of NMR samples. Analysis by 1H, 13C, and 19F
NMR revealed the presence of a mixture of 2 products, (Et2N)3S+SiMe3F2







(Et-TAS-SiMe3F2) and (Et2N)3S+HF2 in approximately 85:15 ratio in
quantitative yield.
Characterization: NMR (13C, THF-d8, 50MHZ): 7.3 ppm (Si-CH3),
13.2 ppm (NCH2CH3 of ((Et2N)3S+SiMe3F2), 42.0 ppm (N-CH2 of
(Et2N)3S+SiMe3F2), 14.7 ppm (NCH2CH3 of TASHF2), 38.7 ppm (NCH2 of
TASHF2).
NMR (IH, THF-dS, 200 MHz): 0.0 ppm(s, Si-CH3), 1.28 ppm (t, N-
CH2CH3 of TASHF2, 1.43 ppm (t, NCH2C.3 of (Et2N)3S+SiMe3F2) 3.22
ppm (q, NCH2 of TASHF2), 3.54 ppm (q, NCH2 of (Et2N)3S+SiMe3F2).
NMR (19F, THF-d8, 188 MHz, CFCI3 = 0.00 ppm): -57.00 ppm
(s,TASSiMe3E2), -149.8 ppm (d, JHF = 120 Hz, TASHE2).


Synthesis and Purification of Initiators


Ketene Trialkylsilvl Acetals
These compounds were prepared according to published
procedures28-30 with some modifications. Compound 1(1-methoxv-1-
trimethylsiloxy-2-methyl-l -propene: Aldrich) was distilled first from CaH2
(350C, 15 mm Hg) and stored under argon. To this was added fresh CaH2
and the round bottom flask A containing it (see Fig. 2-1) was attached to the
side arm B. The apparatus was then attached to the vacuum line and the
contents of A were stirred for an additional 1-2 hours with CaH2 with
occasional degassing. It was finally distilled into C by heating A intermittently
with a low temperature heat gun (actually a hair dryer), while C was immersed
in a dry ice/isopropanol bath (-780C). While A and C were kept cold, the
ampoule was sealed off at the constriction D n vacuo.















HIGH VACUUM


Fig. 2-1. Apparatus used for the in-vacuo distillation of the GTP initiator into
an ampoule.







A number of ketene trialkylsilyl acetals (some are new compounds)
have been prepared according to general published preocedures (see
Table 2-1) in the hopes of using them as GTP initiators. The preparation of
28 and 32 are illustrative general procedures.
Synthesis of Methyl dimethylethylsilyl dimethyl ketene acetal(28). This
was prepared according to a literature procedure28. A three neck round
bottomed flask was flame dried under vacuum. THF (50 ml) was distilled in
through the vacuum line at -780C. Diisopropylamine (5 g; 3.6 ml; 49.4 mmol),
previously distilled from CaH2 was then added to the flask under argon

followed by addition of 30 ml of 1.5 M BuLi (45 mmol) with a syringe. The
mixture was stirred for 30 mins. while the dry ice bath was replaced by an ice-
water bath. Then methyl isobutyrate, 34.
CH(CH3)2CO2Me

34



Table 2-1. Different Silyl Ketene Acetals Synthesized for Use As Initiators in
the GTP of MMA.

(CH3)2C=C(OMe)(OSiR1 R2R3)
Compound No. B1B2B3 Method used (ref)

28 Me2Et 28
29 Me2iPr 28
30 Me2tBu 29, 30
31 Me, C6H5, vinyl 28
32 Me2, C6H5 28
33 Et3 29,30







(Aldrich; 3.0 g; 29.4 mmol), distilled from CaH2 was added dropwise under

argon with a syringe at 0oC. The reaction mixture was stirred for an additional
30 minutes at 0 C and then was treated with excess
chloroethyldimethylsilane (Aldrich) distilled from CaH2 through the vacuum

line. The mixture was stirred for an additional hour at 0oC and then allowed to
warm slowly to room temperature. It was filtered several times to remove the
inorganic salts (LiCI). Evaporation of solvent gave an oily residue which was
filtered again through glass wool. The oil was then distilled in vacuo with a
fractionating column, several fractions were collected and their purity checked
by GC. The purest fraction was determined to be 98% pure. The yield was
approximately 30%.
Compounds 28, 29, and 32 were prepared according to the above
general procedure using methyl isobutyrate and the corresponding silyl
chlorides. The yields of isolated silyl ketene acetals ranged from 20-40%.


Synthesis of methyl triethylsilyl dimethylketene acetal (3).29,30
The reaction is represented by the following equation:


CH2=C(CH3) (CO2Me) + R3SiH ----> CH2=C(CH3)(OSiR3) (2-3)


A mixture of methyl methacrylate (2.6g; 26 mmol, distilled from CaH2),

triethylsilane (3.2 g; 27.5 mole) and tris (triphenylphosphine) rhodium
chloride (Wilkinson's catalyst; Aldrich; gold label; 72.2 mg; 0.0780 mmol) was
heated at 1000C under vacuum for 2 minutes. The ketene silyl acetal was
obtained by fractional vacuum distillation in various fractions (best fraction:
90% pure by GC). The yield of combined fractions was 75%.







Compound Q0 was once unsuccessfully attempted to be prepared by
the method of Ainsworth. However the method of Yoshi29 gave a quantitative
yield of 3Q prior to fractional distillation. The purity of the crude product was
determined by GC to be 77%.
All of the silyl ketene acetals prepared above had reasonable 1H and
13C NMR spectra which are listed below.
Compound 28: 1H NMR (CDCl3, 200 MHz): 0.16 ppm (s,Si-CHL3),0.66
ppm (q, CJt2CH3), 0.99(t, CH2CH3), 1.52, 1.57 ppm (s, (CH3)2C=), 3.49 ppm
(s, OC13).
NMR (13C, CDCl3, 200 MHz): -2.1 ppm (Si-CQH3), 0.64 ppm
(CH2CH3), 0.82 (CH2CH3),16.00 and 18.00 ppm ((CH3)2C=), 56.4 (OCH3),
90.90 ppm ((CH3)2C=), 149.60 ppm (=C(OSiMe3)(OMe)).
Compound 29: 1H NMR (CDCI3, 200 MHz): 0.38 ppm (s, Si(CHi3)2),
1.1-1.38 (m, CHL(CH3)2), 1.75 and 1.80 ppm (s,(CH3)2C=), 3.75 ppm (OCJi3)
NMR (13C, 50 MHz, CDC13): -4.00 ppm (Si-.H3), 14.80 and 16.00
ppm ((CQH3)2C=), 16.80 ppm (HC(QH3)2-), 56.80 ppm (OQH3), 23.40 ppm
((CH(CH3)2), 91.0 ppm ((CH3)2C=), 150.00 ppm (.(OSiMe3)(OMe)).
Compound 30: 1H NMR (CDC13, 60 MHz): 0.22 ppm (s, Si-CH3), 1.05
ppm (s, (CH3)3C), 1.61 and 1.65 ppm ((C03)2C=), 3.57 ppm (s, OCH3).
Compound 31: 1H NMR (200 MHz, CDCI3): 0.55 ppm (Si-CH3), 1.42
and 1.46 ppm (s, (C0.3)2C=), 3.41 ppm (s, OC.3), 5.8-6.46 ppm (m, Si-
CH-=CH2), 7.33-7.80 ppm (aromatic H's).
NMR (13C, 50 MHz, CDCl3): -3.4 ppm (Si-C.H3), 16.0 and 16.4
((CH3)2C=), 57.2 (OH3), 91.40 (Me2Q=), 124.80-135.50 (vinyl and aromatic
C's), 149.50 ppm (=Q(OSiMe3)(OMe)).







Compound U2: 1H NMR (CDCI3, 200MHz): 0.48 ppm (Si-Mh), 1.51
and 1.57 ppm (s, (Ch.3)2C=), 3.42 (OMe), 7.32-7.45 ppm (3 aromatic H's),
7.52-7.72 ppm (2 aromatic H's).
NMR (13C, 50 MHz, CDCI3): -1.40 ppm (Si-CH3), 16.2 and 17.0 ppm
((CH3)2C=), 57.1 ppm (OMt), 91.50 ppm (Me2.=), 127.6, 129.8, 133.30,
137.20 ppm (aromatic C's), 149.3 ppm (=Q(OSiMe3)(OMe).
Compound 3: 1H (200MHz, CDCI3): 0.70 ppm (q, Si-Cj2Me, fine
complicated splitting), 1.00 ppm ( complicated t, Si-CH2Cd3), 1.53, 1.56 ppm
((CH3)2C=), 3.50 ppm (OCC3).
NMR (13C, 50 MHz, CDCI3): -4.8 ppm (Si-CH2CH3), 6.5 ppm
(SiCH2MQ), 15.9 and 16.60 ppm ((CH3)2C=); 56.8 ppm (OSH3), 90.80 ppm
((CH3)2C=), 149.90 ppm (=C(OSiMe3)(OMe).
In the literature, most of the silyl ketene acetals were described as
having been purified by distillation and preparative GC.28-31 They are
sensitive to moisture and decompose upon attempted separation or
purification by column chromatography.31,32 Thus, for example, the
attempted separation of 31 with neutral alumina and very dry solvents resulted
in the cleavage of the O-Si bond and the corresponding silyl alcohol
(Si(C6H5)(CH3)(CH=CH2)(OH)) was isolated as a by-product, identified by
1 H NMR spectroscopy: (60 MHz, CDCI3): 0.5 ppm (s,Si-ChL3), 2.26 ppm
(broad s, Si-OHl), 5.6-6.73 ppm multiplee, CH=CH2), 7.27-7.8 ppm (aromatic
H's). Even the C -silyl compounds decompose upon attempted separation by
column chromatography.32
The scheme for the synthesis of (1-Naphthyl)phenylmethylchlorosilane
was the same as that of Sommer24 and is shown in Fig. 2-2.








Synthesis of (1-Naphthyl) phenvlmethylmethoxvsilane (36)
This was prepared according to the literature procedure24. First the
Grignard reagent (1-NpMgBr), was prepared. A 3-neck round bottom flask
was equipped with a condenser, a drying tube and an addition funnel. Excess
magnesium turnings and a solvent mixture consisting of 2 parts (by volume)
of ether, 3 parts of toluene and one part of THF was placed inside the flask. In
the addition funnel was placed 2.99 g (14.4 mmol) of 1-bromonaphthalene,
35. and 12 ml of the above solvent mixture. The Grignard reagent was
prepared by gradual addition of the contents of the addition funnel to the 3-
neck flask. An oil bath (temp. maintained at 60-70oC) was used to warm
the mixture and iodine was added to initiate the Grignard reaction. The dark
purple color of the iodine faded as the reaction started, together with gradual
depletion of magnesium. At the end of the reaction, the color of the reaction
mixture was yellow. The apparatus was cooled using an ice-water bath and
2.73 g (15.0 mmol) of methylphenyldimethoxysilane dissolved in the above
solvent mixture was added. A white precipitate formed upon the addition of
the reagent. The ice/water bath was removed and replaced with an oil bath,
and the mixture was stirred overnight at 500C. It was then treated wlith cold
aqueous ammonium chloride (saturated) and washed three times with 30 ml
portions of water in a separatory funnel. An inorganic green precipitate was
left behind in the reaction vessel (probably HOMgBr). The organic layer was
dried over anhydrous sodiumsulfate. Filtration and removal of the solvent
provided a viscous syrup which was chromatographed by HPLC using
hexane/THF solvent mixture. At first, pure hexane was used as an eluent.
This eluted the desired compound. Hexane/THF mixture (programmed to
slowly change the composition of solvent mixture to pure THF) removed other










+ Mg


Br


1-NpBr


Ph
I
1 -Np-Si--OCH3
0


,CH(CH3)2


MgBr


2. H30


Menthol
=it----


CH30-Si-Ph
CH
CH3


H3C'


SLiAIH4


Ph
I cy
1-Np--Si-CH3 -
I
H


Ph
1-Np-Si- CH3
Cl


Fig. 2-2. Scheme illustrating the synthesis of (1-naphthyl) phenylmethyl
chlorosilane.







compounds from the column. GC showed the compound (36) to be at least
95% pure. It was characterized by 1H and 13C NMR.
NMR (1H, 60 MHz, CDCl3): 0.80 ppm (s, Si-CH3), 3.55 ppm (s,
OCH3),7.2-8.4 ppm (m, 12 aromatic H's).
NMR (13C, 50 MHz, CDC13) -2.44 ppm (Si-CH3), 50.78 ppm (OCH3),

34.69, 34.79 (C1-Si(1-NP)(Ph)(OMe)),124.96 137.34 (aromatic C's).


Synthesis of (1-Naphthyl) phenylmethylmenthoxysilane (37)
This compound was also prepared according to Sommer's
procedure24. Into a round bottom flask was placed 1.05 g (3.78 mmol) of 8,
0.300 g (1.92 X 10-3 mole) of menthol and a few grains of powdered KOH
(powdered from KOH pellets). Toluene as the reaction solvent was then
added. The reaction mixture was maintained at 125-1350 for six hours while
any MeOH-toluene azeotrope was distilled through a fractionating column.
The toluene was evaporated off from the reaction mixture and the latter was
passed through a column of silica (eluent: CHCI3 first, then THF) to remove

the basic KOH and any other fine inorganic material. Separation of the
product by HPLC (with hexane as the eluent) yielded pure 37. as judged from
GC. The material was a viscous colorless syrup. The reactions involved in
the preparation of 3Z are


Menthol + KOH -------> MenO- K+ + H20 (2-4)


MenO-K++Si(a-Naph)(Ph)(Me)(OMe) ---> Si(1-Naph)(Ph)(Me)(OMen)
+ CH3OH + KOH (2-5)








The procedure in the literature24 called for fractional distillation (173-177oC;
0.07 mm Hg), but in this case, it was obtained simply by HPLC separation.
The 1H and 13C NMR were completely consistent with the structure.
NMR (1H, 200 MHz, CDCI3): 7.2-8.2 ppm (m, 12 aromatic H's), 3.55
ppm (m, OCHR2), 0.4-2.5 ppm (m, 17 aliphatic H's).


Synthesis of (1-Naphthyl)phenvlmethylsilane (3)
This compound was prepared according to Sommer's procedure.24
Compound 3Z (1.92 g; 4.76 mmol) was dissolved in 15 ml of anhydrous ether
and the solution placed in a round bottom flask. LiAIH4 (240 mg; 6.33 mmol;

5.31 equivalents) and 15 ml of di-n-butyl ether were then added. Most of the
diethyl ether was removed by distillation as the mixture was slowly heated to
800C. Heating was continued at 80-900C for 18 hours. After decomposition
of the excess metallic hydride with acetone, treatment with crushed ice and
concentrated hydrochloric acid was followed by drying over sodium sulfate,
and filtering. Solvents and menthol were removed by heating to 1700C at 1.5
mm Hg. The remaining material was further filtered to remove some solid
impurities and chromatographed by HPLC (hexane being the eluent to afford
38 in pure form as judged from GC. The compound was characterized by 1H
and 13C NMR.
NMR (1H, 200 MHz, CDCI3): 5.40 ppm (q, Si-H), 0.70 ppm (d, Si-CJ3),
7.2-8.15 ppm (aromatic H's).
NMR (13C, 50 MHz, CDCI3): -4.0 ppm (Si-CH3), 124-138 ppm
(11 aromatic absorptions).







Synthesis of (1-naphthyl)phenylmethychlorosilane (25)
This compound was prepared using Sommer's procedure.24 In a
three-neck round bottom flask was placed 1 g (4.03 mmol) of 2a dissolved in
dry CCl4. While cooling the reaction flask in an ice bath and magnetically

stirring the contents, chlorine gas was passed into the solution by means of a
fritted glass gas dispersion tube. The reaction was very rapid (approximately
one min.), and it was possible to observe a greenish yellow end point when
the reaction of Si-H was completed. The solvent was removed by a rotary
evaporator to afford the silyl chloride 25, in quantitative yield. The compound
at this stage was 90% pure (GC) and was characterized by both 1H and 13C
NMR which were consistent with the structure.
NMR (1H, 60 MHz, CDCI3): 0.96 ppm (s, Si-Cli3), 6.90-8.33 ppm

(12 aromatic H's).
NMR (13C, 50 MHz, CDCI3): 2.70 ppm (Si-QH3), 125.00-137.00
(13 aromatic C's).
An attempt to improve the purity of 25 by HPLC separation elutingg
solvent being hexane) resulted in a mess, completely degrading the silyl
chloride. Presumably, there is a reaction involving Si-OH group of the silica
gel column with the silyl chloride yielding the corresponding silyl alcohol.


Anionic Initiators
Samples of diphenylmethyllithium (DPML) were kindly donated by
other members of the research group. They were prepared according to
established procedure.33
The DPML initiator concentrations were determined by GC using the
following procedure. About one ml of DPML solution (in THF) was reacted on
the vacuum line with dry Mel (Mel dried over CaH2). The terminated DPML







initiator was analyzed by GC to determine the presence of diphenylmethane
(as unreacted starting material or as unexpected protonated product). From
the relative intensities of the diphenylmethane and 1,1-diphenylethane peaks,
the fraction of the methylated product and therefore that of DPML was
calculated. In most cases, only negligible amounts of diphenylmethane were
detected. To a known amount of diphenylmethane was added a known
volume of the Mel terminated DPML solution. The mixture was analyzed by
GC and the concentration of the unknown DPML initiator was determined by
direct comparison of the Mel terminated DPML with diphenylmethane
standard solution. Even though the GC used employed a flame ionization
detector which responded to the number of carbon atoms a molecule
possessed (diphenylmethane, 13 C; diphenylethane 14 C), no corrections
were made for detector response in the concentration determination of DPML.
It has been previously verified33 that such analysis without corrections (for
compounds having a lot of carbon atoms but differing from each other by only
one carbon atom) results in practically no error.


Monomer Syntheses and Purification


Methyl Methacrylate (MMA)

In a 100 ml round bottom flask equipped with a reflux condenser and
CaSO4 drying tube, 50 ml MMA (Aldrich) was stirred over CaH2 at room

temperature for several hours (12-24). The MMA was then distilled under
atmospheric pressure (99-101oC) discarding the first and last few milliliters of
distillate. Fresh CaH2 was added to the distillate. The distillated was then

attached to the vacuum line and distilled in vacuo into flask A (cooled at








-780C) of the apparatus shown in Fig. 2-3. The flask was warmed to room
temperature and after the introduction of argon into the appartus,
triethylaluminum34-36 was added dropwise into A through sidearm B by
means of a double tipped needle, until the MMA turned slightly but a
persistent greenish yellow color. The flask A was cooled to -780C and
evacuated, and the constriction in B sealed off under vacuum. Then the MMA
was distilled into the side ampoule C after A was warmed to room temperature
and C cooled at -780C. Ampoule C was sealed off from the vacuum line and
the MMA later subdivided into smaller breakseal-equipped ampoules. The
MMA ampoules were stored in the freezer at -200C.


Silver Methacrylate
This was prepared according to literature procedures.33,37
Methacrylic acid was first distilled in vacuo to remove the hydroquinone
monomethyl ether inhibitor. Some of it (50 ml; 50.75 g; 590 mmol) was placed
in a 500 ml three-neck round bottom flask equipped with a mechanical stirrer
and two addition funnels. At room temperature, 35.82 ml (590 mmol) of
aqueous 28% ammonium hydroxide solution was added dropwise. The 100.2
g (590 mmol) of silver nitrate (dissolved in 200 ml ofdeionized water) was
added dropwise to the ammonium methacrylate. The silver methacrylate
(AgMA) precipitated as a grey solid.
The reaction was stirred for an additional two hours. The AgMA was
separated by filtration and recrystallized from boiling water. The final product
was either greyish or slightly purple in color. It was first dried in the vacuum
oven overnight at room temperature and then further dried on the vacuum line
(10-6 torr) for 48 hours. It was stored under high vacuum in flasks equipped








I HIGH VACUUM


Fig. 2-3. Apparatus used for the purification of MMA.







with high vacuum stopcocks (Fig. 2-4). The yield after recrystallization was
about 70%.


Triphenylmethyl (Tritvl) Methacrylate
Thils was prepared according to literature procedures33,37,38 AgMA
(10.56 g; 54.7 mmol) suspended in dry ether was placed in a 500 ml three-
neck round bottom flask equipped with an addition funnel, a reflux condenser
(with calcium sulfate drying tube), a magnetic stirrer and an oil bath.
Trityl chloride (Aldrich; 11.70 g; 42.0 mmol) was dissolved in 150 ml of
dry ether and added to the AgMA-ether suspension. The reaction was
refluxed overnight. The AgCI was collected by vacuum filtrations and the
ether filtrate was condensed on a rotary evaporator. The crude trityl trityl
methacrylate looks slightly yellowish. It is purified by a hot filtration using
celite and dry ether and then a simple recrystallization using ether. Final
product yields were 50% or less. The TrMA was ground to a fine powder and
stored under high vacuum (as in Fig. 2-3). It was characterized by melting
point, elemental analysis and 1H and 13C NMR.
M.P. 99-101oC (lit. 101-103oC).37,38
Elemental analysis: Found : C, 84.08; H, 6.15%. Calculated for
C23H2002: C, 84.12; H, 6.14%.
NMR (1H, 200 MHz, CDCI3): 7.35 ppm (m, 15 aromatic H's); 6.30 ppm
J(s, 1 vinyl H); 5.60 ppm (s, 1 vinyl H), 2.0 ppm (s, C-CJJ3).
NMR (13C, 50 MHz, CDCI3): 18.5 ppm (C-CH3), 90.0 ppm (O-CPh3),
125.5, 137,5 (vinyl C's); 127.3, 127.6, 128.2 (para, meta and ortho aromatic
C's); 143.5 (ipso aromatic C); 165.0 ppm (C=O).















































Fig. 2-4. Apparatus for the storage of solid materials under high vacuum.








According to a previous report,33 the trityl chloride was recrystallized
before use. In the present case, it didn't seem to make any difference whether
the trityl chloride was recrystallized (from a mixture of benzene and acetyl
chloride in a 4:1 v/v ratio) or not. Therefore, in most cases unrecrystallized
trityl chloride (as directly obtained from Aldrich) was used. It appears that both
trityl chloride and trityl methacrylate cannot be analyzed for purity by GC. For
example, although the 1H and the 13C NMR of TrMA looked very good in
terms of purity, the GC of the same sample often showed three peaks with the
major component being about 75%. Thus there appeared to be too much
discrepancy between GC and NMR results, indicating possible decomposition
of the material in the GC column.
The synthesis of TrMA is illustrated in Figure 2-5.


Diphenylmethyl methacrvlate (DMA)
This was prepared according to literature procedures.33,37,38 Silver
methacrylate (AgMA; 17.26 g; 89.4 mmol) and dry ether were placed in a 500
three-neck round bottom flask equipped with a reflux condenser, magnetic
stirrer, addition funnel, and an oil bath. Diphenylmethyl chloride (Aldrich; 15.0
g; 0.074 mole) was added to the flask at room temp. The reaction mixture was
refluxed overnight with stirring. AgCI was separated by vacuum filtration, and
the ether filtrate was concentrated to yield crude diphenylmethyl methacrylate
(DMA). A fraction of the crude product was kept for analysis by melting point,
elemental composition and NMR. The rest of the DMA was recrystallized by
one hot filtration using celite and ether and a simple recrystalllization from
ether. The mother liquors were kept and concentrated to give additional DMA.








CH2gC.COOH + NH CH-- CH2aCCOONH + + 0
CH3 C4l'
Methacryllc add Ammonium methacrylate

SAgNO3

CH2-CCOO- Ag
I + NIJ N03
C Hs
C(C.Hs)2-Cl Silver methacrylate C(C.H), -C



CH,.C.COCH(CH,), CH-gC.COC(C.H,),
C H, C H,
Dlphenylmethyl methacrylate Triphenylmethyl methacrytate



Fig. 2-5. Scheme illustrating the synthesis of DMA and TrMA.



The crude yield was 76%, and the recrystallized yield was about 40%, but

elemental analysis and NMR spectra showed no difference in purity between

the crude and recrystallized product, although the latter looked somewhat

whiter. In view of the analysis results, the DMA obtained from concentration of

the mother liquor filtrate from the recrystallization was kept and used for

subsequent polymerizations.

M.P.: 78-79oC (literature 790C).33,37,38
Elemental analysis: Found : C, 80.86; H, 6.39%. Calculated for C17H1602:

C, 80.95%; H, 6.35%.
1H NMR (200 MHz; CDCI3) : 7.30 ppm (m, 10 aromatic H's); 6.95 ppm (s,

Ph2CH), 6.25 ppm (s, 1 vinyl H); 5.60 (s, 1 vinyl H's); 2.00 ppm (s, C-C.-3).
13C NMR (50 MHz, CDCI3): 18.3 ppm (C-CH3), 126-128.8 ppm (3 aromatic

C's and -CHPh2), 136.6 and 140.5 (vinyl C's), 166.5 (.=O).

The synthesis of DMA is illustrated in Fig. 2-6.




















HIGH VACUUM



(D) /


Fig. 2-6. Apparatus for the GTP of MMA according to Method A








Polymerization Reactions


Group Transfer Polymerization of Methyl methacrylate (MMA)

Method A. In this method, low temperature polymerizations (-100 to -
220C) were carried out, where the MMA was added by in vacuo slow vapor
distillation to a mixture of initiator and catalyst in THF. The apparatus used is
shown in Fig. 2-6. First, the whole apparatus was evacuated on the vacuum
line and flame dried. Then flask A was cooled to -780C with dry
ice/isopropanol. Argon was admitted into the apparatus through the line A
solution of TASHF2 catalyst in acetonitrile was introduced through the side

arm B by a long syringe through the septum into the flask A. A was cooled to -
78 C and vacuum was applied slowly to the apparatus. The constriction in
side arm B sealed off under vacuum and the acetonitrile was slowly pumped
out of A (after removal of the dry ice/isopropanol bath) leaving a very thin film
of the catalyst in A. Dry THF was allowed to distil into A through the line and
the mixture of catalyst and solvent stirred for 30 mins. The break seal of the
ampoule D containing the initiator solution in THF was broken allowing the
initiator to run down into A, the contents of which were being stirred
continuously. Stopcock E was closed and the flask C was cooled with an ice
bath. The MMA ampoule breakseal (F) was broken allowing the monomer to
run down into C. With the MMA stirring in C, the stopcock in C was carefully
opened in order to allow MMA to slowly distill into A. Thirty minutes after the
addition of all of the monomer, the polymer solution in A was terminated by
distillation of methanol through the vacuum line into A.
The polymer solution was precipitated in a 10-fold volume excess of
either hexane or cyclohexane. The polymer was collected by filtration and
dried in vacuum oven at 500 C for at least 2 days.








Method B. In this method, used for higher temperature polymerization
(00C and 25oC) in vacuo, the monomer was slowly poured into a solution of
initiator and catalyst. The apparatus is shown in Fig 2-7. After evacuation and
flame drying of the apparatus, the catalyst was introduced into A under argon

through side tube B as an acetonitrile solution. The constriction in side tube B
was sealed off, and the acetonitrile was slowly pumped off, leaving a thin film
of the catalyst on the glass surface in A. After cooling A to -780C, THF was
distilled in. The apparatus was then sealed off from the vacuum line at
constriction C. The initiator ampoule (F) was then broken and the initiator
solution allowed to run down into A. The contents of A were stirred vigorously.
Flask D was cooled to -78oC and the monomer from the MMA ampoule (G)
was allowed to drain into C. The low temp. dry ice baths cooling A and C
were both removed and the apparatus was allowed to reach room
temperature. While the contents of A were being continuously stirred, the
monomer in flask D was slowly and carefully poured into A intermittently,
allowing it to polymerize. Having allowed enough time for the polymerization,
the apparatus was again hooked up to the vacuum line through side arm E
and the break seal in E broken. MeOH was distilled through the line into flask
A to terminate the reaction.
Method C. The procedure used was the same as in method B except
that a mixed solution of initiator and monomer were slowly added to the
catalyst in THF at room temperature.

Method D. The procedure was the same as in method D except that the
polymerization was carried under argon.

Method E. In this method the catalyst is added as an acetonitrile
solution by means of a syringe to a premixed solution of initiator and
monomer.













HIGH VACUUM


Fig. 2-7. Apparatus for the GTP of MMA according to Method B








Methylation of Chain End of PMMA Prepared by GTP
Several attempts (both at -780C and higher temperature) to methylate
the chain end of PMMA by first using MeLi and then 13CH31 (99% enriched

with carbon 13 label) proved to be unsuccessful (see chapter III). The
successful methylation procedure consisted of addition of 13CH31 (between

one and two equivalents) to the polymerization mixture followed by one
equivalent (with respect to the initiator) of TASSiMe3F2 under argon.

Reversing the order of addition of the reagents resulted in practically no
methylation at all as evidenced by the absence of the labelled methyl end
group in the 13C NMR spectrum of the polymer.


Polymer Isolation for PMMA Prepared by GTP
PMMA prepared by GTP with catalytic amounts (0.1 to 1 mole % with
respect to initiator) of nucleophilic catalysts were precipitated in either hexane
or cyclohexane. Reasons for employing cyclohexane in some cases was that
hexane was often not completely removed even after two days of drying in the
vacuum oven at 50oC, with the result that its signals interfered in the NMR
studies of PMMA. There is only one absorption for cyclohexane in NMR and
so even if it was not completely removed by drying, it did not interfere with
other signals from the PMMA itself. It should be noted that precipitating the
PMMA homopolymers in this way does not remove the trace amount of
catalyst which is insoluble in hexane and remains associated with the
polymer. The catalyst however does not interfere in any way with the NMR
analysis of PMMA samples.
The PMMA homopolymers prepared by GTP which are methylated
contain a much higher amount of catalyst, because of the much higher levels
of catalyst required for methylation (see "Methylation of Chain End of PMMA








prepared by GTP" below). The catalyst (actually TASI) could be removed from
the PMMA by extracting the methylated polymers with water/CHCI3, after all

the THF from the polymer solution is removed. The TASI was removed in the
aqueous layer. The chloroform layer containing the methylated PMMA was
dried with anhydrous sodium sulfate and filtered. It was then precipitated in a
10-fold excess of hexanes or cyclohexane, collected by vacuum filtration, and
dried in a vacuum oven at 50oC for at least two days. The TAS salt can also
be removed by precipitating the polymer into excess methanol (in which the
TAS salt is soluble); however, MeOH precipitation often removes low MW
oligomers of PMMA, hence it is not as good a precipitating solvent as
hexanes.

Group-Transfer Polymerization of Diphenylmethyl and Triphenvlmethyl
Methacrvlate
The apparatus used is shown in Fig. 2-8. After evacuation and flame
drying of the apparatus in vacuo, argon was introduced into the apparatus,
and the rubber septum in side arm (A) removed. A weighed amount of dry
monomer is placed into the flask (B) through a funnel. The septum was
replaced and the apparatus evacuated for 30 minutes. Dry THF was then
distilled into the flask through the vacuum line and any monomer clinging to
the sides of the flask was carefully washed off with THF by application of a
cold dauber to the appropriate regions of the flask. The monomer was made
to dissolve in THF by continuous stirring. Ampoule C was broken and the
initiator solution was allowed to run down into the flask, the contents of which
were being stirred continuously. Argon was introduced into the flask and a
solution of the catalyst in acetonitrile was injected through the septum into the
flask. Aliquots of samples were removed periodically by a long syringe under
argon to monitor the conversion with time by 1H NMR examination of residual















































Fig. 2-8. Apparatus for the GTP of TrMA or DMA.








monomer (vinyl absorption). The polymerization was complete in less than

one minute at -780C. MeOH was finally introduced into the flask either by
distillation through the vacuum line or under argon to terminate the reaction.


Anionic Polymerization of MMA

The method used was the same as that for GTP, using Method A above,
except that a solution of DPML in THF was used as the initiator which was
added to the reaction vessel by means of an ampoule equipped with a
breakseal.


Anionic Polymerization of TrMA

A predetermined amount of solid monomer was placed in a vessel
under argon illustrated in Fig. 2-9, and the monomer addition opening was
sealed with a torch. The monomer vessel was evacuated on the vacuum line
(10-6 mm Hg) for about one hour. The ampoule was then cooled to -780C
and THF was distilled through the vacuum line into it. The vessel was sealed
from the line and stored int the freezer at -20oC.

Anionic homopolymerization of TrMA was carried out in an apparatus
depicted in Fig. 2-10. The apparatus was placed on the vacuum line (10-6
torr), flame dried and cooled to -78oC. Approximately 75 ml of dry THF was
vacuum distilled into (A). The cold bath was removed and the flask was
allowed to warm to room temperature. A THF solution containing the initiator
(DPML) ampoulee (B)) was then added to the flask through the breakseal.
Any residual initiator clinging to the ampoule was washed into the THF by the
application of a cold dauber to the initiator ampoule. The vessel was cooled
to -780C, and the monomer solution (C) was added to the initiator. After
monomer addition, the initiator solution immediately became colorless. After
















































Fig. 2-9. Apparatus used to prepare THF solutions of TrMA.








HIGH VACUUM


Fig. 2.10. Apparatus used in the anionic homopolmerization of TrMA.








allowing the reaction to proceed for a given time, the polymerization was
terminated by vacuum distillation of MeOH through the line. If the polymer
(poly TrMA) was soluble in THF or CHCI3, it was precipitated in a 10 fold
excess of MeOH to remove the catalyst and unreacted monomer (if any), and
the polymer collected by vacuum filtration. If the poly (TrMA) was insoluble on
account of its high molecular weight, then it was simply collected either by
filtration or centrifugation, and the filtrate from the centrifugation was further
concentrated and precipitated in MeOH to attempt to recover any additional
polymer. The polymer was dried in a vacuum oven at 50oC for at least two
days.

Polymer Hydrolysis


The hydrolysis of PTrMA and PDMA were performed according to
literature procedures.38


Polv(TrMA)
About one gram of poly TrMA was refluxed in 50 ml of methanol
containing 1% aqueous HCI for about 4 hours. During the hydrolysis the
originally insoluble PMMA went into solution (this took less than 1 hour). The
solution then was treated by any one of two methods.
In one method, the polymer solution was condensed and the residue
was dissolved in a minimum amount of MeOH. The resulting poly methacrylicc
acid) (PMA) was precipitated in cold ether and collected by vacuum filtration.
The polymer was dried at 50oC for 2 days in a vacuum oven. NMR
measurements of the corresponding PMMA (see diazomethane methylation)
indicated quantitative hydrolysis.








In the second method, after hydrolysis of poly TrMA with acidified
MeOH, all the solvent was pumped out, and the dry mixture of PMA and trityl
alcohol was kept for diazomethane methylation.


PDMA
Approximately one gram of PDMA was refluxed in 50 ml of methanol
containing 5% HCI for at least 7 days. During the first five days, the originally
insoluble polymer slowly went into solution. After hydrolysis the solution was
condensed and dissolved in a minimum amount of MeOH. The PMA was
precipitated in ether and collected by vacuum filtraation. Alternately, after
hydrolysis, the solution was evaporated to dryness and diazomethane
methylation performed on the mixture of diphenyl methanol and PMA.


Diazomethane Methylation
Poly methacrylicc acid) (PMA) was methylated to form PMMA using the
following procedure:39 A diazomethane (CH2N2) generating apparatus

(Fig. 2-11) was constructed from three 250 ml thick wall centrifuge bottles,
condensing jacket with screw cap fittings, long stem separatory funnel with a
teflon stopcock, rubber stoppers, and fire polished glass tubing.
Approximately 10 ml of ether was placed in both collection vessels (B
and C) which were cooled in an ice-salt bath. KOH (2.0g; 0.036 mole), 4 ml
water, 4 ml ether and 13 ml of 2-(2-ethoxy)-ethoxyethane (Kodak) were placed
in centrifuge bottle A. The solution was heated to 70oC using a water bath.
N-methyl-N-nitroso-p-toluenesulfonamide (Diazald, Aldrich) dissolved in 40
ml of ether was added dropwise to A, and immediately CH2N2 was generated

(it was yellow in color), and distilled with ether into B and C.








54






















































4CC

0
mm




















0

CL
C













CD

LT-








The diazomethane-ether solution was added to dry hydrolyzed

polymer samples with a pasteur pipette. Rapid bubbling due to gas evolution

was detected. Additional diazomethane was added to each sample until there

was no further bubbling with the addition of fresh diazomethane. The PMMA

samples were usually insoluble in ether, so they could be collected either by

filtration or by evaporation of the ether and excess diazomethane. The

samples were dissolved in chloroform and precipitated in a 10 fold excess of

hexane or cyclohexane. They were then collected by vacuum filtration and

dried for several days in a vacuum oven at 50oC.


Titration of Alkyl Lithium Solutions

The concentration of MeLi in diethyl ether (Aldrich) and BuLi in hexane
(Aldrich) did not appear to change much with time as determined by periodic

titrations by the method of Winkle et al.40 employing 2,5-dimethoxybenzyl

alcohol (DMBA, Aldrich). The first equivalent of alkyl lithium deprotonates the

alcohol functionality giving the benzoxide salt which is colorless (equation 2-

6).



OMe OMe
CH20H Oe CH2O LiU
+ n-BuLi + n-BuH


OMe OMe

(2-6)
The dianion is dark red in THF and its presence indicates the end point.

A few drops of DMBA were added to a preweighed dry flask and the
flask again weighed to yield the weight of DMBA. This flask was degassed on








the vacuum line after cooling it with liquid nitrogen. Dry THF was distilled in.
A syringe with a teflon plunger was flushed with argon and the alkyl lithium
solution. The syringe was refilled with the solution. With the DMBA in THF
stirring smoothly under argon at room temperature, the alkyl lithium solution
was added dropwise after pushing the syring needle through a septum. The
persistence of the red coloration longer than 15 seconds indicated the end
point. This procedure was repeated twice and an average molarity was
calculated from these runs. The results were reproducible within 5%.


Instrumentation


Gas Chromatography
Routine analyses of samples for purity determination as well as
quantitative determination of reaction products were done on a Hewlett-
Packard Model 5880A gas chromatograph equipped with a capillary column
and a flame ionization detector. The column used (HP#19091-60750) was a
fused silica capillary (50 m long, 0.2 mm ID) coated with 0.11 uM ffilm of
silicone gum (General Electric Co. SE-54, which was methyl 5% phenyl, 1%
vinyl cross-linked polysiloxane). The carrier gas was helium.
Depending on the nature of the analysis, the column was either heated
to a fixed temperature or various step programs were used to increase the
oven temp. after specified time intervals. This all depended on the relative
importance of the desired speed of analysis and resolution.
The microprocessor reported peak retention time (minutes), integrated
areas, type, and percent of total area. Although retention times were highly
reproducible (+ 0.1% for consecutive injections), standards were nonetheless
kept and used to avoid ambiguity as to the identity of peaks.










Preparative Liquid Chromatographv
Practically all of the HPLC work was done on the separation of the
naphthylsilanes. The high performance liquid chromatograph used was an
Altex Model 332 system (now Beckman Co.) with programmable gradient
elution. The two solvent pumps were fitted with preparative heads. A
preparative cell was used in the constant wavelength (254 nm) UV detector
Model 153 for dection of the highly absorbing naphthylsilanes. The
preparative SiO2 column used was Merck's Lobar B (310 X 25 (ID)mm)

packed with 40-63 uM silica gel. It was a glass column with a pressure limit of
90 psi; the system was fitted with a pressure release valve in-line before the
injection port. The gradient elution for most of the separations was simple in
that only hexane was used to elute the compound of interest. After this, any
material still remaining in the column was eluted by using a mixture of hexane
and THF with solvent composition being 100% THF at the end (15 minutes to
change from 100% hexane to 100% THF). The flow rate was maintained at 2
ml/min. The LC fractions were also analyzed by GC for purity determination.


NMR Spectroscopy
NMR (1 H) spectra were obtained on either a Varian EM-360 L (60 MHz)
or a Varian 200 XL Superconducting spectrometer. Chemical shifts are
expressed in parts per million (ppm) downfield from tetramethylsilane (TMS)
unless otherwise noted.
NMR studies of PMMA stereochemistry were carried out in either
CDCI3 (500C) or deuterated tetrachloroethane (TCE-d2) at 90oC at

concentrations of about 200 mg per ml of solvent. The triad tacticities of the
chain were determined both by integration of the alpha-methyl protons








(0.7-1.3 ppm) as well as by integration of the alpha-methyl carbon signals (17-
23 ppm). For compounds with a carbonyl group the pulse delay was set at 1
sec. The triad fractions of the chain end were calculated from the up-and
downfield NMR signals at 23-24 and 29-30 ppm respectively corresponding to
the diastereotopic labelled methyl end groups. The relative areas of the
peaks for the chain end were determined by electronic integration and direct
determination of the relative peak areas. Enhanced resolution for many
spectra was obtained by use of the resolution enhancement (RE) processing
function. This is a line-narrowing technique that resolves Lorentzian lines that
are highly overlapped, provided the spacing between the lines is greater than
1/AT (AT= acquisition time) and there is sufficient signal-to-noise to
accomplish the desired enhancement.


Size Exclusion Chromatography (SEC)
SEC analyses were carried out at room temperature using a Waters
6000 Liquid Chromatograph. The columns used Phenomenex TSK G3000
(7.8 mm X 30 cm; 103A), TSK gel type G5000 HXL (105A) columns in series
following a filter. THF was the eluent in all cases and the flow rates used were
typically 0.7-1.5 ml/min. Both refractive index and UV (222 nm) detectors were
used. The column set was calibrated with PMMA standards (Polymer
Standards Services, Mainz, W. Germany).
From the SEC chromatogram, number average molecular weights
(Mn), weight average molecular weights (Mw), peak molecular weights (Mp)
and molecular weight distributions (Mw/Mn) were determined.
The Mn and Mw values were determined by computer analysis of a

SEC chromatogram using a PMMA calibration curve. In all analyses,
corrections were made for column band broadening caused by diffusion.














CHAPTER III
GROUP TRANSFER POLYMERIZATION OF METHYL METHACRYLATE



Background
The trapping of stereoisomers of a "living" polymer by a suitable
trapping agent has been demonstrated.26, 41 Thus the meso and racemic
chain ends of a propagating polymer anion, for instance, can be effectively

trapped with an electrophile, E+, (Fig. 3-1) and the stereochemical
compositions of the propagating chain ends can be determined.

Thus the trapping of lithium salts of stereoisomeric "living" poly (2-
vinylpyridine) and poly (4-vinylpyridine) anions by reaction with 13C- labelled
methyl iodide followed by 13C NMR analysis of the labelled end group has

H E|
1-,1,- Tr -- l-- L C(H)(E)
I T Trap C1
R R R R R R R R
m m m m


R H E R
-- A a ^- C(H)(E)
%: Trap I I I I
R R R R R R
m r m r


Fig. 3-1. The trapping of stereosomeric anions with an electrophile.







been reported.26'41 It has also been shown that a comparison of the
stereochemistry of the chain end (as obtained from the 13C NMR of a
labelled methyl end group) with that of the main chain may be an independent
and sensitive method to test chain statistics in vinyl polymerizations.26
There are several stereochemical models42 of chain statistics. The
simplest one is the one-parameter or Bernoulli model (Fig. 3-2).
This model assumes that only the last assymetric center in the
propagating chain is important in determining polymer stereochemistry. The
stereochemistry is not affected by the penultimate asymmetric or preceding
centers.

Pm and Pr, the transition or conditional probabilities of forming meso
and racemic dyads, respectively, are defined by


Pm= Rm/(Rm+ Rr); Pr = Rr/(Rm+ Rr); Pm+Pr = 1 (3-1)


where Rm and Rr are the rates for meso and racemic dyad placements

respectively. The term "n-ad" refers to a unit created by "n" adjacent
asymmetric (chiral) centers. Thus dyad tacticity Pm and Pr are synonomous
with the dyad tacticity fraction fm and fr defined as the fraction of adjacent

repeating units which are meso or isotactic-like and racemic or syndiotactic-
like, respectively. The probabilities of formation of mm (isotactic), mr
(heterotactic) and rr (syndiotactic) triads are given by


fmm = Pm2; mr=2 -Pm); frr(1-Pm)2 (3-2)


Thus the probability of forming a particular triad is the product of the
probabilities of forming the two dyads comprising the triad. The coefficient of 2














RPr R
R


PmPr = 1 R


indicates prochiral propagating center.

Fig. 3-2. The Bernoulli model of chain propagation.


for the heterotactic triad accounts for the formation of both mr and rm triads.
Analogous expressions may be derived for tetrads and higher sequences.
The next more complex stereochemical model is the 1st order Markoff
model42 which describes a polymerization where the penultimate assymetric
center is important in determining the stereochemistry of monomer addition.
Meso and racemic dyads can add in two ways as shown in Figure 3-3.
There are now four probabilities, Pmm, Pmr, Prm and Prr,
characterizing the addition process (the designation Pmr means the

probability that the monomer adds in r-fashion to an m chain end, etc.). We
also have the relationships:


Pmr + Pmm = 1; Prm + Prr = 1 (3-3)


The fractions of triads, tetrads, and higher sequences are given in the
literature.42 *










R R R
I I |rR m m


R R Ir R

m r




S RR
R ? ^ R ',^ P',r m-



R I R

r r



Fig. 3-3. The first order Markoff model of chain cropagation.



Higher order Markoff models have been described to ascribe effects to

assymetric centers further back than the penultimate one. Also, non-

Markoffian and non-Bernoullian models such as the Coleman-Fox43 and the

E-Z models44-46 have also been proposed.

The Coleman-Fox propagation mechanism postulates that a growing

polymer chain has two reaction states in dynamic equilibrium, both capable of

adding monomer, but each with its own stereospecificity. This mechanism is

proposed to explain the "stereoblock" structures which occasionally result

from homogeneous anionic polymerizations initiated by metal alkyls in which

runs of m's and r's are produced.43

The E-Z model will be discussed in some detail as there is ample

experimental evidence for the participation of E and Z geometric isomers as

intermediates in anionic polymerization of vinyl monomers of the type

CH2=C(R)C(Y)=X where X,Y = O, N, or C and R = H or alkyl. The model is

based on the demonstrated presence of E and Z isomers and their slow








interconversion relative to monomer addition. E and Z isomers have been
found as intermediates in anionic vinyl polymerization of 2-vinylpyridine, MMA
and t-butylvinyl ketone and in the group transfer polymerization (GTP) of MMA.
They may also play a role in other systems such as in the cationic
polymerization of vinyl ethers as a result of charge delocalization onto oxygen.
The interconversion between the lithio derivatives of E- and Z-39 (Figure 3-4)
generated by the reaction of n- butyllithium (BuLi) with 2-ethylpyridine in THF
has a half life of about of 5 hours at 50oC.46,47 Thus this interconversion
should not occur at -780C during polymerization since this polymerization is
quite fast even at -78oC. The lithio salt 32 is generated predominantly as the
E isomer in 16:1 (E:Z) ratio at -780C upon treatment of 2-ethylpyridine with n-
BuLi. However, upon the addition of 2-vinylpyridine, the corresponding E:Z
ratio in the dimer anion 40 and the trimer and tetramer anions was found to be
close to one.46,47 In view of the absence of E-Z interconversion in the
polymerization time scale, this distribution therefore reflects the mode of
monomer presentation during the transition state, s-cis and s-trans monomer
generating Z- and E carbanions respectively (Fig. 3-4).

In the case of GTP, propagation has been shown to be about 2000
times as fast as E-Z equilibration. In view of the E:Z ratio (1:1) found in the
anionic oligomerization of 2-vinylpyridine in THF in the presence of lithium
ion, the monomer appears completely unselective with regard to the mode of
monomer presentation (s-cis or s-trans) to the propagating carbanion. Even
though the E-Z isomers do not directly interconvert during polymerization, an
indirect "interconversion" is taking place through monomer addition. Being
diastereomers, the E and Z geometric isomers are expected to behave
differently with regard to the kinetics and stereochemistry of polymerization.
The stereochemical pathways are illustrated in Fig. 3-5.









\--Oi


H3C


Fig. 3-4. Scheme illustrating the the influence of monomer conformation on
stereochemistry of propagating species.
(Source: 44: Editor, Polymer Preprints)


S krET


kmr
E kmEl







km
Z zc


Scheme illustrating the various stereochemical pathways of the E-
Z statistical model of chain propagation. (Source: 44: Editor,
Polymer Preprints)


Fig. 3- 5.








The propagating species may be E or Z and these isomers may be
preceded by a meso or racemic diad. Thus


[mE] + [rE] = [E]; [rE + [rZ] = [Z] (3-4)


If several dyads are specified, the one next to the active center is indicated
last. Thus, rmZ specifies a Z active center preceded by a meso dyad which is
in turn preceded by a racemic dyad. The active center may be E or Z, the
monomer presentation may be s-cis or s-trans and meso or racemic dyads
may be formed. As a result, there are eight different processes specified by
bimolecular rate constants kRx,y in which x specifies the reacting carbanion

isomer (E or Z), y specifies the mode of monomer presentation, s-cis or s-trans
(C or T) and R denotes the formation of new dyad (m or r). There are several
assumptions inherent in the E-Z model:
(a). Only one type of ionic species is present (ion pairs for instance).
(b). The stereochemistry of vinyl addition is only dependent upon the
type of carbanion isomer (E or Z) and the mode of monomer addition, s-cis
(C) or s-trans (T) and not upon the stereochemistry of the chain adjacent to the
carbanion. Thus both sites, E and Z propagate according to Bernoullian
statistics. In other words, the mE and rE centers have the same reactivity and
react in a stereochemically identical manner.
(c). Steady state conditions will hold for a particular species, e.g.,


d[mE]/dt = d[rmZ]/dt = 0 (3-5)


It may be shown46 that the E-Z statistical model does not lead to
Bernoullian or first-order Markoff statistics. It, is, however, reducible to







Bernoullian or first-order Markoff chains under certain limiting conditions
(Table 3-1).
Since E-Z diastereomers have been demonstrated as intermediates in
GTP of MMA,7 we were surprised by the results of Stickler and Mueller49 who
reported the statistics of GTP of MMA to be consistent with a Bernoullian
process. The E and Z sites are each expected with a different stereochemistry
and individually propagate according to Bernoullian model giving overall non-
Bernoullian chain statistics for the GTP process. We therefore decided to
verify the results of Stickler and Mueller49 by our independent and
sensitive method involving the comparison of main chain and chain end
stereochemistry, which has been previously developed in this group.






Table 3-1. Limiting Conditions Reducing the E-Z Scheme to Bernoullian or
First-Order Markoff Chains.

No. Condition (Scheme) Statistics


1 kmET=kmZT, kmEC=kmZC, krET=KrZT, krEC=krZC Bernoulliana
2 kmETkmZC, kmEC=kmZT, krET=krZC,krEC=krZT Bernoullianb
3/4 kmZT=krZT=0 or kmEC=krEC=0 Bernoullianc
5 kmZT=kmZC=krET=krEC=O 1st Markoffd
6 kmET=kmEC=krZT=krZC=- 1st Markoffe

aE and Z sites show identical behavior,
bS-trans addition to E is identical to S-cis addition to Z and vice versa,
cOnly Z or E sites are present respectively,
dE sites lead to m-dyads, Z sites lead to r-dyads,
eE sites lead to r-dyads and Z sites lead to m dyads.
(Source: Ref. 44: Editor, Polymer Preprints)







Stereochemical Kinetics: 13p NMR Analysis of PMMA
Terminated with Labelled End Groups

Initially, conditions had to be found where the living PMMA prepared by
GTP could be successfully trapped by reaction with a suitable electrophile.
Since methylated oligomers of MMA have been separated and the various
stereoisomers completely characterized by NMR spectroscopy in this group,
the stereoisomeric composition of the chain-end could be unambiguously
ascertained for a methylated PMMA. Thus 13CH31 has been used in this

group as the electrophile for trapping anionic living polymers. The obvious
reason for employing labelled material is to be able to detect the methyl end
group of a polymer as a signal of sufficiently high intensity.
Since the living chain end of a PMMA prepared by GTP is not an anion,
but a silyl ketene acetal, mere addition of 13CH31 is not expected to methylate

the chain end. One of the first attempts to methylate the chain end was to use
methyl lithium (MeLi) first, followed by 13CH31. It was expected that MeLi

would generate the lithio enolate of PMMA which would then subsequently
methylate with 13CH31 (Fig. 3- 6).

However, this method was unsuccessful when MeLi was added either
at -780C or room temperature, followed by addition of Mel at the same
temperatures as demonstrated by the lack of a detectable 13CH3 end group

signal in 13C NMR. Experiments with the initiator 1 as a model compound for
the chain-end of PMMA showed that there was practically no methylation
when MeU was added at -78oC, followed by CH31 for several hours at -780C.

However when MeLi was added at room temperature to react with the GTP
initiator, followed by CH31 at 0oC or -780C, methyl pivalate, 41 was isolated

as the methylated product together with some other unidentified side products
(equations 3-6 and 3-7).









PMMA OSi(CH3)3 O-13Li PMMA O.
> -+ Si(CH3)4
CF6 C)CH3 CH3 COOH

13cH 31


+ Lil


aMe


Fig. 3- 6.


Scheme illustrating the attempted methylation of GTP PMMA chain
end with methyl lithium/methyl iodide.


1. MeLi, 250C
(CH3)2C=C(OMe)(OSiMe3)----------------->
2. Mel, OOC or -780C

1. MeLi, -780C
(CH3)2C=C(OMe)(OSiMe3)-------X--------->
2. Mel, -780C


Me3CCO2Me
41


As the Dupont scientists reported successful alkylation of PMMA
prepared by GTP with benzyl bromide and other electrophiles1,3 using an
equivalent of TASSiMe3F2 (with respect to the initiator), the same method

was used for successful methylation of the PMMA chain end. In other words,
13CH31 was added first followed by one equivalent of TASSiMe3F2. Based

on Noyori's reports50-52, the methylation should proceed by means of an
enolate with tris (dimethylamino) sulfonium counterion, i.e. a TAS enolate (Fig.
3-7).


(3-6)


(3-7)


13CH3












SiPMMA O TAS
PMMA OSi(CH3)3 PMMA

C OC3 TASSi(CH3)3F2 3OC H


TAS =[N(CH,),]3S+
PMWA
1. 13CH
2. TASSi(CH) HF2 Me
13cH





Fig. 3- 7. Scheme illustrating the successful methylation of the GTP living
PMMA chain.


Reversing the order of addition of the reagents (i.e. TASSiMe3F2 first
and then 13CH31), fails to give methylation of the PMMA chain end.

Presumably, this is due to to a fast competing side reaction of the fleeting TAS

enolate species resulting ultimately in protonation of the active center.

Methylation experiments with the GTP initiator using the crude THF soluble
ethyl-TAS-SiMe3F2 (i.e. TAS+ = [N(CH2CH3)213S+, actually a 85:15 mixture

of ethyl-TAS-SiMe3F2 and ethyl-TAS HF2) and methyl iodide using vinyl

pivalate as a GC internal standard showed that the yield of the methylated
product, methyl pivalate (4.L) corresponded approximately to the mole
amounts of the limiting reagent. Thus when ethyl-TAS-SiMe3F2 catalyst was

used in slight molar excess of one equivalent compared to the inititor, with a

large molar excess (approx 2-3 equivlants compared to the initiator) of methyl
iodide, the yield of methylated product was essentially quantitative and when

the catalyst was used in amounts less than one equivalent compared to the







initiator, the yield of methyl pivalate reflected approximately the proportion of
the catalyst Methylation results of the GTP initiator with Me-TAS-SiMe3F2

were often not reproducible, presumably as a result of the insolubility of the
catalyst. These results are shown in Table 3- 2.
The results are also consistent with the view that the alkylation
proceeds through a TAS enolate species formed by a 1:1 reaction of the
initiator and the catalyst. In addition, in methylation experiments of the GTP
initiator with the THF insoluble catalyst TASSiMe3F2, both methyl pivalate

and TASI were isolated and characterized, the former by 1H and 13C NMR
and the latter by C,H, N analysis.
Table 3-3 summarizes the experimental conditions of the various
polymerization reactions of MMA and the results obtained from SEC.
The reasons for employing methods C and D in some of these
polymerizations were the frequent low yields due to incomplete monomer

Table 3-2. Methylation of the GTP Initiator Under Various Conditions.


Catalyst moles of initiatora moles of catalyst moles MPb
(X 103) (X 103) (X 103)


Ac 0.60 0.73 0.57
AC 0.73 0.37 0.41
Bd 2.18 1.74 1.23
Bd 0.60 1.05 0.28

ainitiator was methyl trimethylsilyl dimethyl ketene acetal (1);
bMP = methyl pivalate (41); yield determined by GC using vinyl pivalate as
internal standard, added after quenching reaction with methanol.;
cA = Et-TAS-SiMe3F2; dB = Me-TAS-SiMe3F2





71
Table 3-3. Experimental Conditions and Results of SEC Analyses of PMMA
Prepared by GTP


Expt. T/oC Mne Mn Mw Mw/Mn Yieldf
No. (calc.) %


la -96 2255 1847 2390 1.29 100
2a -85 2460 2024 2782 1.37 100
3a -78 1800 1602 1929 1.20 100
4a -40 2200 1944 2220 1.14 100
5a -23 2915 3172 4198 1.32 100
6b 0 2050 1662 1796 1.08 23
7c 25 1500 2799 3917 1.40 91
8d 45 1200 2520 3324 1.32 70

amonomer distilled into mixture of initiator + catalyst in vacuo (method A; see
experimental);
bmonomer poured slowly into initiator + catalyst mixture in vacuo (method B);
initiator and monomer mixture added slowly to catalyst in vacuo (method C);
dinitiator + monomer mixture added to catalyst under argon (method D);
number average molecular weight calculated from mole ratio of monomer to
initiator;
based on weight of polymer isolated.


conversion obtained using method B at 0oC and higher temperatures, when
monomer was slowly added to the initiator and catalyst. This is due to
competing side reactions between the initiator and catalyst (possibly involving
initiator-catalyst complex and initiator or initiator-catalyst complex and catalyst;
see the section on "Side reactions in GTP" in this chapter), resulting in
protonation of the living chain. Although there is no mention of side reactions
in GTP in the very early papers by the Dupont group, they are mentioned in
their more recent publications1 and in publications of other scientists working
in this area.53 The rates of side reactions must be lower than the
polymerization rates however on account of the excellent yields and good MW
control at lower temperatures (runs 1-5, Table 3-3). In addition, the side







control at lower temperatures (runs 1-5, Table 3-3). In addition, the side
reactions at higher temperatures can be avoided by changing the order of
adding the reagents for the polymerization, i.e., by adding the catalyst last to a
mixture of initiator and monomer (batch polymerization) or adding a mixture of
monomer and catalyst to initiator It can be seen from Table 1, runs 7, and 8
that the yields improve drastically by employing one of the latter techniques
(mixture of monomer and initiator added to catalyst) to minimize the side
reactions. In these cases, optimum MW control appears to be lost somewhat
but the polydispersity is still well below 2.

The methylation of the chain end of PMMA prepared by GTP is shown
schematically in Fig. 3-8 .It is clear that the methylation neither creates any
new chiral center nor affects the stereochemistry of the assymetric carbons
adjoining the chain ends. Furthermore, if the stereochemical composition of
the last three dyads is known, the proportions of mm*, rm*, mr* and rr* chain
ends may be determined:


fmm* = fmmm* + frmm*; fmr* = fmmr* + frmr*
frr* = frrr* + fmrr*; frm* = frrm* + fmrm* (3- 8)


Fig. 3-9 is an entire spectrum of PMMA prepared by GTP at -230C. The
assignments of various peaks are indicated in the spectrum. Figures 3-10 and
3-11 are expanded regions of Fig. 3-9 showing the main chain alpha-methyl
region, and methyl end group region respectively.
The fractions of the syndiotactic (frr), heterotactic (fmr) and isotactic
triads (fmm) from the main chain can be obtained directly from the NMR by

integration or direct determination of relative peak areas at 16.5, 18.5 and 21-
22 ppm respectively from Fig. 3-10. The absence of nuclear overhauser effect













R = -CO2CH3

TAS = -(NMe2)3S+


kmm


R R
OCH3"
m '


krm
R-13 R 0-43


OCH3 OSiMe3 kr r


a QCH CHa3 3 13
I I 4I CHI
R R R R
mm*

'1) 3 CH31

2) TASSiMe3F2

CH3 C3 H31 aH3


mm* OHa-I OSIMe3


a2!3 3 R ICH3
R R CH
OCH3 OSiMe3
mr*

1) 3CH31

2) TASSIMe3F2


CH3- &CH3 R CH3 1 CH3
0-1 3 I H1 3 I 3
R R CH3 R
mr*


1) '3 c 31
rm* -
2) TASSiMe3F2


1) 13CH31
rr* -------
2) TASSiMe3F2


rm*




rr*


Fig. 3- 8. Scheme illustrating monomer addition to and methylation of GTP
PMMA.





(NOE) has been shown by the use of model compounds.54 The fractions of

racemic (fr) and meso dyads (fm) from the main chain can be calculated from:


fr = frr + 0.5 fmr; fm = fmm + 0.5 fmr


(3-9)



































-i I




Sco




O( '


-* 5

Q4




flo





CL


C;)
0
3: --| -L

U..)









oi
C6

il
















LL
I






a.
I-
0



m
caJ





CL)



a.
0
C-
(DJ







E
Cb
-o



CL
ca
0

0






t5r
C-
Q.
C

0
0f)
E




Ti






0
LO









cii
T--
Ch








cm,
u,


I .











rrr*











mrr*


mr*

rm* rrr*


mr*
Smrr rm*


30 224 23 PPM





Fig. 3-11. 50MHz 13C NMR spectrum of upfield and downfield regions of
13CH3 end group of PMMA prepared by GTP at -230 in THF.







The persistence ratio, p, is defined as :


p = 2 fmfr/fmr (3-10)


This should be unity for a Bernoullian process. Pmr, and Prm, the first order

Markoff probabilities can be calculated from:42


Pmr = fmr/(2fmm + fmr); Prm = mr(2frr + mr) (3-11)


For a Bernoullian process, the sum of these two probabilities, ,P(= Pmr

+ Prm) should be equal to one. From Fig. 3-11 the two diastereomeric methyl
end groups are seen as expected, to absorb at markedly different fields (23-
24 vs. 29-30 ppm). The downfield absorption is more intense, but the tacticity
signals are somewhat better resolved for the upfield absorption. The triad
tacticity assignments (given in the spectrum) are based on well defined
oligomers of MMA.54 The fraction of the chain end heterotactic triads (fmr*
and frm*) are obtained directly from the NMR as are the fractions of chain-end
tetrads (frrr* and fmrr*). The fraction frr* was calculated from equation (3-7).

The relative peak areas were determined directly by the method of cutting and
weighing of the peaks after extrapolation of overlapping peaks to Lorenzian
peak shapes. Both the methyl end groups were used for direct determination
of relative peak areas.
The first order Markoff conditional probabilites P*mr and P*rm from the
chain end, can be calculated from equation (3-12).26


P*mr = frm*/fm*; P*rm = fmr*/fr* (3- 12)


Equation 3- 12 may be derived as follows:26







Elias and co-workers55-57 have shown that differences in the
stereochemistry of the chain end and main chain of vinyl polymers are
consistent with the occurence of Markoff processes. Thus, using Fig. 3-13
and following steady state conditions, we have for chains of sufficiently high
degree of polymerization:


d[m*]/dt = krm = [r*][M] kmrlm*][M] = 0


(3-13)


so that


fm* = [m*]/([m*] + [r*]) = krm/(krm + kmr)


(3-14)


and


fr* = kmr/ (krm + kmr)


(3-15)


The meso content, fm of the chain itself is given by


fm = krm/(krm + kmr k/ km)


(3-16)


where kr = krr + krm and km = kmm + kmr denote the rate constants of

propagation for r* and m* silyl ketene acetals respectively. We also have the
following relationships, using steady state conditions:


d[mr*/dt = kmr[m*][M] kr(mr*][M] = 0
d[rm*]/dt = krm[r*][M] km[rm*][M] = 0 (3-17)


Substituting equations 3-14 and 3-15 into equation 3-17 leads to


[rm*] = krm [r*]/km or frm* = krmkmr/km(kmr + krm)


(3-18)







where

frm* = [rm*]/([r*] + [m*]). (3-19)


Similarly
fmr* = krmkmr/(kr[kmr + krm]) (3-20)


and using equations 3-14 and 3-15 leads to


frm* = (kmr/km)fm* = P*mrfm* and
fmr = (krmkr)fr* = P*rmfr* (3-21)


Rearrangement of equation 3-21 leads directly to equation 3-12. For a
genuinely Bernoullian process, it can be shown that the qualities


fm = fm*; fr = fr (3-22)
and


Pmr = P*mr; Prm = P*rm (3-23)
will hold true.26 Thus for a Bernoullian process, the stereochemistry of main
chain and chain-end should be the same. Noncompliance with equation 3-22
but compliance with equation 3-23 is consistent with a first order Markov
process. All of the tacticity results and the stereochemical parameters from the
main chain and chain end are listed in Table 3-4.
It can be seen from this table that the persistence ratio (p) and the sum
of the two first order Markov probabilities from the main chain, 7P (= Pmr +

Prm) calculated from the main chain triads are close to one, consistent with
Bernoullian statistics as reported in the literature.







Table 3-4. 13C NMR Triad Tacticity and Stereochemical Parameters for Main
Chain and Chain End of PMMA Prepared by GTP



Main Chain

Expt. T/oC fmma frra fmra frb pC Pmrd Prmd fr/fr*
No.


1 -96 <0.02 0.75 0.25 0.88 0.88 1.00 0.14 0.99
2 -87 <0.02 0.74 0.26 0.87 0.87 1.00 0.15 1.00
3 -78 0.031 0.71 0.26 0.84 1.03 0.81 0.16 0.97
4 -40 0.034 0.66 0.30 0.82 0.99 0.82 0.19 0.95
5 -23 0.027 0.64 0.33 0.81 0.94 0.86 0.20 0.91
6 0 0.054 0.61 0.34 0.78 1.03 0.76 0.22 0.99
7 25 0.046 0.58 0.38 0.77 0.95 0.80 0.25 0.93
8 45 0.054 0.56 0.39 0.75 0.97 0.78 0.26 1.04


Chain End


No..


fr*e


0.89
0.87
0.87
0.86
0.89
0.79
0.83
0.72


frm*a


0.11
0.13
0.14
0.14
0.11
0.21
0.17
0.28


fmr*a


0.12
0.14
0.15
0.15
0.19
0.20
0.21
0.18


frr*a


0.77
0.74
0.72
0.72
0.70
0.59
0.62
0.54


P*rmf


0.13
0.16
0.17
0.17
0.22
0.25
0.25
0.25


aObtained directly from 13C NMR;
calculated from equation (3-9);
Calculated from equation (3-10);
calculated from equation (3-11);
calculated from chain end triads as frr* + fmr* and equation (3-8);
calculated from equation (3-12); P'mr = 1 since frm* = fm*, no detectable
mm* signal.






81
The fraction of racemic dyads in the main chain (fr) agrees very well
with that of the chain end (fm*) at all temperatures, indicating consistency with

Bernoullian propagation statistics. In addition, the conditional first order
Markoff probability (P*rm) calculated from the chain end stereochemistry is in
excellent agreement with that from the main chain (Prm), indicating
consistency with first order Markoff chain propagation statistics. P*mr could not

be determined accurately since there was no detectable end mm* triad signal
(equation 3-8).
The temperature of methylation was either -780C or OoC. This
temperature had no effect on the chain end tacticity but it had a slight effect on
the stereochemistry of methylation. The latter is given by the ratio of the peak
areas at 29-30 ppm to that at 23-24 ppm, corresponding to the two
diastereotopic methyl groups. This ratio at -780C is 5.70 while at 0OC, it is
3.33. The increase in stereoselectivity with decreasing temperature is normal
and expected. Comparison with the analogous model oligomers of 2-
isopropenylpyridine58,59 indicate that the most shielded position at 23-24
ppm should correspond to the (a) methyl group with the (b) methyl group
absorbing at 29-30 ppm (Fig. 3-12).



(a)
03 C-13 R -i3 13,
i--- I II I I a3
R R 0-1 R (b)
mr*


R = -CO2CH3

Fig. 3-12. The two diastereotopic methyl groups at the chain-end of PMMA.
(a) meso-like (b) racemic-like.







Thus the methylation appears to have the same preferred stereochemistry as

the addition of monomer. Both occur predominantly in syndiotactic-like
manner.

It appears from our method of comparison of main chain and chain end
stereochemistry, that GTP of MMA is indeed is consistent with a Bernoullian
process. These results are indeed surprising in view of the reasons cited

earlier. The results appear to conform to the limiting cases in the E-Z model
(Table 3-5). One possibility is that the stereochemical behavior of the E and Z
sites are identical (limiting condition 1). Another possibility is that the rate of s-
trans monomer addition to the E isomer is identical to the rate of s-cis addition
to the Z isomer or vice versa (limiting condition 2). The other limiting condition
reducing the E-Z scheme to Bernoullian sites, namely the presence of only Z
or E sites appears unlikely in this case since a 70/30 E/Z mixture of silyl
ketene acetals is always found in the GTP of MMA.

In conclusion, it has been shown that the stereochemisty of the GTP of
MMA is consistent with simple Bernoullian statistics. In addition, it has been
shown that a comparison of the stereochemistry of the main chain with that of
the chain-end of a polymer is a sensitive and independent method for the
verification of statistics of vinyl polymerization and is applicable to systems
other than anionic polymerization.


Side Reactions in Group Transfer Polymerization
Considerable evidence has been given for the occurence of side
reactions in GTP. For example, in the GTP of MMA, low monomer conversions
were encountered in cases where MMA was added to a mixture of initiator
and catalyst at temperatures 0oC and higher. Also, in the GTP of TrMA
(Chapter IV) it was not possible to methylate the chain end. All these are
illustrative of side reactions involving the silyl ketene acetal functionality and








catalyst, leading to the destruction of the living character of the chain-end of
the polymer or even the ketene acetal end group. In this section, the results
are described for an experiment where the GTP initiator (1) and TASHF2

catalyst are mixed together in the absence of monomer, and changes are
followed by NMR.
The purpose of this experiment was to see what side products could be
formed by reaction of the initiator and TASHF2 catalyst. A 11:1 (mole ratio)
mixture of the initiator and TASHF2 catalyst respectively were mixed together
in vacuo at -780C in CD3CN (The catalyst is insoluble in THF). The mixture

was poured into an NMR tube after filtration in vacuo through a coarse glass
frit and sealed. The course of the reaction was followed by 1H NMR.
Figure 3-13 shows the 1H NMR spectrum of the GTP initiator alone and
the spectra of the reaction mixture after approximately 45, 75 and 120 minutes
at room temperature. The initiator is seen to disappear with time as can be
seen by the diminishing intensity of the two singlets at 1.6 ppm and the
methoxy signal at 3.45 ppm. Additional peaks are seen to be appearing at 0-
0.3, 1-1.5, 2.5, and 3.6-3.7 ppm regions. The TASHF2 catalyst has only one

absorbance at about 3 ppm (in the entire region of the spectrum shown),
corresponding to the N-CH3 protons of the TAS moiety. Comparison of the

final 1H and 13C NMR spectra of the product mixtures at the completion of the
reaction with both 1H and 13C NMR spectra of authentic methyl isobutyrate
indicates one of the reaction products to be methyl isobutyrate in
approximately an 85:15 mixture of of deuterated and nondeuterated
compounds. The presence of methyl isobutyrate was also confirmed by a GC
examination of the product mixtures.









CH3 OCH3

CH, OSi(CH3)3
(d) (a)


(c) (d)




, I. ..1 I .I .I .....
4 J


N(CH3)2
+ (CH3)2N-S + I
N(CH3)2
(e)
(a)


0
II
(CH3)2C-C-OCH3
(1) D(H) (9)
(h)


(iii)



)


K


4


(ii)
(g)


(f)




(c) (d)


i a


(iv)

(g)


U~l


1 I


60 MHz 1H NMR spectrum of the GTP initiator 1 and TASHF2
catalyst and initiator mixture (1:11 molar ratio) after various time
intervals. (i) the initiator, (ii) reaction mixture after 45 mins., (iii)
reaction mixture after 75 mins., (iv) reaction mixture after 120 mins.


Fig. 3-13.


--


"'"''" Al








The mechanism of deuteration is undoubtedly complex owing to the
presence of a number of compounds and although a detailed mechanism
cannot be given at this time, the experiment does demonstrate that there is
indeed a reaction between the initiator and catalyst, leading to the complete
destruction of the silyl ketene acetal group (by deuteration in this case). In
addition, since the initiator was in approximately 11 fold excess with respect to
the catalyst, and none of it remained at the end of the reaction, there is a
strong possibility of the polymerization catalyst acting as a deuteration
(protonation) catalyst as well.
Although several mechanisms may be possible, some of the possible
steps are given in Fig. 3-14. A likely first step is the formation of catalyst-
initiator complex, which is the same as the step involving initiator activation in
the polymerization of MMA (step 1).1 This pentacoordinate species may then
abstract a D+ ion from CD3CN (pKa = 25) to give deuterated methyl

isobutyrate in a number of ways (steps 2,3 and 5). The formation of 15%
nondeuterated methyl isobutyrate may result from the protonation of the
initiator (step 6) with HF formed in the second step or from the protonation of
the TAS enolate ( step 7). It was not possible to arrive at a detailed
mechanism for this complicated system in the context of this research. The
identity of any species other than methyl isobutyrate could not be confirmed
spectroscopically, due to the complexity of the 1H and 13C NMR spectra
resulting from the presence of a number of compounds obtained at the end of
the reaction. The catalyst destruction also could not be observed as there was
only one absorption for the TAS moiety in both the 1H and the 13C NMR,
corresponding to the N-methyl group, and the intensity of this absorption did
not change with time.









CH3 OCH3
1) + TASHF2
CH, OSi(CH 3)3



CH3 OCH3
2)--c0 TAS*
2) CH3, / -SIMe3


F


CD2CN


q


CH3 OCH3
S0 TASc
CH3 OSi(CH3)3


HF

- CD(CH3)2CO2CH3+ (CH3)3SiF

+ CD2HCN + TASF


CH, OCH3

)CH3 M
3) C'g J ^SiMe,

r FHF

D

CD2CN
4) TASF + 1 -


AS+ C 0
S- NCxOCD- SiMe3
L FHF


TAS+ --TAS-CD2CN-
I + HF + Me3SiF


CD(CH3)CO02CH3


CH3 OCHA

CH3, 0O TAS*


+ Si(CH 3)3F


CH3 OCH3
5) CH + CD3CN -TASCD2CN + CD(CH 3)20C0CH3
CH3 O- TAS


CH3 OCH3
6) >-H< + HF --
CH3 OSi(CH3)3


CH3 OCH3

CH7)
OHa 0-


(CH3)2CHCO2CH3 + SIMe3F


+ HF (CH3)2CHCO2CH3 + TASF-


TAS*


HF + TAS*CDCN- TAS-F- + CHD2CN



Fig. 3-14. Possible steps in mechanism of deuteration of GTP initiator in the
presence of TASHF2 in CD3CN.












CHAPTER IV

GROUP TRANSFER POLYMERIZATION OF DIPHENYLMETHYL AND
TRIPHENYLMETHYL METHACRYLATES


Background
The stereospecific polymerization of various methacrylate monomers
has been studied extensively.60 Among these the polymerization of
triphenylmethyl methacrylate (TrMA) is of special interest in that its
polymerization brings about some most unusual and unexpected results. This
monomer forms at -780C highly isotactic polymers (> 90%) with n-
Butyllithium (BuLi) not only in toluene but also in THF.11,38 Even the radical
polymerization of this monomer at 600C gives a highly isotactic (= 60%)
polymer.11,38,61,62 This unique nature of this polymerization has been
ascribed to the bulky trityl group and its interaction with the polymer backbone.
The large ester group also affects greatly its reactivity in the alkyl lithium
initiated copolymerization with other methacrylates, such as methyl
methacrylate (MMA) or alpha-methylbenzyl methacrylate (MBMA) in
toluene.63-65 Some of the copolymers of (S)-MBMA and TrMA show large
positive optical rotations, which are opposite in sign to the homopolymers of
(S)-MBMA. This positive rotation is attributed to the isotactic sequence of
TrMA units which has helical conformation spiraled in a single screw
sense.66-68
It has also been found by Okamoto and coworkers that polymerization
of TrMA with chiral anionic initiators also gives an optically active polymer, the
chirality of which is caused by helicity.69 This was the first







88
example of an optically active vinyl polymer the optical activity of which arose
only from the helicity of the chains.
In contrast to trityl methacrylate, the anionic polymerization of
diphenylmethyl methacrylate (DMA) and benzyl methacrylate (BMA) in THF
using BuLi yields predominantly syndiotactic polymer.38 The tacticity results
are generally very similar to MMA.
As group transfer polymerization is a new technique for polymerization
of acrylates and methacrylates, it was of great interest to look at the tacticity of
poly TrMA (PTrMA) and poly DMA (PDMA) prepared by GTP and compare the
results with those obtained from anionic and radical polymerizations. Thus the
group transfer polymerization of MMA, DMA and TrMA would constitute a
systematic study concerning the effect of the ester group on the tacticity of the
polymer.


Group Transfer Polymerization of TrMA
Initially, the GTP of TrMA was attempted under conditions similar to
those used for MMA at temperatures higher than OOC, i.e. slow addition of
monomer and intiator mixture into the catalyst suspension in THF at -780C. At
initiator to catalyst concentration ratio of about 78, virtually no polymer formed,
and all of the unreacted monomer was recovered, as judged by 1H NMR. This
indicated that conditions different from those of GTP of MMA had to be
employed for the polymerization of TrMA. The first successful polymerimation
resulted by using a molar ratio of initiator to catalyst of about one. The GTP of
TrMA is extremely fast. Even at -70oC, complete monomer conversion was
observed in less than one minute. Since the initial failure of polymerization
could have been due to insufficient catalyst concentration in the reaction
mixture, a variety of catalyst concentrations were used to see what conditions







89
would bring about quantitative conversion of TrMA into polymer. The results
of these experiments are summarized in Table 4-1.
It is apparent from this table that much higher levels of catalyst are
required (i.e. higher values of intiator to catalyst concentration ratios) for
quantitative polymerization of TrMA than for the MMA polymerizations, where
catalyst levels as low as 0.1 mole % with respect to initiator (methyl trimethyl
silyl dimethyl ketene acetal) have been found to yield narrow MW distribution
PMMA in quantitative yield.
All of the polymerizations in Table 1(except run number 17 where a
mixture of monomer and initiator were added to the catalyst) were batch
polymerizations in which the catalyst was added at one time as an acetonitrile
solution (0.1-1.0 mL depending on the mole amount of initiator) to a mixture of
monomer and intitator in THF. Several conclusions can be drawn from the
data in Table 1. The molecular weight control appears very poor in that it far
exceeds the expected molecular weight, indicating to poor initiator efficiency,
which varies from 11-53%. There does appear to be some internal
consistency however, in that the agreement between expected and actual MW
becomes poorer with increasing degree of polymerization. The poor initiator
efficiency indicates that not all of the initiator is used to initiate polymerization.
It is somewhat surprising however, that the MW distribution are fairly narrow
despite the poor initiator efficiencies. This means that initiation is indeed fast
compared to propagation but that much of it is destroyed, before initiation.
This is most likely due to side reactions involving the initiator and catalyst.
The rates of side reaction and initiation must be more competitive in this case
compared to MMA polymerization, where there usually was good agreement
between expected and actual Mn. It is not surprising that the termination

reactions are more severe in the case of the TrMA polymerization than in the









Table 4-1. SEC Results of Group Transfer Polymerization of TrMA.

Run No. Temp Catalyst mole. init. [Init.] Yield Mwe Mne Mw Calcul. Mf
(OC) X 10-3 %_
[Cat.] Mn



1 -97 Aa 0.25 1.0 > 95 4105 3196 1.28 1337
2 -95 0.70 > 95 3027 2695 1.12 1437
3 -82 0.46 > 95 4442 3642 1.21 1870
4 -70 0.29 > 95 39564 26870 1.47 2930
5 -70 0.31 > 95 3391 3094 1.11 1313
6 -70 0.21 2.6 >95 14103 11233 1.26 3030
7 -70 0.27 93.0 =30 31787 21586 1.47 2881
8 -42 0.49 1.0 > 95% 4218 3458 1.22 1440
9 -21 0.37 > 95% 3223 2768 1.16 678
10 0 1.39 >95% 8655 6953 1.24 850
11 0 0.57 >95% 8088 4951 1.63 1130
12 25 0.32 >95% 28423 15134 1.88 2720
13 25 0.69 70 9839 5846 1.68 966
14 35 0.49 <5 32317 11309 2.86 1000
15 50 0.98 <5 25705 8137 3.16 643
16 -70 Bb 0.55 1.0 100 10910 8554 1.28 1231
17 -70 Bb 0.73 78.0 <5 -
18 -70 Cc 0.61 1.0 =10 ND9 ND ND ND
19 -70 Dd 0.18 1.0 -15 NDh ND ND ND


aTASSiMe3F2,
Cn-Bu4NF,


bTASHF2,
dn-Bu40Ac,


molecular weight of the transesterified PMMA samples,
calculated from the mole ratio of monomer to initiator,
sample not transesterified but the elution volume of the poly (TrMA)
corresponded to an approximate molecular weight of 199000 based on
PMMA calibration curve, Mw/Mn > 2.0.
sample not transesterified but the elution volume of the poly (TrMA)
corresponded to a molecular weight of approximately 126000 based on
PMMA calibration curve, Mw/Mn = 1.5







91
case of MMA since in the latter case, it is reported that it is best to use the
minimum amount of catalyst in order to avoid termination reactions and to
obtain polymers of lowest polydispersity, particularly when preparing
polymers of Mn above about 20,000. Also in MMA polymerizations using a

high level of catalyst, it was found that the addition of additional monomer
after 30 minutes to a "living GTP PMMA", gave no polymerization, indicating
that the polymer formed in the presence of a high level of catalyst was no
longer living. Since a high level of catalyst is necessary to obtain complete
conversion of monomer to polymer in the case of TrMA, it is not surprising that
the polymerization is actually self-terminating. The self-termination was
demonstrated by the addition of additional monomer (TrMA) 30 mins. after an
initial polymerization of TrMA at -70oC. The second batch of polymer failed to
polymerize.
The rates of side reactions competing with initiation increases with
temperature as was demonstrated during attempted polymerizations at
temperatures higher than 25oC. Thus attempted polymerizations at 350C and
50oC, resulted in complete failure, with virtually all the monomer recovered.
The failure of the polymerization is apparently due to a lack of initiation
caused by the destruction of the silyl ketene acetal initiator. This was
demonstrated by cooling the reaction mixture of an attempted 50oC
polymerization down to OoC, and adding an additional amount of initiator. In
this case, all the monomer polymerized immediately and the color also
changed to dark orange. Thus it appears that although at higher
temperatures, initiation is inhibited due to destruction of the initiator, there is
still sufficient active catalyst left capable of catalyzing the polymerization upon
the addition of additional initiator at lower temperature. This indicates that the
destruction of the initiator may not be due to a 1:1 equimolar reaction between






92
the initiator and the catalyst. This is completely consistent with our earlier
investigations into the side reactions involving the initiator and TASHF2
catalyst in CD3CN at room temperature without the presence of monomer. In

this case, it was found that even though a large excess of initiator over catalyst
(11:1 mole ratio) was present in the reaction, all the initiator dissapeared in
approximately two hours after the start of reaction. Various color changes
were seen in the case when the initiator was added at OoC to to the reaction
mixture of an unsuccessful 50oC attempted polymerization. As more and
more initiator was added drop by drop, the color changed from slight yellow to
orange to dark brown to red and finally to green in gradual progression. It
was a colorful experiment to say the least! The reasons for these color
changes are not entirely clear at present, but could represent reactions
involving the initiator catalyst complex (2) or enolate species with a TAS
counterion (such as 42 generated in equation (4-1)):


CH3 OSI(CH3)3 CO TAS

(1 >


(4-1)


Such an enolate species, could eliminate a methoxide ion to yield a ketene,
43 in a side reaction (equation 4-2). It is to be noted however, that no direct
evidence was found for the presence of the ketene; such reactions however,
are plausible on the basis of literature findings.28









C000- TAS+
TSC -c O + TAS OMe
> = OMe
43

(4-2)
Good catalytic activity was expected from NBu4F (Aldrich; containing less than

5 weight % water) on account of its being a highly efficient source of
nucleophilic fluoride ion, and its use in the GTP of MMA. However even one
equivalent of NBu4F with respect ot initiator was incapable of catalyzing the

polymerization to complete monomer conversion (run no. 18, Table 4-1).
Undoubtedly, the reagent may still contain moisture which could
terminate the polymerization. When halides or tosylates were treated with
"anhydrous TBAF" (i.e. NBu4F, 3 H20 warmed at 400C under vacuum for

several hours) in the absence of any solvent, hydrolysis to the corresponding
alcohol of the tosylate appeared to be a significant side reaction.70 This was
explained by the presence of traces of moisture remaining in the "anhydrous"
TBAF which are rendered highly nucleophilic by the fluoride ion. However, it
is probably unlikely that the presence of water was the reason for the lack of
activity of NBu4F. If that were the case, it would not be effective in GTP of

MMA either. It is therefore not entirely clear why this catalyst failed to yield a
good yield of poly(TrMA), based on the conventional associaive mechanism of
GTP. Another catayst, NBu40AC, known to catalyse the GTP of MMA failed to

effectively catalyse the polymerization of TrMA in good yield (run no. 19).
These findings hint at the possibility of a mechanism for the GTP of TrMA
different from that postulated by the Dupont group for MMA polymerization.