Chemistry Reference
In-Depth Information
copolymers. This process is applicable to vinyl monomers with electron-with-
drawing substituents, like acrylates and methacrylates.
The initiator is a silyl ketene acetal (11-11) which can be synthesized from
methyl methacrylate and trimethyl- or tetramethylsilane:
CH
3
OCH
3
CH
3
CC
CH
2
C+
CH
3
Si(CH
3
)
3
CH
3
O
Si(CH
3
)
3
(11-26)
C
O
OCH
3
11-11
F
~
,CN
~
,or
selected Lewis acids, proceeds by Michael addition in which the silyl group is
transferred to the new monomer unit to renew the terminal silyl ketene acetal. (A
Michael reaction, in general, is the addition of an enolate to an
Monomer addition, which is catalyzed by anions like HF
2
;
α
,
β
-unsaturated
carbonyl compound.)
CH
3
CH
3
CH
3
CH
3
OCH
3
CH
3
OCH
3
HF
2
CC
+ nCH
2
C
C
CH
3
C
CH
2
C
CH
2
n−1
C
C
OSi(CH
3
)
3
OSi(CH
3
)
3
CH
3
O
C
O
O
C
O
O
OCH
3
CH
3
CH
3
(11-27)
Unlike anionic polymerizations, the reaction sites are not ion pairs; the cata-
lyst is believed to facilitate transfer of the trimethylsilyl group by dipolar interac-
tion with the Si atom. As in anionic polymerizations, however, the reactive end
group is deactivated by compounds carrying labile hydrogens. Group transfer
polymerizations therefore must be carried out under anhydrous conditions.
Group transfer processes have some of the characteristics of living polymeri-
zations. The degree of polymerization is determined by the ratio of monomer to
silyl ketene initiator [cf.
Section 11.2.3.2
] and the molecular weight distribution
of the product is narrow. In the absence of terminating impurities, the terminal
silylketene groups remain active and block copolymers can be produced by intro-
duction of another polymerizable monomer. Functional terminal groups can be
introduced by reacting the silylketene acetal with an appropriate electrophile,
such as aldehydes or acryl halides and active macrospecies can be coupled with
dihalides. Also, terminal functional groups can be introduced as substituents in
the silylketene initiator.
Currently, this polymerization technique is applied primarily for the synthesis
of fairly low-molecular-weight functionalized polymers and copolymers of methyl
methacrylate, for automotive finishes.
