Chemistry Reference
In-Depth Information
9.7.8 Preparation of Block Copolymers by Homogeneous Ionic
Copolymerization
Formation of block copolymers by this method depends upon the ability to form “living” chain ends.
Among the anionic systems, the following polymerizations fit this requirement:
1. Polymerizations of nonpolar monomers with alkali metal-aromatic electron transfer initiators in
ethers [
398
].
2. Polymerizations of nonpolar monomers with organolithium compounds in hydrocarbon solvents [
399
].
3. Acrylonitrile polymerizations in dimethyl formamide initiated by sodium triethylthioisopropox-
yaluminate at
78
C[
340
].
4. Copolymerizations of hexafluoroacetone and cyclic oxides initiated by CsF [
341
].
5. Polymerization of alkyl isocyanates initiated by organoalkali species in hydrocarbons at
78
C[
342
].
Among the cationic “living” polymerizations that can be used for block copolymer formation are:
1. Polymerizations of isobutylene [
361
] and/or vinyl ethers [
363
] with appropriate catalysts. This
includes formation of block copolymers from the two types of monomers [
365
].
2. Polymerizations of tetrahydrofuran with the aid of chlorobenzene diazonium hexafluorophosphate
[
343
], triphenylmethyl hexachloroantimonate [
344
], or phosphorus pentafluoride [
445
].
3. Polymerization of
p
-methyl styrene,
N
-vinylcarbazole, and indene with appropriate catalysts.
The preparations by anionic mechanism of A—B—A type block copolymers of styrene and
butadiene can be carried out with the styrene being polymerized first. Use of alkyl lithium initiators
in hydrocarbon solvents is usually a good choice, if one seeks to form the greatest amount of
-1,4
copolymers from methyl methacrylate and styrene, because “living” methyl methacrylate polymers
fail to initiate polymerizations of styrene [
347
]. The poly(methyl methacrylate) anions may not be
sufficiently basic to initiate styrene polymerizations [
345
].
A “living” cationic polymerization of tetrahydrofuran, using BH
3
as the initiator in the presence of
epichlorohydrin and 3,3-bis(chloromethyl)oxacyclo-butane [
348
], results in formation of block
copolymers. Two types form. One is an A—B type. It consists of polytetrahydrofuran blocks attached
to blocks of poly(3,3-bis(chloromethyl)oxacyclo-butane). The other one is an A—AB—B type [
348
].
The preparation of well-defined sequential copolymers by anionic mechanism has been explored
and utilized commercially for some time now. Initially, the cationic methods received less attention
until it was demonstrated by Kennedy [
424
] that a large variety of block copolymers can be formed.
The key to Kennedy's early work is tight control over the polymerization reaction. The initiation and
propagation events must be fundamentally similar, although not identical [
424
]:
cis
Ion generation:
R
MeX
nþ
1
RX
þ
MeX
n
Ð
Cationization and propagation:
R
+
R
R
