Chemistry Reference
In-Depth Information
In the above scheme, the inifer, XRX, is usually an organic dihalide. If chain transferring to the
inifer is faster than chain transferring to the monomer, the polymer end groups become exclusively
terminated with halogens.
It is also possible to carry out “living” cationic polymerization of isobutylene, initiated by a
difunctional initiator [
435
]. This results in a formation of bifunctional “living,” segments of
polyisobutylene that are soft and rubbery. Upon completions of the polymerization, another mono-
mer, one that yields stiff segments and has a high
T
g
, like indene, is introduced into the living charge.
Polymerization of the second monomer is initiated from both ends of the formed polyisobutylene.
When the reaction is complete, the polymerization is quenched. Preparations of a variety of such
triblock and star block polymers have been described [
435
].
A technique was developed, by introducing cationic to anionic transformation [
438
]. A “living”
carbocationic polymerization of isobutylene is carried out first. After it is complete, the ends of the
chains are quantitatively transformed to polymerization-active anions. The additional blocks are then
built by an anionic polymerization. A triblock polymer of poly(methyl methacrylate)-
polyisobutylene-poly(methyl methacrylate) can thus be formed. The transformation takes several
steps. In the first one, a compound like toluene is Friedel-Craft alkylated by
-chloropolyi-
sobutylene. The ditolylpolyisobutylene, which forms, is lithiated in step two to form
a
,
o
-di-
tert
-di-
benzyllithium polyisobutylene. It is then reacted with 1,1-diphenylethylene to give the corresponding
dianion. After cooling to
a
,
o
78
C and dilution, methyl methacrylate monomer is introduced for the
second polymerization [
438
] in step 3.
Formation of block copolymers from polymers with functional end groups has been used in many
ways. In anionic polymerization, various technique were developed for terminating chain growth
with reactive end groups. These end groups allow subsequent formations of many different block
copolymers. One such active terminal group can be toluene diisocyanate [
439
]. The isocyanate group
located ortho to the methyl group is considerably less reactive toward the lithium species due to steric
hindrance [
438
]. The unreacted isocyanate group can be used for attachment of various polymers that
are terminated by hydroxy, carboxy, or amine groups. Other functional compounds that can be used in
such reactions are alkyl or aryl halides, succinic anhydride,
n
-bromophthalimide [
448
], and
chlorosilanes [
449
].
Because block copolymers can often offer properties that are unattainable with simple blends or
random copolymers [
364
], many efforts were made to combine dissimilar materials, like hydrophilic
with hydrophobic, or hard with soft segments, as was shown earlier. One paper [
432
] describes
formation of block copolymers containing helical polyisocyanide and an elastomeric polybutadiene.
Compound [(
3
-C
3
H
5
)-Ni(OC(O)CF
3
)]
2
was used to carry out “living” polymerization of butadiene
and then followed by polymerization of
tert
-butyl isocyanide to a helical polymer.
Z
9.7.9 Special Reactions for Preparation of Block Copolymers
A special case is the use of the
-phenylene pentadienylene) [
415
] is prepared
by this reaction first. This is utilized in a preparation of a block copolymer [
456
] according to the
following scheme:
Witting
reaction. Poly(
p
