Chemistry Reference
In-Depth Information
Another disadvantage of anionic polymerization of acrylonitrile is formation of cyanoethylate as a
side reaction. It can be overcome, however, by running the reaction at low temperatures. An example
is polymerizations initiated by KCN at
50
C in dimethylformamide [
254
], or by butyllithium in
78
C[
255
]. Both polymerizations yield white, high molecular weight products that are
free from cyanoethylation.
It was suggested that the terminations in anionic polymerizations of acrylonitrile proceed by
proton transfer from the monomer. This, however, depends upon catalyst concentrations [
256
,
257
].
At low concentrations, the terminations can apparently occur by a cyclization reaction [
257
] instead:
toluene at
N
N
N
N
N
N
Industrially, polyacrylonitrile homopolymers and copolymers are prepared mainly by free-radical
mechanism. The reactions are often conducted at low temperatures, in aqueous systems, either in
emulsions or in suspensions, using redox initiation. Colorless, high molecular weight materials form.
Bulk polymerizations are difficult to control on a large scale.
Over half the polymer that is prepared industrially is for use in textiles. Most of these are
copolymers containing about 10% of a comonomer. The comonomers can be methyl methacrylate,
vinyl acetate, or 2-vinylpyridine. The purpose of comonomers is to make the fibers more dyeable.
Polymerizations in solution offer an advantage of direct fiber spinning.
Polyacrylonitrile copolymers are also used in barrier resins for packaging. One such resin contains
at least 70% acrylonitrile and often methyl acrylate as the comonomer. The material has poor impact
resistance and in one industrial process the copolymer is prepared in the presence of about 10%
butadiene-acrylonitrile rubber by emulsion polymerization. The product contains some graft copoly-
mer and some polymer blend. In another process the impact resistance of the copolymer is improved
by biaxial orientation. The package, however, may have a tendency to shrink at elevated temperature,
because the copolymer does not crystallize.
It is possible to form clear transparent polyacrylonitrile plastic shapes by a special bulk polymeri-
zation technique [
258
,
259
]. The reaction is initiated with
-toluenesulfinic acid-hydrogen peroxide.
Initially, heterogeneous polymerizations take place. They are followed by spontaneous
transformations, at high conversion, to homogeneous, transparent polyacrylonitrile plastics [
260
].
A major condition for forming transparent solid polymer is continuous supply of monomer to fill the
gaps formed by volume contraction during the polymerization process [
261
].
Methacrylonitrile
p
C(CH
3
)CN, can also be prepared by several routes. Some commercial
processes are based on acetone cyanohydrin intermediate and others on dehydrogenation (or
oxydehydrogenation) of isobutyronitrile. It is also prepared from isobutylene by ammoxidation:
,CH
2
ΒΌ
3
2
+
+
NH
3
O
2
N
Just like acrylonitrile, methacrylonitrile does not polymerize thermally but polymerizes readily in
the presence of free-radical initiators. Unlike polyacrylonitrile, polymethacrylonitrile is soluble in
some ketone solvents. Bulk polymerizations of methacrylonitrile have the disadvantage of long
reaction time. The rate, however, accelerates with temperature. The polymer is soluble in the
monomer at ambient conditions [
262
].
