Chemistry Reference
In-Depth Information
molecular weights are high [
142
]. A high ratio of the transition metal halide to that of the aluminum
alkyl must be used. Such a ratio might, conceivably, mean that the mechanism of polymerization is
cationic. Also, conventional cationic initiators can be used to yield similar products. The cyclization
occurs during propagation. Unsaturation in the products can vary from none to as high as 80%,
depending upon the initiator used [
142
].
Different mechanisms were offered to explain the cyclization of 1,3 dienes [
142
,
143
].
The cyclization might conceivably occur by a sequential process:
or, perhaps from attacks by the propagating carbon cation on
trans
-1,4 double bonds:
R
R
where R
+
can represent either a propagating carbon cation or an initiating species. The extensive
cyclization may be a result of a sequential process [
142
,
143
].
Cyclopolymerizations typically result in low conversions and dormant reaction mixtures. When
additional monomer is added, the dormant mixtures reinitiate polymerizations that again proceed to
some limited conversions. If the original dormant mixtures are allowed to stand for a long time the
unreacted monomers are slowly consumed [
142
].
Polymerization of 2,3-dimethylbutadiene-1,3 with Ziegler-Natta catalysts consisting of
Al(
-1,4-polydimethylbutadiene as described earlier. This, however, takes
place when the aluminum alkyl is in excess. If, on the other hand, the ratio of Al to Ti is 1 or less,
cyclic polymer forms instead. The product has reduced unsaturation and some
trans-
1,4 units in the
chain [
144
]. A complex catalyst, consisting of Al(
i
-C
4
H
9
)
3
-CoCl
2
, yields polymers that are predomi-
nantly
i
-C
4
H
9
)
3
-TiCl
4
yields
cis
1,4 with about 20% of 1,2 units. On the other hand, acid catalysts, like Al(C
2
H
5
)Cl
2
, yield
cyclic polymers [
143
,
144
]. A polymer formed with the aid of X-ray radiation at low temperatures
also contains cyclic units and some
cis-
trans-
1,4 [
145
]. Butadiene and isoprene also form this type of
polymer at the same conditions [
145
].
6.11 Copolymers of Dienes
Several different elastomers, copolymers of butadiene, are produced commercially. The major ones
are copolymers of butadiene with styrene and butadiene with acrylonitrile. Some terpolymers, where
the third component is an unsaturated carboxylic acid, are also manufactured. Block copolymers of
isoprene with styrene and butadiene with styrene are important commercial elastomers.
6.11.1 GR-S Rubber
Copolymerization of butadiene with styrene by free-radical mechanism has been explored very
thoroughly [
146
]. The original efforts started during World War I in Germany. Subsequent work
