Chemistry Reference
In-Depth Information
Use of Ziegler-Natta catalysts, as seen from Table
6.8
, can yield an almost all
cis-
1,4-polyisoprene
or an almost all
1,4-polyisoprene. The microstructure depends upon the ratio of titanium to
aluminum. Ratios of Ti:Al between 0.5:1 and 1.5:1 yield the
trans-
cis
isomer. A 1:1 ratio is the optimum.
Ratios of Ti:Al between 1.5:1 and 3:1 yield the
structures [
123
]. The titanium to aluminum
ratios also affect the yields of the polymers as well as the microstructures. There also is an influence
on the molecular weight of the product [
124
]. Variations in catalyst compositions, however, do not
affect the relative amounts of 1,4 to 3,4 or to 1,2 placements. Only
trans
arrangements are
affected. In addition, the molecular weights of the polymers and the microstructures are relatively
insensitive to the catalyst concentrations. The temperatures of the reactions, however, do affect the
rates, the molecular weights, and the microstructures.
Use of Ziegler-Natta catalysts, as seen from Table
6.8
, can yield an almost all
cis
and
trans
1,4-polyisoprene
or an almost all
trans-
1,4-polyisoprene. The microstructure depends upon the ratio of titanium to
aluminum. Variations in catalyst compositions, however, do not affect the relative amounts of 1,4
to 3,4 or to 1,2 placements. Only
cis-
arrangements are affected. In addition, the molecular
weights of the polymers and the microstructures are relatively insensitive to the catalyst
concentrations. The temperatures of the reactions, however, do affect the rates, the molecular
weights, and the microstructures.
cis
and
trans
6.7 Methyl Rubber, Poly(2,3-dimethylbutadiene)
Early attempts at preparations of synthetic rubbers resulted in developments of elastomers from
2,3-dimethylbutadiene. The material, called “methyl rubber,” was claimed to yield better elastomeric
properties than polybutadiene. Methyl rubber was produced in Germany during World War I where
the monomer was prepared from acetone. The polymerizations were carried out by free-radical
mechanism and anionically, using sodium metal dispersions for initiation. Later, it was demonstrated
that 2,3-dimethyl polybutadiene can be polymerized to very high
cis
-1,4 polymer with Ziegler-Natta
catalysts [
125
,
126
].
6.8 Chloroprene Rubber, Poly(2-chloro-1,3-butadiene)
2-Chloro-1,3-butadiene (chloroprene) was originally synthesized in 1930. The material can polymerize
spontaneously to an elastomer that has good resistance to oil and weathering. Commercial production
of chloroprene rubber started in 1932. Since then, many types of polymers and copolymers were
developed with the trivial generic name of
neoprene
.
The monomer can be prepared from acetylene:
NH
4
Cl
Cl
HCl
CCl
4
It can also be formed from butadiene.
Chloroprene is polymerized commercially by free-radical emulsion polymerization. The reaction
is carried out at 40
C to a 90% conversion. A typical recipe for such an emulsion polymerization is as
follows [
127
]:
