Chemistry Reference
In-Depth Information
polymerization very similarly to the butadiene-styrene copolymers. Similarly, “hot” and “cold”
processes are employed. “Low,” “medium,” and “high” grades of solvent-resistant copolymers are
formed, depending upon the amount of acrylonitrile in the copolymer that can range from 25 to 40%.
The butadiene placement in these copolymers is approximately 77.5%
1,4, and
10% of 1,2-units. Also, the polymers formed by the “cold” process are less branched and have a
narrower molecular weight distribution than those formed by the “hot” process.
An interesting alternating copolymer of butadiene and acrylonitrile was developed in Japan [
147
].
The copolymer is formed with coordination catalysts consisting of AlR
3
, AlCl
3
, and VOCl
3
in a
suspension polymerization process. The product is more than 94% alternate and is reported to have
very good mechanical properties and good oil resistance.
trans-
1,4, 12.5%
cis-
6.12 Polystyrene and Polystyrene-Like Polymers
Styrene is produced in the United States from benzene and ethylene by a Friedel-Craft reaction that is
followed by dehydrogenation over alumina at 600
C. Polystyrene was first prepared in 1839, though
the material was confused for an oxidation product of the styrene monomer [
148
]. Today polystyrene
is produced in very large quantities and much is known about this material.
6.12.1 Preparation of Polystyrene by Free-Radical Mechanism
Styrene is one of those monomers that lends itself to polymerization by free-radical, cationic, anionic
and coordination mechanisms. This is due to several reasons. One is resonance stabilization of the
reactive polystyryl species in the transition state that lowers the activation energy of the propagation
reaction. Another is the low polarity of the monomer. This facilitates attack by free-radicals,
differently charged ions, and metal complexes. In addition, no side reactions that occur in ionic
polymerizations of monomers with functional groups are possible. Styrene polymerizes in the dark by
free-radical mechanism more slowly than it does in the presence of light [
149
]. Also, styrene formed
in the dark is reported to have greater amount of syndiotactic placement [
150
]. The amount
of branching in the polymer prepared by free-radical mechanism increases with temperature [
136
].
This also depends upon the initiator used [
151
].
The following information has evolved about the free-radical polymerization of styrene:
1. Styrene polymerizes thermally [
151
-
155
]. This is discussed in Chap.
3
.
2. Oxygen retards polymerizations of styrene. At higher temperatures, however,
the rate is
accelerated due to peroxide formation [
156
].
3. The rate of styrene polymerizations in bulk is initially, at low conversions, first order with respect to
monomer concentrations. In solution, however, it is a second order with respect to monomer [
157
].
Polystyrene that is manufactured by free-radical polymerization is atactic. Isotactic polystyrene
formed with Ziegler-Natta catalysts was introduced commercially in the 1960s, but failed to gain
acceptance. Syndiotactic polystyrene is now being produced commercially.
Industrially, free-radical styrene polymerizations are carried out in bulk, in emulsion, in solution,
and in suspension. The clear plastic is generally prepared by mass polymerization. Because polysty-
rene is soluble in the monomer, mass polymerization, when carried out to completion, results in a
tremendous increase in melt viscosity. To avoid this, when styrene is polymerized in bulk in an
agitated kettle, the reaction is only carried out to 30-40% conversion. After that, the viscous syrup is
transferred to another type of reactor for the completion of the reaction. According to one early
