Chemistry Reference
In-Depth Information
isomerizations as well as cyclizations
occur in the process of thermal degradation of 1,4-polyisoprene. It is interesting that the
In addition, spectroscopic evidence shows that
cis-trans
cis-trans
isomerizations were observed at temperatures as low as 200
C[
460
].
9.8.3 Thermal Degradation of Polystyrene and Polystyrene-Like Polymers
degradation of atactic polystyrene
Thermal
results in formation of volatile products that contain as
much as 42% of the monomer and small quantities of toluene, ethylbenzene, and methylstyrene. The
rest of the volatile material is made up of dimers, trimers, and tetramers. No large fractions were
isolated. This suggests that the main mechanism of decomposition is depropagation. The rate,
however, exhibits a maximum and there is a rapid decline in molecular weight. This indicates
scission of the chains. Some evidence was presented that rapid decrease in molecular weight is a
result of scission of weak points in the polymer and is independent of the free-radical depropagation
reaction [
461
-
465
]. Some of the weak spots are believed to be occasional “head-to-head” placement
of monomers [
19
]. It was suggested that some weak spots may also be structures that form as a result
of monomer additions to the aromatic ring during the chain growth (see Chap.
6
)[
466
]. The following
scheme of chain scission was, therefore, proposed [
466
]:
H
Δ
+
Δ
H
+
The identity of the weak spots, however, has not been established with certainty [
467
]. The
evidence does indicate that the “weak points” are head-to-head bonds, branch points, or unsaturated
structure [
467
].
The rate of conversion of polystyrene into volatiles was measured in numerous studies.
In contrast to polystyrene, poly(
-methyl styrene) yields in vacuum pyrolyses at temperatures
between 200 and 500
C 95-100% monomer. By comparison, polystyrene only yields about 40.6%
[
457
]. The difference can be attributed to the fact that hydrogen transfer is completely blocked from
the sites of chain scission by the methyl groups in the
a
a
-positions [
457
]. As a result, the terminal free-
radicals unzip into monomers and dimers.
