Chemistry Reference
In-Depth Information
Propagation:
+
HCl
Cl
+
Cl
Cl
Cl
Subsequent interaction of macromolecular radicals leads to cross-linking. Many additional
investigations demonstrated that thermal decompositions of vinyl polymers with pendant electroneg-
ative groups that we can designate as X, result, after elimination of HX and formation of macromo-
lecular residues, with polyene structures [
488
]. In addition to that, at higher temperatures the polyene
sequences that form rearrange into a large numbers of aromatic hydrocarbons [
489
-
492
]. Formation
of polyenes is common to poly(vinyl chloride), poly(vinyl bromide), poly(vinyl alcohol), and poly
(vinyl acetate) [
493
]. The polyene structures partly decompose at formation due to bond stress and
molecular reorganization processes. The stresses result from formation of conjugated structures along
the polymeric chains. When the chains fracture, the remaining portions of polyene sequences form
aromatic compounds [
47
]. It was postulated that the process initially involves reactions of enone
groups, which are present in poly(vinyl chloride) as anomalies, with chlorine units of a neighboring
polymeric chains [
494
,
495
]:
O
HCl
+
O
O
+
The first step is formation of a dihydropyran ring. This is followed by a retro-Diels-Alder splitting
and leads to regeneration of the
-unsaturated ketone and to formation of a double bond in the poly
(vinyl chloride) molecule. The double bond initiates a subsequent elimination of hydrogen chloride
from that molecule [
495
].
An intramolecular initiation process that explains constant rate of dehydrochlorination was also
proposed [
496
]:
a
,
b
Cl
H
Cl
H
Cl
HCl
+
It was pointed out that poly(vinyl chloride) is, in a sense, its own worst enemy, in that all the
structural defects that are known to contribute appreciably to its thermal instability. They are formed
in the polymer molecules during the process of polymerization by routes involving hydrogen
abstraction from the polymer backbone [
584
].
