Chemistry Reference
In-Depth Information
The main chain scissions are envisioned as involving fragmentation of the polymer alkoxy radicals
that form [
553
]:
CH
2
+
O
O
O
O
R
R
O
O
+
O
O
R
R
O
O
films are irradiated by ultraviolet light at various intensities and
temperatures under a nitrogen atmosphere, dehydrochlorination reaction takes place [
554
]. This
reaction occurs in two parts. During the first hour, the reaction is dependent on the intensity of
light and the temperature, but after that it becomes independent of these two parameters.
When poly(vinyl chloride) films are irradiated in the presence of benzophenone, the initiation is a
result of hydrogen abstraction from the polymer by the excited triplet of the aromatic ketone [
555
]:
This is followed by degradation that takes place by a chain mechanism:
When cast
poly(vinyl chloride)
*
O
OH
+
+
2
Cl
2
Cl
T
1
Cl
+
Cl
In the process, the chlorine radicals abstract hydrogens and form HCl. The quantum yield of
hydrogen chloride from this reaction is
0.11 [
556
]. This indicates that only 1 in every 100
photons that are absorbed causes the dehydrochlorination and is followed by formation of polyenes. It
led to the conclusion that a photochemical process must take place between the polyene sequences
and HCl [
556
,
557
].
When
F
HCl
ΒΌ
poly(phenyl vinyl ketone)
is irradiated by ultraviolet light of 365 nm, it degrades and
exhibits a loss of molecular weight [
558
]. No benzaldehyde was found among the products. That
excludes a Norrish Type I reaction and indicates that the polymer cleaves by a Norrish Type II
mechanism [
558
].
Day and Wiles have pictured the primary steps of photochemical decomposition of poly (ethylene
terephthalate) as follows [
559
]:
