Chemistry Reference
In-Depth Information
was studied by several
investigators [
544
-
547
]. A gas evolution was observed during the irradiation with ultraviolet light.
This gas contains hydrogen as its major portion and methane, ethane as the minor portions. There are
also traces of styrene,
The mechanism of photo-degradation of
poly(p-methylstyrene)
-methylstyrene, and toluene [
101
]. The gas evolution is accompanied by
cross-linking. The start of the process is pictured as follows [
547
]:
p
CH
2
CH
2
hν
+
+
+
+
CH
2
The products
react
to further yield hydrogen,
cross-linked material, monomer,
and
cyclohexadiene.
Photolysis of
with ultraviolet light of 254 nm in vacuum at 10
6
mbar
and room temperature yields hydrogen as the main product and a small quantity of methane, ethane,
and a trace of propane [
548
].
Poly(methyl methacrylate)
depolymerizes at elevated temperature under the influence of ultravio-
let light of 259.7 nm [
549
]. Irradiation of polyacrylonitrile, however, leads to chain scission at the
acrylonitrile units. The difference between thermal and ultraviolet light degradation of polyacryloni-
trile is principally in the different sites of initiation and the fact that the reaction occurs at 160
C in the
presence of light and at 280
C in the darkness [
550
].
When a copolymer of methyl methacrylate and
poly(p-isopropyl styrene)
-butyl acrylate is irradiated in vacuum at elevated
temperature, like 165
C with light of 253.7 nm, the gaseous products are minor [
551
]. These are
hydrogen, carbon monoxide, and methane. The liquid products are predominantly methyl methacry-
late,
n
-butyraldehyde. The degradation is explained by Grassie
and coworkers [
551
] in terms of a radical process that is initiated by scission of pendant acrylate units.
The propagation is actually a combination of depropagation of the chains and intra- and intermolec-
ular chain transfer process. The relative importance of each phase of depropagation reaction depends
upon the composition of the copolymer [
551
].
Poly(vinyl acetate)
n
-butyl acrylate,
n
-butyl alcohol, and
n
was shown to undergo simultaneous cross-linking and chain scission when
irradiated in vacuum with ultraviolet light [
552
]. The following mechanism was proposed as the route
to chain cleavage [
552
]:
+
+
O
O
O
O
*
O
O
O
OH
O
OH
O
O
