Chemistry Reference
In-Depth Information
The same is true of aromatic polyamides where photo-Fries rearrangements are done according to
the following mechanism [
563
]:
O
∗
O
N
hν
N
H
H
O
H
N
+
N
O
H
O
NH
2
O
N
H
H
ON
N
N
H
H
O
O
films was shown to produce products that are also
consistent with the photo-Fries reaction. These are salicylic acid and bisphenol type species [
580
].
This is in agreement with earlier studies that showed that a variety of processes, including
rearrangements (photo-Fries) and photo-oxidation, can occur when bisphenol A-based polycarbonate
is photolyzed [
581
,
582
].
Photolytic decomposition of
polycarbonate
9.9.14 Photo-Oxidative Degradations of Polymers
1
Singlet oxygen, the lowest excited state of molecular oxygen,
D
g
O
2
, is known to cause rapid
degradation of polymers. Singlet oxygen forms by several processes, including one of two
mechanisms: (1) through photosensitization by some impurity; (2) through energy transfer from an
excited triplet state of a chromophore.
Polyolefins that are unprotected from weathering react very readily with oxygen in the presence of
sunlight even at room temperature. This leads to loss of molecular weight and often embrittlement.
The mechanism of
was demonstrated to involve carbonyl groups that
developed in the polymer via oxidation during processing [
564
,
565
]. These carbonyl groups act as
chromophores and their triplet states (
photo-oxidation of polyethylene
*) are quenched by molecular oxygen in the ground
triplet state. Electronically excited singlet oxygen molecules form as a result [
564
].
n ! p
O
3
ðn ! p
Þþ
3
S
g
Þ!
1
O
2
C
¼
O
2
ð
C
¼
O(S
0
Þþ
>
The Norrish Type II reaction then takes place with a cleavage of carbon to carbon bonds, formation
of olefins and, subsequently, formation of hydroperoxides:
