Chemistry Reference
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in many topics [
200
]. The isomerization reaction takes place because many olefins in the excited
singlet and triplet states have a perpendicular instead of a planar geometry. This means that in the
excited state the
isomerism disappears. Upon return to the ground state, S
0
, it is possible for
either isomer to form. The return, however, usually takes place to the more stable form. Generally,
photo-isomerization of chromophores in dilute solutions is a first-order reaction.
cis
-
trans
10.6.2.1 Photo-Isomerization of the Olefinic Group
An example of olefinic groups rearrangement is work by Onciu et al. [
201
] who formed three bis
(trimellitimide)s by condensing three aromatic diamines with trimellitic anhydride. This was
followed by preparation of two series of photo-reactive copoly(amide-imide)s by direct polyconden-
sation of the bis(trimellitimide)s and 1,4-phenylenediacrilic acid with either 4,4
0
-diphenyl-
methanediisocyanate in one case or with 1,6-diisocyanatohexane in another case, respectfully. All
of the copoly(amide-imide)s were found to be soluble in polar aprotic solvents and to yield transpar-
ent, flexible, and tough films [
201
]. When the polymers are irradiated in solution, the
p
-phenylene-
diacryloyl units undergo
photo-isomerization and (2 + 2) photo-cycloadditions [
201
]. The
fully aromatic polyamides also undergo a photo-Fries rearrangement. The photo-Fries reaction,
however, is completely suppressed in polymers that contain an aliphatic amide moiety [
22
]. The
same processes are also observed in the polymer films [
201
].
Polymers prepared by condensation of 4,4
0
-diacetylstiblene as the bis(ketomethylene)monomer
with 4,4
0
-diamino-3,3
0
-dibenzoylstilbene, a bi(amino ketone), exhibit photo-viscosity effects in dilute
solutions due to
trans-cis
isomerization [
202
]. The preparation of the polymers and the photo-
rearrangements can be illustrated as follows:
cis-trans
O
H
2
N
NH
2
O
O
+
O
n
N
N
UV light
n
N
N
The changes in viscosity can vary from 2 to 23% as a result of irradiation.
