Chemistry Reference
In-Depth Information
Coumarin photo-dimerization is a known reaction [
169
]:
hν
2
O
O
O
O
O
O
This was utilized by Tian et al. [
168
] to prepare a new class of liquid crystal homopolymers of poly
{ll-[4-(3-ethoxycarbonyl-coumarin-7-oxy)-carbonyl-phenyloxy]-undecyl methacrylate} containing a
coumarin moiety as a photocross-linkable unit. The preparations included polymers of various chain
lengths. Also, liquid crystalline-coil diblock and liquid crystalline-coil-liquid crystalline triblock
copolymers with polystyrene as the coil segment were formed. The polymers were reported to have
been synthesized with the aid of atom transfer radical polymerization. The dimerization of the
coumarin moieties takes place upon irradiation with light of
l >
300 nm to yield cross-linked
network structures.
Lee et al. [
169
] reported the preparation of new soluble and intrinsically photosensitive
poly(amide-
co
-imide)s containing
p
-phenylenediacryloyl moiety. The copolymers were formed
from
-phenylenediacryloyl chloride, aromatic dianhydrides, and two equivalents of aromatic
diamines. The products were subsequently imidized by reactions with the poly(amide-
p
-amic
acid), acetic anhydride, and pyridine. The polymers were stable up to 350
C, showed good
solubility in polar aprotic solvents, and became insoluble after the irradiation due to the photo-
dimerization of phenylenediacryloyl moiety. The photo-reactivity increases with the irradiation
temperature [
169
].
co
10.5.7 Polymers That Cross-link by Dimerization of Nitrenes and by Other
Combinations of Free-Radicals to Form Covalent Bonds
The aromatic azide groups photo-decompose into nitrenes when irradiated with UV light: The nitrenes,
that form, possess two unpaired electrons, similarly to carbenes, and dimerize readily into
azo
groups
UV light
R
N
N
N
R
N
+
N
2
2
R
N
R
N
N
R
This reaction is utilized in photocross-linking.
10.5.8 Polymers with Pendant Azide Groups
Both azide and sulfonyl azide groups are photo-reactive and decompose into active nitrene groups
and nitrogen upon irradiation. The dissociation of the azide moiety follows almost every transition
