Polymers with Pendant Abietate and Dibenzazepine Groups
Poly (vinylbenzyl abietate) in the film state is cross-linkable via photo-dimerization of the conjugated carbon-carbon double bonds of the abietic acid moieties [166]. What the photo-dimerization product looks like is not clear. Formation of a cyclobutane rings in photo-dimerization of steroids was represented as follows [166]:
Also, judging from the photoreaction of cholestra-3,5-diene [166], one might expect some internal cyclization:
The cyclodimerization reaction of dibenzazepine was reported by Hyde et al. [167]. When attached to a polymer, the cross-linking reaction can be visualized as follows:
The cross-linking can result from either direct absorption of the light or in the presence of triple sensitizers like Michler’s ketone [168]. The 2 π +2 π addition leads to a formation of a cyclobutane cross-link.
Coumarin photo-dimerization is a known reaction [169]:
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 λ > 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 p-phenylenediacryloyl chloride, aromatic dianhydrides, and two equivalents of aromatic diamines. The products were subsequently imidized by reactions with the poly (amide-co-amic acid), acetic anhydride, and pyridine. The polymers were stable up to 350C, 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].
