Photocondensation Polymerization

Photocondensation Polymerization.—This term is used in the sense that the polymerization requires the absorption of a photon for every propagation step, as distinct from the photoinitiation of the free-radical chain addition polymerization discussed above. The photodimerization reaction (14) of maleimides in 

Not all such compounds can be polymerized, however, and the authors reasonably argue that the monomer crystal structure dictates the fate of the molecule upon u.v. irradiation in the solid state. 

A luminescence study of the solid-state photopolymerization of p-diethynyl-benzene,58 the photopolymerization of epoxides,57 other photochemical poly- 

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Figure 6.12 The principle of a photocondensation polymerization reaction. With bichromophoric monomers long chains of polymers can be formed...

A very different process of photopolymerization relies on the reaction of photocondensation which is an addition of two molecules to form a longer adduct. There is no radical intermediate in this case and one photon (at least) is required for each step in the polymerization process. Some male-imide derivatives can be polymerized in this way, to form an insoluble crystalline polymer. It should be noted that the monomer molecules must have two reactive groups, one at each end, so that the polymer chains can extend in principle indefinitely (Figure 6.12). 

Cyclic imides with carbon-carbon double bonds are susceptible to polymerization by a radical mechanism or by several other mechanisms, such as photocondensation, Diels-Alder addition, or nucleophilic substitution. Although these latter reactions yield step-growth polymers, they will be considered in this part because traditionally they are considered addition polyimides. Furthermore, in many cases they are indeed cross-linkable through polymerizable imide double bonds. 

Although the investigation of polymaleimides and copolymaleimides has served to solve several theoretical aspects on radical polymerization of these unsaturated rings [265,266] and to give a great deal of data on maleimide polymers, they have not yet found end-use applications. The same is true for polyimides synthesized by the so-called photocondensation method, that consists of step-growth polymerization of bisimides with benzene or alkylbenzenes [267-269]. The reaction is induced by UV radiation (Scheme 52). 

Photocondensation is a procedure limited to monomers that do not easily polymerize by a photoinitiated radieal mechanism. Otherwise the UV radiation would induce the formation of chain growth homopolymers. 

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