Photochemical Syntheses of Graft Copolymers
Photo labile groups on polymers can serve as sites for photoinitiated graft copolymerizations. For instance, when polymers and copolymers of vinyl ketone decompose in ultraviolet light in the presence of acrylonitrile, methyl methacrylate or vinyl acetate graft copolymers form [357]:
The free radicals that are unattached to the backbone polymers, like the methyl radical shown above, also initiate polymerizations and considerable amounts of homopolymers form as well. In some instances, graft copolymers form as a result of chain transferring that takes place after photodecomposition of the photo labile materials. An example is formation of graft copolymers of polyacrylamide on natural rubber, poly(vinyl pyrrolidone), or dextrin with the aid of benzophenone and ultraviolet light [358]. The free radicals from photodecomposition of benzophenone react with the polymers by chain transferring. The growth of acrylamide is subsequently initiated from the polymer backbones. Photo tendering dyes can be used in this manner with cellulose [359]. Thus, anthraquinone dyes can be adsorbed to cellulose. Upon irradiation, proton abstractions take place, creating initiating radicals on the backbone polysaccharide:
It is believed that the above dye mono radicals disproportionate to hydroquinones and quinones. Transfer reactions to solvent lead to formations of homopolymers. This gives high yields of graft copolymers of methyl methacrylate with cellulose. The same is true of acrylonitrile [360]. On the other hand, only small quantities of graft copolymers form with styrene or vinyl acetate monomers [360]. It is also possible to form graft copolymers on the surface of fibers by coating them with photoinitiators, like benzophenone together with a monomer and then irradiating them with ultravio let light [414]. Similar to the action of the anthraquinone dyes shown above, benzophenone in the excited triplet state mainly abstracts hydrogens and forms radicals on the surface [415].
