SynopsisSeveral kinds of copolymers bearing 0-acyloxyimino groups were prepared and their photodegradabilities were examined. Copolymers of 0-methacryloyl ketone (acetophenone, benzophenone, or 2-acetonaphthone) oxime and methyl methacrylate (or styrene) were photodegradable even in the solid phase but in the case of copolymers of 0-acryloyl acetophenone oxime and methyl methacrylate (or styrene) both photodegradation and photocrosslinking were observed. From these results it was found that the photodegradability of the copolymers was not dependent on the comonomer (methyl methacrylate or styrene) but dependent on the structure of the monomer bearing 0-acyloxyimino groups. From detailed analysis of photodegradation of the copolymer of 0-methacyloyl acetophenone oxime and methyl methacrylate in the presence of benzophenone it was found that the benzophenone worked not only as a sensitizer for the decomposition of 0-acyloxyimino groups but also as a plasticizer to assist segment motions of polymer chains and the diffusion of oxygen in the film.