Copolymers derived from 7-acryloyloxy-4-methylcoumarin and acrylates: 2. Reversible photocrosslinking and photocleavage

Abstract

Photosensitive copolyacrylates with pendant 4-methylcoumarin chromophores were derived from 7-acryloyloxy-4-methylcoumarin and various acrylates (methyl methacrylate: MMA, methyl acrylate: MA, ethyl acrylate: EA, butyl acrylate: BA). The pendant 4-methylcoumarins dimerize under irradiation with 300nm or 350 nm light (photocrosslinking), and the dimers split into original 4-methylcoumarins under 254 nm light (photocleavage). The reversible photocrosslinking (300 nm or 350 nm) and photocleaving (254 nm) reactions of these copolymers were investigated in the film state by tracing their ultraviolet (u.v.) absorption at 310 nm (maximum absorption of 4-methylcoumarin chromophore). These photochemical reactions are mainly affected by the copolymer composition, wavelength of irradiation light, and photosensitizer. For copolyacrylates with glass transition temperature (T_g) higher than room temperature, photocrosslinking rate is mainly determined by the content of 4-methylcoumarin chromophore. However, for those with T_g near or lower than room temperature, the rate is mainly influenced by the chain segmental mobility. The irradiation of 300 nm of u.v. is more efficient than 350 nm because the maximum absorption of 4-methylcoumarin chromophore is about 310 nm. The rate of photocrosslinking is increased in the presence of the triplet photosensitizer benzophenone.