When irradiated at >400 nm in air, the benzil carbonyl groups (BZG) of 1-phenyl-2-(4-propenoylphenyl)-1,2-ethanedione/styrene (PCOCO/S) copolymer films are transformed almost quantitatively to benzoyl peroxide carbonyl groups (BPG). Subsequent additional irradiation (at 366 nm) or heating (at 91 degreesC) of the BPG-containing copolymer films generates ester moieties, and significantly more cross-linking than main-chain cleavage is manifested in the copolymer. At 91 degreesC, the rate of the thermal decomposition of the pendant BPG is 3 times slower than that of noncovalently attached benzoyl peroxide molecules in polystyrene films.