The copolymers containing photoreactive 4-methyl coumarin pendant groups have been prepared by radical copolymerization of N-(1-phenylethyl) acrylamide (Ml) and 7-acryloyloxy-4-methyl coumarin (M2) in DMF at 70-degrees-C. The monomer reactivity ratios (MRR) of M1 and M2 are estimated by Finenam-Ross, Kelen-Tudos, Mayo-Lewis, and YBR methods to be about r1 = 0.12-0.16 and r2 = 1.13-1.28. Dilute dioxane solutions of the copolymers (10(-4)-10(-5) M of 4-methyl coumarin chromophores) are first photocrosslinked intramolecularly by irradiating 350 nm light (dimerization of 4-methyl coumarin) and then photocleaved at 254 nm (symmetric cleavage of cyclobutane). Tracing by an UV-visible spectrophotometer, it is found that photocrosslinking and photocleavage proceed simultaneously when 254 nm is the light source and finally reaches to a dynamic equilibrium. However, only photocrosslinking can be observed under 350 nm light. Kinetic results show that both photocrosslinking and photocleavage are first-order reactions. Moreover, photocrosslinking rate constant increases with mole fraction Of M2 in the copolymer (F2). On the contrary, rate constant of photocleavage decreases with increasing F2.