Photoreactive copolymers containing 4-methylcoumarin pendant groups were prepared by radical copolymerization of 7-acryloyloxy-4-methylcoumarin (M(1)) with acrylates [M(2) = methyl methacrylate (MMA), methyl acrylate (MA), ethyl acrylate (EA) and butyl acrylate (BA)] in dimethyl formamide (DMF) at 65 degrees C using azobisisobutyronitrile as initiator. The monomer reactivity ratios (MRRs) of M(1) and M(2) were estimated by the Fineman-Ross and Kelen-Tudos methods, by analysing copolymer compositions which were determined by H-1 nuclear magnetic resonance (n.m.r.) spectra. It was found that the MRRs are r(1) = 0.45, r(2) = 1.68 for M(1)-MMA; r(1) = 2.16, r(2) = 0.48 for M(1)-MA; r(1) = 2.04, r(2) = 0.42 for M(1)-EA; r(1) = 1.82, r(2) = 0.53 for M(1)-BA, respectively. The order of monomer reactivity estimated from 1/r(1) values is MMA > M(1) > BA > EA > MA. The higher reactivity of MMA can be attributed to its extra hyperconjugating stabilization by methyl groups in addition to C = O conjugation. Furthermore, from C-13 n.m.r. spectra, it was found that pi-electron density of the beta-carbon is in the order of MA > EA > BA > M(1). The lower electron density of beta-carbons in a monomer leads to easier electron transfer from propagating radical to the monomer and consequently a higher reactivity. Photocrosslinking (300 nm) of the copolymers in the film state were investigated by their characteristic curves (W/W-0 versus time plot).