The free radical copolymerization of MMA and CR39 was investigated using Fourier transform infra-red spectroscopy, electron spin resonance (e.s.r.), nuclear magnetic resonance and gel permeation chromatography. The data obtained from these techniques were used to evaluate the rate constants, the reactivity ratios and chain transfer to the CR39 allyl groups. The copolymerization reaction proceeds in two major steps or regions, first the predominant polymerization of MMA with only a few CR39 units entering the polymer molecules. This is followed by predominate CR39 polymerization and crosslinking, with the remaining few MMA molecules entering the polymer molecule. In the first region, the propagation rate constant of MMA was found to be unaffected by the presence of CR39, while the termination rate constant decreased with increasing CR39 content. Very little chain transfer from the propagating MMA radical to the CR39 molecule was found during this stage. The presence of CR39 also results in the propagating MMA radical being found to be predominantly in the 13-line e.s.r. confirmation up to a much higher conversion that in the case of homopolymerization of MMA.