Photoinitiated free-radical crosslinking copolymerization of methyl methacrylate and ethylene glycol dimethacrylate has been investigated in toluene at a monomer concentration of 35w/v%. The photoinitiators used were benzoin, benzoin methyl ether, benzoin ethyl ether and 2,2-dimethoxy-2-phenyl-acetophenone. Conversion of monomer and pendant vinyl groups and the size of the pregel polymers were measured as a function of the reaction time up to the onset of macrogelation. Approximately 30% of the pendant vinyl groups were found to be consumed by cyclization reactions. The fraction of units in cycles is independent of the EGDM concentration. As a result of the cyclization reactions, the accessibility of the radical centres and pendant vinyl groups for other polymer molecules is strongly reduced. Consequently, both termination and crosslinking reactions are controlled by the segmental diffusion of the polymer radicals. Calculations indicate a 1-2 orders of magnitude decrease in the average reactivity of pendant vinyls for intermolecular reactions compared to the monomeric vinyls. This drastic decrease in pendant reactivity is mainly responsible for the delay in the gel point. Contrary to gelation theories, the size distribution curves of the pregel polymers change from monomodal to bimodal distributions as polymerization proceeds. This finding confirms the coagulation-type gelation mechanism of compact primary particles and indicates the shape polydispersity of the pregel polymers. (C) 1997 Elsevier Science Ltd.