AbstractBranched copolymers of methyl methacrylate with ethylene dimethacrylate or pentaerythritol tetramethacrylate with high weight average molecular weightsMwwere prepared by free radical polymerization. The double bond conversion of these polymers (β) was measured dilatometrically and the gel point (βc) was located by extrapolation of bulk viscosity measurements. The polymers were studied by light‐scattering,1H NMR and viscometry. Hence it was possible to investigate the relationship between the molecular weightMwand the reduced conversion quantity (1 − β/βc)−1and also the molecular weight dependence of the z‐average mean square radius of gyration 〈S2〉z. Considerable deviations from the classical Stockmayer theory but also from percolation theory were observed.1H NMR showed a relative small extent of ring formation (10 to 24%) and a reduced apparent reactivity of the pendant double bonds, while the delay in the gel formation appeared to be caused by a much higher ring formation (80 to 90%). This apparent inconsistency is explained by a size‐dependent ring formation, where the extent of ring formation is increased with the molecular dimensions; simultaneously, the pendant double bonds become progressively less accessible to cross‐linking by growing radicals. The molecular weight dependence of 〈S2〉zis found to be in agreement with this heter