The polymerization kinetics of HEMA-grafted silica nanoparticles and HEMA monomer mixtures was studied under thermal and UV cures. These two kinds of cure, which correspond to very different initiation rates, lead to opposite effects on the polymerization kinetics as methacrylate-grafted silica nanoparticles are introduced. When the polymerization was done under UV, i.e., in the case of a high initiation rate, the introduction of grafted silica nanoparticles increases the polymerization rate. On the contrary, when the polymerization rate is thermally activated, an opposite effect is observed after the introduction of the grafted silica nanoparticles. Two phenomena having opposite effects on the polymerization kinetics are involved: i) the high functionality of silica nanoparticles leads to an increase of the polymerization rate of the reactive system; ii) the methacrylate groups at the silica surface have a lower reactivity, due to their reduced mobility, and are responsible for the formation of shielded radicals. The large difference in the initiation rates between the two types of polymerization gives evidence of these opposite effects: a high radical concentration leads to a high UV polymerization rate, whereas for the thermal polymerization, shielded radicals still exist but cannot completely mask the presence of the effect of high-functionality species, i.e., grafted silica. (C) 2004 Society of Plastics Engineers.