The effects of radicals on silica surface, which were formed by gamma-ray irradiation, on the polymerization of vinyl monomers were investigated. It was found that the polymerization of styrene was remarkably retarded in the presence of gamma-ray-irradiated silica above 60 degrees C, at which thermal polymerization of styrene is readily initiated. During the polymerization, a part of polystyrene formed was grafted onto the silica surface but percentage of grafting was very small. On the other hand, no retardation of the polymerization of styrene was observed in the presence of gamma-ray-irradiated silica below 50 degrees C the polymerization tends to accelerate and polystyrene was grafted onto the silica surface. Poly(vinyl acetate) and poly(methyl methacrylate) (MMA) were also grafted onto the surface during the polymerization in the presence of gamma-ray-irradiated silica. The grafting of polymers onto the silica surface was confirmed by thermal decomposition GC-MS. It was considered that at lower temperature, the grafting based on the propagation of polystyrene from surface radical ("grafting from" mechanism) preferentially proceeded. On the contrary, at higher temperature, the coupling reaction of propagating polymer radicals with surface radicals ("grafting onto" mechanism) proceeded to give relatively higher molecular weight polymer-grafted silica. (c) 2006 Wiley Periodicals, Inc.