Nanosilica particles (SiO2) were modified with 3-(trimethoxysilyl)propyl methacrylate (MPS) and octyltrimethoxysilane (OTMS)

respectively. The density of anchored silanes could be conveniently adjusted through addition of various amounts of silane. The saturated densities for both silanes are about 3 mol (m2 SiO2)-1. The bulk free-radical polymerization of methyl methacrylate (MMA) was carried out in the presence of SiO2 with different surface modifications to prepare SiO2/poly(methyl methacrylate) (SiO2/PMMA) composites. Influences of the type of anchored silanes and the content of SiO2 on the polymerization rate

molecular weight of PMMA

and hardness of the SiO2/PMMA composites were investigated. The polymerization rate is obviously retarded by addition of SiO2 nanoparticles. The order of retardation effect on polymerization is unmodified SiO2 MPS-modified SiO2 OTMS-modified SiO2. Moreover

the retardation effect increases with the increase of the amount of SiO2 nanoparticles. In addition

the molecular weight of PMMA and the hardness of SiO2/PMMA composites are clearly reduced with the addition of SiO2 nanoparticles. We assume that the silanol groups on the SiO2 surface play a role of chain transfer agent to reduce the molecular weight of PMMA and subsequently to reduce the hardness of SiO2/PMMA composites. In addition

the produced SiO may be not as active as the primary or propagating radicals

and therefore retard the polymerization rate. For the MPS-modified SiO2. It is in addition to the chain transfer effect of silanols groups

the grafting of propagating radicals to MPS units will confine the radicals on the surface of SiO2

which may also decrease the activity of radicals and subsequently retard the polymerization rate and decrease the molecular weight of PMMA. These assumptions are confirmed by the fact that the retardation effect on the polymerization rate is alleviated and the molecular weight of PMMA is increased by using OTMS-modified SiO2 to replace unmodified or MPS-modified SiO2. It is because on one hand

the active silanol groups are consumed by OTMS

leading to the suppression of chain transfer reaction to SiO2

and on the other hand

the use of OTMS can avoid the grafting reaction to SiO2 nanoparticles. This study aims to understand how the surface properties of inorganic nanoparticles may influence the free-radical polymerization of acrylate monomer and the corresponding products.