Effects of Surface Groups on SiO2 Nanoparticles on in Situ Solution Polymerization: Kinetics and Mechanism

In situ free-radical polymerization in the presence of inorganic nanoparticles (NPs) continues to attract intense research interest for the preparation of nanocomposites. In this study, in situ solution polymerization of methyl methacrylate (MMA) in the presence of pristine or surface-modified SiO2 NPs has been successfully carried out. The effects of SiO2 NPs on the initiator decomposition and polymerization rates, as well as the final polymerization conversions, have been systematically investigated. Specifically, the influences of silicon hydroxyl (SiOH), double bonds, and an in situ generated graft layer on the particle surface on initiator decomposition and the in situ polymerization process have been studied in detail. On this basis, a mechanistic rationale for the influence of surface groups on SiO2 NPs on the in situ solution polymerization is proposed. During the in situ polymerization process, the SiOH on the surface of the SiO2 NPs not only accelerates the decomposition of benzoyl peroxide and increases its ineffective consumption, but also enhances the wastage of radicals through chain-transfer reaction. However, when the SiO2 surface bears reactive double bonds, the in situ generated grafted polymer layer is effective in preventing the initiator and free radicals from contacting the SiOH to any considerable extent, thereby enhancing the polymerization rate and the final monomer conversion. The results of this study may also lay down scientific and theoretical foundations for developing more organic–inorganic hybrid nanocomposites through in situ free radical polymerization.