Two types of initiators, water-soluble (potassium persulphate, KPS) and oil-soluble (2,2'-azobisisobutylonitrile, AIBN) initiators, were applied to initiate the copolymerization of gamma-methacryloxypropyltrimethoxysilane (MPS) and styrene (St) in miniemulsion mode. The influences of the nature of initiators on the kinetics including the hydrolysis of methoxysilyl groups and free radical copolymerization, the stability of latex particles and the microstructure of copolymer were investigated. The overall conversion was measured by gravimetry; the methanol content and individual monomer conversion during the process of reaction were followed by gas chromatogram; the size of mini-droplets and latex particles were measured by DLS; the microstructure of copolymer was characterized by FTIR and Si-29 solid state NMR, respectively. The interfaces of mini-droplets or latex particles were the main loci of hydrolysis reactions attested by the independence of hydrolysis rate of MPS on the nature of initiator which could determine the distribution of radicals between aqueous and oil phases. The hydrolysis rate increased with the increase of the MPS/St ratio because of the increase of the amount of MPS at the interface, but the final hydrolysis degree was independent on the MPS/St ratio for both systems initiated by AIBN and KPS as a result of the simultaneous increase of the amount of MPS at the interfaces and in the interior of droplets or latex particles. Compared to the system initiated by KPS, the higher final hydrolysis degree appeared in the system initiated by AIBN could be attributed to the lower reaction rate and longer reaction time. The kinetic behaviors of copolymerization of MPS and St and homopolymerization of St initiated by KPS showed in the completely different way, but in the similar way for the system initiated by the AIBN. This phenomenon could be ascribed to the different water solubility of monomers and the different origin of radicals of KPS and AIBN. The introduction of MPS accelerated the copolymerization rate especially in the initial stage due to its higher propagation constant and reactivity ratio than that of St. Besides the participation in the free radical copolymerization, MPS molecules underwent the hydrolysis and then condensation reactions concurrently. Consequently, the formation of the hydrogen bonding among the resultants of hydrolysis, Si-OH, increased the rigidity of the interface and retarded the polymerization rate. In contrast to the system initiated by KPS, the latex particles of system initiated by AIBN were more stable, and In the system with 1 :4 ratio of MPS to St initiated by AIBN, the ratio of N-P and N-d was 0.97, but was only 0.55 in the system with the same ratio initiated by KPS. The results of FTIR and solid-state Si-29-NMR showed that un-condensed silicone species dominated in the final product in the system with I : I ratio of MPS to St initiated by KPS, and MPS molecules were incorporated in the copolymer via free radical (co) polymerization. We reported that a small part of Si-OH condensed in the system initiated by AIBN before. This could be attributed to the fast incorporation of Si-OH into the polymer chains which confined the mobility of Si-OH and reduced the possibility of condensation reaction as a result of the fast copolymerization rate in the system initiated by KPS.
