Miniemulsion Copolymerization of Styrene and γ-Methacryloxypropyltrimethoxysilane: Kinetics and Mechanism
Résumé
The kinetics of the miniemulsion copolymerization of styrene (St) and gamma-methacryloxypropyltrimethoxysilane (MPS) was investigated by studying the effects of the MPS/St weight ratio, the nature and amount of initiator, the suspension pH, and the surfactant concentration on the overall and individual monomer conversions. It was found that the introduction of MPS accelerates the free radical copolymerization reaction due to its higher propagation rate constant and higher water solubility compared to styrene. On the other hand, MPS decreases the copolymerization rate mainly during the second half of the reaction. This is presumed to be due to the formation of alkoxysitane-rich copolymers on the particles surface providing a barrier to radical's entry. The rate of hydrolysis is pH-dependent, and for a given pH, the influence of the surfactant concentration on the hydrolysis rate suggests that the interface between the latex particles and water is the main locus of hydrolysis. Si-29 solid-state NMR analysis showed that the hydrolyzed alkoxysilyl moieties did not condense under neutral conditions even for high MPS/St feed ratios. Premature cross-linking could be also minimized under basic conditions but could not be avoided in acidic media.