On the limitations of cationic polymerization of vinyl monomers in aqueous dispersed media

In the cationic polymerization of vinyl monomers water is generally avoided due to the reactivity of carbocations towards water as well as the high water sensitivity of the most commonly used Lewis acid catalysts. However, the development of various water tolerant Lewis acid catalysts has led to a number of reports of polymers synthesized by cationic polymerization directly in aqueous dispersed media. In this work we evaluate the fundamental limitations of cationic polymerizations conducted in water, highlighting the effects of inherent chain transfer reactions, whose importance is heightened at elevated temperatures, as well as the extent of chain transfer to water. The results suggest that cationic polymerizations conducted in dispersed media proceed under pseudo-bulk conditions and thus the molar mass of the polymer is dependent on the saturated water content of the organic phase, as well as the rate coefficient of chain transfer to water. For some monomers, such as styrene, it is shown that the effect of transfer to water is negligible and molar mass is largely determined by transfer to monomer reactions. However, for other monomers, such as β-pinene, attempts to polymerize in aqueous dispersed media result in almost exclusive formation of oligomers. While this work highlights that cationic polymerizations conducted in water are necessarily limited to fairly low molecular weights, the results show that in many cases polymers produced by cationic polymerization in dispersed media are within the range required in commercial applications.