Anionic emulsion polymerization of siloxanes

Abstract

AbstractIn 1959, Hyde and Wehrly described an emulsion polymerization of permethylcyclosiloxanes. This unique example of an emulsion polymerization proceeding via an ionic mechanism involved base catalysis with a cationic surfactant. An analogous acid‐catalyzed anionic emulsion polymerization has now been developed which employs dodecylbenzene‐sulfonic acid as the catalyst and surfactant. Polydimethylsiloxane emulsions with particle diameters of 500–5000 Å are obtained on heating aqueous dispersions of permethylcyclosiloxanes with dodecylben‐zenesulfonic acid at 50–100°C. Polymerization is completely arrested by neutralization of the catalytic surfactant. As in the earlier cationic emulsion polymerization, the molecular weight of the polymer is easily controlled by the temperature of polymerization via the equilibrium between siloxane, water, and silanol. Polymers with molecular weights of 49,000 and 172,000 are obtained at 90°C and 25°C, respectively. The use of alkyltrimethoxysilanes in this anionic emulsion polymerization makes available a new system of colloidal silsesquioxanes with extremely small and uniform particle size. Colloidal methylsilsesquioxanes with particle diameters of 70–1000 Å are obtained by the addition of methyltrimethoxysilane to an aqueous dodecylbenzenesulfonic acid solution. Methylsilanetriol and methylsiloxanols were shown to be intermediates in this latter reaction.