Emulsion polymerization of dimethylcyclosiloxane in cationic emulsion: mechanism study utilizing two phase liquid–liquid reaction kinetics

Abstract

Abstract Dimethylcyclosiloxane emulsion polymerization (EP) was studied using heterogeneous two phase liquid–liquid kinetic theory combined with particle size measurements during reaction. A typical octamethylcyclotetrasiloxane EP was found to have a reaction half-life of about 1400 sec; much greater than the time (less than about 10 sec) for the cyclosiloxane to diffuse through the droplet surface into the aqueous phase. Previously published research proposing a mechanism where the cyclosiloxane EP begins by opening the cyclosiloxane ring at the droplet surface appears improbable because a reaction half-life less than a few seconds would be required. The aqueous solubility of D4, D5 and D6 was determined as a function of temperature. Most significantly; the rate of reaction of these cyclosiloxanes in EP was then determined to be a linear function of their aqueous solubility at the reaction temperature. Dimethylcyclosiloxane EP was determined to fully conform to the four characteristics of two phase heterogeneous liquid–liquid reaction kinetics wherein the reaction occurs in the aqueous phase. The kinetic rate equation was developed and the effect of cyclosiloxane mass transfer limitations on reaction rate was shown. (The influence of mass transfer limitations can give an erroneous appearance that reaction occurs at the droplet surface.) Evidence is presented that polymerization of cyclosiloxane in micelles is not favorable. The mechanism of cyclosiloxane emulsion polymerization was concluded to be coagulative homogeneous nucleation in the aqueous phase.