Study of the rate of heterogeneous polymerization of methyl acrylate in aqueous solution

Abstract

Study of the heterogeneous polymerization of methyl acrylate in dilute aqueous solution in a redox-initiated system consisting in Na₂S₂O₃=K₂S₂0₈ reveals the fact that the initial rate of polymerization increases with the increase in concentration of either of the redox pair. The rate, however, suffers a rapid deceleration in the course of polymerization in the region of high concentrations of the catalysts. At a low concentration of the initiators, addition of a small quantity of neutral electrolyte increases the rate, which falls off rapidly on further increase of the electrolyte concentration. Addition of emulsifiers in a concentration less than their critical micelle concentration inwater remarkably increases both the rate and the molecular weight. The rate of polymerization depends more or less linearly on monomer concentration when the concentration of the initiators is low in the medium. A monomer dependence tending to zero order is found when the catalyst concentration is high. Water-soluble alcohols and glycols lower the rate when the insoluble phase in the heterogeneous polymerization is a stable colloid. Solvents such as dimethylformamide and benzene, which have strong affinity for the polymers produced in the medium, also reduce the rate. The peculiarities in the rate of this heterogeneous polymerization seem to be connected with the physical state of the insoluble polymer phase. The latter depends mainly on the ionic strength of the aqueous medium. At a low concentration of electrolytes, which may be either a neutral salt or water soluble electrolytic initiators such as K₂S₂0₈/Na₂S₂0₄, the insoluble phase is a stable colloid which changes via an unstable latex to a coarse precipitate on progressive increase of the electrolyte concentration in the medium. It appears that a kinetics similar to that of an emulsion polymerization operates when the insoluble phase is a stable colloid, suspension polymerization kinetics hold good when the insoluble phase is an unstable latex, and the kinetics of a typical "explosive" polymerization is the mechanism when the insoluble phase precipitates.