Abstract
The emulsion polymerization of vinylidene chloride is divisible into three stages: (I) a normal emulsion polymerization at a rate dependent on the 0.6 power of both the catalyst and surfactant concentration; (II) at a rate less than half that of I; (III) at a rate which increases in direct proportion to the catalyst concentration and independently of surfactant concentration. A careful examination of stirring rate has shown that the slowdown in stage II probably results from monomer starvation caused by slow diffusion of coalesced droplets, and that recovery in stage III is the result of transfer of monomer at the time of collision between a polymer particle and a monomer droplet. Extension of this work to copolymers of vinylidene chloride and acrylonitrile has shown a similar kinetic effect at high ratios of vinylidene chloride. The rate of polymerization for a system containing 90% vinylidene chloride decreases sharply after 15% conversion. The polymer formed increases in acrylonitrile content with conversion, reaches a maximum, and then decreases during stage III of the reaction. A run containing 70% vinylidene chloride in the charge did not show a slowdown but gave evidence of a maximum in the curve relating conversion and composition. At higher acrylonitrile content, no slowing occurred, and the polymer composition changed monotonically with conversion. Since acrylonitrile is partially soluble in water, the partition coefficient of acrylonitrile between vinylidene chloride and water was determined at concentrations similar to those used in the polymerizations. The partition coefficient decreases with increasing acrylonitrile concentration in the water phase. Apparent reactivity ratios were calculated on the basis of the monomer composition corrected for the water solubility of acrylonitrile and the initial composition of the copolymer. These values are r~1~ = 0.390 and r~2~ = 0.585, vinylidene chloride being considered to be the first monomer. The existence of a unique reactivity ratio indicates that the composition of the initial polymer is controlled only by the relative reactivities of the comonomers and the water solubility of acrylonitrile. The polymer formed later is also influenced by the rate of transport of vinylidene chloride to the reaction site. This is confirmed by the conversion curves and the manner in which composition changes with conversion.