Toughened mercaptosiloxane modified emulsion polymerized vinylic polymers

A thermoplastic vinylic polymer having improved impact strength made by the method which comprises polymerizing (1) a vinylic monomer selected from the group consisting of styrenic monomers, methyl methacrylate, mixtures of major amounts of styrenic monomers and minor amounts of monomers selected from the group consisting of acrylic monomers, alpha-methylstyrene, vinyl halide monomers, vinyl acetate, butadiene monomers and acrylonitrile monomers and mixtures of major amounts of methyl methacrylate and minor amounts of monomers selected from the group consisting of other acrylic monomers, styrenic monomers, butadiene monomers, vinyl halide monomers, vinyl acetate, alpha-methylstyrene, and acrylonitrile monomers, in an aqueous emulsion of (2) a copolymer of dimethylsiloxane, from 0.3 to 10 mol percent mercaptosiloxane of the unit formula HSR'SiR.sub.n O.sub.3-n/2 in which R' is a divalent or trivalent saturated aliphatic hydrocarbon radical of 1 to 18 carbon atoms, two valences of R' being attached to the same silicon atom when R' is trivalent, R is a monovalent hydrocarbon radical free of aliphatic unsaturation of 1 to 6 carbon atoms and n is 0 to 2 and sufficient R"SiO.sub.3/2 or SiO.sub.2 units to give a percent volume swell of the copolymer of 500 to 1,600 percent in benzene, R" being of the group HSR'-- and R groups, in the presence of a free radical generator, whereby a thermoplastic matrix having dispersed therein particles of (2) is obtained, the proportions of (1) and (2) being from 1 to 40 percent by weight (2) based on the combined weights of (1) and (2).