Vinyl halide polymer impact modifiers

A three-stage, sequentially produced graft polymer comprises (A) a non-rubbery, hard first stage polymer formed by the sequential polymerization of a monomer charge of 50 to 100 weight percent of a vinylaromatic compound, 0 to 50 weight percent of a different monovinylidene monomer interpolymerizable therewith, and 0 to 10 weight percent of a polyfunctional crosslinking monomer based on the weight of the monomer charge; (B) a second stage rubbery polymer formed by sequentially polymerizing in the presence of the hard polymer Stage (A) a second monomer charge of 50 to 100 weight percent of butadiene, isoprene, chloroprene, an alkyl acrylate or mixtures thereof wherein the alkyl group of the alkyl acrylate has about 3 to 8 carbon atoms, 0 to 50 weight percent of a monovinylidene monomer interpolymerizable therewith, and 0 to 10 weight percent of a polyfunctional crosslinking agent; and (C) a third stage polymer formed by sequentially polymerizing in the presence of the Stage (A) and Stage (B) polymer product a monomer charge of 50 to 100 weight percent of an alkyl methacrylate wherein the alkyl group has about 1 to 4 carbon atoms, 0 to 50 weight percent of a vinylidene monomer interpolymerizable therewith, and 0 to 10 weight percent of a polyfunctonal crosslinking monomer, and/or 0 to 1.0 weight percent of an alkyl mercaptan of 3 to 12 carbon atoms. Preferably the polymer is used as a modifier for vinyl halide polymers. When so used, the refractive index of the first two stages of the graft polymer is substantially equal to the refractive index of the vinyl halide polymer.