Provided are processes for making a polymer composition comprising (i) providing a multistage polymer latex by emulsion polymerization in the presence of a phosphate surfactant, wherein the multistage polymer comprises (a) a first stage polymer having a Tg of −20° C. or lower, and (b) a final stage polymer having a Tg of 50° C. or higher, (ii) coagulating the multistage polymer latex, (iii) treating the coagulated multistage polymer with an alkaline phosphate, (iv) drying the coagulated multistage polymer to a water content of less than 1 weight % based on the dry weight of the multistage polymer, wherein the weight of the dried multistage polymer comprises 100 ppm or more, based on the dry weight of the multistage polymer, of phosphorous that is in the form of an alkaline phosphate. Also provided are processes for making matrix resin compositions comprising such polymer compositions and a matrix resin.