Provided are polymer compositions comprising (i) one or more multistage polymers comprising (a) a first stage polymer, wherein the first stage polymer comprises polymerized units derived from at least one organo-phosphorus monomers, and (b) a final stage polymer, and (ii) at least one organo-phosphorus soap in an amount of 50 ppm or more, based on the dry weight of the multistage polymer. Also provided are matrix resin compositions comprising such polymer compositions and a matrix resin.