Copolymers of from 65 to 75%, by weight, of .alpha.-methyl styrene and from 35 to 25%, by weight, of .alpha.-methyl styrene and from 35 to 25%, by weight, of acrylonitrile, obtainable by radical emulsion polymerization, the monomers being metered in during the course of the polymerization, wherein the molar ratio: ##EQU1## of the unreacted monomers in the reaction mixture is maintained during the metering phase between 0.7 and 1.25.