Process for producing thermoplastic resins

A process for producing a thermoplastic terpolymer resin by emulsion polymerization of a monomer mixture consisting of .alpha.-methylstyrene, methyl methacrylate and acrylonitrile, characterized in that the polymerization is carried out in two steps, in the first step of which a monomer mixture consisting of 60-85% by weight of .alpha.-methylstyrene, 2-30% by weight of methyl methacrylate and 5-20% by weight of acrylonitrile is used in an amount of 30-95% by weight based on the amount of the monomers necessary for the final terpolymer and subjected to polymerization until 25-85% by weight of a terpolymer based on the weight of the final terpolymer is formed, and in the second step of which at least one of the above-mentioned three monomers is added to the polymerization system so that the composition ratio between the three monomers present in the polymerization system falls in the region surrounded by the lines (including the points on the lines) connecting point A (.alpha.-methylstyrene 60% by weight, methyl methacrylate 10% by weight, acrylonitrile 30% by weight), point B (.alpha.-methylstyrene 70% by weight, methyl methacrylate 10% by weight, acrylonitrile 20% by weight), point C (.alpha.-methylstyrene 55% by weight, methyl methacrylate 40% by weight, acrylonitrile 5% by weight), point D (.alpha.-methylstyrene 45% by weight, methyl methacrylate 50% by weight, acrylonitrile 5% by weight) and point E (.alpha.-methylstyrene 40% by weight, methyl methacrylate 50% by weight, acrylonitrile 10% by weight) in the triangular coordinate graph indicating the quantitative relationship of the three monomers, provided that said composition ratio differs from that in the first step, and the polymerization system thus obtained is subjected to polymerization conditions to complete the polymerization..