Process for producing a vinyl terpolymer using reactors arranged in series

A method for the production of a thermoplastic copolymer comprising the following components:

• an aromatic vinyl compound unit in an amount of 30-70% by weight,
• an acrylonitrile or methacrylonitrile unit in an amount of 4-40% by weight, and
• an N-substituted maleimide unit in an amount of 26-50% by weight of the thermoplastic copolymer;
• wherein the content of each of the components in each molecular weight fraction of the thermoplastic copolymer is 80-120% of the content of the components in said thermoplastic copolymer, said thermoplastic copolymer having substantially no acetone-insoluble content, having a weight average molecular weight of not less than 100,000 and not more than 300,000 and a number average molecular weight of not less than 50,000 and not more than 150,000, and having a residual N-substituted maleimide monomer content of not more than 50 ppm in the copolymer, which comprises copolymerizing a monomer mixture comprising 30 to 90% by weight of an aromatic vinyl compound, 4 to 40% by weight of acrylonitrile or methacrylonitrile, and 5 to 50% by weight of an N-substituted maleimide by continuous solution polymerization in a complete mixing type reaction vessel and a laminar flow reaction vessel arranged in series, wherein not less than 30% by weight of the monomer mixture is copolymerized in the complete mixing type reaction vessel and not less than 50% by weight of the monomer mixture is copolymerized in the laminar flow reaction vessel.