Process for the preparation of copolymers containing low monomer content

Claim 1

In a process for the preparation of a copolymer by the polymerization of about 25 to 85 parts by weight of a monomer selected from group A consisting of styrene, α-methylstyrene and a combination thereof, and about 15 to 75 parts by weight of a monomer selected from group B consisting of acrylonitrile, methacrylonitrile and a combination thereof, in an aqueous dispersion of said monomers in the presence of a first at least one compound yielding free radicals, thereby forming an aqueous reaction medium containing said copolymer as well as remaining non-converted monomer, the improvement comprising carrying out said polymerization in said dispersion in the additional presence of a synthetic rubber-like polymer and adding to said reaction medium while at a temperature of between 30° and 120° C., a second at least one compound yielding water-soluble free radicals, having a half-life of less than five hours and in a quantity of between about 0.05 to 2.5 parts per 100 parts by weight of monomer and copolymer in said reaction medium, said addition taking place at a point of time at which (a) the rate of conversion of monomer to said copolymer is less than 15% per hour, (b) the degree of conversion of monomer to said copolymer is greater than 75%, and (c) the amount of non-converted monomer from said group B remaining in said reaction medium is at least 0.1% by weight relative to the total weight of said reaction medium, and whereafter no additional monomer is added to said reaction medium.