Heat-resistant copolymer of alpha-methylstyrene and acrylonitrile, process for preparing the same, and thermoplastic resin composition containing the same

A heat-resistant copolymer which contains 74-82% by weight of (A) alpha-methylstyrene and 18-26% by weight of (B) acrylonitrile and which contains (a) 0-10% by weight of the monomer chain:

• --(A)-- (A)--(A)--, (b) at least 50% by weight of the monomer chain:
• --(A)--(A)--(B)--, and (c) no more than 50% by weight of the monomer chain:
• --(B)--(A)--(B)-- wherein the total amount of chains (a), (b) and (c) is 100% by weight, said copolymer being prepared by a process, comprising:
• feeding (A) 25-82% by weight of alpha-methylstyrene and (B) 3-12% by weight of acrylonitrile in a (A) to (B) weight ratio of less than 9 into a reactor;
• emulsifying the charged monomer;
• initiating the polymerization of the monomers at a temperature of 67.degree.-90.degree. C.;
• continuing the polymerization reaction at said temperature and simultaneously, continuously or intermittently feeding acrylonitrile or a mixture of alpha-methylstyrene and acrylonitrile into the polymerization medium after the polymerization reaction has progressed for at least 1 hour so that the weight ratio of alpha-methylstyrene to acrylonitrile in the unreacted monomers within the system is maintained at at least 7, while continuing the polymerization at the said temperature until the amounts of acrylonitrile and alpha-methylstyrene reach their predetermined levels within the above stated ranges.