Rubber-modified styrene-based copolymer

A rubber-modified styrene-based copolymeric resin comprising a matrix phase consisting of a copolymeric resin of an aromatic monovinyl monomer and a vinyl cyanide monomer and a dispersed phase consisting of particles of a rubbery polymer dispersed in the matrix phase, in which:

• (a) the weight ratio of the moiety of the aromatic monovinyl monomer to the moiety of the vinyl cyanide monomer in the copolymeric resin of the matrix phase is in the range from 60:40 to 95:5;
• (b) the rubbery polymer is an S-B type block-copolymeric rubber of styrene and butadiene, which rubbery polymer is free of graft copolymers, containing from 25 to 40% by weight of the styrene moiety, of which a 5% by weight solution in styrene has a viscosity in the range from 20 to 60 centipoise at 25.degree. C. and the butadiene segments contain from 5 to 25% moles of 1,2-vinyl linkages, and
• (c) the rubbery polymer is dispersed in the matrix of the copolymeric resin of the aromatic monovinyl monomer and the vinyl cyanide monomer in the form of particles having an area-average particle diameter in the range from 0.03 to 0.5 .mu.m in an amount in the range from 3 to 35% by weight based on the rubber-modified styrene-based copolymer, the ratio of the area-average particle diameter to the number-average particle diameter being 2.0 or smaller.