Cationic sizing agent for paper, and a process for its preparation

New, highly effective cationic sizing agents for paper have been provided. They can be obtained by a method in which a water-soluble cationic chemically pure terpolymer compound consisting of (a) 7-40% by weight of N,N-dimethylaminoethyl acrylate and/or methacrylate, (b) 40-80% by weight of styrene and (c) 4-40% by weight of acrylonitrile is dissolved in an aqueous medium, the sum of the components (a) to (c) always being 100% by weight and at least 10% of the N,N-dimethylamino groups of the terpolymer being quaternized and the remainder being protonated, and, in the presence of 10 to 70% by weight, relative to the monomer mixture below, of this emulsifier, (d) 0 to 90% by weight of acrylonitrile and/or methacrylonitrile, (e) 5 to 95% by weight of styrene and (f) 5 to 95% by weight of acrylates and/or methacrylates having 1 to 12 C atoms in the alcohol radical, the sum of the components (d) to (f) always being 100% by weight, are emulsified, and the emulsion thus obtained is subjected to free radical-initiated emulsion polymerization at temperatures from 20 DEG to 150 DEG C.