Abstract: A process for producing a copolymer latex for use as a binder in a paper coating composition by emulsion polymerization while monomers and a molecular weight modifier are continuously introduced into a reactor, characterized in that(a) the molecular weight modifier is used in an amount of X.sub.o parts by weight (in which O<X.sub.o .ltoreq.10) per 100 parts by weight of a monomeric mixture composed of (1) 20 to 50% by weight of an aliphatic conjugated diene monomer, (2) 15 to 70% by weight of a vinyl aromatic monomer, (3) 5 to 40% by weight of an alkyl ester of an ethlenically unsaturated carboxylic acid, (4) 0.

Abstract: A copolymer latex is produced by the emulsion polymerization of 20 to 50% by weight of a conjugated diolefin, 15 to 50% by weight of an aromatic vinyl compound, 0 to 25% by weight of a vinyl cyanide, 5 to 40% by weight of an alkyl ester of an unsaturated carboxylic acid, and 0.5 to 10% by weight of an ethylenically unsaturated acid. First, a monomeric mixture composed of the entire aromatic vinyl compound, the entire vinyl cyanide, at least a part of the conjugated diolefin and at least a part of the ethylenically unsaturated acid, and optionally at least a part of the alkyl ester of an unsaturated carboxylic acid, the amount of the monomeric mixture being 55 to 95% by weight based on the weight of all the monomers used in the process, is polymerized in emulsion. Then, the remainder of the monomers which amounts to 5 to 45% by weight of the weight of all the monomers is added to the above polymerization system and the mixture is polymerized in emulsion.