A reaction product is obtained by free-radical emulsion polymerization in an aqueous medium of ethylenically unsaturated monomers including at least one conjugated diene. The free-radical emulsion polymerization is conducted in the presence of a degraded polysaccharide having a DE of 38 to 70 measured according to ISO 5377 (1981 Feb. 15) and optionally an alkyl mercaptan in an amount of no more than 2.5 weight percent based on the total weight of ethylenically unsaturated monomers. A coating composition includes the reaction product, and a paper or a board can be coated with such a coating composition.