A stable copolymer dispersion is prepared by addition copolymerization of (1) a monomeric adduct of an active hydrogen compound, e.g., a polyether polyol or polyether monool, and an ethylenically unsaturated N-chlorosulfonyl carbamyl compound such as acryloxyethylchlorosulfonylcarbamate or an unsaturated N-chlorocarbonyl carbamyl compound with (2) ethylenically unsaturated monomer(s), e.g., styrene or a mixture thereof with acrylonitrile, while (1) and (2) are dispersed or dissolved in an excess of an active hydrogen polyfunctional compound such as a polyether polyol. The molar ratio of carbamyl moiety to active hydrogen compound is preferably less than about 0.1:1. The resulting dispersion has a controlled particle size and a viscosity such that it is suitably employed in the production of polyurethane foams.