Graft polymer dispersions are prepared by the in situ polymerization in the presence of a free radical catalyst of an ethylenically unsaturated monomer or mixture of monomers in a liquid polyol employing as the free radical catalyst a liquid unsymmetrically-substituted azo compound having a melting point less than 30.degree. C. and a ten hour half-life between 55.degree. C. and 100.degree. C. The dispersions are low viscous liquids which may be advantageously employed in the preparation of polyurethane foams.