Liquid polymerization products are prepared by aqueous emulsion polymerization of conjugated diolefines with or without a minor proportion of an unsaturated copolymerizable monomer containing an ethylenic double bond in the presence of an emulsifying agent and 3-12 per cent by weight on the monomers of one or more C6 to C16 aliphatic mercaptans. Specified diolefines are butadiene, isoprene, piperylene, dimethyl butadiene and methyl pentadiene. Listed copolymerizable monomers include styrene, a -methyl styrene, a -p-dimethyl styrene, halogenated styrenes, acrylic and methacrylic acid esters and nitriles, ethyl fumarate, methyl vinyl ketone and methyl isopropenyl ketone. Preferred emulsifying agents are soaps of linseed and soybean oil fatty acids. The preferred mercaptan is a tert. octyl mercaptan derived from diisobutylene, others specified are dodecyl mercaptans and tertiary mercaptans derived from the trimer and tetramer of isobutylene. Polymerization promotors may be used, listed are amines, alkali ferricyanides and inorganic cyanides. The polymeric oil produced is soluble in benzene and may be hydrogenated or reacted with halogens, halogen acids, maleic anhydride, stannic chloride, sulphur, sulphur chloride and phosphorous pentasulphide. It may be used to plasticise natural and synthetic rubbers, e.g. butadiene-acrylonitrile and butadiene-styrene copolymers. Polyisobutylenes may be softened by the hydrogenated polymer. Drying oils and paints may be obtained by addition of cobalt and manganese naphthenates.