0.1 to 25 molar equivalents of a trichloroalkene having 3 to 5 carbon atoms per molecule and all the chlorine atoms attached to the same carbon atom are copolymerized with one or more monomeric olefinic compounds containing 1 to 4 ethylenic linkages, but excluding dipropenyl and alkyl and chlorine substituted dipropenyl esters of alpha, beta-unsaturated dicarboxylic acids, by heating in the presence of a peroxidic catalyst. Trichloroalkenes specified are 3,3,3-trichloropropene, 1,1,3-trichloropropene, 4,4,4 - trichloro - 1 - butene, 1,1,1-trichloro - 2 - butene, and 4,4,4 - trichloro - 2-methyl - 1 - butene. Copolymerizable monomers specified are ethylene, propylene, butene-1, isobutylene, styrene, alpha-methyl styrene, para-methyl styrene, divinyl benzene, vinyl naphthalene, para-chlorostyrene, para-fluorostyrene, 2,5-dichlorostyrene, vinyl fluoride, vinyl chloride, vinylidene fluoride, vinylidene chloride, trichloroethylene, allyl chloride, methallyl chloride, 2,3-dichloropropene, 1,3-butadiene, 2-methyl-1,3-butadiene, 1,5-hexadiene, 2-chloro-1,3-butadiene, allyl, methallyl, ethallyl, chloroallyl, crotyl, and cinnamyl alcohols, vinyl ethyl, vinyl butyl, vinyl hexyl, vinyl cyclohexyl, vinyl phenyl, vinyl benzyl, vinyl tolyl, divinyl, methyl isopropenyl, allyl ethyl, methallyl ethyl, methally phenyl, methallyl benzyl, 2-chloroallyl ethyl, allyl 2-naphthyl, diallyl, dimethallyl, and dichloroallyl ethers, betaallyloxyethanol, 1,2-diallyloxyethane, 1,4-diallyloxybenzene, vinyl acetate, chloroacetate, propionate, n-butyrate, and benzoate, divinyl oxalate, divinyl phthalate, isopropenyl acetate, allyl acetate, chloroacetate, glycollate, butyrate, stearate, benzoate, para-chlorobenzoate, and para-methoxy benzoate, methallyl acetate, acrylate, allyl methacrylate, beta-allyloxyethyl acrylate, chloroallyl acrylate, allyl chloroacrylate, allyl ethacrylate, methallyl cinnamate, allyl crotonate, crotyl acrylate, diallyl adipate, triallyl tricarballylate, diallyl, dimethallyl, dichloroallyl, and diallyloxyethyl phthalates, diallyloxyethyl fumarate, dimethallyloxyethyl fumarate, diallyloxyethyl itaconate, triallyl aconitate, diallyl oxalate, diallyl succinate, dichloroallyl adipate, dicrotyl succinate, dimethallyl adipate, diallyloxyethyl adipate, and the polyalkenyl esters of malonic, alpha-methyl malonic, glutaric, pimelic, and azelaic acids, diallyl carbonate, diallyl sulphate, triallyl, trimethallyl, and triallyloxyethyl phosphates, tetraallyl silicate, tetraallyl titanate, tetramethallyl silicate, and tetramethyllyl stannate, vinyl methyl, isopropenyl methyl, allyl methyl, allyl phenyl, and diallyl ketones, mesityl oxide, acrylic, methacrylic, and ethacrylic acids, methyl, beta-chloroethyl, n-butyl, 2-ethylhexyl, n-octyl, vinyl, allyl, phenyl, tolyl, and benzyl acrylates, methyl, 2-ethylhexyl and n-octyl methacrylates, methyl chloroacrylates, ethyl chloroacrylate, methyl ethacrylate, methyl crotonate, acrylamide, N - methylmethacrylamide, N-phenyl acrylamide, N-allyl acrylamide, acrylonitrile, and methacrylonitrile, diethyl, dibutyl, di-2-ethyl hexyl, and di-n-octyl maleates, maleic anhydride, maleimide, N-methyl maleimide, N-butyl maleimide, N-phenyl maleimide, N-benzyl maleimide, diethyl, di-2-ethyl hexyl, di-n-octyl, and diallyloxyethyl fumarates, fumaronitrile, itaconic anhydride, di-2-ethyl hexyl itaconate, dihexyl itaconate, itacononitrile, dimethyl mesaconate and citraconate, 2-vinyl pyridine, 2-vinyl furan, and 2-vinyl thiophen. Peroxidic catalysts specified, which may be added incrementally during the reaction, are acetyl peroxide, benzoyl peroxide, and tert. butyl hydroperoxide. The preferred copolymers are the soluble, unsaturated copolymers of the trichloroalkenes with polyolefinic esters, which may be further polymerized with the aid of heat and catalysts, or dissolved in liquid copolymerizable compounds such as butyl, tolyl, and allyl acrylates, vinyl butyrate, styrene, and diethyl and diallyl fumarates, and further copolymerized. The copolymers may be used in the preparation of rods, blocks, sheets, and coating and laminating compositions.