A composition comprises (1) more than 50% by weight of at least one homopolymer and/or at least one copolymer of a vinyl and/or a vinylidene halide, and (2) a copolymer of which at least 45% by weight consists of at least one diene and at most 55% by weight of at least one ester of a bivalent or multivalent propylene carboxylic acid, at least one carboxyl group of which is attached to the non-doubly-bound carbon atom and at least one carboxyl group to a doubly-bound carbon atom, or an ester of a homologue of said carboxylic acid. Chlorinated polyvinyl chloride and copolymers of vinyl chloride with vinyl pivalate, or with vinyl esters of aliphatic carboxylic acids of 9-11 carbon atoms are included in (1). The copolymer (2) may also comprises a vinyl aromatic, e.g. styrene. The composition may be prepared by (i) mixing the latices obtained from the emulsion polymerization of the monomers, and coagulating, or (ii) mixing the polymer dispersion obtained by suspension polymerization with a latex prepared by emulsion polymerization, and coagulating, or (iii) mixing using dry polymers and, e.g. rolling mills, in which case part of the halide polymer may be incorporated at a relatively low temperature, e.g. 100 DEG C., and the remainder at an elevated temperature, e.g. 180 DEG C. The diene-ester copolymers may be prepared by emulsion, in solution, in bulk, or in a liquid, possibly halogenated, hydrocarbon. Several emulsifiers are used in the examples. The copolymerization may be terminated by adding a solution of dimethyl dithiocarbamate, and the latices coagulated using ethanol. As antioxidant, 1,3,5 - trimethyl - 2,4,6 - tri(3,5 - ditert -butyl-4-hydroxybenzyl) benzene may be used. Suitable dienes are 1,3-butadiene, isoprene, dimethylbutadiene-1,3, and piperylene. Suitable acids are itaconic, glutaconic, aconitic and isaconitic acids.