A heat resistant sulfor-modified polychloroprene copolymer of 2-chloro-1,3-butadiene and 2,3-dichloro-1,3-butadiene characterized in that said polychloroprene has a Mooney viscosity ML 1 +4' (100°C) of from about 25-75, contains from about 3-25% by weight of units derived from 2,3-dichloro-1,3-butadiene, and of the units derived from 2-chloro-1,3-butadiene, 92-96% by weight of said units have a 1,4-trans configuration, said polychloroprene copolymer being curable with 4 parts MgO and 5 parts ZnO per 100 parts of copolymer at about 160°C for 30 minutes to yield a vulcanizate having a tensile strength of at least 10 MPa.
Mercaptan, dialkyl xanthogen disulfide or sulfur modified chloroprene based copolymers are disclosed. At least 92% of the chloroprene derived units present in the copolymer have a l,4-trans configuration. The copolymer comprises units derived from chloroprene, up to 20 parts and preferably 2 or more parts per l00 parts chloroprene derived units of units derived from 2,3-dichloro-l,3-butadiene, and 2 to 25 parts per l00 parts of chloroprene and 2,3-dichlorobutadiene of randomly distributed units derived from styrene; α-methyl styrene; acrylonitrile; methacrylonitrile; acrylic acid, methacrylic acid; maleic acid; fumaric acid; and lower alkyl acrylamides, methacrylamides, acrylates, methacrylates, maleates or fumarates.