Abstract: An saturated nitrile-conjugated diene copolymer obtained by copolymerizing acrylonitrile with butadiene in the presence of a free radical initiator and an alkylthiol compound as a molecular weight modifier. A part of the molecular weight modifier is incorporated in a monomer charge before polymerization, and the remainder thereof is incorporated in a polymerization mixture during polymerization. The alkylthiol compound has 12 to 16 carbon atoms, which include at least three tertiary carbon atoms, and further has a sulfur atom directly bound to at least one of the tertiary carbon atoms. The copolymer has a Mooney viscosity of 15 to 150 and contains 3 to 20 weight % of a fraction having a number average molecular weight of not larger than 35,000. The copolymer is used in combination with a sulfur-containing vulcanizer.

Abstract: An saturated nitrile-conjugated diene copolymer obtained by copolymerizing acrylonitrile with butadiene in the presence of a free radical initiator and an alkylthiol compound as a molecular weight modifier. A part of the monomers is incorporated in a monomer charge before polymerization, and the remainder thereof is added in a polymerization mixture during polymerization. The alkylthiol compound has 12 to 16 carbon atoms, which include at least three tertiary carbon atoms, and further has a sulfur atom directly bound to at least one of the tertiary carbon atoms. The copolymer has a Mooney viscosity of 15 to 450 and a bound nitrile content of 10 to 60% by weight. The breadth (.DELTA.AN) of compositional distribution of the unsaturated nitrile in the copolymer is 3 to 20. The copolymer is used in combination with a sulfur-containing vulcanizer.

Abstract: An unsaturated nitrile-conjugated diene copolymer having at least 0.03 mole, per 100 moles of the monomeric units constituting the molecule, of an alkylthio group having 12 to 16 carbon atoms, which include at least three tertiary carbon atoms, and having a sulfur atom which is directly bound to at least one of the tertiary carbon atoms; said copolymer having a Mooney viscosity of 15 to 65. This copolymer is produced by copolymerizing an unsaturated nitrile and a conjugated diene in the presence of a free-radical initiator by using as a molecular weight modifier an alkylthiol compound having the above-mentioned alkylthio group. The copolymer is used as a rubber composition having incorporated therein a sulfur-containing vulcanizer.