Unsaturated nitrile-conjugated diene copolymer, process for producing same and vulcanizable rubber composition

A process for producing an unsaturated nitrile-conjugated diene copolymer wherein an unsaturated nitrile and a conjugated dienes are copolymerized in the presence of a molecular weight modifier and a free radical initiator,

• characterized in that the molecular weight modifier used is an alkylthiol compound with 12 to 16 carbon atoms, which include at least three tertiary carbon atoms, and further with a sulfur atom directly bound to at least one of the tertiary carbon atoms, and
• further in that copolymerization is commenced with 30 to 80% by weight of the total amount of the monomers, and, when the polymerization conversion reaches a value of 20 to 70%, the remainder of the monomers is incorporated in the polymerization mixture.

A vulcanizable rubber composition comprising 100 parts by weight of an unsaturated nitrile-conjugated diene copolymer and 0.01 to 100 parts by weight of a sulfur-containing vulcanizer;

• said copolymer having at least 0.03 mole, per 100 moles of the monomeric units constituting the copolymer molecule, of an alkylthio group with 12 to 15 carbon atoms, which include at least three tertiary carbon atoms, and further with a sulfur atom directly bound to at least one of the tertiary carbon atoms;
• said copolymer having a Mooney viscosity of 15 to 150 and a bound unsaturated nitrile content of 10 to 60% by weight;
• and the breadth (.DELTA.AN) of compositional distribution of the unsaturated nitrile in the unsaturated nitrile-conjugated diene copolymer being in the range of 3 to 20.