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 diene 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 10 to 95% by weight of the entire amount of the molecular weight modifier is incorporated in a monomer mixture before commencement of copolymerization, and, when the polymerization conversion reaches a value of 20 to 70%, the remainder of the molecular weight modifier is incorporated in a polymerization mixture.

A vulcanizate 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 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;
• said copolymer having a Mooney viscosity of 15 to 150 and containing 3 to 20% by weight of a low-molecular-weight fraction having a number average molecular weight of not larger than 35,000.