Method for producing nitrile group-containing copolymer rubber

A method comprising:

• copolymerizing a monomer mixture consisting of an α,β-ethylenically unsaturated nitrile monomer and a conjugated diene monomer,
• wherein
• a method for the copolymerization is an emulsion polymerization method, and a temperature of the emulsion polymerization is 5 to 30° C.,
• in the copolymerization, a polymerization initiator is added in an amount of 0.05 parts by weight or less relative to 100 parts by weight of the monomer mixture used in the copolymerization,
• a molecular weight adjuster is added at the start of the copolymerization in an amount of 0.29 to 0.5 parts by weight relative to 100 parts by weight of the monomer mixture, and a molecular weight adjuster is not added thereafter,
• the copolymer has a Mooney viscosity (ML1+4, 100° C.) in a range of 30 to 60 at a time point when a polymerization conversion rate is 60%,
• the copolymerization is carried out until the polymerization conversion rate reaches 85% or more to obtain a nitrile group-containing copolymer rubber, and
• the obtained nitrile group-containing copolymer rubber has a Mooney viscosity (ML1+4, 100° C.) of 85 to 150.