Method of producing a copolymer having improved oil retention properties

A method of producing a copolymer, the method comprising

• polymerizing monomers including 1,3-diene structure and having 4 to 20 carbons in a solvent in a presence of an anionic polymerization initiator to produce a copolymer,
• wherein a temperature change during the polymerizing is less than 15° C., or
• the method further comprises cooling a temperature during the polymerizing so that a consistency of the copolymer is 6 mol % or less.