Isoprene oligomer, polyisoprene, processes for producing these materials, rubber composition, and pneumatic tire

An isoprene oligomer comprising a substituted trans structural moiety and a cis structural moiety, wherein at least one atom or group of an unsubstituted trans structural moiety A of a preliminary isoprene oligomer having the following formula (1) selected from a hydrogen atom, a methyl group, a methylene group, a carbon atom, or a methine group is replaced by a different atom or group to provide the substituted trans structural moiety, and the different atom or group is selected from the group consisting of a nitrogen atom, an oxygen atom, a sulfur atom, a silicon atom, a carbon atom, an acetoxy group, an alkoxy group, a hydroxy group, an aryl group, an alkyl group, an acetyl group, an N-alkyl-acetamino group, and an azide group:

• wherein n represents an integer from 1 to 10;
• m represents an integer from 1 to 30;
• the preliminary isoprene oligomer has a cis structural moiety B;
• and Y represents a hydroxy group, a formyl group, a carboxy group, an ester group, a carbonyl group, or a group having the following formula (2):