Butadiene rubber or styrene-butadiene rubber is widely used as synthetic rubber for use as a material of a tire. While butadiene serving as a raw material of butadiene rubber or styrene-butadiene rubber is produced as a by-product in production of ethylene by cracking of naphtha, as a method for producing ethylene, a method where a natural gas such as ethane is used as a raw material has been recently extended and therefore a reduction in production volume of butadiene has been predicted. Therefore, there have been advanced various studies about use of synthetic rubber which is obtained without using butadiene as a raw material, as an alternative material of butadiene rubber or styrene-butadiene rubber.
As one synthetic rubber studied as the alternative material of butadiene rubber or styrene-butadiene rubber, a cyclopentene ring-opening polymer which is obtained by ring-opening polymerization of cyclopentene may be mentioned. For example, Patent Document 1 proposes a technique where cyclopentene is subjected to ring-opening polymerization in the presence of a compound having a functional group and an ethylenically unsaturated bond, thereby introducing the functional group to an end of a cyclopentene ring-opening polymer so as to improve affinity between the cyclopentene ring-opening polymer and an inorganic particle. Further, Patent Document 2 proposes a technique where an alkoxysilyl group is bonded to an end of a polymer chain via a urethane bound group to thereby improve affinity between a cyclopentene ring-opening polymer and an inorganic particle.
However, if considering the rising demands for safety from tires in recent years, for example, in a rubber composition using the cyclopentene ring-opening polymer specifically described in Patent Document 1 and Patent Document 2, the obtained cross-linked rubber have been sometimes insufficient in abrasion resistance and low heat buildup property. Therefore, the rubber composition containing the cyclopentene ring-opening polymer has been demanded to be improved for giving a cross-linked rubber excellent in abrasion resistance and low heat buildup property.
Aside from the above, Patent Document 3 proposes, as a rubber composition aimed to be improved in abrasion resistance and low heat buildup property, a rubber composition for a tire tread which contains a diene rubber including 30 wt % or more of an end modified group-containing conjugated diene copolymer which is end-modified, silica, carbon black and a specified silane coupling agent.