It is desirable for rubber compositions for tire components to have better performances including fuel economy and wet grip performance. Known as a technique for improving these performances is a technique of introducing a polar functional group into a main chain or an end of rubber (diene polymer) to control the properties of rubber. Particularly in the case where a polar functional group is introduced into an end of rubber, the energy loss caused by movement of the end is reduced, thereby resulting in significant improvement in fuel economy.
Rubber is generally polymerized through ionic polymerization or radical polymerization. Since ionic polymerization has high flexibility in the molecular design of the microstructure of rubber, the above-mentioned technique of introducing a polar functional group into a main chain or an end of rubber has been accomplished via ionic polymerization. For example, Patent Document 1 discloses a rubber in which a polar functional group is introduced into an end, and Patent Document 2 discloses a rubber in which a polar functional group is introduced into both a main chain and an end. However, the ionic polymerization still has room for improvement, generally, in that the processability is likely to deteriorate because the obtained molecular weight distribution is narrow, and that the production cost is high.
On the other hand, radical polymerization has been industrially widely used because of its easy handling. Use of radical polymerization enables production of a polymer having a broad and unimodal molecular weight distribution and good processability. However, with respect to the radical polymerization which has no livingness, although there is known a technique of introducing a polar functional group into a main chain of rubber by copolymerization of a diene monomer and a polar functional group-containing monomer, a technique of introducing a polar functional group into an end to increase the functions of a diene polymer has not been accomplished.
Patent Document 1: JP 2005-126604 A
Patent Document 2: JP 2010-116546 A