Increased interest in the global environmental issues has led to a demand for superior fuel consumption performance in pneumatic tires, along with high wet performance and superior safety performance. As a result, by compounding silica in rubber compositions that form tread portions, heat build-up has been suppressed, rolling resistance reduced, and fuel consumption performance improved and, also, dynamic visco-elasticity characteristics of the tread rubber has been improved which has led to enhancements in wet performance. Efforts have also been made to use silica having a small particulate diameter for the purpose of further increasing the effects associated with the compounding of silica. However, silica has poor affinity with diene rubber and dispersibility tends to be insufficient. Particularly, when the particle diameter of the silica is small, dispersibility worsens and, as a result, the effects of improving low rolling resistance and wet performance by modifying dynamic visco-elasticity characteristics such as the loss tangent (tan δ) of the rubber composition and the like have not been achievable. Additionally, reinforcing effects tend to be lower when compounding silica in a rubber component than when compounding carbon black. As a result, there is a problem in that when dispersibility is poor, wear resistance is insufficient.
To resolve this problem, Japanese Unexamined Patent Application Publication No. 2009-091498 as well as patent applications WO/2005/021637 and WO/2003/102053 propose improving the dispersibility of silica by compounding silica in a rubber composition with a terminal-modified solution polymerization styrene butadiene rubber where the terminals are modified by a polyorganosiloxane or the like, thereby reducing heat build-up (tan δ at 60° C.) and enhancing wet grip performance (tan δ at 0° C.). However, the low rolling resistance and wet performance obtained by the technology proposed in Patent Documents 1 to 3 is below that demanded by users and there is a need for further improvement in low rolling resistance and wet performance.