Rubber composition and pneumatic tire

1. A rubber composition comprising a diene rubber and a filler;

the diene rubber containing a styrene-butadiene copolymer and a natural rubber, wherein the content of the styrene-butadiene copolymer is not less than 40 mass % and less than 50 mass % of the total mass of the diene rubber;

a radical generation index of the styrene-butadiene copolymer being not greater than 1.0 and being smaller than a value Y calculated from an equation:Y=−0.0186×(SV mass %+vinyl unit mol %)+1.5

where the radical generation index is an index representation of a radical concentration using the radical concentration of a copolymer having a molar ratio of styrene units/1,2 vinyl units/1,4 trans units/1,4 cis units of 13/14/15/58 as a 1.0 reference value; and

“SV mass %” in the equation is a mass % of a partial structure where a styrene single chain having one styrene unit and a vinyl single chain having one vinyl unit are directly linked relative to an amount of bonded styrene of the styrene-butadiene copolymer, and “vinyl unit mol %” in the equation is a mol % of vinyl units when a total of styrene units, 1,2 vinyl units, 1,4 trans units, and 1,4 cis units in the styrene-butadiene copolymer is defined as 100 mol %, and

wherein the styrene single chain content having one styrene unit is not greater than 30 mass % of the amount of bonded styrene in the styrene-butadiene copolymer.

2. The rubber composition according to claim 1 , wherein the styrene-butadiene copolymer has an amount of bonded styrene of not greater than 25 mol % and a vinyl unit content of not less than 30 mol %.

3. The rubber composition according to claim 2 , wherein a partial structure content in which a styrene single chain having one styrene unit and a vinyl single chain having one vinyl unit are directly linked is not greater than 10 mass % of the amount of bonded styrene in the styrene-butadiene copolymer.

4. The rubber composition according to claim 3 , wherein the filler is silica and/or carbon black.

5. The rubber composition according to claim 2 , wherein the filler is silica and/or carbon black.

6. The rubber composition according to claim 1 , wherein a partial structure content in which a styrene single chain having one styrene unit and a vinyl single chain having one vinyl unit are directly linked is not greater than 10 mass % of the amount of bonded styrene in the styrene-butadiene copolymer.

7. The rubber composition according to claim 6 , wherein the filler is silica and/or carbon black.

8. The rubber composition according to claim 1 , wherein the filler is silica and/or carbon black.

9. The rubber composition according to claim 1 , wherein the content of the styrene-butadiene copolymer is less than 45 mass % of the total mass of the diene rubber.

10. The rubber composition according to claim 1 , wherein the content of the natural rubber is greater than the content of the styrene-butadiene copolymer in the total mass of the diene rubber.

11. The rubber composition according to claim 1 , wherein the content of the styrene-butadiene copolymer is not less than 47 mass % and less than 50 mass % of the total mass of the diene rubber.

12. A pneumatic tire comprising the rubber composition described in claim 1 .

13. A pneumatic tire comprising the rubber composition described in claim 2 .

14. A pneumatic tire comprising the rubber composition described in claim 3 .

15. A pneumatic tire comprising the rubber composition described in claim 4 .

16. A pneumatic tire comprising the rubber composition described in claim 5 .

17. A pneumatic tire comprising the rubber composition described in claim 6 .

18. A pneumatic tire comprising the rubber composition described in claim 7 .

19. A pneumatic tire comprising the rubber composition described in claim 8 .