Rubber composition for tire and pneumatic tire

1. A rubber composition for a tire, comprising: diene-based rubber and silica,

the diene-based rubber containing natural rubber and specific conjugated diene-based rubber, a content of the natural rubber in the diene-based rubber being 30 to 45 mass %, and a content of the specific conjugated diene-based rubber in the diene-based rubber being from 30 to 70 mass %; and

the specific conjugated diene-based rubber being conjugated diene-based rubber produced according to a method of producing conjugated diene-based rubber including the following steps of A, B, and C in this order, the conjugated diene-based rubber having: an aromatic vinyl unit content of from 38 to 48 mass %, a vinyl bond content of from 20 to 35 mass %, and a weight average molecular weight of from 500000 to 800000,

a CTAB adsorption specific surface area of the silica being 180 m 2 /g or greater, and

a content of the silica being less than 60 parts by mass per 100 parts by mass of the diene-based rubber;

step A: polymerizing a monomer mixture containing isoprene and aromatic vinyl to form a polymer block A having an active terminal, the polymer block A having an isoprene unit content of from 80 to 95 mass %, an aromatic vinyl unit content of from 5 to 20 mass %, and a weight average molecular weight of from 500 to 15000;

step B: mixing the polymer block A with a monomer mixture containing 1,3-butadiene and aromatic vinyl and continuing polymerization reaction so as to form a polymer block B having an active terminal continuously with the polymer block A to thereby obtain a conjugated diene-based polymer chain having an active terminal, the conjugated diene-based polymer chain having the polymer block A and the polymer block B; and

step C: reacting the active terminal of the conjugated diene-based polymer chain with polyorganosiloxane represented by Formula (1):

where R 1 to R 8 , which are alkyl groups having from 1 to 6 carbons or aryl groups having from 6 to 12 carbons, may be the same as or different from one another; X 1 and X 4 , which are any group selected from the group consisting of alkyl groups having from 1 to 6 carbons, aryl groups having from 6 to 12 carbons, alkoxy groups having from 1 to 5 carbons, and epoxy group containing groups having from 4 to 12 carbons, may be the same as or different from one another; X 2 is an alkoxy group having from 1 to 5 carbons or an epoxy group containing group having from 4 to 12 carbons, and X 2 being plural may be the same as or different from one another; X 3 is a group containing from 2 to 20 repeating alkylene glycol units, and upon X 3 being plural, these may be the same as or different from one another; m is an integer from 3 to 200; n is an integer from 0 to 200; and k is an integer from 0 to 200.

2. The rubber composition for a tire according to claim 1 , further comprising aromatic modified terpene resin having a softening point of from 100 to 150° C.,

wherein a content of the aromatic modified terpene resin is from 1 to 50 parts by mass per 100 parts by mass of the diene-based rubber.

3. A pneumatic tire, wherein the rubber composition for a tire according to claim 2 is arranged in a cap tread.

4. A pneumatic tire, wherein the rubber composition for a tire according to claim 1 is arranged in a cap tread.

5. The rubber composition for a tire according to claim 1 , wherein the content of the specific conjugated diene-based rubber in the diene-based rubber is from 30 to 39 mass %.

6. The rubber composition for a tire according to claim 1 , wherein the content of the specific conjugated diene-based rubber in the diene-based rubber is from 30 to 37 mass %.