Rubber composition for tires and pneumatic tire

A rubber composition for tires comprising a diene rubber, silica, an alkyltrialkoxysilane represented by Formula (I), and a silane coupling agent;

• the diene rubber containing a butadiene rubber and a particular conjugated diene rubber, a content of the butadiene rubber in the diene rubber being 20 mass % or greater and a content of the particular conjugated diene rubber in the diene rubber being from 30 to 80 mass %;
• an average glass transition temperature of the diene rubber being from −65 to −45° C.;
• a content of the silica being from 90 to 150 parts by mass per 100 parts by mass of the diene rubber;
• a content of the alkyltrialkoxysilane being from 0.1 to 8 mass % relative to the content of the silica;
• the silane coupling agent and the silica satisfying a relationship:
• [content of silane coupling agent (g)×100/{content of silica (g)/CTAB adsorption specific surface area (m2/g)}]/100=3.5 to 7.5;
• wherein, R11 represents an alkyl group having from 1 to 20 carbons, and R12 each independently represents a methyl group or an ethyl group;
• the particular conjugated diene rubber being a conjugated diene rubber produced by a method of producing a conjugated diene rubber, the method comprising steps A, B, and C below in this order;
• the particular conjugated diene rubber having an aromatic vinyl unit content of 38 to 48 mass %, a vinyl bond content of 20 to 35 mass %, and a weight average molecular weight of 500000 to 800000;
• Step A:
• a step of forming a polymer block A having an active terminal, the polymer block A having an isoprene unit content of 80 to 95 mass %, an aromatic vinyl unit content of 5 to 20 mass %, and a weight average molecular weight of 500 to 15000, by polymerizing a monomer mixture containing isoprene and an aromatic vinyl;
• Step B:
• a step of obtaining a conjugated diene-based polymer chain having an active terminal, the conjugated diene-based polymer chain having the polymer block A and a polymer block B, by forming the polymer block B having an active terminal by mixing the polymer block A with a monomer mixture containing 1,3-butadiene and an aromatic vinyl to continue polymerization reaction to form the polymer block B in series with the polymer block A;
• and
• Step C:
• a step of reacting a polyorganosiloxane represented by Formula (1) with the active terminal of the conjugated diene-based polymer chain;
• wherein, R1 to R8 are the same or different and are each an alkyl group having from 1 to 6 carbons or an aryl group having from 6 to 12 carbons;
• X1 and X4 are the same or different and are groups 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 groups having an epoxy group and from 4 to 12 carbons;
• X2 is an alkoxy group having from 1 to 5 carbons or a group having an epoxy group and from 4 to 12 carbons, and a plurality of the X2 moieties are the same or different;
• X3 is a group having from 2 to 20 alkylene glycol repeating units, and when a plurality of the X3 moieties exists, the X3 moieties are the same or different;
• and 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.