Rubber composition and pneumatic tire

A rubber composition, comprising a rubber component and silica,

• wherein the rubber component contains, based on 100% by mass of the rubber component,
• 10 to 70% by mass of a high-cis polybutadiene having a cis content of 95% by mass or more and satisfying the following requirements (A) to (C):
• (A) a Mooney viscosity (ML) of 40 to 49;
• (B) a molecular weight distribution (weight-average molecular weight (Mw)/number-average molecular weight (Mn)) of 3.0 to 3.9;
• and (C) an index (n-value in Expression (1) below) of velocity dependence of Mooney viscosity of 2.3 to 3.0, and
• 30 to 90% by mass of a conjugated diene polymer containing a constituent unit based on a conjugated diene and a constituent unit represented by formula (I) below, at least one terminal of the polymer being modified with at least one compound selected from the group consisting of a compound represented by formula (II) below, a compound represented by formula (III) below, and a compound represented by formula (IVa) below, and
• an amount of the silica is 5 to 150 parts by mass per 100 parts by mass of the rubber component,
• log(ML)=log(K)+n−1×log(RS) (Expression 1)
• wherein RS indicates the number of revolutions per minute of a rotor, K indicates an arbitrary number, and ML indicates a Mooney viscosity,
• wherein X1, X2, and X3 each independently represent a hydroxyl group or a C1-4 alkyl group, and two of X1, X2, and X3 are hydroxyl groups;
• wherein n represents an integer of 1 to 10;
• R11, R12, and R13 each independently represent a C1-4 alkyl group or a C1-4 alkoxy group, and at least one of R11, R12, and R13 is an alkoxy group;
• and A1 is a group represented by the formula (IIa):
• wherein R14 and R15 each independently represent a C1-6 alkyl group, a C1-6 alkylene oxide group, a C1-6 alkylene oxide alkyl group, or a C1-6 trialkylsilyl group;
• wherein m represents an integer of 1 to 10;
• R21 represents a hydrogen atom, a C1-6 alkyl group, or a C1-6 alkoxyalkyl group;
• A2 represents the following group:
• —NR22— where R22 represents a hydrogen atom or a C1-4 alkyl group;
• and A3 is a group represented by the formula (IIIa):
• wherein R23 and R24 each independently represent a C1-6 alkyl group, a C1-6 alkylene oxide group, a C1-6 alkylene oxide alkyl group, or a C1-6 trialkylsilyl group;
• wherein q represents an integer of 0 to 10;
• R31 represents a C1-5 alkyl group;
• R32, R33, R34 and R35 each independently represent a hydrogen atom, a C1-5 alkyl group, a C1-5 alkoxyalkyl group, or a C1-5 alkoxy group, and when a plurality of R32's and a plurality of R33's are present, the plurality of R32's and the plurality of R33's may be the same as or different from one another;
• and R36 and R37 each independently represent a C1-6 alkyl group.