Rubber composition and pneumatic tire

A rubber composition, comprising a rubber component, silica, and a processing aid compound represented by formula (1) below,

• wherein the rubber component contains, based on 100% by mass of the rubber component, not less than 5% 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 containing a group represented by formula (III) below, a compound represented by formula (IV) below, a silicon compound containing at least one of a group represented by formula (V) below and a group represented by formula (VI) below, and a compound containing a group represented by formula (VII) below,
• wherein the silica is present in an amount of 5 to 150 parts by mass per 100 parts by mass of the rubber component,
• formula (I):
• wherein X1, X2, and X3 each independently represent a group represented by formula (Ia) below, a hydroxy group, a hydrocarbyl group, or a substituted hydrocarbyl group, and at least one of X1, X2, and X3 is a hydroxy group or a group represented by the following formula (Ia):
• wherein R1 and R2 each independently represent a C1-6 hydrocarbyl group, a C1-6 substituted hydrocarbyl group, a silyl group, or a substituted silyl group, and R1 and R2 may be bonded to each other to form a cyclic structure together with the nitrogen atom;
• formula (II):
• wherein n represents an integer of 1 to 10;
• R11, R12, and R13 each independently represent a C1-4 hydrocarbyl group or a C1-4 hydrocarbyloxy group, and at least one of R11, R12, and R13 is the hydrocarbyloxy group;
• and A1 represents a nitrogen atom-bearing functional group;
• formula (III):
• wherein p represents an integer of 0 or 1;
• T represents a C1-20 hydrocarbylene group or a C1-20 substituted hydrocarbylene group;
• and A2 represents a nitrogen atom-bearing functional group;
• formula (IV):
• wherein g represents an integer of 1 to 10;
• R21 represents a hydrogen atom, a C1-6 hydrocarbyl group, or a C1-6 substituted hydrocarbyl group;
• A3 represents an oxygen atom or the following group:
• —NR22— where R22 represents a hydrogen atom or a C1-10 hydrocarbyl group;
• and A4 represents a functional group bearing at least one of a nitrogen atom and an oxygen atom;
• formulas (V), (VI) and (VII):
• wherein w represents an integer of 1 to 11, and A5 represents a nitrogen atom-bearing functional group;
• formula (1):
• wherein R101 represents a hydrogen atom, a branched or unbranched C1-30 alkyl group, a branched or unbranched C2-30 alkenyl group, a branched or unbranched C6-30 aryl group, or a hydroxy group;
• R102 and R103 are the same as or different from each other and each represent a hydrogen atom, a branched or unbranched C1-30 alkyl group, a branched or unbranched C2-30 alkenyl group, a branched or unbranched C2-30 alkynyl group, a branched or unbranched C6-30 aryl group, or a group represented by formula (2) below, wherein a hydrogen atom of the alkyl group, alkenyl group, alkynyl group, or aryl group may be substituted by a hydroxy group or a carboxyl group;
• a pair of R101 and R102, a pair of R101 and R103, or a pair of R102 and R103 may form a cyclic structure;
• and d1 is zero,
• wherein R104 represents a branched or unbranched C1-30 alkylene group;
• R105 represents a hydrogen atom, a branched or unbranched C1-30 alkyl group, a branched or unbranched C2-30 alkenyl group, a branched or unbranched C6-30 aryl group, or a hydroxy group;
• and d2 represents an integer of 0 to 10.