A polymer composition comprising a star-branched copolymer having a plurality of arms is disclosed. Each polymer arm has a molecular weight Mp of from 1 kg/mol to 50 kg/mol and comprises polymerized units (i) derived from a first vinyl aromatic monomer comprising a radical-reactive group, wherein from greater than 10 mol % to 100 mol % of the units (i) are unhydrogenated; and optionally, polymerized units (ii) comprising hydrogenated and unhydrogenated forms of polymerized units derived from a high Tg monomer, and hydrogenated form of polymerized units (i) or hydrogenated form of polymerized styrene units; and optionally, polymerized units (iii) comprising (a) hydrogenated form of polymerized units derived from one or more acyclic conjugated dienes, and (b) polymerized units derived from one or more of a second vinyl aromatic monomer; wherein less than 10 wt. % of units (a) are unhydrogenated.

A polymer composition comprising a star-branched copolymer having a plurality of arms is disclosed. Each polymer arm has a molecular weight Mp of from 1 kg/mol to 50 kg/mol and comprises polymerized units (i) derived from a first vinyl aromatic monomer comprising a radical-reactive group, wherein from greater than 10 mol % to 100 mol % of the units (i) are unhydrogenated; and optionally, polymerized units (ii) comprising hydrogenated and unhydrogenated forms of polymerized units derived from a high Tg monomer, and hydrogenated form of polymerized units (i) or hydrogenated form of polymerized styrene units; and optionally, polymerized units (iii) comprising (a) hydrogenated form of polymerized units derived from one or more acyclic conjugated dienes, and (b) polymerized units derived from one or more of a second vinyl aromatic monomer; wherein less than 10 wt. % of units (a) are unhydrogenated.

Provided is a tire for a motorcycle having an improved turning stability on wet road surface, the tire comprises a tread composed of a rubber composition, which comes into contact with a road surface, wherein Hs being a rubber hardness and M300 being a tensile stress (MPa) at 300% elongation of the rubber composition of the tread satisfy Equations 1, 2, and 3:
60≤Hs≤80,  Equation 1:
2≤M300≤12, and  Equation 2:
(M300−2)/(Hs−60)≤0.75.  Equation 3:

Provided is a tire for a motorcycle having an improved turning stability on wet road surface, the tire comprises a tread composed of a rubber composition, which comes into contact with a road surface, wherein Hs being a rubber hardness and M300 being a tensile stress (MPa) at 300% elongation of the rubber composition of the tread satisfy Equations 1, 2, and 3:
60≤Hs≤80,  Equation 1:
2≤M300≤12, and  Equation 2:
(M300−2)/(Hs−60)≤0.75.  Equation 3:

Provided are copolymers which provide improved overall performance in terms of fuel economy and wet grip performance, as well as rubber compositions and tires containing the copolymers. Included is a copolymer obtained by copolymerizing an aromatic vinyl compound, a conjugated diene compound, and a compound represented by the formula (1) below or by copolymerizing a conjugated diene compound and a compound represented by the formula (1) below, the copolymer having a weight average molecular weight of 1.0×10.sup.3 to 2.5×10.sup.6, ##STR00001##
wherein R.sup.1 and R.sup.2 may be the same or different and each represents a functional group containing at least two selected from the group consisting of carbon, hydrogen, and silicon, and R.sup.1 and R.sup.2 may be joined together to form a ring structure.

Provided are copolymers which provide improved overall performance in terms of fuel economy and wet grip performance, as well as rubber compositions and tires containing the copolymers. Included is a copolymer obtained by copolymerizing an aromatic vinyl compound, a conjugated diene compound, and a compound represented by the formula (1) below or by copolymerizing a conjugated diene compound and a compound represented by the formula (1) below, the copolymer having a weight average molecular weight of 1.0×10.sup.3 to 2.5×10.sup.6, ##STR00001##
wherein R.sup.1 and R.sup.2 may be the same or different and each represents a functional group containing at least two selected from the group consisting of carbon, hydrogen, and silicon, and R.sup.1 and R.sup.2 may be joined together to form a ring structure.

Provided is a hydrogenated block copolymer, which is a hydrogenation product of a block copolymer including a polymer block (A) containing more than 70 mol % of a structural unit derived from an aromatic vinyl compound and a polymer block (B) containing 30 mol % or more of a structural unit derived from at least one selected from the group consisting of a conjugated diene compound and isobutylene, the hydrogenated block copolymer being satisfied with the following requirements (1) and (2): Requirement (1): the content of the polymer block (A) in the block copolymer is 1 to 30% by mass; and Requirement (2): when the polymer block (B) is regarded as having a structure with a hydrogenation rate of 100 mol %, an average value of a methylene chain length of a main chain of the structural unit derived from at least one selected from the group consisting of a conjugated diene compound and isobutylene is 1.0 to 6.0.

Provided is a hydrogenated block copolymer, which is a hydrogenation product of a block copolymer including a polymer block (A) containing more than 70 mol % of a structural unit derived from an aromatic vinyl compound and a polymer block (B) containing 30 mol % or more of a structural unit derived from at least one selected from the group consisting of a conjugated diene compound and isobutylene, the hydrogenated block copolymer being satisfied with the following requirements (1) and (2): Requirement (1): the content of the polymer block (A) in the block copolymer is 1 to 30% by mass; and Requirement (2): when the polymer block (B) is regarded as having a structure with a hydrogenation rate of 100 mol %, an average value of a methylene chain length of a main chain of the structural unit derived from at least one selected from the group consisting of a conjugated diene compound and isobutylene is 1.0 to 6.0.

A composition for seeded polymerization includes: a seed copolymer which has a compound having dithioate, a mercaptan compound, a first aromatic vinyl monomer, and a first conjugated diene monomer, and has an average particle diameter of 10-30 nm; a second aromatic vinyl monomer; and a second conjugated diene monomer.

A composition for seeded polymerization includes: a seed copolymer which has a compound having dithioate, a mercaptan compound, a first aromatic vinyl monomer, and a first conjugated diene monomer, and has an average particle diameter of 10-30 nm; a second aromatic vinyl monomer; and a second conjugated diene monomer.