Styrene-butadiene copolymer and rubber composition comprising the copolymer

A styrene-butadiene copolymer wherein:

• a total styrene content is in the range of 33 to 42% by weight of the styrene-butadiene copolymer;
• a content of styrene sequences having 4 to 20 styrene units is in the range of 40 to 65% by weight of the total styrene content, as measured by nuclear magnetic resonance spectroscopy and gel permeation chromatography in combination;
• a content of styrene sequences having more than 20 styrene units is in the range of 5% by weight or less of the total styrene content, as measured by gel permeation chromatography;
• a content of vinyl unit is in the range of 30% by mol or less of the butadiene part of the styrene-butadiene copolymer;
• a sum of the total styrene value and ½ of the content of vinyl units is in the range of 46 to 52%;
• and
• wherein the styrene-butadiene copolymer is obtained by copolymerizing styrene and butadiene in a hydrocarbon solvent in the presence of
• (1) an organolithium initiator selected from the group consisting of methyllithium, ethyllithium, propyllithium, n-butyllithium, sec-butyllithium, t-butyllithium, hexyllithium, octyllithium, tetramethylenedilithium, pentamethylenedilithium, hexamethylenedilithium, phenyllithium, tolyllithium, lithium naphthylide and benzyllithium and dilithium formed by the reaction of diisopropenylbenzene and butyllithium,
• (2) at least one type of potassium compound selected from the group consisting of potassium alcoholates, potassium sulfonates, and potassium carboxylates, and
• (3) at least one type of compound selected from the group consisting of ether compounds and amine compounds, and adding a compound selected from the group consisting of tin compounds, isocyanate compounds, imidazolidinone compounds, and aminobenzophenone compounds as a coupling agent or a modifier to the copolymerization product after copolymerization has been completed.