Block copolymer

A block copolymer prepared by coupling of the block copolymer,

• wherein the block copolymer comprises a polymer block (A) and a polymer block (B), the block copolymer being produced by forming the polymer block (A) by reversible addition-fragmentation chain transfer polymerization in the presence of a thiocarbonylthio group-containing compound, and then by forming the polymer block (B),
• wherein the polymer block (A) is prepared by (co)polymerizing 50% to 100% by weight of at least one monomer selected from the group consisting of acrylonitrile and methacrylonitrile and 50% to 0% by weight of at least one monomer selected from the group consisting of methacrylate esters, styrene, and α-methylstyrene, and the polymer block (B) is prepared by (co)polymerizing at least one monomer selected from the group consisting of acrylic acid, methacrylic acid, acrylate esters, methacrylate esters, vinyl acetate, styrene, α-methylstyrene, butadiene, isoprene, and vinyl chloride,
• wherein the thiocarbonylthio group of the block copolymer is converted into a mercapto group or a mercaptide group, and
• wherein said coupling is performed by a reaction of the mercapto group of the block copolymer or the mercaptide group of the block copolymer with another mercapto group of the block copolymer or another mercaptide group of the block copolymer.