Mono vinyl aromatic conjugated diene block copolymer and polymer composition comprising said block copolymer and a mono vinylarene acrylate copolymer

A block copolymer comprising at least one mono vinyl aromatic monomer and at least one conjugated diene monomer,

• wherein the block copolymer comprises at least a first and a second homo block of a mono vinyl aromatic monomer and, between these vinyl aromatic blocks, at least one homo block of a conjugated diene and at least one random block of at least one mono vinyl aromatic monomer and at least one conjugated diene,
• wherein the block copolymer comprises from 20 to 50% by weight of conjugated diene monomer, based on the total block copolymer,
• wherein the block copolymer has a star shaped structure and wherein the block copolymer is obtained by a process that comprises the steps of forming by sequential anionic polymerization and coupling at least one branch with the block sequence S1-S2-B1-S3/B2-S4 and/or S2-B1-S3/B2-S4, wherein S represents homo blocks of mono vinyl aromatic monomer, B represents homo blocks of conjugated diene, and S/B represents a random block consisting of vinyl aromatic monomer and conjugated diene, where at least the polymerization step of the random block S/B takes place in the presence of a potassium salt as randomizer.