ASYMMETRICAL BUTADIENE-STYRENE BLOCK COPOLYMER, MANUFACTURE AND USE

Block copolymers of the formula A-B-A' having a molecular weight of less than 300,000 and built up from 18 to 40 percent by weight of a vinylaromatic hydrocarbon and 60 to 82 percent by weight of one or more, optionally partially or fully hydrogenated diene hydrocarbon(s), and in which A and A' are each a polymer block of a vinylaromatic hydrocarbon and B is a polymer block of a diene hydrocarbon, the weight ratio between the polymer blocks A and A' being greater or equal to 1:5 and less than 1:10, and the mean molecular weight of the polymer block A being from 2000 to 20,000 and that of the polymer block A' being from 10,000 to 110,000, and the molecular weight of block B being at least 18,000, are prepared by a process in which, in a first polymerisation step, the vinylaromatic hydrocarbon is fully polymerised to form a block A or A', the diene hydrocarbon is fully reacted in the second polymerisation step to form block B, and the second portion of the vinylaromatic hydrocarbon subsequently polymerised on to form block A or A', the amount of vinylaromatic compound used to prepare block A' being at least five times, at most ten times, the amount used to form block A. The block copolymers are used to prepare moulding compositions.