Preparation of conjugated diene-monoalkenyl arene block copolymers having a low polydispersity index

A block copolymer which corresponds to the formula ABA wherein A comprises units derived from one or more monoalkenyl arenes;

• and B comprises units derived from one or more conjugated dienes and the residuum of a difunctional initiator;
• wherein the block copolymer is prepared by the process comprising:
• I) contacting (a) a difunctional initiator capable of initiating the polymerization of a compound containing a double bond with (b) a conjugated diene in an aliphatic or cycloaliphatic hydrocarbon solvent wherein the concentration of difunctional initiator is from about 0.002 moles/liter to about 0.06 moles/liter and the molar ratio of conjugated diene to difunctional initiator is from about 30:1 to about 1000:1 under conditions such that the conjugated diene polymerizes to a degree of polymerization of 30 or greater wherein the difunctional initiator is soluble in the solvent;
• II) contacting with the polymerization mixture of I a sufficient amount of conjugated diene to prepare a polymer having a number average molecular weight of from about 30,000 to about 300,000 and of an aliphatic or cycloaliphatic hydrocarbon solvent such that the polymerization mixture has a solids content of from 5 to 30 percent by weight, under conditions such that the added conjugated diene polymerizes;
• and
• III) contacting with the polymerization mixture of II a sufficient amount of monoalkenyl arene to prepare a triblock copolymer having a number average molecular weight of about 31,500 to about 400,000 and an average weight percent of monoalkenyl arenes incorporated in the block copolymer of from about 5 percent to about 50 weight percent under conditions such that the monoalkenyl arene polymerizes, wherein the block copolymer has a polydispersity index of 1.4 or lower, a percent elongation of 1120 or greater and a tensile strength of 1500 psi or greater wherein the block copolymer contains no residual co-initiator.