Process for preparation of block copolymers

In the process of preparing block copolymers predominantly of the structure S-B-S with one or two small polybutadiene terminal blocks, wherein S represents a block of polystyrene and B represents a block of polybutadiene, by the steps of polymerizing the individual monomers to form the respective blocks, such polymerizations being initiatated by a lithium compound, the improvement whereby:

• (a) said polymerization is effected in a solvent consisting essentially of hexane;
• (b) said polymerization is initiated by a hydrocarbyl lithium compound having up to 10 carbon atoms;
• (c) as a precursor to the preparation of a first block of polystyrene, there is prepared initially a block of homo-polybutadiene representing 2-12 parts by weight per 100 parts by weight of the block of polystyrene to which this initial block of polybutadiene will be attached, by reacting said hydrocarbyl lithium with an appropriate amount of butadiene to give said size block and thereby produce a polybutadiene*-Li product, wherein * indicates said small size block;
• (d) sufficient styrene is added to the product of step (c) to give the desired molecular weight of the desired first S block and effecting polymerization of said styrene with said polybutadiene*-Li to give the product polybutadiene*-polystyrene-Li while still dispersed in said hexane;
• (e) thereafter adding an appropriate amount of butadiene to the dispersed product of step (d) to form a desired size block of polybutadiene and effecting polymerization of said butadiene with said polybutadiene*-polystyrene-Li to give the product polybutadiene*-polystyrene-polybutadiene-Li while still dispersed in said hexane, and
• (f) thereafter effecting the addition of a second block of polystyrene onto said polybutadiene*-polystyrene-polybutadiene-Li.