Tetrafunctional initiator

A process for polymerizing a vinyl aromatic monomer comprising from 5 to 45 weight % of star branched vinyl aromatic polymer comprising feeding a mixture comprising:

• i) from 60 to 100 weight % of one or more C8-12 vinyl aromatic monomers;
• and
• ii) from 0 to 40 weight % of one or more monomers selected from the group consisting of C1-4 alkyl esters of acrylic or methacrylc acid and acrylonitrile and methacrylonitrile;
• which polymer optionally grafted onto or occluded within from 0 to 12 weight % of one or more rubbery polymers selected from the group consisting of:
• iii) co- and homopolymers of C4-5 conjugated diolefins;
• and
• iv) copolymers comprising from 60 to 85 weight % of one or more C4-5 conjugated diolefins and from 15 to 40 weight % of a monomer selected from the group consisting of acrylonitrile and methacrylonitrile, and from 0.01 to 0.1 weight % of a tetrafunctional peroxide initiator of the formula:
• wherein R1 is selected from the group consisting of C4-6 t-alkyl radicals and R is a neopentyl group, in the absence of a cross linking agent to a series of two or more continuous stirred tank reactors, to provide a relatively low temperature initial reaction zone at a temperature from 100 to 130° C. and a relatively higher temperature subsequent reaction zone at a temperature from 130 to 160° C. and maintaining a ratio of residence time in said relatively lower temperature reaction zone to said relatively higher temperature reaction zone from 1:1 to 4:1 and recovering the resulting polymer.