NOVELTY - 2-15 weight percent of blowing agent(s) selected from 4-6C alkanes, chlorofluorocarbons, hydrochlorofluorocarbons, hydrofluorocarbons, carbon dioxide and/or nitrogen is injected into molten mass of 8-12C vinyl aromatic polymer at preset condition. The mixture is then extruded. Vinyl aromatic polymer comprises 10-45 wt.% of star branched polymer and has VICAT softening temperature more than 100 degreesC.    USE - For preparing closed cell foam used as slab for construction, as thin walled tubes, coffee cups, meat trays.    ADVANTAGE - The foamed polymer melt has excellent melt strength than conventional foamed polymer melts. The foam collapse in the foamed polymer is reduced.    DETAILED DESCRIPTION - The method involves injecting 2-15 weight percent of one or more blowing agents selected from 4-6C alkanes, chlorofluorocarbons, hydrochlorofluorocarbons, hydrofluorocarbons, carbon dioxide and/or nitrogen into a molten mass of 8-12C vinyl aromatic polymer at 140-235 degreesC and pressure of 1500-3500 psi. The mixture is then extruded. 8-12C vinyl aromatic polymer comprises 60-100 wt.% of 8-12C vinyl aromatic monomer(s) and 0-40 wt.% of monomer(s) selected from 1-4C alkyl esters of (meth)acrylic acid and acrylonitrile and methacrylonitrile.    The polymers are grafted onto 0-12 wt.% of one or more rubbery polymers selected from co- and homo-polymers of 4-5C conjugated diolefins and copolymers comprising 60-85 wt.% of 4-5C conjugated diolefins and 15-40 wt.% of monomer selected from acrylonitrile and methacrylonitrile.    The vinyl aromatic polymer comprises 10-45 wt.% of star branched polymer and have VICAT softening temperature more than 100 degreesC.    An INDEPENDENT CLAIM is also included for polymerizing vinyl aromatic monomer which involves feeding a monomer mixture same as that used for preparation of closed cell foam and 0.01-0.1 wt.% of tetrafunctional peroxide initiator of formula (R1OOC(O)O)4R (I) in the absence of crosslinking agent to a series of two or more continuous stirred tank reactors.    R1 = 4-6C t-alkyl;    R = neopentyl group.    The initial reactor zone is at a temperature of 100-130 degreesC and the subsequent reaction zones are at a temperature of 130-160 degreesC.    The ratio of residence time in relative low temperature reaction zone to relative high temperature reaction zone is maintained at 1.1-4.1 and the resultant polymer is recovered.