Abstract: A process for the preparation of expandable polymer particles containing a polymer of a vinylarene monomer and a foaming agent, which process comprises:1) polymerizing, by suspension polymerization, 100 parts by weight of a vinylarene monomer, in the presence of 1 to 20 parts by weight of a radical initiator having a half life of 1 hour in benzene at 70 to 110.degree. C., until at least 60% of the vinylarene monomer is polymerized;400 to 4,900 parts by weight additional vinylarene monomer as well as 0.05 to 1% wt, based on the amount of vinylarene monomer added in step 2, of a radical initiator having a half life of 1 hour in benzene at 70 to 110.degree. C. and polymerizing to completion, in which process the blowing agent can be added before, during or after the polymerization process.

Abstract: The present invention relates to expandable beads based on a polymer of a vinylaromatic compound containing a blowing agent and, in an amount of 0.1 up to 2% wt, preferably 0.3 up to 1.5% wt calculated on the beads, of a plasticizer that is based on oligomers of styrene and/or alpha-methylstyrene having a number-average molecular weight (M.sub.n) in the range of 200 to 500, preferably of 260 to 460; and a process for preparing these expandable beads.