Styrene oligomers are formed by a free-radical mechanism during the thermal polymerization of styrene in storage. The effects of these compounds on the preparation of expandable polystyrene (EPS) were investigated with respect to Suspension polymerization behavior and the properties of the impregnated Polystyrene beads produced. Styrene dimers and trimers LIP to concentrations of 0.2 wt % did not affect the stability of the Suspension during the polymerization and impregnation Stages. Besides differentiated effects on the particle size distributions of the polymers and on the polymerization rate, no chain-transfer activity of the oligomers was observed, and this confirmed the assignment of chain transfer to the Diels-Alder dimer in the literature. The investigation of the foaming behavior of the pentane-impregnated EPS beads indicated a significant reduction of the prefoaming density caused by styrene dimers and trimers. This behavior resulted from a decrease in the glass-transition temperatures of these Polymers. The effects of high-molecular-weight Polystyrene, formed in addition to oligomers during storage by the thermal polymerization of styrene, on the polymerization behavior and polymer properties of EPS were also investigated. The results showed a significant impact on the suspension stability that was dependent on the concentration of the high-molecular-weight polystyrene. (C) 2008 Wiley Periodicals, Inc.