The extremely rapid anionic polymerization rate of polystyrene has made polymerization difficult to control and has prevented large-scale development. This invention uses batch polymerization to utilize the rapid rate of polymerization to complete polymerization from zero to 100% conversion in minutes and turns this rapid rate into an advantage. Polystyrene itself is used as a heat sink to limit and control the maximum temperature reached during polymerization. Adiabatic polymerization increases the polymer temperature to the desired processing temperature of the polymer for either removal of volatile compounds or for direct pellet formation when no solvent is employed. The polystyrene to be used as a heat sink is added either as pellets or is first formed at a low controllable temperature. Extremely low levels of residual monomer are found in the product. This invention also demonstrates techniques to prevent gel formation or to reduce formation to an acceptable level.