The thermal, bulk polymn. of styrene is conducted in a pilot plant consisting of a recycle tubular reactor followed by a tubular reactor. The conversion of the pre-polymn. part is in the range of 0 to 60% and at the outlet of the pilot plant ?96%. Studies on the residence time distribution show plug-flow behavior for a variety of different conditions with respect to viscosity and d. gradient. The polymers obtained are characterized by gel permeation chromatog. and compared to com. products. [on SciFinder (R)]