Thermal, bulk polymn. of styrene [100-42-5] in a pilot plant composed of a recycle tubular reactor for prepolymn. and a tubular reactor, entirely filled with static mixer elements, is discussed. Conversion in the prepolymn. reactor was 0-60% and at the outlet of the pilot plant ?96%. The residence time distribution showed low backmixing in the tubular reactor under a variety of conditions with respect to viscosity and d. gradients. Flow approached plug flow, with radical mixing complete and axial mixing negligible. The recycle ratio in prepolymn. was 10-80:1. At recycle ratio 10:1 and conversion 60%, the recycle section still behaved as an ideal, homogeneous, continuously-stirred tank reactor. The pilot plant gave polystyrene with mol. wt. ?360,000 and polydispersity comparative to com. samples. Pressure drop was studied for scaleup. [on SciFinder (R)]