The title reactor, of simple construction and suppressing segregation and overheating, consists of a tube filled with static mixing elements, sepd. into a 1st segment which is sepd. by a recirculating pump from the 2nd and 3rd segments, which form a piston-flow reactor. Thus, the polymn. of styrene in such a reactor with the 1st segment and recycle loop at 158 Deg and recycle ratio 10 gave a 60% conversion to polystyrene [9003-53-6] with no.-av. mol. wt. 93,000, polydispersity 1.9, viscosity of the polymn. medium .apprx.10 Pa-s, and energy consumption 0.00073 kW-h/kg. [on SciFinder (R)]