AbstractDue to the ever growing and more complex demands on the properties of polymer materials, raw material producers and plastic processing industries are more and more interested in the continuous synthesis and modification of polymers. In this article the production of polystyrene and styrene–isoprene copolymers by anionic s‐butylithium initiated bulk polymerization in a co‐rotating close intermeshing twin screw extruder is described. In spite of high reaction temperatures of above 200°C the polystyrene stays “living,” which makes it possible to modify it in the melt by adding more monomers. The following processing methods were developed and analyzed: (1) homopolymerization of styrene; (2) copolymerization of styrene–isoprene monomer mixtures; (3) sequential polymerization of styrene and isoprene. The polymerized material resulting from the above‐mentioned processes differed considerably in structural features and final properties depending on the process parameters. As for homopolymerization of styrene, the influence of the process parameters screw rotation speed and screw geometry on the range of molecular weight distribution was investigated. Products made according to the two copolymerization methods were analyzed with regard to their structure and mechanical properties. The tests revealed that for sequential copolymerization the temperature in the polymerization zone has a decisive influence on the product quality. The influence of the screw rotation speed is less important. © 1993 John