Abstract Reactive extrusion is the term used to denote a plastics processing method in which an extruder is used as a reactor for the continuous synthesis and modification of polymers. This paper describes the manufacture of polystyrene and styreneisoprene copolymers by a reactive extrusion process: anionic “living” s-butyl lithium initiated bulk polymerization was performed in a co-rotating closely intermeshing twin screw extruder. The results of the process analysis show that living polymerization of styrene can be performed in a screw-type reactor, despite the high reaction temperatures of over 200 °C. The polystyrene melt can be modified in bulk with comonomers or coupling reagents immediately after polymer synthesis. Depending on the raw material, formulation, and process parameters, the process variants developed and analyzed (homopolymerization of styrene, copolymerization of styrene-isoprene mixtures, sequential polymerization of styrene and isoprene) resulted in styrene polymers with widely differing structural characteristics and properties. For example, the copolymerization or styrene-isoprene monomer mixtures produced poly [(isoprene-co-styrene)-b-styrene]. The sequential polymerization of styrene and isoprene led to poly(styrene-b-isoprene) contaminated with partly crosslinked low molecular weight polyisoprene. The polyisoprene content is presumably formed by side reactions due to the high reaction temperatures.