AbstractSemi‐interpenetrating polymer networks based on polystyrene and polystyrene‐polyisoprene three‐block copolymers (SIS) were prepared by anionic polymerization. Factors affecting synthesis, such as composition, molecular weight of either the unattached SIS or the polystyrene between crosslinks, elastomer content, have been varied. Their effect on network formation was examined by extraction experiments and swelling measurements. Their morphology was investigated by means of electron micro