Abstract: Styreneisoprene copolymers were synthesized through the anionic bulk polymerization in a corotating intermeshing twinscrew extruder. Sodium dodecyl benzene sulfonate and tetrahydrofuran were used as the polar regulators. The oxidative degradation method with hydrogen peroxide and osmium tetraoxide was used to cut off the double bonds of the polyisoprene in the copolymer chains. Based on GPC, 1H-NMR, DMA and TEM analysis, it was found that the copolymer chains were composed of a long polystyrene block and a lot of short styreneisoprene blocks. While adding SDBS and THF into the polymerization system, 3,4-polyisoprene structure was greatly increased. Furthermore, the polymerization of styrene was significantly restrained and the molecular weight of the first long block of polystyrene was comparatively decreased, which could hardly wrap up big isoprene bubble. As a result, the produced copolymers were composed of the styreneisoprene short blocks. The compatibility of two phases was enhanced and the randomization degree of the copolymer increased too.