In this study, styrene–isoprene diblock and multiblock copolymers were synthesized with n-butyllithium as the initiator, in an intermeshing, corotating twin-screw extruder. The diameter (D) of this extruder was 36 mm, and the ratio of length/diameter was 56. The weight content of polyisoprene in these copolymers was above 50% although in the past studies it had not been possible to accomplish levels higher than 30%. Gel permeation chromatography results of samples and their degraded products show that there is only one long block polystyrene in the diblock copolymer chains; while in the multiblock copolymer molecules, there is a long block and large numbers of small blocks. Dynamic mechanical analysis and transmission electron microscopy show that the two phases in the diblock copolymer are completely incompatible, leading to sharp phase separation. In the multiblock copolymer, the two phases are partly compatible without an obvious phase boundary. The successful syntheses of styrene/isoprene diblock and multiblock copolymers with high-isoprene contents provide a novel method to synthesize polystyrene rubbers and styrene–diene–styrene thermoplastic elastomers. Traditionally, these products were mainly synthesized by solution polymerization. The present work in this article provides the possibility to synthesize them with very little or no solvent using bulk polymerization. This method fits the environmentally friendly trend to use low amounts of carbon while allowing commercial profitability. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 39429.