Styrene/isoprene/butadiene integrated rubber (SIBR) was synthesized using anionic bulk polymerization in a corotating intermeshing twin-screw extruder. In order to study the relationship between microstructure and physical properties, three SIBRs were synthesized using different initiator complexes: n-BuLi (N-butyl lithium), n-BuLi/THF (tetrahydrofuran), and n-BuLi/TMEDA (tetramethylethylenediamine). The microstructure of the rubber products was analyzed by gel permeation chromatography and H nuclear magnetic resonance. SIBR prepared with pure n-BuLi exhibited a multiblock microstructure. With the addition of THF or TMEDA, the vinyl content of the alkadiene increased; and the microstructure of SIBRs was randomized, indicated by the presence of a high weight content of styrene microblocks. Transmission electron microscopy demonstrated similar morphology in both the SIBRs and SSBR2003 (an industrial product). Dynamic mechanical analysis of the vulcanized SIBRs showed that the randomized SIBRs were quite suitable for the preparation of high-performance tires. Tensile and tear test results of SIBRs were equivalent to those of SSBR2003. The low manufacturing cost of reactive extrusion technology will allow the large-scale industrialization of SIBRs. POLYM. ENG. SCI., 55:1163-1169, 2015. (c) 2014 Society of Plastics Engineers