Three poly(4-trimethylsilylstyrene)-block-polyisoprenes (TIs), the molecular weights of which were 82,000, 152,000 and 291,000 (TI-82K, TI-152K, and TI-291K), were synthesized by sequential anionic polymerizations. The component polymers were a miscible pair that presented a lower critical solution temperature phase diagram if blended. The TI phase behavior was investigated with transmission electron microscopy. The order-disorder transition could be observed at a temperature between 200 degrees C (the ordered state) and 150 degrees C (the disordered state) for the block copolymer TI-152K. The block copolymer TI-82K presented the disordered state at 200 degrees C, whereas TI-291K was in the ordered state at 150 degrees C. With the Flory-Huggins interaction parameter between poly(4-trimethylsilylstyrene) and polyisoprene, which was evaluated by small-angle neutron scattering for the block copolymers, the TI phase behavior could be reasonably explained by mean-field theory. (c) 2005 Wiley Periodicals, Inc.