The insertion of soft polysiloxane segments into a polyimide backbone introduces changes in its properties (processability, low surface tension, gas permeability, and lower dielectric constant). Generally, these polyimide-polysiloxane copolymers are synthesized by the condensation of a dianhydride with an aromatic diamine and an amine telechelic polysiloxane, or by transimidization between an aminopyridine-terminated oligoimide and an amine end-capped oligosiloxane. This study investigated another route to obtain perfectly alternating polyimide-polyhybridsiloxane (PI-PHSX) block copolymers. The hydrosilylation, widely studied previously, was performed to elaborate copolymers from an allyl telechelic polyimide and a hydrosilane telechelic polyhybridsiloxane. The use of a telechelic polyhybridsiloxane as a soft segment brought better thermostability and better chemical resistance in comparison with an oligosiloxane based on Si(CH3)(2)-O- units. Using the same allyl telechelic polyimide moiety but varying the size of the hybrid siloxane part, we obtained different PI-PHSX block copolymers, leading to thermoplastic elastomers (TPE). We investigated the effect of the soft-segment length on the thermal resistance, activation energy of thermal degradation, mechanical behavior, and surface properties of a series three PI-PHSX block copolymers containing 36, 54, and 75 wt % polyimide.