The synthesis of perfectly alternating fluorinated polyimide-fluorinated polyhybridsiloxane block copolymers (FPI-FPHSX) was achieved through polyhydrosilylation of alpha,omega-diallylfluorinated polyimides (AT-FPI) and alpha,omega-dihydrosilane fluorinated-polyhybridsiloxanes (HT-FPHSX). A series of three FPI-FPHSX containing 15, 38, and 56 wt% of polyimide was synthesized and characterized by tuning the number-average molecular weight either of the hard polyimide segments or of the soft polyhybridsiloxane segments. The influence of the soft and hard segment lengths on the behavior of the thermoplastic elastomer material was studied (hardness, surface tension, thermal stability). The FPI-FPHSX block copolymers thermomechanical properties are also reported. (C) 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 200-207, 2004.