Kinetics of the anionic ring opening copolymerization of 2,4,6-tris(3,3,3-trifluoropropyl)-2,4,6-trimethylcyclotrisiloxane (F-3) with hexamethylcyclotrisiloxane (D-3) was studied in THF using BuLi as the initiator. The apparent reactivity ratios were estimated by computer simulation to r(D3) = 0.10 +/- 0.02, r(F3) = 51.8 +/- 5.5, which were used to predict the copolymer composition. As a result of so much different reactivities, simultaneous copolymerization leads to copolymers of blocky structure containing a narrow intermediate fragment with gradient distribution of siloxane units. Polymers having more uniform distribution of the trifluoropropyl groups along the chain were obtained by the semibatch process, adding F-3 to the polymerization of less reactive D-3. The resulted copolymers were characterized by SEC chromatography, Si-29 NMR, DSC, DMA, and SAXS. Thermal and mechanical properties of the copolymers obtained by simultaneous copolymerization were similar to those of block copolymers. Only the copolymers obtained by semibatch method showed properties typical for gradient copolymers. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1204-1216, 2009