A series of narrow molecular weight distribution (MWD) polystyrene-bpoly[methyl(3,3,3-trifluoropropyl)siloxane] (PS-b-PMTFPS) diblock copolymers were synthesized by the sequential anionic polymerization of styrene and trans-1,3,5trimethyl-1,3,5-tris(3',3',3'-trifluoropropyl)cyclotrisiloxan e in tetrahydrofuran (THF) with n-butyllithium as the initiator. The diblock copolymers had narrow MWDs ranging from 1.06 to 1.20 and number-average molecular weights ranging from 8.2 x 103 to 37.1 x 10(3). To investigate the properties of the copolymers, diblock copolymers with different weight fractions of poly[methyl(3,3,3-trifluoropropyl)siloxane] (15.4-78.8 wt %) were prepared. The compositions of the diblock copolymers were calculated from the characteristic proton integrals of H-1 NMR spectra. For the anionic ring-opening polymerization (ROP) of 1,3,5-trimethyl-1,3,5-tris(3',3',3'-trifluoropropyl)cyclotrisiloxane (F-3) initiated by polystyryllithium, high monomer concentrations could give high polymer yields and good control of MWDs when THF was used as the polymerization solvent. It was speculated that good control of the block copolymerization under the condition of high monomer concentrations was due to the slowdown of the anionic ROP rate of F-3 and the steric hindrance of the polystyrene precursors. There was enough time to terminate the ROP of F-3 when the polymer yield was high, and good control of block copolymerization could be achieved thereafter. The thermal properties (differential scanning calorimetry and thermogravimetric analysis) were also investigated for the PS-b-PMTFPS diblock copolymers. (c) 2005 Wiley Periodicals, Inc.