Anionic and cationic ring-opening polymerizations of two novel cyclotrisiloxanes, tetramethyl-1-(3'-trifluoromethylphenyl)-1-phenylcyclotrisiloxane (I) and tetramethyl-1-[3',5'-bis(trifluoromethyl)phenyl]-1-phenylcyclotrisiloxan e (II), are reported. Anionic ring-opening polymerization of I or II leads to copolymers with highly regular microstructures. Copolymers obtained by cationic polymerizations of I or II, initiated by triflic acid, have less regular microstructures characteristic of chemoselective polymerization processes. The composition and microstructure of copolymers have been characterized by H-1 and Si-29-NMR, the molecular weight distributions by GPC, and the thermal properties by DSC and TGA. (C) 2004 Wiley Periodicals, Inc.