Triflic acid and P-4-t-Bu superbase both catalyze the ring-opening polymerization (ROP) of 2,2,4,4,6,6-hexamethyl-8,8-diphenylcyclotetrasiloxane (1) to yield a mixture of linear copoly(dimethylsiloxane/diphenylsiloxane) (3:1 mole ratio), low molecular weight oligomer and cyclic siloxanes. The molecular weight distribution, microstructure and thermal properties of the copolymers have been determined. The composition of the low molecular weight fractions have been analyzed. Triflic acid catalyzed ROP of I yields a copolymer whose microstructure can be accounted for by chemoselective ring opening of a single silyloxonium ion intermediate. In addition, 40% of I is recovered. On the other hand, P4-t-Bu superbase catalyzed ROP of I yields a copolymer whose microstructure is random. The low molecular weight fraction, in this case, is made up of oligomer and a number of cyclotetra-, cyclopenta- and cyclohexasiloxanes. (C) 2003 Elsevier Science Ltd. All rights reserved.