Poly(diethylsiloxane) and its copolymers with various kinds of R1R2SiO (R-1 = R-2 = methyl or phenyl, or R-1 = methyl and R-2 = phenyl) units have been prepared by the equilibrium polymerization of cyclosiloxanes. All the polymers have been characterized by H-1 and Si-29 NMR, gel permeation chromatography, differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA) measurements. The results indicate that a random distribution of different units has been obtained in the structures of copolymers containing 50 mol % diethylsiloxane units content. DSC and DMA show that the presence of 2.5 mol % diphenylsiloxane units or 5.0 mol % methylphenylsiloxane units in the copolymer can disrupt the crystallinity and lead to noncrystalline copolymers with low glass-transition temperatures (ranging from -133 to -137 degreesC according to DSC). (C) 2003 Wiley Periodicals, Inc.