1-[2'-(Heptaphenylcyclotetrasiloxanyl)ethyl]-1,3,3,5,5-pentamethylcyclot etrasiloxane (II) was prepared from 1-[2'-(methyldichlorosilyl)ethyl]-1,3,3,5,5,7,7-heptaphenylcyclotetrasil oxane (I) and tetramethyldisiloxane-1,3-diol. Acid-catalyzed ring-opening of II in the presence of tetramethyldisiloxane gave 1,9-dihydrido-5-[2'(heptaphenylcyclotetrasiloxanyl)ethyl]nonamethylpenta siloxane (III) and 1,9-dihydrido-3-[2'-(heptaphenylcyclotetrasiloxanyl)ethyl]nonamethylpent asiloxane (IV). Both acid- and base-catalyzed ring-opening polymerization of II gives highly viscous, transparent polymers. The structures of I-TV and polymers were determined by UV, IR, H-1, C-13, and Si-29 NMR spectroscopy. In addition, molecular weights obtained by GPC and NMR end group analysis were confirmed with mass spectrometry. On the basis of Si-29 NMR spectroscopy, the polymers appear to result exclusively from ring-opening of the cyclotrisiloxane ring. No evidence for ring-opening of the cyclotetrasiloxane ring was observed. Polymer properties were determined by DSC and TGA. (c) 2005 Wiley Periodicals, Inc.