1-Hydrido-1-vinyldimethylsiloxy-3,3,5,5 -tetramethylcyclotrisiloxane (IV) was prepared by reaction of tetramethyldisiloxane-1,3-diol and 1,1-dichloro-1-hydrido-3,3-dimethyl-3-vinyldisiloxane (III). Ring-opening polymerization of IV catalyzed by either triflic acid or dilithium diphenylsilanediolate yields high molecular weight poly[1-hydrido-1-vinyldimethylsiloxy-3,3,5,5-tetramethy 1,3,5-trisiloxanylene] (V), which has an exceptionally low T-g. On the other hand, hydrosilylation polymerization of IV catalyzed by the 1,3-divinyltetramethyldisiloxane platinum complex (Karstedt catalyst) gives high molecular weight alt-copoly(3,3,5,5-tetramethylcyclotrisilloxane-1-1-ylene/2,2-dimethyl-1-oxa-2-sila-1,4-butanylene) (VI). The structures of V and VI were confirmed by IR, H-1, C-13, and Si-29 NMR spectroscopy as well as by elemental analysis. Differences in the microstructure of V, formed by acid- or base-catalyzed ring opening of IV, were detected by Si-29 NMR. The molecular weight distributions of V and VI were determined by GPC. The thermal stability and glass transition temperatures of V and VI were measured by TGA and DSC, respectively. As expected, these polymers are easily cross-linked.