2,5,5-Trimethyl-2-trimethylsiloxy-1-oxa-2,5-disilacyclopentane (I), cis- and trans-2,5-dimethyl-2,5-bis(trimethylsiloxy)-1-oxa-2,5-disilacyclopentane (11), and 2-methyl-2,5,5-tris(trimethylsiloxy)-1-oxa-2,5-disilacyclopentane (III) have been prepared from 1,3,5,7-tetramethylcyclotetrasiloxane (D-4(H)) and vinylpentamethyldisiloxane, vinylmethylbis(trimethylsiloxy)silane, and vinyltris(trimethylsiloxy)silane, respectively, by a two-step synthetic process. I, II, and III undergo anionic ring-opening polymerization (AROP) to yield poly [2,2,5-trimethyl-5-trimethylsiloxy-1-oxa-2,5-disila-1,5-pentanylene] (IV), poly [2,5-dimethyl-2,5-bis(trimethylsiloxy)-1-oxa-2,5-disila-1,5-pentanylene] (V), and poly[2-methyl-2,5,5-tris(trimethylsiloxy)-1-oxa-2,5-disila-1,5-pentanylene] (VI), respectively. I-VI have been characterized by H-1, C-13, and Si-29 NMR and IR spectroscopy. Polymer microstructures have been determined by Si-29 NMR. Molecular weights of I-III have been determined by GC/MS and by high-resolution mass spectrometry. Molecular weight distributions of IV-VI have been determined by GPC. M-n's have been independently determined by H-1 N-MR analysis. The glass transition temperatures (T-g's) and thermal stability of these polymers have been determined by DSC and TGA, respectively. Chain transfer apparently limits the molecular weight of VI.