The synthesis of unsaturated poly(carbosiloxane)s, a new class of siloxane polymers possessing a perfectly alternating siloxane and alkenylene main chain, is presented. 1,5-Bis(allyl)-1,1,3,3,5,5hexamethyltrisiloxane (4), l,3-bis(4-pentenyl)tetramethyldisiloxane (6), 1,1,3,, and telechelic a,a)-di-4-pentenylpoly(l,l,3,3-tetramethyldisiloxanylpentylene) (10) undergo acyclic diene metathesis (ADMET) polymerization catalyzed by [(CF.ihCHsCOLUV-^.e-CeH.ri-Pr2)Mo=CHC(CH:i)2Ph (1). These polymerizations, which are performed under bulk conditions and at low temperatures, continuously release ethylene to give poly(l,l,3,3,5,5-hexamethyltrisiloxanyl-2-butenylene) (5), poly(1,1,3,3-tetramethyldisiloxanyl-4-octenylene) (7), poly(1,1,3,3-tetramethyldisiloxanyl-p-phenylenel,l,3,3-tetramethyldisiloxanyl-2-butenylene) (9), and poly(l,1,3,3-tetramethyldisiloxanylpentylene-co-l,1,3,3tetramethyldisiloxanyl-4-octenylene) (11), respectively. Bis(vinyl)tetramethyldisiloxane fails to homopolymerize under ADMET conditions, and bis(allyl)tetramethyldisiloxane (2) releases ethylene when catalyzed by 1 in the absence of solvent, to give, exclusively, the ring-closed product, 1,1,3,3-tetramethyldisiloxacyclohept-5-ene (3). Extending the methylene units or the siloxane linkage in the monomer results in facile ADMET polycondensation, in essentially quantitative conversions, affording well-defined, low-Tg, linear polymers with known vinylic end groups. The polymerizations are void of competing reactions except when back-biting reactions are favorable. Polymer 5 undergoes back-biting to generate the nine-membered cyclic silaoxalkene 13, when active polymer is diluted or, to a lesser extent, when in the bulk. All monomers and polymers were characterized by infrared spectroscopy and , 13C, and 29Si NMR spectroscopy. Number average molecular weights were determined by gel permeation chromatography and quantitative 13C NMR end group analysis when possible. Synthesis, characterization, thermal analysis, and the current scope of this polymerization are discussed.