Ring-opening polymerization (ROP) of 2,2,4,4,6,6-hexamethyl-8,8-divinylcyclotetrasiloxane (I) initiated by both 1-tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino)phosphoranylidenamino]-2lambda(5),4lambda(5)-catenadi(phosphazene) (C22H63N13P4, P-4-t-Bu superbase) and trifluoromethanesulfonic acid (CF3SO3H, triflic acid) has been studied. Both reactions lead to mixtures of linear copolymer, low molecular weight co-oligomers and monomeric cyclosiloxanes. The composition, molecular weight distribution, microstructure, and thermal properties of the copolymers have been determined. The copolymer microstructure has been determined by Si-29 NMR spectroscopy. Monomeric cyclosiloxanes have been identified by GC/MS. Both copolymer microstructure and cyclosiloxanes formed depend on the particular catalyst system utilized. P-4-t-Bu superbase-initiated anionic ROP of I leads to a copolymer with a random microstructure and to a series of monomeric cyclotetra-, cyclopenta-, and cyclohexasiloxanes formed by random combination of dimethylsiloxane (D) and divinylsiloxane (V) units. On the other hand, triflic acid-initiated ROP of I occurs in a chemoselective manner. This leads to a copolymer with a more ordered microstructure. In this case, I is the only monomeric cyclosiloxane found.