The ring-opening polymerization of 2,4,6,8-tetramethyltetrahydro-cyclotetrasiloxane, D-4(H), in the presence of tris(pentafluorophenyl)borane, B(C6F3)(3), has been studied in dilute toluene solution. The mechanism of this polymerization was investigated. The initiation and propagation steps are the B(C6F5)(3) catalyzed ring-opening of D-4(H), by the SiH group resulting in its addition to a growing chain, which occurs with hydride ion transfer to the chain end. Since there are multiple SiH groups present in the monomer, the formed polymer has extensive branching. Although the bulk polymerization leads to gel formation at an early stage of the process, no gelation is observed when the reaction is performed in 10 wt % solution in toluene. This unexpected observation is due to two reaction features. First, an extensive cyclization occurs leading to a closed multicyclic polymer structure. Second, the initiation leads to the formation of very reactive -OSiMeH2 chain end, which is the real propagation center. Branching does not occur in the propagation but in the initiation and termination steps. Gaseous MeSiH3 is released in the latter step.