Oligomer and polymer formation in hexamethylcyclotrisiloxane (D3) -: Hydrosilane systems under catalysis by tris(pentafluorophenyl)borane

Reactions of hexamethylcyclotrisiloxane, D-3, with 1,1,3,3-tetramethyldisiloxane, (MMH)-M-H, 1,1,1,3,3-pentamethyldisiloxane, (MM)-M-H, phenyldimethylsilane and phenylmethylsilane catalyzed by tris( pentafluorophenyl) borane were studied. These reactions lead to ring opening of D-3 by the SiH reactant producing open chain oligomers with hydrosilane functionality at one or both chain ends. The reactivity of the hydrosilanes toward D-3 decreases in the series: PhMeSiH2 > (MMH)-M-H > PhMe2SiH > (MM)-M-H. Competitive self-oligomerization of (MMH)-M-H and H MM also occurs. Primary products of these processes are able to enter into reactions with the SiH and D-3 reactants; some also undergo cyclization. Thus, consecutive and competitive processes lead to a series of various oligohomologues. Gas chromatography in conjunction with chemical ionization mass spectroscopy permitted identification of structure and determination of the basic directions of these oligomerization processes. Polysiloxanes of higher molecular weight may be also formed in some of these systems. The reactions, which occur in the systems studied, are rationalized on the basis of the mechanism involving the hydride transfer from silicon to trivalent boron. This includes the transient formation of tertiary trisilyloxonium borate which decomposes by the hydride transfer to one of the silicon atoms of the trisilyloxonium center.