A mechanistic investigation of gelation. The sol-gel polymerization of precursors to bridged polysilsesquioxanes

The study of a homologous series of precursors to bridged polysilsesquioxan es has uncovered striking discontinuities in gelation behavior. An investig ation of the chemistry during the early stages of the polymerization has pr ovided a molecular basis for these observations. Monomers containing from o ne to four bridging carbon atoms exhibit a pronounced tendency to undergo r apid intra- or bimolecular cyclization. The cyclic and bicyclic intermediat es have been characterized by Si-29 NMR spectroscopy, chemical ionization m ass spectrometry, and isolation from the reaction solution. These carbosilo xanes are local thermodynamic sinks that produce kinetic bottlenecks in the production of high-molecular-weight silsesquioxanes. The formation of cycl ic carbosiloxanes results in slowing, or in some cases completely shutting down, gelation. An additional finding is that the cyclic structures are inc orporated intact into the final xerogel.