Sol-gel polymerization studied through silicon-29 NMR with polarization transfer

Silicon tetramethoxide (TMOS) monomers have been dissolved in mixtures of methanol and water, hydrolyzed, and condensed to form siloxane polymers. The various oligomers which form during the early stages of the reaction have been identified through high-resolution {sup 29}Si NMR with polarization transfer. At low water/alkoxide ratios the hydrolysis is incomplete and limits the condensation reaction. Small cyclics are formed in addition to linear polymers. At high water/alkoxide ratios, the hydrolysis is fast and complete and the condensation is rate limiting. The polymers are largely branched as indicated by the high proportion of Q{sup 3} chain branching sites and Q{sup 4} three dimensionally cross-linked groups. So highly condensed species including small rings or cagelike structures can be expected in these mixtures.