(608d) The Early-Time Polymerization Kinetics in Acidic Water/Alcohol Solutions of Bis(Trialkoxysilyl)Alkanes with Short Bridging Chains

Bridged precursors are being used extensively for the synthesis of hybrid organic-inorganic materials which have a wide range of applications. Various studies have been performed in the past to develop new synthesis procedures and to observe their effect on the gelation of these monomers. Literature reports have shown that there is a striking difference in the gelation behavior of these precursors when compared to tri- and tetra- functional silanes, but there have not been any detailed polymerization studies to date that address the reason for this behavior. The role of different types of cyclization in the gelation of these silanes is still under speculation. The present study addresses this issue with a kinetic investigation of the polymerization of the simplest bridged hexa-functional silanes with varying alkoxide and bridging organic groups. The precursors under consideration are 1,2 -bis(triethoxysilyl)ethane, 1,2- bis(trimethoxysilyl)ethane and bis(triethoxysilyl)methane.

Quantitative modeling of these silanes is performed and their kinetic trends are compared with their counterpart tri-functional silanes, methyltriethoxysilane and methyltrimethoxysilane. The precursors are reacted with acidic water in the presence of parent alcohol and the transients are monitored in situ using ²⁹Si NMR. These experimental data are used to fit a kinetic model of hydrolytic polycondensation which accounts for first-shell substitution effects, and both intra- and inter- molecular cyclization. Reaction conditions are chosen so that (a) solutions remain homogeneous through out the data collection time, (b) total integrated intensities of the peaks in the NMR spectra remain constant with time, and (c) reactions are slow enough to facilitate the unambiguous assignment of the NMR peaks. Meeting these requirements is complicated by surprisingly strong chemical shift coupling between the ends of the monomers and by microphase separation of the products. Kinetic features of bridged and non-bridged silanes are compared ? especially the importance of (Si-O-)_n rings vs. (-O-Si-O-C_x ) rings.