(273e) Dynamic Monte Carlo Simulation of Sol-Gel Polymerization of Bridged Alkoxysilanes

This study deals with polymer structure and gelation during sol-gel polymerization of alkylene-bridged alkoxysilanes such as bis(triethoxysilyl)ethane (BTESE). These precursors are of expanding interest for forming organic-inorganic hybrid materials because the structure and properties of the materials synthesized from them can be customized to suit many applications of porous materials. Experimental studies of the reaction kinetics of these precursors have proven to be challenging not only due to the large number of polymerization pathways possible due to their multifunctional nature, but also because of difficulty preventing loss of signal and identifying specific species by ²⁹Si NMR. FTIR spectroscopy is a viable approach, but it does not provide a high level of information about the molecular structure of oligomers. Thus, while reaction pathways have been proposed for these silanes in the literature, there have been no direct in-situ studies to confirm them. In the present study we investigate the implications of organic bridges between silane polycondensation sites using dynamic Monte Carlo simulations. These simulations include the first shell substitution effects and siloxane cyclization reactions known to be important features of polymerization of non-bridged silanes. Here, we model two contradictory effects of introducing permanent alkylene bridges between silane sites. First, the bridges can facilitate gelation by forming connected pathways between the highly cyclic, cage-like units formed by the silane sites. On the other hand, certain bridged silanes (such as BTESE) are able to form carbosiloxane rings involving one or two monomeric units. The resulting cyclic monomer and bicyclic dimers then serve as building blocks for slow gel formation. The two effects lead to a maximum in gel time with respect to alkylene bridge length.