(162r) NMR Investigation of Polymerization-Induced Microphase Separation in Bis(Triethoxysilyl)Ethane

Bis(triethoxysilyl)ethane(BTESE) is an alkylene bridged precursor that has specific applications in the fields of coatings and adhesives, and in the synthesis of periodic mesoporous materials. It is reported that BTESE is one of a few precursors exhibiting a striking discontinuity in gelation behavior when compared to other members of the homologous series of bis(trialkoxysilyl)alkanes. Intra-molecular cyclization is suspected to be the cause of this anomalous behavior. However, our studies on the polymerization kinetics of BTESE have revealed additional interesting phenomena. BTESE in the presence of acidic water and ethanol is found to be prone to gradual NMR signal loss at certain initial concentrations. Because no macroscopic phase separation or settling is apparent, we hypothesize that this is the result of the formation of a microphase in the reacting solution. The intermediate silica species may form colloidal droplets that do not tumble quickly enough to average out dipolar coupling and chemical shift anisotropy, which leads to signals too broad to be detected. Another likely cause is the dramatic change in the scalar couplings and the relaxation times of the NMR active nuclei which disturb their relaxation mechanism and in turn the observed signal intensities. In order to enhance the sensitivity of the nuclei causing signal loss and also to investigate the role of the above mentioned parameters in signal loss, various NMR pulse sequences and techniques are utilized. The effect of temperature, solvent and concentration of the monomer on the couplings is examined. It is observed that BTESE exhibits unexpected coupling patterns, the analysis of which could at least in part explain its anomalous behavior. In order to aid this interpretation, other related precursors, bis(trimethoxysilyl)ethane, bis(triethoxysilyl)methane, bis(triethoxysilyl)ethene, bis(trichlorosilyl)ethane, bis(trimethoxysilyl)hexane and methyltriethoxysilane are also investigated using the same NMR techniques. Molecular modeling techniques are employed to gain insights into the structure and NMR spectra of these materials.