Polysilicic Acid Esters as Precursors to New Ceramic Materials

162 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1987.Two simple siloxane ester chains, (Si$\sb2$O) (OCH$\sb3$)$\sb6$ and (Si$\sb3$O$\sb2$) (OCH$\sb3$)$\sb8$, and a polysilicic acid ester based on the cubic (Si$\sb8$O$\sb $) core, (Si$\sb8$O$\sb $) (OCH$\sb3$)$\sb8$ were prepared. Synthesis of the di- and trisiloxane esters was achieved by reaction of their commercially available chlorosiloxane precursors with either CH$\sb3$ONO or HC(OCH$\sb3$)$\sb3$. Similar reactions of (Si$\sb8$O$\sb $) Cl$\sb8$, prepared by photochlorination of (Si$\sb8$O$\sb $) H$\sb8$, with CH$\sb3$ONO or HC(OCH$\sb3$)$\sb3$ produce (Si$\sb8$O$\sb $) (OCH$\sb3$)$\sb8$. $\sp{29}$Si NMR spectroscopy was used to confirm the structure of each ester, and in the case of (Si$\sb8$O$\sb $) (OCH$\sb3$)$\sb8$, the structure was determined by single-crystal X-ray analysis.The behavior of these silicic acid esters towards polymerization by hydrolysis-condensation was investigated by $\sp{29}$Si NMR spectroscopy. Spectra obtained demonstrated that for (Si$\sb2$O) (OCH$\sb3$)$\sb6$ and (Si$\sb3$O$\sb2$) (OCH$\sb3$)$\sb8$, monomer degradation had occurred, although at low rates. Similar experiments following the behavior of (Si$\sb8$O$\sb $) (OCH$\sb3$)$\sb3$ showed that these reactions appeared to proceed without cage degradation.Solid-state NMR techniques were used to follow the later stages of gelation and drying. $\sp{29}$Si FTMAS and CPMAS NMR experiments showed that gels prepared from Si(OCH$\sb3$)$\sb4$, (Si$\sb2$O) (OCH$\sb3$)$\sb6$, and (Si$\sb3$O$\sb2$) (OCH$\sb3$)$\sb8$ exhibited similar distributions of Q$\sp2$, Q$\sp3$, and Q$\sp4$ sites. The observance of Q silicon centers demonstrated that degradation of the (Si$\sb8$O$\sb $) cage had occurred. Further distinctions between the gels were made based on spin-lattice relaxation (T$\sb1$) values for the Q$\sb4$ silicons.A series of (Si$\sb8$O$\sb $) -based molecules bearing a single reactive site were synthesized in order to study the condensation behavior of the Q$\sp3$- (Si$\sb8$O$\sb $) centers. (Si$\sb8$O$\sb $) (OSi(CH$\sb3$)$\sb3$) $\sb7$H was prepared from (Si$\sb8$O$\sb $) H$\sb8$ by reaction with (CH$\sb3$)$\sb3$NO and excess (CH$\sb3$)$\sb3$SiCl. Reaction of this monosilane with (CH$\sb3$)$\sb3$SnOCH$\sb3$ produced (Si$\sb8$O$\sb $) (OSi(CH$\sb3$)$\sb3$) $\sb7$OSn(CH$\sb3$)$\sb3$ with essentially quantitative conversion. Hydrolysis of the stannoxy adduct with HCl yielded the stable, crystalline monosilicic acid (Si$\sb8$O$\sb $) OSi(CH$\sb3$)$\sb3$) $\sb7$OH. Silicic acid self-condensation was found to be catalyzed by bases but not by acids. No evidence of cage degraded products were observed. The monosilicic acid was found to readily react with (Si$\sb8$O$\sb $) (OCH$\sb3$)$\sb8$ to yield the mixed dimer ((CH$\sb3$)$\sb3$SiO) $\sb7$ (Si$\sb8$O$\sb $) O (Si$\sb8$O$\sb $) (OCH$\sb3$)$\sb7$. All monomeric and dimeric products were fully characterized by MS and NMR techniques.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD