Polysilicic Acid Esters as Precursors to New Ceramic Materials

Millar, Dean Michael

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Description

Title

Polysilicic Acid Esters as Precursors to New Ceramic Materials

Author(s)

Millar, Dean Michael

Issue Date

1987

Doctoral Committee Chair(s)

Klemperer, Walter G.

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Chemistry, Inorganic
• Engineering, Materials Science

Abstract

• 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.

Graduation Semester

1987

Type of Resource

text

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