The nuclear magnetic resonance spectra of a number of organosilieon compounds are examined. The 7.95 Mcs. silicon-29 spectra are obtained for these compounds and the chemical shifts are considered in detail. It is proposed that the trends observed are the result of a competition between two opposing effects; a paramagnetic shift influenced by changes in the effective nuclear charge on silicon and a diamagnetic shift influenced by (p→d)π bonding. The concept of the competitive shift mechanism rationalizes the chemical shifts observed for series of the type MexSiX₄₋x (where: X = 0Me,0Et, 0Ac, Cl, F and C₆H₅) but does not explain the shifts of silicon hydrides. This concept also rationalizes the trends observed in ¹¹⁹Sn chemical shifts but, logically, does not apply to ¹³C chemical shifts where the d-orbital contribution must vanish. None of the observed coupling constants correlate with the ²⁹Si chemical shifts. Available theories are found to be inadequate to describe the coupling constants in these molecules. A new long range coupling is observed between the methyl groups on silicon attached to oxygen and chlorine with J[subscript]HCSiCH of about 0.35 c.p.s. / Science, Faculty of / Chemistry, Department of / Graduate

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Hybrid inorganic-organic resins may provide higher temperature performance in oxidizing environments than their organic counterparts. Phthalonitrile (PN) polymers are excellent candidates for hybridization due to their high thermal stability and glass transition temperatures and their need for improved long-term oxidative stability and toughness. In this work phenyl-substituted organosilicon linkages were incorporated into PN monomers to investigate their effect on the processing, thermo-mechanical properties, and thermal and oxidative stability. Three hybrid silicon-containing phthalonitrile monomers were synthesized incorporating diphenoxydiphenylsilane, tetraphenylsilane, and hexaphenyldisiloxane moieties. Processability of the polymers was highly dependent on catalyst content and an ideal concentration was determined. The impact on glass transition, coefficient of thermal expansion, stability in TGA, and long-term oxidative stability at 250 °C was evaluated. As-synthesized materials performed significantly better than polymers produced from purified monomers. Degradation of the tetraphenylsilane phthalonitrile monomer was examined in detail via IR-TGA and analysis of aged samples. Multiple degradations were identified involving both the organic and hybrid sections of the polymer. Synthesized materials are compared with commercial phthalonitrile reference materials and to other silicon-phthalonitriles in recent literature. Explanations of behavior and suggestions for future improvements are provided. / PHD / High temperature plastics and plastic composites are needed for electronics and aerospace components. Phthalonitriles (PNs) are one chemistry that shows promise for these applications. PN materials show excellent stability and strength at high temperatures. In this work, the inclusion of silicon-containing linkages into PN plastics was investigated with the intention of improving the properties and long-term stability in air at high temperatures. Three silicon-containing PN compounds were produced. The processing of un-cured resins was characterized and optimized. Resins were then cured at high temperatures. Each polymer’s softening point, thermal expansion, and stability in air and under inert conditions were evaluated. The effect of purity was considered, and it was found that as-produced PN plastics behaved better than highly purified PN plastics. The degradation reactions were studied during long-term exposure to high temperatures and short-term exposure to even higher temperatures. These silicon-containing PN materials were also compared with commercial PN plastics and with other PN literature. Explanations of behavior and suggestions for future improvements are provided.

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The aims of this work were two-fold - to enhance the 'latent functionality' of a silicon centre by expanding the range of functionalisation reactions available to such compounds, and to develop novel silicon-tethered transformations in order to increase the utility of this attractive synthetic strategy. 1. Aryle Silane Oxidation. Building upon the earlier work of Tamao and co-workers, we have developed a mild, functional group-tolerant oxidation of arylsilanes, allowing a wide range of phenols to be readily accessed. One key insight uncovered during this work was the observation that this oxidation could be acheived with sub-stoichiometric quantities of a fluoride promoter, thus allowing several TBS-protected substrates to be oxidised without any concomitant loss of the protecting group. 2. Silicon-Tethering Methodology. In order to utilise our recently acquired expertise in the field of alkoxy arylsilane synthesis, we sought to develop a novel silicon-tethered iron-catalysed biaryl coupling. Unfortunately, despite our considerable efforts, this methodology was found to suffer from reproducibility issues, and thus our attentions subsequently turned to silicon-tethered palladium- and platinum-catalysed processes. These investigations proved to be more fruitful, with the palladium-catalysed methodology affording a small range of silicon-tethered products. Finally, a novel platinum-catalysed hydro-silylation/electrocyclisation cascade was also developed, allowing a substituted arene to be accessed from a dienyne precursor.

Evidence is presented that apparent silene products obtained from the metalation of cyclopentadienyldimethyl - chlorosilane either with tert-butyl1ithium or with methylenetriphenylphosphorane actually arise from the metalated starting material, a silenoid, rather than from a silafulvene intermediate. Trimethylmethoxysi1ane is shown to be an effective trap for dimethylsilafulvene. A new dimethylsilafulvene precursor, bis(dimethylmethoxysi1yl) cyclopentadiene, which gives high yields of dimethyldimethoxysi1ane and the silafulvene at temperatures as low as 240°C is reported.