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One and two-dimensional NMR characterization of organofunctionalized silica gels

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Title: One and two-dimensional NMR characterization of organofunctionalized silica gels
Author: Aroca, Patricia Paulina
Degree Doctor of Philosophy - PhD
Program Chemistry
Copyright Date: 1995
Abstract: An alternative preparation is proposed in this thesis for the synthesis of organofunctionalized silica gels based on the copolymerization of tetraethoxysilane (TEOS) and an organofunctionalized triethoxysilane. The principal analytical techniques used to determine the integrity and distribution of the functionality in the copolymer matrix were solution and solid state NMR. The model copolymer chosen for these studies is methyltriethoxysilane (MTES)/TEOS. One-dimensional 29Si solid state NMR was used to establish the integrity of the system and relative proportions of the different silicon environments present. Two-dimensional ¹H-29Si heteronuclear correlation NMR experiments unambiguously demonstrated that the monomers in the copolymer were “mixed and not phase-separated. To quantify the extent of mixing of the two monomers in the copolymer, kinetic studies were undertaken with the objective of determining reactivity ratios for each monomer. The acid catalyzed hydrolysis and dimer formation reactions of the TEOS and MTES monomers were characterized at a number of catalyst concentrations in order to determine the pH independent kinetic rate constants. With both homopolymers well characterized, the hydrolysis and dimer formation kinetic rate constants were determined for the copolymer. From these kinetic data reactivity ratios were calculated which suggest that the MTES/TEOS system tends to form a random copolymer. One interesting possible application of organofunctionalized silica gels is in the manufacturing of low temperature functionalized glasses. The synthesis of large pieces of glasses from the copolymerization of TEOS and MTES requires the usage of a drying chemical control agent (DCCA) to prevent cracking. The most commonly used DCCA is formamide. Solution NMR was used to determine the integrity and interaction of formamide in the polymerization process. Another step taken to characterize the suitability of these compounds for the manufacturing of low temperature functionalized glasses was to characterize the thermal stability of numerous organic functionalities by thermal analysis techniques and NMR. Of all the functionalities studied the methyl group had the highest thermal stability, up to 700°C when thermally treated under nitrogen, showing promise for the synthesis of methyl functionalized glasses. The cross polarization (CP) dynamics of a number of MTES/TEOS copolymers were then studied, to determine the distance range between the methyl group and the fully condensed unfunctionalized silane (Si(OSi)4). These results further support the conclusion that the functionality in the MTES/TEOS copolymer is quite evenly distributed throughout the copolymer matrix. [Scientific formulae used in this abstract could not be reproduced.]
URI: http://hdl.handle.net/2429/8744
Series/Report no. UBC Retrospective Theses Digitization Project [http://www.library.ubc.ca/archives/retro_theses/]

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