| This thesis involves the study of the stereochemistry of functionalized silsesquioxanes in nucleophilic substitution, and in the grafting of silsesquioxanes to the polymers. The first part of this thesis includes the investigation of cleavage of Si-O-Si linkage of silsesquioxanes by strong acids (e.g., triflic acid, tetrafluoroboric acid) and bases (e.g., tetraethylammonium hydroxide) that can selectively cleave the frameworks. The incompletely-condensed silsesquioxane frameworks from acid cleavage of Si-O-Si linkage possess good leaving groups, such as the triflate group, for subsequent functionalization via nucleophilic substitution reactions. Fluoride and triflate silsesquioxane derivatives react cleanly with many nucleophiles. Nucleophilic substitution on fluoride-containing frameworks requires strong nucleophiles, such as lithium reagents, and proceeds with retention of stereochemistry, while triflate derivatives are more reactive and generally lead to inversion. In the investigation of acid cleavage of Si-O-Si linkages of the framework (c-C6H 11)6Si6O9 (5) by triflic acid, we discovered the rearrangement of silicon-triflate bonds (i.e., Si-OTf) to form a unique structure of C2-(c-C6H11) 6Si6O8(OTf)2 (20) because a preference for either inversion or retention of configuration in nucleophilic substitution reaction does not lead to a different product. In general, the functionalization of silsesquioxane frameworks via nucleophilic substitution is now very well understood after the study of this thesis. Different stereochemical isomers, which were synthesized in high yields and could be employed in polymer syntheses, can be predicted beforehand if the synthesis of certain isomers is of great interest.; Recent studies of silsesquioxane-containing polymers show thermally enhanced and mechanical reinforcing properties in many thermoset and thermoplastic materials. With such a positive result, the copolymerization of silsesquioxane monomers with commercially available monomers, such as methylmethacrylate and hydride polydimethylsiloxanes, is studied in the second part of this thesis. Two kinds of polymers were investigated: methylmethacrylate-silsesquioxane-containing polymers (i.e., POSS-MMA polymers) and polydimethylsiloxane-silsesquioxane-containing polymers (i.e., POSS-PDMS polymers). POSS-MMA polymers were made via radical polymerization reactions while POSS-PDMS were synthesized via hydrosilylation reaction and condensation reactions of silanols with chlorosilanes. The results showed that the size of the silsesquioxane framework affects the rate of polymerization and the polymer properties, such as reaction time, molecular weight, glass transition temperature and physical appearance. |
