Metal-mediated interfacial cross coupling reactions
Huddleston, Noah Eric
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In this dissertation, the surface-initiated polymerization of conjugated polymers using Kumada-catalyst transfer polycondensation (KCTP) combined with new catalysts is presented. Covalently immobilized films of poly(3-methylthiophene) (P3MT) are fabricated on glass slides coated with indium tin oxide (ITO). The “grafting from” technique has allowed for the fabrication of uniform and mechanically robust conjugated polymer films not previously accessible via other methods. Grafting conjugated polymers using surface-initiated Kumada-catalyst transfer polycondensation (SI-KCTP) requires the use of a highly reactive Ni(0) catalyst, as well as the immobilization of a bidentate phosphine ligand to the substrate. Secondly, this methodology often results in low grafting densities of polymers due to a possible disproportionation reaction at the interface. Palladium-mediated SI-KCTP is introduced as a new method for the development of thin films of P3MT. A Pd(0) catalyst is proposed to decrease the overall disproportionation energy of the system in order to fabricate thicker films with higher grafting densities. The Pd(PtBu3)2 catalyst system effectively increases the grafting densities of the initiated species thereby forcing growing chains to adopt a “stretched” conformation, leading to vertically oriented conjugated polymers adapting a “brush-like” architecture. To further understand the nature of interfacial homocoupling reactions, a re-initiation strategy was explored. This methodology yields valuable insight of the initiator environment through the use of an indirect electrochemical method for the quantification of Ni coverage. Secondly, owing to the chain-growth character of KCTP, a re-initiation strategy is employed as a method to chain extend existing P3MT films. A study is detailed using a Ni(0) catalyst to fabricate thin films of P3MT. The films are subsequently re-initiated with fresh catalyst and further polymerized with P3MT. As an effort to broaden the scope and functional group tolerability of the chain-growth synthesis of conjugated polymers, a new methodology employing the Stille coupling reaction is investigated. Several experiments suggest strong association of the Pd(0) catalyst to the growing polymer chain even at elevated temperatures, a feature that is paramount if a chain-growth mechanism is in operation. In an effort to probe the scope of monomer, several substrates were run under general Stille conditions and analyzed with H NMR spectroscopy to determine whether or not they favor π-complexation. The results of this study indicate that aryl halides with strongly electron-donating substituents favor intramolecular coupling while those with electron-withdrawing substituents favor intermolecular coupling reactions.