Interfacial cross-coupling reactions
Sontag, Stephen Kyle
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In this thesis, the surface-initiated polymerization of conjugated polymers using Kumada Catalyst-transfer Polycondensation (KCTP) is presented. Covalently immobilized films of conjugated polymers are fabricated on a variety of substrates and extensively characterized using spectroscopic and electrochemical techniques. This “grafting from” technique has allowed for the formation of smooth, covalently immobilized, and mechanically robust conjugated polymer films, which is not possible to obtain using grafting to and grafting through methodologies. The grafting of conjugated polymers requires the immobilization of phosphine ligated nickel catalysts to form the polymerization initiator, which is a difficult task due to the two-dimensional nature of the interface and the close proximity of neighboring reactive sites. A technique of characterizing the grafting density using indirect electrochemical methods is presented, which has allowed for the first time to probe the behavior of surface-bound nickel species. It was found that the grafting density changes based on the ligand environment, and side reactions between neighboring initiating sites is the primary cause of the observed low grafting density upon exchange to the necessary bidentate phosphine ligands for polymerization. Furthermore, kinetic information is also obtained from the technique and it was found that the rate of cross coupling with the immobilized initiators is dependent on the ligand environment. To further understand the oxidative addition of Ni(0) to haloarenes, a series of kinetic isotope effect (KIE) experiments were carried out. Simple haloarenes were coupled to alkyl or aryl Grignard reagents via nickel catalysts, and analyzed for 13C enrichment using inverse gated decoupling NMR. It was found that the first irreversible step of the coupling reaction is dependent on the type of halide and position of the methyl substituent on the arene. The results presented here rationalize observations at surfaces, and also provide additional insight into the highly debated mechanism of organometallic oxidative addition reactions.