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The central goal of our PRF-funded research project is to use insights gained through mechanistic studies to develop nickel-catalyzed chain-growth polymerization methods for conjugated polymers. In chain-growth polymerizations, an initiator reacts with a monomer to start polymerization and subsequent monomer additions occur solely at the chain end. This method allows the polymer molecular weight to be controlled by the ratio of monomer to initiator. In addition, the distribution of molecular weights is narrow and copolymer microstructure can be controlled by the relative reactivity of the monomers and their rate of addition to the reaction. Chain-growth syntheses of conjugated polymers were first reported in 2004 using nickel catalysts. This method has since been modified to polymerize a small set of other monomers, including (2,5-bishexyloxy)phenylene, 9,9-dioctylfluorene, (2,3-dihexyl)thienopyrazine, N-octylcarbazole, 3-alkoxythiophene, and N-hexylpyrrole. However, each monomer has required empirical development of unique reaction conditions to achieve chain growth. Preliminary attempts at synthesizing block copolymers have highlighted the challenges involved when each monomer requires highly specific conditions. In order to rationally expand this methodology, and to develop improved catalysts with a broader substrate scope, a detailed understanding of the reaction mechanism is essential.

In the first year of funding, our research was focused on understanding the mechanistic influences of ligand, monomer, and additives on these transformations. Specifically, we performed rate and spectroscopic studies on the polymerization of 2,5-bis(hexyloxy)benzene and 3-hexylthiophene catalyzed by Ni(dppe)Cl₂. We found strong evidence for rate-determining reductive elimination for both monomers, indicating that the monomer structure (arene versus thiophene) has no influence on the rate-determining step of the catalytic cycle. Notably, McCullough found evidence for a rate-determining transmetallation for polymerization of 3-hexylthiophene using a different catalyst – Ni(dppp)Cl₂ (Macromolecules 2005, 38, 8649-8656). Combined, these results point to a significant mechanistic influence of the ligand on the polymerization. Moreover, these results suggest that alternative ligand structures may lead to catalysts with improved reactivities. Our observation of a rate-determining reductive elimination reaction indicate that the proposed key intermediate - a Ni⁰ π-complex - if formed, is only a fleeting, post-rate-limiting intermediate. Moreover, our extensive spectroscopic studies identified the catalyst species both during and after polymerization (i.e., Ni^II-biaryl/Ni^II-bithiophene complexes and Ni^II(polyarene)halide/Ni^II(polythiophene)halide complexes, respectively). We also showed that an additive (LiCl) reported to be beneficial in controlled polymerizations of 2,5-bis(hexyloxy)benzene had no effect on the rate-determining step or on the molecular weight distribution when the catalyst was pre-initiated with 5 equiv of monomer. Altogether, these results provide a strong foundation for future studies aimed at preparing novel polymers and developing improved catalysts.

This research project has positively impacted the careers of the involved researchers in the following ways: (1) These PRF-funded studies have thus far resulted in one publication (a full paper currently in revision). (2) The student involved has gained highly interdisciplinary training in organic synthesis, inorganic synthesis, polymer synthesis and characterization, organometallic reaction mechanisms, kinetics, and spectroscopy. (3) The data generated through these studies were used as preliminary results in a pending NSF CAREER grant application (submitted in July 2009). (4) Both the PI and graduate student have presented these results at several conferences, including the 2009 National Organic Symposium (Boulder, CO) and recent American Chemical Society National (Philadelphia, PA) and Regional (Cleveland, OH) Meetings.