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The central goal of our PRF-funded research project is to use insights gained through mechanistic studies to develop improved polymerization methods for conjugated polymers. Organic π-conjugated polymers are promising materials for thin-film solar cells, light-emitting diodes, and transistors. Our focus is on developing controlled, chain-growth methods for polymer synthesis. 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. However, efforts toward expanding the scope and utility of this method have been hindered by the highly monomer-specific reaction conditions necessary to achieve chain-growth. As a result, even simple block copolymers have been difficult to synthesize. 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, 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. In the second year of funding, we explored the effect of ligand structure on the mechanism by performing rate and spectroscopic studies on the Ni(dppp)Cl₂-catalyzed polymerization of 2,5-bis(hexyloxy)benzene and 3-hexylthiophene. We found that the rate-determining step is transmetallation, indicating that modifications to the ligand structure results in a change in the rate-determining step. Our observation of a rate-determining reductive elimination (with dppe) and rate-limiting transmetallation (with dppp) indicate that the proposed key intermediate - a Ni⁰ π-complex - if formed, is only a fleeting, post-rate-limiting intermediate in these polymerizations. We also showed that an additive (LiCl) reported to be beneficial in controlled polymerizations of 2,5-bis(hexyloxy)benzene had little effect on the molecular weight distribution when either catalyst was pre-initiated with 5 equiv of monomer. Nevertheless, the influence of LiCl on the polymerization rate depended on which catalyst was used. For example, LiCl had no effect on the rate with dppe whereas it led to rate accelerations and attenuations with dppp. Given the important mechanistic influence of ligand, these results suggest that modifying the ligand structure may lead to new catalysts that are effective for a broader range of monomers. However, to rationally select new ligands, it will be necessary to understand the influence of ligand structure on the stability and reactivity of key intermediates – including the hypothesized Ni⁰-polymer π-complexes  – and the competing reaction pathways. As a result, our future efforts (funded through a recent NSF CAREER award) are largely focused on identifying ligands with improved substrate scope using mechanistic studies to guide the ligand design. Altogether, the PRF-funded results provided a strong foundation for future studies aimed at developing improved catalysts and preparing novel polymers.

This research project has positively impacted the careers of the involved researchers in the following ways: (1) The PRF-funded studies have resulted in two publications (J. Am. Chem. Soc. and Macromolecules) and another manuscript is in preparation. (2) The students involved 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 successful NSF CAREER grant application. (4) Both the PI and graduate students have presented these results at several national and international conferences, including the 2009 National Organic Symposium (Boulder, CO), the 2009 UM-Nagoya University Joint Symposium (Nagoya, Japan), the 2009 Gordon Research Conference on Polymers (Mt. Holyoke, MA) and the American Chemical Society National Meetings (Philadelphia, PA, and San Francisco, CA) and Central Regional Meeting (Cleveland, OH).