46749-G7
Novel Routes to Well-Defined Conjugated Polymers and Their Block Copolymers
Research overview. The objective of the research supported by this ACS PRF-G grant is to develop a new synthetic methodology for the preparation of well-defined conjugated polymers. During the first year of the project, we focused on the synthesis of new cyclic diene monomers and their controlled polymerization, as precursors for conjugated polymers. Specifically, we synthesized a range of bicyclic monomers with exo diene functionalities and studied their nitroxide-mediated radical polymerization. We also developed a protocol for clean oxidation of the obtained diene polymers to introduce ketone functionalities in the backbone.
Our strategy for the preparation of conjugated polymers consists of three stages: controlled polymerization of cyclic dienes, oxidation of the obtained polymers to install 1,4-diketone groups in the backbone, and subsequent transformation of diketone groups into pyrrole and thiophene units via Paal-Knorr reactions. So far, we have synthesized diene monomers with norbornane and cyclohexane frameworks and studied their polymerization behavior. We identified conditions under which radical polymerization of these monomers proceeded in a controlled manner to provide polymers with defined molecular weights and narrow molecular weight distributions. Importantly, polymerization of these cyclic monomers proceeded almost exclusively via 1,4-addition to install double bonds in the backbone, a necessary precondition for our synthetic strategy. Taking advantage of living polymerization, we were also able to prepare block copolymers of cyclic diene monomers with other monomers amenable to radical polymerization. In the next step, oxidation of backbone unsaturations was investigated as a way to introduce ketone functionalities into the backbone. The use of strong oxidation agents, such as potassium permanganate, osmium tetroxide and ruthenium tetroxide, resulted in the quantitative formation of ketone groups, but often led to overoxidations with concurrent backbone cleavage. This side reaction dramatically decreased molecular weights and broadened molecular weight distributions of the polymers. However, we found that a milder procedure involving epoxidation of the double bonds and subsequent cleavage with periodic acid was more successful and did not lead to overoxidations. We also undertook initial studies of the formation of pyrrole groups from the backbone 1,4-diketone functionalities.
Currently, we are optimizing the oxidation protocol to obtain quantitative conversions without backbone cleavage. We also investigating Paal-Knorr reactions to introduce pyrrole and thiophene group in the polymer backbone. Our preliminary results indicate that this is a very efficient polymer modification reaction that can proceed and very mild conditions. Ultimately, we target preparation of a variety polypyrrole and polythiophene based polymers with narrow molecular weight distributions and their block copolymers.
Significance of research. Our new methodology will provide access to well-defined conjugated polymers and their block copolymers. The self assembly of these block copolymers will result in nanostructured organic materials with semiconducting domains, eagerly sought in the design of more efficient photovoltaics. Also, our synthetic protocol will enable the synthesis of low bandgap polymers unattainable by any other technique. Key synthetic accomplishments described above will serve as a foundation for the new protocol.
Impact of PRF funding. To date, the PRF funding has been used for the acquisitions of materials and supplies necessary for the synthesis and characterization of new conjugated polymers. In the coming year, PRF funds will be used to support one graduate student to devote full-time effort on this project. Thus, the funding will continue to be instrumental in supporting our research team.
