Initiation Strategies From Common Functional Groups in Photocontrolled Polymerizations

Cornell University, Ithaca NY

Investigators

Abstract

With the support of the Macromolecular, Supramolecular, and Nanochemistry program in the Division of Chemistry, Brett P. Fors of Cornell University is developing initiation strategies from common chemical functional groups to prepare polymers with controlled structures. Polymers, or long chains consisting of repeating units derived from monomers, are essential and ever-present in our everyday lives. We encounter them in industrially manufactured plastics, naturally occurring cellulose in plants, and even plant-derived materials like cotton. Controlled polymerizations have revolutionized the way the scientific community makes polymeric materials, enabling the synthesis of well-defined macromolecules in which the size and functionality can be precisely tailored. However, controllably grafting polymers from small molecules, biomolecules, or other polymers, requires the synthesis of initiating species on those molecules. This typically involves tedious multistep syntheses that are inefficient and often the limiting factor in the plastics that one can access. This research will utilize photocatalysis to directly initiate radical polymerizations from common functional groups. Specifically, polymerization processes that are directly grafted from either hydridic C–H bonds or carboxylic acids will be developed and studied. The ability to graft polymers from C–H bonds will offer an opportunity to circumvent pre-functionalization, holding opportunities in polymer upcycling and surface modification. Polymer initiation from carboxylic acids will open an avenue to controlled and direct site-selective formation of protein-polymer conjugates for potential applications in protein therapeutics. Additionally, a photocontrolled radical polymerization using a simple nitroalkane initiator will be examined. The nitroalkane imposes unique reactivity on the polymer chain end, allowing the pairing of radical and anionic processes to make well-defined graft polymers. Overall, the initiation strategies in this research are expected to provide new avenues for the synthesis of advanced functional polymers. This work will offer a substantial training opportunity for students in Ithaca, New York that represents a cross-pollination between polymer chemistry and photocatalysis. It will expand the existing educational outreach activities to include virtual laboratory tours and polymer science tutorial videos. These approaches will distribute high-quality remote STEM (science, technology, engineering and mathematics) educational materials to schools across the country and extend the impact of the research beyond the scientific community to the general public. This research will focus on the development of initiation strategies from common functional groups in photocontrolled polymerizations. In the first objective, photocontrolled radical polymerizations initiated selectively from hydridic C-H bonds will be developed to circumvent challenges of initiator synthesis and enable the streamlined formation of functional polymers. Model studies will include dioxane as the hydrogen-atom source and methyl acrylate as the monomer in reversible addition-fragmentation chain-transfer (RAFT) polymerization. In the second objective, the methodology will be extended to carboxylic acid-based initiators. Strong emphasis will be placed on understanding the catalytic cycle including the oxidative decarboxylation and chain capping steps and the position of RAFT equilibrium. Lastly, nitroalkane initiators will be explored as alternatives to conventionally used alkyl bromides in atom transfer radical polymerization (ATRP). This approach to new chain-end functionality will take advantage of reversible mesolytic cleavage of a nitroalkane to generate an alkyl radical and corresponding nitrite anions. This work represents an important new approach to the initiation of controlled polymerizations, with the potential to directly graft polymers from a vast array of commodity, specialty, macro, and biological molecules without pre-functionalization. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.