Dual Catalysis in Atom Transfer Radical Polymerization

With the support of the Macromolecular, Supramolecular, and Nanochemistry Program in the Division of Chemistry, Professor Krzysztof Matyjaszewski of Carnegie Mellon University will develop new dual catalytic systems for atom transfer radical polymerization (ATRP). Radical polymerization is industrially used to produce annually more than 100 million tons of polymers worldwide. However, these polymers are ill-defined and serve as commodities rather than specialty materials. ATRP, discovered by Matyjaszewski, is an advanced radical polymerization technique used to prepare uniform polymers with precisely controlled chain lengths, compositions, and shapes. ATRP is also used to prepare bioconjugates and nanocomposites. Professor Matyjaszewski’s team will develop powerful catalytic systems that can operate with very low Cu-based catalyst concentration (in the range of parts per million). However, the catalysts lose activity due to unavoidable radical termination and must be regenerated by various reducing agents such as ascorbic acid (vitamin C) or sulfites. This study aims to develop a dual catalytic system in the presence of sub-stoichiometric amounts of co-catalysts. They will be driven by light or other external stimuli such as electrical current or mechanical forces. This should provide a benign pathway to prepare precisely controlled polymers and also expand the range of monomers polymerized under mild conditions with diminished environmental impact. The new dual catalytic system is expected to also depolymerize polymers back to monomers. In addition to the planned scientific activities, this work will have broader impacts through education and outreach. The broader impact plans include training undergraduate and graduate students in polymer science and educating the public about controlled radical polymerization via instructional webpages and videos. Under this award, the Matyjaszewski research team will explore various organic and inorganic co-catalysts to diminish Cu-catalyst concentration with preserved polymerization control in aqueous media and in open-to-air conditions. This will also facilitate high throughput photoATRP synthesis of bioconjugates in 96-well plates. ATRP is based on a reversible atom transfer process between dormant alkyl halides and growing radicals, catalyzed by redox-active transition metal complexes and now co-catalyzed by reducing agents that will be activated by low energy light (red and near-infrared) and weak tribochemical mechanical forces rather than ultrasound. The compounds used as co-catalysts in Cu-mediated ATRP can act as reducing agents, as supplemental activators and also as reagents enabling oxygen tolerant procedures. The fundamental understanding and optimization of dual and plausibly triple catalytic systems requires a detailed photochemical investigation of kinetics and thermodynamics of the involved reactions using electrochemical, spectroscopic, and computational methods, followed by complete analyses of the obtained polymers. This will primarily include photocatalysts but also other reducing reagents. The effect of temperature on the kinetics and thermodynamics of the involved reactions will be studied in detail because special emphasis will be placed on the depolymerization of polymethacrylates at temperatures below 200 C and temporarily activated by external stimuli. 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.