61st Annual Report on Research 2016

This program was aimed to develop chemical transformations that are particularly useful in polymer chemistry, focusing on oxygen-transfer agents and the synthesis of novel ionic liquid building blocks. While our program on controlled oxidations of thiophenes has been successful, the research conducted on the synthesis and properties of a new class of polyelectrolytes based on cyclopropenium ions has been remarkably fruitful. This report will focus on highlighting our efforts in this area, which has now ignited a full research program.

Phase I:

Modular polyelectrolytes have the potential to be transformative in applications such as energy storage and electronic devices, and materials that possess both inherent compositional modularity and accessibility via robust and scalable synthetic pathways are of particular import to the field. To date, development of cationic polyelectrolytes has focused on a limited menu of monomers, most of which bear charge formally localized on heteroatoms and lack chemical handles to tune their physical properties (e.g. imidazolium, ammonium, and phosphonium). The cyclopropenium (CP) ion scaffold could address these challenges, while offering a distinct structural architecture and electronic properties from the aforementioned cationic liquids. This report will focus on the facile synthesis of a series of polymers incorporating cyclopropenium building blocks with various functional groups that acutely affect physical properties, and the applications of these materials. Synthetic routes to cyclopropenium ion-containing monomers are robust and scalable, and these monomers are easily polymerized by reversible addition-fragmentation chain transfer polymerization. Notably, we have developed a modular polymeric precursor that allows access to CP macromolecules via a post polymerization strategy with efficiency levels approaching those attained by click chemistry. Macromolecular assemblies of these materials can be used as ion-conducting membranes that offer mechanical integrity and well-defined conducting paths for ionic transport. Our first papers were published in Nature Communications and ACS Macro Letters in 2015. Future studies are aimed at determining how counter ions impact ionic conductivity in CP-based polyelectrolytes.

Phase II:

Building on our work on using aromatic ions for the synthesis of polyelectrolytes that can be used as a materials platform for various applications, we developed the click functionalization of amine-containing polymers. This work was published in Angewandte Chemie. The primary objective of this research is to develop a new post-polymerization click reaction that provides facile access to cyclopropenium (CP) ion-functionalized macromolecules of various architectures. Traditionally, cationic polyelectrolytes had been primarily restricted to materials bearing formal charges on heteroatoms and their synthesis can be accomplished by polymerization of ionic liquid monomers, or post-polymerization functionalization. In such materials, Coulombic interactions are mainly tuned through variation of the counterion. By contrast, the cyclopropenium ion bears a carbon-centered formal charge, which is delocalized across the tris-amino-stabilized scaffold. This unique feature allows for greatly enhanced tunability of Coulombic interactions, as the amino substituents surrounding the cyclopropenium ion offer convenient handles to tune the physical properties of the polymeric materials. Our work focuses on the modification of a neutral polymer backbone with cyclopropenium-based ionic liquids – a reaction that falls under the realm of click chemistry. This transformation offers a straightforward route to well-defined macromolecules of more complex structure and functionality than is tolerated by conventional controlled polymerization techniques.

The students funded through the PRF grant have been remarkably successful. Spencer Brucks and Andrew Pun were able to recruit their own funds by means of NSF Predoctoral Fellowships, while Jessica Freyer received Honorable Mention by NSF. Since all three students were on TA-ships during their first two years, only their summer salary was paid by the PRF. Jessica Freyer will then became fully funded by this grant, in addition to the summer salary of the other two students. Following their success, we are now taking advantage of our results published in Nature Chemistry, Angewadnte Chemie, and Macromolecules to build a program and recruit funds from national government agencies, aiming for the Army Research Office and the National Institutes of Health.

References:

Jiang, Y.; Freyer, J. L.; Cotanda, P.; Brucks, S. D.; Killops, K. L.; Bandar, J. S.; Torsitano, C.; Balsara, N. P.; Lambert, T. H.; Campos, L. M. 'The Evolution of Cyclopropenium Ions into Functional Polyelectrolytes.' Nat. Commun. 2015, 6, 5950.

Killops, K. L.; Brucks, S. D.; Rutkowski, K. L.; Freyer, J. L.; Jiang, Y.; Valdes, E. R.; Campos, L. M. 'Synthesis of Robust Surface-Charged Nanoparticles based on Cyclopropenium Ions.'Macromolecules 2015, 48, 2519-2525.

Freyer, J. L.; Brucks, S. D.; Gobieski, G.; Russell, S. T.; Yozwiak, C. E.; Sun, M.; Chen, Z.; Jiang, Y.; Bandar, J. S.; Stockwell, B. R.; Lambert, T. H.;* Campos, L. M.* 'Clickable Poly(ionic liquids): A Materials Platform for Transfection.' Angew. Chem. Int. Ed. 2016, 128, 12570-12574.