60th Annual Report on Research 2015

This program aims 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 to collect sufficient data for the submission of our first proposal this Fall.

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.

Our exploration of the cyclopropenium functional group in the context of cationic polyelectrolytes was originally inspired by its ionic liquid properties and the straightforward elaboration of the CP ion with various functional groups. Derivatives of the CP ion are made from inexpensive reagents and can be easily prepared on a multi-gram scale under ambient conditions. Notably, synthetic routes to aminocyclopropenium derivatives are modular and highly scalable.

After synthesizing block copolymers of various compositions, we characterized the morphology of bulk films comprising various CPR building blocks. Recent studies suggest that nanostructured BCPEs have broad implications in materials chemistry, specifically for fuel cells and batteries, if they undergo microphase segregation. Interestingly, we found that our materials form cylindrical morphologies, where the matrix is the polyelectrolyte. Moreover, we found the ionic conductivity to be quite high, 4 x 10^-3 S/cm at room temperature. Comparing these initial results to those reported in literature, we found that the conductivity of our system is higher than those previously reported for solid polyelectrolytes (10^-7-10^-5 S/cm at RT), and in the regime of ionic liquids and gels (10^-5-10^-2 S/cm at RT).

Building on this foundation, we will modulate polymer composition by varying the functional groups decorating the CP moieties to include hydrophilic units, such as ethylene glycol, which could suppress glass transition temperature and in turn improve ion mobility. Furthermore, we will study anion metathesis for bulkier, plasticizing ions, which can have a large impact on the thermal and mechanical properties of the polymer, effectively broadening the temperature window in which materials can be processed. Thus, ion exchange may have implications for the application of these materials in energy conversion technologies. We will exchange the native chloride counterion in a PS-b-PCPiP block copolymer for anions with various structural and electronic characteristics, such as hexafluorophosphate, tetrafluoroborate, and bis(trifluoromethane)sulfonamide (TFSI). Finally, we have begun exploring new chemistries to efficiently couple the CP ionic liquid moiety to amine-functional polymers.

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. Next year, Jessica Freyer will be fully funded by this grant, in addition to the summer salary of the other two students. Following their success, we will take advantage of our results published in Nature Chemistry 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.