Fundamental Charge Transfer Processes in Stable Free-Radical Organic Polymer Systems

Abstract

Polymers with stable pendant radical groups are a unique class of redox-active materials emerging as potentially the next generation energy storage breakthrough. These polymers facilitate apparently remarkably rapid, efficient and reversible multi-step charge-transfer processes. The focus of this project is to advance the fundamental understanding of the structure-property relationships associated with the mechanism(s) of electron transfer and ion transport, along with associated interfacial mass-transfer processes that impact the charge-transfer processes of a unique class of organic free-radical polymeric redox active materials. The project involves an integrated approach of chemical synthesis, electrochemistry, spectroscopy and theoretical modeling of a series of stable organic radical materials. Initially, we have focused on the 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) organic radical moiety incorporated into a complex materials set of poly(4-methacryloyloxy-2,2,6,6-tetramethylpiperidine-N-oxyl) (PTMA). We focused our work to probe, and establish, the charge-transfer mechanisms of a series of the TEMPO-containing macromolecules with varying chain lengths and side-chain compositions. We have investigated the model compound, 4-oxo-2,2,6,6-tetramethylpiperidine-N-oxyl (4-oxo-TEMPO), and compared the results to those for non-cross-linked and cross-linked PTMA oligomers and polymers with: (i) various oligomer chain-lengths; and (ii) controlled densities of the TEMPO radical moieties along the chain (i.e. with some TEMPO-methacrylate units replaced by methyl methacrylate). Our presentation will discuss a complete electrochemical analysis of the TEMPO polymeric materials. The discussion will include the correlation of electron-transfer rates and ionic mobility in various electrochemical solvents, and the role that radical content has on both the heterogeneous electron-transfer rate and the self-exchange charge transfer mechanism of the various materials.