Unexpected Direct Synthesis of Tunable Redox-Active Benzil-Linked Polymers via the Benzoin Reaction

Abstract

Strategies for the sustainable synthesis of redox-active organic polymers could lead to next-generation organic electrode materials for electrochemical energy storage, electrocatalysis, and electro-swing chemical separations. Among redox-active moieties, benzils or aromatic 1,2-diones are particularly attractive due to their high theoretical gravimetric capacities and fast charge/discharge rates. Herein, we demonstrate that the cyanide-catalyzed polymerization of simple dialdehyde monomers unexpectedly leads to insoluble redox-active benzil-linked polymers instead of the expected benzoin polymers, as supported by solid-state nuclear magnetic resonance spectroscopy and electrochemical characterization. Mechanistic studies suggest that cyanidemediated benzoin oxidation occurs by hydride transfer to the solvent and that the insolubility of the benzil-linked polymers protects them from subsequent cyanolysis. The thiophene-based polymer poly(BTDA) is an intriguing organic electrode material that demonstrates two reversible one-electron reductions with monovalent cations such as Li + and Na + but one two-electron reduction with divalent Mg 2+ . As such, the tandem benzoin-oxidation polymerization reported herein represents a sustainable method for the synthesis of highly tunable and redox-active organic materials.

Redox-Active Organic Materials: From Energy Storage to Redox Catalysis

“…5,127−129 However, solid-state CVs typically are performed at much slower sweep rates (0.05−1 mV/s) to allow for the diffusion of ions and the conduction of electrons through the material to access the redoxactive sites. 28,130−133 Although fast sweep rates have been used with insoluble polymers, 128 only qualitative observations such as retention of redox-activity over many CV cycles and changes in behavior with respect to the counterion can typically be examined in this way. Arguably, the most significant difference between solution-and solid-state electrochemistry is that heterogeneous materials require charge compensation via the significantly slower diffusion of counterions into a solid-state matrix.…”

“…One key difference in CV measurements between soluble and insoluble organic compounds is the sweep rate (ν). Solution-state measurements typically are carried out at sweep rates of 10–100 mV/s or higher. ,− However, solid-state CVs typically are performed at much slower sweep rates (0.05–1 mV/s) to allow for the diffusion of ions and the conduction of electrons through the material to access the redox-active sites. ,− Although fast sweep rates have been used with insoluble polymers, only qualitative observations such as retention of redox-activity over many CV cycles and changes in behavior with respect to the counterion can typically be examined in this way.…”