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 cyanide-mediated 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 Mg2+. As such, the tandem benzoin-oxidation polymerization reported herein represents a sustainable method for the synthesis of highly tunable and redox-active organic materials.