Thermally-Induced in Situ Synthesis of Ethylene-Linked Viologen Ionic Radical Polymers for Photocatalytic CO2 Cycloaddition

Abstract

This study presents a facile one-pot quaternization strategy for constructing conjugated ethylene-linked viologen ionic radical polymers (designated as EVIRPs) to enable visible-light-enhanced photocatalytic CO₂ cycloaddition under ambient conditions. The optimized polymer EVIRP-180 was synthesized by thermally induced in situ quaternization between commercially available monomers 1,2-bis(4-pyridyl)ethylene (BPE) and 1,2,4,5-tetrakis(bromomethyl)benzene (TBMB) in the high-boiling-point solvent N-methylpyrrolidone (NMP) at 180 °C for 24 h without requiring external catalysts and reducing agents. Remarkably, electron paramagnetic resonance (EPR) spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses revealed a temperature-dependent enhancement of radical intensity, with EVIRP-180 exhibiting stronger radical signals compared to the control polymers EVIRP-100 and EVIRP-140 prepared at lower temperatures (100 and 140 °C, respectively). This phenomenon arises from two synergistic effects: (1) a higher temperature promotes the formation of more ethylene-linked viologen ionic radicals via a thermally induced process; (2) the solvent NMP can be partially converted into activated NMP (denoted as NMP*) at elevated temperatures, which serves as an effective reducing agent for facilitating one-electron reduction of viologen dications to cationic radicals. The optimized polymer EVIRP-180 demonstrated an enhanced visible-light-harvesting ability and superior photoinduced charge transfer capability. As a metal-free heterogeneous photocatalyst, EVIRP-180 achieved exceptional photocatalytic efficiency in the photocatalytic cycloaddition of CO₂ and epoxides to cyclic carbonates under ambient conditions (room temperature, atmospheric pressure) without using cocatalysts or solvents. This work establishes a sustainable pathway for developing multifunctional ionic radical polymers as efficient photocatalysts for CO₂ conversion under ambient conditions.