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

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.