Urea and Thiourea-Functionalized, Pyridinium-Based Ionic Polymers Convert CO₂ to Cyclic Carbonate under Mild Conditions

The employment of hydrogen-bond donors has become an important metal-free method for activating organic molecules considering their competency in epoxide activation and stabilization of reaction intermediates. In this work, pyridinium-based ionic polymers (PIP) were synthesized, which readily reacted with 3,5-bis(trifluoromethyl)phenyl isothiocyanate and 3,5-bis(trifluoromethyl) phenyl isocyanate under ambient conditions to introduce thiourea and urea structures into the porous polymer. Consequently, bifunctional polymers PIP-thiourea and PIP-urea containing quaternary ammonium centers and dual hydrogen-bond donors were fabricated. The polymers were tested as catalysts for the cycloaddition reaction of CO2 and epoxides. The results reveal a higher catalytic activity of both catalysts PIP-thiourea and PIP-urea than that of catalyst PIP, which confirms the efficacy of the hydrogen bonds introduced by the post-modification in epoxide activation and catalytic activity improvement. A substantial yield of the corresponding cyclic carbonate products can be obtained with only 0.3 mol % catalysts. In addition, the as-synthesized heterogeneous catalysts exhibited good stability and excellent recyclability. Hopefully, the designed polymers will prove to play a powerful role in catalytic reactions for CO2 utilization.