Triazine-imidazole polymers with electronic push-pull synergy for efficient metal-free CO2 cycloaddition under mild conditions.

Abstract

The cycloaddition of CO2 with epoxides to produce cyclic carbonates typically requires harsh conditions, motivating the development of efficient heterogeneous catalysts for mild operation. In this work, four triazine-based organic polymers (CTP-TPT, CTP-TPB, CTP-TIT, and CTP-TIB) were synthesized via one-step quaternization polycondensation of cyanuric chloride with four nitrogen-containing ligands. Their structures were characterized by FT-IR, solid-state 13C NMR, XPS, SEM, and BET, and their catalytic performances in CO2-epoxide cycloaddition were evaluated. CTP-TIT exhibited the best activity, achieving 95.6% epichlorohydrin yield at 80 °C, 0.5 MPa CO2, and 6 h, along with good substrate generality and 90.6% yield retention after five cycles. Electrostatic potential calculations revealed the largest potential gradient (+34.8/-28.15 kcal/Mol) in CTP-TIT, arising from the synergy between the electron-withdrawing triazine framework and the positive potential of imidazole C2H. NPA charge analysis confirmed the coexistence of positive charge at imidazole C2H (+0.284 e) and negative charge at imidazole N (-0.395 e) within the same ring, forming a spatially separated push-pull polarization. DFT calculations showed that CTP-TIT exhibits the lowest ring-opening energy barrier (12.14 kcal/Mol) and the largest potential difference in the transition state, establishing a direct link between the push-pull effect and barrier reduction. In situ FTIR combined with DFT supported a synergistic pathway involving epoxide activation by imidazole C2H, CO2 activation by triazine N, and nucleophilic ring-opening by Cl-. This work provides a systematic theoretical basis for designing metal-free triazine-based polymers for efficient CO2 yield under mild conditions.