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

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.