Activity Enhancement of Phenoxy-Imine Titanium Catalysts via σ–π Synergy and Para-Substituent Effect

Polyethylene, the world’s predominant polyolefin, exhibits tunable material properties dictated by the architecture of the polymerization catalyst. Although phenoxy-imine titanium (FI–Ti) catalysts have attracted considerable attention due to their high activity and structurally tunable features, the mechanistic basis of their electronic effects during polymer chain propagation remains incompletely understood. This study systematically investigates ethylene polymerization mechanisms catalyzed by three phenoxy-imine titanium complexes─FI–Ti–H, FI–Ti-m-OAll, and FI–Ti-p-OAll─using density functional theory (DFT) calculations. The results demonstrate a strong correlation between the coordination energy for the second ethylene insertion and catalytic activity. The para-allyloxy substituted FI–Ti-p-OAll catalyst exhibits optimal performance, with its superior activity originating from a distinctive σ-π synergistic mechanism wherein olefin σ-electron donation and metal center π-back-donation operate cooperatively. Molecular orbital and electrostatic potential analyses collectively reveal that the para-substituent directly modulates the metal center’s electronic structure through strong electron-donating effects, manifested by a pronounced electrostatic potential gradient, improved Frontier molecular orbital alignment and overlap, and a reduced HOMO–LUMO gap. The synergistic integration of these effects substantially lowers the coordination energy required for the second ethylene insertion, thereby enhancing overall catalytic performance. This study establishes a clear structure–activity relationship between substituent position and catalytic activity, providing a theoretical foundation for the rational design of high-performance olefin polymerization catalysts.