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.