Synthesis, Characterization, and Catalytic Properties of Regioisomeric β-Ketoimine Titanium Complexes for Ethylene Polymerization

Non-metallocene catalysts have shown great potential for modulating the polymer microstructure by changing the ligand structure, but the effects of ligand structure and electronic property of metal center on their catalytic performance remain underexplored. Herein, we designed and synthesized two pairs of isomeric tridentate β-ketoimine complexes TiL_a/TiL_b and TiL_c/TiL_d with methylthioaniline and methylthioethylamine side arms, respectively, which exhibited moderate to extremely high activity over 10⁶ g (mol Ti)⁻¹· h⁻¹·atm⁻¹ for the polymerization of ethylene and the copolymerization of ethylene with α-olefins (1-hexene, 1-octene) and polar comonomer (9-decen-1-ol) in the presence of MAO, with the catalytic activities in the order: TiL_d > TiL_c > TiL_a > TiL_b. The regioisomers TiL_c and TiL_d with methylthioethylamine side arm showed significantly higher activity for ethylene (co)polymerization than that of the isomeric TiL_a and TiL_b with methylthioaniline side arm, especially for ethylene copolymerization with α-olefins or polar comonomer 9-decen-1-ol, indicating an obvious positive “comonomer effect”. The complex TiL_d exhibited the highest ethylene (co)polymerization activity and comonomer insertion rate. Structural optimization using density functional theory (DFT) calculations suggested that the “side arms” and the electronic properties of titanium in β-ketoimine titanium complexes played a key role for ethylene polymerization and copolymerization.