Asymmetric bis-salicylaldiminato binuclear titanium complexes for ethylene polymerization and copolymerization

Abstract

Polyolefins with high molecular weight and broad molecular weight distribution have attracted great attention due to their ease of processing and wide applications. In this paper, methylene-bridged asymmetric disalicylaldimine ligands bearing phenylthio and alkylthio sidearms (4a–4c) were synthesized from methylene-bridged disalicylaldehyde that reacted successively with 2-phenylthio and 2-alkylthio anilines, which then directly coordinated with TiCl₄ to afford asymmetric bis-salicylaldiminato binuclear titanium complexes 5a–5c in one step. The asymmetric complexes were detailedly characterized by FTIR, ¹H NMR, ¹³C NMR, and elemental analysis. Under the activation of modified methylaluminoxane (MMAO), these asymmetric complexes displayed high activity over 10⁶ g mol(Ti)⁻¹ h⁻¹ atm⁻¹ for ethylene polymerization and copolymerization with 1-hexene or norbornene. Compared with the corresponding mononuclear complexes, the asymmetric binuclear catalysts showed higher catalytic activity (up to 2.29 × 10⁶ g mol(Ti)⁻¹ h⁻¹ atm⁻¹) for ethylene polymerization to produce higher molecular weight polyethylenes with much wider molecular weight distribution due to the presence of two different active centers. For ethylene polymerization and copolymerization with 1-hexene, the catalytic activity of binuclear 5a which carries phenylthio and methylthio sidearms and the properties of the obtained polyolefins were more similar to those of mononuclear 6b which bears a phenylthio sidearm, indicating that the phenylthio group exerted major influences. However, for ethylene/norbornene copolymerization, 5a showed an activity significantly higher than that of 6b, yet similar to that of mononuclear 6a which bears a methylthio sidearm, indicating that the methylthio sidearm in 5a played a major role in this case. A smaller sidearm in the ligand was more conducive to the polymerization reaction when large comonomers were involved, and a balance of the steric hindrance between the ligand and the comonomer might be needed for achieving high catalytic performance.