Titanium(IV) complexes with strategically substituted naphthoxy-imine ligands: design, synthesis and potentiality in ethylene polymerization

Abstract

Naphthoxy-imine based ligands with the general structure of (E)-1-((2,6-dialkylphenylimino)methyl)naphthalen-2-ol (L1, L2, L3 where, alkyl = -Me, -Et, -iPr respectively) and (E)-1-((mesitylimino)methyl)naphthalen-2-ol (L4) were strategically employed to obtain mononuclear bis(phenoxy-imine)-typeTi(IV) complexes with systematic variation of substituents. Deprotonation of L1–L3 using sodium hydride followed by metalation with TiCl4 afforded the corresponding Ti(IV) complexes Ti1–Ti3, while effectively suppressing the formation of undesired protonated zwitterionic species Ti′–Ti3′. In contrast, mesityl-substituted ligand L4 afforded the desired complex Ti4via a direct HCl-elimination route from TiCl4 and L4. Single crystal X-ray diffraction studies confirmed the molecular structures of ligands L1–L4 and provided insight into the steric and electronic features governing metal coordination. The catalytic performance of the Ti(IV) complexes toward ethylene polymerization was investigated using AliBu3/[Ph3C][B(C6F5)4] as the cocatalyst. The effect of reaction temperature, aluminum to titanium molar ratio and ligand environment on catalyst performance was systematically investigated. The resulting polyethylene was characterized by HT-GPC, TGA, differential scanning calorimetry and FTIR analyses, confirming the formation of high to ultrahigh molecular weight polyethylene. Interestingly, catalyst Ti3, with the bulky isopropyl substituent, produced the highest molecular weight polyethylene with the lowest activity, whereas Ti1, with the least steric hindrance, exhibited the highest activity and lowest molecular weight.