Titanium(IV) complexes featuring fluorenyl-tethered oxazoline ligands: Design, synthesis and catalytic performance in ethylene polymerization
摘要
A series of new fluorenyl-tethered oxazoline based ligands, L = 2-(1-(9 H-fluoren-9-yl)alkyl)-4,5-dihydrooxazole [alkyl = butyl (5a), propyl (5b), ethyl (5c), methyl (5d)] were prepared via an effective synthetic strategy starting from fluorene and ethyl 2-bromoalkanoates, followed by four consecutive reaction steps. Ligands were thoroughly characterized by 1H NMR, 13C NMR, FTIR, HRMS and elemental analyses. The molecular structure of 5a and 5d were determined by single crystal X-ray diffraction analysis. Deprotonation of the ligands 5a-d with n-BuLi, followed by subsequent metallation with TiCl4 afforded the corresponding titanium(IV) complexes Ti1–Ti4. These complexes were characterized unambiguously by NMR spectroscopy, mass spectrometry, and elemental analysis. Upon activation with TIBA/[Ph3C][B(C6F5)4], these half-sandwich titanium(IV) complexes showed catalytic activity toward ethylene polymerization producing polyethylene with ultrahigh molecular weight up to 2.09 × 106 g.mol− 1. The relatively broad PDI range (4–6) for the polyethylene obtained by these catalytic systems may help to improve flow properties of the polymers. Notably, complex Ti1, containing a bulky propyl substituent on the bridging carbon, exhibited the highest catalytic activity among the synthesized titanium complexes under the optimized conditions of 50 °C and an Al/Ti molar ratio of 200. The well-defined melting temperature (Tm) of the resulting polymers, observed in the range of 131–134 °C, indicates the formation of predominantly linear polyethylene chain architecture.