High temperature ethylene polymerization catalyzed by titanium(IV) complexes with tetradentate aminophenolate ligands in cis-O, N, N chelating mode
A series of titanium trichloride complexes 1a–4a, 7a–10a ligated with claw-type tetradentate aminophenolate ligands were synthesized from the direct reaction of TiCl4(THF)2 with 1 equiv. of the corresponding aminophenol in the presence of triethylamine. For comparison purposes, titanium isopropoxide complexes 5e–8e were also synthesized via the reaction of Ti(OiPr)4 and 1 equiv. of the proligand. Similar reactions of ZrCl4(THF)2 with the corresponding aminophenol ligands in the presence of triethylamine only allowed the isolation of zirconium complex 8b. The X-ray diffraction studies reveal that titanium trichloride complexes 2a, 9a and titanium triisopropoxide complex 5e all possess a distorted octahedral geometry with the tetradentate aminophenolate ligand in cis-O, N, N chelating mode, where the methoxy group of the aryl unit does not coordinate with the metal center in the solid state. Upon activation with MMAO, these titanium and zirconium(IV) complexes exhibited moderate to high catalytic activities for ethylene polymerization at 30–120 °C, producing high-molecular-weight polyethylenes with broad distributions (Mw/Mn = 10.2–34.8). The activities of titanium trichloride complexes are significantly higher than those of titanium isopropoxide and zirconium trichloride complexes at high temperatures. The highest activity of 15456 kg (mol-Ti h)−1 could be achieved by titanium trichloride complex 8a with bromo groups on both ortho- and para-positions of the phenolate ring of the ligand at 120 °C.