Large ultra-high molecular weight polyethylene spherical particles produced by AlR3 activated half-sandwich chromium(III) catalysts

A series of half-sandwich pentamethylcyclopentadienyl chromium(III) complexes bearing a salicylaldiminato ligand, Cp*[2-R-1-4-R-2-6-(CH=NR3)C6H2O] CrCl [R-1 = Pr-i (1, 4), Bu-t (2, 3, 5), Ad (6); R-2 = H (1, 2, 3), Bu-t (4, 5, 6); R-3 = Pr-i (1, 2, 5, 6), Bu-t (3, 4)], were synthesized. All complexes were characterized by elemental analyses and the structures of complexes 1-4 and 6 were determined by X-ray diffraction analysis. These complexes adopt a pseudo-octahedral coordination environment with a three-legged piano stool geometry. Upon activation with a small amount of AlR3, complexes 1-6 all catalyze the polymerization of ethylene in a quasi living fashion with good to high catalytic activity under mild conditions and produce ultra-high molecular weight polyethylene as spherical particles with a diameter of 1-6 mm. The catalytic activity of these complexes and the molecular weight of the produced polyethylene can be tuned in a broad range by changing the R-1, R-2, and R-3 groups as well as the AlR3 cocatalyst. It was found that complex 6 with R-1 = Ad, R-2 = Bu-t, and R-3 = Pr-i shows the highest catalytic activity and produces polyethylene with the highest molecular weight.