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==NR(3))C(6)H(2)O]CrCl [R(1) = (i)Pr (1, 4), (t)Bu (2, 3, 5), Ad (6); R(2) = H (1, 2, 3), (t)Bu (4, 5, 6); R(3) = (i)Pr (1, 2, 5, 6), (t)Bu (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 AlR(3), 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 AlR(3) cocatalyst. It was found that complex 6 with R(1) = Ad, R(2) = (t)Bu, and R(3) = (i)Pr shows the highest catalytic activity and produces polyethylene with the highest molecular weight.