Preparation, structure, and ethylene polymerization behavior of bis(imino)pyridyl chromium(III) complexes

Abstract

The synthesis, characterization, and ethylene polymerization behavior of a family of chromium(III) complexes of formula [2,6-bis(imino)pyridyl]CrCl3 is reported. The X-ray diffraction studies of two of the new compounds show that the geometry around the chromium atom is octahedral, with the three chlorine ligands in a mer disposition. The distance between the metal and one of the trans-disposed chlorine atoms is significantly longer than the other two Cr-Cl distances. Treatment of the complexes [2,6-bis(imino)pyridyl]CrCl3 with methylaluminoxane (MAO) leads to very active ethylene polymerization catalysts that afford highly linear polyethylene. The substituents at the ortho position of the N-aryl groups of the 2,6-bis(imino)pyridyl ligands modulate both the catalytic activity and the molecular weights of the resulting polyethylene. The most active catalysts are those with two substituents at the ortho position of the N-aryl groups (activities up to 4 × 107 g (mol of Cr)-1 bar-1 h-1 are achieved). Regarding the size of the substituents, the activity and the molecular weights follow an opposite trend. Systems with two small substituents lead to very active systems, but the molecular weight of the polyethylene is lower than when bulkier substituents are present.