Ethylene polymerization by sterically and electronically modulated Ni(II) α-diimine complexes

A series of highly active ethylene polymerization catalysts based on bidendate alpha-diimine ligands coordinated to nickel are reported. The ligands are prepared via the condensation of bulky ortho-substituted anilines bearing remote push-pull substituents with acenaphthenequinone, and the precatalysts are prepared via coordination of these ligands to (DME)NiBr2 (DME = 1,2-dimethoxyethane) to form complexes having general formula [ZN = C(An)-C(An) = NZ]NiBr2 [Z = (4-NH2-3,5C(6)H(2)R(2))(2)CH(4-C6H4Y); An, acenaphthene quinone; R, Me, Et, iPr; Y = H, NO2, OCH3]. When activated with methylaluminoxane (MAO) or common alkyl aluminiums such as ethyl aluminium sesquichloride (EAS) all catalysts polymerize ethylene with activities exceeding 10(7) g-PE/ mol-Ni h atm at 30 degrees C and atmospheric pressure. Among the cocatalysts used EAS records the best activity. Effects of remote substituents on ethylene polymerization activity are also investigated. The change in potential of metal center induced by remote substituents, as evidenced by cyclic voltammetric measurements, influences the polymerization activity. UV-visible spectroscopic data have specified the important role of cocatalyst in the stabilization of nickel-based active species. A tentative interpretation based on the formation of active and dormant species has been discussed. The resulting polyethylene was characterized by high molecular weight and relatively broad molecular weight distribution, and their microstructure varied with the structure of catalyst and cocatalyst. (c) 2007 Wiley Periodicals, Inc.