Comparative ethylene polymerization using FI-like zirconium based catalysts

Abstract

Three FI-like Zr-based catalysts, Bis[1-[(phenylimino)methyl]-2-naphtholato]zirconium(IV) dichloride (1), Bis[1-[(mesitylimino)methyl]-2-naphtholato]zirconium(IV) dichloride (2) and Bis[1-[(2,6-diisopropylphenyl)imino]methyl-2-naphtholato]zirconium(IV) dichloride (3) were prepared by changing the ligand from salicylaldehyde imine ligand, which is a well known FI catalysts, to 2-hydroxynaphthalene-1-carbaldehyde imine ligand and used for polymerization of ethylene. Triisobutylaluminum (TIBA) and methylaluminoxane (MAO) were used as scavenger and cocatalyst, respectively. Introduction of the bulky substitution phenyl ring on the N of the phenoxy imine ligand enhanced the viscosity average molecular weight of the obtained polymer strongly. Catalyst 3 produced the highest viscosity average molecular weight (Mv) of the obtained polyethylene, but showed the lowest catalytic activity. The activity of all the catalysts was increased with the increase of [Al]/[Zr] molar ratio to an optimum value followed by a slight decrease at higher [Al]/[Zr] molar ratios. Optimum activity of catalyst 1 was obtained at about 30 °C while the highest activity of catalysts 2 and 3 was obtained at about 40 °C following a sharp decrease at higher temperatures. The rate/time profile of the polymerization decayed with a short acceleration period for all of the catalysts. The polymerization activity was increased with increasing the hydrogen concentration due to the fast hydrogenation of sterically more hindered and less reactive intermediates such as those resulting from 2,1-insertions. It is noteworthy that the experimental results indicated that the hydrogen does not act as a chain transfer agent in the case of sterically hindered catalyst (3), while H2 can reduce the molecular weight of the polymer produced by the catalysts (1) and (2). The resulting polymer was characterized and the data for its physical properties are reported.