Activation of Bis(pyrrolylaldiminato) and (Salicylaldiminato)(pyrrolylaldiminato) Titanium Polymerization Catalysts with Methylalumoxane

Abstract

Cationic intermediates formed upon activation of an olefin polymerization catalyst based on bis[ N -phenylpyrrolylaldiminato]titanium(IV) dichloride (L 2 TiCl 2, I ) and [ N -(3- tert -butylsalicylidene)-2,3,4,5,6-pentafluoroanilinato- N ‘-phenylpyrrolylaldiminato]titanium(IV) dichloride (L‘LTiCl 2, II ) with methylalumoxane (MAO) have been identified. Outer-sphere ion pairs of the type [L 2 TiMe(S)] + [MeMAO] - and [L‘LTiMe(S)] + [MeMAO] - capable of ethene polymerization have been characterized by 1 H and 13 C NMR spectroscopy. Unlike methyl metallocenium cations, the barrier of the first ethene insertion into the Ti−Me bonds of these species is not significantly higher than that of subsequent insertions. Surprisingly, whereas homoligated catalyst precursors L 2 TiCl 2 in the presence of MAO are prone to ligand transfer to aluminum, under the same conditions the heteroligated system L‘LTiCl 2 /MAO proved resistant to ligand scrambling.