Activation of bis(pyrrolylaldiminato) and (salicylaldiminato)(pyrrolylaldiminato) titanium polymerization catalysts with methylalumoxane

Cationic intermediates formed upon activation of an olefin polymerization catalyst based on bis[N-phenylpyrrolylaldiminato]titanium(IV) dichloride (L₂TiCl₂, I) and [N-(3-tert-butylsalicylidene)-2,3,4,5,6-pentafluoroanilinato-N‘-phenylpyrrolylaldiminato]titanium(IV) dichloride (L‘LTiCl₂, II) with methylalumoxane (MAO) have been identified. Outer-sphere ion pairs of the type [L₂TiMe(S)]⁺[MeMAO]^- and [L‘LTiMe(S)]⁺[MeMAO]^- capable of ethene polymerization have been characterized by ¹H and ¹³C 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 L₂TiCl₂ in the presence of MAO are prone to ligand transfer to aluminum, under the same conditions the heteroligated system L‘LTiCl₂/MAO proved resistant to ligand scrambling.