Exploring the effects of phenolic compounds on bis(imino)pyridine iron-catalyzed ethylene oligomerization

Abstract

In order to reduce the simultaneous production of insoluble polymers during the bis(imino)pyridine (BIP) iron-catalyzed ethylene oligomerization, a series of phenolic compounds were introduced as modifiers. It was found that the polymer share in the total products would be largely reduced with the increasing dosage of the phenols and the enlargement of para-substituent size from methyl to tert-butyl. Further ¹H NMR studies showed that the phenols could provide methylaluminoxane (MAO) profound structural modifications, giving rise to larger MAO aggregates and decoration of phenoxy groups on its surface. This would thus facilitate the active ion pair separation, leading the phenols to become effective polymer-retarding modifiers. Starting from the reaction between 4-tert-butylphenol, AlMe₃ and water, a novel phenoxy-aluminoxane could be prepared. Its combination with AlMe₃ enabled the catalyst activation, and gave us a further verification about the important role of phenoxy groups on the MAO surface. Furthermore, the introduction of electron-withdrawing groups would improve the reactivity of the –OH group, promoting the interaction between the phenols and MAO. A series of para-halogen substituted phenols were thus developed. With the relatively large size of the bromo group and the highest reactivity of the –OH group, 4-bromophenol was proved to be the most efficient polymer-retarding modifier among the studied phenols in this work. An almost polymer-free ethylene oligomerization could be achieved by this strategy without altering the mono ortho-methyl substituted BIP ligand.