EPR Monitoring of Vanadium(IV) Species Formed upon Activation of Vanadium(V) Polyphenolate Precatalysts with AlR₂Cl and AlR₂Cl/Ethyltrichloroacetate (R = Me, Et)

Abstract

Reactions of the vanadyl polyphenolate complexes {VO[p-tert-butylcalix[4]arene(O)3(OMe)]} (1) and [VO(OArCH2Ar′CH2OAr)]2 (Ar = 4,6-tert-butylphenol; Ar′ = 4-tert-butylphenol) (2) with AlR3 and AlR2Cl, in the absence and in the presence of reactivator (ethyltrichloroacetate, ETA), were monitored by EPR (R = Me, Et). It was shown, that vanadium(IV) complexes with proposed structures L1VIVR(AlR3) and L1VIVR(AlR2Cl) are formed upon interaction of 1 with AlR3 and AlR2Cl, respectively (20 °C, L1 is the initial oxo-calix[4]arene ligand). Similarly, vanadium(IV) complexes with proposed structures L2VIVR(AlR3) and L2VIVR(AlR2Cl) are formed upon interaction of 2 with AlR3 and AlR2Cl (L2 is the initial ligand of 2). The growth of the concentration of L1VIVR(AlR2Cl) and L2VIVR(AlR2Cl) correlates with the increase of the ethylene polymerization activity of the catalyst systems 1,2/AlR2Cl/ETA. It is proposed therefore that vanadium(IV) species of the type L1VIVR(AlR2Cl) and L2VIVR(AlR2Cl) can be the active species (or their immediate precursors) of the catalyst system 1,2/AlR2Cl/ETA.