Probing the interactions between all components of the catalytic pool for homogeneous olefinpolymerisation by diffusion NMR spectroscopy

Abstract

Diffusion NMR spectroscopy was applied to investigate all individual components and combinations thereof for the Cp₂ZrMe₂/MAO (DMAO)/TBP (MAO = methylaluminoxane, DMAO = AlMe₃ depleted MAO, TBP = 2,6-di-tert-butylphenol) ternary system, selected as a prototypical catalytic pool for homogeneous olefin polymerisation. Both MAO and DMAO were found to self-aggregate in C₆D₆ with the latter having a higher propensity. TBP reacts with DMAO affording MeAl(2,6-di-tert-butylphenoxide)₂ and causing a structural modification of DMAO, whose aggregates become much larger. The actual dimensions and self-aggregation tendency of (D)MAO, which depend on Al concentration and the possible presence of TBP, turned out to carry over to [Cp₂Zr(μ-Me)₂AlMe₂]MeMAO (1) OSIP (outer sphere ion pair) and [Cp₂Zr⁺Me⋯MeMAO⁻] (2) ISIP (inner sphere ion pair) that form upon activation of Cp₂ZrMe₂. Once the intrinsic self-aggregation tendency of MAO has been subtracted, OSIP 1 and ISIP 2 behave exactly as analogous ion pairs with borate ions: ISIP 2 does not self-aggregate, whereas OSIP 1 exhibits the same self-aggregation trends of zirconocene OSIPs with borate counterions.