Probing the interactions between all components of the catalytic pool for homogeneous olefin polymerisation 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.