Toward the Identification of Dormant and Active Species in MAO (Methylaluminoxane)-Activated, Dimethylzirconocene-Catalyzed Olefin Polymerization

Possible structural candidates for the active (III) and dormant (IV) species in dimethylzirconocene-catalyzed, MAO (methylaluminoxane)-activated olefin polymerization have been studied via density functional theory (DFT). These species are [Cp2ZrMeAlMe3]+[MeMAO]- (III) and [Cp2ZrMe]+[MeMAO]- (IV), respectively, where Me = CH3 and Cp = n5-C5H5. Relative energies in gas phase and toluene solution as well as 1H and 13C NMR chemical shifts have been calculated. These chemical shifts have been compared with 1H and 13C NMR data from new experimental investigations studying the interaction between Cp2ZrMe2 and MAO in toluene solution.1 By combining experimental and theoretical results, we have been able to identify the most likely candidates for both the dormant and active species. Moreover, we are able to predict that an Al/Zr ratio of ∼275:1 at 298.15 K is necessary in order for all of the Cp2ZrMe2 to bind to MAO as the structures we propose for III and IV. This ratio corresponds to the disappearance of the following species from solution: free Cp2ZrMe2, the weak complex of Cp2ZrMe2 and MAO (I), and [Cp2ZrMe(μ-Me) Cp2ZrMe]+[MeMAO]- (II). Such a mixture would have a IV/III ratio of ∼1.8:1.