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 dimethyl-zirconocene-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-C ₅H₅. 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 Cp₂ZrMe ₂ and MAO (I), and [Cp₂ZrMe(μ-Me) Cp ₂ZrMe]⁺-[MeMAO]^- (II). Such a mixture would have a IV/III ratio of ~1.8:1.