A theoretical study of the insertion barrier of MAO (methylaluminoxane)-activated, Cp2ZrMe2-catalyzed ethylenepolymerization: further evidence for the structural assignment of active and dormant species

Abstract

Density functional theory has been used to calculate the ethylene insertion barrier into the Zr–C bonds of the [Cp₂ZrMe]⁺[AlMe₃Me(A-MAO)]⁻ and [Cp₂ZrMe]⁺[Me(A-MAO)]⁻ ion pairs where (AlOMe)₆ was used to model the active forms of MAO (A-MAO). The results support the proposal that the former is the active and the latter the dormant species in MAO-activated olefin polymerization. For the first insertion, with the active species the trans approach has a lower barrier to insertion than the cis approach in solution, due to the larger ion pair separation. It is likely that the separated ion pairs recombine between subsequent insertions. For the second insertion, we have considered frontside, backside and combined insertion mechanisms showing that the total barrier to insertion is approximately the same in all cases ranging from 13.8 kcal mol⁻¹–14.8 kcal mol⁻¹. For the frontside and combined insertion this barrier is a result of the rotation of the propyl chain; for the backside mechanism it is stems from the ethylene insertion barrier. Comparison of the results for the ion pair and for the naked cation show that in the former case olefin complexation is an endothermic process whereas for the latter it is exothermic. In general, the large ion pair separation prevents the orientation of the anion from exerting much influence on the geometry or energy of the cation. However, when structures with α-agostic interactions are considered the ion pair separation is small enough so that the orientation has some influence on the reaction mechanism. For the second insertion, dissociated transition states were found to have lower barriers than associated ones.