Counter-Ion-Assisted Site Epimerization

In the regular and real olefin polymerization processes with metallocene catalysts, the anion is an ever-present participant and an integral part of the functioning catalyst system. It is systematically displaced and moves (in and) out of the coordination sphere in a concerted action, while the monomer units approach for coordination to be paired again with the cation after the insertion process is complete. Assuming that the monomer approach proceeds along the most favorable path (i.e., via a cis approach with respect to the anion), immediately after the monomer insertion the anion is positioned at the outer coordination sphere, on the side of the polymer chain (Fig. 20a). 

Intuitively, it can be reasonably assumed that the anion reorganization in the outer coordination sphere is spontaneous and occurs freely its rearrangement is governed purely by diffusion. Indeed, our gas-phase calculatirms [146] show that for the cation/ anion model system, MeaCfCpFlulZrMe-Me-BfCeFsla, the energy barrier is about 0.5 kcal/mol, and well within the error associated with the computational method. On the other hand, the formation of the close-contact ion pair is also an exothermic and 

The counter-anion is MeBfCgFsfa. All energies are relative to the respective parent close-contact ion-pair precursor and propylene. Relative energies in kcal/mol 

As can be seen from Table 14, for all three systems, in general, the effect of solvation is to stabilize the transition state species involved, relative to the reactants, i.e., the L2Zr-iBu MeB(C6F5)3 close-contact ion pair plus propylene. 

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In this context, it is also noteworthy to mention another ligand/transition metal related, dynamic behavior, namely, the lateral displacement of the whole ligand system around the imaginary axis connecting the transition metal bonds to the centroids, which was first described by Petersen [164]. This movement can be described as a kind of windshield wiper-type oscillation of the MCI2 moiety within the fixed ligand framework and could facilitate or influence the steps involved in the counter-ion-assisted site epimerization and chain migratory insertion processes (Fig. 19, top). 

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