| Abstract
| - The DFT results of the polymerization propagation step for the studied niobium metallocene catalyst suggest a concerted reaction mechanism, due to the very weak ethene complexation. The termination has a higher barrier and occurs either by hydrogen transfer to the coordinated ethene or by β-hydrogen abstraction from the alkyl resting state.
- We have studied the ethylene polymerization catalyst C5H5Nb(butadiene)Cl2 + MAO usingprimarily density functional theory (DFT). The active species was assumed to be C5H5Nb(butadiene)R+. Chain initiation and propagation as well as different termination processeswere modeled. The ethene coordination is very weak, and no free energy minimum was found.Insertion into the metal−alkyl bond has an energy barrier of 4 kcal/mol for R = CH3 and 6kcal/mol for R = C2H5. The ethene insertion transition state is clearly stabilized by agosticinteraction, with metal−hydrogen distances of 2.07−2.16 Å. However, in alkyl conformationsthese bonds are longer and correspond to only weak agostic interaction. In the absence ofstrong agostic interaction the resting state alkyl complexes are floppy and differentconformations interconvert easily. Termination via hydrogen transfer to a coordinated ethenemolecule ejecting a terminal alkene has a high energy barrier of 17 kcal/mol. An alternativetermination process via β-elimination and subsequent alkene ejection is also very expensive,43 kcal/mol. The propagation free energy barrier for the concerted reaction is 21 kcal/mol,which consists mostly (80%) of ethene coordination. The termination free energy barrier viahydrogen transfer to coordinated alkene is 30 kcal/mol and that via β-elimination is 28 kcal/mol. The free energies have been determined in a vacuum using the harmonic approximation.The key intermediates were also optimized using MP2 supplemented with single-pointcalculations using CCSD. These methods gave stronger complexation energies, resulting inlowering the propagation barrier by approximately 3−4 kcal/mol and increasing theβ-elimination barrier by 6−7 kcal/mol. The BSSEs in MP2 and DFT complexation energieswere estimated to be 15−20 and 1−3 kcal/mol, respectively, using DZ and DVZP bases.
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