Combined Density Functional Theory and Molecular Mechanics (QM/MM) Study of Single-Site Ethylene Polymerization and Chain Termination for the Catalysts [(C_6R_5N=CH)C_4H_3N]_2PrTi~+ (R=F,H) and [bis(eta~5-1-indenyl)dimethylsilane]PrZr~+ in the Presen

摘要

Calculations,using density functional theory,have been carried out to investigate two chain termination processes,(i) unimolecular hydrogen transfer to the metal center and (ii) bimolecular hydrogen transfer to the incoming ethylene monomer,for the PI systems [(C_6R_5N=CH)C_4H_3N]_2PrTi-mu-CH_3-B(C_6F_5)_3 (R=F,H).A validated QM/MM model was used to represent the counterion.The barriers for the unimolecular termination process were about 36.5 kcal/mol for both the PI systems studied,while the rate-determining uptake barriers for the bimolecular chain termination process were about 18 kcal/mol lower for both PI systems.A difference of only about 0.5 kcal/mol was observed between the corresponding barriers for each of the termination processes for the two systems.The insertion of the ethylene monomer into the Ti-C bond was also investigated for the two PI systems,and the rate-determining uptake barrier was found to be about 6 kcal/mol lower than the corresponding rate-determining uptake barriers for bimolecular termination.The effect of increasing the steric bulk on the phenyl groups attached to the imine nitrogen was investigated for the [(C_6R_5N=CH)C_4H_3N]_2PrTi-mu-CH_3-B(C_6F_5)_3,with the hydrogen at the ortho position of the phenyl rings being replaced by tertiary butyl groups,modeled by MM atoms.The rate-determining uptake barrier for the bimolecular chain termination process for this system was found to be only marginally higher than the corresponding uptake barrier for the non-bulky system.Insertion and uni- and bimolecular termination processes were also investigated for the ansa-indenyl system [bis(eta~5-1-indenyl)dimethylsilane]ZrPr-mu-Me-B(C_6F_5)_3.The barriers for insertion and bimolecular chain termination for this system were comparable to those for the PI systems,but the barrier to unimolecular chain termination was found to be about 5 kcal/mol higher.