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Computer Design of Living Olefin Polymerization Catalysts:  A Combined Density Functional Theory and Molecular Mechanics Study

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journal contribution
posted on 26.06.1998, 00:00 by Liqun Deng, Tom Ziegler, Tom K. Woo, Peter Margl, Liangyou Fan
Ethylene polymerization catalyzed by group-4 diamide complexes has been studied by molecular modeling. The modeling was based on pure density functional theory (DFT) in the case of the generic [RNCH2CH2CH2NR]MCH2CH2CH3+ complexes with R = H and M = Ti, Zr, and Hf. For the substituted systems with R = 2,6-iPr2C6H3 and M = Ti and Zr, a combined DFT and molecular mechanics (MM) scheme (QM/MM) was employed. The generic systems revealed the following trends with respect to the three group-4 metals:  ethylene complexation energies are Ti (19.2 kcal/mol) < Hf (21.3 kcal/mol) ≤ Zr (22.0 kcal/mol); the overall insertion barriers ΔEinsertion are Zr (7.4 kcal/mol) ≤ Ti (8.1 kcal/mol) < Hf (9.6 kcal/mol); and the chain termination barriers, ΔEtermination, relative to the most stable π complexes are Hf (8.1 kcal/mol) ≈ Zr (8.2 kcal/mol) < Ti (10.3 kcal/mol). The QM/MM calculations on the substituted systems gave the following energies:  for ethylene complexation 17.0 (Ti) and 22.4 (Zr) kcal/mol; for the insertion barriers 9.4 (Ti) and 11.8 (Zr) kcal/mol; and for the chain termination barriers 19.2 (Ti) and 11.7 (Zr) kcal/mol. The calculated ratio for the rate of insertion versus termination (ΔEtermination − ΔEinsertion) are 42:1 (Ti), 6:1 (Zr), 1:13 (Hf) for R = H and 20 000 000:1 (Ti) and 1:1 (Zr) for R = 2,6-iPr2C6H3. The differences in the predicted performance of the substituted systems for titanium and zirconium are in agreement with experimental findings by McConville et al. The poor performance of the zirconium catalyst is rationalized. On the basis of this analysis, new zirconium catalysts are suggested with increased steric bulk on the diamide chain or the aryl groups. The new complexes were shown by QM/MM calculations to be potential living polymerization catalysts with higher activity than the titanium diamide systems suggested by McConville et al.