COMPUTER DESIGN OF LIVING OLEFIN POLYMERIZATION CATALYSTS - A COMBINED DENSITY-FUNCTIONAL THEORY AND MOLECULAR MECHANICS STUDY

Ethylene polymerization catalyzed by group-4 diamide complexes has bee n studied by molecular modeling. The modeling was based on pure densit y functional theory (DFT) in the case of the generic [RNCH2CH2CH2NR]MC H2CH2CH3+ complexes with R = H and M = Ti, Zr, and Hf For the substitu ted systems with R = 2,6-(Pr2C6H3)-Pr-i and M = Ti and Zr, a combined DFT and molecular mechanics (MM) scheme (QM/MM) was employed. The gene ric systems revealed the following trends with respect to the three gr oup-4 metals: ethylene complexation energies are Ti (19.2 kcal/mol) < Hf (21.3 kcal/mol) less than or equal to Zr (22.0 kcal/mol); the overa ll insertion barriers Delta E double dagger(insertion) are Zr (7.4 kca l/mol) less than or equal to Ti (8.1 kcal/mol) < Hf (9.6 kcal/ mol); a nd the chain termination barriers, Delta E double dagger(termination), relative to the most stable pi complexes are Hf (8.1 kcal/mol) approx imate to Zr (8.2 kcal/mol) < Ti (10.3 kcal/mol). The QM/MM calculation s on the substituted systems gave the following energies: for ethylene complexation 17.0 (Ti) and 22.4 (Zr) kcal/mol; for the insertion barr iers 9.4 (Ti) and 11.8 (Zr) kcal/mol; and for the chain termination ba rriers 19.2 (Ti) and 11.7 (Zr) kcal/mol. The calculated ratio for the rate of insertion versus termination (Delta E double dagger(terminatio n) - Delta E double dagger(insertion)) 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-(Pr2C6H3) -Pr-i. 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 c atalyst is rationalized. On the basis of this analysis, new zirconium catalysts are suggested with increased steric bulk on the diamide chai n or the aryl groups, The new complexes were shown by QM/MM calculatio ns to be potential living polymerization catalysts with higher activit y than the titanium diamide systems suggested by McConville et al.