Comparison of ab Initio and DFT Methods for Studying Chain Propagation and Chain Termination Processes with Group 4 Polymerization Catalysts. 1. The ansa-Bis(cyclopentadienyl)zirconium Catalyst

Abstract

In this paper we present a systematic comparison of the performance of different computational approaches to study the propagation and termination reactions of olefins with a prototype homogeneous group 4 ansa-metallocene catalyst. Chain propagation, β-H transfer to the monomer, and β-H elimination to the metal have been investigated for the H 2 Si-(Cp) 2 ZrR + (R ) ethyl, n-butyl) + C 2 H 4 system using ab initio and density functional theory (DFT) techniques. For all the species investigated, all the computational approaches we considered result in substantially similar geometries. A comparison of the DFT and Møller-Plesset theory (MP2) propagation and termination barriers with extrapolated coupled-cluster calculations with inclusion of single, double, and perturbatively connected triple excitation (CCSD(T)) values indicates that all the pure functionals considered underestimate the difference between termination and propagation by roughly 3-4 kcal/mol. In contrast, hybrid functionals are within 1 kcal/mol from extrapolated CCSD(T) values. For a comparison with experimental results inclusion of zero-point energy contributions and the use of an alkyl group longer than ethyl to simulate the growing chain in both termination reactions are mandatory.

Calculating accurate barriers for olefin insertion and related reactions

“…This latter deviation is expected, since we have shown the reliability of BP86 for BHT predictions to be poor, at best (in earlier work, Talarico et al also reported that DFT methods tend to underestimate H transfer barriers [80]). Combining the two sets of data, we now arrive at a BHT/propagation difference of 14.1, 14.8 and 8.3 kcal/mol (M06-2X), compared to the BP86 prediction of 3.4, 1.9 and 0.9 kcal/mol.…”

“…As mentioned earlier, and summarized in Scheme 1 for Cl 2 TiEt þ þ ethene, even for relative barriers errors can be large. For the difference between propagation and chain transfer to monomer, averaged over all ETM systems studied, we find that errors are typically in the range of 1e2 kcal/mol, with notably poorer performance for the pure functionals BP86 (known to underestimate H transfer reactions [80]), PBEPBE and B97D.…”

Formation and Rearrangement of Reduced Diiminepyridine Complexes of Zr and Hf

“…DFT appears to underestimate H transfer barriers,, and the models used are strongly simplified, so our results should be considered indicative at best and do not allow definitive assignment of the mechanism(s) responsible for rearrangement of the real complexes 2a , 3b and 3c , nor do they explain the absence of such rearrangements for 2b and 4b . However, the involvement of a low‐concentration pseudo‐catalytic anionic or radical species would be consistent with the somewhat capricious nature of the reaction.…”

Tuning the Relative Energies of Propagation and Chain Termination Barriers in Polyolefin Catalysis through Electronic and Steric Effects

“…Selectivity is important in polymerization, and the main side‐reaction is chain termination, which determines the molecular weight (MW) of the resulting polymer . As the MW is such an important property of the product, the factors that affect it have been studied extensively.…”