On Ethylene Polymerisation with Aluminium and Scandium Complexes: DFT and Post‐HF Calculations

Abstract

Summary: The performance of Density Functional Theory (DFT) methods in predicting ethylene polymerisation and/or oligomerisation activity in selected aluminium and scandium based complexes was studied using both DFT and post‐Hartree‐Fock CCSD(T) calculations. Whereas previous reports have drawn attention to the underestimation of the barrier for the β‐hydrogen termination process for a few aluminium based species, we found that the same holds for the corresponding scandium complexes. New, however, is the observation that apart from underestimating the energy barrier connected to β‐hydrogen termination, the insertion of ethylene is also severely underestimated by the DFT methods applied compared to post‐Hartree‐Fock calculations up to the CCSD(T) level.

Synthesis, Characterization, and Applications of Group 13 Cationic Compounds

“…All calculation methods agree that the energy barrier for chain transfer, through a bimolecular β-hydrogen transfer reaction to the monomer, is substantially easier than that for propagation, i.e., insertion of the monomer into the Al−C bond. In fact, the propagation/chain transfer balance for the modeled three-coordinate Al cations appears to be worse (higher preference for chain tranfer) than that for Me 2 AlEt, which is an ethylene oligomerization catalyst (Aufbau reaction) …”

“…Often seen as a potential source of three-coordinate Al cations but yet stable, these highly Lewis acidic cations have attracted much attention for their applications in polymerization catalysis of various substrates. While several initial and independent studies in this area reported that Al alkyl cations {LX}Al(R)(L) + (type E ) or {L 2 X}AlR + (type F ) may polymerize ethylene, ,, subsequent experimental and theoretical studies have ruled out this proposal. ,, Type E and F Al alkyl cations appear to be much more reactive with polar monomers such as methylmethacrylate (MMA), propylene oxide, and cyclic esters like ( D , L )-lactide and ε-caprolactone. In this regard, preliminary studies by Lappert et al showed that the Al cation 74c + catalyzes the conversion of MMA to syndiotactic PMMA of low dispersity .…”

Theoretical Investigation into the Mechanism of Ethylene Polymerization by a Cationic Dinuclear Aluminum Complex: A Longstanding Unresolved Issue

“…It was initially thought that the mononuclear cationic species 3 (Scheme ), formed by the cleavage of 1 , is the initial active catalyst for ethylene polymerization . By way of contrast, many theoretical studies demonstrated that a cationic species such as 3 could not be the active catalyst because the termination step for such a species via β-hydrogen transfer (BHT) is energetically favored over the propagation step (insertion of ethylene into the Al–alkyl bond) (Scheme ). − …”