“Dormant” secondary metal-alkyl complexes are not omnipresent

This theoretical study was inspired by the perpetual debate over the so-called “dormancy” of the active sites in propylene polymerization, i.e., a drop in their activity after a regioerror (2,1-insertion), which was reported to occur in many (although not all) catalytic systems. To explore the range of possible situations, we have selected two homogeneous systems of fundamentally different structure: an octahedral system of C(2) symmetry with a tetradentate [Image: see text] O [Image: see text] N [Image: see text] N [Image: see text] O [Image: see text] ligand and a bridged indenyl catalyst. This choice was not accidental; it is in these two systems where the experimentalists cannot reach a consensus about dormancy. Our density-functional theory calculations explain why in certain catalytic systems both primary and secondary alkyl complexes can be equally reactive toward propylene polymerization, despite the intuitive concept of dormancy. To understand such a behavior, it was imperative to build an extensive model, including the counteranion and solvent effects. The discussion is also supplemented by our latest calculations on the classical second-generation Ziegler–Natta system

Theoretical Investigation of Active Sites at the Corners of MgCl2 Crystallites in Supported Ziegler-Natta Catalysts

We present a theoretical study on possible models of catalytic active species corresponding to Ti-chloride species adsorbed at the corners of MgCl2 crystallites. First we focused our efforts on the interaction between prototypes of three industrially relevant Lewis bases used as internal donors (1,3-diethers, alkoxysilanes and succinates) and MgCl2 units at the corner of a MgCl2 crystallite. Our calculations show that the energetic cost to extract MgCl2 units at the corner of (104) edged MgCl2 crystallites is not prohibitive, and that Lewis bases added during catalyst preparation make this process easier. After removal of one MgCl2 unit, a short (110) stretch joining the (104) edges is formed. Adsorption of TiCl4 on the generated vacancy originates a Ti-active species. In the second part of this manuscript, we report on the stereo- and regioselective behavior of this model of active species in the absence as well as in the presence of the three Lewis bases indicated above. Surface reconstruction due to the additional adsorption of an extra MgCl2 layer is also considered. We show that, according to experimental data, Lewis bases coordinated in the proximity of the active Ti center confer a remarkable stereoselectivity. Moreover, surface reconstruction as well as donor coordination would improve regioselectivity by disfavoring secondary propene insertion. While still models of possible active species, our results indicate that defects, corners and surface reconstruction should be considered as possible anchoring sites for the catalytically active Ti-species