We present a theoretical study of the basic interactions occurring at supported hetereogenous Ziegler-Natta catalytic systems. We first investigated the interaction between prototypes of each class of industrially relevant internal donors (1,3-diethers, alkoxysilanes, phthalates, succinates) and the MgCl 2 support. Our analysis indicates that donors can be separated into two classes. 1,3-Diethers and alkoxysilanes belong to the former because they have a short spacer between the coordinating O atoms and coordinate preferentially to the same Mg atom of the (110) lateral cut. Conversely, phthalates and succinates belong to the latter class because they have a longer spacer between the coordinating O atoms and thus can adopt a variety of coordination modes. Indeed, they can coordinate to both the ( 100) and ( 110) lateral cuts. In the last part of this manuscript we report on the stereo-and regioselective behavior of possible active Ti species with and without two succinate molecules coordinated in the proximity of the Ti atom. We show that the two succinate molecules confer a remarkable stereoselectivity in both primary and secondary propene insertions. This model very simply rationalizes the effect of the donors, and it is consistent with the models so far developed to rationalize the stereoselectivity of metallocene and octahedral nonmetallocene catalysts.

Three diethers, 2,2-diisobutyl-1,3-dimethoxypropane, 2,2-dicyclopentyl-1,3-dimethoxypropane, and 2-ethyl-2-butyl-1,3-dimethoxypropane, differing in the bulkiness of the alkyl substituents in position 2, have been used as internal donors in MgCl2-TiCl4-diether/AlR3 catalysts for the polymerization of propylene. The performance of the three diether-based catalysts is compared with that of a "traditional" MgCl2-supported catalyst based on the phthalate/silane donor pair and with that of a "donor-free" catalyst by coupling the temperature rising elution fractionation (TREF) analysis with a study of the stereoregularity of the first monomeric unit insertion. On the basis of the experimental data obtained, the action mechanisms of the different donors are discussed taking into account the various models presented in the literature.

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.

We propose a model for MgCl 2 -supported Ziegler−Natta catalysts, which is capable to reconcile the discrepancies emerging in the last 20 years, in which attempts have been made to rationalize experimental data by molecular models. We show that step defects on the thermodynamically more stable (104) facet of MgCl 2 can lead to sites for strong TiCl 4 adsorption. The corresponding Ti-active site is stereoeselective, and its stereoselectivity can be enhanced by coordination of Al-alkyls or Lewis bases in the close proximity. The surface energy of the step defected (104) MgCl 2 facet is clearly lower than that of the well accepted (110) facet.

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