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.

In this work, we report on the structure and formation energy of uncovered MgCl2 crystallites of different shapes (hexagonal and square), sizes (up to crystallites composed of 157 MgCl2 units), and edges (crystallites presenting the (104) and (110) edges). Both uncovered crystallites and crystallites covered by dimethyl ether were considered. Our results indicate that the formation energy of uncovered crystallites, irrespective of shape, size, and edges, linearly depends on the density of vacancies (measured as the ratio between the number of Mg vacancies and the number of MgCl2 units in the crystallite) and that larger crystallites that present (104) edges are favored. In the case of crystallites completely covered by dimethyl ether, our results indicate that the formation energy of crystallites, again irrespective of shape, size, and edges, inversely depends on the dimethyl ether/Mg ratio. As opposed to uncovered crystallites, in the presence of dimethyl ether, smaller crystallites presenting (110) edges are favored. The knowledge acquired with both uncovered and dimethyl ether-covered crystallites was used to achieve insight into the behavior of carbon monoxide-covered crystallites by performing calculations on a limited number of small crystallites.

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In this work, we present a systematic DFT analysis of the effect of surface coverage on the coordination properties of alkoxysilanes to the (104) and (110) surfaces of MgCl 2 . Furthermore, we investigated several possible migration pathways for alkoxysilane migration on the same surfaces. Our study clearly shows that complete coverage of the Mg vacancies on the surface by coordinating alkoxysilanes is hampered by steric repulsion between vicinally coordinated donor molecules. Our study clearly indicates that alkoxysilane migration between different MgCl 2 monolayers on the (104) and (110) surfaces requires donor dissociation. The same holds for alkoxysilane migration on a single (110) MgCl 2 monolayer. However, in the case of the (104) surface we found a very low energy pathway for alkoxysilane migration along the same monolayer. ■ INTRODUCTIONHeterogeneous Ziegler−Natta (ZN) catalysts are the most important catalysts in the industrial production of isotactic polypropylene. The typical catalysts used are MgCl 2 /TiCl 4 / donor systems where the donor is a Lewis base (LB) that can be added during catalyst preparation (the so-called internal donor, ID) or during activation (the so-called external donor). 1 Alkoxysilanes, 1,3-diethers, aromatic esters (benzoates and phthalates in particular), and recently aliphatic esters (succinates in particular) were shown to be particularly effective donors. 1 The resulting active system possesses extreme chemical complexity, and the polypropylene that is obtained presents very different properties depending on the specific components and recipe used in the preparation. Focusing on the role of the LB is fundamental in the overall catalyst performance because it can significantly impact (i) the microstructure of the obtained polypropylene; (ii) the molecular mass distribution; and (iii) the response to molecular hydrogen, and it can also have an impact on the morphology of the catalyst because they can stabilize small primary crystallites of MgCl 2 and/or influence the amount and distribution of TiCl 4 in the final catalyst. 2−16 The characterization of heterogeneous Ziegler−Natta catalysts has been the subject of several studies, which underlines the difficulties inherent in the detailed understanding of these catalysts. 13,15−36 Nevertheless, these studies allowed us to clarify several points that are now well accepted. For example, it is clearly accepted that the primary particles of activated MgCl 2 are composed of a few irregularly stacked Cl−Mg−Cl sandwichlike monolayers. 37 These MgCl 2 layers should be terminated by the (104) and (110) lateral cuts 24,38 that contain coordinatively unsaturated Mg 2+ ions with coordination numbers of 4 and 5 on the (110) and (104) cuts, respectively, as shown in Figure 1. 38,39 The problems start with the quantification of the relative numbers of (104) and (110) lateral cuts, which of course also depends on the recipe used for catalyst preparation. MgCl 2 monolayers forming the (104) lateral cut were suggested to be more stable than the...

A method was de®eloped that obser®es polymer particles during a polymerization reaction using optical-and infrared imaging. The yielding images gi®e good insight into catalyst-specific properties as shape replication, distribution of acti®ity, and the acti®ation. The ad®antage of this method is the possibility to link beha®ior of indi®idual particles to its own specific properties. The infrared method measures surface temperatures of growing particles. Such data are useful to feed mathematical models describing the growth of single particles. The temperature᎐ time cur®es showed maximum temperature to be reached after some minutes; a simple model was used to describe this effect qualitati®ely. The maximum particle temperature rise was 20 K, but depended strongly on reaction rate and particle size. Temperature, prepolymerization method, catalyst recipe, and catalyst acti®ation time were ®aried for a fourth-generation Ziegler-Natta catalyst in polymerization of propylene and ethylene.