Theoretical Investigation of Active Sites at the Corners of MgCl2

Article pubs.acs.org/Macromolecules

Theoretical Investigation of Active Sites at the Corners of MgCl2 Crystallites in Supported Ziegler−Natta Catalysts Andrea Correa,†,‡,* Raffaele Credendino,†,∥ Jochem T. M. Pater,§ Giampiero Morini,§ and Luigi Cavallo†,‡,∥ †

Dipartimento di Chimica e Biologia, Università di Salerno, Via Ponte don Melillo Fisciano (SA), I−84084 Italy Dutch Polymer Institute, Eindhoven, The Netherlands § Centro Ricerche “Giulio Natta”, Basell Poliolefine It. SrL (a LyondellBasell Company), Piazzale Guido Donegani 12, Ferrara, I−44100 Italy ∥ King Abdullah University of Science and Technology (KAUST), Chemical and Life Sciences and Engineering, Kaust Catalysis Center, Thuwal 23955-6900. Saudi Arabia ‡

S Supporting Information *

ABSTRACT: 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.

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these two lateral cuts contain coordinatively unsaturated Mg2+ ions with coordination number 4 and 5 on the (110) and (104) cuts, respectively, as shown in Figure 1. On these coordinatively

INTRODUCTION Ziegler−Natta catalysts have been used in the commercial manufacture of various polymeric materials since 1956.1,2 In 2010, the total volume of plastics, elastomers, and rubbers produced from alkenes with these catalysts worldwide exceeded 100 million metric tons. Polymerization using the latest generation MgCl2-supported Ziegler−Natta catalysts is the most important method for industrial production of isotactic polypropylene.1−3 Unfortunately, they are multisite catalysts, which makes their understanding and design extremely complicated. Nevertheless the catalytic systems currently used have achieved exceptional levels of performance, which allows for the design of versatile, clean and economical industrial processes.3 Indeed, the characterization of heterogeneous Ziegler−Natta catalysts is a long-standing challenge, tackled in several experimental,3−10 and theoretical9,11−23 studies, but definitive answers have not been achieved yet. As for the support structure, the primary particles of activated MgCl2 are composed of a few irregularly stacked Cl−Mg−Cl sandwiches. Observations on MgCl2 microcrystals by optical microscopy3 and by HRTEM24 showed crystallites with corners between the lateral edges of 90° and 120°. This would support the co-presence of the two most accepted lateral cuts in MgCl2 crystallites, the (104) and (110) lateral cuts.25−27 For electroneutrality reasons, © 2012 American Chemical Society

Figure 1. Schematic representation of a MgCl2 monolayer fragment. The Mg atoms are colored in orange. The Cl atoms above the Mg plane are dark green colored, whereas the Cl atoms below the Mg plane are light green colored. The (104) and (110) lateral faces are indicated.

unsaturated Mg2+ cations, chemisorption of TiCl4 molecules is supposed to occur and a variety of active site structures were proposed.28,29 Received: January 24, 2012 Revised: March 19, 2012 Published: April 19, 2012 3695

dx.doi.org/10.1021/ma3001862 | Macromolecules 2012, 45, 3695−3701

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sentence: the most stereoselective Ti−active species probably look very similar to a Lewis base modif ied Ti−species adsorbed on the (110) lateral face, but the largely prevailing lateral face, in the absence of a Lewis base during catalyst preparation, probably is the (104). In order to expand our knowledge of these catalysts, and offer a possible solution to the formation of isotactic polypropylene from catalysts prepared in the absence of any Lewis base, we decided to investigate the behavior of Ti-species adsorbed at the corners. Although experiments show that corners such as those investigated here are formed in these catalysts, the philosophy of this study should be considered more as a shift from the paradigm that Ti-active species are only formed on perfect MgCl2 lateral edges. Indeed, each crystal has defects, and it cannot be excluded that Ti-active species can be formed on these defects. Here, we focus on the simplest defect occurring in all MgCl2 crystallitesthe corners. Specifically, we will report on the stereo and regioselective behavior of possible Ti-active species formed at the corners of MgCl2 crystallites, both in the absence as well as in the presence of Lewis bases. The manuscript is split into two parts. The former is focused on the adsorption of the internal donors in Chart 1 at the MgCl2 corners. The second part is focused on

Recently, Busico, Causà, and coauthors reported a systematic study about the relative stability of lateral faces in MgCl2 crystals.9 On the basis of periodic DFT calculations, they concluded that the dominant face is the (104), thus presenting unsaturated 5−coordinated Mg sites, and just a small fraction of the Mg sites at the surface are more unsaturated; they should correspond to (110) edge, and/or to crystal corners, or other defective locations. This conclusion is supported by a DRIFT study of a carbonyl compound adsorbed on MgCl2 samples activated in different ways,20 which suggested that the (104) lateral face significantly prevails over the (110) face also in ball−milled activated catalytic systems. These conclusions are further supported by the DFT analysis of MgCl2 crystallites of different size, which again indicated that the (104) face is definitely more stable, because it normally results in a reduced number of Mg vacancies at the surface.30 However, it is worth noting that the same study demonstrated that crystallization in the presence of a Lewis base can promote the formation of Lewis base covered MgCl2 crystallites with (110) edges. These results led us to revise the active site models proposed in the literature. In fact, calculations clearly indicated that TiCl4 is predominantly adsorbed on the (110) MgCl2 lateral face,12,13,18,22 which is in contradiction with the experimental and computational indication that the dominant adsorption sites are on the (104) MgCl2 lateral face.9,20 The same problem emerges from analysis of kinetic data and polymer structures. In fact, according to data on the number of active centers in the simplest TiCl 4 /MgCl 2 + AlEt 3 catalyst, aspecific sites significantly prevail over the stereospecific ones,31 although a remarkable amount of isotactic polypropylene is still formed. On the other hand, it is quite accepted that the (104) surface can contain stereospecific sites, resulting from the coordination of dinuclear Ti−chloride species as Ti2Cl8, and that the (110) surface, in the absence of Lewis bases, contains aspecific sites resulting from coordination of mono−nuclear TiCl4. Moreover, for the active sites on the (110) surface, the mechanism through which the Lewis bases enhance stereoselectivity was explained by us by observing that on the (110) surface, donors coordination can occur close to the active site, see Scheme 1.19

Chart 1

the stereo and regioselectivity of the proposed active sites. Effects of absorbed donors and surface reconstruction are also discussed.

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Scheme 1

COMPUTATIONAL DETAILS

Computational Details. DFT calculations were carried out using the Turbomole package,32 version 6.1. All energies and geometries were obtained at the BP86 level of theory.33−35 The electronic configuration of all the atoms was described by a triple-ζ basis set augmented with two polarization functions (Turbomole basis set TZVPP).36 The active site is supposed to be a Ti(III) specie, so that unrestricted DFT calculations were performed when stereo and regioselectivity was considered. Because of the size of the systems considered, full transition state searches were not performed, and the transition state for primary and secondary propene insertion into a Ti−iBu bond were approximated through a linear scan of the potential energy surface in the region of 2.10−2.20 Å of the new forming C−C bond, with a step of 0.05 Å. All other degrees of freedom were optimized. This choice was based on the fact that the potential energy surface in the region of the newly forming C−C bond 2.10−2.20 Å is quite flat.13,14,22,37,38 In each case, the geometry that is higher in energy is considered to be a reasonable approximation of the real transition state. A comparable approach was used by Ziegler and coworkers in the similarly difficult case of acrylates polymerization,39 and by us in a previous study on heterogeneous Ziegler−Natta catalysts.19 Crystallite Corners Model. The MgCl2 support was described using the cluster approach validated by Ziegler and co-workers.13 The

On the opposite, for the (104) surface it is not possible to hypothesize a direct effect on stereoselectivity because donors coordinate apart from active sites, see again Scheme 1. All this knowledge can be summarized in the following antithetic 3696


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MgCl2 bulk was assumed to be in the α crystalline phase, and the semiflexible (MgCl2)35 cluster sketched in Figure 2 was used. During

Table 1. Energy, in kcal/mol, of Lewis Base Adsorption on the MgCl2 Unit at the Corner of the Cluster of Figure 2a, Dissociation of the Corner MgCl2 in the Absence as Well as in the Presence of the Coordinated Lewis Base (MgCl2 Diss. Column), and Overall Thermodynamics If the Dissociated Mg Atom Is Coordinated by a Second Lewis Base (Overall Process Column) no LB 1,3-diether succinate alkoxysilane

Figure 2. Sketch of the (MgCl2)35 cluster used to simulate a corner of a MgCl2 crystallite. Mg atoms are in orange, Cl atoms in green. The Mg and Cl atoms that were not relaxed during geometry optimizations are reported in gray.

LB coord.

MgCl2 diss.

overall process

n.a. −29.8 −42.2 −30.5

21.2 14.6 15.2 10.8

21.2 −15.2 −27.1 −19.7

of the Lewis bases on the MgCl2 unit at the corner of the structure of Figure 2a is highly favored (see the rather high Lewis base coordination energy reported in Table 1). The calculated adsorption energies are close to those previously obtained on a large (110) surface for the same type of donors (30.8, 39.9, and 31.4 kcal/mol for the diether, the succinate and the alkoxysilane, respecitvely).19 This is not surprising, considering that coordination around the single Ti atom at the corner of the MgCl2 cluster of Figure 3b is extremely similar to that of a Ti atom on a long (110) stretch. Dissociation of a simple MgCl2 unit costs 21.2 kcal/mol, and a coordinated Lewis base is able to reduce this value down to 10.8 kcal/mol in case of the alkoxysilane. Starting from the cluster in the presence of the Lewis bases coordinated at the corner, dissociation of the MgCl2/LB fragment assisted by the coordination of another LB donor to provide the Mg-hexacoordinated MgCl2/ 2LB becomes energetically favored (see the rather negative overall process energies in Table 1). The Lewis base that improves more the thermodynamics of MgCl2 dissociation is the succinate, with an energy gain of 27.1 kcal/mol. Of course, the energy gain is related to the stabilizing effect of the Lewis base on the dissociated MgCl2 unit by formation of an hexacoordinated Mg complex. The overall process would be made energetically favored by coordination of additional Lewis bases to the unsaturated Mg atoms on the small (110) stretch formed on the cluster after MgCl2 dissociation. All energy values reported in Table 1 are not remarkably high. Thus, considering the temperatures at which these catalysts are prepared and used, the values we calculated suggest that the Lewis bases could contribute to surface reconstruction. Possible Active Species at the Corners of MgCl2 Crystallites. In this part of the manuscript, we report on the behavior of possible Ti−active species at the corners of MgCl2 crystallites, and the influence that surface reconstruction or coordination of the Lewis bases can have on the stereo and regioselectivity of these Ti−species. As a model of the active site, we have considered a mononuclear Ti species adsorbed on the short (110) MgCl2 stretch of the cluster reported in Figure 3b. This active species strongly resembles the active site proposed by Corradini and co-workers for the (110)−MgCl2 lateral cuts. The active Ti(III) atom is 6-fold coordinated and sits epitaxially on the MgCl2 surface in a configuration similar to bulk Mg atoms. The resulting octahedral Ti center is chiral, and its chirality can be labeled Δ or Λ. For the sake of simplicity, in all the calculations the configuration of the Ti atom is Λ.40 All energies discussed in this section are collected in Table 2. The approximated transition states for primary propene insertion into the Ti−iBu bond for this model, without any

the geometry optimizations, the atoms of the cluster colored in gray in Figure 2 were frozen in the position of a perfect crystal.

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RESULTS AND DISCUSSION Corners Stability. Figure 3 displays two possible structures for a MgCl2 crystal corner joining two (104) lateral cuts. Both

Figure 3. Schematic representations of the possible MgCl2 corner structure. Mg atoms of (104) surfaces are colored in orange, whereas Mg atoms of the corner (that is 4-coordinated) are colored in violet. The Cl atoms above the Mg plane are dark green colored, whereas the Cl atoms below the Mg plane are light green colored.

structures are characterized by an overall angle of 120° between the (104) lateral cuts. The two corners can be easily connected by elimination/ addition of one MgCl2 unit. The structure of Figure 3b thus presents a very short (110) MgCl2 sequence, forming angles of 150° with the (104) lateral cuts. The short (110) sequence of the cluster reported in Figure 3b is well suited to epitaxially adsorb a single TiCl4 molecule. Formation of the cluster reported in Figure 3b requires elimination of a MgCl2 unit, an endothermic process calculated to cost 21.2 kcal/mol. Of course, elimination of the single MgCl2 unit can be assisted by coordination of the Lewis bases, as shown in Scheme 2. This process has been calculated for the three Lewis Scheme 2

bases shown in Chart 1, and the corresponding energy values are reported in Table 1. In all cases, we found that adsorption 3697


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Table 2. Relative Energies, in kcal/mol, of the Transition States Corresponding to Primary (or 1,2) and Secondary (or 2,1) Insertion of Propene into the Ti−iBu Bond of the Models Depicted in Figure 4, Naked Entry, Figure 5, ExtraMgCl2 Entry, and Figures 6, 1,3-Diether, Succinate, and Alkoxysilane Entries, Respectively naked extra-MgCl2 1,3-diether succinate alkoxysilane

1,2-re

1,2-si

2,1-re

2,1-si

0.0 0.0 0.0 0.0 0.0

−0.2 1.4 3.2 3.8 4.1

1.3 5.2 5.8 5.3 5.6

1.1 1.8 1.7 2.1 1.7

enantioface (Figure 4a) corresponds to the insertion of the re-propene on a (−) growing chain.40 The energy difference between these two approximated transition states, ΔE‡Stereo, is a measure of the stereoselectivity of the active site. The very low ΔE‡Stereo we calculated, 0.2 kcal/mol, is well below the accuracy that can be expected from this kind of calculation. Nevertheless, consistent with previous results of isolated Ti-active species adsorbed on the (110) lateral cut,19 it clearly indicates that an isolated C2-symmetric Ti atom, also at the crystal corner, is essentially nonstereoselective. The approximate transition states for secondary propene insertion (Figure 4c,d) correspond to re- and si-propene insertion on a (−) growing chain, respectively. These two transition states are approximately 1.3− 1.1 kcal/mol higher in energy relative to the transitions states leading to primary insertion. These energy differences, ΔE‡Regio, are a measure of the regioselectivity of the active site. The rather low value we calculated for both propene enantiofaces indicated that the model we proposed should introduce some regiomistakes into the polypropylene chain, and these regiomistakes should be nonenantioselective due to the very low energy difference between the re- and si-secondary propene insertion, 0.2 kcal/mol. Thus, these results indicate that the polypropylene produced by an isolated Ti species at the MgCl 2 crystal corner should be atactic and present some nonenantioselective regiodefects. In short, the polypropylene formed by these active species should be consistent with the atactic polypropylene obtained in the absence of Lewis bases.14,41,42 The next step was to consider surface reconstruction by coordination of extra MgCl2 units on top of the (104) lateral cuts of the cluster shown in Figure 3b. The approximate transition states for primary and secondary propene insertion for this active site are reported in Figure 5. Focusing on stereoselectivity, the presence of the extra MgCl2 layers resulted in a small increase in stereoselectivity, ΔE‡Stereo = 1.4 kcal/mol. This is due to the presence of steric stress between the growing chain and a nearby MgCl2 unit in the transition state for primary re-propene insertion on (+) the growing chain (distances slightly below 4 Å were found to be between one methyl growing chain and atoms on the surface). Moving to secondary propene insertion, the effect of surface reconstruction is relevant only for one of the two enantiofaces. In fact, transition states for secondary insertion of si- and re-propene are calculated to be 1.8 and 5.2 kcal/mol higher in energy relative to the most favored primary insertion, respectively. Overall, this possibly active Ti-species could form isotactic polypropylene, although isotacticity would not be remarkably high. This model could offer an explanation for the formation of scarcely isotactic polypropylene in the absence of Lewis basees, both during catalyst preparation and polymerization. To date, the only proposed active species capable of forming isotatctic polypropylene in the absence of Lewis basees are dimeric Ti-species adsorbed on the (104) lateral cuts, a model introduced in seminal works by Corradini and co-workers.25,43−45 Of course, we are not claiming that the corner model proposed here corresponds to real active species. Rather, we believe that the model we proposed indicates that surface reconstruction and defects should be considered when heterogeneous Ziegler−Natta catalysts are investigated. Indeed, similar MgCl2 reconstruction could also occur around Ti-active species coordinated on the (110) lateral cut. Finally, we report on the influence of a Lewis base on the stereo and regioselective behavior of the Ti active species on

Lewis bases coordinated around it, are sketched in Figure 4. The most favored transition state (Figure 4b) corresponds to

Figure 4. Top and side views (left and right) of the transition states leading to primary, parts a, and b, and secondary, parts c and d, propene insertion into the Ti−iBu bond. For the sake of clarity, in parts a-d only a section of the MgCl2 cluster used in the calculations is reported. Part e, view of the used cluster perpendicular to the (001) basal plane.

the primary insertion of si-propene on a (+) growing chain. The transition state for primary insertion of the other propene 3698


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due to the short distance between the coordinating O atoms, prefer to chelate to Mg atoms on the (110) lateral cut, rather than to coordinate on the (104) lateral cut.19 For this reason,

Figure 5. Top and side views (left and right) of the transition states leading to primary, parts a, and b; and secondary, parts c and d, propene insertion into the Ti−iBu bond. An additional layer of MgCl2 is adsorbed on the (104) lateral faces. For the sake of clarity, in parts a−d only a section of the MgCl2 cluster used in the calculations is reported. Part e, view of the used cluster perpendicular to the (001) basal plane. The Mg and Cl atoms of the additional MgCl2 layers are colored in dark green and dark orange, respectively. A closer view of secondary the transition states for secondary propene insertion is reported in Figure S3 of the Supporting Information.

Figure 6. Top and side views (left and right) of the transition states leading to primary, parts a and b, and secondary, parts c and d, propene insertion into the Ti−iBu bond. A Lewis base is coordinated to the (104) MgCl2 atoms on both sides of the Ti-species. In parts a−d, only a section of the MgCl2 cluster used in the calculations is reported. Part e shows a view of the used cluster perpendicular to the (001) basal plane, with a better visualization of the coordinated Lewis bases. A closer view of secondary the transition states for secondary propene insertion is reported in Figure S4 of the Supporting Information.

the corner. Within this scope, we considered the Lewis bases in Chart 1. We hypothesized a donor molecule coordinated on both sides of the isolated Ti active specie, thus preserving the overall C2-symmetry of the Ti-active species.19 In the case of the 1,3-diether and the succinate, the donor coordinates to two vicinal Mg atoms of the (104) lateral cut on both sides, a coordination mode named (104)-bridge in a previous study.19 It is worthy to note that one of the Mg atoms to which the Lewis base is coordinated also interacts with a Cl atom of the Ti species,19 see Figure 6. As for the alkoxysilane, we already demonstrated that alkoxysilanes,

we coordinated the two alkoxysilanes to a single Mg atom. The approximate transition states calculated for primary and secondary propene insertion in the presence of the succinate and the alkoxysilane are displayed in Figures S1 and S2, Supporting Information. The calculated stereo and regioselectivity values are reported in Table 2 for all of the systems considered. 3699


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that can be formed in the real heterogeneous Ziegler−Natta catalytic system.

The numbers reported in Table 2 indicate that all three donors considered strongly increase the stereoselectivity of the Ti-active specie at the corner, and this influence is quite larger than that calculated when an additional MgCl2 layer is present, see the ΔEStereo values in Table 2. In all cases, primary insertion of re-propene is clearly favored with respect to si-propene insertion; for the considered donors ΔEStereo is in the range of 3−4 kcal/mol. Examination of the structures in Figure 6 explains this influence. The most stable transition state for primary insertion in the presence of 1,3-diether, see Figure 6b, shows that the growing chain once again assumes a chiral orientation, (−) in this case to minimize steric interactions with the bulky diether molecules. Alternatively, in the transition state leading to a stereomistake, see Figure 6a, propene insertion is disfavored by steric interaction between the (+) growing chain and one of the diethers. Focusing on regiomistakes, in all cases, 2,1 insertion of a repropene molecule is strongly disfavored in the presence of two Lewis bases coordinated near the Ti-species, while 2,1 insertion of a si-propene is scarcely influenced. Consequently, coordination of the Lewis bases makes both primary and secondary propene insertion stereoselective. These results are consistent with the experimental evidence of the beneficial effect of the Lewis bases on increasing stereoselectivity of isotactic polypropylene production,46−49 as well as in enhancing regioselectivity by increasing the stereoselectivity of regiomistakes.50

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ASSOCIATED CONTENT

S Supporting Information *

Results of linear transit calculations, views of the transition states, and DFT optimized geometries and absolute energies. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.C. thanks LyondellBasell for continuous financial support. This work is part of the Research Programme of the Dutch Polymer Institute, Eindhoven, The Netherlands, Project No. 707 SD−ZN. L.C. thanks the HPC team of Enea for using the ENEA-GRID and the HPC facilities CRESCO in Portici (Italy) and for access to remarkable computational resources.

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REFERENCES

(1) Natta, G. Nobel Lectures in Chemistry, 1963−1970; Elsevier: Amsterdam, 1972; p 27. (2) Ziegler, K. Nobel Lectures in Chemistry, 1963−1970; Elsevier: Amsterdam, 1972; p 6. (3) Albizzati, E.; Giannini, U.; Collina, G.; Noristi, L.; Resconi, L. In Polypropylene Handbook; Moore, E. P., Ed.; Hanser: Munich, Germany, 1996; p 11. (4) Brant, P.; Speca, A. N.; Johnston, D. C. J. Catal. 1988, 113, 250. (5) Kashiwa, N.; Yoshitake, J. Makromol. Chem. 1984, 185, 1133. (6) Paukkeri, R.; Lehtinen, A. Polymer 1993, 34, 4083. (7) Randall, J. C. Macromolecules 1997, 30, 803. (8) Busico, V.; Cipullo, R.; Monaco, G.; Talarico, G.; Vacatello, M.; Chadwick, J. C.; Segre, A. L.; Sudmeijer, O. Macromolecules 1999, 32, 4173. (9) Busico, V.; Causa, M.; Cipullo, R.; Credendino, R.; Cutillo, F.; Friederichs, N.; Lamanna, R.; Segre, A.; VanAxel Castelli, V. J. Phys. Chem. C 2008, 112, 1081. (10) Potapov, A. G.; Kriventsov, V. V.; Kochubey, D. I.; Bukatov, G. D.; Zakharov, V. A. Macromol. Chem. Phys. 1997, 198, 3477. (11) Boero, M.; Parrinello, M.; Weiss, H.; Hüffer, S. J. Phys. Chem. A 2001, 105, 5096. (12) Boero, M.; Parrinello, M.; Terakura, K. J. Am. Chem. Soc. 1998, 120, 2746. (13) Seth, M.; Margl, P. M.; Ziegler, T. Macromolecules 2002, 35, 7815. (14) Cavallo, L.; Guerra, G.; Corradini, P. J. Am. Chem. Soc. 1998, 120, 2428. (15) Jensen, V. R.; Børve, K. J.; Ystenes, M. J. Am. Chem. Soc. 1995, 117, 4109. (16) Sakai, S. Int. J. Quantum Chem. 1997, 65, 739. (17) Puhakka, E.; Pakkanen, T. T.; Pakkanen, T. A. J. Mol. Cat. A: Chem. 1997, 120, 143. (18) Monaco, G.; Toto, M.; Guerra, G.; Corradini, P.; Cavallo, L. Macromolecules 2000, 33, 8953. (19) Correa, A.; Piemontesi, F.; Morini, G.; Cavallo, L. Macromolecules 2007, 40, 9181. (20) Stukalov, D. V.; Zakharov, V. A.; Potapov, A. G.; Bukatov, G. D. J. Catal. 2009, 266, 39. (21) Zakharov, I. I.; Zakharov, V. A.; Potapov, A. G.; Zhidomirov, G. M. Macromol. Theory Simul. 1999, 8, 272. (22) Seth, M.; Ziegler, T. Macromolecules 2003, 36, 6613.

CONCLUSION In this manuscript, we proposed models of possible catalytic active species corresponding to Ti−chloride species adsorbed at the corners of MgCl2 crystallites, and we investigated their stereo and regioselective behavior. The main conclusions are that removal of MgCl2 units at the corner of (104) edged MgCl2 crystallites, considering the harsh conditions used for catalysts preparation, cannot be excluded; this reconstruction process can be aided by the Lewis bases that are added during catalyst preparation and polymerization. Removal of a MgCl2 unit creates a short (110) stretch joining the (104) lateral cuts. Adsorption of TiCl4 on the created vacancy creates a Ti-species that can possibly promote propene polymerization. The stereo and regioselective behavior of this Ti-species was also investigated. The main conclusions we obtained are: (i) isolated Ti-species at the crystal corners would lead to atactic polypropylene. The regioregularity of this polypropylene should not be very high and regiomistakes would be not enantioselective. (ii) Adsorption of an additional MgCl2 layer on the (104) lateral cuts around the Ti-species at the corner, as well as the adsorption of Lewis bases in the proximity of the Ti-active species would improve steroselectivity. This effect is predicted to be stronger in the case of the Lewis bases. (iii) Adsorption of an additional MgCl2 layer on the (104) lateral cuts around the Ti-species, as well as adsorption of Lewis bases in the proximity of the Ti-active species, would improve regioselectivity by disfavoring one of the two 2,1-propene insertion modes. As final remark, please note that the models proposed herein are hypothetical, as any model of active species in this unique field. Nevertheless, we believe that considering models of active species different from those deriving from standard TiCl4 adsorption on perfect (104) and (110) lateral cuts should also be considered. Any support has limited size and defects, and it cannot be excluded that TiCl4 adsorption on corners and defects could enrich the variety of possible active centers 3700


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Theoretical Investigation of Active Sites at the Corners of MgCl2

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