Investigating Alkoxysilane Coverage and Dynamics on the (104) and

Article pubs.acs.org/JPCC

Investigating Alkoxysilane Coverage and Dynamics on the (104) and (110) Surfaces of MgCl2‑Supported Ziegler−Natta Catalysts Raffaele Credendino,† Jochem T. M. Pater,‡ Dario Liguori,‡ Giampiero Morini,‡ and Luigi Cavallo*,†,§ †

Chemical and Life Sciences and Engineering, Kaust Catalysis Center, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia ‡ LyondellBasell Polyolefins, G. Natta Research Center, P. le G. Donegani 12, 44100 Ferrara, Italy § Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, The Netherlands S Supporting Information *

ABSTRACT: 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 MgCl2. 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 MgCl2 monolayers on the (104) and (110) surfaces requires donor dissociation. The same holds for alkoxysilane migration on a single (110) MgCl2 monolayer. However, in the case of the (104) surface we found a very low energy pathway for alkoxysilane migration along the same monolayer.

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Mg2+ ions with coordination numbers of 4 and 5 on the (110) and (104) cuts, respectively, as shown in Figure 1.38,39

INTRODUCTION Heterogeneous Ziegler−Natta (ZN) catalysts are the most important catalysts in the industrial production of isotactic polypropylene. The typical catalysts used are MgCl2/TiCl4/ 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 MgCl2 and/or influence the amount and distribution of TiCl4 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 MgCl2 are composed of a few irregularly stacked Cl−Mg−Cl sandwichlike monolayers.37 These MgCl2 layers should be terminated by the (104) and (110) lateral cuts24,38 that contain coordinatively unsaturated © 2012 American Chemical Society

Figure 1. Schematic representation of a MgCl2 crystallite presenting (104) and (110) lateral cuts.

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. MgCl2 monolayers forming the (104) lateral cut were suggested to be more stable than the (110) lateral cut because of the lower unsaturation of the surface Mg atoms on the (104) monolayer. A clear quantification of this old concept was provided by Busico and co-workers, who used periodic DFT calculations to Received: August 31, 2012 Revised: October 3, 2012 Published: October 3, 2012 22980

dx.doi.org/10.1021/jp308658c | J. Phys. Chem. C 2012, 116, 22980−22986

The Journal of Physical Chemistry C

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move on the MgCl2 surfaces. To fill this gap, we present here a DFT study focused on the two points above. This work is split into three parts. In the first part, we analyze the effect of coverage on the coordination of rather small alkoxysilane D1; see Chart 1. For this part, we used a small

estimate the energy preference for (104)-terminated versus (110)-terminated MgCl2 crystallites.40 This result was confirmed recently by us and further suggested that for uncovered crystallites composed of the same number of MgCl2 units the (104)-terminated crystallite is more stable than the (110)terminated crystallite because it can minimize the number of Mg vacancies. Our results also suggested that the density of vacancies (i.e., the number of vacancies divided by the total number of MgCl2 units composing a given crystallite) can be used as a general descriptor to place crystallites of different size and shape on the same scale.41 However, we also demonstrated that the coordination of the donor can completely change the stability order of the two faces. Indeed, the high-energy gain associated with the coordination of typical donors42 results in a clear stabilization of the (110) face because it maximizes the number of Mg−O interactions. Again, the density of vacancies can be used as a general descriptor to place crystallites of different size and shape as well as uncovered and donor-covered crystallites on the same scale. Finally, our analysis indicated that larger crystallites should be obtained in the absence of any donor, whereas strongly coordinating donors should induce the formation of smaller crystallites to maximize the Mg−O interaction. Our results are consistent with the general idea that the specific characteristics of the catalyst strongly depend on the recipe used to prepare the catalyst, as clearly demonstrated by Andoni and coauthors.35 Our study is consistent with recent results by Thüne and co-workers. They demonstrated that in the absence of donors the sintering of MgCl2 leads to very large MgCl2 platelets that offer only a very small number of coordination sites whereas in the presence of donors the MgCl2 crystallites remain much smaller because of the stabilization of the surface sites by donor adsorption.36 However, all of these studies offered a static picture of the most likely situation with and without donors, but it did not give insight into the dynamics of these processes. In other words, they provided no clue as to the dynamics of MgCl2 reconstruction or the capability of the donors to move on the MgCl2 surfaces and to induce MgCl2 reconstruction or the capability of the donors to move on the MgCl2 surfaces even during polymerization. Indeed, there is evidence that donors can move on the MgCl2 surface. First, it is well accepted that the microstructure of iPP from ZN catalysts presents most of the concentrated stereodefects, thus forming stereoirregular stereoblocks.43 This is a remarkable difference from iPP produced by classical single-site metallocenes, which instead present the stereodefects randomly distributed along the macromolecule.44 To explain this peculiarity, Busico proposed a three-site model for ZN catalysts, and within the three-site dynamic model, one of the mobile ligands could indeed be a donor. Furthermore, HR-MAS experiments by Busico and coauthors clearly indicated that alkoxysilanes coordinated to the MgCl2 surfaces are mobile, and this mobility could impact the catalyst structure and catalytic performance.45 Finally, the same study showed that the alkoxysilane presents low or high mobility as a function of surface coverage.45 Although the interaction between the donors and the MgCl2 crystallites were investigated theoretically,42,46−48 these studies are limited to the analysis of a single donor coordinated on the surface or to the effect of the donors on stereoselectivity. To our knowledge, no study has systematically investigated the effect of increasing surface coverage on the coordination ability of the donors or the mechanism through which a donor can

Chart 1

alkoxysilane to reduce the computational cost. Nevertheless, we also made test calculations using industrially relevant alkoxysilane D2. In the second part, we investigate the dynamic stability of a single D2 molecule when coordinated on the (104) and (110) surfaces. In the third part, we investigated several possible migration pathways for D2 on both the (104) and (110) surfaces.

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COMPUTATIONAL DETAILS DFT static and dynamics simulations were performed using the Born−Oppenheimer scheme as implemented in the CP2K Quickstep code.49 The electronic structure calculations were carried out at the DFT level by using the Perdew−Burke− Ernzerhof exchange and correlation functional.50 The CP2K program employs a mixed basis set approach with Gaussiantype orbitals (GTO) and plane waves (PWs).49 GTO functions are used to expand the molecular orbitals and the charge density in real space, whereas PWs are used for the representation of the charge density in reciprocal space. An energy cutoff of 300 Ry is used for the plane-waves basis set. A double-ζ basis set with a polarization function, in conjunction with the Goedecker−Teter−Hutter pseudopotentials,51 was used for all of the atoms. Pseudopotentials of the GTH form for all of the elements were used.51 A cubic box of 14.5 × 21.7 × 35.0 Å3 was used to model the (104) surface, and a box of 18.8 × 30.0 × 17.6 Å3 was used to model the (110) surface. In both cases, five layers of MgCl2 were considered in the direction perpendicular to the surface to model the bulk. For all systems, the first bulk layer was frozen. The two systems are composed of 60 and 45 units of MgCl2, respectively. In the dynamic simulation, the equations of motion were integrated by using a time step of 0.5 fs. The MgCl2/silane adduct was equilibrated at 300 K for 10 ps by using a canonical-sampling-through-velocityrescaling thermostat.52 The energy barriers for the donor migration were evaluated by an improved tangent-nudged elastic band (NEB) method,53 which allows us to find the minimum-energy path (MEP) between a given initial and final state of a transition or reaction. The MEP is found by constructing a set of images (replicas) of the system (for migrations, we adopted 11 replicas) between the initial and the final state. A spring interaction between adjacent images is added to ensure the continuity of the path, thus mimicking an elastic band. In one case, corresponding to intralayer donor migration on the (110) surface, the energy barrier found with the NEB method was tested versus the energy barrier found using the classical cluster method, an approach validated by Ziegler and co-workers.54 These calculations were carried out using Gaussian 09 software.55 22981



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Energies, geometries, and vibrational frequencies were obtained at the PBEhPBE level56 of theory in conjunction with Ahlrichs' triple-ζ basis set57 incorporating a polarization function. A cluster of 11 MgCl2 units was chosen, and all of the MgCl2 units were frozen. The transition state (TS) was located by the synchronous transit-guided quasi-Newton approach58 (QST3 method) implemented in Gaussian 09. The genuineness of the transition state was ensured by the presence of one imaginary frequency.

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Figure 2. Top view of the (a) (104) and (b) (110) surfaces completely covered by donor D1.

RESULTS AND DISCUSSION Coverage. The saturation of coordinative vacancies, or in other sense the donor coverage, is an important factor controlling the relative stability of surfaces with an impact on the TiCl4 adsorption process, several catalytic aspects and the crystallite morphology because Mg pentacoordinated (104) lateral surfaces should be dominant without donors, whereas the Mg tetracoordinated (110) surfaces should dominate in presence of a large donor excess. Taking these aspects into account, we model different coverage for both surfaces using the smallest silane, dimethoxy(dimethyl)silane, D1, in the following. We define the coverage function θ as the ratio between the number of oxygen atoms adsorbed and the number of Mg vacancies on the surface (eq 1). noxygen θ= nv (1)

observed at around θ = 0.5, with ⟨dMg−O⟩ increasing rapidly at θ > 0.5. Nevertheless, in all cases, all of the oxygen atoms remain attached to the surface. The influence of surface coverage on the (110) surface is rather similar, although the effects are more pronounced. The average distance ⟨dMg−O⟩ is rather stable up to θ = 0.5, and then it increases to 2.95 Å for θ = 1. This sharp increase in ⟨dMg−O⟩ at high θ again indicates congestion on the surface at high coverage. Interestingly, the congestion is so high that at θ > 0.67 the added silane is not able to chelate the Mg atoms on the surface and monocoordination occurs, with one Mg−O distance close to 2.3 Å and the other instead being around 3.8 Å. Moving to the energy of donor adsorption, we find that the average adsorption energies reported in Table 1 indicate a large impact of surface coverage on the Ead values, mirroring the trend in the ⟨dMg−O⟩ values. When we focus on the (104) surface, the Ead of an isolated silane, θ = 0.17, is −25.6 kcal/ mol. By increasing the coverage at θ = 0.33, with two D1 molecules on different MgCl2 layers, the average binding energy decreases by only 1.5 kcal/mol. A similar effect is observed when a third D1 molecule is adsorbed on the third layer, θ = 0.50. This indicates that up to one-half of the vacancies can be covered by donor molecules without any sizable impact on the adsorption energy. This situation is reported in Figure S1. Beyond this θ value, the adsorption of additional silane molecules inevitably increases the congestion at the surface, and as a consequence, the binding energy decreases significantly. This aspect is evident when the coverage increases to θ = 0.67 or more, with Ead decreasing to only 14.3 kcal/mol for θ = 1.0. An even stronger effect is predicted for donor coordination on the (110) surface, with Ead decreasing from 29.0 kcal/mol when an isolated donor is coordinated to 7.2 kcal/mol only at complete coverage. Of course, the extremely low Ead at full coverage also reflects the presence of monocoordinated donors, as evidenced by the analysis of the ⟨dMg−O⟩ distances. In short, our geometrical and energy analysis indicates that complete coverage of the surfaces by donor molecules is hampered by steric repulsion between donors coordinated on vicinal Mg atoms. Furthermore, the effect we reported in Table 1 was obtained with the rather small alkoxysilane presenting two methyl groups on the Si atom. To test for the effect of silane bulkiness, we compared the Ead of D1 with that of the industrially relevant dicyclopentyl-dimetoxysilane. For the sake of computational power, we compared the situations with two donors coordinated only on vicinal Mg atoms on the same layer. For small dimethyl dimethoxysilane D1, Ead decreases by 3.1 kcal/mol when the two donors are coordinated on vicinal Mg atoms on the (104) surface, whereas for the large dicyclopentyl dimethoxysilane Ead decreases by 3.2 kcal/mol.

The average donor adsorption energy Ead is calculated according to eq 2 Ead =

EMg/D − EMg − nDE D nD

(2)

where EMg/D is the energy of the system composed of donor molecule adsorbed on the MgCl2 surface, EMg and ED are the energies of the uncovered MgCl2 system and the free donor, respectively, and nD is the number of adsorbed donor molecules. On the basis of previous work, the (104)-bridge and (110)-chelate coordination modes were considered.42 Focusing on geometry, the most important parameter we considered is the average distance of the adsorbed oxygen atoms of D1 from the surface, ⟨dMg−O⟩, reported in Table 1 and Table 1. Average Mg−O Distance, ⟨dMg−O⟩ in Å, and Average Adsorption Energy, Ead in kcal/mol, for the (104)Bridged and (110)-Chelated Adsorption of D1 on MgCl2 coverage θ

⟨dMg−O⟩

0.17 0.33 0.50 0.67 0.83 1.00

2.18 2.20 2.20 2.27 2.30 2.35

Ead

⟨dMg−O⟩

25.6 24.1 23.8 19.3 16.6 14.3

2.17 2.21 2.23 2.24 2.56 2.95

(104)

Ead (110) 29.0 26.5 22.3 21.4 16.4 7.2

Figure 2. With regard to the (104) surface, ⟨dMg−O⟩ = 2.18 Å at the lowest coverage (θ = 0.17 corresponding to one molecule of D1 adsorbed on the surface) increases to 2.35 Å at θ = 1, corresponding to a (104) surface that is completely covered with D1. The sizable increase in ⟨dMg−O⟩ at high coverages indicates that donors interact repulsively at high coverage. As for intermediate θ values, a sharp increase in ⟨dMg−O⟩ is 22982



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consistent with our previous analysis, which indicated that the alkoxysilane has to stretch to reach the (104)-bridge coordination mode. Also of interest is the behavior of the Mg−Cl bonds trans to the Mg−O bonds. In the case of the (104) surface, the average ⟨dMg−Cl⟩ distance trans to the Mg−O bonds is 3.20 ± 0.18 Å, a value remarkably longer than the ⟨dMg−Cl⟩ distance in the case of an uncovered Mg atom on the (104) surface, 2.57 ± 0.10 Å, and with respect to the ⟨dMg−Cl⟩ distance in a bulk Mg atom, 2.71 ± 0.04 Å. Moving to the (110) surface, we find the average ⟨dMg−Cl⟩ distance trans to the Mg−O bonds to be 2.70 ± 0.10 Å, again remarkably longer than the ⟨dMg−Cl⟩ distance in the case of an uncovered Mg atom on the (110) surface, 2.48 ± 0.09 Å, and much closer to the ⟨dMg−Cl⟩ distance in a bulk Mg atom, 2.70 ± 0.10 Å. The tendency of Mg atoms on the surface to be attracted to the bulk of the crystal was noticed by Parrinello and co-workers. In conclusion, these simulations suggest that alkoxysilane coordination on both surfaces is also rather stable from a dynamic point of view and that problems with donor coordination can arise only at high coverage. Furthermore, it is tempting to hypothesize that eventual incomplete coverage of the surfaces by the donors could create vacancies where even the weakly coordinating TiCl4 could bind without having to compete with the more aggressively binding donors. Migration. In this section, we examine possible mechanisms for the migration of D2 coordinated on the (104) and (110) surfaces. Specifically, we investigate the following possible migration pathways (Figure 4): (i) The interlayer pathway,

This small difference is understandable, considering that in the case of (104)-bridge coordination the Si substituents on vicinal donors are at rather large distance; see Figure S2. Much larger is the impact for vicinal coordination on the (110) face. In this case, Ead decreases by 2.1 kcal/mol for D1 and by 8.2 kcal/mol for the dicyclopentyl dimethoxysilane. This is a consequence of steric clashes between the larger cyclopentyl substituents of one donor with the vicinally coordinated donor; see Figure S3. This result indicates that the problems encountered in reaching the high coverage that we derived for D1 are probably even stronger with bulkier donors. Furthermore, the small Ead at high coverage coupled with an unfavorable entropic term not considered in the present calculations results in low free energies of adsorption and suggests that complete coverage of the Mg lateral cuts is difficult to achieve. Coordination Stability. Next we examined the dynamic behavior of a single donor molecule coordinated to the (104) and (110) surfaces. For this study, we used industrially relevant alkoxysilane D2 in the (104)-bridge and (110)-chelate geometries. In Figure 3, we report the Mg−O distances during

Figure 4. Possible migration pathways for donor D2 coordinated on the (a) (104) and (b) (110) surfaces.

with D2 moving between two layers on two different pathways, indicated as A1 and A2 in Figure 4. (ii) The intralayer pathway, with D2 moving on the same layer, indicated as B and C in Figure 4. Pathways A1, A2, and B essentially correspond to a donor jump that requires the breaking and formation of two Mg−O bonds. This kind of migration is possible for both the (104) and (110) surfaces; see Figure 4. Pathway C is a rotation that requires the breaking and formation of only one Mg−O bond; see Figure 4. We start by discussing interlayer migration on the (104) surface along pathways A1 and A2. The NEB -approximated transition state for D2 migration along the A1 pathway shows alkoxysilane D2 between the two layers; see Figure 5a. The two oxygen atoms are parallel to the ab crystallographic plane, with an average distance of 5.1 Å with respect to the Mg atoms in the starting geometry and an average distance of 7.5 Å with respect to the Mg atoms in the final geometry. The transition state along the A2 pathway is rather similar, again with alkoxysilane D2 between the two MgCl2 layers. The main difference with respect to the A1 pathway is that the

Figure 3. Time evolution of the Mg−O distances for D2 coordination to the (a) (104) and (b) (110) surfaces.

the DFT molecular dynamics simulations. An inspection of Figure 3 clearly indicates that during the 10 ps of simulation alkoxysilane D2 remains adsorbed strongly on both surfaces with average Mg−O distances of 2.3 ± 0.1 Å on the (104) surface (Figure 3a) and 2.2 ± 0.1 Å on the (110) surface (Figure 3b). Fluctuations are larger in the case of coordination to the (104) surface, with peaks reaching values of roughly 2.6 Å. Nevertheless, after these large fluctuations the donors also rebind to the surface. The larger fluctuation in the Mg−O distances in the case of coordination on the (104) surface is 22983



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Figure 5. Geometry of the NEB-approximated transition state for D2 migration along the A1 pathway on the (104) surface and of the intermediate structure along the C pathway on the (104) surface.

alkoxysilane has to move diagonally to reach the final geometry. The average distances of the O atoms from the Mg atoms in the starting and final geometries are 5.1 and 7.7 Å, respectively. Overall, the transition state along the A1 and A2 pathways is best described as a geometry in which the donor is completely dissociated from the MgCl2 surface and is forced to jump over the Cl atoms of the starting layer to reach the vicinal layer. Regarding the intralayer migrations on the (104) surface, the transition state along the B pathway shows alkoxysilane D2 sliding on the same (104) layer by two exposed Mg atoms. This sliding movement requires the breaking of the Mg−O bonds in the starting geometry, with the consequent detachment of the donor from the surface. The last pathway on the (104) surface, pathway C, instead requires the simpler rotation of the donor around one Mg−O bond, which acts as a pivot point. During this rearrangement, only one Mg−O bond is broken, and there is an intermediate structure with both O atoms interacting with the same Mg atom; see Figure 5b. The Mg−O bond distance in this intermediate structure, 3.7 Å, is much longer than in the starting and final geometries, 2.1 Å. In short, the movement along pathway C can be assimilated to a crawling movement of the donor on the (104) layer because the donor always maintains contact with surface Mg atoms. We discuss now donor migration on the (110) surface. In this case, both interlayer and intralayer migration pathways require the breaking of both the Mg−O bonds in the starting geometry in which alkoxysilane D2 is chelated to one Mg atom. For these reasons, all of the transition states are rather similar, with the donor detached from the (110) surface midway between the starting and final geometries. The Mg−O distances in the transition state along interlayer pathways A1 and A2 are 5.6 and 5.8 Å, respectively, whereas that along interlayer pathway B is 5.3 Å. The NEB energy profile corresponding to D2 migration along the various pathways is shown in Figure 6. The reaction coordinate, γ, is defined in eq 3 i f γ = dMg −O − dMg−O

diMg−O

Figure 6. NEB energy profile for D2 migration on the (a) (104) and (b) (110) layers.

barrier, about 8 kcal/mol, with the minimum-energy intermediate of Figure 6a being slightly less stable than that of the starting and final geometries. As for donor migration on the (110) surface, Figure 6b, all of the pathways considered show remarkably high energy barriers, roughly 35 kcal/mol, close to complete donor dissociation (34.6 kcal/mol). These results are in qualitative agreement with the HR-MAS NMR experiments of Busico, Segre, and co-workers, which indicated a rather high mobility of alkoxysilanes on what was hypothesized to be the (104) face, whereas hardly mobile donors where hypothesized to be coordinated on the (110) face.45

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CONCLUSIONS In this work, we investigated the effect of increasing the number of donor molecules (alkoxysilanes) coordinated to Mg atoms on the (104) and (110) surfaces of MgCl2, the dynamics of a single alkoxysilane coordinated on both surfaces, and finally possible pathways corresponding to alkoxysilane migration from one coordination site to the next. Both interlayer and intralayer migration pathways were considered. The main conclusions of this work can be summarized as follows. (1) Increasing surface coverage results in decreasing the average coordination ability of the donor. Specifically, alkoxysilanes coordinated on vicinal sites interact repulsively, which results in decreased coordination energy. This effect is found both for the (104) and (110) surfaces, and test calculations indicated that this effect is clearly more relevant for bulkier donors. The conclusion is that complete coverage of the surface by the donors is difficult to achieve.

(3)

dfMg−O

where and are the distances of the O atoms from the Mg atoms in the donor coordinates of the initial and final geometries, respectively. As clearly appears in Figure 6a, where the energy profiles corresponding to migration on the (104) surface are reported, interlayer pathways A1 and A2 show high migration barriers of roughly 25 kcal/mol, which is very close to complete donor dissociation (26.0 kcal/mol). Intralayer pathway B presents a much smaller energy barrier, roughly 19 kcal/mol, due to some stabilizing interaction between the jumping donor and the (104) layer. Finally, the migration along pathway C is characterized by a remarkably small energy 22984



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(14) Matsuoka, H.; Liu, B.; Nakatani, H.; Terano, M. Macromol. Rapid Commun. 2001, 22, 326. (15) Rodriguez, L. A. M.; van Looy, H. M. J. Polym. Sci., Part A: Polym. Chem. 1966, 4, 1951. (16) Galli, P.; Luciani, L.; Cecchin, G. Angew. Makromol. Chem. 1981, 94, 63. (17) Giannini, U.; Giunchi, G.; Albizzati, E.; Barbè, P. C. NATO ASI Ser. C 1987, 215, 473. (18) Chien, J. C. W.; Kuo, C.-I. J. Polym. Sci., Part A: Polym. Chem. 1986, 24, 1779. (19) Chien, J. C. W.; Bres, P. L. J. Polym. Sci., Part A: Polym. Chem. 1986, 24, 2483. (20) Chien, J. C. W.; Weber, S.; Hu, Y. In Transition Metals and Organometallics as Catalysts for Olefin Polymerization; Sinn, W. K. a. H., Ed.; Springer-Verlag: Berlin, 1988; p 45. (21) Brant, P.; Tornqvist, E. G. M. Inorg. Chem. 1986, 25, 3776. (22) Brant, P.; Speca, A. N.; Johnston, D. C. J. Catal. 1988, 113, 250. (23) Mori, H.; Tashino, K.; Terano, M. Macromol. Rapid Commun. 1995, 16, 651. (24) Mori, H.; Sawada, M.; Higuchi, T.; Hasebe, K.; Otsuka, N.; Terano, M. Macromol. Rapid Commun. 1999, 20, 245. (25) Mori, H.; Hasebe, K.; Terano, M. J. Mol. Catal. A 1999, 140, 165. (26) Mori, H.; Saito, H.; Yamahiro, M.; Kono, H.; Terano, M. Macromol. Chem. Phys. 1998, 199, 613. (27) Mori, H.; Iguchi, H.; Hasebe, K.; Terano, M. Macromol. Chem. Phys. 1997, 198, 1249. (28) Bukatov, G. D.; Goncharov, V. S.; Zakharov, V. A. Macromol. Chem. Phys. 1995, 196, 1751. (29) Bukatov, G. D.; Zakharov, V. A. Macromol. Chem. Phys. 2001, 202, 2003. (30) Potapov, A. G.; Kriventsov, V. V.; Kochubey, D. I.; Bukatov, G. D.; Zakharov, V. A. Macromol. Chem. Phys. 1997, 198, 3477. (31) Sergeev, S. A.; Poluboyarov, V. A.; Zakharov, V. A.; Anufrienko, V. F.; Bukatov, G. D. Makromol. Chem. 1985, 186, 243. (32) Cavallo, L.; Del Piero, S.; Ducéré, J.-M.; Fedele, R.; Melchior, A.; Morini, G.; Piemontesi, F.; Tolazzi, M. J. Phys. Chem. C 2007, 111, 4412. (33) Brambilla, L.; Zerbi, G.; Piemontesi, F.; Nascetti, S.; Morini, G. J. Mol. Catal A 2007, 263, 103. (34) Trubitsyn, D. A.; Zakharov, V. A.; Zakharov, I. I. J. Mol. Catal. A 2007, 270, 164. (35) Andoni, A.; Chadwick, J. C.; Niemantsverdriet, H. J. W.; Thüne, P. C. J. Catal. 2008, 257, 81. (36) Cheruvathur, A. V.; Langner, E. H. G.; Niemantsverdriet, J. W. H.; Thüne, P. C. Langmuir 2012, 28, 2643−2651. (37) ; Nakatani, H. In Progress and Development of Catalytic Olefin Polymerization; Sano, T., Uozumi, T., Eds.; Technology and Education Publishers: Tokyo, 2000; p 7. (38) Corradini, P.; Barone, V.; Fusco, R.; Guerra, G. Gazz. Chim. Ital. 1983, 113, 601. (39) Giannini, U.; Giunchi, G.; Albizzati, E.; Barbè, P. C. In Recent Advances in Mechanistic and Synthetic Aspects of Polymerization; Fontanille, M., Guyot, A., Eds.; D. Reidel: Boston, 1987; p 473. (40) Credendino, R.; Busico, V.; Causà, M.; Barone, V.; Budzelaar, P. H. M.; Zicovich-Wilson, C. Phys. Chem. Chem. Phys. 2009, 11, 6525. (41) Credendino, R.; Pater, J. T. M.; Correa, A.; Morini, G.; Cavallo, L. J. Phys. Chem. C 2011, 115, 13322. (42) Correa, A.; Piemontesi, F.; Morini, G.; Cavallo, L. Macromolecules 2007, 40, 9181. (43) Busico, V.; Cipullo, R.; Monaco, G.; Talarico, G.; Vacatello, M.; Chadwick, J. C.; Segre, A. L.; Sudmeijer, O. Macromolecules 1999, 32, 4173. (44) Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem. Rev. 2000, 100, 1253. (45) Busico, V.; Causa, M.; Cipullo, R.; Credendino, R.; Cutillo, F.; Friederichs, N.; Lamanna, R.; Segre, A.; Van Axel Castelli, V. J. Phys. Chem. C 2008, 112, 1081.

(2) Molecular dynamics simulation of a single alkoxysilanecoordinated (104)-bridge or (110)-chelate resulted in rather stable systems, even for the rather stretched (104)bridge coordination mode. (3) Interlayer migration of the considered alkoxysilane on the (104) and (110) surfaces is very expensive, and it corresponds to complete donor dissociation from the surface rather than to a migration process. A similar highenergy migration pathway is found for sliding intralayer movements on both the (104) and (110) surfaces. However, we found that an intralayer crawling migration pathway on the (104) surface is possible by using one Mg−O bond as a pivot. The energy barrier for this migration pathway is around 8 kcal/mol and could explain the high mobility of alkoxysilanes evidenced by HR-MAS NMR experiments.

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ASSOCIATED CONTENT

S Supporting Information *

Coordinates and energy of key structures discussed in the text. Figures sketching selected geometries at different surface coverages. Complete reference 55. 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 This work is part of the Research Programme of the Dutch Polymer Institute, Eindhoven, The Netherlands, project no. 707.

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REFERENCES

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