Theoretical study of selectivity mechanisms in propylene polymerization with metallocene catalysts

Journal of Molecular Structure (Theochem) 541 (2001) 227±235

www.elsevier.nl/locate/theochem

Theoretical study of selectivity mechanisms in propylene polymerization with metallocene catalysts L. Petitjean a, D. Pattou a, M.-F. Ruiz-LoÂpez b,* a Groupement de Recherches de Lacq, Ato®na, BP 34, 64170 Artix, France Laboratoire de Chimie TheÂorique, UMR CNRS-UHP 7565, Institut NanceÂien de Chimie MoleÂculaire, Universite Henri PoincareÂ, Nancy I, BP 239, 54506 Vandoeuvre-les-Nancy Cedex, France

b

Received 28 September 2000; accepted 25 October 2000

Abstract The insertion mechanism in metallocene-catalyzed propylene polymerization is investigated through density functional theory calculations. Three paths corresponding to different orientations of the methyl group of the ole®n have been studied and compared. The activation energy of the most favorable path is found to be 7.6 kcal/mol, which compares well with the experimental estimation, 10 kcal/mol. The activation energies for the other, less favorable, processes are 12.1 and 12.3 kcal/ mol. The calculations show that propylene is coordinated very asymmetrically to the metal center. This asymmetry is assumed to be at the origin of the regio-selectivity of the reaction. On the other hand, the present computations con®rm the importance of the ªrelay mechanismº, which correlates stereo-selectivity to steric interactions with the chain rather than to interactions with the ligand. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Selectivity mechanisms; Propylene; Polymerization; Metallocene catalysts

1. Introduction There is currently a substantial interest in homogenous catalysts as an alternative to classical Ziegler± Natta heterogenous catalysts [1]. These new compounds allow a-ole®n polymerization to proceed under mild conditions as well as the formation of regular polymers such as linear polyethylene or isotactic polypropylene, which are of great industrial interest. A special feature of homogenous catalysts is that their structure is well de®ned so that the proper-

* Corresponding author. Tel.: 133-3-8391-2527; fax: 133-3-8391-2530. E-mail address: [email protected] (M.-F. Ruiz-LoÂpez).

ties of the product are closely related to it. This very interesting characteristic has been the source of numerous theoretical works that have also been devoted to the understanding of the polymerization mechanism and to the search of new potentially active systems [2±21]. In fact, while ethylene insertion has been extensively studied, few high level calculations (ab initio or DFT) have been carried out on propylene insertion [22±25]. This is due to the fact that propylene insertion is a much more complicated process. It involves in principle the study of four different paths corresponding to the four different possible positions of the methyl group during the reaction (Scheme 1). Additionally, in order to get an insight into the factors in¯uencing the selectivity of the reaction, the calculations must be carried out with a system that involves a

0166-1280/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0166-128 0(00)00803-4

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L. Petitjean et al. / Journal of Molecular Structure (Theochem) 541 (2001) 227±235

Zr

Pol

Me

+

Me H2 Si secondary insertion

primary insertion

Pol

Zr

1,2-si

Zr

1,2-re

Pol

Zr

Pol

Zr

2,1-si

Pol

2,1-re

Scheme 1.

large number of atoms and consequently a considerably large computational effort is needed. Molecular mechanics (MM) calculations allow dealing with very large systems and indeed they have been very helpful in the understanding of propylene polymerization. Since steric hindrance of the ligand is believed to play a major role in the control mechanism, MM studies have been able to reproduce the trends of the activity [26±29]. The control mechanism is described as a relay mechanism, where the ligand has the ability to favor a speci®c orientation of the chain. The so-called stereo-control (discrimination between the two primary insertions in Scheme 1) comes therefore from the chain and is due to the steric repulsion between the growing polymer and the methyl of the propylene (see Scheme 2). This interpretation has been validated by several experimental works [30,31] and is now commonly accepted. However, it does not explain how the regio-control of the reaction occurs (control between primary and secondary insertions). In particular, it is dif®cult to justify experimentally why the 1,2 insertion is preferred to the 2,1 one. Indeed, steric repulsion with the chain would favor the 2,1 process where

Zr

Pol HC CH3 CH2

Zr

1,2-si sterically unfavored

Pol HC CH3 CH2

1,2-re sterically favored Scheme 2.

Me

Zr

HC

CH2 Me

CH3 CH3

Scheme 3.

the carbon atom to be linked is not substituted. There must then be some electronic factors that also play a major role in the reactivity of metallocene with propylene. Such factors cannot be taken into account through standard molecular mechanics. In previous works, we have investigated the mechanisms for ethylene insertion [32] and hydrogenolysis [33] using Density Functional Theory (DFT) computations. We have proposed a reaction model in which rotation of the ole®n during the process is a key feature both in ethylene insertion and hydrogenolysis. Our computations compare well with available experimental data demonstrating that the DFT level represents a good compromise between accuracy and computer cost. Our aim in the present study is to enlarge our research in the ®eld of metallocene catalysis by examining the highly complex mechanisms leading to propylene insertion. The main objective is to determine which electronic and steric factors are at the origin of both stereo- and regio-control of the reaction. Besides, the reaction model proposed for ethylene insertion must be con®rmed (or refuted) in the case of larger ole®ns. That point is inspected here too. 2. Models and computational details As stated above, we need for this study a larger ligand and model chain than in previous works. The system chosen is a C2-symmetric ligand, as shown in Scheme 3. The model for the isotactic polypropylene chain is an isobutyl group. The choice of such a model involves a substantial increase of the computational effort needed. Since, as shown by our previous calculations [32], the activation barrier for polymerization lies on the insertion step, we shall not consider explicitly the complexation of the ole®n to the active species in the present work.

L. Petitjean et al. / Journal of Molecular Structure (Theochem) 541 (2001) 227±235

Zr

Cn H2n+1

Zr

Cn H2n+1

rotation

Zr

229

Cn H2n+1

insertion Scheme 4.

Rather, we start with a perpendicular complex. This species leads to a parallel complex by rotation around the Zr±ole®n bond. From this parallel complex, the creation of the C±C bond is easy, as indicated in Scheme 4. In principle, to study the selectivity of the reaction, one needs to consider four paths. However, due to the size of the system, we have limited our computations to the investigation of only three paths. The ®rst path studied is considered to be the regular insertion (1,2re) [34]. The second one (1,2-si) is considered to be the source of stereo-errors. Finally, we have described the (2,1-si) path, assumed to be a source of regioerrors. We have considered only one 2,1 insertion path, since it allows to evaluate electronic and steric effects in regio-selectivity. The present calculations were carried out using the DFT approach and the gaussian 94 [35] package. The basis set D95 [36] was employed for C and H atoms. For Zr, a double-j basis set together with Los Alamos effective core potentials (ECPs) [37±39] for the description of core electrons was used. All calculations were performed with the BLYP exchangecorrelation functional [40,41]. The BLYP functional uses an LDA approximation with gradient corrections for the exchange functional, as proposed by Becke [40], and for the correlation functional, as proposed by Lee, Yang, and Parr [41]. All the structures were fully optimized without any constraints or symmetry conditions. For optimizations to a transition structure, we used a quasi-Newton approach method [42]. Due to the large size of the present computations, frequency calculations would have been too long to perform after the geometry optimizations. However, many of the structures described here are comparable to those characterized in the case of ethylene insertion by us [32] and other authors [14]: we may then reasonably conclude that the stable structures and transition structures obtained in our work correspond to true minima and saddle points on the potential energy surface.

3. Results and discussion 3.1. 1,2-re insertion The relevant structures located along the 1,2-re insertion path are described in Fig. 1. Selected structural parameters and energetic data are summarized in Table 1. The structure of the starting complex (1) is roughly the same than in the ethylene case [32]. The major difference appears for the monomer±metal binding. In the case of ethylene, the two carbon atoms are symmetrically linked to the metal. This is not the case for propylene, where a strong asymmetry is present. The carbon atom bearing the methyl group (C1) is at a greater distance from the metal than the other carbon atom (C2). This can be explained by a charge stabilization effect. Thus, conformation (a) in Scheme 5 is expected to be more favorable than conformation (b). The parallel complex (3) in Fig. 1, which is the intermediate in the insertion step, has been also localized on the potential energy surface and is only 0.95 kcal/mol higher than the perpendicular structure (1). The a-agostic bond is weaker in (3) than in (1). Compared to the ethylene case, the conformation of the chain is less optimal for insertion, the a-agostic H atom being situated between the ole®n and the Ca . The product of insertion is a g-agostic species (5), similar to that found in the ethylene case. Transition structures (TS) joining these three species have been localized and the corresponding energy barriers computed. The ®rst TS (2) characterizes the monomer rotation, whereas the other TS

Scheme 5.

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L. Petitjean et al. / Journal of Molecular Structure (Theochem) 541 (2001) 227±235

Fig. 1. Structures located along the 1,2-re insertion path.

(4) holds for the C±C bond creation, i.e. insertion. In Ê , which is in the latter TS, the C±C bond is 2.235 A Ê ). The line with results for ethylene insertion (2.240 A a-agostic bond is here stronger than in the p-complex

 as already observed in other forms Zr±H ˆ 2:171 A; insertion processes [14,32,33]. The barrier for ole®n rotation is quite small (1.5 kcal/mol) in comparison with the insertion one

L. Petitjean et al. / Journal of Molecular Structure (Theochem) 541 (2001) 227±235

231

Table 1 Ê and degrees) and energetic data (in kcal/mol) calculated along different insertion paths in metallocene-catalyzed Structural parameters (in A propylene polymerization Structures

Energies a

Structural parameters Zr±C1 b

Zr±C2

Col ±Ca c

C1 ±C2

Zr±Hg

Zr±Ha

Zr±Ca ±Ha

Zr±Ca ±Cb

1,2-re path 1 2 (TS) 3 4 (TS) 5

0.00 1.40 0.90 7.55 25.65

3.194 3.338 3.301 2.715 ±

2.656 2.745 2.742 2.354 2.247

3.922 3.669 3.720 2.235 1.584

1.379 1.377 1.376 1.457 1.596

± ± ± ± 2.278

2.645 2.488 2.460 2.171 d ±

86.1 87.6 86.1 65.0 ±

122.3 125.5 128.1 149.0 96.5

1,2-si path 6 7 (TS) 8

20.35 11.75 23.80

3.233 2.724 ±

2.666 2.327 2.250

3.947 2.220 1.583

1.378 1.470 1.593

± ± 2.290

2.508 2.141 d ±

88.6 63.2 ±

125.9 154.2 98.1

2,1-si path 6 9 (TS) 10 11 (TS) 12

20.35 1.55 0.75 11.95 23.35

3.233 3.388 3.321 2.656 ±

2.666 2.679 2.661 2.476 2.255

3.686 3.538 3.401 2.263 1.587

1.378 1.377 1.377 1.435 1.611

± ± ± ± 2.285

2.508 2.905 2.620 2.177 d ±

88.6 96.5 94.9 66.6 ±

125.9 119.9 121.5 151.7 86.5

a b c d

Energies in kcal/mol relative to 1. C1 is the carbon that brings the methyl of the propylene. In the case of 1,2-si Col ˆ C1 and in the case of 2,1-si Col ˆ C2 This distance does not refer to the same hydrogen atom as previous structures in the path.

(6.65 kcal/mol). Besides, the backward reaction from 3 to 1 is quite easy so that the complex 3 is not expected to play a role in the kinetics of the insertion reaction. Therefore, the overall insertion barrier is roughly given by the difference 4 2 1, i.e. 7.55 kcal/ mol. This barrier is much higher than the one corresponding to the ethylene process (3.1 kcal/mol). The difference can be due to the fact that the a-agostic interaction in p-complexes 1 and 3 does not favour a C±C distance decrease. Besides, steric interactions with the methyl groups of the ligand may destabilize the transition structure. Such an effect does not exist in the case of ethylene polymerization [32]. The reaction releases 5.65 kcal/mol, which is also quite small in comparison with the 9.55 kcal/mol calculated in the case of ethylene (again, this may come from steric interactions). 3.2. 1,2-si insertion Fig. 2 depicts the structures of this path. Selected structural parameters and energetics are shown in

Table 1. We have not been able to localize the parallel intermediate in this case. The reaction proceeds with a concerted rotation and insertion of the monomer. This feature is certainly due to the fact that the rotation of the monomer is much more dif®cult. During the rotation, there will be a strong repulsion between the methyl group of the monomer and the chain. The starting structure (6) is quite close to (1). It seems that the monomer complexation is a little bit weaker since the metal±carbon distances are larger. The a-agostic Zr±H distance is a little bit shorter, and at the same time the Zr±Ca ±Cb angle is slightly larger, so that we can imagine that this interaction is slightly stronger in (6) than in (1). Again the a-agostic H atom is not placed ideally for the insertion. The energy of (6) is a little bit lower than (1) (0.35 kcal/ mol), certainly because it minimizes the steric interactions between the monomer and the ligand. Ê . The aIn TS (7), the C±C distance is 2.220 A agostic hydrogen atom is very close to the metal Ê ), showing that the agostic interaction is (2.141 A also stronger in (7) than in (4). After creation of the

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Fig. 2. Structures located along the 1,2-si insertion path.

C±C bond, one can see in the product (8) that the chain avoids steric repulsion by turning around the new C±C bond. The reaction proceeds with an activation energy of 12.1 kcal/mol and is exothermic by only 3.45 kcal/ mol. Thus, this reaction is, as expected, less favorable than the previous one.

Ê . Note also that the a-agostic interaction is 2.263 A Ê , and here weaker with a Zr±H distance of 2.177 A that the product (12) displays g-agostic interactions. The reaction exhibits a barrier of 12.3 kcal/mol. The product is only 3.00 kcal/mol more stable than the starting point.

3.3. 2,1-si insertion

4. Conclusions

The structures located along this path are depicted in Fig. 3. As before, some selected structural parameters and energetics are shown in Table 1. The path also starts with complex (6). In this case, we have been able to locate a parallel complex (9). That structure is comparable to complex (3) although the asymmetry is in fact more marked. This can be due to the steric in¯uence of a methyl group of the ligand. However, the energy of this structure is comparable to (3) (0.15 kcal/mol more stable). The asymmetric complexation will play a major role in this path and will disfavor the 2,1 insertion path. Indeed, in order to create the new C±C bond, the metal must release the carbon atom closer to it in the p-complex form (i.e. (9)). However, if one looks at the TS (10), the asymmetry is not yet reversed  and Zr±C1 ˆ 2:656 A†:  Besides, …Zr±C2 ˆ 2:476 A one can see that the C±C bond length is large,

The comparison between the energetics of the three paths clearly shows that the 1,2-re insertion is most favorable, with an activation barrier of 7.5 kcal/mol. The two other paths display similar activation barriers (12.1 and 12.3 kcal/mol). These values are consistent with the recent experimental evaluation for the insertion barrier of propylene that amounts to about 11 kcal/mol [43], as well as with other theoretical calculations of similar processes that predict a barrier between 5 and 9 kcal/mol [23,24]. The energies of the two starting p-complexes, (1) and (6), are very close, differing by only 0.35 kcal/ mol. Therefore, at the stage of the complexation, there is no real preference in favor of one of the paths. This result underlines that the ligand framework itself cannot determine the reaction path, as already mentioned in the literature [30]. According to our calculations, the chain plays a destabilizing role in

L. Petitjean et al. / Journal of Molecular Structure (Theochem) 541 (2001) 227±235

233

Fig. 3. Structures located along the 2,1-si insertion path.

the early stage of the 1,2-si path, so that a 1,2-si parallel p-complex cannot be located. Therefore, both rotation and insertion are more dif®cult in this path. It supports the existence of a relay mechanism in

which the chain plays the major role in stereoselectivity [30,31]. A quite interesting feature is that 2,1 insertion seems to be prevented essentially by electronic

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reasons. The strong asymmetry of the complexation may be the main explanation for its high-energy barrier. This property is not observed in the polymerization of propylene with Pd-based catalysts. A theoretical study [24] has shown that this asymmetry does not exist in that case, where 2,1 insertion is favored because it involves a smaller deformation of the propylene molecule during the insertion. This trend can be reversed by the use of relatively bulky ligands, which by steric repulsion destabilizes the 2,1 path. It is worth also noting that the activation barrier for stereo (1,2-si) and regio (2,1-si) errors is very similar in our model. This can be related to the fact that isospeci®c metallocene has been demonstrated to produce systematically, easily detectable amounts of regio-irregularities (Ref. [28] and references therein). This property has been interpreted (Ref. [28] and references therein) as the possibility of 1,2-si and 2,1-si paths to inter-convert easily by rotation from one parallel p-complex to the other. Indeed, our results show that the two paths come from the same perpendicular complex. Since the activation barriers of the two insertions are very close to each other, there is no way to avoid secondary insertions in such a case. This could con®rm, as recently demonstrated [25], that the use of speci®c bulky ligands prevents the 2,1 insertion from happening. However, in our scheme, this property can also be explained by the presence of a bulky group that destabilizes the formation of the 2,1 parallel complex and favors 1,2-si rotation. These later results show the importance of the initial orientation of the coordinated ole®n in the understanding of propylene polymerization mechanisms and selectivity. Acknowledgements We thank Ato®na for authorization to publish this work. We are also grateful to Dr Saudemont and Dr Malinge from Groupement de Recherches de Lacq (Ato®na) for their collaboration in this project. References [1] H.H. Brintzinger, D. Fischer, R. MuÈlhaupt, B. Rieger, R.M. Waymouth, Angew. Chem. Int. Ed. Engl. 34 (1995) 1143. [2] C. Janiak, J. Organomet. Chem. 452 (1993) 63.

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