Intermetallic Cooperation in Olefin Polymerization Catalyzed by a

Article pubs.acs.org/Organometallics

Intermetallic Cooperation in Olefin Polymerization Catalyzed by a Binuclear Samarocene Hydride: A Theoretical Study Gen Luo,† Yi Luo,*,† Zhaomin Hou,*,†,‡ and Jingping Qu† †

State Key Laboratory of Fine Chemicals, School of Pharmaceutical Science and Technology, Dalian University of Technology, Dalian 116024, People’s Republic of China ‡ Organometallic Chemistry Laboratory and Center for Sustainable Resource Science, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan S Supporting Information *

ABSTRACT: The cooperative effect in bi- and multinuclear metal complexes is of great interest in catalysis since such a cooperative effect often gives the complexes unique catalytic performance unavailable in mononuclear analogues. However, the related mechanism of bi- and multinuclear cooperative catalysis remained almost unexplored. Herein, the detailed mechanism of ethylene polymerization by a binuclear samarocene hydride complex has been computationally modeled. The results have not only revealed new aspects of the mechanism of olefin insertion reactions but also provided theoretical evidence for electronic communication between the metal centers during the polymerization, where the bridging hydride ligand plays an important role in such an intermetallic cooperation.

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branch formation mechanism (Chart 1a).19 On the basis of a conjugated linker (Chart 1b), an event of electronic communication between two metal centers was proposed to exist during the dinickel-catalyzed ethylene oligomerization or polymerization,20 although there was no direct evidence. In spite of much interest and recent progress in this field, studies on intermetallic cooperation in olefin polymerization remained limited. Theoretical clarification of the intermetallic synergistic effects would therefore be helpful for a better understanding of the reaction mechanism and for the development of more efficient binuclear metal catalysts. A binuclear samarocene hydride complex, [Me2Si(C5H3-3Me3Si)2SmH]2 (SmH, Chart 1c), was reported to have a good performance for ethylene polymerization.21 This binuclear catalyst possesses moderate molecular size, proper metal−metal distance (3.762 Å Sm···Sm distance, being slightly longer than twice the covalent radius (3.70 Å) of the Sm atom22), and simplest bridging ligand (hydride), which provided a good model system to investigate the cooperative effect between the two metal centers during the reaction. As we know, the insertion of an alkene into the metal−alkyl bond generally follows a traditional four-center transition state (TS) mechanism in mononuclear-catalyzed systems.1 In the case of binuclear systems, there are four possible olefin insertion TSs, viz., a four-center TS (A), a metal-assisted four-center TS (B), and two kinds of five-center TSs (C and D), as illustrated in Chart 2.1,23−25 In this study, the detailed mechanism of ethylene polymerization catalyzed by SmH was computation-

INTRODUCTION In the past decades, numerous studies have shown that mononuclear transition and rare-earth metal complexes are effective for polymerization of olefin or other monomers, and so far the related mechanisms have been well documented both experimentally and theoretically.1−3 In this context, theoretical studies have successfully explained experimental observations (such as activity and selectivity) and effectively promoted the design and development of new catalysts.4−7 Cooperative effects in multinuclear metal complexes are of great current interest8,9 and were recently emphasized by preparation of highly active catalysts for (co)polymerizations,10 including that of cyclic esters (lactide, caprolactone) and epoxides11,12 as well as olefins.13 Compared to mononuclear polymerization catalysts, multinuclear analogues often show high activity and unique selectivity, especially for binuclear metal complexes.14 For instance, the homobimetallic group 4 constrained geometry catalysts (CGCs)15,16 and group 4 or group 10 phenoxyiminato olefin polymerization catalysts17,18 exhibit remarkably enhanced polyethylene Mn’s, chain branching, and comonomer enchainment selectivity in comparison with corresponding monometallic counterparts, respectively. Such superiority may benefit from their unique substrate activation and novel reactivity patterns on the basis of the possible cooperation between adjacent active metal centers. DFT calculations on the reaction of a linked CGC bimetallic Zr complex with 1-octene reported by Marks et al. strongly suggest that the close proximity of two Zr sites in such a bimetallic system promotes a non-negligible agostic interaction between an oligomeric π-bonded vinyl-terminated oligoethylene chain and the second metal site, a prerequisite for the proposed ethyl © XXXX American Chemical Society

Received: January 11, 2016

A

DOI: 10.1021/acs.organomet.6b00018 Organometallics XXXX, XXX, XXX−XXX

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Chart 1. π-Complex of 1-Octene with Binuclear Zr Complex (a), Binuclear Ni Catalysts (b), and Binuclear Sm Catalyst (c)

Chart 2. Possible Modes for Alkene Insertion into the M−C Bond in a Binuclear Complexa

a

M = metal, L = ligand. 3/2 ⎤ ⎡⎛ −15/2 solvent ⎞⎛ 10 V free 2πMRTe 5/3 ⎞ ⎥ analyte ⎟ ⎜ ⎟ Strans = R ln⎢⎜⎜ ⎢⎝ NA 4[X ] ⎟⎠⎝ h2 ⎠ ⎥⎦ ⎣

ally modeled. The results obtained could deepen the understanding of the olefin insertion patterns and cooperative effects in binuclear metal complex systems.

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solvent = 11.1 + 12.5ln(T ) + 12.5ln(M ) + 8.3ln V free

COMPUTATIONAL METHODS

in which

Due to the relatively large molecular size, each methyl of the linked cyclopentadienyl ligand [Me2Si(η5-C5H4SiMe3)2] was replaced by H and the model ligand [H2Si(η5-C5H4SiH3)2] was used for the calculations.26 The B3LYP functional27 was used for geometry optimizations with the 6-31G(d) basis set for C and H atoms of the cyclopentadienyl ligand and the 6-31+G(d,p) basis set for the C and H atoms of the monomer (ethylene) and H ligands connected to Sm atoms, and the Stuttgart/Dresden relativistic effective core potentials (RECPs)28 as well as the associated valence basis sets were used for Si and Sm (large-core RECPs)29 atoms. The basis set of the Si atom was also augmented by one d polarization function (exponent of 0.284).30 Each optimized structure was analyzed by harmonic vibrational frequencies obtained at the same level and characterized as a minimum (NImag = 0) or a transition state (NImag = 1). To obtain more reliable relative energies, single-point energy calculations were carried out with a larger basis set. In such single-point energies, the M06-L functional, which often shows better performance in the treatment of transitionmetal systems,31 was used together with the CPCM model32 (in toluene solution with UFF atomic radii33) for considering the solvation effect, the Stuttgart/Dresden RECP (MWB51) together with associated basis sets was used for Sm atoms, and the 6311+G(d,p) was used for the remaining atoms. The free energies obtained from gas-phase calculation often overestimate the translational entropy in comparison with that in solution. Therefore, such a destabilization energy was partially due to the larger decrease in translation entropy calculated in the gas phase. The method of Whitesides34 was applied to estimate the decrease in translational entropy in this work. In the system investigated, we may assume that the free volume of the solution is dominated by the free volume of the solvent. We also assume that the molecules are cubic and are in a 3D cubic array. According to the Whitesides method, the translational entropy of an analyte in solution can be therefore estimated by the following equation:

solvent V free

⎛ = 8⎜⎜ 3 ⎝

⎛ 1027 ⎞ ⎜ ⎟ − ⎝ [X ]NA ⎠

3

⎞3 Vmolec ⎟⎟ ⎠

where the temperature (T, K), mass (M, g/mol), and concentration (X, mol/L) of the analyte, Planck’s constant (h, J s2), the fundamental constant (e, unitless), and Avogadro’s number (NA, unitless) are included. The numerical term 10−15/2 converts units of m3 and kg into L and g, respectively. Herein, the temperature is 298.15 K and the volume of one toluene molecule (Vmolec) was computed to be 142.1 Å3 via the Gaussian program, and the Vfree was further computed to be 0.49 Å3 for toluene solvent. All calculations were performed with the Gaussian 09 program.35

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RESULTS AND DISCUSSION Marks and co-workers previously reported that R2SiCp′Mbased binuclear lanthanocene hydrides exhibited good stability in THF solvent.36 Therefore, the possibility of the dinuclear complex SmH dissociating into two monomeric species was not considered in the current system. As shown in Figure 1, at the chain initiation stage, the reaction proceeds from the coordination of ethylene to one of metal centers of SmH, leading to the π-complex C1. Subsequently, the CC double bond of ethylene could insert into the Sm−H bond via transition state TS1 to yield the CH2-bridged complex P1. This process has an energy barrier of 12.1 kcal/mol and is exergonic by 9.5 kcal/mol. It is noted that TS1 shows a four-center structure constructed by Sm1, C1, C2, and H1 atoms, as suggested by the interatomic distances, viz., 2.70 Å (Sm1···C1), 1.40 Å (C1···C2), 1.53 Å (C2···H1), and 2.31 Å (Sm1···H1). All of these geometric features are similar to a traditional fourB


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in Figure 3. Paths A, B, and C undergo the transition states TS2A, TS2B, and TS2C, respectively, to give the insertion products. As illustrated in Figure 3, in TS2A, the distance of Sm2···C1 (3.11 Å) is significantly longer than Sm1···C1 (2.90 Å), and the structure was stabilized by the agostic interaction Sm2···H3(C1). TS2A could be thus regarded as a traditional four-center (Sm1, C3, C4, and C1) TS with the Sm2 simply acting as part of a metalloligand, which is similar to mode A shown in Chart 2. In TS2B, the C1 alkyl group remains interacting with both Sm1 and Sm2 metal centers (2.89 and 2.81 Å), and the ethylene moiety (C3 atom) interacts only with the Sm1 metal center. It is similar to TS1 shown in Figure 1 and is a metal-assisted four-center TS (see mode B in Chart 2). As for TS2C,37 the C3 atom of the incoming ethylene directly interacts not only with Sm1 (2.91 Å) but also with Sm2 (3.00 Å) metal centers, and the Sm1, C3, C4, C1, and Sm2 atoms form a five-center structure (see mode C in Chart 2). It is noteworthy that the distance of C4···C1 in TS2C (2.58 Å) is significantly longer than that in TS2A (2.17 Å) and TS2B (2.19 Å) and the bond length of C3−C4 in TS2C (1.37 Å) is shorter than that in TS2A (1.41 Å) and TS2B (1.40 Å). All attempts to simulate the other kind of five-center TS (mode D in Chart 2), which was described in binuclear aluminum complexes,25 were fruitless but led to TS2B instead. This could be ascribed to the different coordination capability between Sm and Al atoms. The relative energies of insertion transition states TS2A, TS2B, and TS2C are 30.7, 19.3, and 18.1 kcal/mol, respectively, suggesting that the traditional four-center TS (TS2A) is significantly unfavorable compared to the metal-assisted fourcenter TS (TS2B) and the five-center TS (TS2C). This could benefit from the bimetallic cooperative effect and is in line with

Figure 1. Energy profile (free energy in kcal/mol) for the first ethylene insertion into SmH and the structure of TS1 (distances in Å). All the [H2Si(η5-C5H4SiH3)2] ligands were omitted for clarity.

center TS in mononuclear systems. However, the H1 atom in TS1 not only interacts with C2 and Sm1 atoms but also connects with Sm2 atom (2.37 Å (Sm2···H1)). In this sense, this process could be regarded as a metal-assisted four-center TS mechanism. Such a bridging feature of H1 in TS1 may play an important role in the stability of the structure, like an agostic interaction does.24 Many attempts to locate other modes of ethylene insertion TSs were fruitless. Starting with the CH2-bridged complex P1, at the chain propagation stage, several possible mechanisms (Chart 2) have been investigated. As shown in Figure 2 (left part), for the second ethylene insertion, three mechanisms of CC bond insertion into the Sm−C bond were successfully located (paths A to C), and their corresponding structures of TSs were shown

Figure 2. Computed reaction pathways of the second and third ethylene insertion together with the relative free energies (kcal/mol). C


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Figure 3. Computed structures of transition states involved in the second ethylene insertion together with selected structural parameters (distances in Å).

fourth ethylene insertion, viz., the energy of TS4C is lower than TS4B by 1.0 kcal/mol (see Figure S1 in the SI). All the results suggest that paths B and C have a small difference in energy barrier. On the basis of the aforementioned results, paths B and C are the favorable pathways during chain propagation and they are competitive. On the basis of the results obtained thus far, we know that the actual activation barrier for ethylene polymerization by binuclear catalyst SmH is ca. 27 kcal/ mol.39 As for the mononuclear samarocene complex, the energy barrier for ethylene insertion is as high as 38.0 kcal/mol (see Figure S9 in the SI), which is in agreement with the experimental observation that the mononuclear complex is inactive.21 The activity discrepancy between bi- and mononuclear complexes could account for the intermetallic cooperation in the binuclear system. The frontier molecular orbitals of TSs display the interaction between the insertionrelated carbon species and the metal centers (see Figure S13 in the SI). The frontier occupied orbitals of TS1, TS2B, and TS2C suggest that the two metal centers play an important role in the bonding of TS structures, and the HOMO of TS2A suggests a traditional four-center TS feature. It is noteworthy that the Sm···H−(α-C) agostic interaction plays an important role in the chain growth stage. In TS2C (Figure 3), Sm2 has an unambiguous interaction with H−(αC), as suggested by the short distance of H3−Sm2 (2.61 Å) and the slight elongation of the C1−H3 bond (ca. 0.02 Å). To estimate the role of this interaction, we tried to model the TS without such an agostic interaction. Fortunately, a five-center TS, TS2′C, without the Sm2···H−(α-C) agostic interaction was located (Figure 3). The relative energies of the two five-center TSs (18.1 kcal/mol for TS2C and 25.5 kcal/mol for TS2′C) clearly suggest that TS2C was significantly stabilized by the Sm···H−(α-C) agostic interaction. To get more information about the electronic aspect, the change of natural charges on Sm1 and Sm2 atoms along with paths B and C is illustrated in Figure 4. As shown in this figure, the change of natural charges on Sm1 and Sm2 atoms indicates a complementary tendency in both metal-assisted four-center TS and five-center TS pathways (paths B and C, respectively).40 This suggests that electronic communication

our previous results on the mechanism of olefin polymerization by a cationic binuclear yttrium complex.23 Paths B and C give the same insertion products (P2B = P2C), and TS2C is slightly more favorable than TS2B, by 1.2 kcal/mol. In fact, the insertion of a second ethylene into the Sm−H bond in P1 could also occur. As shown in Figure 2, TS2D and TS2E display ethylene insertion into the Sm2−H and Sm1−H bonds, respectively. Similar to TS1, both TS2D and TS2E are metalassisted four-center TSs, and their products P2D and P2E contain strong interactions among the H2 atom and the two metal centers (Figure S5 in the SI).38 The relative energies of TS2D (10.8 kcal/mol) and TS2E (7.4 kcal/mol) are lower than that of TS2C (18.1 kcal/mol). However, the products P2D (7.6 kcal/mol) and P2E (0.4 kcal/mol) are significantly less stable than P1 (−9.5 kcal/mol) and P2C (−21.4 kcal/mol). Thus, the reverse reactions easily occur during these processes, and then they also convert to the energetically favorable product P2C. The intermediates P2D and P2E could further evolve to more stable intermediates P2D′ and P2E′ with two bridging ethyl groups. The computed results suggest that ethylene insertion into the diethyl-bridging complexes (P2D′ and P2E′) need to overcome energy barriers of more than 37 kcal/mol, which is unlikely to occur in the current system (see Figures S10 and S11 and related description in the SI). In fact, prior to ethylene insertion into the Sm−H bond of P1, it could be easy to go through TSC2 with a low energy barrier (5.9 kcal/mol) to form a π-complex C2,37 which further converts to more stable complex P2C via TS2C. Additionally, it is noteworthy that the flexible bridging μ-H ligand is capable of adjusting the intermetallic distance (from 3.65 to 4.10 Å; see Figure S12 in the SI) and facilitating the cooperation during the ethylene insertion. To corroborate the proposed mechanism for the chain propagation stage, the insertion of the third ethylene was also computed. As shown in Figure 2 (right part), although the fourcenter TS could not be located in this process, all other pathways are similar to that of the corresponding second ethylene insertions. The five-center TS pathway (via TS3C) is also slightly more favorable than the metal-assisted four-center TS one (via TS3B) by 2.3 kcal/mol. The same is true for the D


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Article

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00018. Computed pathway for the fourth ethylene insertion, natural charge, 3D structures, frontier orbitals, distances of Sm···Sm, ethylene polymerization by mononuclear complex, energy corrections, and electronic energies for each stationary point (PDF) File giving all optimized Cartesian coordinates of stationary points (XYZ)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail (Y. Luo): [email protected]. *E-mail (Z. Hou): [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by the NSFC (Nos. 21174023, 21231003, 21429201) and a Grant-in-Aid for Scientific Research (S) from the JSPS (No. 26220802). The authors also thank the RICC (RIKEN Integrated Cluster of Clusters) and the Network and Information Center of the Dalian University of Technology for part of the computational resources.

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Figure 4. Change of natural charges on Sm1 and Sm2 atoms along with the reaction pathways.

REFERENCES

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exists between the metal centers during the reaction. In path B, the reaction mainly occurs at the Sm1 metal center. Sm1 obtains electron while Sm2 loses electron. In contrast, in path C, Sm1 loses electron while Sm2 obtains electron. Comparing Figure 4a and b, it is obvious that the extent of electronic communication of the five-center TS pathway (path C) is larger than that of the metal-assisted four-center TS pathway (path B). To the best of our knowledge, this is the first evidence for electronic communication between the metal centers in olefin polymerization catalyzed by binuclear metal complexes.

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CONCLUSION In summary, the present study has revealed the detailed mechanism of ethylene polymerization by a binuclear samarocene hydride complex. The result suggests that, at the chain initiation stage, ethylene insertion into the Sm−H bond goes through a metal-assisted four-center transition state. At the chain propagation stage, ethylene continuously inserts into the Sm−C bond via a five-center transition state and metal-assisted four-center transition state. The flexible bridging hydride ligand is capable of adjusting the intermetallic distance and facilitating the cooperation during the ethylene insertion. It is also found that the Sm···H−(α-C) agostic interaction plays an important role in stabilizing the five-center transition state. More interestingly, the change in natural charges on two metal atoms offers the first evidence for the intermetallic electronic communication during olefin polymerization. This study could add to a better understanding of the bimetallic cooperative catalysis. E


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(26) The first ethylene insertion with the full-ligand catalyst SmH′ was also calculated as a test case. The relative energies of the stationary points involved in the full-ligand system are slightly higher than that of the model system (0−3 kcal/mol), and there is almost no effect on the geometric structure of the transition state (≤0.02 Å; see Figures S3 and S4 in the SI). Therefore, the model complexes would be suitable for investigating the mechanism of this system. (27) (a) Beck, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (b) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789. (28) (a) Dolg, M.; Wedig, U.; Stoll, H.; Preuss, H. J. Chem. Phys. 1987, 86, 866−872. (b) Schwerdtfeger, P.; Dolg, M.; Schwarz, W. H. E.; Bowmaker, G. A.; Boyd, P. D. W. J. Chem. Phys. 1989, 91, 1762− 1774. (c) Dolg, M.; Stoll, H.; Savin, A.; Preuss, H. Theor. Chim. Acta 1989, 75, 173−194. (d) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123−141. (e) Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1993, 85, 441−450. (f) Bergner, A.; Dolg, M.; Kuechle, W.; Stoll, H.; Preuss, H. Mol. Phys. 1993, 80, 1431−1441. (29) Large-core RECPs have been successfully used for studying the mechanism of reactions of lanthanide complexes, including samarium complexes. See: (a) Kefalidis, C. E.; Perrin, L.; Maron, L. Dalton Trans. 2014, 43, 4520−4529. (b) Perrin, L.; Maron, L.; Eisenstein, O.; Tilley, T. D. Organometallics 2009, 28, 3767−3775. (c) Iftner, C.; Bonnet, F.; Nief, F.; Visseaux, M.; Maron, L. Organometallics 2011, 30, 4482− 4485. (30) Höllwarth, A.; Böhme, M.; Dapprich, S.; Ehlers, A. W.; Gobbi, A.; Jonas, V.; Köhler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys. Lett. 1993, 208, 237−340. (31) (a) Averkiev, B. B.; Truhlar, D. G. Catal. Sci. Technol. 2011, 1, 1526−1529. (b) Gusev, D. G. Organometallics 2013, 32, 4239−4243. (c) Zhao, Y.; Truhlar, D. G. Acc. Chem. Res. 2007, 41, 157−167. (32) (a) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995− 2001. (b) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem. 2003, 24, 669−681. (33) (a) Miertus, S.; Scrocco, E.; Tomasi, J. Chem. Phys. 1981, 55, 117−129. (b) Cammi, R.; Mennucci, B.; Tomasi, J. J. Phys. Chem. A 2000, 104, 5631−5637. (34) Mammen, M.; Shakhovich, E. I.; Deutch, J. M.; Whitesides, G. M. J. Org. Chem. 1998, 63, 3821−3830. (35) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A. Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision A.01; Gaussian, Inc.: Wallingford, CT, 2009. (36) The experimental results suggested that heating a Et2SiCp′2Lubased dinuclear hydride in THF for 3 days at 60 °C gave no NMR spectroscopic evidence for dissociation/formation of Et2SiCp′2Lu(H)based species. See: Stern, D.; Sabat, M.; Marks, T. J. J. Am. Chem. Soc. 1990, 112, 9558−9575. (37) In paths A and B, the incoming ethylene directly inserts into the Sm−C bond without preforming a π-complex. However, in path C, the cleavage of the Sm1−C1 bond could make space for the coordination of the incoming ethylene to form a π-complex C2 and further goes through TS2C to give insertion product P2C. (38) Ethylene insertion into the Sm−H bond of Sm2 directly leads to alkyl-bridged product P1, while the ethylene insertion into the Sm−H bond of P1 forms P2D or P2E, having a strong interaction among the

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Organometallics

H2 atom and the two metal centers. See Figure S8 in the SI for more details. (39) The catalytic activity of the neutral catalyst SmH (27 g mmol−1 h−1 atm−1, ref 21) is low to moderate for ethylene polymerization, and the activation barrier of ca. 27 kcal/mol should be acceptable. As for the grade of activity for ethylene polymerization catalysts, see: Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed. 1999, 38, 428−447. (40) The change of natural charge on the other ligands (linked-Cp′ ligands, hydride ligand, and alkyl) is also displayed in Figure S14 in the SI. The result shows that the charge of the ligands is almost constant during the chain propagation stage.

G


Intermetallic Cooperation in Olefin Polymerization Catalyzed by a

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