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Ethylene Polymerization Initiated by Tertiary Diamine/n-Butyllithium Complexes: An Interpretation from Density Functional Theory Study Huayi Li,*,† Liaoyun Zhang,‡ Zhi-Xiang Wang,*,‡ and Youliang Hu† Joint Laboratory of Polymer Science and Materials, Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China, and College of Chemistry and Chemical Engineering, Graduate UniVersity of Chinese Academy of Sciences, Beijing 100049, China ReceiVed: NoVember 10, 2009; ReVised Manuscript ReceiVed: January 7, 2010
The mechanism of ethylene insertions into eight tertiary diamine/n-butyllithium complexes has been studied at the BLYP/DNP level. In contrast to the cationic coordination polymerization in which a strong coordination complex between ethylene and the metal center is formed prior to ethylene insertion, there is only a weak van der Waals complex between ethylene and tertiary diamine/n-butyllithium complex. After crossing a fourmembered-ring transition state, ethylene inserts into the Li-C bond. The insertion barriers for the eight reactions are in the range of 6.9-11.0 kcal/mol, comparable to those of ethylene cationic coordination polymerizations. However, the polymerization activities of ethylene anionic polymerizations are much lower than those of cationic coordination polymerizations. Comparing the energy profiles of these ethylene anionic polymerizations with those of cationic coordination polymerizations, it can be found that the transition states in the ethylene anionic polymerizations are higher in energy than the reactants, while the transition states in ethylene cationic coordination polymerizations are lower than the reactants. Therefore, ethylene anionic polymerizations need additional energy to climb the energy barriers, while the energies for overcoming the transition states in the cationic coordination polymerizations can be obtained from reactants that are higher in energy than the reactants. We reason the differences in their energy profiles could be one of the reasons for the lower activity of ethylene anionic polymerization than ethylene cationic coordination polymerization despite their comparable insertion barriers. Introduction Polyethylene is widely used in our daily life. In industry, polyethylene is generally produced via coordination polymerization catalyzed by Ziegler-Natta catalysts under low pressure or radical polymerization under high pressure.1 It has been found that alkyl lithium complexes can also promote ethylene polymerization via the anionic polymerization mechanism. The first study on anionic polymerization of ethylene was reported by Friedrich who used n-butyllithium to initiate the reaction.2 After that, most anionic polymerizations of ethylene were initiated by tertiary diamine/alkyl and aryl lithium complexes, which were proved to be more effective than noncoordinated organolithium complexes.3-10 The initial rates of ethylene polymerization initiated by tertiary diamine/alkyl lithium complexes were assumed to relate to the structures of tertiary diamine compounds.11 The anionic polymerizations of ethylene initiated by tertiary diamine/alkyl lithium complexes exhibit somewhat living nature, but do not satisfy the criteria for a standard living polymerization. The polydispersity index of polyethylene prepared by tertiary diamine/n-butyllithium complexes was often in the range of 1.4-2.0. The molecular weight of polyethylene increased with polymerization time nonlinearly.11 Several chain transfer and chain termination reactions were assumed.11 However, signals corresponding to the side reactions were not observed * Corresponding author. Tel.: 86-10-62562697 (H.L.); 86-10-88256674 (Z.-X.W.). Fax: 86-10-62554061 (H.L.); 86-10-88256674 (Z.-X.W.). E-mail:
[email protected] (H.L.);
[email protected] (Z.-X.W.). † Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences.
in the 13C NMR spectrum of polyethylene, and the intensities of the signals from both the initiation and the termination chain ends are almost the same.12 End functional polyethylene13-15 and block copolymers of ethylene with styrene, isoprene, and methyl methacrylate16-18 could be prepared using anionic polymerization of ethylene. The polymerization activity of ethylene anionic polymerization is generally much lower than that of ethylene coordination polymerization catalyzed by Ziegler-Natta catalysts. We are intrigued by the experimental facts and attempt to explain why the ethylene anionic polymerizations initiated by the tertiary diamine/n-butyllithium complexes are so low, on the basis of energetics of the reactions. On the other hand, we noticed that, while the mechanisms of ethylene coordination polymerizations by Ziegler-Natta catalysts have been extensively investigated using density functional theory (DFT) calculations,19,20 to our best knowledge, the mechanism of ethylene anionic polymerizations by tertiary diamine/n-butyllithium complexes has never been studied. Thus, the study could be helpful for understanding the mechanism of not only ethylene anionic polymerization but also ethylene coordination polymerization. In this study, ethylene insertions into Li-C bonds of eight tertiary diamine/n-butyllithium complexes were investigated using DFT calculations. A four-membered-ring transition state was found to play an important role in each of the insertion reactions. The influences of the structures of tertiary diamine compounds on the insertion barriers were also studied. The energy profiles of ethylene anionic polymerizations are compared to the reported energy profiles of cationic coordination polymerizations catalyzed by Ziegler-Natta catalysts. The results are used to explain why the activity of ethylene anionic
10.1021/jp910712v 2010 American Chemical Society Published on Web 02/02/2010
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Figure 1. Structures of the eight tertiary diamine compounds.
TABLE 1: Results for the Ethylene Insertions into Eight Tertiary Diamine/n-Butyllithium Complexes tertiary diamine
∆Ec(NLi)a (kcal/mol)
∆Ei(NLi-C2)b (kcal/mol)
DLi-Cmc (Å)
∆Eb (kcal/mol)
IFd (cm-1)
∆Er (kcal/mol)
DIPYE sparteine DIPIM DIPIE DIPIP TMEDA TMPDA TEEDA
-34.4 -37.0 -26.2 -34.5 -29.7 -31.6 -34.4 -33.3
-2.9 -2.8 -2.5 -2.8 -2.4 -2.9 -2.5 -2.1
5.537 5.193 5.864 5.791 5.118 4.535 8.180 6.278
7.8 11.0 6.9 9.4 9.9 8.5 7.9 7.8
287.5i 291.8i 285.3i 302.2i 290.7i 305.8i 300.8i 287.9i
-18.9 -18.8 -18.9 -16.1 -16.4 -15.5 -16.3 -16.7
a
The coordination energy between tertiary diamine compound and n-butyllithium. b The interaction energy of the van der Waals complex relative to the separated reactants. c The distance between the Li atom and the middle point of the CdC double bond of ethylene in the van der Waals complex of tertiary diamine/n-butyllithium and ethylene. d Imaginary frequency.
polymerization is much lower than that of coordination polymerization. Computation Details All DFT computations were performed using the Dmol3 program (Accelrys Co.). The electronic structures of the molecular systems were described by double-numerical basis sets with polarization functions (DNP). Geometry, energy, and vibrational frequency calculations were carried out using the general gradient approximation correction models with the exchange functional of Becke21 and correlation functional of Lee, Yang, and Parr22 (BLYP). Transition states were located by using the complete LST/QST (linear synchronous transit/ quadratic synchronous transit) method. In this method, a QST method is performed first to search for an energy maximum with constrained minimization. Next, a LST method is performed to search for a single interpolation to a maximum energy. The process repeats until a stationary point is located. Subsequently, the saddle point is fully optimized following the negative eigenvector along the reaction path. Frequency calculations were performed to characterize the nature of the identified stationary points. Transition states were confirmed to have only one imaginary frequency and no imaginary frequency for minima. The convergence thresholds for energy, maximum force, and maximum displacement are 1 × 10-6 hartree, 0.002 hartree/Å, and 0.005 Å, respectively. The structures of the eight tertiary diamine compounds are displayed in Figure 1. In the following discussion, the interaction
energy (∆Ei) is defined as the difference of the electronic energy between the van der Waals complex and the separated reactants. The coordination energy (∆Ec) is defined as the difference of the electronic energy between the complex and the separated reactants. The insertion barrier (∆Eb) is defined as the difference of the electronic energy between the transition state and the corresponding van der Waals complex or ethylene/transition metal complex. Reaction energy (∆Er) is defined as the difference of the electronic energy between the product and the reactants (i.e., the tertiary diamine/n-butyllithium complex and ethylene). All of the energetic results are compiled in Table 1. Because of the C-C bond rotation, the butyl group in the tertiary diamine/n-butyllithium complex can adopt several minimum conformations, among which the zigzag conformation is the lowest minimum. To simplify the calculations, only the zigzag conformation of the butyl group in the tertiary diamine/ n-butyllithium complex was considered. Results and Discussion In a typical experimental anionic ethylene polymerization, n-butyllithium solution and tertiary diamine are added into an autoclave, and ethylene is then introduced. The complexation between n-butyllithium and tertiary diamine is quick and finished in a very short time. Therefore, the initiator for the anionic polymerization is assumed to be the tertiary diamine/n-butyllithium complex, instead of n-butyllithium. DFT calculations indicate, as shown in Table 1, that the complexation energies of tertiary diamine compounds to n-butyllithium are more than
Ethylene Polymerization
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Figure 2. Optimized structures of (A) DIPYE-BuLi, (B) DIPYE-BuLi/ ethylene van der Waals complex, (C) transition state of ethylene insertion into DIPYE-BuLi, and (D) product DIPYE-n-hexyllithium. For clarity, hydrogen atoms are omitted.
26.0 kcal/mol. The large complexation energies explain the quick reactions between tertiary diamines and n-butyllithium. We use the ethylene polymerization initiated by DIPYE/nbutyllithium (DIPYE-BuLi) complex as an example for the following discussion. The optimized structures of the DIPYEBuLi complex and the DIPYE-BuLi/ethylene van der Waals complex are shown in Figure 2. The distance between Li atom and the middle point of CdC bond of ethylene is 5.537 Å, which indicates that the interaction between Li atom and ethylene is not due to molecular orbital interactions, but due to van der Waals interaction (the interaction energy (∆Ei) is only 2.9 kcal/ mol). Therefore, ethylene cannot form coordination complexes similar to those in the ethylene cationic polymerizations. As compared in Figure 2A, the structure of the DIPYE-BuLi fragment in the DIPYE-BuLi/ethylene van der Waals complex (Figure 2B) only changes slightly; the bond lengths of Li-N and Li-C are slightly lengthened from 2.166 and 2.073 Å to 2.167 and 2.075 Å, respectively. The double bond of ethylene is slightly elongated from 1.336 to 1.340 Å. The transition state (Figure 2C) of the insertion reaction of ethylene into the Li-C bond of the DIPYE-BuLi complex has a four-membered-ring structure. In the structure of the transition state, the Li-N and Li-C1 bond lengths are 2.241 and 2.162 Å, respectively. The CdC double bond of ethylene also is elongated to 1.423 Å. The Li-C2 distance is shorter than the C1-C3 distance. The insertion barrier is 7.8 kcal/mol. After the ethylene insertion, the product (DIPYE-n-hexyllithium complex) is obtained. The structure of the product is shown in Figure 2D. The reaction energy (∆Er) is 18.9 kcal/mol. The energy profile of the ethylene insertion reaction is shown in Figure 3A. The energy profiles of the ethylene insertions into the other seven tertiary diamine/n-butyllithium complexes are similar to that in the case of the DIPYE-BuLi complex. The energetic results are included in Table 1. In each of the cases, ethylene
Figure 3. (A) The energy profile for the insertion of ethylene into DIPYE-BuLi complex. (B) A typical energy profile of the insertion of ethylene into C-metal (the metal center of the Ziegler-Natta catalysts) in cationic coordination polymerization.
has a weak van der Waals interaction with the lithium complex. The distance between the Li atom in the tertiary diamine/nbutyllithium complex and the middle point of the CdC bond of ethylene is longer than 5 Å except for that of TMEDA (4.535 Å). The ∆Ei is less than 3 kcal/mol. After a four-memberedring transition state, ethylene inserts into the Li-C bond. The insertion barrier is in the range of 6.9-11.0 kcal/mol. The reaction energy ranges from 16 to 19 kcal/mol. In the coordination polymerization catalyzed by transition metal Ziegler-Natta catalysts, the insertion of ethylene into the transition metal-C bond involves a coordination state and a four-membered-ring transition state.23 The major differences between ethylene anionic and coordination polymerizations include the nature of active site and the coordination energy. In Ziegler-Natta coordination polymerization, the active center is a cation, rather than an anion. The coordination energy between ethylene and Ziegler-Natta catalyst is usually large (more than 20 kcal/mol).19 In the coordination state, the distance between ethylene and transition metal is usually less than 3 Å, and the coordination between ethylene and transition metal is due to Lewis acid-base interaction. In contrast, there is only a weak van der Waals complex formed between ethylene and the anionic complex, which is indicated by the long distance between the ethylene and the Li atom in the tertiary diamine/ n-butyllithium complex. The insertion barriers of ethylene anionic polymerizations are close to or even lower than those of cationic coordination
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polymerization (in most cases, 6-11 kcal/mol).19 However, the reaction rate of cationic coordination polymerization is generally thousands times higher than that of ethylene anionic polymerization, which cannot be rationalized on the basis of the heights of reaction barriers. Their different energy profiles can interpret this “discrepancy”. A typical energy profile (Figure 3B) of ethylene insertion into Ziegler-Natta catalyst19,23,24 is compared to that of ethylene into the DIPYE-BuLi complex in Figure 3B. In the case of ethylene insertion into the DIPYE-BuLi complex (Figure 3A), the transition state lies above the reactants, DIPYE-BuLi + ethylene, which indicates that the ethylene insertion needs additional energy input to climb the transition state. In contrast, the transition state in Figure 3B lies below the reactants, cat + ethylene. The energy for climbing the transition state thus can be obtained from the reactants; no additional energy is demanded. In the process to produce a long polyethylene chain, hundreds to thousands of ethylene insertions occur. In the case of anionic polymerization, the polymerization needs to overcome hundreds to thousands of energy barriers, which needs substantial energy input. However, in the case of cationic coordination polymerization, the energy profile, which goes down and up, indicates that the coordination energy can be utilized to overcome the energy barrier, without requiring additional energy input. This could be one of the main reasons why the ethylene polymerization rate of cationic coordination polymerization is much higher than that of anionic polymerization, while they have comparable insertion barriers. It is worth noticing that an adiabatic process assumption was applied to the ethylene coordination reaction. If the coordination energy is released and then transferred to the reaction medium, part of the energy is needed to be transferred from the reaction medium back to the reactants to drive the ethylene insertion. The energy exchange process should be finished in a very short time, because, if not, the coordination state would become a stable state and act as reactants. However, the ethylene coordination state is a very unstable state in cationic coordination polymerization. It has never been captured in any experimental study. Thus, the quick energy exchange process can occur, and the reaction can be considered as an adiabatic process. In other words, the energy released due to coordination is not released to the reaction medium, but transformed into other forms of energies such as vibrational and kinetic energies stored in the molecules, which drive the reaction forward. This also explains why the coordination complexes cannot be captured experimentally. Conclusion The reaction mechanism of ethylene insertions into eight tertiary diamine/n-butyllithium complexes has been investigated using density functional theory at the level of BLYP/DNP. The complexation energies between tertiary diamine compound and n-butyllithium are in the range of 26.2-37.0 kcal/mol. Each of the insertions of ethylene into Li-C bond of the tertiary diamine/ n-butyllithium complexes takes place via a four-membered-ring transition state after a van der Waals complex. The interaction energies of the van der Waals complexes are less than 3 kcal/
Li et al. mol, the insertions are in the range of 6.9-11.0 kcal/mol, and the products are 15.5-18.9 kcal/mol lower the initial reactants. The energy profiles of ethylene anionic polymerization are different from those of cationic coordination polymerizations. In terms of energy, the reactants of ethylene anionic insertion lie above the transition state, while the reactants of ethylene cationic coordination insertion lie below of the transition state. Therefore, each insertion of ethylene into Li-C bond in ethylene anionic polymerization needs additional energy to overcome the energy barrier. In contrast, the energy in the case of cationic coordination polymerization for crossing the transition state can be obtained from the energy release due to the coordination between ethylene and the transition center of the Ziegler-Natta catalysts. These could explain the low activity of ethylene anionic polymerization and high activity of ethylene cationic coordination polymerization even though they have comparable insertion barriers. Acknowledgment. We express thanks for financial support from the National Science Foundation of China (no. 20334030 and no. 50703044). Z.-X.W. acknowledges financial support from the Chinese Academy of Sciences. References and Notes (1) Vasile, C. Handbook of Polyolefins, 3rd ed.; Marcel Dekker: New York, 2000. (2) Friedrich, M. E. P.; Marvel, C. S. J. Am. Chem. Soc. 1930, 52, 376. (3) Eberhardt, G. G.; Butte, W. A. J. Org. Chem. 1964, 29, 2928. (4) Eberhard, G.; Davis, W. R. J. Polym. Sci. 1965, 3, 3753. (5) Hay, J. N.; McCabe, J. F.; Robb, J. C. J. Chem. Soc., Faraday Trans. 1 1972, 68, 1227. (6) Hay, J. N.; Harris, D. S.; Wiles, M. Polymer 1976, 17, 613. (7) Rodriguez, F.; Abadie, M.; Schu, F. Eur. Polym. J. 1976, 12, 17. (8) Rodriguez, F.; Abadie, M.; Schue, F. J. Polym. Sci., Polym. Chem. Ed. 1976, 14, 773. (9) Magnin, H.; Rodriguez, F.; Abadie, M.; Schue, F. J. Polym. Sci., Polym. Chem. Ed. 1977, 15, 875. (10) Aldissi, M.; Schu, F.; Geckeler, K.; Abadie, M. Makromol. Chem. 1980, 181, 1413. (11) Marshall, W. B.; Brewbaker, J. L.; Delaney, M. S. J. Appl. Polym. Sci. 1991, 42, 533. (12) Endo, K.; Otsu, T. Makromol. Chem. Rapid Commun. 1993, 14, 1. (13) Magnin, H.; Rodriguez, F.; Abadie, M.; Schue, F. J. Polym. Sci., Part A: Polym. Chem. 1977, 15, 901. (14) Aldissi, M.; Schu, F.; Liebich, H.; Geckeler, K. Polymer 1985, 26, 1096. (15) Bergbreiter, D. E.; Blanton, J. R.; Chandran, R.; Hein, M. D.; Huang, K. J.; Treadwell, D. R.; Walker, S. A. J. Polym. Sci., Part A: Polym. Chem. 1989, 27, 4205. (16) Endo, K.; Otsu, T. J. Chem. Soc., Chem. Commun. 1990, 1372. (17) Endo, K.; Otsu, T. Makromol. Chem. Rapid Comm. 1992, 13, 135. (18) Endo, K.; Otsu, T. Macromol. Rapid Commun. 1994, 15, 233. (19) Rappe, A. T.; Skiff, W. M.; Casewit, C. J. Chem. ReV. 2000, 100, 1435. (20) Vasile, C. Handbook of Polyolefins, 3rd ed.; Marcel Dekker: New York, 2000. Morokuma, K.; Musaev, D. G. Computitional Modeling for Homogeneous and Enzymatic Catalysis; Wiley-VCH: Weinheim, 2008. (21) Becke, A. D. J. Chem. Phys. 1988, 88, 2547. (22) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (23) Kuran, W. Principles of Coordination Polymerization; John Wiley & Sons: Chichester, 2000. (24) Yang, S. H.; Jo, W. H.; Noh, S. K. J. Chem. Phys. 2003, 119, 1824.
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