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aluminoxanes
European Polymer Journal 36 (2000) 1265±1270
Study on polymerization of ethylene with Cp2ZrCl2/ aluminoxanes Qi Wang*, Jianhua Weng, Zhiqiang Fan, Linxian Feng Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, People's Republic of China Received 27 August 1998; received in revised form 10 February 1999; accepted 12 May 1999
Abstract Ethylene polymerization with Cp2ZrCl2/dierent aluminoxanes was investigated. Based on comparison of activity, molecular weight and molecular weight distribution of polyethylene obtained by dierent aluminoxanes, the eect of aluminoxanes on ethylene polymerization were discussed and the co-aluminoxane phenomenon was proposed. The introduction of a dierent alkyl group into aluminoxane greatly changes its regular cluster structure and therefore dierent coordinated aluminum atoms are presented. The increment of multiple types of aluminum atoms in aluminoxane improves not only the higher conversion of metallocene into the active site, but also the high stability of the counterion of the aluminoxane±anion complex in metallocene catalyst which leads to a high activity of mixed aluminoxane and a broad molecular weight distribution of related products. # 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction The discovery of homogeneous Ziegler±Natta catalytic systems comprising of metallocene and aluminoxane opened a new era in the world of polyole®ns. New homo- and copolymers have been prepared by changing the structure of the metallocene. The high activity of metallocene leads to its high value of commercial application. Compared to the wide study on the relationship between the structure of metallocene and the structure or properties of related products, the role that cocatalyst aluminoxanes play in the homogeneous catalyst has not been studied extensively. The eect of aluminoxane has recently received considerable attention in the ole®n polymerization. A few articles [1±6] deal with the role that residual TMA plays in MAO in the polymerization of ethylene which
* Corresponding author. Fax: +86-571-795-1773.
leads to contradictory conclusions. Addition of common aluminoxane such as ethylaluminoxane (EAO) and iso-butylaluminoxane (i-BAO) to MAO was also explored [7]. Recently, aluminoxane such as isooctylaluminoxane and benzylaluminoxane [8] were also applied to the ole®n polymerization which exhibit similar or better properties compared with MAO. With the help of mixed methyl-iso-butylaluminoxane, a soluble polynorbornene was produced [9]. A new concept, `latent Lewis acidity' was proposed by Barron [10] to account for the reactivity of aluminoxane, based on the research of monodispersed t-BAO and the catalytic activity of t-BAO was determined by the structure of the aluminoxanes. So the cocatalyst aluminoxane not only aects the activity of metallocene, but also the properties of the products obtained. By varying the composition of cocatalyst, many new results of polymerization can be achieved. From the dierent performance of aluminoxanes, their eect on the homogeneous catalyst system may be deduced. In
0014-3057/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 9 9 ) 0 0 1 8 4 - 6
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this paper, the role that aluminoxane cocatalyst plays in the ethylene polymerization has been explored. In addition, the in¯uence of dierent aluminoxanes on the activity and properties of related polymers were investigated. The reason for the co-aluminoxane phenomenon was also proposed. 2. Experimental 2.1. Materials MAO (Schering Co.), iso-Bu3Al, Et3Al (Aldrich Co.) and Cp2ZrCl2 (Fluka Co.) were used as received. Toluene was puri®ed by re¯uxing over Na±K alloy under nitrogen atmosphere and distilled prior to use. Polymerization grade ethylene was further puri®ed by passing over 4 AÊ activated molecular sieves. 2.2. Synthesis of mixed aluminoxane Mixed aluminoxane was prepared by hydrolysis of a mixture of Et3Al and iso-Bu3Al with water in toluene at low temperature. For example, to a 100 ml toluene solution of 0.07 mol triisobutylaluminum and 0.03 mol triethylaluminum, 1.8 ml water (0.1 mol) was directly added dropwise at ÿ208C with vigorous stirring. The reaction mixture was allowed to warm slowly to room temperature and stirred for 3 h. The resulting mixture was re¯uxed for 15 min and then cooled to room temperature. The white solid was obtained by evaporation of the solvent at reduced pressure (yield 70±95%). 13 C-NMR (C6D4Cl2): d (in ppm)=27.5 (methyl of iso-butyl), 24.7 (methine of iso-butyl), 20.2 (methylene of iso-butyl), 9.3 (methyl of ethyl), 0.6 (methylene of ethylene). EAO and i-BAO were synthesized with the same method. 13C-NMR (C6D4C12) for EAO: 8.37 (methyl of ethyl), 0.88 (methylene of ethylene). 13C-NMR (C6D4Cl2) for i-BAO: d=28.6 (methyl of iso-butyl), 25.7 (methine of iso-butyl), 19.7 (methylene of isobuty1). 2.3. Polymerization procedure The polymerization was carried out in a 100-ml
glass autoclave equipped with ethylene inlet, magnetic stirrer and vacuum line. The autoclave was ®lled with 50 ml dry toluene and certain quantities of aluminoxane. The mixture was stirred at 308C and then saturated with ethylene (1 atm). The reaction was initiated by adding the solution of Cp2ZrCl2 in toluene. After 30 min the polymerization was terminated by addition of acidi®ed ethanol. The resulting polymer was separated by ®ltration and dried under vacuum until constant weight. The stirring speed was 1500 rmp at which the diusion eect on polymerization was minimized. 2.4. Measurement The aluminum content of aluminoxanes was determined by titration with EDTA. 13C-NMR spectra of EBAO were performed on JEOL FT NMR 90 spectrometer at room temperature. The properties of aluminoxanes are listed in Table 1. High temperature GPC was performed with a Waters 150 C instrument at 1358C using four styragel (103, 104, 105, 106 AÊ) pore size columns in series and o-dichlorobenzene as the solvent. Polystyrene was used for calibration. 3. Results and discussion 3.1. Comparison among dierent aluminoxanes 3.1.1. Eect of aluminoxane on activity Table 2 shows the results for ethylene polymerization with Cp2ZrCl2/EAO, BAO, EBAO and MAO. It is evident that the polymerization activities of metallocene comprising the alummoxanes e.g. EAO, BAO and EBAO with various compositions are rather low, comparing with MAO. The most important result is that the activities of mixed aluminoxanes EBAOs are much higher than those of homoaluminoxane such as EAO and BAO under the same conditions. Fujita [7] also reported that in the propylene polymerization, methylisobutylaluminoxane (MBAO) showed a higher activity than pure MAO. The similar result is called `coalurninoxane phenomenon' here as compared to `comonomer eect' in ole®n polymerization. 3.1.2. Eect of aluminoxane on MW and MWD The molecular weight distribution (MWD) of poly-
Table 1 Properties of mixed ethylene±iso-butylaluminoxane (EBAO) AlEt3/Al(iso-BU)3 (mol/mol)
1/0
7/3 EBAOl
1/1 EBAO2
3/7EBAO3
0/1
Yield (wt%) [Al] (10ÿ2mol/g) [Et] (mol%)
51.7 ± 100
74.6 1.38 50
76.9 1.22 10
94.4 1.34 5
88.7 ± 0
Qi Wang et al. / European Polymer Journal 36 (2000) 1265±1270
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Table 2 Polymerization of ethylene with Cp2ZrCl2/Aluminoxanesa Aluminoxane
No.
Al/Zr
Activity (106g/mol Zr/H)
Mnb 104g/mol
Mw/Mnb
MAOc
1 2 3 4 5 6 7 8 9 10 11 12 13 14
1000 2000 4000 2000 4000 8000 2000 4000 8000 4000 4000 1000 2000 4000
2.66 5.22 12.57 0.22 0.27 0.59 0.10 0.17 0.57 0.35 0.66 0.57 1.37 1.53
± 21.0 18.5 ± 4.82 ± ± 6.64 8.88 4.89 5.55 ± ± 6.20
± 2.64 2.60 ± 2.90 ± ± 5.77 3.85 2.95 3.68 ± ± 3.56
EAOd BAOd EBAO1d EBAO2d EBAO3d
a
General conditions: T = 508C, t = 30 min. Tested by GPC. c [Zr]=10ÿ6M. d [Zr]=10ÿ5M. b
ethylene, as illustrated in Fig. 1 and Table 2, produced by EAO, BAO and EBAOs is wider than that of MAO. Furthermore, in BAOs apparent bimodal distribution of molecular weight is observed. Similar wide MWD of polymers produced by metallocenes under a certain polymerization condition was also reported [6]. These facts may suggest that multiple active sites were formed in the metallocene catalysts containing aluminoxanes e.g. EAO, BAO and EBAO. Mixed-aluminoxanes are comprised of at least two types of alkyl groups and the active sites formed by the reaction of
Cp2ZrCl2 and EBAO must be varied in the view of alkyl group and counterion. The dierent active sites will produce polymers with dierent molecular weights, which leads to the broad MWD of the ®nal product. The molecular weight of polyethylene produced by EBAO, EAO and BAO is close but lower than when produced by MAO. 3.1.3. Polymerization kinetics The kinetic study on the ethylene polymerization catalyzed by dierent aluminoxanes was also investigated and was reported in our previous paper [11], in which a multiple active sites kinetic model for simulation of ethylene polymerization was developed. Good agreements of polymerization rate and polydispersity of molecular weight were achieved for ®tting the kinetic pro®les with the model. 3.2. Possible explanation for Co-aluminoxane phenomenon
Fig. 1. GPC curves of polyethylene produced by Cp2ZrCl2/ aluminoxanes. (a) run 14, (b) run 8 and (c) run 3.
Aluminoxane as cocatalyst in the metallocene catalyst system activates the metallocene in two steps. The ®rst step of reaction is the alkylation of metallocene with aluminoxane to form a metal±alkyl bond and then the alkylated metallocene was changed into the cation site by releasing an anion. The anion is captured by aluminoxane to form a counterion. Both steps shown in Eqs. (1) and (2) are essential for the formation of the active site in ole®n polymerization. Common trialkyl aluminums may carry on the alkylation reaction, but they cannot seize the anion to
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form the counterion, so they show no activity with metallocene. The functions of aluminoxane in the homogeneous Ziegler±Natta catalyst are greatly determined by its unique property. Their relationship will be discussed with examples in the following. K1
Cp2 MtX2 AO$Cp2 MtRX AO X
formance. The structure of aluminoxanes is complex and has been investigated by various methods. As proved by 27Al-NMR analysis [12], the aluminoxane is not linear as people assumed previously, but has a three-dimensional structure as a cluster, because most aluminum atoms in aluminoxanes are tetra-coordinated. The new architectural feature may explain the unique polymerization ability of MAO versus Me3Al. Even though whether Me3A1 remaining in the MAO is essential to the polymerization is not con®rmed, the MAO mainly contributes to the high activity performance of homogeneous Ziegler±Natta catalyst. The three-dimensional structure of aluminoxanes is responsible for dierent performances of aluminoxanes. As proposed by Sugano [12], the cluster structure for MAO, EAO and BAO are not the same because of their dierent 27Al-NMR spectra. MAO and EAO have the same architecture while that of BAO is dierent. Due to the sterical eect, the aluminum atoms in BAO can not coordinate with the other alkyl group which is common in MAO and EAO, thus these atoms alternatively contact with the oxygen atoms to form a tetra-coordinated structure which is a saturated state for aluminum atoms. The ability of aluminoxanes to transfer metallocene to the stable cation type active site is mainly related to their cluster structure. Most aluminum atoms in aluminoxanes are tetra-coordinated, but there are still some tri-coordinated aluminum atoms in the aluminoxanes; the Lewis acidity of these atoms is higher than that of tetra-coordinated aluminum atoms. The tri-coordinated aluminum atoms with high Lewis acidity can ÿ ÿ seize anions such as CHÿ OR, etc., to form a 3, X , tetra-coordinate stable state [13]. The anion transfer reaction forms the active sites for metallocene and the counterion of aluminoxane±anion, which can distribute
1
K2
Cp2 MtRX AO$Cp2 MtR AO X2 ÿ
2
3.2.1. Composition of aluminoxanes The obvious reason for performance of dierent aluminoxanes probably lies in the composition of the aluminoxane. In view of composition, MAO contains the segment of ±Al(Me)±O± and trace Me3Al, while the alkyl group for EAO, BAO and EBAO are ethyl and iso-butyl, separately. The reactivity of the Me±Al bond is apparently higher than either Et±Al or iso-Bu±Al bonds. So the conversion of metallocene into active site is in the sequence of MAO > EAO > BAO. The dierence of the alkyl±metal bond formed by alkylation may be partly attributed to the dierent polymerization activity during the formation of active cation sites of metallocene catalyst and the early period of propagation, because the alkyl±metal bonds are similar for dierent aluminoxane systems after insertion of several monomers. The dierence in aluminoxane composition also can not explain the reason that mixed EBAO is more active than EAO and BAO. 3.2.2. Structure of aluminoxane On the other side, the dierent structure of aluminoxanes may also be the reason of their unique perTable 3 27 Al-NMR chemical shift for dierent aluminoxanes [7] No.
1 2 3 4 5 6 7
Aluminoxane
MAO MAO BAO MBAOb MBAOc MBAOd MBAOe a
Chemical shift (ppm) Al±Me
Al±i-Bu
152 154 ± 174 179 156 155
± ± 67 68 68 ± 66
In 104 g PP/g catalyst. Produced by hydrolysis of AlMe3 and Al±i-Bu3. c Produced by hydrolysis of i-BAO and MAO at 708C. d Produced by hydrolysis of i-BAO and Me3A1 at room temperature. e Produced by reaction of i-BAO and MAO at room temperature. b
Activitya
Feature
19.5 17.3 ± 35.4 37.1 8.1 11.6
from TOSO±AKZO from Schering Co. from TOSO±AKZO i-Bu/Me=1.35 i-Bu/Me=0.86
Qi Wang et al. / European Polymer Journal 36 (2000) 1265±1270
the electron over the whole cage and stabilize the active cation site. Common trialkylaluminum compounds such as Me3Al usually exist in dimmer form, both aluminum atoms in tetra-coordinated state, so they show no activity in the metallocene catalyst system. The polymerization activity can be improved by the addition of trace B(C6F5)3 to MAO [14], which suggest that the anion transfer reaction is also critical for the polymerization. The tri-coordinated aluminum, atoms exist in a very small portion in the aluminoxane, which may be the reason that a high Al/Mt ratio is critical for a high activity of the metallocene system. In BAO, each aluminum atom is coordinated with two oxygen atoms, so the Lewis acidity of aluminoxane decreases, which leads to the decrement of the ability to capture anions. This may explain the low activity of BAO compared to EAO and EBAO. 3.2.3. Agostic±H interaction The b and g-agostic H interactions play an important role in ole®n polymerization with metallocene catalyst. It in¯uences stability of the active sites, transfer reaction and stereoselectivity of insertion. When the active site is formed, the hydrogen atom of the alkyl group linked to the center metal atom may react with the metal atom of the cation to stabilize it. There are several agostic interactions such as a, b and g agostic± H interactions existing in the catalyst system. In view
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of MAO, only the a-agostic interaction may be present as the active site is formed. For EAO and BAO, b and g agostic±H interaction will appear additionally. There may be an equilibrium among various forms of active sites for dierent aluminoxanes which are illustrated in Scheme 1. As the three-membered ring is less stable, so the a-agostic H eect is weaker than the b and g agostic±H interactions comprising of a four- and ®ve-membered ring separately. Thus the b and g-agostic hydrogen interaction will be obvious during the formation of active sites. The b and g-agostic eect enhances the stability of active sites and also decrease the reactivity of the metal±alkyl bonds, which leads to a low activity of EAO, BAO and EBAO. The strong agostic interaction not only decreases the reactivity of the active sites, but also increases the probability of the b±hydrogen transfer reaction. Both of them can decrease the molecular weight of the polymer. So the negative variation of molecular weight of polyethylene can also be attributed to the strong agostic hydrogen interaction in non-MAO aluminoxane systems. 3.2.4. Distribution of alkyl groups of mixed aluminoxane As discussed above, some aspects attributed to the performance of aluminoxane are outlined. However, none of them can perfectly explain the co-aluminoxane phenomenon.
Scheme 1. Possible agostic hydrogen interaction and equilibrium among various active sites of (a) MAO, (b) EAO and (c) BAO systems.
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The experiment of polymerization of propylene with MBAO reported by Fujita [7] may give us some suggestions. In Fujita's patent, two kinds of MBAO prepared by dierent methods were used as cocatalyst. One kind of MBAO (here called MBAO1) is prepared by hydrolyzing the solution of Me3A1 and (iso-Bu)3Al or MAO and i-BAO at 708C. The other one (here called MBAO2) is prepared by the reaction of isoBAO and MAO or Me3Al at room temperature. In 27 Al-NMR spectra of MBAOs, the chemical shifts for Al of MABO2 are similar to that of MAO and BAO, suggesting the block structure of MBAO2. The spectra of MBAO1 are dierent from those of MAO and BAO, which suggests that MBAO1 has a random alkyl group distribution. The main variation is that signals for aluminum atoms in Me±Al are shifted to down®eld. This variation implies an increase of Lewis acidity of aluminum atoms in Me±Al. Like a random copolymer, MBAO1 shows a distinguished feature as compared to MAO and BAO. Its activity is nearly twice as much as that of MAO [7]. According to the 13C-NMR spectra of EAO, i-BAO and EBAO, it can be concluded that the EBAO was a randomly mixed aluminoxane; in other words, the ethyl and iso-butyl groups are randomly distributed. Even though Et3Al is more active than iso-Bu3Al, the EAO block or BAO block can not be formed since the hydrolysis reaction is a condensation reaction instead of an addition reaction when the Et3A1 and iso-Bu3Al reacted with water in an equal stoichiometric ratio. So the most possible explanation for the co-aluminoxanes phenomenon is derived from the random distribution of alkyl groups of mixed aluminoxanes. In mixed aluminoxane, the incorporation of various alkyls greatly change the regular three-dimensional structure of homo-aluminoxanes, e.g. MAO, EAO and BAO and more irregular structures, in other words, `defects' are formed. The random distribution of the alkyl group in aluminoxanes, alike random copolymer, produces some distinguished types of aluminum atoms with dierent coordination states, some of which are unsaturated aluminum atoms with a tri-coordinated state in the mixed-aluminoxane. The increment of tricoordinated aluminum atoms greatly enhances the Lewis acidity of the whole aluminoxane, which leads to not only the higher eciency of conversion of metallocene into the active site, but also the high stability of the counterion of the aluminoxane±anion complex. 3.2.5. Latent Lewis acidity As proposed by Barron [10], latent Lewis acidity may explain the cocatalytic activity of aluminoxanes. In our case, the random distribution of alkyl groups of mixed aluminoxanea will increase their latent Lewis
acidity, which also leads to the improvement of the activity of mixed aluminoxanes.
4. Conclusions Based on study of ethylene polymerization catalyzed by Cp2ZrCl2/dierent aluminoxanes, the comparison of activity, molecular weight and molecular weight distribution of polyethylene obtained from dierent aluminoxanes was made. The co-aluminoxane phenomenon was proposed. The introduction of dierent alkyl groups into aluminoxane greatly changes its regular cluster structure and, therefore, dierent coordinated aluminum atoms are presented. The increment of unsaturated aluminum atoms in aluminoxane will improve not only the higher conversion of metallocene into the active site, but also the high stability of the counterion of the aluminoxane±anion complex. According to this principle, a better performance of current aluminioxane may be achieved if it is modi®ed by some methods.
Acknowledgements The ®nancial support of National Natural Science Foundation of China (No. 29734140) is gratefully acknowledged.
References [1] Cherdron H, Brekner MJ, Osan F. Angew Maromol Chem 1994;223:121. [2] Resconi S, Bossi S, Abis L. Macromolecules 1990;23:4489. [3] Tritto I, Li S, Sacchi MC, Zannoni G. Macromolecules 1993;26:7111. [4] Chien JCW, Sugimoto R. J Polym Sci A 1991;29:459. [5] Reddy S, Shashidhar G, Sivaran S. Macromolecules 1993;26:1180. [6] Chien JCW, Wang BP. J Polym Sci A 1988;26:3089. [7] EP 498,657. Mitsubishi Petroch. Co. Inv.: Fujita, Takashi, et al., 1992. [8] Maurizio G, Tiziano D, Fabrizio P, Gianni C. Worldwide Metallocene Conference, 1996. [9] JP 08,165,309. Mistsubishi Chem. Corp. Inv.: Nishizawa U, Takasaki K, Aritomi M. CA: 125,196683, 1996. [10] Barron AR. J Am Chem Soc 1995;117:6465. [11] Wang Q, Weng JH, Fan ZQ, Feng LX. Polymer 1999;40:1683. [12] Sugano T, Matsubara K, Fujita T. J Mol Catal 1993;82:93. [13] Kurokawa H, Sugano T. Macromol Symp 1995;97:143. [14] Michiels N, Antorio ME. Macromol Chem Macromol Symp 1995;97:171.
Study on polymerization of ethylene with Cp2ZrCl2/ aluminoxanes Qi Wang*, Jianhua Weng, Zhiqiang Fan, Linxian Feng Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, People's Republic of China Received 27 August 1998; received in revised form 10 February 1999; accepted 12 May 1999
Abstract Ethylene polymerization with Cp2ZrCl2/dierent aluminoxanes was investigated. Based on comparison of activity, molecular weight and molecular weight distribution of polyethylene obtained by dierent aluminoxanes, the eect of aluminoxanes on ethylene polymerization were discussed and the co-aluminoxane phenomenon was proposed. The introduction of a dierent alkyl group into aluminoxane greatly changes its regular cluster structure and therefore dierent coordinated aluminum atoms are presented. The increment of multiple types of aluminum atoms in aluminoxane improves not only the higher conversion of metallocene into the active site, but also the high stability of the counterion of the aluminoxane±anion complex in metallocene catalyst which leads to a high activity of mixed aluminoxane and a broad molecular weight distribution of related products. # 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction The discovery of homogeneous Ziegler±Natta catalytic systems comprising of metallocene and aluminoxane opened a new era in the world of polyole®ns. New homo- and copolymers have been prepared by changing the structure of the metallocene. The high activity of metallocene leads to its high value of commercial application. Compared to the wide study on the relationship between the structure of metallocene and the structure or properties of related products, the role that cocatalyst aluminoxanes play in the homogeneous catalyst has not been studied extensively. The eect of aluminoxane has recently received considerable attention in the ole®n polymerization. A few articles [1±6] deal with the role that residual TMA plays in MAO in the polymerization of ethylene which
* Corresponding author. Fax: +86-571-795-1773.
leads to contradictory conclusions. Addition of common aluminoxane such as ethylaluminoxane (EAO) and iso-butylaluminoxane (i-BAO) to MAO was also explored [7]. Recently, aluminoxane such as isooctylaluminoxane and benzylaluminoxane [8] were also applied to the ole®n polymerization which exhibit similar or better properties compared with MAO. With the help of mixed methyl-iso-butylaluminoxane, a soluble polynorbornene was produced [9]. A new concept, `latent Lewis acidity' was proposed by Barron [10] to account for the reactivity of aluminoxane, based on the research of monodispersed t-BAO and the catalytic activity of t-BAO was determined by the structure of the aluminoxanes. So the cocatalyst aluminoxane not only aects the activity of metallocene, but also the properties of the products obtained. By varying the composition of cocatalyst, many new results of polymerization can be achieved. From the dierent performance of aluminoxanes, their eect on the homogeneous catalyst system may be deduced. In
0014-3057/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 9 9 ) 0 0 1 8 4 - 6
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this paper, the role that aluminoxane cocatalyst plays in the ethylene polymerization has been explored. In addition, the in¯uence of dierent aluminoxanes on the activity and properties of related polymers were investigated. The reason for the co-aluminoxane phenomenon was also proposed. 2. Experimental 2.1. Materials MAO (Schering Co.), iso-Bu3Al, Et3Al (Aldrich Co.) and Cp2ZrCl2 (Fluka Co.) were used as received. Toluene was puri®ed by re¯uxing over Na±K alloy under nitrogen atmosphere and distilled prior to use. Polymerization grade ethylene was further puri®ed by passing over 4 AÊ activated molecular sieves. 2.2. Synthesis of mixed aluminoxane Mixed aluminoxane was prepared by hydrolysis of a mixture of Et3Al and iso-Bu3Al with water in toluene at low temperature. For example, to a 100 ml toluene solution of 0.07 mol triisobutylaluminum and 0.03 mol triethylaluminum, 1.8 ml water (0.1 mol) was directly added dropwise at ÿ208C with vigorous stirring. The reaction mixture was allowed to warm slowly to room temperature and stirred for 3 h. The resulting mixture was re¯uxed for 15 min and then cooled to room temperature. The white solid was obtained by evaporation of the solvent at reduced pressure (yield 70±95%). 13 C-NMR (C6D4Cl2): d (in ppm)=27.5 (methyl of iso-butyl), 24.7 (methine of iso-butyl), 20.2 (methylene of iso-butyl), 9.3 (methyl of ethyl), 0.6 (methylene of ethylene). EAO and i-BAO were synthesized with the same method. 13C-NMR (C6D4C12) for EAO: 8.37 (methyl of ethyl), 0.88 (methylene of ethylene). 13C-NMR (C6D4Cl2) for i-BAO: d=28.6 (methyl of iso-butyl), 25.7 (methine of iso-butyl), 19.7 (methylene of isobuty1). 2.3. Polymerization procedure The polymerization was carried out in a 100-ml
glass autoclave equipped with ethylene inlet, magnetic stirrer and vacuum line. The autoclave was ®lled with 50 ml dry toluene and certain quantities of aluminoxane. The mixture was stirred at 308C and then saturated with ethylene (1 atm). The reaction was initiated by adding the solution of Cp2ZrCl2 in toluene. After 30 min the polymerization was terminated by addition of acidi®ed ethanol. The resulting polymer was separated by ®ltration and dried under vacuum until constant weight. The stirring speed was 1500 rmp at which the diusion eect on polymerization was minimized. 2.4. Measurement The aluminum content of aluminoxanes was determined by titration with EDTA. 13C-NMR spectra of EBAO were performed on JEOL FT NMR 90 spectrometer at room temperature. The properties of aluminoxanes are listed in Table 1. High temperature GPC was performed with a Waters 150 C instrument at 1358C using four styragel (103, 104, 105, 106 AÊ) pore size columns in series and o-dichlorobenzene as the solvent. Polystyrene was used for calibration. 3. Results and discussion 3.1. Comparison among dierent aluminoxanes 3.1.1. Eect of aluminoxane on activity Table 2 shows the results for ethylene polymerization with Cp2ZrCl2/EAO, BAO, EBAO and MAO. It is evident that the polymerization activities of metallocene comprising the alummoxanes e.g. EAO, BAO and EBAO with various compositions are rather low, comparing with MAO. The most important result is that the activities of mixed aluminoxanes EBAOs are much higher than those of homoaluminoxane such as EAO and BAO under the same conditions. Fujita [7] also reported that in the propylene polymerization, methylisobutylaluminoxane (MBAO) showed a higher activity than pure MAO. The similar result is called `coalurninoxane phenomenon' here as compared to `comonomer eect' in ole®n polymerization. 3.1.2. Eect of aluminoxane on MW and MWD The molecular weight distribution (MWD) of poly-
Table 1 Properties of mixed ethylene±iso-butylaluminoxane (EBAO) AlEt3/Al(iso-BU)3 (mol/mol)
1/0
7/3 EBAOl
1/1 EBAO2
3/7EBAO3
0/1
Yield (wt%) [Al] (10ÿ2mol/g) [Et] (mol%)
51.7 ± 100
74.6 1.38 50
76.9 1.22 10
94.4 1.34 5
88.7 ± 0
Qi Wang et al. / European Polymer Journal 36 (2000) 1265±1270
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Table 2 Polymerization of ethylene with Cp2ZrCl2/Aluminoxanesa Aluminoxane
No.
Al/Zr
Activity (106g/mol Zr/H)
Mnb 104g/mol
Mw/Mnb
MAOc
1 2 3 4 5 6 7 8 9 10 11 12 13 14
1000 2000 4000 2000 4000 8000 2000 4000 8000 4000 4000 1000 2000 4000
2.66 5.22 12.57 0.22 0.27 0.59 0.10 0.17 0.57 0.35 0.66 0.57 1.37 1.53
± 21.0 18.5 ± 4.82 ± ± 6.64 8.88 4.89 5.55 ± ± 6.20
± 2.64 2.60 ± 2.90 ± ± 5.77 3.85 2.95 3.68 ± ± 3.56
EAOd BAOd EBAO1d EBAO2d EBAO3d
a
General conditions: T = 508C, t = 30 min. Tested by GPC. c [Zr]=10ÿ6M. d [Zr]=10ÿ5M. b
ethylene, as illustrated in Fig. 1 and Table 2, produced by EAO, BAO and EBAOs is wider than that of MAO. Furthermore, in BAOs apparent bimodal distribution of molecular weight is observed. Similar wide MWD of polymers produced by metallocenes under a certain polymerization condition was also reported [6]. These facts may suggest that multiple active sites were formed in the metallocene catalysts containing aluminoxanes e.g. EAO, BAO and EBAO. Mixed-aluminoxanes are comprised of at least two types of alkyl groups and the active sites formed by the reaction of
Cp2ZrCl2 and EBAO must be varied in the view of alkyl group and counterion. The dierent active sites will produce polymers with dierent molecular weights, which leads to the broad MWD of the ®nal product. The molecular weight of polyethylene produced by EBAO, EAO and BAO is close but lower than when produced by MAO. 3.1.3. Polymerization kinetics The kinetic study on the ethylene polymerization catalyzed by dierent aluminoxanes was also investigated and was reported in our previous paper [11], in which a multiple active sites kinetic model for simulation of ethylene polymerization was developed. Good agreements of polymerization rate and polydispersity of molecular weight were achieved for ®tting the kinetic pro®les with the model. 3.2. Possible explanation for Co-aluminoxane phenomenon
Fig. 1. GPC curves of polyethylene produced by Cp2ZrCl2/ aluminoxanes. (a) run 14, (b) run 8 and (c) run 3.
Aluminoxane as cocatalyst in the metallocene catalyst system activates the metallocene in two steps. The ®rst step of reaction is the alkylation of metallocene with aluminoxane to form a metal±alkyl bond and then the alkylated metallocene was changed into the cation site by releasing an anion. The anion is captured by aluminoxane to form a counterion. Both steps shown in Eqs. (1) and (2) are essential for the formation of the active site in ole®n polymerization. Common trialkyl aluminums may carry on the alkylation reaction, but they cannot seize the anion to
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form the counterion, so they show no activity with metallocene. The functions of aluminoxane in the homogeneous Ziegler±Natta catalyst are greatly determined by its unique property. Their relationship will be discussed with examples in the following. K1
Cp2 MtX2 AO$Cp2 MtRX AO X
formance. The structure of aluminoxanes is complex and has been investigated by various methods. As proved by 27Al-NMR analysis [12], the aluminoxane is not linear as people assumed previously, but has a three-dimensional structure as a cluster, because most aluminum atoms in aluminoxanes are tetra-coordinated. The new architectural feature may explain the unique polymerization ability of MAO versus Me3Al. Even though whether Me3A1 remaining in the MAO is essential to the polymerization is not con®rmed, the MAO mainly contributes to the high activity performance of homogeneous Ziegler±Natta catalyst. The three-dimensional structure of aluminoxanes is responsible for dierent performances of aluminoxanes. As proposed by Sugano [12], the cluster structure for MAO, EAO and BAO are not the same because of their dierent 27Al-NMR spectra. MAO and EAO have the same architecture while that of BAO is dierent. Due to the sterical eect, the aluminum atoms in BAO can not coordinate with the other alkyl group which is common in MAO and EAO, thus these atoms alternatively contact with the oxygen atoms to form a tetra-coordinated structure which is a saturated state for aluminum atoms. The ability of aluminoxanes to transfer metallocene to the stable cation type active site is mainly related to their cluster structure. Most aluminum atoms in aluminoxanes are tetra-coordinated, but there are still some tri-coordinated aluminum atoms in the aluminoxanes; the Lewis acidity of these atoms is higher than that of tetra-coordinated aluminum atoms. The tri-coordinated aluminum atoms with high Lewis acidity can ÿ ÿ seize anions such as CHÿ OR, etc., to form a 3, X , tetra-coordinate stable state [13]. The anion transfer reaction forms the active sites for metallocene and the counterion of aluminoxane±anion, which can distribute
1
K2
Cp2 MtRX AO$Cp2 MtR AO X2 ÿ
2
3.2.1. Composition of aluminoxanes The obvious reason for performance of dierent aluminoxanes probably lies in the composition of the aluminoxane. In view of composition, MAO contains the segment of ±Al(Me)±O± and trace Me3Al, while the alkyl group for EAO, BAO and EBAO are ethyl and iso-butyl, separately. The reactivity of the Me±Al bond is apparently higher than either Et±Al or iso-Bu±Al bonds. So the conversion of metallocene into active site is in the sequence of MAO > EAO > BAO. The dierence of the alkyl±metal bond formed by alkylation may be partly attributed to the dierent polymerization activity during the formation of active cation sites of metallocene catalyst and the early period of propagation, because the alkyl±metal bonds are similar for dierent aluminoxane systems after insertion of several monomers. The dierence in aluminoxane composition also can not explain the reason that mixed EBAO is more active than EAO and BAO. 3.2.2. Structure of aluminoxane On the other side, the dierent structure of aluminoxanes may also be the reason of their unique perTable 3 27 Al-NMR chemical shift for dierent aluminoxanes [7] No.
1 2 3 4 5 6 7
Aluminoxane
MAO MAO BAO MBAOb MBAOc MBAOd MBAOe a
Chemical shift (ppm) Al±Me
Al±i-Bu
152 154 ± 174 179 156 155
± ± 67 68 68 ± 66
In 104 g PP/g catalyst. Produced by hydrolysis of AlMe3 and Al±i-Bu3. c Produced by hydrolysis of i-BAO and MAO at 708C. d Produced by hydrolysis of i-BAO and Me3A1 at room temperature. e Produced by reaction of i-BAO and MAO at room temperature. b
Activitya
Feature
19.5 17.3 ± 35.4 37.1 8.1 11.6
from TOSO±AKZO from Schering Co. from TOSO±AKZO i-Bu/Me=1.35 i-Bu/Me=0.86
Qi Wang et al. / European Polymer Journal 36 (2000) 1265±1270
the electron over the whole cage and stabilize the active cation site. Common trialkylaluminum compounds such as Me3Al usually exist in dimmer form, both aluminum atoms in tetra-coordinated state, so they show no activity in the metallocene catalyst system. The polymerization activity can be improved by the addition of trace B(C6F5)3 to MAO [14], which suggest that the anion transfer reaction is also critical for the polymerization. The tri-coordinated aluminum, atoms exist in a very small portion in the aluminoxane, which may be the reason that a high Al/Mt ratio is critical for a high activity of the metallocene system. In BAO, each aluminum atom is coordinated with two oxygen atoms, so the Lewis acidity of aluminoxane decreases, which leads to the decrement of the ability to capture anions. This may explain the low activity of BAO compared to EAO and EBAO. 3.2.3. Agostic±H interaction The b and g-agostic H interactions play an important role in ole®n polymerization with metallocene catalyst. It in¯uences stability of the active sites, transfer reaction and stereoselectivity of insertion. When the active site is formed, the hydrogen atom of the alkyl group linked to the center metal atom may react with the metal atom of the cation to stabilize it. There are several agostic interactions such as a, b and g agostic± H interactions existing in the catalyst system. In view
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of MAO, only the a-agostic interaction may be present as the active site is formed. For EAO and BAO, b and g agostic±H interaction will appear additionally. There may be an equilibrium among various forms of active sites for dierent aluminoxanes which are illustrated in Scheme 1. As the three-membered ring is less stable, so the a-agostic H eect is weaker than the b and g agostic±H interactions comprising of a four- and ®ve-membered ring separately. Thus the b and g-agostic hydrogen interaction will be obvious during the formation of active sites. The b and g-agostic eect enhances the stability of active sites and also decrease the reactivity of the metal±alkyl bonds, which leads to a low activity of EAO, BAO and EBAO. The strong agostic interaction not only decreases the reactivity of the active sites, but also increases the probability of the b±hydrogen transfer reaction. Both of them can decrease the molecular weight of the polymer. So the negative variation of molecular weight of polyethylene can also be attributed to the strong agostic hydrogen interaction in non-MAO aluminoxane systems. 3.2.4. Distribution of alkyl groups of mixed aluminoxane As discussed above, some aspects attributed to the performance of aluminoxane are outlined. However, none of them can perfectly explain the co-aluminoxane phenomenon.
Scheme 1. Possible agostic hydrogen interaction and equilibrium among various active sites of (a) MAO, (b) EAO and (c) BAO systems.
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The experiment of polymerization of propylene with MBAO reported by Fujita [7] may give us some suggestions. In Fujita's patent, two kinds of MBAO prepared by dierent methods were used as cocatalyst. One kind of MBAO (here called MBAO1) is prepared by hydrolyzing the solution of Me3A1 and (iso-Bu)3Al or MAO and i-BAO at 708C. The other one (here called MBAO2) is prepared by the reaction of isoBAO and MAO or Me3Al at room temperature. In 27 Al-NMR spectra of MBAOs, the chemical shifts for Al of MABO2 are similar to that of MAO and BAO, suggesting the block structure of MBAO2. The spectra of MBAO1 are dierent from those of MAO and BAO, which suggests that MBAO1 has a random alkyl group distribution. The main variation is that signals for aluminum atoms in Me±Al are shifted to down®eld. This variation implies an increase of Lewis acidity of aluminum atoms in Me±Al. Like a random copolymer, MBAO1 shows a distinguished feature as compared to MAO and BAO. Its activity is nearly twice as much as that of MAO [7]. According to the 13C-NMR spectra of EAO, i-BAO and EBAO, it can be concluded that the EBAO was a randomly mixed aluminoxane; in other words, the ethyl and iso-butyl groups are randomly distributed. Even though Et3Al is more active than iso-Bu3Al, the EAO block or BAO block can not be formed since the hydrolysis reaction is a condensation reaction instead of an addition reaction when the Et3A1 and iso-Bu3Al reacted with water in an equal stoichiometric ratio. So the most possible explanation for the co-aluminoxanes phenomenon is derived from the random distribution of alkyl groups of mixed aluminoxanes. In mixed aluminoxane, the incorporation of various alkyls greatly change the regular three-dimensional structure of homo-aluminoxanes, e.g. MAO, EAO and BAO and more irregular structures, in other words, `defects' are formed. The random distribution of the alkyl group in aluminoxanes, alike random copolymer, produces some distinguished types of aluminum atoms with dierent coordination states, some of which are unsaturated aluminum atoms with a tri-coordinated state in the mixed-aluminoxane. The increment of tricoordinated aluminum atoms greatly enhances the Lewis acidity of the whole aluminoxane, which leads to not only the higher eciency of conversion of metallocene into the active site, but also the high stability of the counterion of the aluminoxane±anion complex. 3.2.5. Latent Lewis acidity As proposed by Barron [10], latent Lewis acidity may explain the cocatalytic activity of aluminoxanes. In our case, the random distribution of alkyl groups of mixed aluminoxanea will increase their latent Lewis
acidity, which also leads to the improvement of the activity of mixed aluminoxanes.
4. Conclusions Based on study of ethylene polymerization catalyzed by Cp2ZrCl2/dierent aluminoxanes, the comparison of activity, molecular weight and molecular weight distribution of polyethylene obtained from dierent aluminoxanes was made. The co-aluminoxane phenomenon was proposed. The introduction of dierent alkyl groups into aluminoxane greatly changes its regular cluster structure and, therefore, dierent coordinated aluminum atoms are presented. The increment of unsaturated aluminum atoms in aluminoxane will improve not only the higher conversion of metallocene into the active site, but also the high stability of the counterion of the aluminoxane±anion complex. According to this principle, a better performance of current aluminioxane may be achieved if it is modi®ed by some methods.
Acknowledgements The ®nancial support of National Natural Science Foundation of China (No. 29734140) is gratefully acknowledged.
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