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Polyethylene with bimodal molecular weight distribution synthesized by 2,6-bis(imino)pyridyl complexes of Fe(II) activated with various activators
European Polymer Journal 40 (2004) 1881–1886
EUROPEAN POLYMER JOURNAL www.elsevier.com/locate/europolj
Polyethylene with bimodal molecular weight distribution synthesized by 2,6-bis(imino)pyridyl complexes of Fe(II) activated with various activators Qi Wang *, Lidong Li, Zhiqiang Fan Department of Polymer Science and Engineering, Zhejiang University, Zheda Road, Hangzhou 310027, PR China Received 19 October 2003; received in revised form 19 February 2004; accepted 3 March 2004 Available online 15 April 2004
Abstract Polyethylenes with bimodal molecular weight distribution (MWD) were synthesized by 2,6-bis(imino)pyridyl complexes of Fe(II) combined with different activators, which were prepared from alkylaluminium. It is found that the molecular weight (MW) and MWD was influenced by not only iron complexes but activators as well. The activator plays key important role in determination of the MW and MWD of final polymer and the MWD shape could be regulated by selection of various activators and polymerization conditions. The study on the variation of the MWD with the polymerization time and fitting of bimodal MWD with two Flory distributions suggests that bimodal MWD is caused by chain transfer reaction to activator or two active sites. 2004 Elsevier Ltd. All rights reserved. Keywords: Bimodal molecular weight distribution; Polyethylene; Iron complex; Activator
1. Introduction The molecular weight (MW) and molecular weight distribution (MWD) are important factors in determining the mechanical and rheological properties of polymers. It is believed that the polymer fraction of low MW improves the flow properties, while the fraction of high MW enhances melt strength and good mechanical properties. Therefore polymers with bimodal MWD may simultaneously show enhanced mechanical and rheological properties. Different techniques have been proposed to produce different polymer materials with bimodal MWD. All these techniques could be summarized as: stepwise polymerization in tandem reactor [1–4], polymerization using mixtures of different catalysts [5–10], or a sudden
*
Corresponding author. Tel./fax: +86-571-879-52400. E-mail address: [email protected] (Q. Wang).
variation of reaction conditions [11], especially the concentration of chain transfer agent. Several iron and cobalt catalysts based on tridentate diiminopyridine ligands have been reported to produce linear polyethylene with high MW [12,13]. With an activator such as methylaluminoxane (MAO), polyethylene with bimodal MWD is synthesized by iron catalyst at relatively high molar ratio of Al/Fe [12,14]. Unfortunately, large amount of low MW polymer is yielded under such reaction condition. The bimodal MWD with predominant low MW fraction is not an optimal MWD. Kumar and Sivaram [15], Radhakrishnan et al. [16,17] and our group [18] have reported the influence of activator for iron complex on the ethylene polymerization and bimodal MWD could be obtained under certain polymerization condition. In this article we report the polyethylene with bimodal MWD containing predominant high MW fraction produced by 2,6-bis(imino)pyridyl complexes of Fe(II) activated with different aluminium compounds.
0014-3057/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.03.003
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2. Experimental 2.1. General methods All manipulations were carried out under purified argon atmosphere by using standard Schlenk or high vacuum line techniques. The alkyl group and aluminium content of aluminium compounds was measured by GC and titration methods. Gel permeation chromatography (GPC) analyses were performed on PL-GPC 220 at 150 C with a polymer solution in 1,2,4-trichlorobenzene. DSC measurement was performed under a nitrogen atmosphere at a heating rate of 10 C/min on PE Pyris-1 DSC and the second run was reported. Gas chromatography (GC) was performed over Shimadzu GC8APF. 2.2. Materials Toluene was distilled under argon from sodium benzophenone ketyl. Methylene chloride was distilled under argon from P2 O5 . Et3 Al and i-Bu3 Al(Aldrich) were used as received. 4-fluorobenzeneboric acid was purchased from Acros. Ethylene (polymer grade) was purified by triisobutylaluminium in toluene. 2.3. Synthesis [(ArN ¼ C(Me))2 C5 H3 N]FeCl2 , Ar ¼ 2,6-dimethylphenyl (1) and Ar ¼ 2,6-diisopropylphenyl (2) were prepared according to reference [14]. Tetraethylaluminoxane (TEAO) was prepared as following procedure. 0.45 ml (0.025 mol) pure water was slowly added to 50 ml toluene solution of triethylaluminium (0.05 mol) at )78 C with vigorous stirring. The solution was allowed to warm up to room temperature and further stirred for 2 h. Tetraalkylaluminoxane (TAAO) was prepared as following procedure. 0.45 ml (0.025 mol) pure water was slowly added to 50 ml toluene solution of triethylaluminium (0.025 mol)/triisobutylaluminium (0.025 mol) at )78 C with vigorous stirring. The solution was allowed to warm up to room temperature and further stirred for 2 h. Et2 AlOB(p-FC6 H4 )OAlEt2 (BTEAO) was prepared as follows: 2.85 ml triethylaluminium (0.02 mol) was slowly added to a 20 ml toluene suspension of 1.40 g 4-fluorobenzeneboric acid (0.01 mol) at 0 C. The mixture was allowed to warm up to room temperature and stirred for 2 h. After the solid boric acid disappeared, the toluene solution of BTEAO was obtained. 2.4. Polymerization procedure Toluene and activator were injected into a 250 ml Schlenk flask with magnetic stirring at the desired tem-
peratures. The solution was then saturated with 1 atm of ethylene. Polymerization started with the addition of a dichloromethane solution of the iron complex into the flask. The polymerization was terminated by the addition of acidified ethanol. The polymer was washed with ethanol and dried under vacuum at 50 C.
3. Results and discussion Two known iron complexes shown in Scheme 1 were selected as precatalyst and three aluminium compounds as activators for ethylene polymerization. The synthetic methods of activators are illustrated in Scheme 2. By method A, involving direct reaction between trialkylaluminium and water at low temperature, two tetraalkylaluminoxanes were prepared. Tetraethylaluminoxane (TEAO) was prepared by reaction of triethylaluminium and water with molar ratio of 2:1. The Et/Al molar ratio of TEAO is 2.1. Tetraalkylaluminoxane (TAAO) was prepared by the reaction of triethylaluminium/triisobutylaluminium (1:1) and water with same molar ratio. The Et/iso-Bu ratio of TAAO measured by GC is 1:2.97. The synthesis of TEAO was previously studied by Siergiejczyk and Synoradzki [19]. There are multiple equilibriums among different components, including Et3 Al, TEAO and oligomeric aluminoxanes, existing in the final product of TEAO. The 1 H-NMR of TEAO confirms the presence of certain amount of Et3 Al in TEAO. By method B, BTEAO was synthesized through reaction between boric acid and triethylaluminium with molar ratio of 1:2.
Me
Me N
R N
R N
Fe Cl
1 R=Me
Cl
2 R=iso-Pr
R
R
Scheme 1. Structure of iron complex 1 and 2.
Method A: 2R3Al + H2O
R -78°C
R
Al-O-Al
R R
TEAO or TAAO
R=Et or iso-bu
Method B: R 2Et3Al+ HO-B-OH
R 0°C
Et2Al-O-B-O-AlEt 2 BTEAO
R= p-FC6 H4
Scheme 2. Synthesis of aluminium compounds as activator for iron complex.
Q. Wang et al. / European Polymer Journal 40 (2004) 1881–1886
Two iron complexes were combined with three different activators to polymerize ethylene. The results are summarized in Table 1. All catalyst groups show desirable polymerization activity and the MWD of polyethylene rely on both the catalyst group and Al/Fe molar ratio. When ethylene polymerization was conducted in the presence of complex 1, two activators gave bimodal MWD and one yielded unimodal. The bimodal MWD of polyethylene prepared by 1/TEAO and 1/TAAO are shown in Figs. 1 and 2. For the catalyst system 1/TEAO, the shape of MWD varies gradually with the molar ratio of Al/Fe. The low MW peak increases and the high MW peak reduces as the Al/Fe molar ratio varied from 500 to 2000, while the position of each peak hardly shifts. When BTEAO was employed as activator for complex 1, the resulting polyethylene’s MW was high and MWD was unimodal. The double melting points are observed for samples 1–3 and 5–6 because of large difference between the MWs of two fractions. Two activators yielded bimodal MWD and one yielded broad MWD when complex 2 was employed as precatalyst. The MWDs are shown in Figs. 3 and 4 respectively. When BTEAO, which was prepared from phenyl boric acid, is used as activator, the MWD of polyethylene is also influenced by molar ratio of Al/Fe. With the increment of Al/Fe molar ratio, the low MW peak reduces and the bimodal MWD is changed into unimodal one. The position of each peak is almost
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Fig. 1. The MWD of PE obtained by the 1/TEAO system as function of Al/Fe molar ratio (a) 500, (b) 1000 and (c) 2000.
identical. Such variation of MWD is different from MWD of polymer generated by normal aluminium activators used in this study. By different combinations of precatalysts and activators, various MWD shapes could be obtained. The MWD of resulting polymer changes from bimodal to unimodal when ethylene polymerization is conducted in the presence of same precatalyst activated with different aluminium compounds, e.g. 1/TEAO vs 1/BTEAO and 2/TEAO vs 2/TAAO. For the same activator, such
Table 1 Ethylene polymerization catalyzed by iron complexes and different aluminoxanea Activity (106 g PE/mol Fe h)
Mn b (104 g/mol)
Tm c (C)
2.62 1.51 1.30
17.6 12.6 13.7
79.7, 129.4 81.9, 127.9 93.2, 125.4
0.29 0.20 0.11
2.27 1.26 0.43
7.8 6.4 3.9
129.1 106.0, 126.8 88.7, 122.3
3.17 4.57 6.92
1.74 1.23 1.76
4.14 3.55 4.21
2.4 2.9 2.4
133.2 132.4 132.8
500 1000 2000
4.28 6.80 11.02
0.40 0.25 0.12
3.55 3.84 1.89
8.9 15.4 15.8
131.1 129.6 126.3
2/TAAO
500 1000 2000
2.91 3.74 6.66
1.04 0.48 0.49
4.42 3.51 3.69
4.3 7.3 7.5
132.4 131.4 130.9
2/BTEAO
500 1000 2000
0.14 0.62 2.26
1.14 1.97 3.79
9.47 12.43 12.54
8.3 6.3 3.3
133.3 134.4 134.4
Catalyst
Al/Fe
1 2 3
1/TEAO
500 1000 2000
2.22 3.13 5.87
0.15 0.12 0.09
4 5 6
1/TAAO
500 1000 2000
2.31 2.83 3.21
7 8 9
1/BTEAO
500 1000 2000
10 11 12
2/TEAO
13 14 15 16 17 18 a
Mw b (104 g/mol)
Mw =Mn b
Entry
General polymerization conditions: [Fe] ¼ 2 · 105 M, T ¼ 30 C, t ¼ 30 min, Pethylene ¼ 1 atm, in 50 ml toluene. Measured by means of GPC. c Measured by DSC. b
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Fig. 2. The MWD of PE obtained by the 1/TAAO system as function of Al/Fe molar ratio (a) 500, (b) 1000 and (c) 2000.
c a b
1
2
3
4 5 Log (MW)
6
7
Fig. 3. The MWD of PE obtained by the 2/TEAO system as function of Al/Fe molar ratio (a) 500, (b) 1000 and (c) 2000.
variation could be also found, e.g. 1/BTEAO vs 2/ BTEAO and 1/TAAO vs 2/TAAO. Therefore the MWD of resulting polymer is determined by not only precatalyst but activator as well, suggesting that the MW and MWD of polymer synthesized by iron complexes could be regulated by varying activators. Gibson’s and coworkers [14] have proposed that chain transfer to aluminium is mainly responsible for the MW of polymer prepared by iron complex activated by MAO. At high Al/Fe molar ratio, 2/MMAO yields polyethylene with bimodal MWD and low MW polymer fraction increases with the increment of Al/Fe ratio. Kumar and Sivaram [15], Radhakrishnan et al. [17] and our group [18] have reported the influence of alkylaluminium and mixture thereof as activator for 2 on the ethylene polymerization and bimodal MWD could be obtained under certain polymerization conditions.
Fig. 4. The MWD of PE obtained by the 2/BTEAO system as function of Al/Fe molar ratio (a) 500, (b) 1000 and (c) 2000.
Chain transfer reaction to alkylaluminium is believed as the key factor determining the MWD. From the comparison of TEAO with TAAO, it is concluded that Bu-Al group is efficient to improve the MW of final polymer, since it is less active chain transfer agent than Et-Al group. It is worth mention that the MW of polymer produced by activators modified with phenylboric acid is high with respect to other aluminium activators. The MW of low MW fraction of polymer prepared by 2/BTEAO is substantially increased and higher than those of corresponding fraction of polymer prepared by TEAO, TAAO and MAO [12,14]. For example, the peak MWs of sample 17 are 1.52 · 104 (one order of magnitude higher than those of samples 1–3) and 1.36 · 105 g/mol respectively. Furthermore, the MW difference between low and high MW fractions of bimodal MWD generated by boric acid based activators is smaller than those by other aluminium activators. The result is explained by the strong Lewis acidity of boron site, which retards the chain transfer reaction to activator. Polymer with bimodal MWD is usually generated by the presence of two active sites, one site generates low MW fraction and the other produces high MW fraction. Table 2 gives the detailed MW data of two peaks of bimodal MWD of polyethylene prepared by 1/TEAO. When the molar Al/Fe ratio varies from 500 to 2000, the number average molecular weights ðMn Þ and the polydispersity indexes (PDI) of low MW or high MW peaks are very close and only relative weight fraction of each peak changes. The PDIs of low MW peaks are around 1.2 and those of high MW peaks are close to 2.0. The polyolefin prepared by homogeneous catalyst, such as metallocene, normally has a PDI of 2.0. The quite narrow MWD of low MW fraction might be generated by three ways. The first is that the polymer with MW lower
Q. Wang et al. / European Polymer Journal 40 (2004) 1881–1886
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Table 2 Molecular weight distribution of polyethylene prepared by iron 1/TEAO Catalyst
Al/Fe
Peak 1 Mn
1/TEAO
500 1000 2000
a
0.04 0.06 0.07
Peak 2 Mw =Mn 1.22 1.20 1.22
b
c
Total
wt.%
Mn
Mw =Mn
wt.%
Mn
Mw =Mn
76 40 65
1.9 2.2 2.9
1.85 1.66 1.78
24 60 35
0.15 0.12 0.09
17.6 12.6 13.7
a
Number average molecular weight. Polydispersity index. c Weight percentage of the fraction. b
than 100 could not be precipitated by ethanol, therefore we get narrow MWD. The second is that the narrow MWD is caused by the fast chain transfer reaction. The third is that the low MW fraction is produced by distinctive active site other than the active site generating high MW fraction. If the first possibility is excluded, the last two possibilities could be discriminated by studying the variation of the MWD of polymer with polymerization time. If the bimodal MWD is generated by two active sites, the amount of polymer with low and high MW will both increase with the polymerization time. If it is caused by chain transfer reaction, polymer with low MW will be formed earlier than that of high MW. The experiment result is that the low MW fraction is produced at early stage of polymerization and the high MW fraction formed latter. So it is reasonable to make a conclusion that the low MW fraction is most probably generated by chain transfer reaction. It is suggested that the activator functions as chain transfer agent at the beginning of polymerization and polymer with low MW is produced. As the activator is consumed by the chain transfer reaction during the polymerization, polymer with high MW is generated. Such result has been reported by Gibson’s and coworkers as well [14]. The variation of MWD of polymer produced by 2/ BTEAO could not be well interpreted by chain transfer reaction, since the MWD becomes narrow when the Al/ Fe molar ratio is high. The Flory distribution has been applied for deconvolution of broad or bimodal MWD of polyethylene prepared by Ziegler–Natta [20] and metallocene [21] catalysts. As shown in Fig. 5, the MWD of polymer produced by 2/BTEAO could be fitted well with superposition of two Flory distributions, each having a PDI of 2.0. It is commonly accepted that the presence of one Flory distribution corresponds to one active site. So it is reasonable to say that the bimodal MWD of polyethylene produced by 2/BTEAO is caused by two different active sites. Multiple active sites model has been proposed for homogenous metallocene catalyst for ethylene polymerization [22,23]. For the mechanism of formation of multiple active centers, Chien and Wang [24] proposed that the metallocene can be coordinated to zero, one, or two MAOs, resulting in active centers of different kp. We also proposed that there must be a
(a)
2
3
4 5 Log (MW)
6
(b)
2
3
4
5
6
Log( MW)
Fig. 5. The MWD of PE obtained by the 2/BTEAO system fitted with Flory distributions (a) Al/Fe ¼ 500 and (b) Al/ Fe ¼ 1000 (circle: experimental data, dash line: Flory distribution, solid line: sum of Flory distributions).
maximum number of coordinated aluminoxane owing to steric effect and a minimum number owing to the stability of the center [25]. In the case of iron complex, it might also interact with different amount of activator molecules, resulting in different type of active sites. At low Al/Fe molar ratio, some iron complexes are coordinated with more activator molecules than others, which leads to two kinds of active sites. When the amount of
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activator is increased and in excess, the latter active site interacts with more activator molecules and is changed to former one. The overall result is that the active sites become uniform and the MWD of product is varied from bimodal to unimodal as the Al/Fe molar ratio increases.
4. Conclusion We report preparation of polyethylene with bimodal MWD involving single iron complex activated with different activators. The MWD of polymer could be regulated by selection of various activators and polymerization conditions in single reactor, which offers the most economic way to produce bimodal MWD polyolefins. The bimodal MWD generated by 1/TEAO is suggested to be caused by chain transfer to activator and the bimodal MWD by 2/BTEAO is proved to be generated by two active sites.
Acknowledgements Financial support from ‘‘State Key Fundamental Research Project of China (G1999064801)’’ is appreciated.
References [1] Stricklen PM (invs.). US 4939217 (1990), Phillips Petroleum. [2] Stricklen PM (invs.). US 5064797 (1991), Phillips Petroleum. [3] Lo FY, Nowlin TE, Shirodkar PP (invs.). US 5032562 (1991), Mobil Oil Corporation. [4] Alt FP, Bohm LL, Enderle HF, Berthold J. Macromol Symp 2001;163:135.
[5] Heiland K, Kaminsky W. Makromol Chem-Macromol Chem Phys 1992;193:601. [6] D’Agnillo L, Soares JBP, Penlidis A. J Polym Sci Polym Chem 1998;36:831. [7] Kim JD, Soares JBP, Rempel GL. Macromol Rapid Commun 1998;19:197. [8] Soares JBP, Kim JD. J Polym Sci Polym Chem 2000; 38:1408. [9] Shan CLP, Chu KJ, Soares J, Penlidis A. Macromol Chem Phys 2000;201:2195. [10] Kim Y, Han Y, Lee MH, Yoon SW, Choi KH, Song BG, et al. Macromol Rapid Commun 2001;22:573. [11] Han TK, Choi HK, Jeung DW, Ko YS, Woo SI. Macromol Chem Phys 1995;196:2637. [12] Small BL, Brookhart M, Bennett AMA. J Am Chem Soc 1998;120:4049. [13] Britovsek GJP, Gibson VC, Kimberley BS, Maddox PJ, McTavish SJ, Solan GA, et al. Chem Commun 1998: 849. [14] Britovsek GJP, Bruce M, Gibson VC, Kimberley BS, Maddox PJ, Mastroianni S, et al. J Am Chem Soc 1999;121:8728. [15] Kumar KR, Sivaram S. Macromol Chem Phys 2000; 201:1513. [16] Radhakrishnan K, Cramail H, Deffieux A, Francois P, Momtaz A. Macromol Rapid Commun 2002;23:829. [17] Radhakrishnan K, Cramail H, Deffieux A, Francois P, Momtaz A. Macromol Rapid Commun 2003;24:251. [18] Wang Q, Yang HX, Fan ZQ. Macromol Rapid Commun 2002;23:639. [19] Siergiejczyk L, Synoradzki L. J Organomet Chem 1986; 311:253. [20] Kissin YV. J Polym Sci Polym Chem 1995;33:227. [21] Kim JD, Soares JBP, Rempel GL. J Polym Sci Polym Chem 1999;37:331. [22] Estrada JMV, Hamielec AE. Polymer 1994;35:808. [23] Soga K, Uozumi T, Kishi N. Macromol Rapid Commun 1995;16:793. [24] Chien JCW, Wang BP. J Polym Sci Polym Chem 1990; 28:15. [25] Wang Q, Weng JH, Xu L, Fan ZQ, Feng LX. Polymer 1999;40:1863.
EUROPEAN POLYMER JOURNAL www.elsevier.com/locate/europolj
Polyethylene with bimodal molecular weight distribution synthesized by 2,6-bis(imino)pyridyl complexes of Fe(II) activated with various activators Qi Wang *, Lidong Li, Zhiqiang Fan Department of Polymer Science and Engineering, Zhejiang University, Zheda Road, Hangzhou 310027, PR China Received 19 October 2003; received in revised form 19 February 2004; accepted 3 March 2004 Available online 15 April 2004
Abstract Polyethylenes with bimodal molecular weight distribution (MWD) were synthesized by 2,6-bis(imino)pyridyl complexes of Fe(II) combined with different activators, which were prepared from alkylaluminium. It is found that the molecular weight (MW) and MWD was influenced by not only iron complexes but activators as well. The activator plays key important role in determination of the MW and MWD of final polymer and the MWD shape could be regulated by selection of various activators and polymerization conditions. The study on the variation of the MWD with the polymerization time and fitting of bimodal MWD with two Flory distributions suggests that bimodal MWD is caused by chain transfer reaction to activator or two active sites. 2004 Elsevier Ltd. All rights reserved. Keywords: Bimodal molecular weight distribution; Polyethylene; Iron complex; Activator
1. Introduction The molecular weight (MW) and molecular weight distribution (MWD) are important factors in determining the mechanical and rheological properties of polymers. It is believed that the polymer fraction of low MW improves the flow properties, while the fraction of high MW enhances melt strength and good mechanical properties. Therefore polymers with bimodal MWD may simultaneously show enhanced mechanical and rheological properties. Different techniques have been proposed to produce different polymer materials with bimodal MWD. All these techniques could be summarized as: stepwise polymerization in tandem reactor [1–4], polymerization using mixtures of different catalysts [5–10], or a sudden
*
Corresponding author. Tel./fax: +86-571-879-52400. E-mail address: [email protected] (Q. Wang).
variation of reaction conditions [11], especially the concentration of chain transfer agent. Several iron and cobalt catalysts based on tridentate diiminopyridine ligands have been reported to produce linear polyethylene with high MW [12,13]. With an activator such as methylaluminoxane (MAO), polyethylene with bimodal MWD is synthesized by iron catalyst at relatively high molar ratio of Al/Fe [12,14]. Unfortunately, large amount of low MW polymer is yielded under such reaction condition. The bimodal MWD with predominant low MW fraction is not an optimal MWD. Kumar and Sivaram [15], Radhakrishnan et al. [16,17] and our group [18] have reported the influence of activator for iron complex on the ethylene polymerization and bimodal MWD could be obtained under certain polymerization condition. In this article we report the polyethylene with bimodal MWD containing predominant high MW fraction produced by 2,6-bis(imino)pyridyl complexes of Fe(II) activated with different aluminium compounds.
0014-3057/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.03.003
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Q. Wang et al. / European Polymer Journal 40 (2004) 1881–1886
2. Experimental 2.1. General methods All manipulations were carried out under purified argon atmosphere by using standard Schlenk or high vacuum line techniques. The alkyl group and aluminium content of aluminium compounds was measured by GC and titration methods. Gel permeation chromatography (GPC) analyses were performed on PL-GPC 220 at 150 C with a polymer solution in 1,2,4-trichlorobenzene. DSC measurement was performed under a nitrogen atmosphere at a heating rate of 10 C/min on PE Pyris-1 DSC and the second run was reported. Gas chromatography (GC) was performed over Shimadzu GC8APF. 2.2. Materials Toluene was distilled under argon from sodium benzophenone ketyl. Methylene chloride was distilled under argon from P2 O5 . Et3 Al and i-Bu3 Al(Aldrich) were used as received. 4-fluorobenzeneboric acid was purchased from Acros. Ethylene (polymer grade) was purified by triisobutylaluminium in toluene. 2.3. Synthesis [(ArN ¼ C(Me))2 C5 H3 N]FeCl2 , Ar ¼ 2,6-dimethylphenyl (1) and Ar ¼ 2,6-diisopropylphenyl (2) were prepared according to reference [14]. Tetraethylaluminoxane (TEAO) was prepared as following procedure. 0.45 ml (0.025 mol) pure water was slowly added to 50 ml toluene solution of triethylaluminium (0.05 mol) at )78 C with vigorous stirring. The solution was allowed to warm up to room temperature and further stirred for 2 h. Tetraalkylaluminoxane (TAAO) was prepared as following procedure. 0.45 ml (0.025 mol) pure water was slowly added to 50 ml toluene solution of triethylaluminium (0.025 mol)/triisobutylaluminium (0.025 mol) at )78 C with vigorous stirring. The solution was allowed to warm up to room temperature and further stirred for 2 h. Et2 AlOB(p-FC6 H4 )OAlEt2 (BTEAO) was prepared as follows: 2.85 ml triethylaluminium (0.02 mol) was slowly added to a 20 ml toluene suspension of 1.40 g 4-fluorobenzeneboric acid (0.01 mol) at 0 C. The mixture was allowed to warm up to room temperature and stirred for 2 h. After the solid boric acid disappeared, the toluene solution of BTEAO was obtained. 2.4. Polymerization procedure Toluene and activator were injected into a 250 ml Schlenk flask with magnetic stirring at the desired tem-
peratures. The solution was then saturated with 1 atm of ethylene. Polymerization started with the addition of a dichloromethane solution of the iron complex into the flask. The polymerization was terminated by the addition of acidified ethanol. The polymer was washed with ethanol and dried under vacuum at 50 C.
3. Results and discussion Two known iron complexes shown in Scheme 1 were selected as precatalyst and three aluminium compounds as activators for ethylene polymerization. The synthetic methods of activators are illustrated in Scheme 2. By method A, involving direct reaction between trialkylaluminium and water at low temperature, two tetraalkylaluminoxanes were prepared. Tetraethylaluminoxane (TEAO) was prepared by reaction of triethylaluminium and water with molar ratio of 2:1. The Et/Al molar ratio of TEAO is 2.1. Tetraalkylaluminoxane (TAAO) was prepared by the reaction of triethylaluminium/triisobutylaluminium (1:1) and water with same molar ratio. The Et/iso-Bu ratio of TAAO measured by GC is 1:2.97. The synthesis of TEAO was previously studied by Siergiejczyk and Synoradzki [19]. There are multiple equilibriums among different components, including Et3 Al, TEAO and oligomeric aluminoxanes, existing in the final product of TEAO. The 1 H-NMR of TEAO confirms the presence of certain amount of Et3 Al in TEAO. By method B, BTEAO was synthesized through reaction between boric acid and triethylaluminium with molar ratio of 1:2.
Me
Me N
R N
R N
Fe Cl
1 R=Me
Cl
2 R=iso-Pr
R
R
Scheme 1. Structure of iron complex 1 and 2.
Method A: 2R3Al + H2O
R -78°C
R
Al-O-Al
R R
TEAO or TAAO
R=Et or iso-bu
Method B: R 2Et3Al+ HO-B-OH
R 0°C
Et2Al-O-B-O-AlEt 2 BTEAO
R= p-FC6 H4
Scheme 2. Synthesis of aluminium compounds as activator for iron complex.
Q. Wang et al. / European Polymer Journal 40 (2004) 1881–1886
Two iron complexes were combined with three different activators to polymerize ethylene. The results are summarized in Table 1. All catalyst groups show desirable polymerization activity and the MWD of polyethylene rely on both the catalyst group and Al/Fe molar ratio. When ethylene polymerization was conducted in the presence of complex 1, two activators gave bimodal MWD and one yielded unimodal. The bimodal MWD of polyethylene prepared by 1/TEAO and 1/TAAO are shown in Figs. 1 and 2. For the catalyst system 1/TEAO, the shape of MWD varies gradually with the molar ratio of Al/Fe. The low MW peak increases and the high MW peak reduces as the Al/Fe molar ratio varied from 500 to 2000, while the position of each peak hardly shifts. When BTEAO was employed as activator for complex 1, the resulting polyethylene’s MW was high and MWD was unimodal. The double melting points are observed for samples 1–3 and 5–6 because of large difference between the MWs of two fractions. Two activators yielded bimodal MWD and one yielded broad MWD when complex 2 was employed as precatalyst. The MWDs are shown in Figs. 3 and 4 respectively. When BTEAO, which was prepared from phenyl boric acid, is used as activator, the MWD of polyethylene is also influenced by molar ratio of Al/Fe. With the increment of Al/Fe molar ratio, the low MW peak reduces and the bimodal MWD is changed into unimodal one. The position of each peak is almost
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Fig. 1. The MWD of PE obtained by the 1/TEAO system as function of Al/Fe molar ratio (a) 500, (b) 1000 and (c) 2000.
identical. Such variation of MWD is different from MWD of polymer generated by normal aluminium activators used in this study. By different combinations of precatalysts and activators, various MWD shapes could be obtained. The MWD of resulting polymer changes from bimodal to unimodal when ethylene polymerization is conducted in the presence of same precatalyst activated with different aluminium compounds, e.g. 1/TEAO vs 1/BTEAO and 2/TEAO vs 2/TAAO. For the same activator, such
Table 1 Ethylene polymerization catalyzed by iron complexes and different aluminoxanea Activity (106 g PE/mol Fe h)
Mn b (104 g/mol)
Tm c (C)
2.62 1.51 1.30
17.6 12.6 13.7
79.7, 129.4 81.9, 127.9 93.2, 125.4
0.29 0.20 0.11
2.27 1.26 0.43
7.8 6.4 3.9
129.1 106.0, 126.8 88.7, 122.3
3.17 4.57 6.92
1.74 1.23 1.76
4.14 3.55 4.21
2.4 2.9 2.4
133.2 132.4 132.8
500 1000 2000
4.28 6.80 11.02
0.40 0.25 0.12
3.55 3.84 1.89
8.9 15.4 15.8
131.1 129.6 126.3
2/TAAO
500 1000 2000
2.91 3.74 6.66
1.04 0.48 0.49
4.42 3.51 3.69
4.3 7.3 7.5
132.4 131.4 130.9
2/BTEAO
500 1000 2000
0.14 0.62 2.26
1.14 1.97 3.79
9.47 12.43 12.54
8.3 6.3 3.3
133.3 134.4 134.4
Catalyst
Al/Fe
1 2 3
1/TEAO
500 1000 2000
2.22 3.13 5.87
0.15 0.12 0.09
4 5 6
1/TAAO
500 1000 2000
2.31 2.83 3.21
7 8 9
1/BTEAO
500 1000 2000
10 11 12
2/TEAO
13 14 15 16 17 18 a
Mw b (104 g/mol)
Mw =Mn b
Entry
General polymerization conditions: [Fe] ¼ 2 · 105 M, T ¼ 30 C, t ¼ 30 min, Pethylene ¼ 1 atm, in 50 ml toluene. Measured by means of GPC. c Measured by DSC. b
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Fig. 2. The MWD of PE obtained by the 1/TAAO system as function of Al/Fe molar ratio (a) 500, (b) 1000 and (c) 2000.
c a b
1
2
3
4 5 Log (MW)
6
7
Fig. 3. The MWD of PE obtained by the 2/TEAO system as function of Al/Fe molar ratio (a) 500, (b) 1000 and (c) 2000.
variation could be also found, e.g. 1/BTEAO vs 2/ BTEAO and 1/TAAO vs 2/TAAO. Therefore the MWD of resulting polymer is determined by not only precatalyst but activator as well, suggesting that the MW and MWD of polymer synthesized by iron complexes could be regulated by varying activators. Gibson’s and coworkers [14] have proposed that chain transfer to aluminium is mainly responsible for the MW of polymer prepared by iron complex activated by MAO. At high Al/Fe molar ratio, 2/MMAO yields polyethylene with bimodal MWD and low MW polymer fraction increases with the increment of Al/Fe ratio. Kumar and Sivaram [15], Radhakrishnan et al. [17] and our group [18] have reported the influence of alkylaluminium and mixture thereof as activator for 2 on the ethylene polymerization and bimodal MWD could be obtained under certain polymerization conditions.
Fig. 4. The MWD of PE obtained by the 2/BTEAO system as function of Al/Fe molar ratio (a) 500, (b) 1000 and (c) 2000.
Chain transfer reaction to alkylaluminium is believed as the key factor determining the MWD. From the comparison of TEAO with TAAO, it is concluded that Bu-Al group is efficient to improve the MW of final polymer, since it is less active chain transfer agent than Et-Al group. It is worth mention that the MW of polymer produced by activators modified with phenylboric acid is high with respect to other aluminium activators. The MW of low MW fraction of polymer prepared by 2/BTEAO is substantially increased and higher than those of corresponding fraction of polymer prepared by TEAO, TAAO and MAO [12,14]. For example, the peak MWs of sample 17 are 1.52 · 104 (one order of magnitude higher than those of samples 1–3) and 1.36 · 105 g/mol respectively. Furthermore, the MW difference between low and high MW fractions of bimodal MWD generated by boric acid based activators is smaller than those by other aluminium activators. The result is explained by the strong Lewis acidity of boron site, which retards the chain transfer reaction to activator. Polymer with bimodal MWD is usually generated by the presence of two active sites, one site generates low MW fraction and the other produces high MW fraction. Table 2 gives the detailed MW data of two peaks of bimodal MWD of polyethylene prepared by 1/TEAO. When the molar Al/Fe ratio varies from 500 to 2000, the number average molecular weights ðMn Þ and the polydispersity indexes (PDI) of low MW or high MW peaks are very close and only relative weight fraction of each peak changes. The PDIs of low MW peaks are around 1.2 and those of high MW peaks are close to 2.0. The polyolefin prepared by homogeneous catalyst, such as metallocene, normally has a PDI of 2.0. The quite narrow MWD of low MW fraction might be generated by three ways. The first is that the polymer with MW lower
Q. Wang et al. / European Polymer Journal 40 (2004) 1881–1886
1885
Table 2 Molecular weight distribution of polyethylene prepared by iron 1/TEAO Catalyst
Al/Fe
Peak 1 Mn
1/TEAO
500 1000 2000
a
0.04 0.06 0.07
Peak 2 Mw =Mn 1.22 1.20 1.22
b
c
Total
wt.%
Mn
Mw =Mn
wt.%
Mn
Mw =Mn
76 40 65
1.9 2.2 2.9
1.85 1.66 1.78
24 60 35
0.15 0.12 0.09
17.6 12.6 13.7
a
Number average molecular weight. Polydispersity index. c Weight percentage of the fraction. b
than 100 could not be precipitated by ethanol, therefore we get narrow MWD. The second is that the narrow MWD is caused by the fast chain transfer reaction. The third is that the low MW fraction is produced by distinctive active site other than the active site generating high MW fraction. If the first possibility is excluded, the last two possibilities could be discriminated by studying the variation of the MWD of polymer with polymerization time. If the bimodal MWD is generated by two active sites, the amount of polymer with low and high MW will both increase with the polymerization time. If it is caused by chain transfer reaction, polymer with low MW will be formed earlier than that of high MW. The experiment result is that the low MW fraction is produced at early stage of polymerization and the high MW fraction formed latter. So it is reasonable to make a conclusion that the low MW fraction is most probably generated by chain transfer reaction. It is suggested that the activator functions as chain transfer agent at the beginning of polymerization and polymer with low MW is produced. As the activator is consumed by the chain transfer reaction during the polymerization, polymer with high MW is generated. Such result has been reported by Gibson’s and coworkers as well [14]. The variation of MWD of polymer produced by 2/ BTEAO could not be well interpreted by chain transfer reaction, since the MWD becomes narrow when the Al/ Fe molar ratio is high. The Flory distribution has been applied for deconvolution of broad or bimodal MWD of polyethylene prepared by Ziegler–Natta [20] and metallocene [21] catalysts. As shown in Fig. 5, the MWD of polymer produced by 2/BTEAO could be fitted well with superposition of two Flory distributions, each having a PDI of 2.0. It is commonly accepted that the presence of one Flory distribution corresponds to one active site. So it is reasonable to say that the bimodal MWD of polyethylene produced by 2/BTEAO is caused by two different active sites. Multiple active sites model has been proposed for homogenous metallocene catalyst for ethylene polymerization [22,23]. For the mechanism of formation of multiple active centers, Chien and Wang [24] proposed that the metallocene can be coordinated to zero, one, or two MAOs, resulting in active centers of different kp. We also proposed that there must be a
(a)
2
3
4 5 Log (MW)
6
(b)
2
3
4
5
6
Log( MW)
Fig. 5. The MWD of PE obtained by the 2/BTEAO system fitted with Flory distributions (a) Al/Fe ¼ 500 and (b) Al/ Fe ¼ 1000 (circle: experimental data, dash line: Flory distribution, solid line: sum of Flory distributions).
maximum number of coordinated aluminoxane owing to steric effect and a minimum number owing to the stability of the center [25]. In the case of iron complex, it might also interact with different amount of activator molecules, resulting in different type of active sites. At low Al/Fe molar ratio, some iron complexes are coordinated with more activator molecules than others, which leads to two kinds of active sites. When the amount of
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activator is increased and in excess, the latter active site interacts with more activator molecules and is changed to former one. The overall result is that the active sites become uniform and the MWD of product is varied from bimodal to unimodal as the Al/Fe molar ratio increases.
4. Conclusion We report preparation of polyethylene with bimodal MWD involving single iron complex activated with different activators. The MWD of polymer could be regulated by selection of various activators and polymerization conditions in single reactor, which offers the most economic way to produce bimodal MWD polyolefins. The bimodal MWD generated by 1/TEAO is suggested to be caused by chain transfer to activator and the bimodal MWD by 2/BTEAO is proved to be generated by two active sites.
Acknowledgements Financial support from ‘‘State Key Fundamental Research Project of China (G1999064801)’’ is appreciated.
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