- Email: [email protected]
Preparation of polyethylene with controlled bimodal molecular weight distribution using zirconium complexes
Journal of Industrial and Engineering Chemistry 18 (2012) 429–432
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Preparation of polyethylene with controlled bimodal molecular weight distribution using zirconium complexes Junseong Lee a, Youngjo Kim b,* a b
Department of Chemistry, School of Basic Science, Chonnam National University, Gwangju 500-757, Republic of Korea Department of Chemistry, Chungbuk National University, Cheongju, Chungbuk 361-763, Republic of Korea
A R T I C L E I N F O
Article history: Received 23 March 2011 Accepted 2 May 2011 Available online 10 November 2011 Keywords: Zirconium Phenoxybenzotriazole Bidentate ligands Polyethylene Bimodal molecular weight distribution Broad molecular weight distribution
A B S T R A C T
Four zirconium complexes containing fully deprotonated 2-(2H-benzo[d][1,2,3]triazol-2-yl)-4,6-di-tertpentylphenol were used as catalysts for the polymerization of ethylene. In the presence of methylalumoxane (MAO) as a cocatalyst, the precursors were highly active for polyethylene with bimodal or multimodal molecular weight distribution. ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of Ziegler–Natta catalysts in 1960s and metallocene catalysts in 1980s, many catalytic systems for ethylene polymerization have been reported in the literature [1]. Unlike heterogeneous Ziegler–Natta systems, polyethylene (PE) produced by homogeneous metallocene catalysts has been hampered by significant processing problems, owing to its narrow molecular weight distribution (MWD). Thus, the enhancement of the processibility of PE has been an important research subject in recent years [2,3], with particular focus on modulating the MWD [4–6]. The adjustment of MWD for PE could be tackled by using a series of reactors under different polymerization conditions, employing mixed catalytic systems in one-pot and designing precatalysts that produce multiactive species during the polymerization process [7–10]. All researches for PE with broad MWD have focused on dinuclear complexes [11,12], mixed usage of more than two mononuclear complexes [13–15], the treatment of hydrogen during polymerization [16–18], and silica supported metallocene catalysts [19]. Surprisingly, the homogeneous half-sandwich zirconium systems remain undeveloped in this filed though some half-sandwich titanium complexes producing PE with bimodal or broad MWD have been reported in the literature [9]. Recently, we have reported on the synthesis, characterization, and their X-ray structures of three half-sandwich zirconium
* Corresponding author. Tel.: +82 43 261 3395; fax: +82 43 267 2279. E-mail address: [email protected] (Y. Kim).
complexes 1–3 and a non-organometallic complex 4 with fully deprotonated 2-(2H-benzo[d][1,2,3]triazol-2-yl)-4,6-di-tert-pentylphenol (Lig) shown in Fig. 1 [20]. According to their preliminary ethylene polymerization at the polymerization temperature of 90 8C [20], complexes 1–3 in the presence of MAO as a cocatalyst produced PE with better activity and higher molecular weight than CpZrCl3 did. As expected, non-organometallic complex 4 showed somewhat lower catalytic activity than CpZrCl3; however, 4 gave PE with high molecular weight comparable to 1–3. Like other homogeneous zirconium catalysts, 1–3/MAO systems gave PE with narrow MWD. Interestingly, complex 4 gave polyethylene with very broad MWD even at the high polymerization temperature of 90 8C [20]. In addition, we have observed that some titanium complexes could generate PE with bimodal MWD as the polymerization temperature gradually went down [9]. In order to control MWD of PE produced by 1–4/MAO systems we applied the stepwise decrease of the polymerization temperature. In this manuscript, we report the synthetic behaviors of PE with bimodal or multimodal MWD using 1–4/MAO systems under the condition of various polymerization temperatures. 2. Experimental 2.1. General procedure All manipulations of air- and moisture-sensitive materials were carried out under a dinitrogen atmosphere using the standard Schlenk-type glassware on a dual manifold Schlenk line and glovebox [21]. Complexes 1–4 were synthesized by the literature
1226-086X/$ – see front matter ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2011.11.105
J. Lee, Y. Kim / Journal of Industrial and Engineering Chemistry 18 (2012) 429–432
430
N
N
N
Zr O
Cl
Cl Cl
N
O
N
N
Cl
Zr O
N N
N
N
N
Cl
2
Cl Cl Cl O
Zr
Cl
1
Zr
O
N Zr
N Cl
N
O
Cl
N
N N
Cl
Zr O
Cl
N
4
3 Fig. 1. Schematic diagram of complexes 1–4.
procedure [20]. All other chemicals were purchased from Aldrich and were used as supplied unless otherwise indicated. MAO with 10% in heptane solution was purchased from Witco. Molecular weights and MWD of PE polymers were determined at 140 8C in 1,2,4-trichlorobenzene by PL 220 + 220R GPC calibrated with standard polystyrenes. 2.2. Polymerization procedure Polymerizations were carried out in 250 mL Schlenk flask with magnetic stirring. Toluene, the polymerization solvent, was distilled from sodium/benzophenone ketyl under dinitrogen atmosphere just before use and stored in a Schlenk tube containing activated 3 A˚ molecular sieves under dinitrogen atmosphere [22]. MAO was used as a solid form, which is obtained by the removal of all volatiles under vacuo from solution MAO purchased from Witco. Polymerizations were carried out as following: solid MAO (0.58 g, 10 mmol), toluene (50 mL), and the zirconium compound (10 mmol) were injected into a 250 mL Schlenk flask with magnetic stirring in that order at the desired temperatures of 30 8C, 50 8C, 70 8C, and 90 8C. And then, 1 bar pressure of ethylene gas was introduced into the reaction vessel. After 10 min was reached, the reaction was terminated by the addition of 50 mL of 10 wt% HCl in ethanol, affording polyethylene as a precipitate. The polymer obtained was filtered and washed with ethanol several times and dried in vacuum at 60 8C for 12 h. 3. Results and discussion Catalyst precursors 1–4 are examined as catalysts for ethylene polymerization in the presence of MAO as a cocatalyst. The polymerization results are summarized in Table 1 in terms of the activity of the catalyst, Mw, and Mw/Mn. To probe the nature of the
polymerization reaction, we carried out polymerizations at various temperatures such as 30, 50, 70, and 90 8C under the condition of the fixed [Al]/[Zr] content of 1000. The polymerization temperature affects considerably the activities of the catalysts, the molecular weights, and molecular weight distribution of PE polymers. The polymerization data reveal the activities for 1–4/ MAO systems showed the different patterns. In the case of complexes 1–3, the catalytic activities gradually increased as the polymerization temperature up to 90 8C; however, non-Cp type
Table 1 Summary of polymerization of ethylene using 1–4. Entrya
Cat.
Tp
Act.b
Mw (10
3 c
1 2 3 4
1
30 50 70 90
240 260 290 370
439 367 304 185
58 68 17 2.7
5 6 7 8
2
30 50 70 90
120 290 460 510
763 606 472 229
88 140 19 2.09
9 10 11 12
3
30 50 70 90
68 160 390 650
536 666 352 234
85 73 24 2.02
13 14 15 16
4
30 50 70 90
520 240 270 44
36 181 246 54
)
a Polymerization conditions: toluene solvent = 50 mL; Tp = 10 min; [Zr] = 0.010 mmol, [s-MAO] = 0.58 g (10.0 mmol). b Activity: kg of PE/(mol of Zr h). c Determined by GPC.
Mw/Mn c
34 120 61 31 [Al]/[Zr] = 1000;
J. Lee, Y. Kim / Journal of Industrial and Engineering Chemistry 18 (2012) 429–432
complex 4 showed the opposite tendency with the increase of polymerization temperature. The coupled effect of the presence of Cp ligand and steric bulkiness in bidentate ligand Lig in 1–3 could generate much more thermally stable active species than 4 containing only Lig ligand. Complexes 1–4 yield PE with Mw in the range of 185,000– 439,000, 229,000–763,000, 234,000–666,000, and 36,000– 246,000, respectively. As expected, the rapid decrease of molecular weight of PE obtained by 1–4/MAO was observed, along with the increase of polymerization temperature. The MWD and polydispersities produced by 1–4/MAO are quite distinctive because of their homogenous, mono/dinuclear, and half/non-metallocene properties. In general, PE polymers produced by single-site catalysts show a unimodal MWD and polydispersities of Mw/ Mn = 2–3, whereas PE made by heterogeneous systems have broad MWD with high PDIs of 5–10 [1,23]. In the case of the catalytic system 1/MAO shown in Fig. 2(a), the asymmetric unimodal MWD with small tail in the low molecular weight side is observed at 70 8C and 90 8C. Lowering the temperature to 50 8C resulted in the appearance of bimodal MWD. The maximum was shifted to the high molecular weight side and an additional unimodal distribution in the low molecular weight area appeared. At 30 8C, the maximum at high molecular weight side was shifted to the higher molecular weight part and the peak of low molecular distribution is broader than that of high molecular distribution. For 2/MAO system, the similar trend has been shown with 1/MAO (see Fig. 2(b)). At 90 8C, the MWD with long tail in the low molecular weight side is observed. As long as
431
the temperature was lowered from 90 8C to 70 8C, 50 8C, and finally 30 8C, the distinct bimodal MWD appeared. At the same time, the peak intensity of low molecular weight side gradually increased; however, the amount of high molecular side was unchanged. The maximum of both low and high molecular weight side was shifted to the higher molecular weight part. In the case of 3/MAO system, complicated multimodal MWD of PE polymers produced by 3/MAO was observed in Fig. 2(c). The symmetric unimodal MWD is detected at 90 8C. As the polymerization temperature gets lowered, the tail in the low molecular weight area appeared and gradually grew bigger and bigger. At all temperature ranges, high molecular weight side remains unimodal peak but low part becomes multimodal MWD. Unlike 1–3/MAO system, 4/MAO system is distinct in the aspect that 4/MAO system showed the tails with higher molecular weight side (see Fig. 2(d)). Asymmetric unimodal MWD with small tail in the higher molecular weight side is observed at 30 8C. At 50 8C, the intensity of high molecular weight side increased and the peak separation occurred, resulting in multimodal MWD. The peak of high molecular weight side showed the maximum intensity at 70 8C, and then the peak intensity decreased rapidly. Except for 70 8C, the peak intensity of low molecular weight side is constant. In order to verify the reason why 1–4/MAO gave PE with bimodal or multimodal MWD, we carried out the ethylene polymerization experiments using precatalyst 1 at various [Al]/ [Zr] contents of 250, 500, 1000, and 2000 under the polymerization temperature of 30 8C. Like the temperature dependence of MWD, [Al]/[Zr] ratio dependence of MWD was also observed. As [Al]/[Zr]
Fig. 2. GPC curves of PE obtained by 1–4/MAO systems at various temperatures (black line, 30 8C; red line, 50 8C; green line, 70 8C; blue line, 90 8C). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
432
J. Lee, Y. Kim / Journal of Industrial and Engineering Chemistry 18 (2012) 429–432
goes up, the number average molecular weight (Mn) of PE obtained increased rapidly from 350,000 ([Al]/[Zr] = 250) to the maximum value of 460,000 ([Al]/[Zr] = 500) and then went down to 439,000 ([Al]/[Zr] = 1000) and finally to 290,000 ([Al]/[Zr] = 2000). Also, MWD values change from 7.6 ([Al]/[Zr] = 250) to 68 ([Al]/ [Zr] = 500) to 58 ([Al]/[Zr] = 1000) and to 56 ([Al]/[Zr] = 2000). Interestingly, unimodal MWD pattern of PE was observed at low [Al]/[Zr] ratio of 250. This observation means the formation of more than two active species via acid–base interaction between MAO and N atoms of precursor 1. Among these active species, much thermally stable specie generated the portion of the high molecular weight side while unstable specie gave the portion of the low molecular weight side, leading to the generation of PE with bimodal molecular weight distribution. More than two different species are likely to show different thermal stabilities and thus to give the temperature dependence of MWD illustrated in Fig. 2. In addition, this polymerization data may be open to another possibility of chain transfer reaction from MAO to polymer chains. Overall, the chelating bidentate ligand with the unique steric bulkiness seems to play an important role in generating PE with a bimodal or multimodal molecular weight distribution. 4. Conclusions Catalytic behaviors for ethylene polymerization using monoand dinuclear zirconium catalytic systems 1–4 were investigated. Although 1–4/MAO are homogeneous half-metallocenes, they were highly active for the polymerization of ethylene with bimodal or multimodal MWD.
Acknowledgment This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MEST) (No. 2010-0007092). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
V.C. Gibson, S.K. Spitzmesser, Chem. Rev. 103 (2002) 283. H.L. Kim, D. Rana, H. Kwag, S. Choe, J. Ind. Eng. Chem. 6 (2000) 115. J.-W. Lim, Y. Choi, H.-S. Yoon, Y.-K. Park, J.-H. Yim, J. Ind. Eng. Chem. 16 (2010) 51. J.-W. Kim, K.-D. Suh, J. Ind. Eng. Chem. 14 (2008) 1. Y. Tohi, H. Makio, S. Matsui, M. Onda, T. Fujita, Macromolecules 36 (2003) 523. L. D’Agnillo, J.B.P. Soares, A. Penlidis, Macromol. Chem. Phys. 199 (1998) 955. S. Murtuza, O.L. Casagrande, R.F. Jordan, Organometallics 21 (2002) 1882. C.T. Owen, P.D. Bolton, A.R. Cowley, P. Mountford, Organometallics 26 (2007) 83. Y. Kim, Y. Han, M.H. Lee, S.W. Yoon, K.H. Choi, B.G. Song, Y. Do, Macromol. Rapid Commun. 22 (2001) 573. C. Muller, D. Lilge, M.O. Kristen, R. Jutzi, Angew. Chem. Int. Ed. 39 (2000) 789. J.Q. Sun, Y.J. Nie, H. Ren, X.H. Xiao, Z.Z. Cheng, H. Schumann, Eur. Polym. J. 44 (2008) 2980. X. Li, X.T. Zhao, B.C. Zhu, F. Lin, J.Q. Sun, Catal. Commun. 8 (2007) 2025. L. D’Agnillo, J.B.P. Soares, A. Penlidis, Polym. Int. 47 (1998) 351. L. D’Agnillo, J.B.P. Soares, A. Penlidis, J. Polym. Sci. A: Polym. Chem. 36 (1998) 831. J.B.P. Soares, J.D. Kim, J. Polym. Sci. A: Polym. Chem. 38 (2000) 1408. J.D. Kim, J.B.P. Soares, G.L. Rempel, Macromol. Rapid Commun. 19 (1998) 197. R. Blom, I.M. Dahl, Macromol. Chem. Phys. 202 (2001) 719. K.J. Chu, J.B.P. Soares, A. Penlidis, Macromol. Chem. Phys. 201 (2000) 552. J.S. Chung, H.S. Cho, Y.G. Ko, W.Y. Lee, J. Mol. Catal. A: Chem. 144 (1999) 61. K.M. Lee, S. Yoon, J. Lee, J. Kim, Y. Do, T.-S. You, Y. Kim, Polyhedron 30 (2011) 809. D.F. Shriver, The Manipulation of Air-Sensitive Compounds, McGraw-Hill, New York, 1969. W.L.F. Armarego, C.L.L. Chai, Purification of Laboratory Chemicals, 6th ed., Elsevier, New York, 2009. G.J.P. Britovsek, V.C. Gibson, D.F. Wass, Angew. Chem. Int. Ed. 38 (1999) 428.
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Preparation of polyethylene with controlled bimodal molecular weight distribution using zirconium complexes Junseong Lee a, Youngjo Kim b,* a b
Department of Chemistry, School of Basic Science, Chonnam National University, Gwangju 500-757, Republic of Korea Department of Chemistry, Chungbuk National University, Cheongju, Chungbuk 361-763, Republic of Korea
A R T I C L E I N F O
Article history: Received 23 March 2011 Accepted 2 May 2011 Available online 10 November 2011 Keywords: Zirconium Phenoxybenzotriazole Bidentate ligands Polyethylene Bimodal molecular weight distribution Broad molecular weight distribution
A B S T R A C T
Four zirconium complexes containing fully deprotonated 2-(2H-benzo[d][1,2,3]triazol-2-yl)-4,6-di-tertpentylphenol were used as catalysts for the polymerization of ethylene. In the presence of methylalumoxane (MAO) as a cocatalyst, the precursors were highly active for polyethylene with bimodal or multimodal molecular weight distribution. ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of Ziegler–Natta catalysts in 1960s and metallocene catalysts in 1980s, many catalytic systems for ethylene polymerization have been reported in the literature [1]. Unlike heterogeneous Ziegler–Natta systems, polyethylene (PE) produced by homogeneous metallocene catalysts has been hampered by significant processing problems, owing to its narrow molecular weight distribution (MWD). Thus, the enhancement of the processibility of PE has been an important research subject in recent years [2,3], with particular focus on modulating the MWD [4–6]. The adjustment of MWD for PE could be tackled by using a series of reactors under different polymerization conditions, employing mixed catalytic systems in one-pot and designing precatalysts that produce multiactive species during the polymerization process [7–10]. All researches for PE with broad MWD have focused on dinuclear complexes [11,12], mixed usage of more than two mononuclear complexes [13–15], the treatment of hydrogen during polymerization [16–18], and silica supported metallocene catalysts [19]. Surprisingly, the homogeneous half-sandwich zirconium systems remain undeveloped in this filed though some half-sandwich titanium complexes producing PE with bimodal or broad MWD have been reported in the literature [9]. Recently, we have reported on the synthesis, characterization, and their X-ray structures of three half-sandwich zirconium
* Corresponding author. Tel.: +82 43 261 3395; fax: +82 43 267 2279. E-mail address: [email protected] (Y. Kim).
complexes 1–3 and a non-organometallic complex 4 with fully deprotonated 2-(2H-benzo[d][1,2,3]triazol-2-yl)-4,6-di-tert-pentylphenol (Lig) shown in Fig. 1 [20]. According to their preliminary ethylene polymerization at the polymerization temperature of 90 8C [20], complexes 1–3 in the presence of MAO as a cocatalyst produced PE with better activity and higher molecular weight than CpZrCl3 did. As expected, non-organometallic complex 4 showed somewhat lower catalytic activity than CpZrCl3; however, 4 gave PE with high molecular weight comparable to 1–3. Like other homogeneous zirconium catalysts, 1–3/MAO systems gave PE with narrow MWD. Interestingly, complex 4 gave polyethylene with very broad MWD even at the high polymerization temperature of 90 8C [20]. In addition, we have observed that some titanium complexes could generate PE with bimodal MWD as the polymerization temperature gradually went down [9]. In order to control MWD of PE produced by 1–4/MAO systems we applied the stepwise decrease of the polymerization temperature. In this manuscript, we report the synthetic behaviors of PE with bimodal or multimodal MWD using 1–4/MAO systems under the condition of various polymerization temperatures. 2. Experimental 2.1. General procedure All manipulations of air- and moisture-sensitive materials were carried out under a dinitrogen atmosphere using the standard Schlenk-type glassware on a dual manifold Schlenk line and glovebox [21]. Complexes 1–4 were synthesized by the literature
1226-086X/$ – see front matter ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2011.11.105
J. Lee, Y. Kim / Journal of Industrial and Engineering Chemistry 18 (2012) 429–432
430
N
N
N
Zr O
Cl
Cl Cl
N
O
N
N
Cl
Zr O
N N
N
N
N
Cl
2
Cl Cl Cl O
Zr
Cl
1
Zr
O
N Zr
N Cl
N
O
Cl
N
N N
Cl
Zr O
Cl
N
4
3 Fig. 1. Schematic diagram of complexes 1–4.
procedure [20]. All other chemicals were purchased from Aldrich and were used as supplied unless otherwise indicated. MAO with 10% in heptane solution was purchased from Witco. Molecular weights and MWD of PE polymers were determined at 140 8C in 1,2,4-trichlorobenzene by PL 220 + 220R GPC calibrated with standard polystyrenes. 2.2. Polymerization procedure Polymerizations were carried out in 250 mL Schlenk flask with magnetic stirring. Toluene, the polymerization solvent, was distilled from sodium/benzophenone ketyl under dinitrogen atmosphere just before use and stored in a Schlenk tube containing activated 3 A˚ molecular sieves under dinitrogen atmosphere [22]. MAO was used as a solid form, which is obtained by the removal of all volatiles under vacuo from solution MAO purchased from Witco. Polymerizations were carried out as following: solid MAO (0.58 g, 10 mmol), toluene (50 mL), and the zirconium compound (10 mmol) were injected into a 250 mL Schlenk flask with magnetic stirring in that order at the desired temperatures of 30 8C, 50 8C, 70 8C, and 90 8C. And then, 1 bar pressure of ethylene gas was introduced into the reaction vessel. After 10 min was reached, the reaction was terminated by the addition of 50 mL of 10 wt% HCl in ethanol, affording polyethylene as a precipitate. The polymer obtained was filtered and washed with ethanol several times and dried in vacuum at 60 8C for 12 h. 3. Results and discussion Catalyst precursors 1–4 are examined as catalysts for ethylene polymerization in the presence of MAO as a cocatalyst. The polymerization results are summarized in Table 1 in terms of the activity of the catalyst, Mw, and Mw/Mn. To probe the nature of the
polymerization reaction, we carried out polymerizations at various temperatures such as 30, 50, 70, and 90 8C under the condition of the fixed [Al]/[Zr] content of 1000. The polymerization temperature affects considerably the activities of the catalysts, the molecular weights, and molecular weight distribution of PE polymers. The polymerization data reveal the activities for 1–4/ MAO systems showed the different patterns. In the case of complexes 1–3, the catalytic activities gradually increased as the polymerization temperature up to 90 8C; however, non-Cp type
Table 1 Summary of polymerization of ethylene using 1–4. Entrya
Cat.
Tp
Act.b
Mw (10
3 c
1 2 3 4
1
30 50 70 90
240 260 290 370
439 367 304 185
58 68 17 2.7
5 6 7 8
2
30 50 70 90
120 290 460 510
763 606 472 229
88 140 19 2.09
9 10 11 12
3
30 50 70 90
68 160 390 650
536 666 352 234
85 73 24 2.02
13 14 15 16
4
30 50 70 90
520 240 270 44
36 181 246 54
)
a Polymerization conditions: toluene solvent = 50 mL; Tp = 10 min; [Zr] = 0.010 mmol, [s-MAO] = 0.58 g (10.0 mmol). b Activity: kg of PE/(mol of Zr h). c Determined by GPC.
Mw/Mn c
34 120 61 31 [Al]/[Zr] = 1000;
J. Lee, Y. Kim / Journal of Industrial and Engineering Chemistry 18 (2012) 429–432
complex 4 showed the opposite tendency with the increase of polymerization temperature. The coupled effect of the presence of Cp ligand and steric bulkiness in bidentate ligand Lig in 1–3 could generate much more thermally stable active species than 4 containing only Lig ligand. Complexes 1–4 yield PE with Mw in the range of 185,000– 439,000, 229,000–763,000, 234,000–666,000, and 36,000– 246,000, respectively. As expected, the rapid decrease of molecular weight of PE obtained by 1–4/MAO was observed, along with the increase of polymerization temperature. The MWD and polydispersities produced by 1–4/MAO are quite distinctive because of their homogenous, mono/dinuclear, and half/non-metallocene properties. In general, PE polymers produced by single-site catalysts show a unimodal MWD and polydispersities of Mw/ Mn = 2–3, whereas PE made by heterogeneous systems have broad MWD with high PDIs of 5–10 [1,23]. In the case of the catalytic system 1/MAO shown in Fig. 2(a), the asymmetric unimodal MWD with small tail in the low molecular weight side is observed at 70 8C and 90 8C. Lowering the temperature to 50 8C resulted in the appearance of bimodal MWD. The maximum was shifted to the high molecular weight side and an additional unimodal distribution in the low molecular weight area appeared. At 30 8C, the maximum at high molecular weight side was shifted to the higher molecular weight part and the peak of low molecular distribution is broader than that of high molecular distribution. For 2/MAO system, the similar trend has been shown with 1/MAO (see Fig. 2(b)). At 90 8C, the MWD with long tail in the low molecular weight side is observed. As long as
431
the temperature was lowered from 90 8C to 70 8C, 50 8C, and finally 30 8C, the distinct bimodal MWD appeared. At the same time, the peak intensity of low molecular weight side gradually increased; however, the amount of high molecular side was unchanged. The maximum of both low and high molecular weight side was shifted to the higher molecular weight part. In the case of 3/MAO system, complicated multimodal MWD of PE polymers produced by 3/MAO was observed in Fig. 2(c). The symmetric unimodal MWD is detected at 90 8C. As the polymerization temperature gets lowered, the tail in the low molecular weight area appeared and gradually grew bigger and bigger. At all temperature ranges, high molecular weight side remains unimodal peak but low part becomes multimodal MWD. Unlike 1–3/MAO system, 4/MAO system is distinct in the aspect that 4/MAO system showed the tails with higher molecular weight side (see Fig. 2(d)). Asymmetric unimodal MWD with small tail in the higher molecular weight side is observed at 30 8C. At 50 8C, the intensity of high molecular weight side increased and the peak separation occurred, resulting in multimodal MWD. The peak of high molecular weight side showed the maximum intensity at 70 8C, and then the peak intensity decreased rapidly. Except for 70 8C, the peak intensity of low molecular weight side is constant. In order to verify the reason why 1–4/MAO gave PE with bimodal or multimodal MWD, we carried out the ethylene polymerization experiments using precatalyst 1 at various [Al]/ [Zr] contents of 250, 500, 1000, and 2000 under the polymerization temperature of 30 8C. Like the temperature dependence of MWD, [Al]/[Zr] ratio dependence of MWD was also observed. As [Al]/[Zr]
Fig. 2. GPC curves of PE obtained by 1–4/MAO systems at various temperatures (black line, 30 8C; red line, 50 8C; green line, 70 8C; blue line, 90 8C). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
432
J. Lee, Y. Kim / Journal of Industrial and Engineering Chemistry 18 (2012) 429–432
goes up, the number average molecular weight (Mn) of PE obtained increased rapidly from 350,000 ([Al]/[Zr] = 250) to the maximum value of 460,000 ([Al]/[Zr] = 500) and then went down to 439,000 ([Al]/[Zr] = 1000) and finally to 290,000 ([Al]/[Zr] = 2000). Also, MWD values change from 7.6 ([Al]/[Zr] = 250) to 68 ([Al]/ [Zr] = 500) to 58 ([Al]/[Zr] = 1000) and to 56 ([Al]/[Zr] = 2000). Interestingly, unimodal MWD pattern of PE was observed at low [Al]/[Zr] ratio of 250. This observation means the formation of more than two active species via acid–base interaction between MAO and N atoms of precursor 1. Among these active species, much thermally stable specie generated the portion of the high molecular weight side while unstable specie gave the portion of the low molecular weight side, leading to the generation of PE with bimodal molecular weight distribution. More than two different species are likely to show different thermal stabilities and thus to give the temperature dependence of MWD illustrated in Fig. 2. In addition, this polymerization data may be open to another possibility of chain transfer reaction from MAO to polymer chains. Overall, the chelating bidentate ligand with the unique steric bulkiness seems to play an important role in generating PE with a bimodal or multimodal molecular weight distribution. 4. Conclusions Catalytic behaviors for ethylene polymerization using monoand dinuclear zirconium catalytic systems 1–4 were investigated. Although 1–4/MAO are homogeneous half-metallocenes, they were highly active for the polymerization of ethylene with bimodal or multimodal MWD.
Acknowledgment This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MEST) (No. 2010-0007092). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
V.C. Gibson, S.K. Spitzmesser, Chem. Rev. 103 (2002) 283. H.L. Kim, D. Rana, H. Kwag, S. Choe, J. Ind. Eng. Chem. 6 (2000) 115. J.-W. Lim, Y. Choi, H.-S. Yoon, Y.-K. Park, J.-H. Yim, J. Ind. Eng. Chem. 16 (2010) 51. J.-W. Kim, K.-D. Suh, J. Ind. Eng. Chem. 14 (2008) 1. Y. Tohi, H. Makio, S. Matsui, M. Onda, T. Fujita, Macromolecules 36 (2003) 523. L. D’Agnillo, J.B.P. Soares, A. Penlidis, Macromol. Chem. Phys. 199 (1998) 955. S. Murtuza, O.L. Casagrande, R.F. Jordan, Organometallics 21 (2002) 1882. C.T. Owen, P.D. Bolton, A.R. Cowley, P. Mountford, Organometallics 26 (2007) 83. Y. Kim, Y. Han, M.H. Lee, S.W. Yoon, K.H. Choi, B.G. Song, Y. Do, Macromol. Rapid Commun. 22 (2001) 573. C. Muller, D. Lilge, M.O. Kristen, R. Jutzi, Angew. Chem. Int. Ed. 39 (2000) 789. J.Q. Sun, Y.J. Nie, H. Ren, X.H. Xiao, Z.Z. Cheng, H. Schumann, Eur. Polym. J. 44 (2008) 2980. X. Li, X.T. Zhao, B.C. Zhu, F. Lin, J.Q. Sun, Catal. Commun. 8 (2007) 2025. L. D’Agnillo, J.B.P. Soares, A. Penlidis, Polym. Int. 47 (1998) 351. L. D’Agnillo, J.B.P. Soares, A. Penlidis, J. Polym. Sci. A: Polym. Chem. 36 (1998) 831. J.B.P. Soares, J.D. Kim, J. Polym. Sci. A: Polym. Chem. 38 (2000) 1408. J.D. Kim, J.B.P. Soares, G.L. Rempel, Macromol. Rapid Commun. 19 (1998) 197. R. Blom, I.M. Dahl, Macromol. Chem. Phys. 202 (2001) 719. K.J. Chu, J.B.P. Soares, A. Penlidis, Macromol. Chem. Phys. 201 (2000) 552. J.S. Chung, H.S. Cho, Y.G. Ko, W.Y. Lee, J. Mol. Catal. A: Chem. 144 (1999) 61. K.M. Lee, S. Yoon, J. Lee, J. Kim, Y. Do, T.-S. You, Y. Kim, Polyhedron 30 (2011) 809. D.F. Shriver, The Manipulation of Air-Sensitive Compounds, McGraw-Hill, New York, 1969. W.L.F. Armarego, C.L.L. Chai, Purification of Laboratory Chemicals, 6th ed., Elsevier, New York, 2009. G.J.P. Britovsek, V.C. Gibson, D.F. Wass, Angew. Chem. Int. Ed. 38 (1999) 428.