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Effects of alkylaluminum as cocatalyst on the active center distribution of 1-hexene polymerization with MgCl2-supported Ziegler–Natta catalysts
Catalysis Communications 62 (2015) 104–106
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Effects of alkylaluminum as cocatalyst on the active center distribution of 1-hexene polymerization with MgCl2-supported Ziegler–Natta catalysts Hongrui Yang a, Letian Zhang b, Dandan Zang a, Zhisheng Fu a,⁎, Zhiqiang Fan a a b
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China Shanghai Research Institute of Chemical Industry, Shanghai 200062, China
a r t i c l e
i n f o
Article history: Received 17 October 2014 Received in revised form 20 December 2014 Accepted 23 January 2015 Available online 24 January 2015 Keywords: Ziegler–Natta catalyst Cocatalyst Active center distribution
a b s t r a c t Effect of AlEt3 on 1-hexene polymerization with MgCl2-supported Ziegler–Natta catalyst was studied. Complete activation of active centers (C*) producing medium to low molecular weight polymer requires relatively higher Al/Ti than C* producing high molecular weight polymer. Raising Al/Ti from 30 to 300 caused continuous shifting of the active center distribution from centers of smaller number, fast chain propagation and slow chain transfers to centers of larger number, slow chain propagation and fast chain transfers. Chain transfer with AlEt3 is more evident in C* producing lower molecular weight polymer. © 2015 Elsevier B.V. All rights reserved.
1. Introduction More than 100 million tons of polyolefins are manufactured annually by MgCl2-supported Ziegler–Natta catalysts. However, the mechanism of cocatalyst effects on polymerization kinetics and polymer structure hasn't been fully clarified so far, though many studies on it have been reported [1–6]. In MgCl2/TiCl4-alkylaluminum type catalyst, the roles of alkylaluminum include reduction of TiCl4 to lower oxidation states and subsequent alkylation of the Ti species [1,2], as well as scavenging the reaction system. After contacting with alkylaluminum, the catalyst shows a distribution of Ti(IV), Ti(III) and Ti(II), and the distribution varies with the alkylaluminum concentration and the reaction time [3,7–9]. Both Ti(III) and Ti(II) species may be active centers (C*) for ethylene polymerization, while Ti(III) and possibly Ti(IV) species are C* for propylene polymerization [8]. When the catalyst contains internal electron donor (Di), complexation of Di with alkylaluminum happens, and even chemical reactions between alkylaluminum and Di have reported [10]. Meanwhile, products of the reactions between the catalyst and trialkylaluminum, like AlEt2Cl, may also complex with Ti or Mg atoms on the catalyst surface, leading to changes in activity and stereoselectivity of C* [4,11]. As the stereoregularity, molecular weight distribution (MWD) and chemical composition distribution of polyolefins are determined by the properties and distribution of C*, it is necessary to systematically study and understand the mechanism of cocatalyst effects in the catalyst system. Determination of the number of C* in a heterogeneous catalysis system is the key step of a complete mechanistic study. Owing to lack of ⁎ Corresponding author. E-mail address: [email protected] (Z. Fu).
http://dx.doi.org/10.1016/j.catcom.2015.01.023 1566-7367/© 2015 Elsevier B.V. All rights reserved.
suitable methods of counting the active centers that produce different polymer chains [12], only very limited research work has been devoted to the experimental study of active center distribution of MgCl2supported Ziegler–Natta catalysts [13–15]. Recently, we provided a method of active center determination using cinnamoyl chlorides as the quenching agent [16]. This special acyl chloride has very strong UV–vis absorption in the range of 240–320 nm, however, both the polyolefin chains and hydrocarbon solvents have no absorptions. Through this method the effects of ethylene as comonomer on the active center distribution of 1-hexene polymerization with MgCl2supported Ziegler–Natta catalysts were investigated [17]. In this work, the effects of alkylaluminum on the catalytic properties and distribution of C* in 1-hexene polymerization with a TiCl4/Di/MgCl2-AlEt3 catalyst system are investigated. 1-Hexene was chosen as monomer because of the solubility of poly(1-hexene) at room temperature. 2. Experimental 2.1. Chemicals MgCl2/dibutylphthalate/TiCl4 catalyst (SINOPEC) containing 2.7 wt.% of Ti was used. 1-Hexene (98%, Acros Organics) was purified by refluxing over Na for 6 h and distilled before use. AlEt3 (Albemarle) and cinnamoyl chloride (98%, Alfa Aesar) were diluted with n-heptane to 1 M before use. 2.2. Polymerization and quenching reaction 1-Hexene polymerization was conducted and then quenched by cinnamoyl chloride as described previously [16] with about 20 mg of
H. Yang et al. / Catalysis Communications 62 (2015) 104–106
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catalyst and [1-hexene]0 = 1 mol/L at 40 °C. For more detailed information please refer to the supporting information. 2.3. Polymer characterization The molecular weights of the samples were measured by gel permeation chromatography (PL 220, Polymer Laboratories Ltd.) at 30 °C in THF. Universal calibration against narrow polystyrene standards was adopted. The content of carbonyl in the quenched polymers was determined spectrophotometrically with a Varian Cary 100 Bio UV–vis spectrometer, using n-heptane as the solvent [16]. For more detailed information please refer to the supporting information. 3. Results and discussion 3.1. Polymerization activity and molecular weight of poly(1-hexene) Fig. 1. The effect of Al/Ti on [C*]/[Ti] and kp.
The results about the influence of Al/Ti on 1-hexene polymerization were summarized in Table 1. It indicates that the polymerization activity is significantly influenced by Al/Ti. As Al/Ti increased from 30 to 150, the activity increased, but further increase in Al/Ti causes slight decrease in activity. Similar phenomenons have been reported in propylene or ethylene polymerization [18,19]. Molecular weight of poly(1-hexene) undergoes rapid decrease as Al/ Ti increased from 30 to 200, and then slightly decreases when Al/Ti increased from 200 to 300. Besides, the polydispersity index of poly(1hexene) increased firstly and then decreased as Al/Ti increased. 3.2. The number and activity of active centers The effect of Al/Ti on the concentration and intrinsic activity of active centers was investigated by counting the active centers of each polymerization run and calculating the chain propagation rate constant kp according to kinetic equation Rp = kp[C⁎][M], where Rp is the average polymerization rate, and [M] is the monomer concentration. As shown in Fig. 1, the plot of Al/Ti vs. [C*]/[Ti] % increases till it reaches Al/Ti ratio of 150 then it tends to be stable. However, the kp value decreases till the Al/Ti ratio reaches 150 then tends to be stable. Compared with those data in Table 1, it indicates that the changing trend of the polymerization activity is accord with [C*]/[Ti] %. 3.3. Analysis of active center distribution Strong change of kp value with Al/Ti implies that there are more than one kind of active centers in the catalyst. Deconvolution of MWD curves of the samples was made to study the active center distribution [20,21]. The MWD curves of all the five samples in Table 1 were deconvoluted into 5 Flory components respectively, and these components are named as component I, II, III, IV and V. Each Flory component corresponds to polymer produced by a certain type of C*. Thus there were five types of active centers, C⁎I , CII⁎, CIII⁎, CIV⁎ and CV⁎, among them C⁎I produces poly(1-hexene) with the highest Mw and CV⁎ produces polymer with the lowest Mw. The results of MWD deconvolution, namely, the weight fraction and weight average molecular weight of each Flory component, are
Table 1 Effect of Al/Ti molar ratio on 1-hexene polymerization.
summarized in Table 2. By multiplying the fraction of the component by the total activity of polymerization, the polymerization activity of each type of active center has also been calculated. The weight average molecular weight (Mw) and weight fraction (Fr) of each Flory component presented the frequency of chain transfer reaction and the polymerization activity of the corresponding C*, respectively. The polymerization activity of each C* was calculated by multiplying the Fr of Flory component by the total activity of polymerization. As seen in Table 2, increase in Al/Ti caused marked decrease of the weight of components I and II, but the weight of components IV and V increased. As shown in Fig. 2, catalytic activity of the five C* undergoes systematical but divergent changes as Al/Ti increased. The activity of C⁎I did not change much before Al/Ti rises to 150, and evidently decay when Al/Ti was higher. The activity of CII⁎ rises rapidly when Al/Ti rises from 30 to 50, and then decreases evidently. The activities of CIII⁎, CIV⁎ and CV⁎ reached the highest level at a higher Al/Ti ratio of 150, and decay rather moderately when Al/Ti becomes higher. It is clear that C⁎I and CII⁎ can be fully activated at relatively low Al/Ti, while the other Table 2 Results of MWD curve deconvolution. Al/Ti (mol/mol)
Flory component
Mw × 10−4
Fractiona (%)
Activityb (kg PH/g Ti·h)
30
I II III IV V I II III IV V I II III IV V I II III IV V I II III IV V
370.4 104.7 36.1 11.4 3.7 400.0 87.7 26.4 8.5 2.5 392.2 77.8 20.5 6.2 1.8 303.0 67.8 18.3 5.7 1.7 344.8 73.8 20.0 6.2 2.0
27.6 30.0 30.3 10.1 2.0 10.6 23.8 29.6 26.3 9.7 7.7 18.3 27.5 31.4 15.2 6.2 16.9 27.3 33.7 16.0 6.7 17.6 27.9 33.5 14.4
0.88 0.96 0.97 0.33 0.06 1.07 2.40 2.99 2.65 0.98 0.95 2.26 3.40 3.88 1.87 0.62 1.70 2.75 3.40 1.62 0.62 1.63 2.59 3.12 1.34
50
150
200
300
Run
Al/Ti (mol/mol)
Activity (kg PH/g Ti·h)
Mw × 10−4
Mw/Mn
1 2 3 4 5
30 50 150 200 300
3.21 10.09 12.36 10.09 9.30
146.9 74.2 54.1 40.5 46.5
6.9 10.6 12.6 10.5 10.4
a
Weight fraction of the Flory component. Activity of the Flory components, which is calculated by multiplying the weight fraction of Flory component by the total activity of polymerization. b
106
H. Yang et al. / Catalysis Communications 62 (2015) 104–106
For C⁎I , molecular weight of its product changes only slightly with Al/Ti, meaning that this type of center has rather slow chain transfer with TEA. The other four types of active centers show evident chain transfer with the cocatalyst, as molecular weight of their product decreases with increasing Al/Ti. However, the decrements of molecular weight in raising Al/Ti from 150 to 300 are much smaller than those in raising Al/Ti from 50 to 150. It is possible that complexation of TEA and DEAC (product of the reactions between the catalyst and TEA) on the active sites enhanced their stereospecificity [4], meanwhile increased polymer's molecular weight, as positive relationship between stereospecificity of active center and molecular weight of its polymer product has been reported [22]. This effect partly counteracts the increased chain transfer with TEA when the Al/Ti ratio is raised. 4. Conclusions Fig. 2. Effect of TEA concentration on activity of the Flory components.
three types of centers need higher Al/Ti for complete activation, and they are relatively more stable at high Al/Ti. Fig. 3 showed the effect of Al/Ti on the molecular weight of the five Flory components in each sample. As Al/Ti increased from 30 to 200, molecular weight of component I did not change much, but that of the components II, III, IV and V decreased evidently. Further increase of Al/Ti from 200 to 300 caused slight increase in molecular weight of all the five components. 3.4. Mechanism of the cocatalyst effects Combining the results of active center counting and MWD deconvolution, it is possible to make rational deductions on the effects of cocatalyst in the polymerization system. First, from Figs. 1 and 2 we can see that C⁎I and CII⁎ contributed more than half of the activity when Al/Ti was only 30. Meanwhile, the total concentration of active centers was rather low, and the kp value was rather high. It means that C⁎I and CII⁎ have small number but high kp value than the other types of active centers. As Al/Ti rose to 150 and higher, contributions of CIII⁎, CIV⁎ and CV⁎ in the total activity increased rapidly, meanwhile the [C*]/[Ti] ratio increased markedly, and the kp value dropped rapidly. All these results indicate that CIII⁎, CIV⁎ and CV⁎ have much larger number and lower kp value than C⁎I and CII⁎, but they can be activated only when Al/Ti is high enough. Therefore, raising Al/Ti caused continuous shifting of the active center distribution from centers of fast chain propagation and slow chain transfers to centers of slow chain propagation and fast chain transfers. Changes of polymer molecular weight of the different active centers with Al/Ti can also provide hints for the mechanism of cocatalyst effects.
Fig. 3. Effect of Al/Ti on the molecular weight of the Flory components.
In 1-hexene polymerization with MgCl2/TiCl4-alkylaluminum type Ziegler–Natta catalyst, raising Al/Ti markedly changed [C*]/[Ti], kp and active center distribution. Raising Al/Ti ratio from 30 to 300 caused continuous shifting of the active center distribution from centers of smaller number, fast chain propagation and slow chain transfers to centers of larger number, slow chain propagation and fast chain transfers. These changes lead to fast increase of [C*]/[Ti] and polymerization activity as well as decrease of polymer's molecular weight when Al/Ti increased from 30 to 150. Complete activation of C* producing polymer with medium to low molecular weight requires relatively higher Al/Ti than C* producing polymer with high molecular weight. Chain transfer with cocatalyst is more evident in C* producing polymer with relatively lower molecular weight. Acknowledgments Support by the Major State Basic Research Programs (grant no. 2011CB606001) and the National Natural Science Foundation of China (grant no. 21374094) is gratefully acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2015.01.023. References [1] N. Kashiwa, J. Yoshitake, Die Makromol. Chem. 185 (1984) 1133–1138. [2] J.C.W. Chien, S. Weber, Y. Hu, J. Polym. Sci. A Polym. Chem. 27 (1989) 1499–1514. [3] N. Senso, P. Praserthdam, B. Jongsomjit, T. Taniike, M. Terano, Polym. Bull. 67 (2011) 1979–1989. [4] J. Hu, B. Han, X.R. Shen, Z.S. Fu, Z.Q. Fan, Chin. J. Polym. Sci. 31 (2013) 583–590. [5] E.S. Gnanakumar, R.R. Gowda, S. Kunjir, T.G. Ajithkumar, P.R. Rajamohanan, D. Chakraborty, C.S. Gopinath, ACS Catal. 3 (2013) 303–311. [6] E.S. Gnanakumar, K.S. Thushara, R.R. Gowda, R. Gowda, S.K. Raman, T.G. Ajithkumar, P.R. Rajamohanan, D. Chakraborty, C.S. Gopinath, J. Phys. Chem. C 116 (2012) 24115–24122. [7] K. Soga, S.I. Chen, R. Ohnishi, Polym. Bull. 8 (1982) 473–478. [8] D. Fregonese, S. Mortara, S. Bresadola, J. Mol. Catal. A Chem. 172 (2001) 89–95. [9] A.S.N. Al-arifi, J. Appl. Polym. Sci. 93 (2004) 56–62. [10] Y.V. Kissin, A.J. Sivak, J. Polym. Sci. Polym. Chem. Ed. 22 (1984) 3747–3758. [11] T. Nitta, B. Liu, H. Nakatani, M. Terano, J. Mol. Catal. A Chem. 180 (2002) 25–34. [12] M.M. Marques, P.J.T. Tait, J. Mejzlik, A.R. Dias, J. Polym. Sci. A Polym. Chem. 36 (1998) 573–585. [13] Z.Q. Fan, L.X. Feng, S.L. Yang, J. Polym. Sci. A Polym. Chem. 34 (1996) 3329–3335. [14] G.D. Bukatov, V.A. Zakharov, A.A. Barabanov, Kinet. Catal. 46 (2005) 166–176. [15] I. Nishiyama, B.P. Liu, H. Matsuoka, H. Nakatani, M. Terano, Macromol. Symp. 193 (2003) 71–80. [16] L.T. Zhang, Z.S. Fu, Z.Q. Fan, Macromol. Res. 18 (2010) 695–700. [17] H.R. Yang, L.T. Zhang, Z.S. Fu, Z.Q. Fan, J. Appl. Polym. Sci. 132 (2015). http:// dx.doi.org/10.1002/app.41264. [18] H. Trischler, M. Ruff, C. Paulik, J. Mater. Sci. Eng. A 2 (2012) 511–518. [19] B. Zhang, Q. Dong, Z.S. Fu, Z.Q. Fan, Polym. 55 (2014) 4865–4872. [20] Y.V. Kissin, Makromol. Chem. Macromol. Symp. 66 (1993) 83–94. [21] D.E. Thompson, K.B. McAuley, P.J. McLellan, Macromol. React. Eng. 1 (2007) 264–274. [22] Z.Q. Fan, L.T. Zhang, S.J. Xia, Z.S. Fu, J. Mol. Catal. A Chem. 351 (2011) 93–99.
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Effects of alkylaluminum as cocatalyst on the active center distribution of 1-hexene polymerization with MgCl2-supported Ziegler–Natta catalysts Hongrui Yang a, Letian Zhang b, Dandan Zang a, Zhisheng Fu a,⁎, Zhiqiang Fan a a b
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China Shanghai Research Institute of Chemical Industry, Shanghai 200062, China
a r t i c l e
i n f o
Article history: Received 17 October 2014 Received in revised form 20 December 2014 Accepted 23 January 2015 Available online 24 January 2015 Keywords: Ziegler–Natta catalyst Cocatalyst Active center distribution
a b s t r a c t Effect of AlEt3 on 1-hexene polymerization with MgCl2-supported Ziegler–Natta catalyst was studied. Complete activation of active centers (C*) producing medium to low molecular weight polymer requires relatively higher Al/Ti than C* producing high molecular weight polymer. Raising Al/Ti from 30 to 300 caused continuous shifting of the active center distribution from centers of smaller number, fast chain propagation and slow chain transfers to centers of larger number, slow chain propagation and fast chain transfers. Chain transfer with AlEt3 is more evident in C* producing lower molecular weight polymer. © 2015 Elsevier B.V. All rights reserved.
1. Introduction More than 100 million tons of polyolefins are manufactured annually by MgCl2-supported Ziegler–Natta catalysts. However, the mechanism of cocatalyst effects on polymerization kinetics and polymer structure hasn't been fully clarified so far, though many studies on it have been reported [1–6]. In MgCl2/TiCl4-alkylaluminum type catalyst, the roles of alkylaluminum include reduction of TiCl4 to lower oxidation states and subsequent alkylation of the Ti species [1,2], as well as scavenging the reaction system. After contacting with alkylaluminum, the catalyst shows a distribution of Ti(IV), Ti(III) and Ti(II), and the distribution varies with the alkylaluminum concentration and the reaction time [3,7–9]. Both Ti(III) and Ti(II) species may be active centers (C*) for ethylene polymerization, while Ti(III) and possibly Ti(IV) species are C* for propylene polymerization [8]. When the catalyst contains internal electron donor (Di), complexation of Di with alkylaluminum happens, and even chemical reactions between alkylaluminum and Di have reported [10]. Meanwhile, products of the reactions between the catalyst and trialkylaluminum, like AlEt2Cl, may also complex with Ti or Mg atoms on the catalyst surface, leading to changes in activity and stereoselectivity of C* [4,11]. As the stereoregularity, molecular weight distribution (MWD) and chemical composition distribution of polyolefins are determined by the properties and distribution of C*, it is necessary to systematically study and understand the mechanism of cocatalyst effects in the catalyst system. Determination of the number of C* in a heterogeneous catalysis system is the key step of a complete mechanistic study. Owing to lack of ⁎ Corresponding author. E-mail address: [email protected] (Z. Fu).
http://dx.doi.org/10.1016/j.catcom.2015.01.023 1566-7367/© 2015 Elsevier B.V. All rights reserved.
suitable methods of counting the active centers that produce different polymer chains [12], only very limited research work has been devoted to the experimental study of active center distribution of MgCl2supported Ziegler–Natta catalysts [13–15]. Recently, we provided a method of active center determination using cinnamoyl chlorides as the quenching agent [16]. This special acyl chloride has very strong UV–vis absorption in the range of 240–320 nm, however, both the polyolefin chains and hydrocarbon solvents have no absorptions. Through this method the effects of ethylene as comonomer on the active center distribution of 1-hexene polymerization with MgCl2supported Ziegler–Natta catalysts were investigated [17]. In this work, the effects of alkylaluminum on the catalytic properties and distribution of C* in 1-hexene polymerization with a TiCl4/Di/MgCl2-AlEt3 catalyst system are investigated. 1-Hexene was chosen as monomer because of the solubility of poly(1-hexene) at room temperature. 2. Experimental 2.1. Chemicals MgCl2/dibutylphthalate/TiCl4 catalyst (SINOPEC) containing 2.7 wt.% of Ti was used. 1-Hexene (98%, Acros Organics) was purified by refluxing over Na for 6 h and distilled before use. AlEt3 (Albemarle) and cinnamoyl chloride (98%, Alfa Aesar) were diluted with n-heptane to 1 M before use. 2.2. Polymerization and quenching reaction 1-Hexene polymerization was conducted and then quenched by cinnamoyl chloride as described previously [16] with about 20 mg of
H. Yang et al. / Catalysis Communications 62 (2015) 104–106
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catalyst and [1-hexene]0 = 1 mol/L at 40 °C. For more detailed information please refer to the supporting information. 2.3. Polymer characterization The molecular weights of the samples were measured by gel permeation chromatography (PL 220, Polymer Laboratories Ltd.) at 30 °C in THF. Universal calibration against narrow polystyrene standards was adopted. The content of carbonyl in the quenched polymers was determined spectrophotometrically with a Varian Cary 100 Bio UV–vis spectrometer, using n-heptane as the solvent [16]. For more detailed information please refer to the supporting information. 3. Results and discussion 3.1. Polymerization activity and molecular weight of poly(1-hexene) Fig. 1. The effect of Al/Ti on [C*]/[Ti] and kp.
The results about the influence of Al/Ti on 1-hexene polymerization were summarized in Table 1. It indicates that the polymerization activity is significantly influenced by Al/Ti. As Al/Ti increased from 30 to 150, the activity increased, but further increase in Al/Ti causes slight decrease in activity. Similar phenomenons have been reported in propylene or ethylene polymerization [18,19]. Molecular weight of poly(1-hexene) undergoes rapid decrease as Al/ Ti increased from 30 to 200, and then slightly decreases when Al/Ti increased from 200 to 300. Besides, the polydispersity index of poly(1hexene) increased firstly and then decreased as Al/Ti increased. 3.2. The number and activity of active centers The effect of Al/Ti on the concentration and intrinsic activity of active centers was investigated by counting the active centers of each polymerization run and calculating the chain propagation rate constant kp according to kinetic equation Rp = kp[C⁎][M], where Rp is the average polymerization rate, and [M] is the monomer concentration. As shown in Fig. 1, the plot of Al/Ti vs. [C*]/[Ti] % increases till it reaches Al/Ti ratio of 150 then it tends to be stable. However, the kp value decreases till the Al/Ti ratio reaches 150 then tends to be stable. Compared with those data in Table 1, it indicates that the changing trend of the polymerization activity is accord with [C*]/[Ti] %. 3.3. Analysis of active center distribution Strong change of kp value with Al/Ti implies that there are more than one kind of active centers in the catalyst. Deconvolution of MWD curves of the samples was made to study the active center distribution [20,21]. The MWD curves of all the five samples in Table 1 were deconvoluted into 5 Flory components respectively, and these components are named as component I, II, III, IV and V. Each Flory component corresponds to polymer produced by a certain type of C*. Thus there were five types of active centers, C⁎I , CII⁎, CIII⁎, CIV⁎ and CV⁎, among them C⁎I produces poly(1-hexene) with the highest Mw and CV⁎ produces polymer with the lowest Mw. The results of MWD deconvolution, namely, the weight fraction and weight average molecular weight of each Flory component, are
Table 1 Effect of Al/Ti molar ratio on 1-hexene polymerization.
summarized in Table 2. By multiplying the fraction of the component by the total activity of polymerization, the polymerization activity of each type of active center has also been calculated. The weight average molecular weight (Mw) and weight fraction (Fr) of each Flory component presented the frequency of chain transfer reaction and the polymerization activity of the corresponding C*, respectively. The polymerization activity of each C* was calculated by multiplying the Fr of Flory component by the total activity of polymerization. As seen in Table 2, increase in Al/Ti caused marked decrease of the weight of components I and II, but the weight of components IV and V increased. As shown in Fig. 2, catalytic activity of the five C* undergoes systematical but divergent changes as Al/Ti increased. The activity of C⁎I did not change much before Al/Ti rises to 150, and evidently decay when Al/Ti was higher. The activity of CII⁎ rises rapidly when Al/Ti rises from 30 to 50, and then decreases evidently. The activities of CIII⁎, CIV⁎ and CV⁎ reached the highest level at a higher Al/Ti ratio of 150, and decay rather moderately when Al/Ti becomes higher. It is clear that C⁎I and CII⁎ can be fully activated at relatively low Al/Ti, while the other Table 2 Results of MWD curve deconvolution. Al/Ti (mol/mol)
Flory component
Mw × 10−4
Fractiona (%)
Activityb (kg PH/g Ti·h)
30
I II III IV V I II III IV V I II III IV V I II III IV V I II III IV V
370.4 104.7 36.1 11.4 3.7 400.0 87.7 26.4 8.5 2.5 392.2 77.8 20.5 6.2 1.8 303.0 67.8 18.3 5.7 1.7 344.8 73.8 20.0 6.2 2.0
27.6 30.0 30.3 10.1 2.0 10.6 23.8 29.6 26.3 9.7 7.7 18.3 27.5 31.4 15.2 6.2 16.9 27.3 33.7 16.0 6.7 17.6 27.9 33.5 14.4
0.88 0.96 0.97 0.33 0.06 1.07 2.40 2.99 2.65 0.98 0.95 2.26 3.40 3.88 1.87 0.62 1.70 2.75 3.40 1.62 0.62 1.63 2.59 3.12 1.34
50
150
200
300
Run
Al/Ti (mol/mol)
Activity (kg PH/g Ti·h)
Mw × 10−4
Mw/Mn
1 2 3 4 5
30 50 150 200 300
3.21 10.09 12.36 10.09 9.30
146.9 74.2 54.1 40.5 46.5
6.9 10.6 12.6 10.5 10.4
a
Weight fraction of the Flory component. Activity of the Flory components, which is calculated by multiplying the weight fraction of Flory component by the total activity of polymerization. b
106
H. Yang et al. / Catalysis Communications 62 (2015) 104–106
For C⁎I , molecular weight of its product changes only slightly with Al/Ti, meaning that this type of center has rather slow chain transfer with TEA. The other four types of active centers show evident chain transfer with the cocatalyst, as molecular weight of their product decreases with increasing Al/Ti. However, the decrements of molecular weight in raising Al/Ti from 150 to 300 are much smaller than those in raising Al/Ti from 50 to 150. It is possible that complexation of TEA and DEAC (product of the reactions between the catalyst and TEA) on the active sites enhanced their stereospecificity [4], meanwhile increased polymer's molecular weight, as positive relationship between stereospecificity of active center and molecular weight of its polymer product has been reported [22]. This effect partly counteracts the increased chain transfer with TEA when the Al/Ti ratio is raised. 4. Conclusions Fig. 2. Effect of TEA concentration on activity of the Flory components.
three types of centers need higher Al/Ti for complete activation, and they are relatively more stable at high Al/Ti. Fig. 3 showed the effect of Al/Ti on the molecular weight of the five Flory components in each sample. As Al/Ti increased from 30 to 200, molecular weight of component I did not change much, but that of the components II, III, IV and V decreased evidently. Further increase of Al/Ti from 200 to 300 caused slight increase in molecular weight of all the five components. 3.4. Mechanism of the cocatalyst effects Combining the results of active center counting and MWD deconvolution, it is possible to make rational deductions on the effects of cocatalyst in the polymerization system. First, from Figs. 1 and 2 we can see that C⁎I and CII⁎ contributed more than half of the activity when Al/Ti was only 30. Meanwhile, the total concentration of active centers was rather low, and the kp value was rather high. It means that C⁎I and CII⁎ have small number but high kp value than the other types of active centers. As Al/Ti rose to 150 and higher, contributions of CIII⁎, CIV⁎ and CV⁎ in the total activity increased rapidly, meanwhile the [C*]/[Ti] ratio increased markedly, and the kp value dropped rapidly. All these results indicate that CIII⁎, CIV⁎ and CV⁎ have much larger number and lower kp value than C⁎I and CII⁎, but they can be activated only when Al/Ti is high enough. Therefore, raising Al/Ti caused continuous shifting of the active center distribution from centers of fast chain propagation and slow chain transfers to centers of slow chain propagation and fast chain transfers. Changes of polymer molecular weight of the different active centers with Al/Ti can also provide hints for the mechanism of cocatalyst effects.
Fig. 3. Effect of Al/Ti on the molecular weight of the Flory components.
In 1-hexene polymerization with MgCl2/TiCl4-alkylaluminum type Ziegler–Natta catalyst, raising Al/Ti markedly changed [C*]/[Ti], kp and active center distribution. Raising Al/Ti ratio from 30 to 300 caused continuous shifting of the active center distribution from centers of smaller number, fast chain propagation and slow chain transfers to centers of larger number, slow chain propagation and fast chain transfers. These changes lead to fast increase of [C*]/[Ti] and polymerization activity as well as decrease of polymer's molecular weight when Al/Ti increased from 30 to 150. Complete activation of C* producing polymer with medium to low molecular weight requires relatively higher Al/Ti than C* producing polymer with high molecular weight. Chain transfer with cocatalyst is more evident in C* producing polymer with relatively lower molecular weight. Acknowledgments Support by the Major State Basic Research Programs (grant no. 2011CB606001) and the National Natural Science Foundation of China (grant no. 21374094) is gratefully acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2015.01.023. References [1] N. Kashiwa, J. Yoshitake, Die Makromol. Chem. 185 (1984) 1133–1138. [2] J.C.W. Chien, S. Weber, Y. Hu, J. Polym. Sci. A Polym. Chem. 27 (1989) 1499–1514. [3] N. Senso, P. Praserthdam, B. Jongsomjit, T. Taniike, M. Terano, Polym. Bull. 67 (2011) 1979–1989. [4] J. Hu, B. Han, X.R. Shen, Z.S. Fu, Z.Q. Fan, Chin. J. Polym. Sci. 31 (2013) 583–590. [5] E.S. Gnanakumar, R.R. Gowda, S. Kunjir, T.G. Ajithkumar, P.R. Rajamohanan, D. Chakraborty, C.S. Gopinath, ACS Catal. 3 (2013) 303–311. [6] E.S. Gnanakumar, K.S. Thushara, R.R. Gowda, R. Gowda, S.K. Raman, T.G. Ajithkumar, P.R. Rajamohanan, D. Chakraborty, C.S. Gopinath, J. Phys. Chem. C 116 (2012) 24115–24122. [7] K. Soga, S.I. Chen, R. Ohnishi, Polym. Bull. 8 (1982) 473–478. [8] D. Fregonese, S. Mortara, S. Bresadola, J. Mol. Catal. A Chem. 172 (2001) 89–95. [9] A.S.N. Al-arifi, J. Appl. Polym. Sci. 93 (2004) 56–62. [10] Y.V. Kissin, A.J. Sivak, J. Polym. Sci. Polym. Chem. Ed. 22 (1984) 3747–3758. [11] T. Nitta, B. Liu, H. Nakatani, M. Terano, J. Mol. Catal. A Chem. 180 (2002) 25–34. [12] M.M. Marques, P.J.T. Tait, J. Mejzlik, A.R. Dias, J. Polym. Sci. A Polym. Chem. 36 (1998) 573–585. [13] Z.Q. Fan, L.X. Feng, S.L. Yang, J. Polym. Sci. A Polym. Chem. 34 (1996) 3329–3335. [14] G.D. Bukatov, V.A. Zakharov, A.A. Barabanov, Kinet. Catal. 46 (2005) 166–176. [15] I. Nishiyama, B.P. Liu, H. Matsuoka, H. Nakatani, M. Terano, Macromol. Symp. 193 (2003) 71–80. [16] L.T. Zhang, Z.S. Fu, Z.Q. Fan, Macromol. Res. 18 (2010) 695–700. [17] H.R. Yang, L.T. Zhang, Z.S. Fu, Z.Q. Fan, J. Appl. Polym. Sci. 132 (2015). http:// dx.doi.org/10.1002/app.41264. [18] H. Trischler, M. Ruff, C. Paulik, J. Mater. Sci. Eng. A 2 (2012) 511–518. [19] B. Zhang, Q. Dong, Z.S. Fu, Z.Q. Fan, Polym. 55 (2014) 4865–4872. [20] Y.V. Kissin, Makromol. Chem. Macromol. Symp. 66 (1993) 83–94. [21] D.E. Thompson, K.B. McAuley, P.J. McLellan, Macromol. React. Eng. 1 (2007) 264–274. [22] Z.Q. Fan, L.T. Zhang, S.J. Xia, Z.S. Fu, J. Mol. Catal. A Chem. 351 (2011) 93–99.