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1-hexene copolymerization
Journal of Organometallic Chemistry xxx (2015) 1e7
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Effects of comonomer on active center distribution of TCl4/ MgCl2eAlEt3 catalyst in ethylene/1-hexene copolymerization Tao Xu, Hongrui Yang, Zhisheng Fu, Zhiqiang Fan* MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 December 2014 Received in revised form 25 February 2015 Accepted 6 April 2015 Available online xxx
Ethylene/1-hexene copolymerization with MgCl2/TiCl4eAlEt3 catalyst has been conducted under different initial 1-hexene concentration (0e0.5 mol/L). Number of polymerization active centers was determined by quenching the reaction with 2-thiophenecarbonyl chloride and measuring sulfur content of the quenched polymer. Each copolymer sample was fractionated into boiling n-heptane soluble and insoluble fractions, and active centers in these fractions were also counted. The rate constants of ethylene and 1-hexene insertion in the active centers were calculated, respectively. Molecular weight distribution (MWD) curves of the polymers were deconvoluted with 4e5 Flory components, and changes of activity of the Flory components with [1-hexene] were analyzed. The polymerization activity and the number of active centers were significantly enhanced by increasing [1-hexene]. Large number of active centers were revived by small amount of 1-hexene. With the increase of [1-hexene], the number of active centers producing polymer chains with lower molecular weight and higher 1-hexene content was increased more than those producing polymer chains with higher molecular weight and lower 1-hexene content, and the MWD curve continuously inclined to the low molecular weight side. The active centers with higher 1-hexene incorporation rate have relatively smaller rate constant of ethylene insertion. When [1hexene] was increased, the rate constant of ethylene insertion was only slightly changed, but the rate constant of 1-hexene insertion was markedly lowered, meaning that the active centers revived by 1hexene have relatively lower ability of incorporating 1-hexene. © 2015 Elsevier B.V. All rights reserved.
Keywords: Ethylene 1-Hexene Comonomer effect ZieglereNatta catalyst Active center distribution
Introduction Copolymers of ethylene with a-olefins (propylene, 1-butene, 1hexene etc.) are important polymer materials with wide applications. At present, more than 80% of these copolymers are produced by MgCl2-supported ZieglereNatta catalysts. As an important feature, ZieglereNatta catalysts produce ethylene-a-olefin copolymer with very broad chemical composition distribution (CCD), which is caused by multiple active centers in the catalyst, and the aolefin units are highly enriched in the part of polymer with low molecular weight [1e7]. The broad CCD strongly influences the application properties of the copolymer. As an example, in the production of high density polyethylene (HDPE) resin by copolymerizing ethylene with small amount of a-olefin, increasing comonomer content of the high molecular weight part while
* Corresponding author. E-mail address: [email protected] (Z. Fan).
keeping low a-olefin content in the low molecular weight part can greatly improve the mechanical properties of the HDPE resin. In ethylene/a-olefin copolymerization with ZieglereNatta catalysts, metallocene catalysts or Cr-based Phillips catalysts, enhancement of reaction activity by the a-olefin comonomer (socalled “comonomer effect”) has long been reported and studied [1,2,8e20]. Though several theories have been developed to rationalize this kind of kinetic phenomenon, there is still not a generally accepted explanation or mechanistic model. In our previous work, strong influences of a-olefin comonomer on the polymer molecular weight distribution (MWD) had been reported [18]. In ethylene-1-hexene copolymerization with a MgCl2/TiCl4 type Z-N catalyst, besides the strong activity enhancement, small amount of 1-hexene ([1-Hexene] ¼ 0.05 M) also caused strong broadening and shifting of the MWD curve as compared to the ethylene homopolymerization system [18]. This was explained by unequal activation of different types of active centers by the a-olefin. The active centers that produce low molecular weight polymer were activated more than those produce
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high molecular weight polymer. In other words, the comonomer also influences the distribution of active centers in the catalyst. This result was in keeping with the explanation of comonomer effect based on chemical sources, rather than physical sources. However, only the changes in MWD curve are not enough to disclose the full scenario of comonomer effect. In this paper, comonomer effect in ethylene/1-hexene copolymerization with a MgCl2/TiCl4eAlEt3 catalysts has been investigated, with special attention being paid to the influences of 1hexene on the distribution and intrinsic activity of the multiple types of active centers. A method using 2-thiophenecarbonyl chloride as quenching reagent has been adopted for counting the active centers in two copolymer fractions [21,22]. The results obtained in this work can be new evidences for understanding the mechanism of the comonomer effect, and can provide clues for figuring out structure of different types of active centers.
Results and discussion
Fig. 1. Influence of initial 1-hexene concentration on polymerization activity and copolymer composition.
Copolymerization activity and CCD of the copolymer Using a TiCl4/MgCl2 catalyst activated by TEA, a series of ethylene/1-hexene copolymerization were conducted under 1 atm ethylene pressure and different initial 1-hexene concentrations. All the copolymer samples were fractionated into two parts: the boiling n-heptane soluble fraction (C7-sol) and insoluble fraction (C7-ins). The changes of copolymerization activity and fraction weight of C7-sol and C7-ins with 1-hexene concentration are shown in Table 1. It is seen that the activity increased with the 1hexene concentration, and was more than tripled when [1hexene] was raised to 0.5 mol/L. The comonomer activation effect was evident even at [1-hexene] as low as 0.02 mol/L (see Fig. 1). Meanwhile, the weight fraction of C7-sol also quickly increased with [1-hexene]. This can be attributed to increase of the copolymer's 1-hexene content (see Fig. 1 and Table 2). The 1-hexene content in the C7-sol fraction was always much higher than that in the C7-ins fraction, meaning that incorporation of enough 1hexene in the polymer chains can make them soluble in boiling n-heptane. The gradual decrease of melting enthalpy with [1hexene] (see Table 1) also indicates that the incorporated 1hexene units can disturb crystallization of the polymer chains and increase the amount of amorphous phase that is soluble in nheptane. The 1-hexene content of the C7-ins fractions were all lower than 3 mol-%, while 1-hexene content of the C7-sol fractions were all higher than 4 mol-%, so the critical 1-hexene content determining the solubility of copolymer chains should be between 3 and 4 mol-%. Because the difference in 1-hexene content of the C7-sol and C7-ins fractions was evidently enlarged with increasing
[1-hexene], the CCD of the copolymer became broader at high 1hexene incorporate rate. The molecular weight of the polymer samples and their fractions were measured by GPC, and the results are summarized in Table 2. The molecular weight of the whole polymer sample decreased sharply with increasing [1-hexene], but molecular weights of both the C7-sol and C7-ins fractions gradually increased. Because the molecular weights of C7-sol were much lower than that of the C7-ins fraction, the decrease of molecular weight of the whole polymer sample is mainly caused by increase of the C7-sol fraction. Number of active centers and chain propagation rate constant The number of active centers in the whole polymer sample and their fractions has been determined by a method based on quenching the catalysis system with TPCC, and the results are listed in Table 3. As seen in Table 3 and Fig. 2, the total number of active centers ([C*]/[Ti], where C* represents active center) increased sharply with [1-hexene], implying that introducing 1-hexene in the catalysis system caused formation of new active centers or revival of some types of dormant active sites. The comonomer effect can thus be largely attributed to this activation of dormant centers. The number of active centers in the C7-sol fraction also increased with [1-hexene], but that of the C7-ins fraction tended to decrease with [1-hexene]. This is understandable when we know that the weight fraction of C7-ins decreases with [1-hexene].
Table 1 Results of copolymerization under different 1-hexene concentration.a Run
[1-Hexene] (mol/L)
Yield (g)
Activity (kg/gTi h)
Tmb ( C)
△Hmc (J/g)
C7-sold (wt-%)
C7-inse (wt-%)
1 2 3 4 5 6 7
0 0.02 0.05 0.1 0.2 0.3 0.5
1.60 4.42 4.25 4.93 5.15 5.25 4.72
2.56 3.68 5.69 6.73 7.36 7.69 8.20
134.1 127.8 126.1 125.5 124.8 124.3 124.0
178.6 152.7 144.5 131.4 113.7 112.2 67.4
e 4.9 10.1 20.6 42.1 51.1 60.2
e 95.1 89.9 79.4 57.9 48.9 39.8
a b c d e
Polymerization conditions: Al/Ti ¼ 100 (mol/mol), TP ¼ 50 C, Pethylene ¼ 1 atm, tP ¼ 30 min. Melting temperature. Melting enthalpy. Fraction of boiling n-heptane soluble part. Fraction of boiling n-heptane insoluble part.
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Table 2 Composition and molecualar weight of the polymer samples and their fractions. Run
[1-Hexene] (mol/L)
1 2 3 4 5 6 7 a b
0 0.02 0.05 0.1 0.2 0.3 0.5
Mw (105)
Hexene content (mol-%)
PDIa
Whole
C7-sol
C7-insol
Whole
C7-sol
C7-ins
Whole
0 2.3 3.0 3.5 4.6 7.0 12
e –b –b –b 7.5 12 20
e 2.2 2.3 2.6 2.7 2.8 3.0
8.1 3.0 3.3 2.3 2.1 1.7 1.2
e 0.07 0.14 0.19 0.36 0.35 0.34
e 2.5 2.8 2.5 4.1 3.9 4.0
5.3 8.0 14.9 10.7 10.9 8.7 8.0
Polydispersity index of the polymer. Not determined.
Table 3 Number of active centers and chain propagation rate constants of the polymer samples and their fractions. Run
1 2 3 4 5 6 7 a b c
[H]a mol/L
Whole [C*]/[Ti] (mol-%)
kpEb (L/mol$s)
kpHc (L/mol$s)
[C*]/[Ti] (mol-%)
kpEb (L/mol$s)
kpHc (L/mol$s)
[C*]/[Ti] (mol-%)
kpEb (L/mol$s)
kpHc (L/mol$s)
0 0.02 0.05 0.1 0.2 0.3 0.5
13.0 25.9 35.8 42.2 42.4 46.8 57.2
75 81 89 88 81 82 73
e 7.4 4.3 2.5 1.5 1.6 1.6
e 6.5 17.0 25.3 30.7 36.8 47.6
e 15 15 27 43 46 42
e e e e 1.4 1.6 1.7
e 19.4 18.8 16.9 11.7 10.0 9.6
e 103 155 178 179 211 222
e 9.0 5.7 3.7 1.9 1.6 1.1
C7-sol
C7-ins
Initial [1-hexene]. Propagation rate constant of ethylene insertions. Propagation rate constant of 1-hexene insertions.
to form thick polymer layer around each fragment of the catalyst particle. This will make the actual [Mi] at the location of the active centers much lower than that in the bulk of reaction medium. Therefore, the kp values observed in this work (see Table 3) were much smaller than those measured by stopped-flow method for similar catalytic systems [23], as the polymerization duration in the stopped-flow experiment was only about 0.1 s. However, the kp values obtained in this work can still reflect the relative changes in intrinsic activity of the active centers. The propagation rate constants of ethylene insertion (kpE) and 1-hexene insertion (kpH) of the whole copolymerization system and those of the soluble and insoluble fractions are shown in Table 3, Fig. 3 and Fig. 4. In Fig. 3 we can find that the rate constant of ethylene insertion (kpE) slightly increased when [1-hexene] increased from 0 to
Fig. 2. Influence of 1-hexene concentration on the number of active centers in the whole polymerization system, the boiling n-heptane soluble fraction and the boiling nheptane insoluble fraction.
The propagation rate constants of both ethylene and 1-hexene insertion in the active centers can be calculated by the equation Rpi ¼ kpi [C*][Mi] (i ¼ ethylene or 1-hexene), where Rpi is the insertion rate of monomer Mi. For the copolymerization runs, the Rpi was calculated from the copolymer yield and the copolymer composition data. Because of the difficulty in estimating the real monomer concentrations at the site of active center, we can only use the equilibrium ethylene concentration and initial 1-hexene concentration in the calculations. It can be expected that the diffusion barrier in the catalyst-polymer particles was rather large, because the polymerization reactions were all continued for 30 min
Fig. 3. Influence of 1-hexene concentration on rate constant of ethylene insertion and 1-hexene insertion.
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the copolymer chains moved to the C7-sol part at higher [1-hexene] were produced by active centers with larger kpE, the average kpE of the C7-sol part will be increased. Increase of kpE of the C7-ins part with [1-hexene] means that the remaining copolymer chains of C7ins were produced by active centers with even larger kpE. Therefore, there should be a nearly continuous distribution of active centers that have different kpE and kpH values. Distribution of active centers based on deconvolution of MWD curves
Fig. 4. Influence of 1-hexene concentration on rate constants of ethylene insertion and 1-hexene insertion in C7-sol and C7-ins fractions.
0.05 mol/L, and then slowly decreased as [1-hexene] further increased to 0.5 mol/L. Meanwhile, the rate constant of 1-hexene insertion (kpH) sharply decreased when [1-hexene] increased from 0 to 0.1 mol/L, and then leveled off. This means that small amount of 1-hexene can strongly influence the properties of the active centers. Because [C*]/[Ti] was sharply increased when [1hexene] increased from 0 to 0.1 mol/L, the quick changes of kpE and kpH with [1-hexene] could be attributed to the emerged active centers. They may have larger kpE and smaller kpH than those original active centers present in the ethylene homopolymerization system. This phenomenon is quite interesting, and the mechanism will be discussed later in this article. Because diffusion barrier in the catalyst particles tend to decrease with [1-hexene] as a result of increasing copolymer solubility in the reaction medium, such decrease of kpH with [1-hexene] could be only explained by change of the active centers. Fig. 4 shows the changes of kpE and kpH of both fractions with [1hexene]. The following trends can be concluded: (1) In both the C7sol and C7-ins fractions, the rate constants of ethylene insertion were all much larger than those of 1-hexene insertion; (2) The rate constants of ethylene insertion (kpE) of the insoluble fraction was much larger than kpE of the soluble fraction, but the kpH values of the two fractions were only a little different. As a result, the kpE/kpH ratio of C7-sol was much smaller than that of the C7-ins fraction; (3) kpE in both fractions continuously increase with the 1-hexene concentration, meanwhile kpH continuously decrease with the 1hexene concentration. These results provided important clues that disclose the nature of the two groups of active centers producing the soluble and insoluble copolymer chains. Because kpE/kpH ratio is reversely proportional to the 1-hexene incorporation rate, it is clear that the active centers producing the C7-sol part have much stronger ability of incorporating 1-hexene in the copolymer chains than the active centers producing the C7-ins part. On the other hand, the much larger (kpE)ins than (kpE)sol means that ethylene insertion in the active centers forming the C7-ins chains are much faster than the centers of C7-sol part. It is the faster ethylene insertion that caused larger kpE/kpH ratio and lower 1-hexene content in the C7-ins chains. The continuous changes of (kpE)ins, (kpE)sol, (kpH)ins and (kpH)sol with [1-hexene] can be understood from the change of solubility of the C7-ins chains. As [1-hexene] increases, 1-hexene content of copolymer chains formed on all the active centers will increase, moving more copolymer chains into the soluble fraction. Because
Deconvolution of MWD curves of the seven polymer samples were made to study the active center distribution in details [1,24e27]. As shown in Fig. 5, all the MWD curves were deconvoluted into 5 or 4 Flory components, and these components were named as component A, B, C, D and E, respectively. Each Flory component corresponds to polymer produced by a certain type of active center [24e27]. Thus there were five types of active centers, C*A, C*B, C*C, CD* and C*E. Among them C*E produces polymer with the highest molecular weight and C*A produces polymer with the lowest molecular weight. The results of MWD deconvolution, namely the weight fraction and weight average molecular weight of each Flory component, are summarized in Table 4. By multiplying the fraction of the component by the total activity of polymerization, the polymerization activities of different types of active center were calculated. As shown in Fig. 5, MWD of the ethylene homopolymer was satisfactorily fitted with four Flory components, but MWD curves of the copolymers were deconvoluted into five Flory components, in which the component A was absent in the homopolymer. As [1hexene] increased from 0.02 to 0.2 mol/L, the catalytic activity of all the active centers increased (see Fig. 6). The increments of the catalytic activity of C*A and CB* were relatively higher than those of C*C, C*D and C*E. It indicates that introduction of 1-hexene activated the active centers producing low molecular weight polymer more than those producing high molecular weight polymer. It should be noted that the catalytic activity of C*A tended to decrease at higher [1-hexene] and finally went down to 0 at [1-hexene] ¼ 0.5 mol/L. Fig. 7 shows comparison of the C7-sol fraction with the weight fraction of Flory component A as well as the sum of components A, B and C. When the 1-hexene concentration was lower than 0.1 mol/ L, the C7-sol fraction was very close to the amount of component A, meaning that it may be mainly composed of copolymer formed by C*A. As [1-hexene] was increased to 0.2 mol/L, the C7-sol fraction (42.1%) exceeded the sum of components A and B (41.5%). When [1hexene] was further increased to 0.5 mol/L, the C7-sol fraction reached the sum of components A, B and C. From this phenomenon, it seems that the copolymers produced by C*B and C*C shifted from the C7-ins to the C7-sol fraction as [1-hexene] gradually increased. At [1-hexene] ¼ 0.5 mol/L, the C7-ins fraction was mainly composed of the copolymer chains produced by C*D and C*E. This explanation is supported by the fact that molecular weights of both C7-sol and C7-ins fractions gradually increased with [1-hexene] (see Table 2). Gradual increase of (kpE)ins and (kpE)sol with [1hexene] is another evidence that supports this explanation. In summary, the five types of active centers differ in ability of incorporating 1-hexene in the order of: C*A > C*B > C*C > C*D > C*E. The order of chain transfer rate should be the same, but the kpE values follow an opposite order of C*A < C*B < C*C < C*D < C*E. Besides the continuous shifting of copolymer chains from C7-ins to C7-sol with the increase of [1-hexene], the total number of active centers also increased with [1-hexene]. As seen in Fig. 2 and Table 3, the [C*]/[Ti] ratio quickly increased to a high level (~40%) when [1hexene] increased from 0 to 0.2 mol/L. During this change of [1hexene], the number of active centers in the C7-ins fraction
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Fig. 5. Deconvolution of MWD of different polymer samples with multiple Flory components. a. [1-hexene] ¼ 0; b. [1-hexene] ¼ 0.02 mol/L; c. [1-hexene] ¼ 0.2 mol/L; d. [1hexene] ¼ 0.5 mol/L.
Table 4 Effects of 1-hexene concentration on molecular weights and contents of Flory components. Run
[1-Hexene] (mol/L)
1
0
2
0.02
3
0.05
4
0.1
5
0.2
6
0.3
7
0.5
Center *
A B* C* D* E* A* B* C* D* E* A* B* C* D* E* A* B* C* D* E* A* B* C* D* E* A* B* C* D* E* A* B* C* D* E*
Mw (105)
Fraction (%)
e 0.89 3.6 9.0 31 0.16 0.54 1.7 4.7 16 0.06 0.31 1.0 3.7 13 0.07 0.26 0.85 3.0 9.0 0.09 0.31 0.98 3.4 11 0.06 0.18 0.58 2.1 6.8 e 0.11 0.35 1.3 4.5
0 12.0 33.9 38.2 15.4 4.1 24.2 30.5 27.5 9.0 7.0 16.9 26.8 33.9 13.4 9.6 19.3 26.8 28.9 14.9 13.4 28.1 23.5 23.6 10.3 4.1 29.0 28.2 22.7 15.4 0 28.3 30.6 22.2 18.4
decreased from 19.4 to 11.7% with a decrement of 7.7%, but the number of active centers in the C7-sol fraction increased from 6.5 to 30.7% with a increment of 24.2%. If the 7.7% active centers reduced from the C7-ins fraction had been added to the C7-sol fraction, the extra 16.5% of active centers in C7-sol at [1-hexene] ¼ 0.2 mol/L should be newly formed or revived from dormant states by the added 1-hexene. Therefore, it is reasonable to say that many active centers with low intrinsic activity, high chain transfer rate and high 1-hexene incorporation rate can be activated or revived by 1hexene. This effect continued when [1-hexene] further increased from 0.2 to 0.5 mol/L (see Table 3). The properties of the newly formed active centers can be roughly estimated from the data in Table 3. For example, when [1hexene] was raised from 0.05 to 0.1 mol/L, [C*]/[Ti] of the C7-sol
Fig. 6. Influence of 1-hexene concentration on the polymerization activity of different Flory components.
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insertion of bulky 1-hexene. This can explain the lowering in kpH during raising 1-hexene concentration. To fully clarify the mechanism of comonomer effect as well as the structure and properties of different types of active centers (including those revived by the comonomer), more direct experimental evidences are to be found. Considering that comonomer incorporation rate and chemical composition distribution are so important for developing high-quality polyolefin materials, it is worthy to make more efforts on solving this problem. Conclusions
Fig. 7. Influence of 1-hexene concentration on the amount of C7-sol fraction, the weight fraction of Flory component A and the sum of components A, B and C.
part was increased from 17.0 to 25.3%, but [C*]/[Ti] of the C7-ins part was only reduced from 18.8 to 16.9%. If 1.9% [C*]/[Ti] was shifted from C7-ins to C7-sol during this rising of [1-hexene], then 6.4% out of 25.3% ([C*]/[Ti] of the C7-sol part at [1hexene] ¼ 0.1 mol/L) should be contributed by the newly formed active centers. kpE of these new centers was estimated to be about 16 L/mol$s, which is much lower than kpE value of the insoluble part. Therefore, the active centers revived by the comonomer should be of relatively low intrinsic activity. There are two possible mechanistic models for formation or activation of active centers by a-olefins. Y. V. Kissin et al. proposed that the comonomer effects in ethylene/1-hexene copolymerization can be explained by activation of TidC2H5 type dormant active sites by 1-hexene [1]. Such dormant state has been identified by DFT calculations on a MgCl2-supported model active species with TieC2H5 bond [28]. This type of dormant sites might be formed by alkylation of the supported TiClX species or by insertion of ethylene in TidH. M.P. McDaniel et al. [20], F.J. Karol et al. [29] and Ystenes [30] proposed a “trigger mechanism’’ to explain the comonomer effect. They suggested that certain types of active centers with two vacancies can be coordinated by two monomers, and the coordination of the second monomer was proposed to ‘‘trigger’’ the insertion of the first. It is still impossible to make definitive judgment on the two explanations based on the results in this work. As mentioned before, the quick increase of kpE and decrease of kpH in raising [1hexene] from 0 to 0.1 mol/L were accompanied by strong increase in the number of active centers (see Figs. 2 and 3). These phenomena are more or less in keeping with the “trigger mechanism’’, as the ‘‘trigger’’ effect requires only small amount of a-olefin. According to this mechanism, the active centers revived by 1-hexene may have coordinated two monomers, in which one may be inserted into the propagation chain. When one ethylene and one 1hexene molecule were coordinated with such an active center, ethylene can be quickly inserted while the 1-hexene plays the role of triggering the active center. It means that without the coordinated 1-hexene, the active center will be inactive or stay in a dormant state. When two 1-hexene molecules were coordinated in such active center, insertion of 1-hexene in the propagation chain could be relatively slower as compared to the active centers without two coordinated monomers, because stereochemical hindrance caused by the coordinated monomer may retard the
In copolymerization of ethylene and 1-hexene with TiCl4/ MgCl2eAlEt3 type ZieglereNatta catalyst, the polymerization activity was significantly enhanced by increasing the concentration of 1-hexene, meanwhile the number of active centers also increased. Large number of active centers were formed or revived by adding small amount of 1-hexene comonomer. When 1-hexene concentration of the copolymerization system was increased, the number of active centers producing polymer chains with lower molecular weight and higher 1-hexene content was increased more than those producing polymer chains with higher molecular weight and lower 1-hexene content. As a result, the molecular weight distribution curve continuously inclined to the low molecular weight side. The active centers with higher 1-hexene incorporation rate have relatively smaller rate constant of ethylene insertion. When the 1-hexene concentration was increased, the average rate constant of ethylene insertion was only slightly changed, but the average rate constant of 1-hexene insertion was markedly lowered, meaning that the active centers revived by 1-hexene have relatively lower ability of incorporating 1-hexene. Experimental Materials All reactions and manipulations were carried out under dry nitrogen atmosphere. TiCl4 (Sinopharm Chemical Reagent Co., Ltd.) was purified by vacuum distillation. Anhydrous ethanol (Titan) was purified by distillation and dried with molecular sieves prior to use. 1-Hexene (Acros Organics), n-heptane (Titan) and toluene (Titan) were purified by refluxing over sodium for 6 h and distilled ahead of use. Al(C2H5)3 (TEA, Fluka) was diluted in n-heptane to 2 M. 2Thiophenecarbonyl chloride (TPCC, Alfa Aesar) was diluted in toluene to 2 M. Ethylene (polymerization grade product, Hangzhou Minxing Chemical Technology Co., Ltd.) was used after passing through a column filled with deoxygen reagent and molecular sieves. Anhydrous MgCl2 (Alfa Aesar) was used without any further purification. Other reagents were analytical pure and used as received. Preparation and characterization of catalyst Anhydrous MgCl2 and n-heptane as solvent were added into a two-neck Schlenk flask which was first evacuated and filled with pure nitrogen (99.999%) and stirred sufficiently. Subsequently, a designed volume of anhydrous ethanol (n(EtOH): n(MgCl2) ¼ 4:1) was injected into the flask immersed in a 120 C oil bath. After refluxing for 2 h with magnetic stirring, the products were cooled to room temperature, washed with n-heptane and dried in vacuum at 60 C. The dried MgCl2$nEtOH adduct powder was added into a twoneck Schlenk flask immersed in an ice water bath under the protection of pure nitrogen and fully dispersed in n-heptane by stirring. Then a large excess of titanium tetrachloride (n(Ti):
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n(Mg) ¼ 20:1) was slowly added into the system. The reactor was then transferred to an oil bath. The temperature was gradually raised to 120 C and kept for 2 h with magnetic stirring. Then the flask was cooled to room temperature and transferred to an ice water bath. The liquid in the mixture was removed and the remaining solid was washed three times with dry n-heptane. Then the second crop of titanium tetrachloride (n(Ti): n(Mg) ¼ 20:1) was added to the flask and the above operations were repeated. Finally the solid product was collected and dried in vacuum at 60 C. The content of ethanol in the MgCl2$nEtOH adduct was analyzed by gas chromatography. The average value of n in the MgCl2$nEtOH adduct was 2.41. The content of Ti in the catalyst was determined by conventional spectrophotometry method [31]. Ti contents of the catalyst was 8.53 wt. %. Polymerization and quenching reaction All of the polymerizations were carried out at 50 C in a 100 mL round-bottom flask equipped with magnetic stirrer. The flask immersed in a 50 C thermostat bath was first evacuated and purged with pure nitrogen (99.999%) three times, and then filled with ethylene at 1 atm. n-Heptane as solvent, a designated volume of 1-hexene and Al(C2H5)3 were successively added into the reactor. When the system reached a stable temperature, 15 mg of catalyst was added to start the polymerization. Ethylene at 1.1 atm was continuously supplied into the flask to maintain a constant pressure. 30 min later, ethylene was stopped and a designated volume of TPCC was added into the flask to quench the polymerization. After 5 min, the reaction system was poured into 250 mL ethanol containing 1 mL hydrochloric acid to eliminate the catalyst and settling down the polymer. The polymer particles were filtered, washed with ethanol and dried in vacuum at 50 C. To thoroughly purify the polymer samples for making precise measurement of the number of active centers ([C*]/[Ti]), each polymer sample was treated by refluxing in excess of ethanol/HCl mixture for 60 min, and then purified by one dissolution-precipitation operation using n-octane as the solvent and isopropanol as the precipitant. The polymer was then extracted with dry ethanol in a Soxhlet extractor for 6 h, and dried in vacuum at 60 C. Polymer characterization Each copolymer sample was extracted with boiling n-heptane in a Soxhlet extractor for 12 h, and was fractionated into the boiling nheptane soluble fraction (C7-sol) and insoluble fraction (C7-ins). The content of 1-hexene in the copolymer as well as the copolymer fractions was measured by FTIR spectra. About 30e50 mg polymer powder was used to make polymer film with a Universal Film Maker at 160 C. All FTIR spectra were recorded in a NICOLET 6700 spectrometer. Typical IR acquisition parameters were: 1 cm1 resolution, 64 scans for each sample, 1300e1600 cm1 spectral region. The absorbances at 1377 cm1 (CH3 symmetrical bending) and 1462 cm1 (CH2 scissoring) were used to make quantitative analysis. The standard curve correlating the absorbance ratio A1377/A1462 and 1-hexene content has been prepared using a series of ethylene/1-hexene copolymer samples with known 1-hexene content determined by 13C NMR. The 1hexene content of unknown sample was calculated using the standard curve. Thermal analysis of the polymer samples was performed on a TA Q100 thermal analyzer. The polymer (about 4 mg) was sealed in aluminum pan, heated to 170 C at 10 K/min, kept at that temperature for 5 min, and then cooled to 40 C at 10 K/min and kept for 5 min. Then the sample was scanned from 40 C to 170 C at a heating rate of 10 K/min, and the DSC trace was recorded.
7
Molecular weight and molecular weight distribution (MWD) of polymer samples were measured with gel permeation chromatography (GPC) in a PL-220 GPC instrument (Polymer Laboratories) at 150 C. 1,2,4-Trichlorobenzene stabilized with 125 ppm BHT was used as the mobile phase. Three PL mixed B columns (500e107) were used. Universal calibration against narrow polystyrene standards was adopted to calculate the MWD curve. Sulfur content of the quenched polymers and their fractions was measured in a GLC-200 microcoulometry sulfur analyzer with a lower detection limit of 0.05 ppm (Jiangyan Yinhe Instrument Co., Jiangyan, China). The polymer sample for analysis was solid powder (2e4 mg, weighed to ±0.01 mg), and the average value of three parallel measurements was taken as the sulfur content. Because the polymerization was quenched with 2-thiophenecarbonyl chloride, each propagation chain (the polymer chain connecting with the titanium of active center) was labeled with a thiophenyl group [21,22]. Thus the mole number of sulfur in the sample equals to the number of active centers.
Acknowledgment Support by the National Natural Science Foundation of China (grant no. 21374094) and the Major State Basic Research Programs (grant no. 2011CB606001) is gratefully acknowledged.
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Effects of comonomer on active center distribution of TCl4/ MgCl2eAlEt3 catalyst in ethylene/1-hexene copolymerization Tao Xu, Hongrui Yang, Zhisheng Fu, Zhiqiang Fan* MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 December 2014 Received in revised form 25 February 2015 Accepted 6 April 2015 Available online xxx
Ethylene/1-hexene copolymerization with MgCl2/TiCl4eAlEt3 catalyst has been conducted under different initial 1-hexene concentration (0e0.5 mol/L). Number of polymerization active centers was determined by quenching the reaction with 2-thiophenecarbonyl chloride and measuring sulfur content of the quenched polymer. Each copolymer sample was fractionated into boiling n-heptane soluble and insoluble fractions, and active centers in these fractions were also counted. The rate constants of ethylene and 1-hexene insertion in the active centers were calculated, respectively. Molecular weight distribution (MWD) curves of the polymers were deconvoluted with 4e5 Flory components, and changes of activity of the Flory components with [1-hexene] were analyzed. The polymerization activity and the number of active centers were significantly enhanced by increasing [1-hexene]. Large number of active centers were revived by small amount of 1-hexene. With the increase of [1-hexene], the number of active centers producing polymer chains with lower molecular weight and higher 1-hexene content was increased more than those producing polymer chains with higher molecular weight and lower 1-hexene content, and the MWD curve continuously inclined to the low molecular weight side. The active centers with higher 1-hexene incorporation rate have relatively smaller rate constant of ethylene insertion. When [1hexene] was increased, the rate constant of ethylene insertion was only slightly changed, but the rate constant of 1-hexene insertion was markedly lowered, meaning that the active centers revived by 1hexene have relatively lower ability of incorporating 1-hexene. © 2015 Elsevier B.V. All rights reserved.
Keywords: Ethylene 1-Hexene Comonomer effect ZieglereNatta catalyst Active center distribution
Introduction Copolymers of ethylene with a-olefins (propylene, 1-butene, 1hexene etc.) are important polymer materials with wide applications. At present, more than 80% of these copolymers are produced by MgCl2-supported ZieglereNatta catalysts. As an important feature, ZieglereNatta catalysts produce ethylene-a-olefin copolymer with very broad chemical composition distribution (CCD), which is caused by multiple active centers in the catalyst, and the aolefin units are highly enriched in the part of polymer with low molecular weight [1e7]. The broad CCD strongly influences the application properties of the copolymer. As an example, in the production of high density polyethylene (HDPE) resin by copolymerizing ethylene with small amount of a-olefin, increasing comonomer content of the high molecular weight part while
* Corresponding author. E-mail address: [email protected] (Z. Fan).
keeping low a-olefin content in the low molecular weight part can greatly improve the mechanical properties of the HDPE resin. In ethylene/a-olefin copolymerization with ZieglereNatta catalysts, metallocene catalysts or Cr-based Phillips catalysts, enhancement of reaction activity by the a-olefin comonomer (socalled “comonomer effect”) has long been reported and studied [1,2,8e20]. Though several theories have been developed to rationalize this kind of kinetic phenomenon, there is still not a generally accepted explanation or mechanistic model. In our previous work, strong influences of a-olefin comonomer on the polymer molecular weight distribution (MWD) had been reported [18]. In ethylene-1-hexene copolymerization with a MgCl2/TiCl4 type Z-N catalyst, besides the strong activity enhancement, small amount of 1-hexene ([1-Hexene] ¼ 0.05 M) also caused strong broadening and shifting of the MWD curve as compared to the ethylene homopolymerization system [18]. This was explained by unequal activation of different types of active centers by the a-olefin. The active centers that produce low molecular weight polymer were activated more than those produce
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high molecular weight polymer. In other words, the comonomer also influences the distribution of active centers in the catalyst. This result was in keeping with the explanation of comonomer effect based on chemical sources, rather than physical sources. However, only the changes in MWD curve are not enough to disclose the full scenario of comonomer effect. In this paper, comonomer effect in ethylene/1-hexene copolymerization with a MgCl2/TiCl4eAlEt3 catalysts has been investigated, with special attention being paid to the influences of 1hexene on the distribution and intrinsic activity of the multiple types of active centers. A method using 2-thiophenecarbonyl chloride as quenching reagent has been adopted for counting the active centers in two copolymer fractions [21,22]. The results obtained in this work can be new evidences for understanding the mechanism of the comonomer effect, and can provide clues for figuring out structure of different types of active centers.
Results and discussion
Fig. 1. Influence of initial 1-hexene concentration on polymerization activity and copolymer composition.
Copolymerization activity and CCD of the copolymer Using a TiCl4/MgCl2 catalyst activated by TEA, a series of ethylene/1-hexene copolymerization were conducted under 1 atm ethylene pressure and different initial 1-hexene concentrations. All the copolymer samples were fractionated into two parts: the boiling n-heptane soluble fraction (C7-sol) and insoluble fraction (C7-ins). The changes of copolymerization activity and fraction weight of C7-sol and C7-ins with 1-hexene concentration are shown in Table 1. It is seen that the activity increased with the 1hexene concentration, and was more than tripled when [1hexene] was raised to 0.5 mol/L. The comonomer activation effect was evident even at [1-hexene] as low as 0.02 mol/L (see Fig. 1). Meanwhile, the weight fraction of C7-sol also quickly increased with [1-hexene]. This can be attributed to increase of the copolymer's 1-hexene content (see Fig. 1 and Table 2). The 1-hexene content in the C7-sol fraction was always much higher than that in the C7-ins fraction, meaning that incorporation of enough 1hexene in the polymer chains can make them soluble in boiling n-heptane. The gradual decrease of melting enthalpy with [1hexene] (see Table 1) also indicates that the incorporated 1hexene units can disturb crystallization of the polymer chains and increase the amount of amorphous phase that is soluble in nheptane. The 1-hexene content of the C7-ins fractions were all lower than 3 mol-%, while 1-hexene content of the C7-sol fractions were all higher than 4 mol-%, so the critical 1-hexene content determining the solubility of copolymer chains should be between 3 and 4 mol-%. Because the difference in 1-hexene content of the C7-sol and C7-ins fractions was evidently enlarged with increasing
[1-hexene], the CCD of the copolymer became broader at high 1hexene incorporate rate. The molecular weight of the polymer samples and their fractions were measured by GPC, and the results are summarized in Table 2. The molecular weight of the whole polymer sample decreased sharply with increasing [1-hexene], but molecular weights of both the C7-sol and C7-ins fractions gradually increased. Because the molecular weights of C7-sol were much lower than that of the C7-ins fraction, the decrease of molecular weight of the whole polymer sample is mainly caused by increase of the C7-sol fraction. Number of active centers and chain propagation rate constant The number of active centers in the whole polymer sample and their fractions has been determined by a method based on quenching the catalysis system with TPCC, and the results are listed in Table 3. As seen in Table 3 and Fig. 2, the total number of active centers ([C*]/[Ti], where C* represents active center) increased sharply with [1-hexene], implying that introducing 1-hexene in the catalysis system caused formation of new active centers or revival of some types of dormant active sites. The comonomer effect can thus be largely attributed to this activation of dormant centers. The number of active centers in the C7-sol fraction also increased with [1-hexene], but that of the C7-ins fraction tended to decrease with [1-hexene]. This is understandable when we know that the weight fraction of C7-ins decreases with [1-hexene].
Table 1 Results of copolymerization under different 1-hexene concentration.a Run
[1-Hexene] (mol/L)
Yield (g)
Activity (kg/gTi h)
Tmb ( C)
△Hmc (J/g)
C7-sold (wt-%)
C7-inse (wt-%)
1 2 3 4 5 6 7
0 0.02 0.05 0.1 0.2 0.3 0.5
1.60 4.42 4.25 4.93 5.15 5.25 4.72
2.56 3.68 5.69 6.73 7.36 7.69 8.20
134.1 127.8 126.1 125.5 124.8 124.3 124.0
178.6 152.7 144.5 131.4 113.7 112.2 67.4
e 4.9 10.1 20.6 42.1 51.1 60.2
e 95.1 89.9 79.4 57.9 48.9 39.8
a b c d e
Polymerization conditions: Al/Ti ¼ 100 (mol/mol), TP ¼ 50 C, Pethylene ¼ 1 atm, tP ¼ 30 min. Melting temperature. Melting enthalpy. Fraction of boiling n-heptane soluble part. Fraction of boiling n-heptane insoluble part.
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Table 2 Composition and molecualar weight of the polymer samples and their fractions. Run
[1-Hexene] (mol/L)
1 2 3 4 5 6 7 a b
0 0.02 0.05 0.1 0.2 0.3 0.5
Mw (105)
Hexene content (mol-%)
PDIa
Whole
C7-sol
C7-insol
Whole
C7-sol
C7-ins
Whole
0 2.3 3.0 3.5 4.6 7.0 12
e –b –b –b 7.5 12 20
e 2.2 2.3 2.6 2.7 2.8 3.0
8.1 3.0 3.3 2.3 2.1 1.7 1.2
e 0.07 0.14 0.19 0.36 0.35 0.34
e 2.5 2.8 2.5 4.1 3.9 4.0
5.3 8.0 14.9 10.7 10.9 8.7 8.0
Polydispersity index of the polymer. Not determined.
Table 3 Number of active centers and chain propagation rate constants of the polymer samples and their fractions. Run
1 2 3 4 5 6 7 a b c
[H]a mol/L
Whole [C*]/[Ti] (mol-%)
kpEb (L/mol$s)
kpHc (L/mol$s)
[C*]/[Ti] (mol-%)
kpEb (L/mol$s)
kpHc (L/mol$s)
[C*]/[Ti] (mol-%)
kpEb (L/mol$s)
kpHc (L/mol$s)
0 0.02 0.05 0.1 0.2 0.3 0.5
13.0 25.9 35.8 42.2 42.4 46.8 57.2
75 81 89 88 81 82 73
e 7.4 4.3 2.5 1.5 1.6 1.6
e 6.5 17.0 25.3 30.7 36.8 47.6
e 15 15 27 43 46 42
e e e e 1.4 1.6 1.7
e 19.4 18.8 16.9 11.7 10.0 9.6
e 103 155 178 179 211 222
e 9.0 5.7 3.7 1.9 1.6 1.1
C7-sol
C7-ins
Initial [1-hexene]. Propagation rate constant of ethylene insertions. Propagation rate constant of 1-hexene insertions.
to form thick polymer layer around each fragment of the catalyst particle. This will make the actual [Mi] at the location of the active centers much lower than that in the bulk of reaction medium. Therefore, the kp values observed in this work (see Table 3) were much smaller than those measured by stopped-flow method for similar catalytic systems [23], as the polymerization duration in the stopped-flow experiment was only about 0.1 s. However, the kp values obtained in this work can still reflect the relative changes in intrinsic activity of the active centers. The propagation rate constants of ethylene insertion (kpE) and 1-hexene insertion (kpH) of the whole copolymerization system and those of the soluble and insoluble fractions are shown in Table 3, Fig. 3 and Fig. 4. In Fig. 3 we can find that the rate constant of ethylene insertion (kpE) slightly increased when [1-hexene] increased from 0 to
Fig. 2. Influence of 1-hexene concentration on the number of active centers in the whole polymerization system, the boiling n-heptane soluble fraction and the boiling nheptane insoluble fraction.
The propagation rate constants of both ethylene and 1-hexene insertion in the active centers can be calculated by the equation Rpi ¼ kpi [C*][Mi] (i ¼ ethylene or 1-hexene), where Rpi is the insertion rate of monomer Mi. For the copolymerization runs, the Rpi was calculated from the copolymer yield and the copolymer composition data. Because of the difficulty in estimating the real monomer concentrations at the site of active center, we can only use the equilibrium ethylene concentration and initial 1-hexene concentration in the calculations. It can be expected that the diffusion barrier in the catalyst-polymer particles was rather large, because the polymerization reactions were all continued for 30 min
Fig. 3. Influence of 1-hexene concentration on rate constant of ethylene insertion and 1-hexene insertion.
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the copolymer chains moved to the C7-sol part at higher [1-hexene] were produced by active centers with larger kpE, the average kpE of the C7-sol part will be increased. Increase of kpE of the C7-ins part with [1-hexene] means that the remaining copolymer chains of C7ins were produced by active centers with even larger kpE. Therefore, there should be a nearly continuous distribution of active centers that have different kpE and kpH values. Distribution of active centers based on deconvolution of MWD curves
Fig. 4. Influence of 1-hexene concentration on rate constants of ethylene insertion and 1-hexene insertion in C7-sol and C7-ins fractions.
0.05 mol/L, and then slowly decreased as [1-hexene] further increased to 0.5 mol/L. Meanwhile, the rate constant of 1-hexene insertion (kpH) sharply decreased when [1-hexene] increased from 0 to 0.1 mol/L, and then leveled off. This means that small amount of 1-hexene can strongly influence the properties of the active centers. Because [C*]/[Ti] was sharply increased when [1hexene] increased from 0 to 0.1 mol/L, the quick changes of kpE and kpH with [1-hexene] could be attributed to the emerged active centers. They may have larger kpE and smaller kpH than those original active centers present in the ethylene homopolymerization system. This phenomenon is quite interesting, and the mechanism will be discussed later in this article. Because diffusion barrier in the catalyst particles tend to decrease with [1-hexene] as a result of increasing copolymer solubility in the reaction medium, such decrease of kpH with [1-hexene] could be only explained by change of the active centers. Fig. 4 shows the changes of kpE and kpH of both fractions with [1hexene]. The following trends can be concluded: (1) In both the C7sol and C7-ins fractions, the rate constants of ethylene insertion were all much larger than those of 1-hexene insertion; (2) The rate constants of ethylene insertion (kpE) of the insoluble fraction was much larger than kpE of the soluble fraction, but the kpH values of the two fractions were only a little different. As a result, the kpE/kpH ratio of C7-sol was much smaller than that of the C7-ins fraction; (3) kpE in both fractions continuously increase with the 1-hexene concentration, meanwhile kpH continuously decrease with the 1hexene concentration. These results provided important clues that disclose the nature of the two groups of active centers producing the soluble and insoluble copolymer chains. Because kpE/kpH ratio is reversely proportional to the 1-hexene incorporation rate, it is clear that the active centers producing the C7-sol part have much stronger ability of incorporating 1-hexene in the copolymer chains than the active centers producing the C7-ins part. On the other hand, the much larger (kpE)ins than (kpE)sol means that ethylene insertion in the active centers forming the C7-ins chains are much faster than the centers of C7-sol part. It is the faster ethylene insertion that caused larger kpE/kpH ratio and lower 1-hexene content in the C7-ins chains. The continuous changes of (kpE)ins, (kpE)sol, (kpH)ins and (kpH)sol with [1-hexene] can be understood from the change of solubility of the C7-ins chains. As [1-hexene] increases, 1-hexene content of copolymer chains formed on all the active centers will increase, moving more copolymer chains into the soluble fraction. Because
Deconvolution of MWD curves of the seven polymer samples were made to study the active center distribution in details [1,24e27]. As shown in Fig. 5, all the MWD curves were deconvoluted into 5 or 4 Flory components, and these components were named as component A, B, C, D and E, respectively. Each Flory component corresponds to polymer produced by a certain type of active center [24e27]. Thus there were five types of active centers, C*A, C*B, C*C, CD* and C*E. Among them C*E produces polymer with the highest molecular weight and C*A produces polymer with the lowest molecular weight. The results of MWD deconvolution, namely the weight fraction and weight average molecular weight of each Flory component, are summarized in Table 4. By multiplying the fraction of the component by the total activity of polymerization, the polymerization activities of different types of active center were calculated. As shown in Fig. 5, MWD of the ethylene homopolymer was satisfactorily fitted with four Flory components, but MWD curves of the copolymers were deconvoluted into five Flory components, in which the component A was absent in the homopolymer. As [1hexene] increased from 0.02 to 0.2 mol/L, the catalytic activity of all the active centers increased (see Fig. 6). The increments of the catalytic activity of C*A and CB* were relatively higher than those of C*C, C*D and C*E. It indicates that introduction of 1-hexene activated the active centers producing low molecular weight polymer more than those producing high molecular weight polymer. It should be noted that the catalytic activity of C*A tended to decrease at higher [1-hexene] and finally went down to 0 at [1-hexene] ¼ 0.5 mol/L. Fig. 7 shows comparison of the C7-sol fraction with the weight fraction of Flory component A as well as the sum of components A, B and C. When the 1-hexene concentration was lower than 0.1 mol/ L, the C7-sol fraction was very close to the amount of component A, meaning that it may be mainly composed of copolymer formed by C*A. As [1-hexene] was increased to 0.2 mol/L, the C7-sol fraction (42.1%) exceeded the sum of components A and B (41.5%). When [1hexene] was further increased to 0.5 mol/L, the C7-sol fraction reached the sum of components A, B and C. From this phenomenon, it seems that the copolymers produced by C*B and C*C shifted from the C7-ins to the C7-sol fraction as [1-hexene] gradually increased. At [1-hexene] ¼ 0.5 mol/L, the C7-ins fraction was mainly composed of the copolymer chains produced by C*D and C*E. This explanation is supported by the fact that molecular weights of both C7-sol and C7-ins fractions gradually increased with [1-hexene] (see Table 2). Gradual increase of (kpE)ins and (kpE)sol with [1hexene] is another evidence that supports this explanation. In summary, the five types of active centers differ in ability of incorporating 1-hexene in the order of: C*A > C*B > C*C > C*D > C*E. The order of chain transfer rate should be the same, but the kpE values follow an opposite order of C*A < C*B < C*C < C*D < C*E. Besides the continuous shifting of copolymer chains from C7-ins to C7-sol with the increase of [1-hexene], the total number of active centers also increased with [1-hexene]. As seen in Fig. 2 and Table 3, the [C*]/[Ti] ratio quickly increased to a high level (~40%) when [1hexene] increased from 0 to 0.2 mol/L. During this change of [1hexene], the number of active centers in the C7-ins fraction
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5
Fig. 5. Deconvolution of MWD of different polymer samples with multiple Flory components. a. [1-hexene] ¼ 0; b. [1-hexene] ¼ 0.02 mol/L; c. [1-hexene] ¼ 0.2 mol/L; d. [1hexene] ¼ 0.5 mol/L.
Table 4 Effects of 1-hexene concentration on molecular weights and contents of Flory components. Run
[1-Hexene] (mol/L)
1
0
2
0.02
3
0.05
4
0.1
5
0.2
6
0.3
7
0.5
Center *
A B* C* D* E* A* B* C* D* E* A* B* C* D* E* A* B* C* D* E* A* B* C* D* E* A* B* C* D* E* A* B* C* D* E*
Mw (105)
Fraction (%)
e 0.89 3.6 9.0 31 0.16 0.54 1.7 4.7 16 0.06 0.31 1.0 3.7 13 0.07 0.26 0.85 3.0 9.0 0.09 0.31 0.98 3.4 11 0.06 0.18 0.58 2.1 6.8 e 0.11 0.35 1.3 4.5
0 12.0 33.9 38.2 15.4 4.1 24.2 30.5 27.5 9.0 7.0 16.9 26.8 33.9 13.4 9.6 19.3 26.8 28.9 14.9 13.4 28.1 23.5 23.6 10.3 4.1 29.0 28.2 22.7 15.4 0 28.3 30.6 22.2 18.4
decreased from 19.4 to 11.7% with a decrement of 7.7%, but the number of active centers in the C7-sol fraction increased from 6.5 to 30.7% with a increment of 24.2%. If the 7.7% active centers reduced from the C7-ins fraction had been added to the C7-sol fraction, the extra 16.5% of active centers in C7-sol at [1-hexene] ¼ 0.2 mol/L should be newly formed or revived from dormant states by the added 1-hexene. Therefore, it is reasonable to say that many active centers with low intrinsic activity, high chain transfer rate and high 1-hexene incorporation rate can be activated or revived by 1hexene. This effect continued when [1-hexene] further increased from 0.2 to 0.5 mol/L (see Table 3). The properties of the newly formed active centers can be roughly estimated from the data in Table 3. For example, when [1hexene] was raised from 0.05 to 0.1 mol/L, [C*]/[Ti] of the C7-sol
Fig. 6. Influence of 1-hexene concentration on the polymerization activity of different Flory components.
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T. Xu et al. / Journal of Organometallic Chemistry xxx (2015) 1e7
insertion of bulky 1-hexene. This can explain the lowering in kpH during raising 1-hexene concentration. To fully clarify the mechanism of comonomer effect as well as the structure and properties of different types of active centers (including those revived by the comonomer), more direct experimental evidences are to be found. Considering that comonomer incorporation rate and chemical composition distribution are so important for developing high-quality polyolefin materials, it is worthy to make more efforts on solving this problem. Conclusions
Fig. 7. Influence of 1-hexene concentration on the amount of C7-sol fraction, the weight fraction of Flory component A and the sum of components A, B and C.
part was increased from 17.0 to 25.3%, but [C*]/[Ti] of the C7-ins part was only reduced from 18.8 to 16.9%. If 1.9% [C*]/[Ti] was shifted from C7-ins to C7-sol during this rising of [1-hexene], then 6.4% out of 25.3% ([C*]/[Ti] of the C7-sol part at [1hexene] ¼ 0.1 mol/L) should be contributed by the newly formed active centers. kpE of these new centers was estimated to be about 16 L/mol$s, which is much lower than kpE value of the insoluble part. Therefore, the active centers revived by the comonomer should be of relatively low intrinsic activity. There are two possible mechanistic models for formation or activation of active centers by a-olefins. Y. V. Kissin et al. proposed that the comonomer effects in ethylene/1-hexene copolymerization can be explained by activation of TidC2H5 type dormant active sites by 1-hexene [1]. Such dormant state has been identified by DFT calculations on a MgCl2-supported model active species with TieC2H5 bond [28]. This type of dormant sites might be formed by alkylation of the supported TiClX species or by insertion of ethylene in TidH. M.P. McDaniel et al. [20], F.J. Karol et al. [29] and Ystenes [30] proposed a “trigger mechanism’’ to explain the comonomer effect. They suggested that certain types of active centers with two vacancies can be coordinated by two monomers, and the coordination of the second monomer was proposed to ‘‘trigger’’ the insertion of the first. It is still impossible to make definitive judgment on the two explanations based on the results in this work. As mentioned before, the quick increase of kpE and decrease of kpH in raising [1hexene] from 0 to 0.1 mol/L were accompanied by strong increase in the number of active centers (see Figs. 2 and 3). These phenomena are more or less in keeping with the “trigger mechanism’’, as the ‘‘trigger’’ effect requires only small amount of a-olefin. According to this mechanism, the active centers revived by 1-hexene may have coordinated two monomers, in which one may be inserted into the propagation chain. When one ethylene and one 1hexene molecule were coordinated with such an active center, ethylene can be quickly inserted while the 1-hexene plays the role of triggering the active center. It means that without the coordinated 1-hexene, the active center will be inactive or stay in a dormant state. When two 1-hexene molecules were coordinated in such active center, insertion of 1-hexene in the propagation chain could be relatively slower as compared to the active centers without two coordinated monomers, because stereochemical hindrance caused by the coordinated monomer may retard the
In copolymerization of ethylene and 1-hexene with TiCl4/ MgCl2eAlEt3 type ZieglereNatta catalyst, the polymerization activity was significantly enhanced by increasing the concentration of 1-hexene, meanwhile the number of active centers also increased. Large number of active centers were formed or revived by adding small amount of 1-hexene comonomer. When 1-hexene concentration of the copolymerization system was increased, the number of active centers producing polymer chains with lower molecular weight and higher 1-hexene content was increased more than those producing polymer chains with higher molecular weight and lower 1-hexene content. As a result, the molecular weight distribution curve continuously inclined to the low molecular weight side. The active centers with higher 1-hexene incorporation rate have relatively smaller rate constant of ethylene insertion. When the 1-hexene concentration was increased, the average rate constant of ethylene insertion was only slightly changed, but the average rate constant of 1-hexene insertion was markedly lowered, meaning that the active centers revived by 1-hexene have relatively lower ability of incorporating 1-hexene. Experimental Materials All reactions and manipulations were carried out under dry nitrogen atmosphere. TiCl4 (Sinopharm Chemical Reagent Co., Ltd.) was purified by vacuum distillation. Anhydrous ethanol (Titan) was purified by distillation and dried with molecular sieves prior to use. 1-Hexene (Acros Organics), n-heptane (Titan) and toluene (Titan) were purified by refluxing over sodium for 6 h and distilled ahead of use. Al(C2H5)3 (TEA, Fluka) was diluted in n-heptane to 2 M. 2Thiophenecarbonyl chloride (TPCC, Alfa Aesar) was diluted in toluene to 2 M. Ethylene (polymerization grade product, Hangzhou Minxing Chemical Technology Co., Ltd.) was used after passing through a column filled with deoxygen reagent and molecular sieves. Anhydrous MgCl2 (Alfa Aesar) was used without any further purification. Other reagents were analytical pure and used as received. Preparation and characterization of catalyst Anhydrous MgCl2 and n-heptane as solvent were added into a two-neck Schlenk flask which was first evacuated and filled with pure nitrogen (99.999%) and stirred sufficiently. Subsequently, a designed volume of anhydrous ethanol (n(EtOH): n(MgCl2) ¼ 4:1) was injected into the flask immersed in a 120 C oil bath. After refluxing for 2 h with magnetic stirring, the products were cooled to room temperature, washed with n-heptane and dried in vacuum at 60 C. The dried MgCl2$nEtOH adduct powder was added into a twoneck Schlenk flask immersed in an ice water bath under the protection of pure nitrogen and fully dispersed in n-heptane by stirring. Then a large excess of titanium tetrachloride (n(Ti):
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T. Xu et al. / Journal of Organometallic Chemistry xxx (2015) 1e7
n(Mg) ¼ 20:1) was slowly added into the system. The reactor was then transferred to an oil bath. The temperature was gradually raised to 120 C and kept for 2 h with magnetic stirring. Then the flask was cooled to room temperature and transferred to an ice water bath. The liquid in the mixture was removed and the remaining solid was washed three times with dry n-heptane. Then the second crop of titanium tetrachloride (n(Ti): n(Mg) ¼ 20:1) was added to the flask and the above operations were repeated. Finally the solid product was collected and dried in vacuum at 60 C. The content of ethanol in the MgCl2$nEtOH adduct was analyzed by gas chromatography. The average value of n in the MgCl2$nEtOH adduct was 2.41. The content of Ti in the catalyst was determined by conventional spectrophotometry method [31]. Ti contents of the catalyst was 8.53 wt. %. Polymerization and quenching reaction All of the polymerizations were carried out at 50 C in a 100 mL round-bottom flask equipped with magnetic stirrer. The flask immersed in a 50 C thermostat bath was first evacuated and purged with pure nitrogen (99.999%) three times, and then filled with ethylene at 1 atm. n-Heptane as solvent, a designated volume of 1-hexene and Al(C2H5)3 were successively added into the reactor. When the system reached a stable temperature, 15 mg of catalyst was added to start the polymerization. Ethylene at 1.1 atm was continuously supplied into the flask to maintain a constant pressure. 30 min later, ethylene was stopped and a designated volume of TPCC was added into the flask to quench the polymerization. After 5 min, the reaction system was poured into 250 mL ethanol containing 1 mL hydrochloric acid to eliminate the catalyst and settling down the polymer. The polymer particles were filtered, washed with ethanol and dried in vacuum at 50 C. To thoroughly purify the polymer samples for making precise measurement of the number of active centers ([C*]/[Ti]), each polymer sample was treated by refluxing in excess of ethanol/HCl mixture for 60 min, and then purified by one dissolution-precipitation operation using n-octane as the solvent and isopropanol as the precipitant. The polymer was then extracted with dry ethanol in a Soxhlet extractor for 6 h, and dried in vacuum at 60 C. Polymer characterization Each copolymer sample was extracted with boiling n-heptane in a Soxhlet extractor for 12 h, and was fractionated into the boiling nheptane soluble fraction (C7-sol) and insoluble fraction (C7-ins). The content of 1-hexene in the copolymer as well as the copolymer fractions was measured by FTIR spectra. About 30e50 mg polymer powder was used to make polymer film with a Universal Film Maker at 160 C. All FTIR spectra were recorded in a NICOLET 6700 spectrometer. Typical IR acquisition parameters were: 1 cm1 resolution, 64 scans for each sample, 1300e1600 cm1 spectral region. The absorbances at 1377 cm1 (CH3 symmetrical bending) and 1462 cm1 (CH2 scissoring) were used to make quantitative analysis. The standard curve correlating the absorbance ratio A1377/A1462 and 1-hexene content has been prepared using a series of ethylene/1-hexene copolymer samples with known 1-hexene content determined by 13C NMR. The 1hexene content of unknown sample was calculated using the standard curve. Thermal analysis of the polymer samples was performed on a TA Q100 thermal analyzer. The polymer (about 4 mg) was sealed in aluminum pan, heated to 170 C at 10 K/min, kept at that temperature for 5 min, and then cooled to 40 C at 10 K/min and kept for 5 min. Then the sample was scanned from 40 C to 170 C at a heating rate of 10 K/min, and the DSC trace was recorded.
7
Molecular weight and molecular weight distribution (MWD) of polymer samples were measured with gel permeation chromatography (GPC) in a PL-220 GPC instrument (Polymer Laboratories) at 150 C. 1,2,4-Trichlorobenzene stabilized with 125 ppm BHT was used as the mobile phase. Three PL mixed B columns (500e107) were used. Universal calibration against narrow polystyrene standards was adopted to calculate the MWD curve. Sulfur content of the quenched polymers and their fractions was measured in a GLC-200 microcoulometry sulfur analyzer with a lower detection limit of 0.05 ppm (Jiangyan Yinhe Instrument Co., Jiangyan, China). The polymer sample for analysis was solid powder (2e4 mg, weighed to ±0.01 mg), and the average value of three parallel measurements was taken as the sulfur content. Because the polymerization was quenched with 2-thiophenecarbonyl chloride, each propagation chain (the polymer chain connecting with the titanium of active center) was labeled with a thiophenyl group [21,22]. Thus the mole number of sulfur in the sample equals to the number of active centers.
Acknowledgment Support by the National Natural Science Foundation of China (grant no. 21374094) and the Major State Basic Research Programs (grant no. 2011CB606001) is gratefully acknowledged.
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