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1-Hexene polymerization with supported Ziegler-Natta catalyst: Correlation between catalyst particle fragmentation and active center distribution
Molecular Catalysis 447 (2018) 13–20
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Molecular Catalysis journal homepage: www.elsevier.com/locate/mcat
1-Hexene polymerization with supported Ziegler-Natta catalyst: Correlation between catalyst particle fragmentation and active center distribution Pengjia 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
Article history: Received 12 November 2017 Received in revised form 23 December 2017 Accepted 28 December 2017 Available online 4 February 2018 Keywords: Supported Ziegler-Natta catalyst 1-Hexene polymerization Kinetics Particle fragmentation Active centers
a b s t r a c t 1-Hexene was polymerized with a commercial MgCl2 -supported Ziegler-Natta catalyst using triethylaluminum as cocatalyst at different monomer/catalyst mass ratio (mhexene /mcat = 5, 15, 30), and the number of active centers ([C* ]/[Ti]) was determined by quench-labeling the catalyst with 2-thiophenecarbonyl chloride and monitoring sulfur content of the polymer. Changes of polymerization rate (Rp ) and propagation rate constant (kp ) with time were also determined. [C* ]/[Ti] was found to increase with time for 3 − 5 folds in the first 10 min of polymerization, but the maximum [C* ]/[Ti] value reached after the induction period markedly enhanced with increase of monomer/catalyst mass ratio. Gradual fragmentation of the catalyst particles in parallel with the [C* ]/[Ti] increase was observed by SEM analysis. It means that a large proportion of active center precursors are inaccessible to cocatalyst and monomer at the beginning of reaction, and fragmentation of the particles by hydraulic force of the growing polymer chains leads to exposure and activation of these precursors. Raising the monomer concentration can provide larger hydraulic force to expose those active site precursors that are more tightly buried inside the catalyst particle. The polymer molecular weight distribution and related active center distribution was found to shift with increase of the monomer/catalyst ratio, meaning that there are different types of buried active center precursors, and their exposure requires different extent of particle fragmentation. © 2017 Elsevier B.V. All rights reserved.
1. Introduction MgCl2 -supported Ziegler-Natta (ZN) catalysts are playing dominant role in polyolefin production, with more than 100 million tons of polyethylene and polypropylene resins being produced with ZN catalysts per annum. However, current knowledge on the structure of active centers and their spacial distribution in the supported ZN catalysts in particle form is still rather poor even after more than three decades of foundamental studies. As one of the typical multisite heterogeneous catalysts, supported ZN catalyst contains multiple types of active centers that differ in reactivity, chain transfer rate and comonomer incorporation rate, leading to broad molecular weight distribution (MWD) and chemical composition distribution (CCD) of the produced polymer [1–14]. Different approaches have been explored to disclose the chemical structure of different types of active centers, which enable descriptions of
∗ Corresponding author. E-mail address: [email protected] (Z. Fan). https://doi.org/10.1016/j.mcat.2017.12.040 2468-8231/© 2017 Elsevier B.V. All rights reserved.
some important features of the active centers. Study on TiCl3 /MgCl2 type model catalysts with very low Ti contents (< 0.1%) showed that most Ti species existed in isolated state [15]. In comparison with normal TiCl3 /MgCl2 catalyst (Ti > 1%), the catalyst with low Ti loading showed much lower isospecificity in propylene polymerization, which supports the catalyst model that isolated mono Ti species are non-stereospecific, and clustered Ti species are more stereospecific. Because the isotacticity of ␣-olefin polymer increases with its molecular weight [11,16–19], it can be concluded that the active centers with clustered Ti species are spacially more congested than those with isolated Ti species. Further studies on the active center distribution (ACD) of supported ZN catalysts revealed that the active centers forming low molecular weight polymer chains have larger number and lower reactivity as compared with those forming high molecular weight polymer chains [3,11,14]. Based on these results a basic scenario of active centers and their distribution in MgCl2 -supported ZN catalysts could be proposed: a relatively larger proportion of active centers are formed by isolated Ti species adsorbed on MgCl2 crystallites, which have relatively more open stereochemical environment and show lower catalytic reactivity
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and stereospecificity. In contrast, the active centers formed by clustered Ti species occupy a smaller proportion. They have relatively more congested stereochemical environment and show higher catalytic reactivity and stereospecificity. There are also active centers that show catalytic behaviors in between these two extremes, making a nearly continuous distribution of active centers [3,11,14]. However, such knowledge on supported ZN catalysts is not enough for understanding the real spacial distribution of the active centers in the catalyst particle and their mechanism of formation. Extensive studies have been focused on characterizing the morphology and solid phase structure of supported ZN catalysts [20–30]. The macroscopic catalyst particles (with size of about 1–50 m) were found to have multi-level solid structure, as each particle is composed of smaller sub-particles, which were aggregates of nano-sized primary particles with adsorbed active species on their surface [22,24–27]. Such complex particle structure makes the kinetics of active center activation (or initiation of polymerization) more complicated than that of single-site catalysis systems, as only a small proportion of active center precursors was quickly activated at the beginning of polymerization [26,31], and more active centers were gradually activated in the polymerization process [32–37]. The shape and length of rate curve in the induction period were found to be influenced by the catalyst used. Short induction period in propylene polymerization with a supported ZN catalyst was observed by M. Skoumal et al. [38]. In propylene polymerization with a Mg(OEt)2 based ZN catalyst, the polymerization rate starts to speed up again after a very short stationary stage, which was attributed to catalyst fragmentation in the induction period [39]. The gradual activation can be explained by progressive exposure of more catalyst surfaces that contain large number of active center precursors [40–42]. It was proposed that complete activation of all the potential active species replies on thorough disintegration of the polymer/catalyst particles by the hydraulic forces of the expanding polymer phase, which release the active species buried inside the particles. In our previous work [43], formation of active centers in the induction period of ethylene polymerization with MgCl2 supported ZN catalyst was studied by tracing the change of active center concentration ([C* ]) with reaction time. The [C* ] was found to increase with the polymerization time in two-stages, in which the first stage of quick [C* ] rising corresponds to activation of active center precursors located on the exposed surface of the particles, and the second stage of [C* ] rising was correlated with the observed disintegration of polymer/catalyst particles. This motivated us to further explore the spacial distribution of different active centers in the catalyst particle by controlling the degree of particle fragmentation and tracing the changes of active centers and polymer structure in the reaction process. In this work, 1-hexene polymerizations with a MgCl2 -supported ZN catalyst were conducted at different initial monomer concentration to control the degree of particle fragmentation. By using 1-hexene as the monomer, morphological changes of the catalyst particles can be observed more clearly as compared to the ethylene polymerization system, because the highly soluble poly(1-hexene) will be quickly dissolved into the
polymerization solvent. The investigations on particle morphology, reaction kinetics and polymer chain structure have enabled us to have a deeper insight in the catalyst structure and its multiple types of active centers.
2. Experimetal 2.1. Materials A commercial MgCl2 -supported ZN catalyst (MgCl2 /Di/TiCl4 , Ti content = 2.7 wt%, Di = diisobutyl phthalate, provided by SINOPEC) was used for polymerization. Triethylaluminum (TEA, purchased from Albemarle Co.) was used as received and diluted in n-heptane before use. 1-Hexene (Alfa Aesar Co.) was purified by passing through columns of molecular sieves. 2-Thiophenecarbonyl chloride (TPCC, 98%, J&K Scientific Co.) was distilled and diluted with n-heptane to 2 M before use. n-Heptane was first dried over 4A molecular sieves under dry N2 and then refluxed over Na before use. All other chemicals were obtained commercially and used without further purification unless otherwise stated. All operations with moisture and oxygen sensitive reagents were conducted under N2 using standard Schlenk techniques or glove box unless otherwise noted.
2.2. Polymerization and quenching reaction Three series of 1-hexene polymerization in n-heptane with respectively fixed initial 1-hexene concentration ([1-Hexene]o ) were carried out in Schlenk flask at 40 ◦ C under nitrogen atmosphere for different durations (tp ). To obtain enough product for characterization, when monomer/catalyst mass ratio was 30 ([1-Hexene]o = 1.26 mol/L) or 15 ([1-Hexene]o = 0.63 mol/L), the amount of catalyst was 100 mg at tp > 60 s and 200 mg at tp ≤ 60s, respectively. When monomer/catalyst mass ratio was 5 ([1Hexene]o = 0.21 mol/L), the amount of catalyst was 200 mg at tp > 180 s and 600 mg at tp ≤ 180s, respectively. The total volume of reaction system was regulated accordingly to keep the [1-Hexene]o value of a certain reaction series constant at 0.21, 0.63 or 1.26 mol/L when the catalyst weight was changed. Conditions of the polymerization runs are listed in Table 1. Calculated amount of reagents was added in the order of n-heptane, 1-hexene, TEA and the catalyst with fixed [Ti] = 2 mmol/L and [Al] = 60 mmol/L. The polymerization started after adding the catalyst into the flask under stirring. After the designed reaction duration, calculated amount of TPCC solution was injected into the reactor at TPCC/Al = 2 to quench-label the popagation chains, and the reactor content was further stirred at 40 ◦ C for 2 min. Subsequently isopropanol/HCl mixture (95/5) was added to decompose the catalyst, and the polymer was precipitated with excess of isopropanol. Then the deposited polymer was thoroughly purified by repeated dissolution-precipitation operation for three times, and then dried in vacuum at 50 ◦ C.
Table 1 Reaction conditions of the polymerization runs.a b
Run No.
tp
1–4 5–10 11–12 13–20 21–22 23–30
30 ∼ 180 300 ∼ 7200 30 ∼ 60 120 ∼ 7200 30 ∼ 60 120 ∼ 7200
a b
(s)
Catalyst (mg)
1-Hexene/catalyst (g/g)
[1-Hexene]o (mol/L)
Reaction volume (ml)
600 200 200 100 200 100
5 5 15 15 30 30
0.21 0.21 0.63 0.63 1.26 1.26
170 56.7 56.7 28.3 56.7 28.3
The other conditions are: [Ti] = 2 mmol/L, [TEA] = 60 mmol/L, Tp = 40 ◦ C, n-heptane was used as solvent. Reaction durations (tp ) of all the polymerization runs are shown in Table S1–S3 of the Supplementary Data.
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2.3. Characterization Sulfur content of each quenched polymer was measured in an YHTS-2000 UV fluorescence sulfur analyzer with a lower detection limit of 0.05 ppm (Jiangyan Yinhe Instrument Co., Jiangyan, China). 2 − 5 mg polymer sample (weighed to ±0.01 mg) was used in each measurement, and the average value of three parallel measurements was taken as the sulfur content. Only trace amount of sulfur was observed in blank samples that were not quenched by TPCC and purified in the same procedures, thus 0 ppm of blank sulfur level was adopted. Molecular weight and molecular weight distribution (Mw /Mn ) of the obtained poly(1-hexene) was determined by gel permeation chromatograph (GPC) on a PL GPC220 instrument equipped with refractive index detector (Polymer Laboratories Ltd., U.S.A.) and two PL gel 5 m MIXED-C columns using polystyrene standards and THF eluent at a flow rate of 1.0 mL/min at 40 ◦ C. Scanning electron microscope (SEM) observations of the catalyst particles were made with a Hitachi-4800 SEM. Micrographs were taken at 3-kV acceleration voltage. The samples for SEM observation were prepared by dropping diluted polymerization solution taken from the reactor on conductive glue stuck on the sample stage in glove box. The sample stage was then moved to a vacuum chamber to dry the solution under vacuum. Before the SEM observations, all the sample surfaces were coated with a thin layer of gold. Propagation rate constant (kp ) of polymerization was calculated according to the equation Rp = kp [C* ][M], where Rp is the rate of polymerization detemined according to the equation Rp = −d[M]/dt (t is the polymerization time, and [M] is the instant monomer concentration in the reaction system), [C* ] is the concentration of active centers.
Fig. 1. Changes of polymerization rate (Rp ) with tp at different monomer/catalyst mass ratios.
3. Results and discussion 3.1. Kinetics of catalyzed 1-hexene polymerization Using a commercial MgCl2 -supported Ziegler-Natta catalyst and TEA cocatalyst, three series of 1-hexene polymerization reactions were conducted for different durations ranging from 30 s to 7200 s, in which the monomer/catalyst mass ratio was 30:1, 15:1 and 5:1 (or [1-Hexene] = 1.27, 0.63 and 0.21 M), respectively. Each polymerization run was quenched by TPCC at TPCC/TEA = 2 and 40 ◦ C for 2 min in order to selectively label each propagation chain end with a 2-thiophenecarbonyl group [32]. By measuring sulfur content of the quenched polymer sample, concentration of the active centers in the reaction system at the time of quencher addition was determined. The polymerization rate (Rp ) was determined by calculating the derivative of the yield-polymerization time (tp ) curve. Rate curves of the three polymerization series are shown in Fig. 1. It can be seen that the reaction rate reached the maximum after about 1 min and then quickly decayed with the proceeding of polymerization. This can be largely attributed to rapid decrease of monomer concentration with time, as the monomer conversion was found to be about 70% at tp = 300 s for the polymerization under different 1-hexene/catalyst ratios (see Tables S1, S2 and S3 in the Supplementary Data). After polymerization for 1 h, all the three systems reached the maximum conversion of about 90% (mhexene /mcat = 30, 15) or 80% (mhexene /mcat. = 5), which was almost unchanged by prolonging the reaction duration to 2 h (see detailed data in the Supplementary Data). It means that the residual monomer in the reaction system (about 0.05 − 0.1 M) was unable to take part in further polymerization after 1 h. Fig. 2 shows change of the number of active centers ([C* ]/[Ti]) with reaction time under different monomer/catalyst mass ratios. At mhexene /mcat = 5, [C* ]/[Ti] grew to the maximum level in
Fig. 2. Changes of active center concentration with tp at different monomer/catalyst mass ratios.
about 5 min, then gradually decreased in the next 2 h. At mhexene /mcat = 15, the maximum [C* ]/[Ti] value was slightly higher. When mhexene /mcat was further raised to 30, the maximum [C* ]/[Ti] value became much higher than those at lower monomer/catalyst mass ratios. It is worth noting that the [C* ]/[Ti] value at the very beginning (tp = 30 s) of the reaction was not evidently influenced by the monomer/catalyst mass ratio. It means that the increment of [C* ]/[Ti] in the reaction process is closely related with the amount of polymer and its production rate in the system. Gradual fragmen-
Fig. 3. Changes of chain propagation rate constant with tp at different monomer/catalyst mass ratios.
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Fig. 4. SEM pictures of the catalyst particles (a) and polymer/catalyst particles after 48 s (b), 194 s (c, d), 610 s (e, f) of 1-hexene polymerization at mhexene /mcat = 5:1, and polymer/catalyst particles after 150 s (g) and 630 s (h) of 1-hexene polymerization at mhexene /mcat = 30:1.
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tation of the catalyst particles by the hydraulic force of the formed polymer chains should be responsible for exposure and activation of more active centers in the induction period of polymerization. The degree of particle fragmentation seems to depend on the polymer/catalyst mass ratio. This phenomenon will be further discussed in the next section. When the reaction time was longer than 10 min, the [C* ]/[Ti] value gradually descended to lower level in the later stage of polymerization, especially when the initial mhexene /mcat ratio was low. This phenomenon can be explained by severe retard of chain propagation in the later stage. As seen in Table S3, at mhexene /mcat ratio of 5:1, [1-hexene] was lower than 0.06 mol/L after 10 min of polymerization, and the polymerization was almost ceased (see Tables S1-S3 of the Supplementary Data). Under the mutual effects of very low [1-hexene] and high barrier to monomer diffusion caused by high polymer concentration in the later stage of polymerization, the reactions of Ti − (CH2 )5 CH3 (product of chain transfer with 1-hexene) with 1-hexene to form new propagation chain would be greatly retarded, leading to gradual accumulation of Ti − (CH2 )5 CH3 and Ti − (CH2 CH(C4 H9 ))n H species with very short chain length (n < 10). Even though these active species can be quench-labeled by TPCC, because of very low molecular weight and high solubility in alcohols, their quenching products will be washed out from the polymer product during the purification procedures and thus cannot be counted into the [C* ] value. The value of chain propagation rate constant (kp ) was calculated according to the equation Rp = kp [C* ][hexene], which is generally accepted for describing the kinetics of catalyzed olefin polymerization. The changes of kp in the reaction process are shown in Fig. 3 and Tables S1 − S3 in the Supplementary Data. At different monomer/catalyst mass ratios, kp quickly decayed in the first 5 min of reaction, which can be mainly attributed to increase of monomer diffusion barrier by the growing polymer phase. The kp values of the three systems decreased with increase in mhexene /mcat , also showing the effects of diffusion limitation. 3.2. Fragmentation of the catalyst particles The morphologies of the original catalyst particles and the polymer/catalyst particles formed after different reaction durations were observed by SEM, and the SEM pictures are shown in Fig. 4. The original catalyst particles are spherical granules with big cracks on the surface. After 48 s of polymerization at initial mhexene /mcat = 5:1, most of the particles were fragmented into smaller pieces or partly fragmented, meaning that the growing polymer phase (mPH /mcat was about 0.5:1) had expanded the cracks in the particles. After longer durations of polymerization, higher extent of particle fragmentation can be observed (see Fig. 4c and e), and thin filaments can be seen in the cracks, which can be attributed to presence of poly(1hexene) phase in the particle. It should be noted that not all the produced polymer can be retained in the polymer/catalyst particles, because poly(1-hexene) is highly soluble in the reaction medium, n-heptane. This enabled observation of the catalyst fragments that are mainly composed of inorganic phases (MgCl2 crystallites and their aggregates) at mPH /mcat ratio larger than 2:1 (see Fig. 4c–f). If a large proportion of polymer phase was not dissolved into the solvent, it will be hard to see the morphology of the inorganic part of the particles. It was found that the plain catalyst particles were not fragmented after strring a n-heptane slurry of catalyst for 10 min at 40 ◦ C in the absence of TEA and monomer (SEM picture of the catalyst particles after stirring in n-heptane for 10 min at 40 ◦ C can be seen in Fig. S1 of the Supplementary Data). Therefore, the observed particle fragmentation in the polymerization processes cannot be attributed to mechanical force exerted by the stirrer on the catalyst particles. When the polymerization was conducted at higher mhexene /mcat ratio, severe fragmentation of the particles took shorter time than
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Fig. 5. MWD curve of a typical poly(1-hexene) sample (Run No. 22, mhexene /mcat = 30, tp = 60 s) and its deconvolution into five Flory components. (circles: experimental data points; lines: Flory components and their sum.).
the system with lower mhexene /mcat ratio. As seen in Fig. 4g and h, more filament structures can be seen in the cracks, and the external surface of the particle fragment became smoother as compared with the particles formed at lower mhexene /mcat ratio. It means that more poly(1-hexene) chains were retained in the particles, possibly caused by entanglement of the polymer chains in some narrow cracks. Morphological changes of the catalyst particles during the polymerization process can be taken as important evidence for explaining the continuous increase of active center concentration in the first 5–10 min of polymerization (see Fig. 2). A reasonable explanation is that a large proportion of active center precursors are buried inside the aggregates of MgCl2 crystallites, therefore they are inaccessible to the cocatalyst and/or monomer. When the catalyst particles are fragmented or disintegrated under the hydraulic force of the growing polymer, these active center precursors become accessible to the cocatalyst and monomer and are quickly activated, resulting in increase of [C* ] [43]. The dependence of the maximum [C* ] increment on the mhexene /mcat ratio (see Fig. 2) could be explained by the presence of active center precursors that can be exposed after different extent of particle fragmentation. Under rather lower mhexene /mcat ratio (e.g. mhexene /mcat = 5), because of the limited polymer volume, the weak hydraulic force of the polymer phase can only cause incomplete particle fragmentation, and a large proportion of the buried active sites cannot be released. Increasing the mhexene /mcat ratio can not only provide more polymer to fill the cracks in the particle, but also enhance the absolute chain propagation rate (see Fig. 1) to form stronger dynamic hydraulic force that can destroy the aggregates of MgCl2 crystallites. As a result, more buried active sites can be exposed and activated. 3.3. Distribution of active centers To determine the effects of mhexene /mcat mass ratio on the active center distribution (ACD), deconvolution of the polymer’s MWD curves with several Schulz-Flory most-probable distributions has been applied. This kind of deconvolution treatment has been found useful in differentiating the different active sites in heterogeneous Ziegler-Natta catalysts [2–4,44–48]. The MWD curves of the poly(1hexene) samples synthesized under different mhexene /mcat ratios and different reaction durations were satisfactorily deconvoluted into five Flory components respectively, and these components were named as component A, B, C, D and E in the order of decreasing molecular weight. Fig. 5 shows MWD curve of a typical poly(1-
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Fig. 6. Change of the fractions of different type of Flory components with monomer conversion at different monomer/catalyst mass ratios.
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Scheme 1. Model of clustered Ti species bridging two MgCl2 crystallites and their exposure and subsequent activation after breaking up of the aggregate. (Pale small spheres: Mg, dark small spheres: Ti, big spheres: Cl, 䊐: vacancy of active center, 䊉∼: propagation chain. The oval circle indicate clustered Ti species.).
hexene) sample and its deconvolution into five Flory components (more MWD curves are shown in Fig. S2 and Fig. S3 in the Supplementary Data). Each Flory component can be ascribed to polymer produced by a certain type of active center [3,4]. By calculating the weight percentage of a certain Flory component in the whole polymer and tracing its change in the polymerization process, we have studied correlations between the catalyst particle fragmentation and ACD (the data of weight fractions of the Flory components are listed in Table S4, Table S5 and Table S6 in the Supplementary Data). As shown in Fig. 6, change in mhexene /mcat ratio caused evident difference in distribution of the five Flory components (components A, B, C, D and E). Though the component A accounted for 12 − 20% of the total product when mhexene /mcat ratio was 15 or 30, it dropped to no more than 10% when mhexene /mcat ratio was lowered to 5. Meanwhile, the weight fractions of component D and E were evidently enhanced when mhexene /mcat ratio was lowered from 30 or 15–5. Similar changes of polymer MWD with the monomer concentration have been observed by Barabanov et al. [49]. The weight fractions of component B and C were bascially unchanged by varying the mhexene /mcat ratio. A possible explanation is that the active centers forming component A are formed from clustered Ti species that can only be exposed and activated after sufficient extent of particle fragmentation. According to literatures, the active centers producing high molecular weight ␣-olefin polymer have relatively higher stereospecificity [3,11,16–19]. These active centers are considered to be formed on clustered Ti species [15,50]. Based on this general recognition, we propose that the clustered Ti species on the catalyst surface are less accessible than the isolated Ti species, because the multiple Ti atoms and their Cl ligands congested in a limited space can form stereochemical hindrance to the approaching cocatalyst and monomer molecules. It is also possible that some clustered Ti species may form bridging structures connecting two or more MgCl2 crystallites. The Ti atoms in such structures will be in lack of free Ti − Cl bonds for reacting with the cocatalyst. Activation of such Ti species can only take place after breaking up of the bridging structures by the hydraulic force of the growing polymer chains. A possible bridging structure and activation of its Ti species is shown in the model of Scheme 1. According to this model, there is a positive correlation between activation of clustered and stereospecific Ti species (component A) and particle fragmentation. At higher mhexene /mcat ratios, the higher polymer/catalyst ratio at low conversion provides sufficient hydraulic force of the polymer chains to ensure quick breaking up of the bridging structures and release of more clustered and stereospecific Ti species, resulting in activation of more active centers of component A (e.g. monomer conversion of 20% in the mhexene /mcat = 15 system gives polymer/catalyst ratio of 3, in contrast to 1 for the mhexene /mcat = 5 system). On the other
hand, the Flory components D and E with the lowest molecular weight could be formed by active centers based on isolated Ti species, which are more accessible than the other types of active centers. When the mhexene /mcat ratio is low, these active centers can get relative preference in the competition for limited amount of monomer, because the active centers producing low molecular weight polymer have larger number as compared with the other centers [3,11,14]. This can explain the observed increase in component D and E with decrease of mhexene /mcat ratio. The accessibility of active centers of components B and C seems to fall in between those of component A and components D and E, so their weight fractions are less sensitive to the change in mhexene /mcat ratio. In summary, there are close correlations between the monomer/catalyst mass ratio and the increase of active center concentration in the induction period as well as the active center distribution. Combining with morphological change of the catalyst particles during the polymerization process, it can be concluded that the extent of particle fragmentation depends on the rate of polymer formation and its accumulation effect (namley, the polymer/catalyst mass ratio reached in the reaction process), which constitute the driving force to break up the catalyst particles. The shifting of ACD with intensification of particle fragmentation means that there are different types of active center precursors distributed inside the catalyst particles, and their release through particle fragmentation requires different intensity of polymer expansion in the particle. The present results suggest that the active centers from isolated Ti species can be released by relatively lower extent of particle fragmentation than those based on clustered Ti species. This kind of knowledge can promote in-depth understanding of the catalyst’s microstructure and the mechanism of forming such structure in the process of catalyst preparation.
4. Conclusions By tracing changes of [C* ]/[Ti] and the active center distribution in the process of 1-hexene polymerization with a MgCl2 -supported ZN catalyst, close correlations between monomer/catalyst mass ratio and increase of active center concentration as well as shifting of ACD in the induction period have been discovered. The extent of [C* ]/[Ti] increase in the induction period enhanced with increase of monomer/catalyst mass ratio of the polymerization system, meanwhile the catalyst particles were fragmented more severely in the reactions with larger monomer/catalyst mass ratio. All these phenomena show that the increment of [C* ]/[Ti] is a result of particle fragmentation under the hydraulic force of the growing polymer. In this catalyst, a large proportion of active center precursors are inaccessible to the monomer and cocatalyst as they are buried inside the solid phase of catalyst particles, and can be released
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after the particle fragmentation. The observed shifting of ACD with increase of monomer/catalyst ratio means that there are different types of active center precursors buried inside the catalyst particles, and their release through particle fragmentation requires different extent of polymer phase expansion in the particle. Acknowledgement Support by the National Natural Science Foundation of China (Grant No. 21374094 and U1462114) is gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.mcat.2017.12. 040. References [1] M. Kakugo, T. Miyatake, K. Mizunuma, Macromolecules 24 (1991) 1469–1472. [2] J.B.P. Soares, A.E. Hamielect, Polymer 36 (1995) 2257–2263. [3] Z.Q. Fan, L.X. Feng, S.L. Yang, J. Polym. Sci. A Polym. Chem. 34 (1996) 3329–3335. [4] Y.V. Kissin, R.I. Mink, T.E. Nowlin, J. Polym. Sci.: Part A: Polym. Chem. 37 (1999) 4255–4272. [5] V. Busico, R. Cipullo, G. Monaco, G. Talarico, M. Vacatello, J.C. Chadwick, A.L. Segre, O. Sudmeijer, Macromolecules 32 (1999) 4173–4182. [6] B.P. Liu, T. Nitta, H. Nakatani, M. Terano, Macromol. Symp. 213 (2004) 7–18. [7] Y.P. Chen, Z.Q. Fan, J.H. Liao, S.Q. Liao, J. Appl. Polym. Sci. 102 (2006) 1768–1772. [8] Y.V. Kissin, J.C. Chadwick, I. Mingozzi, G. Morini, Macromol. Chem. Phys. 207 (2006) 1344–1350. [9] D.E. Thompson, K.B. McAuley, P.J. McLellan, Macromol. React. Eng. 1 (2007) 264–274. [10] M.I. Nikolaeva, T.B. Mikenas, M.A. Matsko, L.G. Echevskaya, V.A. Zakharov, J. Appl. Polym. Sci. 115 (2010) 2432–2439. [11] Z.Q. Fan, L.T. Zhang, Z.S. Fu, J. Mol. Catal. A: Chem. 351 (2011) 93–99. [12] S.J. Xia, Z.S. Fu, B. Huang, J.T. Xu, Z.Q. Fan, J. Mol. Catal. A: Chem. 355 (2012) 161–167. [13] H.R. Yang, L.T. Zhang, Z.S. Fu, Z.Q. Fan, J. Appl. Polym. Sci. 132 (2015) 41264. [14] B. Zhang, L.T. Zhang, Z.S. Fu, Z.Q. Fan, Catal. Commun. 69 (2015) 147–149. [15] T. Wada, T. Taniike, I. Kouzai, S. Takahashi, M. Terano, Macromol. Rapid Commun. 30 (2009) 887–891. [16] M. Kakugo, T. Miyatake, Y. Naito, K. Mizunuma, Macromolecules 21 (1988) 314–319. [17] J.T. Xu, Y.Q. Yang, L.X. Feng, X.M. Kong, S.L. Yang, J. Appl. Polym. Sci. 62 (1996) 727–731. [18] J.T. Xu, L.X. Feng, S.L. Yang, Y.Q. Yang, X.M. Kong, Macromolecules 30 (1997) 7655–7660.
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1-Hexene polymerization with supported Ziegler-Natta catalyst: Correlation between catalyst particle fragmentation and active center distribution Pengjia 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
Article history: Received 12 November 2017 Received in revised form 23 December 2017 Accepted 28 December 2017 Available online 4 February 2018 Keywords: Supported Ziegler-Natta catalyst 1-Hexene polymerization Kinetics Particle fragmentation Active centers
a b s t r a c t 1-Hexene was polymerized with a commercial MgCl2 -supported Ziegler-Natta catalyst using triethylaluminum as cocatalyst at different monomer/catalyst mass ratio (mhexene /mcat = 5, 15, 30), and the number of active centers ([C* ]/[Ti]) was determined by quench-labeling the catalyst with 2-thiophenecarbonyl chloride and monitoring sulfur content of the polymer. Changes of polymerization rate (Rp ) and propagation rate constant (kp ) with time were also determined. [C* ]/[Ti] was found to increase with time for 3 − 5 folds in the first 10 min of polymerization, but the maximum [C* ]/[Ti] value reached after the induction period markedly enhanced with increase of monomer/catalyst mass ratio. Gradual fragmentation of the catalyst particles in parallel with the [C* ]/[Ti] increase was observed by SEM analysis. It means that a large proportion of active center precursors are inaccessible to cocatalyst and monomer at the beginning of reaction, and fragmentation of the particles by hydraulic force of the growing polymer chains leads to exposure and activation of these precursors. Raising the monomer concentration can provide larger hydraulic force to expose those active site precursors that are more tightly buried inside the catalyst particle. The polymer molecular weight distribution and related active center distribution was found to shift with increase of the monomer/catalyst ratio, meaning that there are different types of buried active center precursors, and their exposure requires different extent of particle fragmentation. © 2017 Elsevier B.V. All rights reserved.
1. Introduction MgCl2 -supported Ziegler-Natta (ZN) catalysts are playing dominant role in polyolefin production, with more than 100 million tons of polyethylene and polypropylene resins being produced with ZN catalysts per annum. However, current knowledge on the structure of active centers and their spacial distribution in the supported ZN catalysts in particle form is still rather poor even after more than three decades of foundamental studies. As one of the typical multisite heterogeneous catalysts, supported ZN catalyst contains multiple types of active centers that differ in reactivity, chain transfer rate and comonomer incorporation rate, leading to broad molecular weight distribution (MWD) and chemical composition distribution (CCD) of the produced polymer [1–14]. Different approaches have been explored to disclose the chemical structure of different types of active centers, which enable descriptions of
∗ Corresponding author. E-mail address: [email protected] (Z. Fan). https://doi.org/10.1016/j.mcat.2017.12.040 2468-8231/© 2017 Elsevier B.V. All rights reserved.
some important features of the active centers. Study on TiCl3 /MgCl2 type model catalysts with very low Ti contents (< 0.1%) showed that most Ti species existed in isolated state [15]. In comparison with normal TiCl3 /MgCl2 catalyst (Ti > 1%), the catalyst with low Ti loading showed much lower isospecificity in propylene polymerization, which supports the catalyst model that isolated mono Ti species are non-stereospecific, and clustered Ti species are more stereospecific. Because the isotacticity of ␣-olefin polymer increases with its molecular weight [11,16–19], it can be concluded that the active centers with clustered Ti species are spacially more congested than those with isolated Ti species. Further studies on the active center distribution (ACD) of supported ZN catalysts revealed that the active centers forming low molecular weight polymer chains have larger number and lower reactivity as compared with those forming high molecular weight polymer chains [3,11,14]. Based on these results a basic scenario of active centers and their distribution in MgCl2 -supported ZN catalysts could be proposed: a relatively larger proportion of active centers are formed by isolated Ti species adsorbed on MgCl2 crystallites, which have relatively more open stereochemical environment and show lower catalytic reactivity
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and stereospecificity. In contrast, the active centers formed by clustered Ti species occupy a smaller proportion. They have relatively more congested stereochemical environment and show higher catalytic reactivity and stereospecificity. There are also active centers that show catalytic behaviors in between these two extremes, making a nearly continuous distribution of active centers [3,11,14]. However, such knowledge on supported ZN catalysts is not enough for understanding the real spacial distribution of the active centers in the catalyst particle and their mechanism of formation. Extensive studies have been focused on characterizing the morphology and solid phase structure of supported ZN catalysts [20–30]. The macroscopic catalyst particles (with size of about 1–50 m) were found to have multi-level solid structure, as each particle is composed of smaller sub-particles, which were aggregates of nano-sized primary particles with adsorbed active species on their surface [22,24–27]. Such complex particle structure makes the kinetics of active center activation (or initiation of polymerization) more complicated than that of single-site catalysis systems, as only a small proportion of active center precursors was quickly activated at the beginning of polymerization [26,31], and more active centers were gradually activated in the polymerization process [32–37]. The shape and length of rate curve in the induction period were found to be influenced by the catalyst used. Short induction period in propylene polymerization with a supported ZN catalyst was observed by M. Skoumal et al. [38]. In propylene polymerization with a Mg(OEt)2 based ZN catalyst, the polymerization rate starts to speed up again after a very short stationary stage, which was attributed to catalyst fragmentation in the induction period [39]. The gradual activation can be explained by progressive exposure of more catalyst surfaces that contain large number of active center precursors [40–42]. It was proposed that complete activation of all the potential active species replies on thorough disintegration of the polymer/catalyst particles by the hydraulic forces of the expanding polymer phase, which release the active species buried inside the particles. In our previous work [43], formation of active centers in the induction period of ethylene polymerization with MgCl2 supported ZN catalyst was studied by tracing the change of active center concentration ([C* ]) with reaction time. The [C* ] was found to increase with the polymerization time in two-stages, in which the first stage of quick [C* ] rising corresponds to activation of active center precursors located on the exposed surface of the particles, and the second stage of [C* ] rising was correlated with the observed disintegration of polymer/catalyst particles. This motivated us to further explore the spacial distribution of different active centers in the catalyst particle by controlling the degree of particle fragmentation and tracing the changes of active centers and polymer structure in the reaction process. In this work, 1-hexene polymerizations with a MgCl2 -supported ZN catalyst were conducted at different initial monomer concentration to control the degree of particle fragmentation. By using 1-hexene as the monomer, morphological changes of the catalyst particles can be observed more clearly as compared to the ethylene polymerization system, because the highly soluble poly(1-hexene) will be quickly dissolved into the
polymerization solvent. The investigations on particle morphology, reaction kinetics and polymer chain structure have enabled us to have a deeper insight in the catalyst structure and its multiple types of active centers.
2. Experimetal 2.1. Materials A commercial MgCl2 -supported ZN catalyst (MgCl2 /Di/TiCl4 , Ti content = 2.7 wt%, Di = diisobutyl phthalate, provided by SINOPEC) was used for polymerization. Triethylaluminum (TEA, purchased from Albemarle Co.) was used as received and diluted in n-heptane before use. 1-Hexene (Alfa Aesar Co.) was purified by passing through columns of molecular sieves. 2-Thiophenecarbonyl chloride (TPCC, 98%, J&K Scientific Co.) was distilled and diluted with n-heptane to 2 M before use. n-Heptane was first dried over 4A molecular sieves under dry N2 and then refluxed over Na before use. All other chemicals were obtained commercially and used without further purification unless otherwise stated. All operations with moisture and oxygen sensitive reagents were conducted under N2 using standard Schlenk techniques or glove box unless otherwise noted.
2.2. Polymerization and quenching reaction Three series of 1-hexene polymerization in n-heptane with respectively fixed initial 1-hexene concentration ([1-Hexene]o ) were carried out in Schlenk flask at 40 ◦ C under nitrogen atmosphere for different durations (tp ). To obtain enough product for characterization, when monomer/catalyst mass ratio was 30 ([1-Hexene]o = 1.26 mol/L) or 15 ([1-Hexene]o = 0.63 mol/L), the amount of catalyst was 100 mg at tp > 60 s and 200 mg at tp ≤ 60s, respectively. When monomer/catalyst mass ratio was 5 ([1Hexene]o = 0.21 mol/L), the amount of catalyst was 200 mg at tp > 180 s and 600 mg at tp ≤ 180s, respectively. The total volume of reaction system was regulated accordingly to keep the [1-Hexene]o value of a certain reaction series constant at 0.21, 0.63 or 1.26 mol/L when the catalyst weight was changed. Conditions of the polymerization runs are listed in Table 1. Calculated amount of reagents was added in the order of n-heptane, 1-hexene, TEA and the catalyst with fixed [Ti] = 2 mmol/L and [Al] = 60 mmol/L. The polymerization started after adding the catalyst into the flask under stirring. After the designed reaction duration, calculated amount of TPCC solution was injected into the reactor at TPCC/Al = 2 to quench-label the popagation chains, and the reactor content was further stirred at 40 ◦ C for 2 min. Subsequently isopropanol/HCl mixture (95/5) was added to decompose the catalyst, and the polymer was precipitated with excess of isopropanol. Then the deposited polymer was thoroughly purified by repeated dissolution-precipitation operation for three times, and then dried in vacuum at 50 ◦ C.
Table 1 Reaction conditions of the polymerization runs.a b
Run No.
tp
1–4 5–10 11–12 13–20 21–22 23–30
30 ∼ 180 300 ∼ 7200 30 ∼ 60 120 ∼ 7200 30 ∼ 60 120 ∼ 7200
a b
(s)
Catalyst (mg)
1-Hexene/catalyst (g/g)
[1-Hexene]o (mol/L)
Reaction volume (ml)
600 200 200 100 200 100
5 5 15 15 30 30
0.21 0.21 0.63 0.63 1.26 1.26
170 56.7 56.7 28.3 56.7 28.3
The other conditions are: [Ti] = 2 mmol/L, [TEA] = 60 mmol/L, Tp = 40 ◦ C, n-heptane was used as solvent. Reaction durations (tp ) of all the polymerization runs are shown in Table S1–S3 of the Supplementary Data.
P. Yang et al. / Molecular Catalysis 447 (2018) 13–20
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2.3. Characterization Sulfur content of each quenched polymer was measured in an YHTS-2000 UV fluorescence sulfur analyzer with a lower detection limit of 0.05 ppm (Jiangyan Yinhe Instrument Co., Jiangyan, China). 2 − 5 mg polymer sample (weighed to ±0.01 mg) was used in each measurement, and the average value of three parallel measurements was taken as the sulfur content. Only trace amount of sulfur was observed in blank samples that were not quenched by TPCC and purified in the same procedures, thus 0 ppm of blank sulfur level was adopted. Molecular weight and molecular weight distribution (Mw /Mn ) of the obtained poly(1-hexene) was determined by gel permeation chromatograph (GPC) on a PL GPC220 instrument equipped with refractive index detector (Polymer Laboratories Ltd., U.S.A.) and two PL gel 5 m MIXED-C columns using polystyrene standards and THF eluent at a flow rate of 1.0 mL/min at 40 ◦ C. Scanning electron microscope (SEM) observations of the catalyst particles were made with a Hitachi-4800 SEM. Micrographs were taken at 3-kV acceleration voltage. The samples for SEM observation were prepared by dropping diluted polymerization solution taken from the reactor on conductive glue stuck on the sample stage in glove box. The sample stage was then moved to a vacuum chamber to dry the solution under vacuum. Before the SEM observations, all the sample surfaces were coated with a thin layer of gold. Propagation rate constant (kp ) of polymerization was calculated according to the equation Rp = kp [C* ][M], where Rp is the rate of polymerization detemined according to the equation Rp = −d[M]/dt (t is the polymerization time, and [M] is the instant monomer concentration in the reaction system), [C* ] is the concentration of active centers.
Fig. 1. Changes of polymerization rate (Rp ) with tp at different monomer/catalyst mass ratios.
3. Results and discussion 3.1. Kinetics of catalyzed 1-hexene polymerization Using a commercial MgCl2 -supported Ziegler-Natta catalyst and TEA cocatalyst, three series of 1-hexene polymerization reactions were conducted for different durations ranging from 30 s to 7200 s, in which the monomer/catalyst mass ratio was 30:1, 15:1 and 5:1 (or [1-Hexene] = 1.27, 0.63 and 0.21 M), respectively. Each polymerization run was quenched by TPCC at TPCC/TEA = 2 and 40 ◦ C for 2 min in order to selectively label each propagation chain end with a 2-thiophenecarbonyl group [32]. By measuring sulfur content of the quenched polymer sample, concentration of the active centers in the reaction system at the time of quencher addition was determined. The polymerization rate (Rp ) was determined by calculating the derivative of the yield-polymerization time (tp ) curve. Rate curves of the three polymerization series are shown in Fig. 1. It can be seen that the reaction rate reached the maximum after about 1 min and then quickly decayed with the proceeding of polymerization. This can be largely attributed to rapid decrease of monomer concentration with time, as the monomer conversion was found to be about 70% at tp = 300 s for the polymerization under different 1-hexene/catalyst ratios (see Tables S1, S2 and S3 in the Supplementary Data). After polymerization for 1 h, all the three systems reached the maximum conversion of about 90% (mhexene /mcat = 30, 15) or 80% (mhexene /mcat. = 5), which was almost unchanged by prolonging the reaction duration to 2 h (see detailed data in the Supplementary Data). It means that the residual monomer in the reaction system (about 0.05 − 0.1 M) was unable to take part in further polymerization after 1 h. Fig. 2 shows change of the number of active centers ([C* ]/[Ti]) with reaction time under different monomer/catalyst mass ratios. At mhexene /mcat = 5, [C* ]/[Ti] grew to the maximum level in
Fig. 2. Changes of active center concentration with tp at different monomer/catalyst mass ratios.
about 5 min, then gradually decreased in the next 2 h. At mhexene /mcat = 15, the maximum [C* ]/[Ti] value was slightly higher. When mhexene /mcat was further raised to 30, the maximum [C* ]/[Ti] value became much higher than those at lower monomer/catalyst mass ratios. It is worth noting that the [C* ]/[Ti] value at the very beginning (tp = 30 s) of the reaction was not evidently influenced by the monomer/catalyst mass ratio. It means that the increment of [C* ]/[Ti] in the reaction process is closely related with the amount of polymer and its production rate in the system. Gradual fragmen-
Fig. 3. Changes of chain propagation rate constant with tp at different monomer/catalyst mass ratios.
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Fig. 4. SEM pictures of the catalyst particles (a) and polymer/catalyst particles after 48 s (b), 194 s (c, d), 610 s (e, f) of 1-hexene polymerization at mhexene /mcat = 5:1, and polymer/catalyst particles after 150 s (g) and 630 s (h) of 1-hexene polymerization at mhexene /mcat = 30:1.
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tation of the catalyst particles by the hydraulic force of the formed polymer chains should be responsible for exposure and activation of more active centers in the induction period of polymerization. The degree of particle fragmentation seems to depend on the polymer/catalyst mass ratio. This phenomenon will be further discussed in the next section. When the reaction time was longer than 10 min, the [C* ]/[Ti] value gradually descended to lower level in the later stage of polymerization, especially when the initial mhexene /mcat ratio was low. This phenomenon can be explained by severe retard of chain propagation in the later stage. As seen in Table S3, at mhexene /mcat ratio of 5:1, [1-hexene] was lower than 0.06 mol/L after 10 min of polymerization, and the polymerization was almost ceased (see Tables S1-S3 of the Supplementary Data). Under the mutual effects of very low [1-hexene] and high barrier to monomer diffusion caused by high polymer concentration in the later stage of polymerization, the reactions of Ti − (CH2 )5 CH3 (product of chain transfer with 1-hexene) with 1-hexene to form new propagation chain would be greatly retarded, leading to gradual accumulation of Ti − (CH2 )5 CH3 and Ti − (CH2 CH(C4 H9 ))n H species with very short chain length (n < 10). Even though these active species can be quench-labeled by TPCC, because of very low molecular weight and high solubility in alcohols, their quenching products will be washed out from the polymer product during the purification procedures and thus cannot be counted into the [C* ] value. The value of chain propagation rate constant (kp ) was calculated according to the equation Rp = kp [C* ][hexene], which is generally accepted for describing the kinetics of catalyzed olefin polymerization. The changes of kp in the reaction process are shown in Fig. 3 and Tables S1 − S3 in the Supplementary Data. At different monomer/catalyst mass ratios, kp quickly decayed in the first 5 min of reaction, which can be mainly attributed to increase of monomer diffusion barrier by the growing polymer phase. The kp values of the three systems decreased with increase in mhexene /mcat , also showing the effects of diffusion limitation. 3.2. Fragmentation of the catalyst particles The morphologies of the original catalyst particles and the polymer/catalyst particles formed after different reaction durations were observed by SEM, and the SEM pictures are shown in Fig. 4. The original catalyst particles are spherical granules with big cracks on the surface. After 48 s of polymerization at initial mhexene /mcat = 5:1, most of the particles were fragmented into smaller pieces or partly fragmented, meaning that the growing polymer phase (mPH /mcat was about 0.5:1) had expanded the cracks in the particles. After longer durations of polymerization, higher extent of particle fragmentation can be observed (see Fig. 4c and e), and thin filaments can be seen in the cracks, which can be attributed to presence of poly(1hexene) phase in the particle. It should be noted that not all the produced polymer can be retained in the polymer/catalyst particles, because poly(1-hexene) is highly soluble in the reaction medium, n-heptane. This enabled observation of the catalyst fragments that are mainly composed of inorganic phases (MgCl2 crystallites and their aggregates) at mPH /mcat ratio larger than 2:1 (see Fig. 4c–f). If a large proportion of polymer phase was not dissolved into the solvent, it will be hard to see the morphology of the inorganic part of the particles. It was found that the plain catalyst particles were not fragmented after strring a n-heptane slurry of catalyst for 10 min at 40 ◦ C in the absence of TEA and monomer (SEM picture of the catalyst particles after stirring in n-heptane for 10 min at 40 ◦ C can be seen in Fig. S1 of the Supplementary Data). Therefore, the observed particle fragmentation in the polymerization processes cannot be attributed to mechanical force exerted by the stirrer on the catalyst particles. When the polymerization was conducted at higher mhexene /mcat ratio, severe fragmentation of the particles took shorter time than
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Fig. 5. MWD curve of a typical poly(1-hexene) sample (Run No. 22, mhexene /mcat = 30, tp = 60 s) and its deconvolution into five Flory components. (circles: experimental data points; lines: Flory components and their sum.).
the system with lower mhexene /mcat ratio. As seen in Fig. 4g and h, more filament structures can be seen in the cracks, and the external surface of the particle fragment became smoother as compared with the particles formed at lower mhexene /mcat ratio. It means that more poly(1-hexene) chains were retained in the particles, possibly caused by entanglement of the polymer chains in some narrow cracks. Morphological changes of the catalyst particles during the polymerization process can be taken as important evidence for explaining the continuous increase of active center concentration in the first 5–10 min of polymerization (see Fig. 2). A reasonable explanation is that a large proportion of active center precursors are buried inside the aggregates of MgCl2 crystallites, therefore they are inaccessible to the cocatalyst and/or monomer. When the catalyst particles are fragmented or disintegrated under the hydraulic force of the growing polymer, these active center precursors become accessible to the cocatalyst and monomer and are quickly activated, resulting in increase of [C* ] [43]. The dependence of the maximum [C* ] increment on the mhexene /mcat ratio (see Fig. 2) could be explained by the presence of active center precursors that can be exposed after different extent of particle fragmentation. Under rather lower mhexene /mcat ratio (e.g. mhexene /mcat = 5), because of the limited polymer volume, the weak hydraulic force of the polymer phase can only cause incomplete particle fragmentation, and a large proportion of the buried active sites cannot be released. Increasing the mhexene /mcat ratio can not only provide more polymer to fill the cracks in the particle, but also enhance the absolute chain propagation rate (see Fig. 1) to form stronger dynamic hydraulic force that can destroy the aggregates of MgCl2 crystallites. As a result, more buried active sites can be exposed and activated. 3.3. Distribution of active centers To determine the effects of mhexene /mcat mass ratio on the active center distribution (ACD), deconvolution of the polymer’s MWD curves with several Schulz-Flory most-probable distributions has been applied. This kind of deconvolution treatment has been found useful in differentiating the different active sites in heterogeneous Ziegler-Natta catalysts [2–4,44–48]. The MWD curves of the poly(1hexene) samples synthesized under different mhexene /mcat ratios and different reaction durations were satisfactorily deconvoluted into five Flory components respectively, and these components were named as component A, B, C, D and E in the order of decreasing molecular weight. Fig. 5 shows MWD curve of a typical poly(1-
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Fig. 6. Change of the fractions of different type of Flory components with monomer conversion at different monomer/catalyst mass ratios.
P. Yang et al. / Molecular Catalysis 447 (2018) 13–20
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Scheme 1. Model of clustered Ti species bridging two MgCl2 crystallites and their exposure and subsequent activation after breaking up of the aggregate. (Pale small spheres: Mg, dark small spheres: Ti, big spheres: Cl, 䊐: vacancy of active center, 䊉∼: propagation chain. The oval circle indicate clustered Ti species.).
hexene) sample and its deconvolution into five Flory components (more MWD curves are shown in Fig. S2 and Fig. S3 in the Supplementary Data). Each Flory component can be ascribed to polymer produced by a certain type of active center [3,4]. By calculating the weight percentage of a certain Flory component in the whole polymer and tracing its change in the polymerization process, we have studied correlations between the catalyst particle fragmentation and ACD (the data of weight fractions of the Flory components are listed in Table S4, Table S5 and Table S6 in the Supplementary Data). As shown in Fig. 6, change in mhexene /mcat ratio caused evident difference in distribution of the five Flory components (components A, B, C, D and E). Though the component A accounted for 12 − 20% of the total product when mhexene /mcat ratio was 15 or 30, it dropped to no more than 10% when mhexene /mcat ratio was lowered to 5. Meanwhile, the weight fractions of component D and E were evidently enhanced when mhexene /mcat ratio was lowered from 30 or 15–5. Similar changes of polymer MWD with the monomer concentration have been observed by Barabanov et al. [49]. The weight fractions of component B and C were bascially unchanged by varying the mhexene /mcat ratio. A possible explanation is that the active centers forming component A are formed from clustered Ti species that can only be exposed and activated after sufficient extent of particle fragmentation. According to literatures, the active centers producing high molecular weight ␣-olefin polymer have relatively higher stereospecificity [3,11,16–19]. These active centers are considered to be formed on clustered Ti species [15,50]. Based on this general recognition, we propose that the clustered Ti species on the catalyst surface are less accessible than the isolated Ti species, because the multiple Ti atoms and their Cl ligands congested in a limited space can form stereochemical hindrance to the approaching cocatalyst and monomer molecules. It is also possible that some clustered Ti species may form bridging structures connecting two or more MgCl2 crystallites. The Ti atoms in such structures will be in lack of free Ti − Cl bonds for reacting with the cocatalyst. Activation of such Ti species can only take place after breaking up of the bridging structures by the hydraulic force of the growing polymer chains. A possible bridging structure and activation of its Ti species is shown in the model of Scheme 1. According to this model, there is a positive correlation between activation of clustered and stereospecific Ti species (component A) and particle fragmentation. At higher mhexene /mcat ratios, the higher polymer/catalyst ratio at low conversion provides sufficient hydraulic force of the polymer chains to ensure quick breaking up of the bridging structures and release of more clustered and stereospecific Ti species, resulting in activation of more active centers of component A (e.g. monomer conversion of 20% in the mhexene /mcat = 15 system gives polymer/catalyst ratio of 3, in contrast to 1 for the mhexene /mcat = 5 system). On the other
hand, the Flory components D and E with the lowest molecular weight could be formed by active centers based on isolated Ti species, which are more accessible than the other types of active centers. When the mhexene /mcat ratio is low, these active centers can get relative preference in the competition for limited amount of monomer, because the active centers producing low molecular weight polymer have larger number as compared with the other centers [3,11,14]. This can explain the observed increase in component D and E with decrease of mhexene /mcat ratio. The accessibility of active centers of components B and C seems to fall in between those of component A and components D and E, so their weight fractions are less sensitive to the change in mhexene /mcat ratio. In summary, there are close correlations between the monomer/catalyst mass ratio and the increase of active center concentration in the induction period as well as the active center distribution. Combining with morphological change of the catalyst particles during the polymerization process, it can be concluded that the extent of particle fragmentation depends on the rate of polymer formation and its accumulation effect (namley, the polymer/catalyst mass ratio reached in the reaction process), which constitute the driving force to break up the catalyst particles. The shifting of ACD with intensification of particle fragmentation means that there are different types of active center precursors distributed inside the catalyst particles, and their release through particle fragmentation requires different intensity of polymer expansion in the particle. The present results suggest that the active centers from isolated Ti species can be released by relatively lower extent of particle fragmentation than those based on clustered Ti species. This kind of knowledge can promote in-depth understanding of the catalyst’s microstructure and the mechanism of forming such structure in the process of catalyst preparation.
4. Conclusions By tracing changes of [C* ]/[Ti] and the active center distribution in the process of 1-hexene polymerization with a MgCl2 -supported ZN catalyst, close correlations between monomer/catalyst mass ratio and increase of active center concentration as well as shifting of ACD in the induction period have been discovered. The extent of [C* ]/[Ti] increase in the induction period enhanced with increase of monomer/catalyst mass ratio of the polymerization system, meanwhile the catalyst particles were fragmented more severely in the reactions with larger monomer/catalyst mass ratio. All these phenomena show that the increment of [C* ]/[Ti] is a result of particle fragmentation under the hydraulic force of the growing polymer. In this catalyst, a large proportion of active center precursors are inaccessible to the monomer and cocatalyst as they are buried inside the solid phase of catalyst particles, and can be released
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