Bimodal Polyethylene

Article pubs.acs.org/IECR

Advanced Catalyst Technology for Broad/Bimodal Polyethylene, Achieved by Polymer-Coated Particles Supporting Hybrid Catalyst Binbo Jiang,† Yong Yang,† Lijun Du,‡ Jingdai Wang,*,† Yongrong Yang,† and Siegfried Stapf§ †

State Key Laboratory of Chemical Engineering, Department of Chemical and Biochemical Engineering, Zhejiang University, Hangzhou 310027, Zhejiang, China ‡ Shanghai 3F New Material Company, Ltd., No. 4411 Longwu Road, Shanghai 200241, China § Fachgebiet Technische Physik II/Polymerphysik, Institut für Physik, Technische Universität Ilmenau, Postfach 10 05 65, 98684 Ilmenau, Germany ABSTRACT: Polymer-coated particles were used to develop a novel catalyst technology for the production of broad/bimodal polyethylene in a single reactor. The methods for broad/bimodal polyethylene by catalyst technology, such as hybrid catalyst, composite support, and diffusion control, were combined together in this catalyst technology. An improved phase inversion method was used to fabricate poly[styrene-co-(acrylic acid)] (PSA) coated silica particles (SiO2/PSA). Core−shell structure was obviously observed in the resulting SiO2/PSA. SiO2/PSA showed a significant barrier effect to the diffusion of 1-hexene and nhexane but no barrier effect to the diffusion of ethylene. This means the PSA layer has a selective barrier effect to the diffusion of small molecules. PSA-coated silica particles were used to support (n-BuCp)2ZrCl2/TiCl4, TiCl3/TiCl4, and Cp2ZrCl2/Fe based hybrid catalysts. The ethylene polymerization results indicated that the two different active sites can perform independently, and broad/bimodal polyethylene was obtained by all hybrid catalysts. The polydispersity index of the resulting polyethylene was as high as 18.13, even 44.5. The style of the activity profile was determined by whether the active sites in the core needed a cocatalyst or not.

1. INTRODUCTION Advances in manufacturing and catalyst technologies are creating products with improved properties and easier processability.1,2 The latest trend in the polyethylene industry is tailor-made polymers, such as polyethylene with broad/ bimodal molecular weight distribution (MWD). This broad/ bimodal polyethylene has gained considerable popularity over its unimodal counterparts due to a better balance of the mechanical and rheological properties.3−5 At present, cascadereactor technology is the normally used method for production of broad/bimodal polyethylene in industry.6,7 Large capital and energy consumption is needed to build such multiple reactors, with complicated operation procedures. However, the existing processes for producing polyethylene, such as gas phase fluidized bed and slurry phase reactor, mainly contain only one reactor. As a result, custom-designed catalysts to produce broad/bimodal polyethylene in a single reactor, instead of the existing cascade-reactor technology, attracts researchers in various institutes and companies.5,6,8−10 If catalyst technology is realized, the energy consumption and capital will be significantly reduced and the operation procedure will be simplified. In the literature, three means are concluded for broadening the MWD of polyethylene. The most often studied method is a hybrid catalyst which combines two different catalytic sites, which have different kinetic responses such as different hydrogen responses and termination rate constants in one support particle. Ahmadi et al.11 loaded TiCl4 and Cp2ZrCl2 catalyst on MgCl2·nEtOH support produced by a melt quenching method. Ethylene polymerization was carried out respectively with triethylaluminum (TEA) or methylaluminoxane (MAO) as cocatalyst. The © 2013 American Chemical Society

hybrid catalyst needs a high ratio of [Al]/[M] to achieve considerable activity, and only with MAO as cocatalyst could it produce bimodal MWD polyethylene. Cho et al.12 prepared Ziegler−Natta/metallocene hybrid catalysts on recrystallized MgCl2 to get polyethylene with two different lamellar structures, but the MWD was less than 10. In another work, they combined MgCl2 and SiO2 to be one support for the impregnation of Cp2ZrCl2 and TiCl4 catalysts by recrystallization and the sol−gel method.13 When MAO and triisobutylaluminum (TIBA) were used together as cocatalyst, polyethylene with a bimodal MWD pattern was produced. Choi et al.14 reported that they have produced polyethylene with tailored molecular weight and chain branch distributions by immobilizing metallocene and nickel diimine catalyst on SiO2. However, it is hard to achieve the optimized hybrid catalyst technique due to some rigorous requirements for the two different catalysts:9,15 balanced activity in polymerization conditions; long and stable polymerization lifetime; difference in molecular weight of polymers from each active species; good and balanced hydrogen and comonomer response; good polymer morphology. They should also be kinetically and chemically compatible. Therefore, some researchers attempted to synthesize composite carriers with multiple chemical environments to support one catalyst. It is expected that more different active sites are formed by the influence of the various chemical environments of the support. Feng et al.16 Received: Revised: Accepted: Published: 2501

July 24, 2012 January 17, 2013 January 22, 2013 February 6, 2013 dx.doi.org/10.1021/ie301971k | Ind. Eng. Chem. Res. 2013, 52, 2501−2509

Industrial & Engineering Chemistry Research

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synthesized by combining silica (core) with poly(styrene− acrylic acid) membrane (shell) through the phase inversion method to construct two independent regions in one particle. The catalyst can anchor on the poly(styrene−acrylic acid) (PSA) membrane by bonding between the active metal atom of catalyst and the functional group (−COOH) of PSA. The PSA membrane also serves as the barrier to drain off the reactants. The catalyst which is more sensitive to hydrogen is chosen to be supported on the core (SiO2), while the catalyst which is less sensitive to hydrogen is chosen to be supported on the shell (PSA). Ziegler−Natta catalyst, metallocene, and late-transitionmetal catalyst are the candidates to be combined on the polymer-coated particles. The preparation details of hybrid catalysts will be introduced in the Experimental Section. At the beginning of the polymerization, the outer catalyst on the PSA layer is available for all reactants. However, the inner catalyst has a different fate. The PSA layer works as a filter. The small ethylene and hydrogen molecules are able to diffuse through PSA, and the polymerization is initiated when they reach active centers. The larger cocatalyst (triethylaluminium) and comonomer (hexene) molecules will diffuse through the membrane layer much more slowly, and it will take more time for them to reach the inner active centers. As a result, the reactant composition around the inner catalyst is different from that around the outer catalyst. It is possible to supply suitable reaction conditions for the two individual catalysts respectively, and the properties of the resulting polyethylene will improve. Different thicknesses of the membrane will result in varying reactant compositions, and the resulting polyethylene will show different properties.

produced polyethylene with an achieved polydispersity index (PDI) as high as 27.6 by Ziegler−Natta catalyst supported on a composite carrier, which was MgCl2/SiO2. This is an inorganic composite support. Kaur et al.17 synthesized an inorganic/ organic mixed support, which was MgCl2·xEB/poly(methyl acrylate-co-1-octene), and then TiCl4 was immobilized on it. The PDI of the produced polyethylene by the supported catalysts with different compositions was 10.6, 12.0 and 21.4, respectively. Generally speaking, the existing preparation methods of composite support in the existing reports were complicated. Even more, some impurities which were toxic to the active sites were introduced into the composite particles,18−21 leading to low catalyst activity. Besides, Silveira et al.22−24 tried to achieve the diffusion control of reactants by the precise control of the pore size of the support. They synthesized several kinds of mesoporous support, aiming to form different reactant compositions around the active sites along the catalyst particle by selective diffusion features. Subsequently, Serrano et al.25 prepared a silica support with a bimodal pore size distribution and loaded (nBuCp)2ZrCl2. Although the MWD of the polyethylene was still unimodal, the PDI improved to 4.0 from 2.0. The most interesting result was that the branch distribution of polyethylene was bimodal. The active sites on the larger pore have more opportunities to copolymerize ethylene with 1-hexene while the active site on the smaller pore size has less opportunity to touch 1-hexene, resulting in more homopolyethylene. This indicated that the size of the pores determined the diffusion of 1-hexene. Our approach toward designing effective catalyst technology for broad/bimodal polyethylene for industrial gas phase and slurry polymerization processes is based on combining the effects of hybrid catalyst, composite support, and diffusion control. We aim to utilize the advantages and avoid the disadvantages of the existing catalyst technologies. The polymer-coated particles are used to support hybrid catalysts. Our approach is to realize production of broad/bimodal polyethylene in a single reactor. We have researched this catalyst technology for a long time, and part of the discussions are supported by the results which have been published before.26,27 This article aims to completely and deeply interpret the concept of our catalyst technology based on a series of hybrid catalysts supported on polymer-coated particles.

3. EXPERIMENTAL SECTION 3.1. Materials. Silica (grade XPO2485) from W. R. Grace & Co. was dried at 600 °C for 4 h under nitrogen flow. Poly[styrene-co-(acrylic acid)] (PSA), provided by Changchun Institute of Applied Chemistry, Chinese Academy of Science, was dried at 70 °C under nitrogen flow for 24 h before use. Two kinds of PSA with different molecular weights and functional groups −COOH were used. The weight-average molecular weight of PSA is 19 000 g·mol−1, and the molar ratio between styrene and acrylic acid is 1.5. Toluene, n-hexane, and n-heptane were purified by the solvent purification system of Innovative Technology (USA). High-purity nitrogen, polymerization-grade ethylene, and hydrogen were obtained from SINOPEC Shanghai Corp. (Shanghai, China) and purified by sequentially passing them through a copper catalyst column and a alumina column. MgCl2 and 3TiCl3·AlCl3 were donated by SINOPEC Tianjin Corp. (Tianjin, China). Bis(n-butyl cyclopentadienyl) zirconium dichloride ((n-BuCp)2ZrCl2, ACROS Organics, USA) and titanium(IV) chloride (TiCl4, ≥98.0 wt %) were purchased and used without further treatment. Modified methylaluminoxane (MMAO, 7 wt % solution in heptane) and trimethyaluminum (TMA, 2.0 mol/L) were purchased from Akzo Nobel. Triethylaluminum (TEA, 1 mol/L solution in n-heptane), butylmagnesium chloride (BuMgCl, 20 wt % solution in toluene/THF), Fe(acac)3, and 2,6-bis[1-(2-isopropylphenylimino)ethyl]pyridine were purchased from J&K Chemical Corp. 3.2. Synthesis of Polymer-Coated Particles. In a typical procedure, PSA was dissolved in toluene at room temperature. Figure 1 shows the equipment provided for fabricating polymer-coated particles. In a 250 mL glass flask, 10 mL of PSA/toluene solution was stirred with 2.0 g of silica at 0 °C. A

2. CONCEPT A creative design is proposed in Scheme 1, toward broad/ bimodal polyethylene achieved by catalyst technology. The key of the design is the carrier of a core−shell structure, which is Scheme 1. Proposed Mechanism for the Formation of the Hybrid Catalyst

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was completed, resulting in the catalyst precursor TiCl3/ MgCl2/THF. The solution was then cooled to room temperature. The above treated silica was added to this solution. The mixture was agitated for 2 h and excess THF was distilled. The residual solid was dried for 3 h at 65 °C under a nitrogen flow, and finally pink free-flowing powders were obtained. This was a kind of traditional Ziegler−Natta catalyst. A mixture of the catalyst obtained above (4.0 g) and the PSA/MgCl2/THF solution (32 mL) was stirred in a 250 mL Schlenk flask at 150 rpm at 0 °C. According to the previously described technique of coating polymer membrane, the PSAcoated catalyst was obtained. A 2.0 g sample of PSA-coated catalyst and 40 mL of hexane were added to a 200 mL Schlenk flask containing a magnetic stirring bar. TiCl4 (0.26 g) was added via syringe to the stirred mixture. After 30 min, the liquid phase was then removed by evaporation under nitrogen flow at 55 °C, to obtain a freeflowing pale yellow powder. This was hybrid catalyst (HC). 3.4. Ethylene Polymerization. Slurry ethylene polymerization was carried out in a 1 L Büchi stainless steel autoclave reactor, equipped with a mechanical stirrer, a mass flow meter, and a temperature control unit consisting of cooling water and an electric heater. The reactor was heated above 90 °C for more than 3 h and repeatedly pressurized with nitrogen, purged, and evacuated ( ZT-2 > ZT-3. Normally, the molecular weight distribution of the polyethylene produced by metallocene catalyst is narrow.33 However, the polyethylene synthesized by the core−shell hybrid catalysts showed a very broad, even bimodal MWD (Figure 6) due to the combination of zirconocene and titanium based Ziegler−Natta catalysts. In our previous work,34 it was concluded that the PSA-supported TiCl4 could produce polyethylene with a higher MW than (n-BuCp)2ZrCl2 under identical polymerization conditions. The polydispersity index (PDI) of the polyethylene produced by ZT-1, ZT-2, and ZT-3

Figure 5. Activity profiles of polymerization by (n-BuCp)2ZrCl2/TiCl4 hybrid catalysts (a) ZT-1, (b) ZT-2, and (c) ZT-3. Polymerization conditions: P, 0.83 MPa; H2, 0.1 MPa; n-heptane, 350 mL; 1-hexene, 20 mL; T, 75 °C; Al/Ti = 120.

is 7.92, 11.13, and 18.16, respectively. This indicates that the MWD is strongly influenced by the composition of low molecular weight polyethylene and high molecular weight polyethylene. ZT-1 has the highest activity because more polyethylene was produced by (n-BuCp)2ZrCl2. Reasonably, the polyethylene produced by ZT-2 showed a lower molecular weight and PDI. Under normal circumstances, the performance of zirconocene catalysts increases with higher Al/Zr molar ratio.35 (n-BuCp)2ZrCl2 in ZT-3 is thought to produce less polyethylene than in ZT-2 due to the decreased Al/Zr molar

Figure 4. SEM phtographs of silica and (n-BuCp)2ZrCl2/TiCl4 hybrid catalyst particles: (a) silica; (b−d) hybrid catalyst under different magnifications. 2505



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a barrier effect on some reactants. According to the results of diffusion measurements in Table 1, the PSA layer has no barrier effect on the diffusion of ethylene. Reasonably, the diffusion of TEA is the reason for the increasing decaying activity profile. TEA needs more time to diffuse through the PSA layer than ethylene. This leads to low activity at the initial stage of ethylene polymerization. With the diffusion of TEA, the inside active sites are initiated and consequently the activity increases. On the other hand, more active sites are exposed to the reactants due to the fragmentation of the outer catalyst. This also increases the catalyst activity. 4.5. Metallocene/Late-Transition-Metal Hybrid Catalyst. Cp2ZrCl2 hybrid with 2,6-bis[1-(2isopropylphenylimino)ethyl]pyridine/Fe(acac)3 (SiO2−MAO/ Zr/PSA−MgCl2/Fe) was prepared, which Zr was supported on the inner SiO2 and Fe was supported on the outer PSA. For comparison, 2,6-bis[1-(2-isopropylphenylimino)ethyl]pyridine/Fe(acac)3 supported on PSA layer of the polymercoated particles and Cp2ZrCl2 supported on SiO2 (SiO2− MAO/Zr) and were also prepared (SiO2/PSA−MgCl2/Fe). The hybrid catalyst also showed a decaying activity profile because the inner Cp2ZrCl2 needs no cocatalyst. The properties of the resulting polyethylene are listed in Table 3. The molecular weight and molecular weight distribution are critical. The PDI of the polyethylene produced by supported Cp2ZrCl2 was 2.33. The average molecular weight was 1.2 × 105 g·mol−1. The average molecular weight of Fe was low and the PDI was as high as 36.4. The PDI of the hybrid catalyst was 44.5. It is clear that polyethylene with high molecular weight is shown. A bimodal molecular weight distribution is obvious. Also, the molecular weight of final polyethylene produced by hybrid catalyst is between the molecular weights of polyethylene produced by metallocene and polyethylene produced by late transition metal. The GPC chromatograms of the produced polyethylene in Figure 8 are more specific to interpret the variation of

Figure 6. GPC results of the polyethylene produced by the hybrid catalysts (a) ZT-1, (b) ZT-2, and (c) ZT-3.

ratio in ZT-3. As a result, the relative composition of high MW polyethylene is greater in ZT-3, leading to a high average molecular weight. The above GPC analysis results suggest that both (n-BuCp)2ZrCl2 and TiCl4 catalyst are active toward the copolymerization of ethylene/1-hexene. This means that (nBuCp)2ZrCl2 and TiCl4 are compatible on SiO2/PSA core− shell microspheres and perform independently due to the separation effect of the PSA shell. 4.4. Hybrid Titanium Catalyst. The results of hybrid titanium catalyst have been published already.34 Polyethylene with a broad MWD was produced. Here we aim to analyze the activity profile as shown in Figure 7. SiO2/MgCl2/TiCl3

Figure 7. Activity profile of (a) SiO2/MgCl2/TiCl3 and (b) hybrid titanium catalyst.

Figure 8. GPC results of the obtained polyethylene in (a) SiO2− MAO/Cp2ZrCl2, (b) SiO2/PSA−MgCl2/Fe, and (c) Cp2ZrCl2/PSA− MgCl2/Fe.

without a PSA layer showed typical decaying activity profile, while the hybrid titanium catalyst showed an increasing decaying activity profile. This indicates that the PSA layer has

Table 3. Properties of the Resulting Polyethylene Produced by Single and Hybrid Catalysts catalyst

melting point (°C)

crystallinity (%)

MW (×104 g·mol−1)

PDI

SiO2−MAO/Zr SiO2/PSA−MgCl2/Fe SiO2−MAO/Zr/PSA−MgCl2/Fe

135.33 131.63 130.40

71 70 66

35.3 11.9 22.2

2.33 36.4 44.5

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Figure 9. Stages of polymer-coated particle supported catalyst polymerization of olefins. (a) The inner catalyst needs a cocatalyst, and (b) the inner catalyst needs no cocatalyst.

If the active sites in the core are available without a cocatalyst, the catalyst supported on polymer-coated particles would show an activity profile like that in Figure 9b. The inner active sites can catalyze ethylene to be polymerized as soon as they touch ethylene. As we know, the PSA layer has no barrier effect on ethylene diffusion. Initially, the inner active sites touch ethylene and show the highest activity. Large amounts of polyethylene are generated to hinder the diffusion of ethylene. Consequently, the activity will decrease. As a result, a typical decaying-style activity profile is reasonably proposed. The fragmentation of the catalyst particles should be from inside to outside. This is consistent with the activity profile of a metallocene/Ziegler−Natta hybrid catalyst.

molecular weight distribution. Figure 8a is the GPC chromatogram of polyethylene produced by SiO2−MAO/Zr while Figure 8b is the GPC chromatogram of polyethylene produced by SiO2/PSA−MgCl2/Fe. The position of the spectrum of polyethylene produced by hybrid catalyst (Figure 8c) is between the chromatogram of polyethylene produced by single metallocene and that produced by late-transition-metal catalyst. This definitely means that one part of the polyethylene is produced by late-transition-metal catalyst and the other part is produced by metallcoene. 4.6. Proposed Polymerization Process of Catalyst Supported on Polymer-Coated Particles. Based on the results of the activity profiles of metallocene/Ziegler−Natta hybrid catalyst and Ziegler−Natta/Ziegler−Natta hybrid catalyst, two different polymerization processes are proposed here. If the active sites in the core need a cocatalyst, the catalyst supported on polymer-coated particles would show an activity profile like that in Figure 9a. The initial activity is owing to the active sites on the PSA layer. The inner active sites cannot perform until they contact with cocatalyst which has to pass through the PSA layer. Due to the barrier effect of the PSA layer, the diffusion of TEA is much slower than that of ethylene. As a result, the activity will increase with more inner active sites touching the cocatalyst. The activity will achieve its maximum in a period and then start to reduce. The fragmentation of the catalyst particles is from outside to inside. This is consistent with the activity profile of a Ziegler−Natta hybrid catalyst.

5. CONCLUSION A novel catalyst technology was developed for the production of broad/bimodal polyethylene. Poly[styrene-co-(acrylic acid)]coated silica (SiO2/PSA) particles were prepared by an improved phase inversion method which we developed. A core−shell structure was obviously observed in SiO2/PSA. Effective diffusion coefficients of ethylene, 1-hexene, and nhexane in SiO2 and SiO2/PSA particles were obtained by gravimetric adsorption measurements. The effective diffusion coefficients of 1-hexene and n-hexane in SiO2/PSA were less than half of those in SiO2, indicating that the PSA layer had a significant barrier effect on the diffusion of 1-hexene and nhexane but no barrier effect on the diffusion of ethylene. The 2507



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(11) Ahmadi, M.; Jamjah, R.; Nekoomanesh, M.; Zohuri, G. H.; Arabi, H. Ziegler-Natta/metallocene hybrid catalyst for ethylene polymerization. Macromol. React. Eng. 2007, 1 (6), 604−610. (12) Cho, H. S.; Chung, J.; Han, J.; Ko, Y.; Lee, W. Polymerization of ethylene of ethylene and ethylene/1-hexene over Ziegler-Natta/ Metallocene hybrid catalysts supported on MgCl2 prepared by a recrystallization method. J. Appl. Polym. Sci. 1998, 70 (9), 1707−1715. (13) Cho, H.; Chung, J.; Lee, W. Control of molecular weight distribution for polyethylene catalyzed over Ziegler-Natta/metallocene hybrid and mixed catalysts. J. Mol. Catal. A: Chem. 2000, 159 (2), 203−213. (14) Choi, Y. Y.; Soares, J. Supported hybrid early and late transition metal catalysts for the synthesis of polyethylene with tailored molecular weight and chemical composition distributions. Polymer 2010, 51 (21), 4713−4725. (15) Mehildiabadi, S.; Soares, J.; Dekmezian, A. H. Simulation of polymerization and long chain branch formation in a semi-batch reactor using two single-site catalysts. Macromol. React. Eng. 2008, 2 (1), 37−57. (16) Wang, J. F.; Wang, L.; Gao, H. Q.; Wang, W. Q.; Wang, W.; Zhao, Z. R.; Sun, T. X.; Feng, L. F. Ethylene polymerization using a novel MgCl2/SiO2-supported Ziegler-Natta catalyst. Polym. Int. 2006, 55 (2), 299−304. (17) Kaur, S.; Singh, G.; Makwana, U. C.; Gupta, V. K. Immobilization of titanium tetrachloride on mixed support of MgCl2·xEB/poly(methyl acrylate-co-1-octene): catalyst for synthesis of broad MWD polyethylene. Catal. Lett. 2009, 132 (1−2), 87−93. (18) Furusawa, K.; Anzai, C. Preparation of composite particles by hetercoagulations. Colloid Polym. Sci. 1987, 265 (10), 882−888. (19) Sertchook, H.; Avnir, D. Submicron silica/polystyrene composite particles prepared by a one-step sol-gel process. Chem. Mater. 2003, 15 (8), 1960−1694. (20) Qiang, W.; Wang, Y.; He, P.; Xu, H.; Gu, H.; Shi, D. Synthesis of asymmetric inorganic/polymer nanocomposite particles via localized substrate surface modification and miniemulsion polymerization. Langmuir 2008, 24 (3), 606−608. (21) Yu, D. G.; An, J. H. Preparation and characterization of titanium dioxide core and polymer shell hybrid composite particles prepared by two-step dispersion polymerization. Polymer 2004, 45 (14), 4761− 4768. (22) Silveria, F.; Petry, C. F.; Pozebon, D.; Pergher, S. B.; Detoni, C.; Stedile, F. C.; Dos Santos, J. H. Z. Supported metallocene on mesoporous materials. Appl. Catal., A: Gen. 2007, 333 (1), 96−106. (23) Silveria, F.; Pires, G. P.; Petry, C. F.; Pozebon, D.; Stedile, F. C.; Dos Santos, J. H. Z.; Rigacci, A. Effect of the silica texture on grafting metallocene catalysts. J. Mol. Catal. A: Chem. 2007, 265 (1−2), 167− 176. (24) Silveria, F.; Do Carmo Martins Alves, M.; Stedile, F. C.; Pergher, S. B.; Dos Santos, J. H. Z. Microporous and mesoporous supports and their effect on the performance of supported metallocene catalysts. J. Mol. Catal. A: Chem. 2010, 315 (2), 213−220. (25) Serrano, D. P.; van Grieken, R.; Melgares, A. M.; Moreno, J. Synthesis of hard mesoporous macrospheres with silicate and aluminosilicate compositions. J. Porous Mater. 2010, 17 (4), 387−397. (26) Lijun, D.; Wei, Q.; Jingdai, W.; Yongrong, Y.; Wenqing, W.; Binbo, J. An improved phase-inversion process for the preparation of silica/poly[styrene-co-(acrylic acid)] core−shell microspheres: synthesis and application in the field of polyolefin catalysis. Polym. Int. 2011, 60 (4), 584−591. (27) Du, L. J.; Li, W.; Fan, L. N.; Jiang, B. B.; Wang, J. D.; Yang, Y. R.; Liao, Z. W. Hybrid titanium catalyst supported on core-shell silica/ poly(styrene-co-acrylic acid) carrier. J. Appl. Polym. Sci. 2010, 118, 1743−1751. (28) Kärger, J.; Valiulim, R. Diffusion in Porous Media. In eMagRes (Encyclopedia of Magnetic Resonance) [Online]; John Wiley & Sons, Ltd.: New York, 2011. DOI: 10.1002/9780470034590.emrstm0121.pub2. (29) Zechlin, J.; Hauschild, K.; Fink, G. Macromol. Chem. Phys. 2000, 201, 597−603.

PSA layer has a selective barrier effect on the diffusion of small molecules. PSA-coated silica particles were used to support (nBuCp)2ZrCl2/TiCl4, TiCl3/TiCl4, and Cp2ZrCl2/Fe based hybrid catalysts. The polyethylenes produced by these three hybrid catalysts showed very broad or bimodal molecular weight distributions. The PSA layer can isolate the active sites on the SiO2 core from the active sites on the PSA layer. This ensures that the two different catalysts can independently perform and produce polyethylenes with different molecular weights. Finally, the effect of the PSA layer on the activity profiles in ethylene polymerization was discussed. The style of the activity profile is determined by whether the active sites in the core need a cocatalyst or not.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 571 87951227. Fax: +86 571 87951227. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support provided by The Project of National Natural Science Foundation of China (21176208), the National Basic Research Program of China (2012CB720500), and the Fundamental Research Funds for the Central Universities (2011QNA4032) is gratefully acknowledged.

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REFERENCES

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