Ind. Eng. Chem. Res. 2008, 47, 5369–5375
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Effect of Catalysts Supporting on Tandem Polymerization of Ethylene Stock in Synthesis of Ethylene-1-Hexene Copolymer Junwei Zhang,† Hong Fan,*,† Bo-Geng Li,*,† and Shiping Zhu*,‡ Department of Chemical & Biochemical Engineering, State Key Laboratory of Polymer Reaction Engineering, Zhejiang UniVersity, Hangzhou, People’s Republic of China 310027, and Department of Chemical Engineering, McMaster UniVersity, Hamilton, Ontario, Canada L8S 4L7
Ethylene-1-hexene copolymers were synthesized with a tandem catalysis system consisting of a new trimerization catalyst (1), bis(2-dodecylsulfanylethyl)amine-CrCl3, and a copolymerization catalyst (2), Et(Ind)2ZrCl2. Catalysts 1 and 2 were supported on silica particles, and the effects of different supporting strategies on trimerization selectivity, 1-hexene incorporation efficiency, and copolymerization activity were studied and compared to the homogeneous system. It was found that the supported 1 trimerized ethylene with a similar selectivity (>99%) but one-quarter of the activity of the homogeneous 1. The supported 2 gave 40% 1-hexene incorporation efficiency and one-third of the copolymerization activity of the homogeneous 2. The tandem action of the supported 1 and 2 yielded linear low-density polyethylene (LLDPE) materials that contained only C4 side-chains. The dual supported system had activities at a 107 g/(mol Zr h) level, in the same order of the homogeneous counterpart. Adjusting the Cr/Zr ratio yielded various branching densities and, thus, melting temperatures of the resulting polymers. The samples prepared with the supported 2 exhibited broad differential scanning calorimetry (DSC) curves, probably due to multiple active sites. Introduction Recently, the synthesis of linear low-density polyethylene (LLDPE) from ethylene-only stock in a single reactor by a tandem action of oligomerization and copolymerization catalysts has attracted good attention from academic and industrial researchers.1 Compared to the commonly used two-stage process, this single-stage approach has a clear advantage in plant investment, R-olefin purification, storage, and transport. There have been many investigations reported in the literature. Various combinations of ethylene oligomerization catalyst and Z-N or metallocene copolymerization catalyst have been applied to synthesize ethylene/R-olefin copolymers under various reaction conditions. The effects of catalyst ratio as well as other reaction conditions on the catalyst activity, incorporation efficiency, selectivity of oligomerization catalyst, and resulting copolymer properties were extensively studied to obtain efficient, highselectivity, and high-activity catalysis systems.2–6 However, most of the works were on homogeneous tandem catalysis systems. Only a few heterogeneous systems were applied to the tandem synthesis of LLDPE.7–12 As can be seen in Table 1, there were only several studies on supporting copolymerization catalysts. None of them supported both oligomerization and copolymerization catalysts and examined the influence of supporting strategies on the tandem action of two catalysts and properties of resulting polymers. Recently, there were many highly efficient and highly selective homogeneous trimerization catalysts reported in the literature, especially some chromium-based systems.13 However, supported trimerization catalysts received less attention. Severn et al.14 supported (C5H4CMe2Ph)TiCl3 on MgCl2 to determine if the catalyst retained its trimerization ability. The homogeneous catalyst was shown by Hessen et al. to be very effective in * Corresponding authors. E-mail:
[email protected] (H. Fan);
[email protected] (B. Li);
[email protected] (S. Zhu). † Zhejiang University. ‡ McMaster University.
trimerizing ethylene.15 However, Severn et al.’s result showed that the MgCl2-supported trimerization catalyst became a polymerization catalyst. The coordination between pendant aryl ring and metal center, which is essential for trimerization, was no longer operative after the MgCl2 supporting. It was found that the support changed the characteristics of active sites from trimerization to polymerization. So far, there has been no report on the tandem polymerization of ethylene using both supported oligomerization and copolymerization catalysts. The effects of various supporting strategies on trimerization and copolymerization are not yet to be revealed. Recognizing the importance of heterogeneous catalysts in ethylene polymerization with current reactor technologies, we made a good effort to systematically investigate catalyst supporting and its effects on tandem catalysis in ethylene polymerization and thermal properties of the resulting polymers. In our previous work, we developed an effective homogeneous tandem catalysis system (see Scheme 1). The system consisted of the trimerization catalyst bis(2-dodecylsulfanylethyl)amine-CrCl3 (1) and the copolymerization catalyst Et(Ind)2ZrCl2 (2). We used the system for the synthesis of ethylene-1-hexene copolymers under atmospheric pressure.16 The study showed that the polymerization conditions, such as temperature, metal mole fraction, and pretrimerization time, significantly influence catalyst activity and polymer properties. In this study, we carried out ethylene polymerization with various supported tandem catalyst systems and investigated the effects of catalyst supporting on the trimerization selectivity, 1-hexene incorporation efficiency, and copolymerization activity, as well as thermal properties of the resulting copolymers. Three combinations of the supported catalysts were examined, namely, supported 1 with homogeneous 2 (system B), homogeneous 1 with supported 2 (system C), and supported 1 with supported 2 (system D). The results of the supported systems were compared
10.1021/ie7017564 CCC: $40.75 2008 American Chemical Society Published on Web 06/25/2008
5370 Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 Table 1. Synthesis of LLDPE Using Supported Tandem Catalysis Systems no.
copolymerization catalyst support cocatalyst
oligomerization catalyst cocatalyst
oligomerization product
ref.
1
Ti(C5Me4SiMe2NR)Cl2 (R ) Me or tBu) SiO2-Si-CH2CH2C5H4N MAO Et(Ind)2ZrCl2 SiO2 MAO Et(Ind)2ZrCl2 MMT MA0 TiCl4 MgCl2 MAO TiCl4 MgCl2 MAO/TEA Cp2ZrCl2 SMAO MAO
(C5H4NCHdNAr)NiBr2 (Ar ) 2,6-diisopropylphenyl) MAO
1-olefin/2-olefin ) 1:7 MW ) 430
7
Ti(OBu)4 TEA
1-butene
8
[(2-ArNdC(Me))2C5H3N]FeCl2 (Ar ) 2,4-C6H4(Me)2) MAO [(2-ArNdC(Me))2C5H3N]FeCl2 (Ar ) 2-Cl-4-C6H4(Me)2) MAO [(2-ArNdC(Me))2C5H3N]FeCl2 (Ar ) 2-Cl-4-C6H4(Me)2) MAO/TEA TpMsNiCl (TpMs ) HB(3-mesityl-pyrazolyl)3) MAO
1-butene 30%, 1-hexene 20%, g1-octene 45% 1-butene 30%, 1-hexene 20%, g1-octene 45% 1-butene 30%, 1-hexene 20%, g1-octene 45% 1-butene 50%, 1-hexene 20%
9
2 3 4 5 6
Scheme 1. Structures of the Trimerization and Copolymerization Catalysts
to their homogeneous counterpart, i.e., homogeneous 1 with homogeneous 2 (system A). Experimental Section Materials. All the manipulations were performed under nitrogen atmosphere in glovebox or Schlenk techniques. Toluene was refluxed over metallic potassium with benzophenone as an indicator and was distilled under nitrogen atmosphere prior to use. Nitrogen and polymerization-grade ethylene (Sinopec, China) were purified by passing through CuO catalyst and 3 Å molecular sieves. The cocatalyst methylaluminoxane (MAO, 10% in toluene) from Albemarle Corporation was used as received. The trimerization catalyst 1 was synthesized according to the published procedure.17 The copolymerization catalyst 2 Et(Ind)2ZrCl2 from Strem Company was used as received. Silica (Grace 955) was donated by Grace Davison and was activated under nitrogen atmosphere for 5 h at 450 °C. The support was then cooled to room temperature under vacuum and was stored under nitrogen atmosphere in the glovebox. The silica material has the following characteristics: average pore diameter ) 230 Å, pore volume ) 1.62 cc/g, total Brunauer-Emmett-Teller (BET) surface area ) 280 m2/g, and average particle size ) 53.5 µm. Preparation of 1/MAO/SiO2 and 2/MAO/SiO2. The supported catalysts were prepared as described in Scheme 2. SiO2/ MAO was prepared by impregnating 1.0 g of thermally treated silica Grace 955 with 4 mL of MAO-toluene solution at 50 °C for 3 h under stirring. A bis(2-dodecylsulfanylethyl)amine-CrCl3 solution (0.06 g in 30 mL of toluene) or an Et(Ind)2ZrCl2 solution (0.01 g in 30 mL of toluene) was transferred to the SiO2/MAOcontaining toluene slurry at 50 °C, and the mixture was stirred for 4 h. The slurry was then centrifuged and filtered. The resulting
10 11 12
solid was washed with toluene, centrifuged and filtered several times, and dried under vacuum. The final catalyst loadings on silica, determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), were 0.18 wt % Zr/catalyst and 0.35 wt % Cr/catalyst, respectively. Ethylene Trimerization with 1/MAO or (1/MAO/SiO2)/ MAO. The trimerization was carried out in a 250 mL glass reactor equipped with a magnetic stirrer under atmospheric ethylene pressure. After air was evacuated and exchanged with pure and dry nitrogen at 100 °C, the reactor was flashed and pressurized with ethylene. It was then placed into an oil bath set at the operating temperature. Toluene (100 mL) and a desired amount of MAO were introduced to the reactor. After equilibrating for 10 min, a prescribed amount of the 1/toluene stock solution was injected or 1/MAO/SiO2 powder was flashed by toluene into the reactor to start trimerization. The reaction temperature and ethylene pressure were kept constant throughout the trimerization process. Magnetic stirring was applied. After 30 min, the reactor was cooled down and vented. Heptane (1 mL) was injected as an internal standard for analysis. The trimerization product was collected for gas chromatography or GC-MS measurements. Ethylene Polymerization with Tandem Catalysis System. A procedure similar to the trimerization was applied. The trimerization and copolymerization catalysts were injected to the reactor at the same time. After 30 min of polymerization, the reactor was vented and 200 mL of acidified alcohol was added. The polymer materials were collected, washed with alcohol, and dried overnight. Characterization. GC analysis was conducted on Agilent 6890N GC to determine the 1-hexene content in the trimerization product. The polymer melting point (Tm) was measured using Perkin-Elmer DSC 7 in the standard mode. The temperature and heat capacity of the instrument were calibrated with an indium standard. The polymer sample (about 5.2 mg) was first heated to 180 °C at the rate of 10 °C/min to remove thermal history. It was then cooled down to 40 °C at 10 °C/min. A second heating cycle was used for the acquisition of a differential scanning calorimetry (DSC) thermogram at the rate of 10 °C/min. The highest peak was reported as the Tm value. Selected samples (A4, B4, C4, and D4) were analyzed with gel permeation chromatography (GPC) and 13C NMR. Polymer molecular weight (MW) and molecular-weight distribution (MWD) were measured by GPC with a PL-GPC220 coupled with an in-line capillary viscometer. The analyses were per-
Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 5371 Scheme 2. Preparation of Supported Trimerization and Copolymerization Catalysts
formed at 150 °C using 1,2,4-trichlorobenzene as solvent with a flow rate of 1 mL/min. A calibration curve was established with monodisperse polystyrene standards. 13C NMR 300 MHz analysis was conducted on a Varian Mercury 300 pulsed NMR spectrometer at 120 °C. The polymer sample was dissolved in o-dichlorobenzene (about 5 wt % polymer) in a 10 mm NMR tube. At least 2000 scans were applied in each acquisition to obtain a good signal-to-noise ratio. The chemical shift assignments and the estimate of copolymer composition followed the Randall method.18 Results and Discussion The 1/MAO system was found to be highly active and selective for ethylene trimerization. An analysis of the liquid fraction by GC-MS revealed that ethylene trimerization by 1/MAO under various conditions yielded 1-hexene with an excellent selectivity. The product had >99% purity. Only a trace amount of polyethylene was found as a byproduct at low reaction temperatures. Our motivation was to examine if the catalyst is still effective for ethylene trimerization after supporting. To exclude the influence of the leached catalyst from the support, the supported trimerization catalyst was washed with a toluene-MAO mixture at 75 °C, stirred for 10 min, and filtrated. The colorless filtrate was used as a homogeneous trimerization catalyst but showed no reactivity, which was strong proof that the active sites were truly supported and could not be leached off. The experimental data showed that the supported catalyst system, (1/MAO/SiO2)/MAO, under various conditions yielded the same products as the 1/MAO system. The selectivity was also >99% of 1-hexene. Unlike the catalysts reported in the literature, the selectivity of this catalyst was retained after supporting. However, the trimerizatiom activity dropped to one-quarter of that previously, as shown in Figure 1. The support did not change the kinetic feature of the active site but imposed some hindrance that resulted in the activity reduction. The trimerization activity increased with temperature and reached its maximum at about 75 °C. It decreased with a further increase in temperature due to catalyst deactivation. The support also affected the dynamic evolution of the catalyst activity of the studied systems. As for the homogeneous oligomerization catalyst at 75 °C, the reaction rate was very low at the beginning, increased with time, and reached its highest value after 10 min of the reaction, followed by a decrease. In the supported system, though the activity of the supported catalyst was lower than that of the homogeneous counterpart, the reaction rate was
relatively more stable during the reaction. It seemed the support of the oligomerization catalyst increased its thermal stability. Encouraged by this highly selective chromium-ethylene trimerization catalyst, we developed a homogeneous tandem catalysis system, composed of bis(2-dodecylsulfanylethyl)amine-CrCl3 and Et(Ind)2ZrCl2. Our objective is to develop a supported tandem catalysis system. To investigate the effect of various supporting strategies on the tandem action of the 1 and 2 system and the thermal properties of resulting polymers, we designed the experiments as summarized in Table 2. The homogeneous system results are included for comparison. Figure 2 shows the copolymerization activity as a function of the metal mole fraction (Cr/Zr ratio). The activity increased with the Cr/Zr ratio from 0 to 60 with all the catalysis systems. Further increase in the Cr/Zr ratio caused a reduction in the activity with system A and system C. However, there were no such reductions observed with the supported systems. Their activities were relatively stable at the high Cr/Zr levels. This trend can be attributed to a comonomer effect. As we observed from all the experiments, the polymer was insoluble in the ethylene homopolymerization with low Cr/Zr ratios but partially soluble with high Cr/Zr ratios. The higher the Cr/Zr ratio, the higher was the comonomer concentration and the more insertion of comonomer into the polymer chains happened. The insertion of R-olefin comonomer restricted crystallization of polymer chains and, thus, increased solubility of both polymer and propagating metal alkyl, as well as monomer accessibility to the active sites.6,8,19 The high solubility and accessibility increased the chance of monomer chelating, which resulted in an increase in activity. This phenomenon was more profound in the supported copolymerization catalyst systems. With the
Figure 1. Effect of temperature on the trimerization activity of 1/MAO and (1/MAO/SiO2)/MAO; reaction conditions: toluene as solvent; total volume ) 100 mL; nCr ) 5 umol, nAl/nCr ) 600, treaction ) 30 min, P ) 1 atm.
5372 Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 Table 2. Tandem Polymerization of Ethylene with bis(2-Dodecylsulfanylethyl)amine-CrCl3 and Et(Ind)2ZrCl2 Prepared by Different Supporting Strategies runa c
A1 A2 A3 A4 A5 A6 B1c B2 B3 B4 B5 B6 C1d C2 C3e C4e C5e C6e D1d D2 D3e D4e D5e D6e
Zr (umol)
Cr (umol)
Cr/Zr
activityb
Tm,1 (°C)
0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 1.18 0.21 0.202 0.206 0.192 0.204 1.18 0.397 0.2 0.198 0.196 0.192
0 2 4 8 12 16 0 1.99 3.97 8.05 11.96 16 0 2.1 4.04 8.24 11.52 16.32 0 4.01 4 7.97 12.41 15.83
0 10.0 20.0 40.0 60.0 80.0 0 10.0 19.9 40.3 59.8 80.0 0 10.0 20.0 40.0 60.0 80.0 0.0 10.1 20.0 40.3 63.3 82.4
14.79 38.89 55.44 75.31 84.56 55.13 14.79 40.76 47.85 74.88 82.70 86.46 1.64 4.23 13.46 18.30 22.50 22.14 1.64 8.65 16.22 21.99 27.86 30.15
132.3 121.6 116.0 112.9 110.1 103.8 132.3 129.2 125.4 117.4 115.6 111.8 131.8 121.1 106.6 97.1 96.3 86.3 131.8 127.0 111.6 100.3 95.5 94.6
Tm,2 (°C)
117.6 118.8 118.7 118.8 122.3 120.7 116.8 114.4
4H (J/g)
Xc (%)
150.49 120.75 107.53 95.62 90.08 76.38 150.49 135.36 114.61 101.74 98.15 90.70 137.32 80.29 60.70 48.81 39.66 22.52 137.32 110.59 93.51 44.73 35.82 32.49
55.12 44.23 39.39 35.02 33.00 27.98 55.12 49.58 41.98 37.27 35.95 33.22 50.30 29.41 22.23 17.88 14.53 8.25 50.30 40.51 34.25 16.38 13.12 11.90
a Reaction conditions: solvent ) toluene; total volume ) 100 mL; nAl/nCr ) 600, t ) 30 min, P ) 1 atm; reaction temperature ) 75 °C. b In 106 g/ (mol Zr h) c The same experiment nAl/nZr ) 2500; other reaction conditions are the same. d The same experiment, nAl/nZr ) 2500; other reaction conditions are the same. e Multipeaks were found.
Figure 2. Effect of Cr/Zr ratio on the activity of tandem catalysis system with different supporting strategies.
Figure 3. Activity profiles obtained with different support strategies.
soluble catalysts, resistances to the diffusion were mainly in the reaction media, while with the supported catalysts, both reaction media and polymer-wrapped support particles resisted diffusion. On the other side, the incorporation of R-olefin comonomer also introduced hindrance to monomer insertion at the active sites, which resulted in a decrease in activity.6,19–21 The activities of systems A and C increased initially with the Cr/Zr ratio because the solubility and diffusivity issue played a dominating role. The hindrance effect exceeded the solubility and diffusivity advantage with further increase in the Cr/Zr ratio and resulted in the activity reduction at the high Cr/Zr ratio region. With the supported trimerization catalysts (systems B and D), the lowered activity (see Figure 1) gave a low 1-hexene concentration and, thus, a low 1-hexene incorporation level. The hindrance effect became minor, and the increase in solubility and diffusivity dominated. The copolymerization activity increased with the Cr/Zr ratio without a significant reduction in the range of high Cr/Zr ratios. The polymerization runs with homogeneous 2 (systems A and B) gave the same levels of activities at the same Cr/Zr ratios. The runs with supported 2 (systems C and D) also had similar activities. Figure 3 illustrates the activity profiles obtained with different supporting strategies at the same Cr/Zr ratio. Both A4 and B4 reached the highest reaction rate at the beginning of
the reaction followed by a linear decrease. However, the activities of C4 and D4 were relatively stable during the whole reaction process and showed no substantially reduction. It is well-known that supported catalysts are unable to undergo bimetallic reactions between active sites and that the bimetallic reactions often deactivate active sites.22 This observation suggests that the activity of the tandem system depends on the feature of the copolymerization catalyst and is independent of that of trimerization catalyst. The activities of the tandem system with the supported copolymerization catalyst (systems C and D) were about one-quarter that of their homogeneous counterparts (systems A and B). The support imposed some hindrance and blocked the chelating of ethylene and 1-hexene. The melting temperature Tm and crystal fraction Xc are the important materials properties that are determined by the chain microstructure of polyethylene. In general, the Tm and Xc values decreased with an increased branching density (i.e., the incorporated 1-hexene content). Figure 4 shows that the Tm and Xc of system A decreased with the Cr/Zr ratio. The transition also became broader. This suggested a higher level of 1-hexene content generated and incorporated into copolymer chains with an increased Cr/Zr ratio. A similar trend was observed with system B as shown in Figure 5. Compared to system A, system B yielded samples having higher Tm and Xc. The supported
Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 5373
Figure 4. DSC curves of ethylene-1-hexene copolymers obtained with the tandem catalysis system A.
Figure 5. DSC curves of ethylene-1-hexene copolymers obtained with the tandem catalysis system B.
Figure 6. DSC curves of ethylene-1-hexene copolymers obtained with the tandem catalysis system C.
Figure 7. DSC curves of ethylene-1-hexene copolymers obtained with the tandem catalysis system D.
catalyst 1 in the tandem system had a significantly reduced activity that resulted in a lowered 1-hexene concentration. The copolymer samples contained fewer branches and, thus, exhibited higher Tm and Xc values. The thermal properties of the samples produced with systems C and D are more complicated. As shown in Figures 6 and 7, both systems exhibited high Tm and Xc values, as well as single symmetrical melting peaks at low Cr/Zr ratios. However, when the Cr/Zr ratio increased above 20, the transition became very
Figure 8. DSC curves of ethylene-1-hexene copolymers obtained with the tandem catalysis system with different supporting strategies.
broad and multiple peaks appeared. The low melting temperature decreased with the increased Cr/Zr ratio, while the high melting temperature remained unchanged. The melt-recrystallized polymers have several maxima, indicating that crystalline lamellae with different thicknesses were present in the recrystallized product. The complex structure of the endotherm for the polymers at high Cr/Zr ratios showed that the maxima in the endotherms were not the result of random events but were related to the nature of the catalyst and the polymerization conditions. The reasons are probably 2-fold. First, the copolymerization catalyst 2 was supported in both systems. It is known that supporting a metallocene catalyst on SiO2 particle could alter its single-site nature and result in a multiple-site type.23 Different active sites had different abilities to incorporate the comonomer. Second, the composition drifting could also cause broadening in the DSC curve. The 1-hexene concentration was low at the beginning of polymerization and required some time to build up. As a result, polymer chains containing low comonomer contents were generated during this period. With 1-hexene being continuously generated, its concentration increased gradually and leveled off after about 10 min.16,24 The leveling off indicated that the generation and consumption of 1-hexene reached a steady state. Polymer chains of high 1-hexene content started to form. Copolymer chains having different comonomer contents were generated at different times and were blended in situ. The resulting copolymer could actually be a blend of polyethylenes with various comonomer contents and broad composition distributions.16,24,25 These materials formed crystallites having different sizes and, thus, exhibited different melting temperatures and broad transition. While the time required for reaching the steady state could contribute, the multisite nature of the supported copolymerization catalysts would be the major reason for the heterogeneity in the DSC curves. Figure 8 shows the DSC results of four samples prepared under the same conditions but with different catalysis systems. Because of the reduced activity of the supported trimerization catalyst 1, sample B4 exhibited a higher Tm than A4. The latter incorporated more 1-hexene in the copolymer chains. Both C4 and D4 samples gave lower Tm,1 but higher Tm,2 than A4. It was difficult to obtain their relative branch contents from the DSC curves. The hindrance introduced by supported 2 decreased the reactivities of both ethylene and 1-hexene. However, the influence on 1-hexene was more significant. It was difficult for 1-hexene molecules to insert into copolymer chains with supported 2. The chains had a smaller number of branches and, thus, a higher Tm. A 13C NMR analysis showed that the molar percentage of 1-hexene in sample A4 was 3.54 mol %, higher than B4’s 2.91 mol %, C4’s 1.45 mol %, and D4’s 1.04 mol %. Therefore, sample A4 had more braches than C4 and D4, while the latter exhibited lower Tm’s. It should be pointed out that the support particles were fragmented during the copolym-
5374 Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008
Acknowledgment The work is supported by the Special Fund for Major State Basic Research Project (No. 2005CB623804), the National Science Foundation for Distinguished Overseas Young Scholars (No. 20428605), and the National Science Foundation (No. 20476090). Literature Cited Figure 9. GPC curves of ethylene-1-hexene copolymers obtained with the tandem catalysis system with different supporting strategies.
erization. However, the fragments embedded in polymer samples were only 0.5 wt % of the total. This small amount of the support fragments would not significantly influence the thermal properties of the resulting polymers. Figure 9 shows a comparison of the molecular weight curves for four samples produced under similar conditions but with different supported catalyst systems. The GPC analysis showed that both A4 and B4 exhibited narrow distributions with polydispersities about 2.2. Samples C4 and D4 had broad distributions with polydispersities about 2.6. The weight-average molecular weights of A4 and B4 were lower than those of C4 and D4. This suggested that the supported 2 had lower chain transfer and β-hydrogen elimination rate constants. Comparing A4 and C4 to B4 and D4, the latter samples had higher weightaverage molecular weights. With the supported 1, the system had lower 1-hexene concentrations and, thus, lower rates of chain transfer to comonomer, which was one of the major chaintermination mechanisms. Conclusions On the basis of the experimental investigation and analysis on the synthesis of ethylene-1-hexene copolymers from ethylene-only stock by a tandem action of various combinations of homogeneous and silica-supported bis(2dodecylsulfanylethyl)amine-CrCl3 (1) and Et(Ind)2ZrCl2 (2), the following conclusions can be drawn. (1) Supported 1 trimerized ethylene with high selectivity similar to homogeneous 1, but with lower activity. Supporting 1 did not change its selectivity in the tandem catalysis system. Supporting 2 resulted in a reduction in both catalytic activity and incorporation ability of 1-hexene because of a hindrance effect. (2) The activity of the tandem system depended on if 2 was supported but was not affected by 1 supporting. The activity increased with the Cr/Zr molar ratio because of a comonomer effect. (3) Both 1 and 2 supporting influenced the thermal properties of the resulting copolymers. Successive increases in the 1 amount yielded a series of copolymer samples with decreased Tm and Xc. Homogeneous 2 yielded lower molecular weight copolymers with lower polydispersity index and having narrower DSC curves. Supported 2 gave higher molecular weight, higher polydispersity, and broader DSC curves because of its multiplesite nature. (4) The tandem action of supported 1 and supported 2 gave high activities at a 107 g/(mol Zr h) level. The resulting copolymers contained only C4 side-chains with melting temperature that ranged from 95 to 120 °C, similar to commercial LLDPE. The samples had the highest molecular weight among the four catalysis systems under similar reaction conditions.
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ReceiVed for reView December 23, 2007 ReVised manuscript receiVed May 15, 2008 Accepted May 16, 2008 IE7017564
