1-Hexene Copolymers from Ethylene Using a

Article pubs.acs.org/Macromolecules

Preparation of Ethylene/1-Hexene Copolymers from Ethylene Using a Fully Silica-Supported Tandem Catalyst System Fabian F. Karbach,† Tibor Macko,‡ and Robbert Duchateau*,† †

Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands ‡ Material Analytics Group, Fraunhofer Institute for Structural Durability and System Reliability, Schlossgartenstrasse 6, 64289 Darmstadt, Germany ABSTRACT: A silica-supported tandem catalyst system, capable of producing ethylene/1-hexene copolymers from ethylene being the single monomer, has been investigated. As tandem couple a phenoxyimine titanium catalyst for ethylene trimerization was combined with a metallocene catalyst for α-olefin polymerization. Two different approaches were pursued to combine the two catalysts as silica-supported tandem partners. The co-immobilization of the catalysts on the same support particles led to low polymerization activities and yielded products with low comonomer content due to interference of the two catalysts on the support. Immobilization of the two catalysts on separate supports prevented this interaction and led to high polymerization activities while the comonomer content of the product was controlled by the employed catalyst ratio. The copolymers obtained via the latter method were thoroughly analyzed with respect to their chemical composition distribution (CCD) by DSC-SSA, Crystaf, and HT-HPLC. The obtained data indicate a broad and in some cases bimodal CCD, which was explained by the synergy of composition drift during the polymerization and increasing diffusion limitation within the expanding polymer particle.

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INTRODUCTION Polyolefins are widely applied and account for about 60% of the global thermoplastic polymer market.1 Besides the homopolymers of ethylene and propylene, copolymers of ethylene with α-olefins like 1-butene, 1-hexene, or 1-octene form an important class of commodity plastics (linear low density polyethylene or LLDPE). The incorporation of α-olefins in the polymer chain results in short chain branching (SCB) which alters several properties (e.g., crystallinity, density, and flexibility) of the final polymer product. By controlling the comonomer content, the properties of the polymer can be tuned over a broad range to fit the corresponding application.2 Besides the traditional copolymerization of ethylene with αolefins, the preparation of LLDPE from ethylene as a single monomer by tandem catalysis has been studied.3 The concept of tandem catalysis makes use of the fact that linear α-olefins (with an even carbon number) are themselves oligomers of ethylene. Commonly, α-olefins are produced by ethylene oligomerization using either a selective or nonselective catalyst to yield one particular olefin or a distribution of α-olefins.4−6 By coupling an ethylene oligomerization catalyst with a polymerization catalyst (Scheme 1), LLDPE can be obtained from ethylene without the need of adding a comonomer to the reactor. In general, choosing the right combination of catalysts is crucial for a tandem system to work. Both catalysts have to show reasonably good activity under the same reaction © XXXX American Chemical Society

Scheme 1. Concept of Tandem Catalysis by Combining SiO2-Supported Ethylene Trimerization Catalyst 1 with SiO2-Supported Olefin Polymerization Catalyst 2

conditions (pressure, temperature, solvent, and cocatalyst) and should not deactivate each other. Received: November 8, 2015 Revised: January 24, 2016

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DOI: 10.1021/acs.macromol.5b02430 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Table 1. Influence of Scavengers on Polymerization Catalyst 2/SiO2a

1-C6 content

a

entry

scavenger

1 2 3 4

Et3Al iBu3Al/SiO2 Et3Al iBu3Al/SiO2

1-C6

2 2

(g)

−1

−1

productivity [g(PE) g(2/SiO2) ]

Mw (g mol )

Mw/Mn

Tm (°C)

Xc (%)

174 155 284 343

163 000 159 000 93 000 103 000

2.3 2.2 2.4 2.2

133.7 134.6 116.7 101.7/116.7

61.0 64.1 42.2 37.3

13

C (%)

1.9 3.6

1

H (%)

3.0 4.0

Conditions: 50 mg 2/SiO2 (0.7 μmol Zr), 75 mL Isopar, 10 bar C2, 58 °C, 1 h. Scavenger: 0.1 mmol Et3Al or 650 mg iBu3Al/SiO2.

(e.g., Et3Al, iBu3Al, nOct3Al), resulted in low activities and reactor fouling. In contrast, when silica-supported scavengers were employed, high trimerization activities were obtained and reactor fouling was effectively prevented. These promising results encouraged us to combine the heterogeneous trimerization catalyst with a supported polymerization catalyst to produce 1-hexene/ethylene copolymers by an entirely heterogeneous catalyst system. In the current study, we demonstrate the application of the given silica-supported trimerization catalyst as a tandem partner in combination with the similarly immobilized olefin polymerization catalyst (nBuCp)2ZrCl2 (2).

Combining oligomerization and polymerization catalysts is not new. In the presence of alkylaluminum or boron species or silanes, Phillips catalysts give so-called in situ branching affording LLDPE.7−9 With the recent development of highly active and selective ethylene oligomerization catalysts, tandem catalysis of single-site catalysts has received considerable interest.10−19 Homogeneous single-site polymerization catalysts usually produce polymers without any control over the product morphology and induce severe reactor fouling when applied below the melting point of the polymer product. Therefore, to be of industrial interest, these catalysts are usually immobilized on inert carriers (e.g., SiO2 or MgCl2).20−22 Olefin polymerization with heterogeneous catalysts leads to the formation of polymer particles, which by replication resemble the shape of the original support particle and prevent reactor fouling. It was shown that this benefit of particle formation could also be exploited in tandem catalysis. Homogeneous late transition metal ethylene oligomerization catalysts have been combined with either supported single-site polymerization catalysts23−26 or traditional heterogeneous Ziegler−Natta27−30 catalysts to yield LLDPE. However, even the most selective ethylene oligomerization catalysts usually produce small amounts of polyethylene as a side product. Although these traces of polymer are harmless in laboratory scale batch reactions, they can have a strong impact in industrial scale continuous processes as their accumulation unavoidably leads to reactor fouling, clogging of valves and lines and eventually to long down times. Hence, to avoid reactor fouling, also the oligomerization catalyst should be supported. The immobilization of ethylene oligomerization catalysts is a rather unexplored field, and it is therefore not surprising that only limited examples of tandem catalysis are known in which a heterogeneous oligomerization catalyst is combined with a heterogeneous polymerization catalyst. Zhang et al. immobilized the highly selective chromiumbased ethylene trimerization catalyst (Sasol’s SNS-Cr-system) on silica and combined it with a supported zirconocene catalyst.31 Guo et al. reported the combination of a nonselective iron-based oligomerization catalyst with a zirconocene polymerization catalyst both supported on molecular sieves.32 In both publications it was demonstrated that LLDPE was formed and that the degree of branching can be controlled by the ratio between the oligomerization and polymerization catalyst. Since neither of the authors discussed the morphology of the produced polymers or the occurrence of reactor fouling, it is not clear whether catalyst leaching might have taken place. Recently, we have successfully immobilized the Mitsui phenoxyimine titanium ethylene trimerization catalyst (1).33,34 The right choice of scavenger for residual impurities (i.e., moisture and oxygen) in the reaction mixture was found to be crucial for good performance and to prevent leaching of the catalyst from the support. Using “standard” scavengers, commonly employed for supported polymerization catalysts

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RESULTS AND DISCUSSION Individual Catalysts. A good match of the two catalysts is of key importance to achieve successful tandem catalysis. Ideally, the tandem partners should exhibit high activity and selectivity under the same reaction conditions. For the given trimerization system (1/SiO2), the choice of scavenger to remove residual impurities from the solvent and monomer feed is of crucial importance. Since 1/SiO2 exhibits its best performance in the presence of silica-supported scavengers (e.g., iBu3Al/SiO2), tandem catalysis should be conducted using such heterogeneous scavenger. However, silica-supported polymerization catalysts like 2/SiO2 are commonly employed in the presence of homogeneous aluminum alkyls as scavengers. Therefore, first silica-supported iBu3Al was tested as scavenger in combination with 2/SiO2 to elucidate to what extent using this heterogeneous scavenger influences the behavior of the polymerization catalyst. Table 1 summarizes the results obtained with the silicasupported polymerization catalysts (2/SiO2) in the presence of a homogeneous (Et3Al) and a supported scavenger (iBu3Al/ SiO2) in ethylene homopolymerization and copolymerization with 1-hexene (concerning the choice of homogeneous and heterogeneous scavenger see note at the end of this articlea). In the presence of homogeneous Et3Al the catalyst exhibited a productivity of 174 g(PE) g(2/SiO2)−1 while changing to the supported scavenger led to a somewhat lower productivity of 155 g(PE) g(2/SiO2)−1 in ethylene homopolymerization. Based on duplicate experiments, using both homogeneous and supported Et3Al as scavenger, a standard deviation of 3% was calculated, indicating that this difference in productivity cannot be explained by the error of the employed equipment. Since aluminum alkyls play an important role in the activation of metallocene catalysts, 35 the observed difference in productivity could be explained by activation of dormant species by Et3Al when this compound is employed as homogeneous scavenger. Independent of the scavenger, 2/ SiO2 produced linear polyethylene with comparable molecular weights (Mw ∼ 160 000 g mol−1, Mw/Mn = 2.2−2.3), melting temperatures (Tm ∼ 134 °C), and crystallinities (Xc ∼ 63%). B


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From the melting behavior and average comonomer content it can be concluded that the choice of scavenger has an influence on the ability of the catalyst to incorporate the comonomer. In the absence of aluminum alkyls (i.e., using iBu3Al/SiO2) the ability of the catalyst to incorporate 1-hexene is clearly enhanced. Most likely, Et3Al interacts with the active sites of the catalyst hampering comonomer incorporators. Based on the results presented above, it can be concluded that using iBu3Al/SiO2 as scavenger leads to a higher activity as well as improved comonomer incorporation. The broad melting range of the copolymers could be explained by a broad chemical composition distribution (CCD) as a result of composition drift during the polymerization. However, the occurrence of two melting peaks indicates a bimodal CCD, which can originate from several different reasons. A detailed analysis of the CCD using several independent methods (SSA, Crystaf, and HPLC) will be discussed below, and these CCDs will be compared to those of the polymers prepared by tandem catalysis. As mentioned above, it is crucial but also challenging to find tandem partners both exhibiting a reasonably good activity and selectivity under certain reaction conditions. Apart from the choice of scavenger, the reaction temperature was found to be a key parameter in tandem catalysis using the combination of 1/ SiO2 and 2/SiO2. The polymerization catalyst (2/SiO2) exhibits only inferior productivity at low temperatures (1.0 μmol) of 1/SiO2, a potentially bimodal distribution of SCB is obtained by tandem catalysis. Figure 9 compares the melting behavior of the copolymers obtained by tandem catalysis (1/SiO2 + 2/SiO2) with the product obtained from the standard 1-hexene/ethylene copolymerization using 2/SiO2 as the sole catalyst in the presence of 1-hexene. Both standard DSC thermograms as well as the heating curve after SSA treatment compare well with each other. Essentially the tandem system produces a copolymer similar to the one obtained in the “traditional” copolymerization of ethylene with 1-hexene. The good comparability of the two samples, prepared by tandem catalysis and traditional copolymerization, indicates that the initial phase of the tandem reaction, in which the 1-hexene concentration increases slowly, has only a minor influence on the CCD. Apparently the overall distribution of branching in the H


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Macromolecules

Figure 10. Crystaf analysis of copolymers produced by tandem catalysis (approach 1: 1/SiO2 + 2/SiO2) in comparison to standard C6/C2 copolymerization (top right).

Figure 11. HT HPLC analysis of copolymers produced by tandem catalysis (approach 1: 1/SiO2 + 2/SiO2) in comparison to C2 homopolymer and C6/C2 copolymer (top right). Dashed lines represent deconvolution of complex peaks.

the polymer fraction crystallizing at 86.8 °C most likely originates from the initial phase of the polymerization where only a small amount of 1-hexene is provided by 1/SiO2. In the presence of 2.0 μmol of 1/SiO2 polymerization catalyst 2/SiO2 exhibits such a low overall productivity that composition drift due to comonomer consumption can be excluded as a cause for the formation of polymer with low comonomer content. Over the past years significant progress has been made in the application of high performance liquid chromatography

obtained in tandem catalysis combining 2/SiO2 and 1.0 mmol of 1/SiO2. The occurrence of a polymer fraction with low comonomer content in the standard copolymerization can be attributed to composition drift in the later stage of the polymerization, where most of the 1-hexene is consumed. However, in the tandem reaction also the initial phase of the polymerization is likely to lead to the production of comonomer poor polymer. Especially when 2.0 μmol of 1/SiO2 is employed in the tandem reaction, I


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CONCLUSIONS A silica supported tandem catalyst system for the ethylene/1hexene copolymerization from ethylene as a single monomer proved to yield copolymers with very similar architecture as obtained via “traditional” ethylene/1-hexene copolymerization. The described system therefore offers an elegant way of producing 1-hexene/ethylene copolymers from ethylene as a single monomer while using supported catalysts to prevent reactor fouling. Interestingly, co-immobilization of the single-site ethylene trimerization and polymerization catalysts on the same support particles resulted in low polymerization and trimerization productivities due to interference between the two catalysts on the silica surface. By immobilizing the two catalysts on separate supports, this heterobimetallic poisoning was effectively prevented, while the branching content of the obtained copolymers was controlled by the amount of trimerization catalyst employed during the reaction. Reactor fouling by homogeneous polymer formation was completely prevented and polymer products with good particle morphologies were obtained. Only when the average comonomer content exceeded 4.0 mol %, the produced copolymer became soluble under the employed reaction conditions, leading to particle agglomeration, a loss of morphology control, and reactor fouling. Detailed analysis of the obtained copolymers via SSA, Crystaf, and HT-HPLC revealed that the polymer samples with intermediate comonomer content (2.7−4.3 mol %) exhibit a bimodal chemical composition distribution. The copolymers with low (≤2.7 mol %) and high hexene content (≥7.6 mol %), however, showed a monomodal distribution of the comonomer.

(HPLC) for the analysis of polyolefins. Several examples in the literature show the successful application of HPLC to separate polyolefins according to tacticity or comonomer content.53−56 While SSA and Crystaf separate a polymer sample according to the ability to crystallize, HPLC fractionates a given polymer according to the ability of polymer chains to adsorb onto or desorb from a given column material. To gain further insight into the CCD, the copolymers produced via tandem catalysis were analyzed by high temperature gradient HPLC. The polymer sample, dissolved in 2-octanol, was injected on a column containing a porous graphite sorbent (Hypercarb). During the experiment, the solvent was gradually changed from 2-octanol to 1,2,4-trichlorobenzene. Figure 11 shows the chromatograms of the copolymers prepared by tandem catalysis in comparison to a polyethylene homopolymer (Mw = 55 000 g mol−1) and the ethylene/1-hexene copolymer prepared with 2/ SiO2. As expected, the copolymers prepared by tandem catalysis eluted at a lower elution volume when compared to the ethylene homopolymer. The copolymer prepared in the presence of 0.25 μmol 1/SiO2 eluted in one slightly broadened peak (9.9 mL), which has clearly shifted in comparison to the peak of pure polyethylene (10.1 mL). Similarly, the peak corresponding to the polymer produced in the presence of 2.0 μmol 1/SiO2 gave one isolated peak at a much lower elution volume (9.4 mL). Both copolymers represent the extremes of low and high comonomer content and form considerably sharp Gaussian shaped peaks indicative for a monomodal CCD. This is in agreement with the results obtained from SSA and Crystaf, where both polymers appear (at least almost) monomodal. The peaks of the copolymers with intermediate comonomer content (0.5−1.5 μmol 1/SiO2) also shifted toward lower elution volumes with increasing amount of 1/SiO2 present during the polymerization, indicative for increasing comonomer incorporation. Analogous to the results obtained from DSC and Crystaf analysis, the copolymer obtained by standard copolymerization using 2/SiO2 exhibits, according to HPLC, a similar CCD as the polymer obtained by tandem catalysis in the presence of 1.0 μmol 1/SiO2. Overall, the analysis via HPLC is in good agreement with the results from Crystaf analysis. The somewhat deformed shapes of the peaks of the copolymers with intermediate comonomer content could be deconvoluted into two Gaussian curves. From the HPLC analysis it can therefore be concluded that the given copolymers consist of two components with significantly different CCD. The contribution of the Gaussian curve corresponding to comonomer-rich copolymers increases with increasing amount of 1/SiO2. While a broad but monomodal CCD could easily be explained by composition drift during the copolymerization or tandem reaction, the observation of a bimodal distribution of SCB is somewhat surprising but not uncommon. Nonuniform CCDs are commonly explained by the formation of different active sites on the heterogeneous catalyst, which differ in their ability to incorporate comonomers.57,58 However, since the employed polymerization catalyst 2/SiO2 also produced copolymers with considerably monomodal CCD (in the presence of very high and very low amount of 1/SiO2), this explanation does not suffice for the given observations. A combination of composition drift and the change in diffusion limitation due to particle expansion (filter effect)59 during the reaction might be the reason for the formation of polymers with bimodal CCDs.

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EXPERIMENTAL SECTION

Materials and General Considerations. All manipulations of oxygen and moisture sensitive compounds were performed under nitrogen using standard Schlenk and glovebox techniques. Isopar E (isomeric mixture of octanes and nonanes, Brentag), toluene (technical, Biosolve), and ethylene (purity 4.5, Linde) were deoxygenized and dried by passing them subsequently through columns containing BTS catalyst and molecular sieves (3 Å). Neat iBu3Al and Et3Al as well MAO (10 wt % in toluene) were purchased from Sigma-Aldrich and used without further purification. The phenoxiimine titanium ethylene trimerization catalyst (1) was synthesized via published procedures.33 The ethylene polymerization catalyst (nBuCp)2ZrCl2 (2) was purchased from Strem. Silica (Sylopol 948, Grace Davison, particle diameter ∼50 μm) was calcined at 600 °C under a nitrogen stream before use. The calcined SiO2 was stored in a glovebox to prevent any rehydration. Elemental analysis was performed at Mikroanalytisches Laboratorium Kolbe (Mühlheim, Germany). Preparation of 1/SiO2. The immobilization of 1 was achieved via a two-step process. First silica-supported MAO was prepared, while 1 was immobilized in a second step. Because of the short shelf life of the silica-supported 1, its immobilization was performed directly before the ethylene oligomerization/polymerization experiments. Silica-supported MAO (MAO/SiO2) was prepared by adding a 10 wt % solution of MAO in toluene (40 mL) to SiO2 (10 g) at room temperature. The obtained slurry was heated to 80 °C and occasionally agitated manually for 4 h. Finally, the solvent was evaporated at reduced pressure to obtain the supported cocatalyst (MAO/SiO2) as a free-flowing powder. The aluminum content was determined by elemental analysis to be 10.6 wt %. The immobilization of 1 on MAO/SiO2 was conducted by adding a toluene (2 mL) solution of 1 (2 μmol) to MAO/SiO2 (100 mg) at room temperature. The slurry was heated to 50 °C and shaken for 1 h after which the supernatant was decanted. The supported catalyst was washed with toluene (4 × 2 mL) and Isopar E (2 × 2 mL) to obtain a J


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Macromolecules

atmosphere with a standard heating/cooling rate of 10 °C min−1. Polymer samples (3−5 mg) were sealed in aluminum pans and initially heated to 160 °C and held at this temperature for 5 min to erase any thermal history. Standard DSC experiments were conducted by heating the sample from −50 to 160 °C. Thermal fractionation by stepwise self-nucleation and annealing (SSA) was conducted according to the following method. After erasure of the thermal history the sample was cooled to −20 °C to create an initial crystalline state. Subsequently, the sample was heated to the first self-seeding temperature (Ts) of 140 °C and annealed isothermally for 5 min. After cooling down to −20 °C the sample was heated to the second Ts at 135 °C, kept isothermal for 5 min, and cooled again to −20 °C again. The procedure was repeated with constant ΔT of 5 °C between successive annealing steps. In total a temperature range from 60 to 160 °C was covered before a final standard DSC measurement was conducted on the sample. Scanning Electron Microscopy. The morphology of the produced polymers was analyzed by scanning electron microscopy (SEM) on a Jeol JSM-5600. The polymer samples were fixed on a sample holder by means of adhesive carbon tape and sputtered with gold before the analyses. Size Exclusion Chromatography. The molecular weight distributions of the obtained polymers were analyzed by high temperature size exclusion chromatography (HT-SEC) on a PL-XT20 Rapid Polymer Analysis system equipped with an autosampler (PLXT-200) using a series of three PLgel Olexis columns (300 × 7.5 mm, Polymer Laboratories). As a mobile phase 1,2,4-trichlorobenzene was used at a temperature of 160 °C and a flow rate of 1 mL min−1. Molecular weights were calculated with respect to polyethylene standards (Polymer laboratories). 1 H and 13C NMR Spectroscopy. Quantitative 1H and 13C NMR spectroscopy for the analysis of the comonomer content of the obtained copolymers was conducted at the Inorganic Chemistry Department of the University of Bayreuth (Germany). Measurements were conducted on a 300 MHz Varian Inova spectrometer at 120 °C using deuterated 1,1,2,2-tetrachloroethane as solvent. Analysis of 13C NMR spectra was conducted via published procedures.60 Crystaf. Crystaf analysis was conducted on a Crystaf-Tref 300 (Polymer Char). Polymer samples (∼20 mg) were dissolved in odichlorobenzene and measured at a cooling rate of 0.1 °C min−1 in a temperature range of 110−25 °C. High Temperature Gradient HPLC. High temperature gradient HPLC was conducted at 160 °C on a PL GPC 220 equipped with an ELS detector, gradient pump (Agilent), and a Hypercarb column (Thermo Scientific). Measurements were conducted at a flow rate of 0.8 mL min−1. Polymer samples (2−3 mg) were dissolved in 2-octanol (1.5 mL), and 50 μL of the resulting solutions was injected. After 3 min the mobile phase was gradually changed from 100% 2-octanol to 100% 1,2,4-trichlorobenzene over 10 min (linear gradient).

light orange solid. The thus obtained 1/SiO2 was directly used to prevent catalyst decomposition. Peparation of 2/SiO 2 . The polymerization catalyst (nBuCp)2ZrCl2 (2, 36.8 mg, 0.091 mmol) was mixed with MAO in toluene (10 wt %, 9.1 mL) at room temperature and stirred with a magnetic stirrer bar for 30 min. Subsequently, the obtained preactivated catalyst was added to SiO2 (2.28 g) at room temperature. The obtained slurry was heated to 50 °C for 3 h during which time it was resuspended by shaking it every 20 min. Finally the solvent was evaporated under reduced pressure to obtain the supported catalyst as a faint yellow, free-flowing powder. The aluminum and zirconium content was found to be 10.6 wt % (3.9 mmol/g) and 0.12 wt % (13 μmol/g), respectively, as determined by elemental analysis. Preparation of 1 (1 + 2)/SiO2. The co-immobilization of 1 and 2 was performed analogous to the immobilization of 1 described above by using 2/SiO2 as support medium instead of MAO/SiO2. Immobilization of Scavenger on SiO2 (iBu3Al/SiO2). A 500 mL round-bottom flask was charged with calcined (600 °C) SiO2 (33 g) and toluene (300 mL) under nitrogen and stirred with a mechanical stirrer. The slurry was cooled to 0 °C using an ice−water bath and a solution of iBu3Al (11.8 g, 59.4 mmol) in toluene (32 mL) was added dropwise. After stirring at 0 °C for 1 h the mixture was warmed to room temperature and stirred for an additional 2 h. The particles were allowed to settle, and the supernatant was carefully decanted off. The solid was further washed with toluene (3 × 150 mL) followed by decantation. Finally, the solid was dried under vacuum to obtain a freeflowing powder. The aluminum content was found to be 2.2 wt % by elemental analysis. Ethylene Polymerization. Catalyst testing was conducted using a 125 mL stainless steel autoclave (PREMEX) equipped with a mechanical stirrer (anchor-shaped impeller, 500 rpm), temperature control via electrically heated and water-cooled reactor walls, and pressure control using a Bronkhorst mass flow controller. The reactor was dried by heating the reactor wall to 130 °C and conducting four consecutive vacuum/nitrogen cycles (5 min vacuum and 10 min nitrogen, respectively). After drying, the reactor was cooled to the reaction temperature of 58 °C, charged with the solvent (Isopar E, 45 mL) and the scavenger (650 mg iBu3Al/SiO2 in 15 mL Isopar E). The mixture was stirred, and ethylene was fed to the reactor by means of a mass flow controller to obtain an ethylene pressure of 10 bar. After leaving the system to stabilize for 1 h the catalyst slurry in Isopar E (15 mL) was injected. The temperature was maintained at 58 °C ± 1, and the pressure was kept constant by controlled feeding of ethylene. After a reaction time of 1 h the ethylene feed was stopped, and the autoclave was vented to atmospheric pressure. Subsequently, the reaction mixture was poured on a mixture of aqueous hydrochloric acid (10 wt %, 50 mL) and ethanol (150 mL). After it was stirred for 1 h, the solid polymer particles were filtered off, washed with ethanol, and dried at 50 °C in a vacuum oven. In case soluble polymers were produced, additional diluted aqueous hydrochloric acid (∼100 mL) was added, and the mixture was stirred at room temperature in air until the organic phase was evaporated (3−5 days). The precipitated, rubbery material was isolated by filtration, washed with ethanol, and dried at 50 °C in a vacuum oven. Ethylene Trimerization. Catalyst testing of the supported 1 in the absence of the polymerization catalyst was conducted analogous to the procedure described above for ethylene polymerization with the following modification: An internal standard (n-decane, 1 mL) was added together with the scavenger before the reaction. Furthermore, before venting the reactor and quenching the reaction, a small sample of the reaction mixture was drawn into a pressurizable steel tube via the bottom valve of the reaction vessel. The steel tube was subsequently disconnected from the reactor and cooled to −30 °C before it was vented to prevent any evaporation of the 1-hexene produced. The 1-hexene and decene cotrimer content of the reaction mixture was analyzed by gas chromatography on a Varian 450-GC equipped with a C18 factor four column (30 m × 0.25 mm) and quantified using the indicator. Thermal Analysis. Differential scanning calorimetry (DSC) was performed on a DSC Q1000 (TA Instruments) under a nitrogen

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (R.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dr. Winfried Kretschmer is kindly acknowledged for providing NMR spectroscopic data of the produced copolymers. This work is part of the research program of the Dutch Polymer Institute (DPI) under project #706.

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ABBREVIATIONS MAO, methylaluminoxane; Et3Al, triethylaluminum; iBu3Al, triisobutylaluminum; NMR, nuclear magnetic resonance; CCD, chemical composition distribution; DSC, differential scanning calorimetry; SSA, successive self-nucleation and annealing; K


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Macromolecules

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Crystaf, crystallization analysis fractionation; HPLC, highperformance liquid chromatography; LLDPE, linear low density polyethylene; SCB, short chain branches; MWD, molecular weight distribution; SEM, scanning electron microscopy.

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On the choice of homogeneous and heterogeneous scavenger (Et 3 Al vs iBu 3 Al/SiO 2 ) employed with 2/SiO 2 : Both homogeneous iBu3Al and Et3Al are common scavengers in the catalytic polymerization of olefins. Also, in our research on supported metallocenes both aluminum alkyls are employed as scavengers and usually yield comparable polymerization results in combination with 2/SiO2 (at the described low scavenger concentration). However, iBu3Al was found to be a somewhat less efficient scavenger for impurities than Et3Al, leading to lower polymerization activities when solvent and monomer purities are reduced. Furthermore, it was found that under certain reaction conditions the presence of iBu3Al facilitates catalyst leaching and reactor fouling by homogeneous polymer formation. Because of these reasons, Et3Al is the homogeneous scavenger of choice in combination with polymerization catalyst 2/SiO2. The comparison to silica-supported Et3Al might appear more logical. However, in our study concerning the heterogeneous FI-catalyst (1/SiO2) it was found that employing Et3Al/SiO2 or iBu3Al/SiO2 as scavenger lead to essentially the same experimental results. Since a large amount of silicasupported aluminum alkyl had to be prepared for this study, we decided (for the sake of safety) for the somewhat less pyrophoric aluminum alkyl (iBu3Al) to be immobilized and used as supported scavenger.

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1-Hexene Copolymers from Ethylene Using a

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