Note pubs.acs.org/Macromolecules
An Alternative Method for the Preparation of TrialkylaluminumDepleted Modified Methylaluminoxane (dMMAO) Ryo Tanaka, Tomoyasu Kawahara, Yuto Shinto, Yuushou Nakayama, and Takeshi Shiono* Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashihiroshima, 739-8527 Japan S Supporting Information *
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INTRODUCTION Methylaluminoxane (MAO), which is prepared by the condensation of Me3Al and H2O, is a well-known activator for olefin polymerization catalysts.1−3 Although the precise structure is not exactly known, MAO consists of oligomers containing −Al(−Me)−O− repeat units and residual Me3Al. Activation of metal catalysts by MAO consists of two elementary reactions: alkylation and cationization. Modified methylaluminoxane (MMAO), which is obtained from the hydrolysis of a mixture of iBu3Al and Me3Al, is also important as an olefin polymerization cocatalyst. The superiority of MMAO over MAO is the high solubility of MMAO in hydrocarbons such as heptane, which is caused by the introduction of higher alkyl groups. The residual Me3Al often affects olefin polymerization. In general, the amount of Me3Al in MAO strongly affects the activity of the catalyst and molecular weight of the resulting polymer because trialkylaluminums and aluminum hydrides easily coordinate with the metal center of the catalyst to hamper propagation and act as a chain transfer reagent (Scheme 1).4−6
reduce the coordination ability of the aluminum derivative to the metal catalyst as increasing the number of substituted phenoxides. As a result, the use of TBP- or BHT-modified MAO enhances the polymerization activity and produces polymers with high molecular weight and narrow molecular weight distributions.10−13 These effects on polymerization behavior are also observed by the use of BHT-modified MMAO.14 However, phenol-modified MAOs sometimes deactivate the polymerization catalysts, especially lanthanide catalysts, likely because oxygen atoms in phenols can easily coordinate with lanthanides. Vacuum drying is a widely accepted process in academic settings to reduce the amount of free trialkylaluminums in MAO. However, this method is time-consuming and requires high energy input. Additionally, the process of evacuating trialkylaluminums should be closely monitored for safety reasons. For alkylaluminums with a high boiling point, such as iBu3Al in MMAO, they can only be removed by repeating the vacuum drying process after redissolution in heptane, which takes several days even in the lab-scale preparation. From a practical point of view, the development of an alternative method for the removal of iBu3Al is desirable. We previously reported that MMAO supported on SiO2 did not activate the titanium diamide complex, whereas vacuumdried MMAO supported on SiO2 activated the complex to promote propylene polymerization.15,16 This result, indicating that iBu3Al in MMAO was preferentially supported on SiO2, led us to consider using SiO2 to remove trialkylaluminums from MMAO. Herein, we prepared trialkylaluminum-depleted MMAO (dMMAO) utilizing the SiO2 support technique and applied the resulting dMMAO as an olefin polymerization cocatalyst.
Scheme 1. Chain Transfer Reaction Involving Trialkylaluminums
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i
Bu3Al in MMAO also affects the polymerization behavior in a similar manner to Me3Al in MAO. For example, propylene polymerization using a titanium chelate diamide complex does not proceed when MMAO is used as an activator, whereas i Bu3Al-depleted MMAO promotes the polymerization.7 In the propylene polymerization using fluorenylamido-ligated titanium complexes, living polymerization is achieved only when trialkylaluminums are removed from MAO or MMAO. The chain transfer reaction takes place when a suitable amount of i Bu3Al is added to the living system.8,9 Several strategies have been introduced to minimize the effect of residual alkylaluminums in MAO. One approach is the modification of alkylaluminums with bulky phenols, such as 2,6di-tert-butylphenol (TBP) and 2,6-di-tert-butyl-4-methylphenol (BHT). These phenols can preferentially react with Me3Al in MAO to give the corresponding aluminum phenoxides, which © XXXX American Chemical Society
EXPERIMENTAL SECTION
General. All manipulations were performed under an atmosphere of nitrogen using standard Schlenk line techniques. Modified methylaluminoxane (MMAO, 6.5 wt % Al in toluene) was generously donated by Tosoh-Finechem Co. (Japan). Silica gel (Cariact P-10, 273 m2/g surface area, 40.8 μm average particle size) was donated by Fuji Silicia Co. Ltd. (Japan) and calcined at 700 °C for 4 h or dried under vacuum at 150 °C overnight prior to use. The density of hydroxy groups, with or without calcination, was determined by titration with MeLi (calcined SiO2: 1.0 mmol/g; noncalcined SiO2: 3.0 mmol/g). Dry toluene was purchased from Kanto Chemical Co. Inc., and Received: May 15, 2017 Revised: July 6, 2017
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DOI: 10.1021/acs.macromol.7b01003 Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. Structures of complexes 1−3. and the precipitated polymer was collected by filtration. The obtained polymer was washed with MeOH and dried for 6 h at 60 °C under reduced pressure to a constant weight. 3.08 g of polypropylene was obtained as a colorless solid. Postpolymerization of Propylene Using the 1−SiO2-Treated MMAO System. dMMAO in toluene (Al content: 1.24 mol/L, 3.2 mL) was charged to a 100 mL two-necked flask and diluted with toluene (20.8 mL). Nitrogen in the headspace of the flask was removed, and 224 mL (421 mg, 10 mmol) of propylene, of which flow rate was regulated to 200 mL/min, was dissolved into the solution at 0 °C. The amount of dissolved propylene was confirmed weighing the flask. After the addition of propylene, nitrogen was introduced to the flask until ambient pressure was obtained. Polymerization was initiated by addition of 1 (7.4 mg, 20 μmol) in toluene (1.0 mL) under nitrogen. After stirring for 20 min at 0 °C, a propylene solution in toluene (saturated at 25 °C, 1 atm, 0.83 M, 12.2 mL, 421 mg, 10 mmol) was added to the flask, and the reaction was further stirred for 20 min at 0 °C. The resulting solution was poured into MeOH (200 mL) containing concentrated HCl (4 mL), and the precipitated polymer was collected and purified according to the same procedure described above. 829 mg (98%) of polypropylene was obtained as a colorless solid.
residual water was removed with sodium metal. Complexes 1−3 (Figure 1) were prepared according to literature procedures.17−19 Propylene gas (Sumitomo Seika Co. Ltd.) was used by passing the gas stream through Nikka Seiko GC-RP and DC-A4 columns prior to use to remove trace amounts of O2 and water. 1-Hexene was distilled from CaH2 and stored under MS 4A. All other materials were used without further purification. 1H NMR spectra were recorded in benzene-d6 on a Varian 500 NMR spectrometer. The obtained spectra were referenced to the signal of residual protonated solvent [δ = 7.15 ppm]. Molecular weights of polymers were determined using a Viscotec HT-GPC350 chromatograph calibrated with a RI− viscometer−light scattering triple detector (T = 150 °C; eluent: odichlorobenzene) or a Tosoh HLC-8320GPC calibrated with polystyrene standards (T = 40 °C; eluent: THF). Titration Method for the Determination of Hydroxy Group Content in SiO2. To a Et2O (3.0 mL) slurry of SiO2 (ca. 100 mg) was added MeLi (1.0 mL, 1.14 M, 1.14 mmol). The slurry was stirred for 30 min, and the supernatant MeLi solution was titrated with L-menthol using 2,2′-bipyridyl as an indicator (colorless → red). The amount of surface hydroxy group was calculated from the difference in concentration between SiO2-treated MeLi and the original MeLi concentration. Preparation of dMMAO by Vacuum Drying. MMAO in toluene (20.0 mL, 41.8 mmol) was dried under vacuum at 60 °C for 3 h. The resulting colorless solid was redissolved in toluene (20 mL). The drying and redissolving cycle was repeated seven times to obtain 3.32 g of colorless solid. An aliquot of the obtained solid (vide inf ra) was titrated to reveal that the aluminum content was 11.1 mmol/g (88% of aluminum was recovered). The prepared MMAO was used as a stock solution in toluene (2.0 M). Preparation of dMMAO Using the SiO2 Modification. To a MMAO solution (30.0 mL, 62.7 mmol, 16.6 equiv of −OH group in SiO2) was added a toluene slurry (20 mL) of SiO2 (1.26 g, 3.78 mmol of −OH group), and the slurry was stirred for 1 h at 20 °C. The supernatant liquid was decanted, and the remaining solid was washed with another two 30 mL portions of toluene. Evaporation of the combined toluene solutions gave 5.03 g of colorless solid. An aliquot of the obtained solid was titrated to reveal that the aluminum content was 11.6 mmol/g (93% of aluminum was recovered). The prepared MMAO was used as a stock solution in toluene (1.24 M). Titration Method Used To Determine the Aluminum Content in MMAO. MMAO (ca. 100 mg) was precisely weighed in a 20 mL Schlenk flask under nitrogen and decomposed by the slow addition of water (10 mL) over 30 min. The acidity of the solution was adjusted to pH 3 with aqueous acetic acid (2−3 drops). A certain amount of ethylenediaminetetraacetic acid (EDTA) was added, and the solution was refluxed for 2 min. Subsequently, several portions of hexamethylenetetramine (ca. 10 mg) were added until the pH reached 6 (confirmed by pH testing paper). The mixture was titrated with an aqueous ZnBr2 solution (0.05 M) using xylenol orange as an indicator (yellow → red). Polymerization of Propylene Using the 1−SiO2-Treated MMAO System. A freshly prepared solution of dMMAO in toluene (Al content: 1.24 mol/L, 6.5 mL) was charged into a 100 mL twonecked flask and diluted with toluene (22.5 mL). Nitrogen in the headspace of the flask was removed, and propylene (1 atm) was introduced at 0 °C until the solution was saturated with propylene. Polymerization was initiated with the addition of 1 (7.4 mg, 20 μmol) in toluene (1.0 mL) under a steady stream of propylene at 1 atm. After the solution was stirred for 4 min at 0 °C, the resulting mixture was poured into MeOH (200 mL) containing concentrated HCl (4 mL),
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RESULTS AND DISCUSSION In the 1H NMR spectra (C6D6:THF = 5:1) of MMAO, the sharp signals around −0.6 and 0.08 ppm are assigned to Al− CH3 protons and Al−CH2−protons of the isobutyl groups on trialkylaluminums, respectively, according to the literature (Figure 2).15 Multiple sharp signals can be attributed to the presence of several alkylaluminum species, such as Me3Al, Me2iBuAl, and iBu3Al; cationic species, such as [Me2Al(thf)2]+, were not observed.20 The amounts of iBu and Me groups in the free alkylaluminums were calculated separately using the integral ratio of these sharp signals. First, the amount of trialkylaluminum in MMAO during the course of vacuum drying was traced. The overall amount of trialkylaluminums decreased to approximately 75% after three cycles of vacuum drying−redissolution and approximately 25% after seven cycles (Table 1, runs 2 and 3). The ratio of iBu and Me groups in the free alkylaluminum was 1.4:1, regardless of the number of vacuum drying−redissolution cycles. This is probably ascribed to an equilibrium between aluminoxanes and free alkylaluminums, as was shown in previous NMR studies.21,22 Next, treatment of MMAO with SiO2 was attempted. A certain amount of SiO2 was slowly added as a toluene slurry to MMAO solution and stirred for 1 h, and the resulting solution was decanted to give SiO2-treated MMAO. From the toluene solution, 93% of the aluminum was recovered, which was higher than the recovery ratio after seven cycles of vacuum drying−redissolution (88%). The rest of the aluminum was supported on SiO2. The amount of free alkylaluminum was reduced to less than 50%, which was comparable to the treatment with several vacuum drying−redissolution cycles, indicating that the single treatment with SiO2 can effectively remove free alkylaluminums (run 4). The use of silica calcined B
DOI: 10.1021/acs.macromol.7b01003 Macromolecules XXXX, XXX, XXX−XXX
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In the propylene polymerization using 1, SiO2-treated MMAO showed higher polymerization activity than the original trialkylaluminum-containing MMAO (runs 1−3). Narrow PDI and high molecular weight were observed in runs 2 and 3 compared to run 1, and this was mainly attributed to the suppression of chain transfer by the removal of alkylaluminums because the number of polymer chains was smaller in the runs using SiO2-treated MMAO. We previously reported that the controlled polymerization of propylene proceeded with Me2Si(NtBu)(C5Me4)TiMe2 activated by Me3Al-free MAO at 0 °C and that the addition of iBu3Al and Octyl3Al more than doubled the activity in addition to the occurrence of chain-transfer reaction.24 We also confirmed that the addition of iBu3Al and Octyl3Al to the living polymerization of propylene with the 1− B(C6F5)3 system at −50 °C raised the polymer yield. Both the yield and the molecular weight of the produced polymer were increased with increasing concentrations of Octyl3Al.25 Therefore, we consider that the difference in the molecular weight of the produced polymer, i.e., propagation rate, between runs 2 and 3, should be ascribed to the difference in the content of remaining trialkylaluminum. The use of BHT-modified MMAO also gave high molecular weight polypropylene, but PDI of the polymer was larger than that obtained by SiO2-treated MMAO (run 4). As previously mentioned, SiO2-treated MMAO promoted titanium complex 2 catalyzed propylene polymerization and gave high molecular weight polymers with narrow PDI. Whereas, the use of as-supplied MMAO did not promote the polymerization (runs 5 and 6). In 1-hexene polymerization, 3− SiO2-treated MMAO gave much higher molecular weight polymers than 3−MMAO alone, indicating that the chaintransfer reaction was suppressed by the removal of trialkylaluminums from MMAO (runs 7 and 8). Finally, we conducted a postpolymerization of propylene to confirm the living nature of the 1−SiO2-treated MMAO system (Scheme 2). Postpolymerization was performed by the successive addition, after complete monomer consumption in the first polymerization step, of the same amount of propylene as the initial feed. The polymer yield was almost quantitative and the GPC trace of the second step shifted toward doubled molecular weight from the first step while maintaining a narrow molecular weight distribution (Figure 3). These results showed that the amount of free alkylaluminum in the SiO2-treated MMAO was small enough to achieve living polymerization using catalyst 1 under these conditions, similarly to vacuumdried MMAO.
Figure 2. 1H NMR spectrum (500 MHz, C6D6:THF = 5:1) of MMAO before (top, Table 1, run 1) and after SiO2 treatment (bottom, Table 1, run 4).
Table 1. Content of Free Alkylaluminums in MMAO after Various Treatments run 1 2 3 4 5 6 7d
treatment method none vacuum dry × 3 vacuum dry × 7 SiO2 SiO2 SiO2 SiO2
Al/−OH (equiv)
yielda (mol %)
R3Alb (mol %)
− −
− −
− −c
5.0 3.3
1.4:1 1.4:1
−
−
88
1.2
1.4:1
none 700 °C none none
16.7 13.3 8.3 16.7
93 84 88 91
1.9 2.5 1.9 2.8
1.2:1 1.3:1 1.2:1 1.2:1
calcination of SiO2
i
Bu:Meb
a
Determined by titration. bContent of free alkylaluminums calculated by 1H NMR. cNot measured. dSiO2 slurry was added at 0 °C.
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at 700 °C reduced both the recovery ratio and R3Al removal efficiency, indicating that a significant amount of MMAO was trapped in the calcined SiO2 (run 5). Increasing the amount of SiO2 did not improve the removal efficiency (run 6), and the addition of SiO2 at low temperature did not effectively remove R3Al compared with the addition at room temperature (run 7). Unless otherwise specified, the subsequent polymerization reactions used dMMAO obtained from run 4. The prepared dMMAO was applied to propylene polymerizations using group 4 metal catalysts 1−3 (Table 2). These catalysts show living polymerization nature with propylene and 1-hexene when combined with B(C6F5)3 or dMMAO.7,15,17,19,23
CONCLUSION
We successfully removed trialkylaluminum from commercially available MMAO by the selective supporting of trialkylaluminum onto SiO2. The amount of trialkylaluminum remaining after SiO2 treatment was reduced to less than 2 mol %. The obtained dMMAO was used in combination with a titanium catalysts 1 and 2 for the living polymerization of propylene. We have provided a very convenient preparation method for dMMAO that can be used for the living polymerization of olefins. This method is superior to the repeated drying− redissolution methods currently used. C
DOI: 10.1021/acs.macromol.7b01003 Macromolecules XXXX, XXX, XXX−XXX
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Table 2. Effect of SiO2 Treatment on the Cocatalyst Ability of MMAO in Olefin Polymerization with Group 4 Complexesa
run
MMAO treatment
cat.
monomer
Al/Ti
temp (°C)
time (min)
Ab
Mnc (×104)
PDIc
1 2 3d 4 5 6 7 8
none SiO2 SiO2 10% BHT none SiO2 none SiO2
1 1 1 1 2 2 3 3
propylene propylene propylene propylene propylene propylene 1-hexene 1-hexene
400 400 400 200 200 200 200 200
0 0 0 0 0 0 r.t. r.t.
4 4 4 3 10 10 210 210
950 1700 2300 1900 0 37 16 22
3.8 11.4 15.9 28.3 −e 25.8 0.4 1.1
1.32 1.08 1.25 1.32 −e 1.11 1.91 1.84
a
Polymerization conditions: [C3H6] = 1 atm, toluene = 30 mL (runs 1−4); [C3H6] = 1 atm, heptane = 30 mL (runs 5 and 6); and [1-hexene] = 5.0 mL, toluene = 2.0 mL (runs 7 and 8). bActivity as kg mol (Ti)−1 h−1. cMolecular weight measured by GPC equipped with RI−viscometer−light scattering triple detectors (polypropylene) or calibrated with polystyrene standard (poly(1-hexene)). dCalcined SiO2 was used. eNot measured.
Scheme 2. Postpolymerization of Propylene Using 1−SiO2Treated MMAO
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01003. 1 H NMR spectra of SiO2-treated MMAO (PDF)
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ACKNOWLEDGMENTS
We greatly appreciate a generous donation of MMAO from Tosoh-Finechem Co. (Japan) and silica from Fuji Silicia Co. Ltd. (Japan). This work is partly supported by the Center of Research for Environmentally Friendly Smart Materials, Hiroshima University.
Figure 3. GPC traces of polypropylene obtained from Scheme 1 (left: postpolymerization; right: first polymerization).
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AUTHOR INFORMATION
Corresponding Author
*(T.S.) E-mail
[email protected]. ORCID
Ryo Tanaka: 0000-0002-6085-074X Notes
The authors declare no competing financial interest. D
DOI: 10.1021/acs.macromol.7b01003 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules (13) Cipullo, R.; Busico, V.; Fraldi, N.; Pellecchia, R.; Talarico, G. Improving the Behavior of Bis(phenoxyamine) Group 4 Metal Catalysts for Controlled Alkene Polymerization. Macromolecules 2009, 42, 3869−3872. (14) Tanaka, R.; Kamei, I.; Cai, Z.; Nakayama, Y.; Shiono, T. Ethylene−Propylene Copolymerization Behavior of ansaDimethylsilylene(fluorenyl)(amido)dimethyltitanium Complex: Application to Ethylene−Propylene−Diene or Ethylene−Propylene− Norbornene Terpolymers. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 685−691. (15) Hagimoto, H.; Shiono, T.; Ikeda, T. Living Polymerization of Propene with a Chelating (Diamide)dimethyltitanium Complex Using Silica-Supported Methylaluminoxane. Macromolecules 2002, 35, 5744− 5745. (16) Hagimoto, H.; Shiono, T.; Ikeda, T. Supporting Effects of Methylaluminoxane on the Living Polymerization of Propylene with a Chelating (Diamide)dimethyltitanium Complex. Macromol. Chem. Phys. 2004, 205, 19−26. (17) Nishii, K.; Hagihara, H.; Ikeda, T.; Akita, M.; Shiono, T. Stereospecific polymerization of propylene with group 4 ansafluorenylamidodimethyl complexes. J. Organomet. Chem. 2006, 691, 193−201. (18) Scollard, J. D.; McConville, D. H.; Payne, N. C.; Vittal, J. J. Polymerization of α-Olefins by Chelating Diamide Complexes of Titanium. Macromolecules 1996, 29, 5241−5243. (19) Tshuva, E. Y.; Goldberg, I.; Kol, M. Isospecific Living Polymerization of 1-Hexene by a Readily Available Nonmetallocene C2-Symmetrical Zirconium Catalyst. J. Am. Chem. Soc. 2000, 122, 10706−10707. (20) Ghiotto, F.; Pateraki, C.; Tanskanen, J.; Severn, J. R.; Luehmann, N.; Kusmin, A.; Stellbrink, J.; Linnolahti, M.; Bochmann, M. Probing the Structure of Methylalumoxane (MAO) by a Combined Chemical, Spectroscopic, Neutron Scattering, and Computational Approach. Organometallics 2013, 32, 3354−3362. (21) Bryliakov, K. P.; Semikolenova, N. V.; Panchenko, V. N.; Zakharov, V. A.; Brintzinger, H. H.; Talsi, E. P. Activation of racMe2Si(ind)2ZrCl2 by Methylalumoxane Modified by Aluminum Alkyls: An EPR Spin-Probe, 1H NMR, and Polymerization Study. Macromol. Chem. Phys. 2006, 207, 327−335. (22) Babushkin, D. E.; Brintzinger, H. H. Modification of Methylaluminoxane-Activated ansa-Zirconocene Catalysts with Triisobutylaluminum - Transformations of Reactive Cations Studied by NMR Spectroscopy. Chem. - Eur. J. 2007, 13, 5294−5299. (23) Scollard, J. D.; McConville, D. H. Living Polymerization of αOlefins by Chelating Diamide Complexes of Titanium. J. Am. Chem. Soc. 1996, 118, 10008−10009. (24) Ioku, A.; Hasan, T.; Shiono, T.; Ikeda, T. Effects of Cocatalysts on Propene Polymerization with [t-BuNSiMe2(C5Me4)]TiMe2. Macromol. Chem. Phys. 2002, 203, 748−755. (25) Shiono, T.; Yoshida, S.; Hagihara, H.; Ikeda, T. Additive effects of trialkylaluminum on propene polymerization with (tBuNSiMe2Flu)TiMe2-based catalysts. Appl. Catal., A 2000, 200, 145−152.
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DOI: 10.1021/acs.macromol.7b01003 Macromolecules XXXX, XXX, XXX−XXX
