Article pubs.acs.org/Organometallics
Termination Events in Sterically Hindered Metallocene-Catalyzed Olefin Oligomerizations: Vinyl Chain Ends in Oligooctenes Patrick Brant, Peijun Jiang, Jacqueline Lovell, and Donna Crowther* ExxonMobil Chemical Company, 5200 Bayway Drive, Baytown, Texas 77520, United States S Supporting Information *
ABSTRACT: Sterically hindered rac-dimethylsilylbis(2-methyl-3-propylindenyl)hafnium dimethyl (1) activated with dimethylanilinium tetrakis(perfluoronaphthylborate) (2) gave high levels of allylic chain ends in the oligomerization of higher α-olefins such as 1-octene. Solution oligomerization of 1octene in hexanes yielded atactic macromers with roughly 60 mol % vinyl termination and lesser amounts of trisubstituted (25%) and vinylidene (20%) termination. To further probe the mechanism of elimination, oligomerization with 1-octene (1-13C) was performed and the label distribution determined by 13C NMR spectroscopy. Enhancement of the methylene (H2CC−) vinyl resonance was observed, supporting β-hexyl elimination as a possible mechanistic step and ruling out allylic formation by a chain-walking mechanism prevalent in late-transition-metal systems.
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INTRODUCTION
vinylidene, and other unsaturated chain ends in propylene and higher α-olefin polymerizations has been widely documented. Termination by β-methyl elimination was first recognized in neutral lutetium systems5 and later in highly congested group 4 (Cp*)2MCl2 (M = Zr, Hf) (where Cp* is pentamethylcyclopentadienyl) catalyzed propylene oligomerization.6−9 The atactic oligopropene was characterized by its 13C NMR spectrum, which quantitatively indicated one saturated isobutyl chain end per allylic chain end.9 Interestingly, the same (Cp*)2MCl2/MAO catalyst produced only vinylidene-terminated oligobutene. To our knowledge, the formation of high levels of vinyl chain ends in higher α-olefin (greater than propylene) polymerization has not been documented.3,10 The amount of β-methyl to β-hydride elimination in propylene polymerization is highly dependent on the metallocene structure.11 β-hydride termination generally predominates, yielding a macromer with a vinylidene chain end and a corresponding saturated n-propyl chain end.12,13 Mixtures of vinyl, vinylidene, trisubstituted, and/or vinylene chain ends have also been identified and interpreted mechanistically due to β-hydride or β-methyl elimination, rearrangements through either an allylic intermediate or a π-bonded alkene metal hydride, or 2,1-misinsertions, respectively.14 In polymerizations of α-olefins greater than propylene, the formation of vinyl chain ends are found only in nonmetallocene systems and postulated to be formed by chainwalking mechanisms.15 Higher α-olefin polymerizations by group 4 metallocenes predominantly give vinylidene chain ends via β-hydride termination. However, other types of unsatura-
Termination reactions occurring in transition-metal-catalyzed olefin polymerizations with well-defined components have been the subject of many research endeavors. Studies initially focused on termination events in order to elucidate how the catalyst’s structural and electronic parameters influenced relative rates of propagation versus termination. One of the main reasons was to increase polymer molecular weight. More recently, it has been recognized that lower molecular weight macromers or oligomers, specifically those with terminal unsaturated chain ends, are useful materials in themselves.1 These can be transformed through numerous reactions to give polar functionality or used as macromers, especially those containing allylic chain ends.2 The types of unsaturated chain ends formed in olefin polymerizations are dependent on the α-olefin monomer, the activator/transition-metal catalyst pair, and the reaction conditions. The majority of past studies utilized MAO (methylalumoxane) as the activator, lessening the probability of unsaturated chain end formation due to chain transfer to aluminum (Scheme 1).3,4 Even so, the identification of vinyl, Scheme 1. Chain Ends from (Cp*)2MCl2/MAO System
Received: May 20, 2016
© XXXX American Chemical Society
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DOI: 10.1021/acs.organomet.6b00409 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
Scheme 2. Unsaturated Chain Ends from Hf 1 Catalyzed Olefin Oligomerizations and Mode of Termination Proposed for Each Unsaturation Type
tion such as vinylene etc. can occur by mechanisms similar to those previously discussed.16−18
Chart 1. Cooligomerization of Propylene and Octene Catalyzed by 1:a Vinyl Content As Dependent on Octene Content in Oligomer
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RESULTS AND DISCUSSION Borate activated complex 1 polymerized propylene by regioregular 1,2-insertion to yield atactic polypropylene (aPP) with greater than 95% allylic chain ends (Scheme 2). The remaining unsaturated groups were vinylidene chain ends. Analysis by 13C NMR spectroscopy indicated a ratio of one isobutyl chain end per allylic chain end. This supports a βmethyl elimination mechanism as postulated in ref 9. The number-average molecular weight (Mn) of the aPP can be varied from 1 to 60 kg/mol through control of reactor conditions, including temperature and monomer concentration. The Mn decreases with increasing temperature and decreasing propylene concentration, whereas the amount of allylic chain ends remains relatively constant at or above 95 mol %. Select aPP samples were also analyzed by gel permeation chromatography (GPC) analysis, and plots of Mn from 1H NMR analysis versus Mn from GPC analysis show parity (see the Supporting Information). Cooligomerization of octene and propylene with borate activated 1 gave varying amounts of vinyl chain ends, which were highly dependent on the concentration of propylene in the feed. High levels of vinyl chain ends were maintained until about 30 mol % of propylene, suggesting that β-elimination events are the predominant chain release mechanism. Most likely β-methyl elimination occurs to form allylic chain ends, but we cannot rule out β-hexyl elimination. The remaining unsaturated end groups are vinylidenes and smaller amounts of trisubstituted groups; both increased with increasing amounts of octene in the oligomer product. Even with pure octene, high levels of allylic chain ends were identified by 1H NMR spectroscopy (Chart 1). 1-Octene was oligomerized by activated complex 1 to give predominately allylic terminated oligooctene, with Mn controlled through the reaction temperature, conversion, and octene concentration (Table 1). The oligooctene products were atactic, and no 2,1-regioerrors were observed by 13C NMR analysis (see the Supporting Information). Parity of Mn by GPC
a
Reaction conditions: HTE (high-throughput experimentation) runs at 85 °C. The propylene concentration was varied, and octene was maintained at [3.3] mol %. Octene in the macromer was determined by 13C NMR spectroscopy. Runs were quenched at 60 min. Allylic and other chain ends were determined by 1H NMR spectroscopy. Details are given in the Supporting Information.
analysis with 1H NMR analysis is fairly good, except for entry 8, in which MAO was used as activator. In this case, Mn is overestimated by 1H NMR analysis. This indicates chain transfer to aluminum which results in formation of doubly saturated chain ends. Analysis by 13C NMR techniques to identify saturated end groups, specifically −CH(C6H13)2, was not successful due to overlapping methyl signals from regioregular 1,2-octene enchainment. 1-Octene (1-13C) was oligomerized under conditions identical with those of entry 3 in Table 1. Details are provided in the Supporting Information. A comparison of the 13C NMR spectra from the 1-13C-labeled octene run and entry 3 is shown in Figure 1. The enhancement of signals from the backbone methylene resonances in the range from 42.0 to 40.0 ppm is due to the regioregular 1,2-insertion mechanism. The enhancement of the vinyl methylene (CH2CH−) resonance at 116.6 ppm supports the notion that β-hexyl elimination is responsible for its formation and rules out formation by a chain-walking mechanism. Minor enhancement of resonances at 111.2 ppm (vinylidene), as well as at 132.3 ppm (most likely due to a B
DOI: 10.1021/acs.organomet.6b00409 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Table 1. Octene Oligomerization Catalyzed by 1a entry 1 2 3 4 5 6 7d 8e
octene (mol/L) 1.6 3.3 3.3 1.6 3.3 6.3 3.3 6.3
1 (mmol) −4
8 × 10 8 × 10−4 5.2 × 10−4 8 × 10−4 8 × 10−4 8 × 10−4 1.5 × 10−5 1.7 × 10−5
T (°C)
time (min)
conversn (%)
allyl (%)b
vinyl (%)
trisub (%)
Mn (kg/mol)b
Mn (kg/mol)c
30 30 50 85 85 85 85 85
30 30 60 30 30 30 150 30
36 50 64 5 11 16 60.9 29.7
68 64.9 68 61.7 58.1 54.3 57.8 51.5
11.6 6.5 5.4 14.2 16.3 18.5 12.1 29.9
20.4 28.6 27 24.1 25.6 27.2 30.1 18.6
50.4 54.4 10.9 2.8 4.6 6.6 2.5 7.7
46 67 na na 4.4 6.7 2.4 4.9
a
Oligomerizations were carried out in glass scintillation vials using 5 mL of octene solution. bDetermined from 1H NMR spectroscopy. cDetermined by GPC DRI. dThe reaction solvent was toluene. eMAO used as activator at a molar ratio of 130/1 Al/Hf.
Scheme 3. Proposed Mechanism for Oligomerization of Octene Catalyzed by Complex 1 Based on Distribution of 13 C Label
Figure 1. Comparison of 1-octene (1-13C) and 1-octene oligomerizations catalyzed by 1.
minor trisubstitued chain end), are also evident. A major trisubstituted chain end is formed, in addition, with the methine (−CHCR−) resonance at 127.4 ppm. This signal is not enhanced in the spectrum of the labeled oligooctene but can be observed in both 1H and 13C NMR spectra from unlabeled octene oligomerizations. On the basis of the distribution of enriched 13C label, we propose the mechanisms shown in Scheme 3. Allylic chain ends are most likely formed by β-hexyl elimination. In addition, β-hydride elimination yields vinylidene-terminated oligooctenes. The formation of the minor and major trisubstituted unsaturated chains cannot be distinguished as originating from either an allylic intermediate or a 1,2rearranged intermediate. Further support for assignments of unsaturated end groups are provided in the Supporting Information.
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EXPERIMENTAL SECTION
General Details. All syntheses were carried out under inert conditions using a drybox continuously purged with nitrogen. 1Octene (1-13C 99%) was purchased from Cambridge Isotopes and was purified by passing through a basic alumina column and dried over 3A sieves. 1-Octene was purchased from Aldrich Chemical and was purified similarly. Dimethylanilinium tetrakis(perfluoronaphthylborate) (2) was purchased from Albemarle Corp. and used as received. Reaction solvents were purchased as anhydrous versions from Aldrich and dried with activated 3A sieves for at least 24 h prior to use. Propylene was supplied in house and stored over 3A sieves prior to use. Methylalumoxane was purchased from Albemarle as a 30 wt % solution in toluene. The 1H and 13C NMR spectra were recorded on either a Bruker 500 or 400 MHz instrument. Chemical shifts are relative to residual CHDCl2. Elemental analysis was provided by Galbraith Laboratories Inc. Synthesis of rac-Dimethylsilylbis(2-methyl-3-propylindenyl)hafnium Dimethyl (1). (2-Methylindenyl)lithium (10 g, 73.5 mmol) was slurried in Et2O (150 mL) and reacted with propyl bromide (18 g, 146 mmol). THF (50 mL) was added, and the reaction mixture was stirred for 12 h. The reaction was quenched with H2O and the organic layer dried over MgSO4. Removal of volatiles gave the product as a yellow oil (10.5 g, 83% yield). The crude product was dissolved in hexane (200 mL) and deprotonated with nBuLi (6.1 mL, 10 M, 61 mmol). After 12 h, the white solid was filtered onto a glass
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CONCLUSIONS In conclusion, borate activated rac-dimethylsilylbis(2-methyl-3propylindenyl)hafnium dimethyl 1 catalyzes the oligomerization of octene to yield high amounts of allylic terminated macromer. Isotopic experiments with 1-octene (1-13C) support the formation of allylic chain ends by a β-hexyl elimination event and rule out chain-walking events. In addition, β-methyl termination is the predominant elimination event in the oligomerization of propylene catalyzed by 1. Although sterically hindered metallocenes have been shown previously to undergo β-methyl termination with propylene as monomer, these same metallocenes do not β-alkyl terminate with α-olefins greater than propylene or deliver high-molecular-weight products. C
DOI: 10.1021/acs.organomet.6b00409 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics frit, washed with additional hexane (2 × 50 mL), and dried in vacuo (10.8 g). All of the lithiated ligand [2-Me-3-propylC9H5][Li]2 was dissolved in THF (100 mL) and reacted with Me2SiCl2 (3.8 g, 29 mmol) for 10 h. The crude ligand can be purified by column chromatography (hexane/ethyl acetate 75/25 v/v) on silica gel (200− 400 mesh). (2-Me-3-propylC9H5)2SiMe2 (7.4 g, 32.7 mmol) was dissolved in Et2O (70 mL) and reacted with nBuLi (5.2 g, 10 M in hexanes) for 12 h. The solid precipitate was filtered onto a glass frit, washed with hexanes (2 × 30 mL), and dried in vacuo (9.1 g). The compound [(2-Me-3-propylC9H5)2SiMe2][Li2] (6.6 g, 16.0 mmol) was slurried in Et2O (100 mL) and reacted with HfCl4 (4.2 g, 13.1 mmol). After 1 h about 50 mL of Et2O was removed and the bright yellow solid collected on a glass frit (4.2 g). The hafnocene dichloride was slurried in Et2O (50 mL) and toluene (80 mL) and reacted with MeMgI (5.6 g, 3 M in Et2O) at ambient temperature for 16 h. Dimethoxyethane (6 g) was added to the crude reaction mixture and the mixture filtered through a medium glass frit. The filtrate was reduced, pentane added (30 mL), and the filtrate cooled to −35 °C. The first crop collected was pure rac-1 (0.6 g). The second crop collected contained 5% meso and 95% 1 (0.7 g). Anal. Calcd for C30H40HfSi: C, 59.34; H, 6.64; Hf, 29.40; Si, 4.62. Found: C, 60.25; H, 6.91; Hf, 27.6; Si, 4.53. 1H NMR (CD2Cl2, 500 MHz; δ, ppm): 7.4 (d), 7.3 (d), 7.08 (t), 6.75 (t), 2.68 to 2.22 (complex m), 1.87 (s), 1.41 (m), 0.97 (s), 0.81 (t), −1.95 (s). 13C NMR (CD2Cl2, 500 MHz; δ, ppm): 131.0, 129.6, 127.0, 126.1, 124.5, 124.2, 123.6, 123.4, 78.4, 42.9, 28.9, 23.7, 15.2, 14.8, 3.5. HTE General Details. Typical solution polymerizations were carried out using a high-throughput robotic system manufactured by Symyx Technologies (Santa Clara, CA). The experimental details were developed using Library Studio version 7.1.9. The reactions were carried out in parallel with robotic control and typically took less than 2 h for completion. Individual reaction wells were lined with disposable glass inserts and were equipped with Teflon stirring paddles. Stock solutions of metallocene and activator in toluene were added separately to an isohexane reaction solvent containing a specific amount of scavenger, Al(C8H17)3. Monomers were then added and the reactions controlled either by time or in some cases by a prespecified pressure drop. The total volume of monomers, solvent, metallocene, activator, and scavenger was maintained at 5.1 mL. The reactions were quenched with CO2 addition, and the volatiles were removed under reduced pressure at 70 °C.
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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.organomet.6b00409. 13 C and 1H NMR spectra for 1, characterization of oligomers, select NMR spectra for relevant oligomers, and oligomerization procedures (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail for D.C.:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors greatly appreciate the financial support of ExxonMobil Chemical Co.
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REFERENCES
(1) (a) Janiak, C.; Lange, K. C. H.; Marquardt, P. J. J. Mol. Catal. A: Chem. 2002, 180, 43−58. (b) Janiak, C.; Blank, F. Macromol. Symp. 2006, 236, 14−22. D
DOI: 10.1021/acs.organomet.6b00409 Organometallics XXXX, XXX, XXX−XXX
