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High Temperature, Living Polymerization of Ethylene by a Sterically-Demanding Nickel(II) Α-Diimine Catalyst

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High Temperature, Living Polymerization of Ethylene by a Sterically-Demanding Nickel(II) Α-Diimine Catalyst polymers Article High Temperature, Living Polymerization of Ethylene by a Sterically-Demanding Nickel(II) α-Diimine Catalyst Lauren A. Brown, W. Curtis Anderson Jr., Nolan E. Mitchell, Kevin R. Gmernicki and Brian K. Long * ID Department of Chemistry, University of Tennessee, Knoxville, TN 37996, USA; [email protected] (L.A.B.); [email protected] (W.C.A.J.); [email protected] (N.E.M.); [email protected] (K.R.G.) * Correspondence: [email protected]; Tel.: +1-865-974-5664 Received: 15 December 2017; Accepted: 27 December 2017; Published: 2 January 2018 Abstract: Catalysts that employ late transition-metals, namely Ni and Pd, have been extensively studied for olefin polymerizations, co-polymerizations, and for the synthesis of advanced polymeric structures, such as block co-polymers. Unfortunately, many of these catalysts often exhibit poor thermal stability and/or non-living polymerization behavior that limits their ability to access tailored polymer structures. Due to this, the development of catalysts that display controlled/living behavior at elevated temperatures is vital. In this manuscript, we describe a Ni α-diimine complex that is capable of polymerizing ethylene in a living manner at temperatures as high as 75 ◦C, which is one of the highest temperatures reported for the living polymerization of ethylene by a late transition metal-based catalyst. Furthermore, we will demonstrate that this catalyst’s living behavior is not dependent on the presence of monomer, and that it can be exploited to access polyethylene-based block co-polymers. Keywords: polyethylene; living polymerization; nickel α-diimine; catalysis 1. Introduction Controlled/living polymerizations offer a precise means by which polymer structure, co-monomer incorporation levels, and even regio- and stereoselectivity can be tailored [1–7]. Living polymerization methodologies are those that are free of deleterious chain termination and irreversible chain transfer events [8]. Specifically for ethylene and α-olefin polymerizations, these chain transfer and termination events are often suppressed at low temperatures, and it is for this reason that most living polymerizations of ethylene occur at or below ambient temperature [1,9–14]. However, due to the growing demand for polymers with tailored structure and because industrial olefin polymerizations are often conducted at elevated temperatures (70–115 ◦C), the overall utility of most reported late transition metal-based olefin polymerization catalysts are severely limited [15,16]. To date, only a few examples of Ni- and Pd-based olefin polymerization catalysts that exhibit controlled/living polymerization behavior at superambient temperatures have been reported [17–22]. Furthermore, most of these polymerizations are not performed using ethylene as a sole feedstock, but rather utilize higher α-olefins. This lack of Ni- and Pd-based catalysts capable of performing living ethylene polymerizations at elevated temperatures represents a fundamental gap in current knowledge. To address this issue, we report herein that the Ni-based α-diimine catalyst 1 readily polymerizes ethylene in a living fashion at temperatures as high as 75 ◦C (Figure1). To the best of our knowledge, this is one of the highest temperature living ethylene polymerizations using a Ni- or Pd-based catalyst reported to date. Polymers 2018, 10, 41; doi:10.3390/polym10010041 www.mdpi.com/journal/polymers Polymers 2018, 10, 41 2 of 10 our knowledge, this is one of the highest temperature living ethylene polymerizations using a Ni- or Polymers 2018, 10, 41 2 of 9 Pd-based catalyst reported to date. Figure 1. Catalyst 1 used for ethylene polymerizations. 2. Materials and Methods All experimentsexperiments were performedperformed under a drydry nitrogennitrogen atmosphereatmosphere usingusing standardstandard SchlenkSchlenk techniquestechniques oror anan MBraun inert-atmosphere glove box (Stratham, NH, USA), unless otherwise noted. Solvents were purifiedpurified usingusing aa two-columntwo-column solid-statesolid-state InnovativeInnovative TechnologiesTechnologies PureSolvPureSolv SolventSolvent PurificationPurification System (Amesbury, (Amesbury, MA MA USA) USA) and and dega degassedssed via via three three freeze-pump-thaw freeze-pump-thaw cycles cycles prior prior to touse. use. NMR NMR solvents solvents were were purchased purchased from from Cambri Cambridgedge Isotope Isotope Laboratories Laboratories (Andover, (Andover, MA, MA, USA). Catalyst 11 waswas prepared prepared according according toto literature literature [23]. [23 Liquid]. Liquid chromatography-mass chromatography-mass spectrometry spectrometry (LC- (LC-MS)MS) experiments experiments were were performed performed using using a Thermo a Thermo Fisher Fisher Scientific Scientific Exactive Exactive (Waltham, (Waltham, MA, USA) MA, USA)Plus Orbitrap Plus Orbitrap MS (Waltham, MS (Waltham, MA, MA,USA) USA) using using direct direct injection, injection, full-scan, full-scan, electrospray electrospray ionization. ionization. All Allpolymerizations polymerizations were were activated activated using using methylalumin methylaluminoxanesoxanes (PMAO-IP) (PMAO-IP) that that was was purchased purchased from from the theAkzo Akzo Nobel Nobel (Amsterdam, (Amsterdam, the the Netherlands) Netherlands) and and used used as as received. received. All All other other reagents reagents were were purchased fromfrom commercialcommercial vendorsvendors andand usedused withoutwithout furtherfurther purification.purification. GelGel permeationpermeation chromatographychromatography (GPC)(GPC) waswas performedperformed atat 160160 ◦°CC inin 1,2,4-trichlorobenzene at ata a flowflowrate rateof of1.0 1.0 mL/minmL/min using a Malvern ViscotekViscotek HT-GPCHT-GPC (Malvern,(Malvern, UK)UK) equippedequipped withwith tripletriple detection.detection. PolymerPolymer 11H NMR spectra werewere ◦ obtained inin CDClCDCl33 usingusing aa BrukerBruker400 400 MHz MHz NMR NMR at at 50 50 C.°C. All All NMR NMR spectra spectra are are referenced referenced relative relative to theirto their residual residual solvent solvent signal. signal. Branching Branching content content was was determined determined by 1byH NMR1H NMR spectroscopy spectroscopy using using the formulathe formula (CH (CH3/3)/((CH3/3)/((CH + CH+ CH2 +2 CH+ CH3)/2)3)/2)× × 1000 [[24].24]. PolymerPolymer thermalthermal transitiontransition temperaturestemperatures ((TTmm) werewere measuredmeasured using using a a TA TA instruments instruments Q2000 Q2000 Differential Differential Scanning Scanning Calorimeter Calorimeter (DSC, (DSC, New New Castle, Castle, DE, USA)DE, USA) and recordedand recorded on the on second the second heating heating cycle at cycle a heating at a rateheating of 10 rate◦C/min. of 10 Polyethylene°C/min. Polyethylene samples forsamples tensile for testing tensile were testing melt-pressed were melt-pressed using 0.5 gusing of polyethylene 0.5 g of polyethylene sample in sample a Carver in Press a Carver at 10,000 Press lbs. at of10,000 pressure lbs. of for pressure 10 minutes, for 10 followed minutes, by followed slow cooling by slow to roomcooling temperature. to room temperature. Both faces ofBoth the faces Carver of Pressthe Carver were coveredPress were with covered Kapton with film priorKapton to pressingfilm prior to to prevent pressing film to sticking prevent and film contamination. sticking and Tensilecontamination. (stress vs. Tensile strain) analysis(stress vs. was strain) performed analysis using was an Instronperformed 5943 electrousing an mechanic Instron single 5943 columnelectro universalmechanic testingsingle column machine universal (Norwood, testing MA, USA)machine with (Norwood, pneumatic MA, tension USA) grips with connected pneumatic to atension 100 N loadgrips cell connected at a strain to a rate 100 in N accordance load cell at with a strain the rate ASTM in accordance D638 standard. with the ASTM D638 standard. General ethyleneethylene polymerizationpolymerization procedure: Under an inert atmosphere, a Fisher-PorterFisher-Porter bottlebottle waswas chargedcharged withwith catalystcatalyst 1 (5(5 µµmol)mol) dissolveddissolved inin dichloromethanedichloromethane (DCM) (2 mL), toluene (98 mL), and a magnetic stir bar. The Fisher-Porter bottlebottle waswas sealedsealed andand placedplaced inin anan oiloil bath set to the desired temperature.temperature. TheThe vessel was pressurized with ethylene gas while stirring and equilibrated for 10 10 min. min. PMAO-IP (100 equiv) was injected to initiate the polymerization and the polymerization was stirred continuouslycontinuously for thethe desireddesired time.time. All polymerizationspolymerizations were quenched via thethe additionaddition ofof methanolmethanol (MeOH)(MeOH) (10(10 mL).mL). TheThe polymerpolymer waswas precipitatedprecipitated usingusing excessexcess acidicacidic MeOHMeOH (5%(5% HClHCl inin MeOH)MeOH) andand allowed to stirstir inin thatthat solutionsolution forfor 2424 h.h. The resultantresultant polymer was filteredfiltered andand drieddried toto constantconstant weightweight inin aa vacuumvacuum oven.oven. Polymers 2018, 10, 41 3 of 9 3. Results and Discussion 3.1. Synthesis of High Purity Catalyst 1 Catalyst 1 was previously reported by our group and found to exhibit significantly enhanced time-resolved thermal stability due to its acenaphthenequinone-based ligand that contains sterically-demanding N-aryl moieties [23,25]. More specifically, catalyst 1 displayed virtually perfect thermal stability up to reaction temperatures as high as 90 ◦C, which is one of
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