Facile Access to Polar-Functionalized Ultrahigh Molecular Weight Polyethylene at Ambient Conditions

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 May 2022Facile Access to Polar-Functionalized Ultrahigh Molecular Weight Polyethylene at Ambient Conditions Xiaoqiang Hu, Xiaohui Kang, Yixin Zhang and Zhongbao Jian Xiaoqiang Hu State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 University of Science and Technology of China, Hefei 230026 , Xiaohui Kang College of Pharmacy, Dalian Medical University, Dalian 116044 , Yixin Zhang State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 and Zhongbao Jian *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 University of Science and Technology of China, Hefei 230026 https://doi.org/10.31635/ccschem.021.202100895 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The most straightforward and potentially ideal route to produce polar-functionalized polyethylene is direct copolymerization of ethylene with polar monomers. However, access to high-molecular-weight polar copolymers represents one of the biggest challenges in the field of olefin polymerization. In this contribution, we report a family of well-designed nickel catalysts that readily address this issue under convenient and highly desired ambient conditions. Under 1 bar at 30 °C, polar-functionalized ultrahigh number-average molecular weight polyethylenes (UHMWPEs, Mn = 0.83–1.10 × 106 g mol−1) are directly generated. The highest average number of incorporated polar units per polymer chain is 122. This enhances copolymer molecular weights by two orders of magnitude relative to previous reports. Notably, this nickel catalyst family also exceptionally produces the highest number-average molecular weight polyethylenes (Mn = 6.04 × 106 g mol−1) at 1 bar. The Sterimol B1 steric parameter of nickel catalyst quantitatively correlates to polymer molecular weight. Mechanistic insights from density functional theory (DFT) calculation reveal that the low barrier (10.4 kcal mol−1) of ethylene insertion as the rate-limiting step should be responsible for high activity and the formation of UHMWPE. This coordination–insertion approach is a striking contrast to the high energy free-radical approach. Download figure Download PowerPoint Introduction Reactions that occur at mild conditions are economic, low energy-consuming, convenient, safe, and highly desired in chemical synthesis. One notable example of pursuing a mild reaction is the synthesis of ammonia in industry. It is produced mainly via the Haber–Bosch process at high pressure of >100 bar and high temperature of 400–500 °C.1 Significant improvement to very mild conditions as 1 bar and 45 °C has very recently been reported via a mechanochemical method.2 Likewise, a mild reaction condition is particularly important for the synthesis of polyolefin materials, the most produced polymers in modern society. The introduction of functional group (even a small amount of <0.5 mol %)3 into widely used nonpolar polyolefins to produce functionalized polyolefins is of significant importance because it endows paramount end-use properties to polyolefin materials such as adhesion, chemical compatibility, toughness, and dyeability, upgrading polyolefins for value-added and advanced applications.4–8 Direct copolymerization of ethylene with polar monomers is considered to be powerful and energy efficient and the most straightforward method for accessing polar-functionalized polyolefins over the past decades; however, this reaction represents one of the biggest challenges in polymer science thus far.9–16 Two key approaches have been applied to these challenging copolymerization reactions of ethylene with polar monomers (Chart 1): free-radical polymerization and coordination–insertion polymerization. Because of the very low reactivity of ethylene, free-radical polymerization of ethylene and copolymerization of ethylene with polar monomer require harsh reaction conditions (high ethylene pressures [250−3000 bar] and high temperatures [150−375 °C]) either in an autoclave or pressure tube.17–19 Moreover, the polymer architectures obtained are branched and poorly defined such as commercial low-density polyethylene (LDPE) and ethylene–vinyl acetate copolymer (EVA). Recent improvements (<250 bar, <100 °C) on these issues via controlled radical polymerization techniques are appealing, but molecular weights (Mn) of polymers are still low, at most 105 g mol−1 but usually 103–104 g mol−1.20–27 In contrast, since the milestone discovery from Ziegler and Natta, early- and late-transition and rare-earth metal-catalyzed coordination–insertion polymerization of ethylene employs prominently mild reaction conditions (1–100 bar, <100 °C) and is highly controlled to produce polyethylenes (PEs) with a variety of molecular weights (103–107 g mol−1) and well-defined architectures. Nevertheless, when this approach is applied to the copolymerization of ethylene with polar vinyl monomers, issues including drastically reduced molecular weight and catalytic activity, mandatory use of polar monomers bearing a masking reagent, are encountered owing to accelerated chain transfer reactions like β-H elimination and stronger binding of functional groups. In particular, polymer molecular weight is the most serious obstacle to polymer applications, despite significant advancements over the past decades.10,13,28–43 Chart 1 | Polar-functionalized polyethylenes via the copolymerization of ethylene with polar monomers. Download figure Download PowerPoint Molecular weight is one of the key parameters in olefin polymerization (also in any polymerization reaction), which determines the macroscopic chemical and physical properties of polymer materials. For instance, ultra-high molecular weight polyethylene (UHMWPE, Mn > 106 g mol−1), a notably important thermoplastic material, exhibits outstanding chemical and biological stability, high impact toughness, abrasion resistance, and lubricity.44,45 Thus far, UHMWPE has been readily available using early- or late-transition-metal-mediated coordination polymerization; however, polar-functionalized UHMWPE via the copolymerization of ethylene with polar monomers remains a formidable challenge. Typically, these functionalized polyethylenes produced by the most successful late transition-metal nickel and palladium catalysts suffer from molecular weights of Mn < 105 g mol−1 even at high pressures.46–59 In this contribution, we now show how polar-functionalized ultrahigh number-average molecular weight polyethylenes (Mn > 106 g mol−1) can be accessible via the coordination–insertion copolymerization of ethylene with polar monomers using the well-designed α-diimine nickel catalysts. Most notably, to address the issue of molecular weight, these reactions proceed at convenient and highly desired ambient conditions of both pressure (1 bar) and temperature (30 °C) in a glass reactor. This is in sharp contrast to previous severe methods such as free-radical copolymerization. Experimental Methods All syntheses involving air- and moisture-sensitive compounds were carried out using standard Schlenk-type glassware (or in a glovebox) under an atmosphere of nitrogen. All solvents were purified from the MBraun SPS system. NMR spectra for the ligands, complexes, and polymers were recorded on a Bruker AV400 (1H, 400 MHz; 13C, 100 MHz) or a Bruker AV500 (1H, 500 MHz; 13C, 125 MHz). NMR assignments were confirmed by 1H−1H COSY, 1H−13C HSQC, and 1H−13C HMBC experiments when necessary. X-ray diffraction data collections were performed at −100 °C on a Bruker SMART APEX diffractometer with a charge-coupled device (CCD) area detector, using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The molecular weights (Mn) and molecular weight distributions (Mw/Mn) of polyethylenes and copolymers were measured by means of gel permeation chromatography (GPC) on a photoluminescence (PL)-GPC 220-type high-temperature chromatograph equipped with three PLgel 10 μm Mixed-B LS-type columns (Agilent) at 150 °C with 1,2,4-trichlorobenzene as the solvent. Melting points (Tm) of polyethylenes and copolymers were measured through differential scanning calorimetry (DSC) analyses, which were carried out on a Mettler TOPEM TM DSC instrument under nitrogen atmosphere at heating and cooling rates of 10 °C/min (temperature range of 0−160 °C). Mass spectra of the complexes were recorded on a Quattro Premier XE MS with Acquity Ultra Performance Liquid Chromatography (UPLC) system (Waters). Elemental analysis was performed at the National Analytical Research Center of Electrochemistry and Spectroscopy of Changchun Institute of Applied Chemistry. The water contact angle (WCA) of each sample was obtained by using a contact angle goniometer (DSA KRÜSS GMBH, Hamburg 100) at room temperature at least five times. Infrared (IR) spectra of the copolymers were recorded on a VERTEX 70 Fourier transform IR (FTIR) spectrometer. All detailed crude data can be found in the Supporting Information. Computational Methods All quantum chemical computations were performed by using the Gaussian16 package of programs. Each optimized structure was optimized at the GGA B3LYP/BSI level and was subsequently characterized as a minimum (Nimag = 0) or a transition state (Nimag = 1) by harmonic vibration frequencies, which provide thermodynamic data. The transition-state structures were shown to connect the reactant and product on either side by following the intrinsic reaction coordinate (IRC) (for details, see Supporting Information). Results and Discussion Catalyst design Generally, ultrahigh number-average molecular weight polyethylene is relatively easy to produce from ethylene polymerization at ambient conditions using early transition-metal catalysts such as the Ziegler–Natta catalysts and metallocene catalysts. In contrast, UHMWPE (Mn) generated by late transition-metal catalysts, such as nickel and palladium catalysts, at ambient conditions, such as 1 bar of ethylene pressure, is extremely difficult and remains elusive. Although there are no published reports on UHMWPE produced at 1 bar thus far, numerous nickel and palladium catalysts enable the preparation of UHMWPEs at higher pressures.60–82 Herein, we initially synthesized and screened typical nickel catalysts (Chart 2; for details, see Supporting Information Figures S8–S18) chelated by α-diimine, phosphine-sulfonate, phosphine-phenolate, and phenoxy-imine ligands, respectively, that generated very high to ultrahigh molecular weight polyethylenes at more than 8 bar.55,57,58,63,69,83–88 Chart 2 | Representative nickel catalysts for producing high molecular weight polyethylene at high pressure. Download figure Download PowerPoint Under 1 bar at 30 °C, ethylene polymerization was studied using these reference nickel catalysts Ref-Ni1– Ref-Ni7 (Table 1). Number-average molecular weights (Mn) of the obtained polyethylenes generated by α-diimine nickel catalysts Ref-Ni1– Ref-Ni4 were 7.3–32.0 × 104 g mol−1, which are far lower than the Mn of UHMWPE. Phosphine-sulfonate nickel catalyst Ref-Ni5 was even inactive at ambient conditions, although its 2,6-lutidine analogue gave very high Mn of 36.2 × 104 g mol−1 at 8 bar.58 Removal of the chelated pyridine molecule with Lewis acid B(C6F5)3 also only led to the formation of low Mn of 0.3 × 104 g mol−1 at 1 bar. Phosphine-phenolate nickel catalyst Ref-Ni6 featured an outstanding copolymerization with polar monomers and could produce the polyethylene with Mn of 8.4 × 104 g mol−1 at 1 bar without the addition of scavenger, but which again was lower than the Mn value of 48.0 × 104 g mol−1 at 10 bar as anticipated.57 Phenoxy-imine nickel catalysts typically produce UHMWPE at high pressure (>30 bar)63,69–71,78; however, Ref-Ni7 showed no activity toward ethylene polymerization at 1 bar. Adding B(C6F5)3 to Ref-Ni7 resulted in a high Mn of 36.1 × 104 g mol−1, which is still far from the ultrahigh Mn. These events undoubtedly demonstrate that the production of UHMWPE is extremely difficult at ambient conditions of 1 bar and 30 °C. Naturally, the generation of polar-functionalized UHMWPE is much more challenging at ambient conditions. Table 1 | Ethylene Polymerization with Outstanding Reference Nickel Catalysts at Ambient Conditions of 1 bar and 30 °Ca Entry Cat. Yield (g) Act. (106)b Mn (104)c Mw/Mnc brsd Tme (°C) 1f Ref-Ni1 0.92 0.92 30.9 1.18 106.6 — 2f Ref-Ni2 2.32 2.32 17.1 1.61 102.7 — 3f Ref-Ni3 0.43 0.43 32.0 1.06 92.5 — 4f Ref-Ni4 0.22 0.04 7.3 1.10 45.6 66 5 Ref-Ni5 Trace — — — — — 6g Ref-Ni5 0.82 0.16 0.3 2.37 16.2 119 7 Ref-Ni6 0.49 0.10 8.4 1.41 < 1.0 136 8 Ref-Ni7 Trace — — — — — 9g Ref-Ni7 0.62 0.25 36.1 1.17 32.6 90 aReaction conditions: 1 bar, 30 °C, 30 min, toluene/CH2Cl2 (98 mL/2 mL); catalyst loading: entries 1–3 (2 μmol), entries 4–7 (10 μmol), entries 8–9 (5 μmol); all entries are based on at least two runs unless noted otherwise. bActivity is in unit of g mol−1 h−1. cMn is in g mol−1. Determined by GPC in 1,2,4-trichlorobenzene at 150 °C using a light-scattering detector. dbrs = Number of branches per 1000C, as determined by 1H NMR spectroscopy. eDetermined by DSC (second heating). fMMAO (600 equiv). gB(C6F5)3 (2 equiv), toluene (100 mL). We were ultimately interested in improving copolymerization of olefin and polar monomers6,78–82 and were interested in the class of terphenyl-substituted α-diimine nickel catalysts (Chart 2) that were first reported in 2001.64,65 To greatly enhance Mn of polyethylene and reach UHMWPE, we envisioned a strategy of installing secondary, sterically encumbered substituents into terphenyl-substituted α-diimine nickel catalysts and thus synthesized a new family of nickel catalysts Ni1– Ni4 (Scheme 1). In these new nickel catalysts, we initially developed a concerted double-layer steric strategy, namely installing a rigid and planar phenyl group (Ph, blue) as the first layer for favoring ethylene coordination and insertion, and constructing the second layer (R1, R2, and R3, red) for inhibiting chain transfer (Scheme 1). To access the desired terphenyl-type anilines, these starting aniline derivatives with different substituents (CH3, PhCH2, Ph2CH, and Ph3C) at the para position first underwent an iodination reaction in the presence of sodium nitrite and potassium iodide and then a Suzuki coupling with the corresponding arylboronic acids. Subsequently, a series of α-diimine ligands were prepared in excellent yields by the p-toluenesulfonic acid-catalyzed condensation reaction of 2,3-butanedione with 2.1 equiv of the above terphenyl-type anilines in toluene for several days at 130 °C (Scheme 1). All α-diimine ligands were identified by elemental analysis and 1H and 13C NMR spectroscopy (for details, see Supporting Information Figures S21–S39). The corresponding α-diimine nickel complexes Ni1– Ni4 were directly synthesized by a typical reaction of α-diimine ligand with 1.0 equiv of NiBr2(DME) (DME = 1,2-dimethoxyethane) in dichloromethane for several days at room temperature. These pure nickel precatalysts were isolated from the mixture by a simple recrystallization with dichloromethane and n-hexane. Structure and purity of these α-diimine Ni(II) catalysts were fully identified by multiple methods including 1H NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray diffraction analysis (Figure 1; for other details, see Supporting Information Figures S21–S39 and S163–S166). The relationship of catalyst structure and polymer molecular weight will be discussed in the polymerization part (see below). Scheme 1 | Design and synthesis of α-diimine ligands and the related nickel catalysts. Download figure Download PowerPoint Evaluation of catalyst on ethylene polymerization Since UHMWPE is usually available at high pressure, Ni1– Ni4 were initially evaluated for ethylene polymerization at 8 bar (Table 2). With activation by modified methylaluminoxane (MMAO), Ni1 produced polyethylene with an extremely high Mw of 6.08 × 106 g mol−1 at 30 °C for 10 min (Table 2, entry 1). With the addition of phenyl substituents ( Ni2– Ni4), Mw gradually rose to reach an astonishing value of 9.34 × 106 g mol−1 (Mn = 6.49 × 106 g mol−1, Table 2, entry 11). To the best of our knowledge, this is the highest value produced by a nickel catalyst. These improvements on molecular weight reflected an improved strategy of catalyst design and indicated the possibility of the formation of UHMWPE at low pressure. Although ultrahigh molecular weights (106 g mol−1) and high catalytic activities (107 g mol−1 h−1) are retained, increasing temperature from 30 to 90 °C led to decline (Table 2). Table 2 | Ethylene Polymerization with Ni1–Ni4 at the High Pressure of 8 bara Entry Cat. T (°C) Yield (g) Act. (107)b Mw (106)c Mw/Mnc brsd Tme (°C) 1 Ni1 30 2.61 1.57 6.08 1.27 4.7 127 2 Ni1 60 1.54 0.92 3.42 1.42 9.2 118 3 Ni1 90 1.22 0.73 0.87 1.61 16.8 110 4 Ni2 30 2.49 1.49 7.39 1.59 6.3 124 5 Ni2 60 2.26 1.36 3.66 1.63 10.8 117 6 Ni2 90 2.15 1.29 1.19 1.89 17.9 110 7 Ni3 30 2.31 1.39 8.34 1.47 5.5 122 8 Ni3 60 2.14 1.28 4.05 1.38 9.6 117 9 Ni3 90 1.96 1.18 1.30 1.54 15.8 109 10 Ni4 0 0.42 0.25 6.43 1.41 2.3 134 11 Ni4 30 2.18 1.31 9.34 1.44 3.8 122 12 Ni4 60 2.01 1.21 4.28 1.68 10.5 120 13 Ni4 90 1.76 1.06 1.64 1.56 18.7 109 aReaction conditions: Ni catalyst (1 μmol), MMAO (600 equiv), toluene/CH2Cl2 (98 mL/2 mL), 8 bar, 10 min; all entries are based on at least two runs unless noted otherwise. bActivity is in unit of g mol−1 h−1. cMn is in g mol−1. Determined by GPC in 1,2,4-trichlorobenzene at 150 °C using a light-scattering detector. dbrs = Number of branches per 1000C, as determined by 1H NMR spectroscopy. eDetermined by DSC (second heating). Figure 1 | Molecular structures of Ni1–Ni4 (from left to right). Hydrogen atoms are omitted for clarity. Download figure Download PowerPoint UHMWPE produced at ambient conditions The optimized nickel catalyst Ni4 and 30 °C temperature were applied to varied pressure experiments (Table 3). Under otherwise identical conditions, reducing ethylene pressure from 8 bar (Table 2, entry 11) to 4 bar (Table 3, entry 1), 2 bar (Table 3, entry 2), and 1 bar (Table 3, entry 3) led to the decrease of Mn from 6.49 × 106 to 3.73 × × and × 106 g mol−1 as this that UHMWPE is now available with late transition-metal catalysts at ambient conditions of 1 bar and 30 °C. Because of the mild conditions, the of polymer when the reaction from 10 min to a relatively 120 min (Table 3, entries Most Mn also gradually a Mn value of 6.04 × 106 g mol−1 (Table 3, entry This Mn that was produced under 1 bar at 30 °C is with late transition-metal catalysts. notable early transition-metal catalysts are extremely difficult to access such an ultrahigh Under conditions, the Ziegler–Natta catalysts, catalysts, catalysts, and metallocene catalysts usually UHMWPEs with Mn × 106 g mol−1 under 1 bar at 30 °C for 60 with the data in Table 1 using the reference nickel catalysts Ref-Ni1– Ref-Ni7 under the reaction conditions (1 bar, 30 °C, and 30 Ni4 an of one of magnitude in of molecular weight (Table 3, entry 5 Table 1) with higher the and Ni1 also generated UHMWPEs with Mn × 106 g mol−1, with a of Mn with Ni4 (Table 3, entries All UHMWPEs obtained were branched of branches per and = °C). Table 3 | Ethylene Polymerization with Ni1–Ni4 at Ambient Conditions of 1 bar and 30 °Ca Entry Cat. Yield (g) Act. (106)b Mn (106)c Mw (106)c Mw/Mnc brsd Tme (°C) 1f Ni4 10 3.73 1.63 122 Ni4 10 1.57 3 Ni4 10 2.49 1.27 4 Ni4 1.39 119 5 Ni4 30 1.49 119 6 Ni4 60 6.04 1.38 118 7 Ni4 120 — 118 8 Ni3 30 118 9 Ni2 30 119 10 Ni1 30 1.56 1.47 122 aReaction conditions: Ni catalyst (1 μmol), MMAO (600 equiv), 1 bar, 30 °C, 30 min, toluene/CH2Cl2 (98 mL/2 mL); all entries are based on at least two runs unless noted otherwise. bActivity is in unit of g mol−1 h−1. by GPC in 1,2,4-trichlorobenzene at 150 °C using a light-scattering detector. dbrs = Number of branches per 1000C, as determined by 1H NMR spectroscopy. is in g mol−1. Determined by DSC (second heating). (2 weight was the of GPC detector, and the of the crude polymer was extremely With structures of all nickel catalysts Ni1– Ni4 in to provide a of the relationship catalyst structure and polymer molecular weight, the steric of the substituents in these nickel catalysts using the steric parameter as the Sterimol parameter was (for details, see Supporting were found only the Mn of polyethylene and the Sterimol B1 parameter but also the Mw of polyethylene and the Sterimol B1 parameter (Figure 2, also see Supporting Information Figures With higher B1 of the α-diimine nickel catalysts, such as Ni3 and produced polyethylenes with higher molecular This on the polymer molecular weight is because of the steric in the of the nickel Figure 2 | the number-average molecular weight (Mn) and the molecular weight of polyethylenes and the Sterimol B1 parameter of nickel catalysts. Download figure Download PowerPoint Mechanistic insights into the key chain To the of Ni1–Ni4 for the formation of UHMWPE, with high activities under 1 bar at 30 °C, a density functional theory (DFT) calculation was used to the catalyst and the typical catalyst Ref-Ni4 that the and in the substituents of (Figure 3). The was based on the data of these three nickel catalysts, and the condition of was 1 bar at °C in which the conditions (1 bar and 30 °C). The α-diimine ethylene have that either the coordination of ethylene to the nickel or the insertion of ethylene is a rate-limiting step of chain which is on catalyst structure and reaction Figure 3 | energy for the chain of ethylene polymerization by and Download figure Download PowerPoint In the we on the key chain shown in Figure 3, the energy barrier of ethylene coordination transition state was and kcal mol−1 for and on the generated the energy barrier of ethylene insertion ( was and 18.7 kcal mol−1, respectively, to the the energy barrier of ( from to nickel ( 2 was and kcal mol−1 for and These that ethylene insertion is the rate-limiting step of chain with the energy barrier of and kcal mol−1 for and The barrier of chain and the most of 2, and for Ni1 were with the highest activity toward ethylene polymerization (Table 3, entry 10 Table entries 1 and This also should be responsible for the formation of UHMWPE × 106 30.9 × 104 and 7.3 × 104 g For Ref-Ni4 at 1 bar and 30 °C high of both ethylene coordination and insertion a low polymer molecular weight and extremely low It should be that Ref-Ni4 produced UHMWPE of 1.61 × 106 g mol−1 at high pressure but only gave a low Mn of 7.3 × 104 g mol−1 at 1 this is highly of the of the formation of UHMWPE at ambient conditions. To a into such and energy (for details, see Supporting of the rate-limiting step at 1 bar and °C were carried out for the transition and The indicated that the energy kcal mol−1 for kcal mol−1 for and kcal mol−1 for ethylene and in Ni1 was the which could the energy and kcal mol−1) and to the kcal mol−1) with kcal mol−1) and kcal mol−1) (Figure a both the ethylene and the catalyst part and the steric for the highest of the and (Figure we found that although the phenyl group of Ni1 is in than the group in the rigid planar of the group led to the ligand two and from ethylene and the polymer as by the of and in with in Ref-Ni1 Å). In the the of the group the steric The the energy of the catalyst of the Ni in were and respectively, which are with the of energy ethylene and in and The confirmed that the group the of α-diimine catalytic This also for the more in Ni1 and the higher activity at 1 bar of ethylene pressure. Figure 4 | in and energy of and of the rate-limiting Download figure Download PowerPoint Polar-functionalized UHMWPE produced at ambient conditions In of a small amount of functional group into polyolefin is to desired however, this is usually by and significant decline of polymer molecular weight in the copolymerization of ethylene and polar monomers. Because of this low molecular weight is the issue in producing polar-functionalized The improvement of molecular weight at ambient conditions is even more for high molecular weight polar-functionalized polyethylenes (Mn = × 105 g mol−1, = mol higher pressures bar) using phosphine-sulfonate, phosphine-phenolate, or nickel To the example for producing ultrahigh molecular weight

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