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 Google Scholar More articles by this author , Xiaohui Kang College of Pharmacy, Dalian Medical University, Dalian 116044 Google Scholar More articles by this author , Yixin Zhang State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 Google Scholar More articles by this author 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 Google Scholar More articles by this author 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 × 106, 2.82 × 106, and 1.97 × 106 g mol−1 as expected. Remarkably, this verifies 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 yield of polymer increased when prolonging the reaction time from 10 min to a relatively long 120 min (Table 3, entries 3–7). Most importantly, Mn also gradually reached a Mn value of 6.04 × 106 g mol−1 (Table 3, entry 6). This huge Mn that was produced under 1 bar at 30 °C is unprecedented with late transition-metal catalysts. Even notable early transition-metal catalysts are extremely difficult to access such an ultrahigh level. Under quite similar conditions, the benchmark Ziegler–Natta catalysts, constrained geometry configuration (CGC) catalysts, Fujita catalysts, and metallocene catalysts usually give UHMWPEs with Mn <4.00 × 106 g mol−1 under 1 bar at approximately 30 °C for 60 min.44,89–94 Compared with the data in Table 1 using the reference nickel catalysts Ref-Ni1– Ref-Ni7 under the same reaction conditions (1 bar, 30 °C, and 30 min), Ni4 achieved an enhancement of one order of magnitude in terms of molecular weight (Table 3, entry 5 vs Table 1) with higher activity. Furthermore, the less bulky Ni3, Ni2, and Ni1 also consistently generated UHMWPEs with Mn beyond 2.25 × 106 g mol−1, along with a decreasing trend of Mn compared with Ni4 (Table 3, entries 8–10). All UHMWPEs obtained were lightly branched (5.8–9.8 brs [number of branches per 1000C]) and semicrystalline (Tm = 117.7–122.4 °C). Table 3 | Ethylene Polymerization with Ni1–Ni4 at Ambient Conditions of 1 bar and 30 °Ca Entry Cat. Time (min) Yield (g) Act. (106)b Mn (106)c Mw (106)c Mw/Mnc Chains/[Ni] brsd Tme (°C) 1f Ni4 10 1.79 10.74 3.73 6.07 1.63 0.5 5.8 122 2g Ni4 10 1.53 9.18 2.82 4.43 1.57 0.5 6.7 121 3 Ni4 10 1.15 6.90 1.97 2.49 1.27 0.6 6.9 121 4 Ni4 20 1.40 4.20 3.06 4.25 1.39 0.5 7.4 119 5 Ni4 30 1.91 3.82 3.60 5.36 1.49 0.5 7.8 119 6 Ni4 60 3.41 3.41 6.04 8.32 1.38 0.6 8.7 118 7 Ni4 120 4.48 2.24 —h —h —h — 9.1 118 8 Ni3 30 2.09 4.18 3.07 4.64 1.51 0.7 9.8 118 9 Ni2 30 1.77 3.54 2.62 3.82 1.46 0.7 7.6 119 10 Ni1 30 1.56 3.12 2.25 3.31 1.47 0.7 5.9 122 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. cDetermined 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. eMn is in g mol−1. Determined by DSC (second heating). fPressure (4 bar). gPressure (2 bar). hMolecular weight was beyond the limitation of GPC detector, and the filtration of the crude polymer was extremely difficult. With single-crystal structures of all four nickel catalysts Ni1– Ni4 in hand, to provide a deeper insight of the relationship between catalyst structure and polymer molecular weight, the steric effect of the substituents in these nickel catalysts using the steric parameter known as the Sterimol parameter was quantified (for details, see Supporting Information).54,95 Close correlations were found not only between the Mn of polyethylene and the Sterimol B1 parameter but also between the Mw of polyethylene and the Sterimol B1 parameter (Figure 2, also see Supporting Information Figures S1–S4). With higher B1 values of the substituents, α-diimine nickel catalysts, such as Ni3 and Ni4, produced polyethylenes with higher molecular weights. This increase on the polymer molecular weight is presumably because of the steric congestion in the axial positions of the nickel center. Figure 2 | Correlation between the number-average molecular weight (Mn) and the weight-average molecular weight (Mw) of polyethylenes and the Sterimol B1 parameter of nickel catalysts. Download figure Download PowerPoint Mechanistic insights into the key chain propagation To rationalize the preference of Ni1–Ni4 for the formation of UHMWPE, along with high activities under 1 bar at 30 °C, a density functional theory (DFT) calculation was used to evaluate Ni1, the classical Brookhart catalyst Ref-Ni1, and the typical bulky catalyst Ref-Ni4 that contain the same backbone and differ in the substituents of axial sites (Figure 3). The geometric optimization was based on the single-crystal data of these three nickel catalysts, and the condition of DFT simulation was 1 bar at 25 °C in toluene, which mimics the real experimental conditions (1 bar and 30 °C). The α-diimine nickel-mediated ethylene polymerization60,83,96–101 pioneering works have revealed that either the coordination of ethylene to the β-agostic n-propyl nickel intermediate or the next migratory insertion of ethylene is a rate-limiting step of chain propagation, which is dependent on catalyst structure and external reaction condition. Figure 3 | Computed energy profiles for the chain propagation of ethylene polymerization catalyzed by Ni1, Ref-Ni1, and Ref-Ni4. Download figure Download PowerPoint In the calculation, we focused on the key chain propagation step. As shown in Figure 3, the energy barrier of ethylene coordination (the transition state TScoor) was 5.4, 9.7, and 11.7 kcal mol−1 for Ni1, Ref-Ni1, and Ref-Ni4, respectively. Based on the generated intermediate 1coor, the energy barrier of subsequent ethylene insertion ( TSins) was 10.4, 11.9, and 18.7 kcal mol−1, respectively, to form the γ-agostic intermediate 2. Finally, the energy barrier of isomerization ( TSiso) from γ-agostic to β-agostic n-propyl nickel intermediate ( 2 → 2iso) was 2.9, 1.6, and 4.0 kcal mol−1 for Ni1, Ref-Ni1, and Ref-Ni4, respectively. These calculations suggest that ethylene insertion is the rate-limiting step of chain propagation with the total energy barrier of 10.4, 16.4, and 19.8 kcal mol−1 for Ni1, Ref-Ni1, and Ref-Ni4, respectively. The lowest barrier of chain propagation and the most stable intermediates of 1coor, 2, and 2iso for Ni1 were consistent with the highest activity toward ethylene polymerization (Table 3, entry 10 vs Table 1, entries 1 and 4). This also should be responsible for the formation of UHMWPE (2.25 × 106 vs 30.9 × 104 and 7.3 × 104 g mol−1). For Ref-Ni4 at 1 bar and 30 °C significantly high barriers of both ethylene coordination and insertion explain a low polymer molecular weight and extremely low activity. It should be mentioned that Ref-Ni4 produced UHMWPE of 1.61 × 106 g mol−1 at high pressure (9 bar)87 but only gave a low Mn of 7.3 × 104 g mol−1 at 1 bar.86 Again, this is highly persuasive evidence of the difficulty of the formation of UHMWPE at ambient conditions. To obtain a deeper insight into such discrepancy between Ni1, Ref-Ni1, and Ref-Ni4, energy decomposition analyses (for details, see Supporting Information) of the rate-limiting step at 1 bar and 25 °C were carried out for the transition states TSins(Ni1), TSins(Ref-Ni1), and TSins(Ref-Ni4). The results indicated that the interaction energy (ΔEint, −50.2 kcal mol−1 for Ni1, −48.7 kcal mol−1 for Ref-Ni1, and −49.6 kcal mol−1 for Ref-Ni4) between ethylene and 1β in species Ni1 was the strongest, which could offset the smallest deformation energy (ΔEdef, 48.8 vs 50.5 and 54.2 kcal mol−1) and eventually lead to the lowest ΔETS (−1.4 kcal mol−1) compared with TSins(Ref-Ni1) (1.8 kcal mol−1) and TSins(Ref-Ni4) (4.6 kcal mol−1) (Figure 4). As a consequence, both the strongest interaction between ethylene and the catalyst part and the smallest steric hindrance accounted for the highest stability of TSins(Ni1). Regarding the geometries TSins(Ni1), TSins(Ref-Ni1), and TSins(Ref-Ni4) (Figure 4), we further found that although the phenyl group of Ni1 is larger in size than the isopropyl group in Ref-Ni1, the rigid planar effect of the terphenyl group led to the auxiliary ligand (especially two phenyls Ph1 and Ph2) being distant from ethylene and the polymer chain, as suggested by the extended distances of H···H and H···C (>2.59 Å) in comparison with those in Ref-Ni1 (>2.16 Å). In Ref-Ni4, the phenyls replacing the methyls of the isopropyl group further increased the steric hindrance. The geometrical characters explained well the lowest deformation energy of the catalyst Ni1. Furthermore, natural bond orbital (NBO) charges of the Ni center in species 1β were 0.655, 0.586, and 0.603, respectively, which are consistent with the trend of interaction energy ΔEint between ethylene and 1β in species Ni1, Ref-Ni1, and Ref-Ni4. The results confirmed that the terphenyl group increased the electron-withdrawing ability of α-diimine ligand, further enhancing catalytic activity. This electronic character also accounted for the more stable ethylene-coordinated complex 1coor in Ni1 and the higher activity at 1 bar of ethylene pressure. Figure 4 | Optimized geometries (distance in Å) and energy decomposition analyses of TSins(Ni1), TSins(Ref-Ni1), and TSins(Ref-Ni4) of the rate-limiting step. Download figure Download PowerPoint Polar-functionalized UHMWPE produced at ambient conditions In principle, incorporation of a small amount of functional group into polyolefin is enough to render desired PE properties; however, this is usually accompanied by unavoidable and significant decline of polymer molecular weight in the copolymerization of ethylene and polar monomers. Because of this contradiction, low molecular weight is continually the largest issue in producing polar-functionalized polyolefins. The improvement of molecular weight enabled at ambient conditions is even more challenging. Some pioneering works for obtaining high molecular weight polar-functionalized polyethylenes (Mn = 0.68–1.77 × 105 g mol−1, incorporation = 0.3–0.7 mol %) required higher pressures (8–30