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Concerted Steric and Electronic Strategy in Thermostable Salicylaldiminato Nickel Catalysts for Ethylene (Co)polymerization
RESEARCH ARTICLE | Updated：2024-08-06
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    • Concerted Steric and Electronic Strategy in Thermostable Salicylaldiminato Nickel Catalysts for Ethylene (Co)polymerization

    • Ji Hong-Yu

      a ,  

      Mu Hong-Liang

      b ,  

      Tang Chun-Feng

      a ,  

      Zhang Yu-Xing

      b ,  

      Chi Yue

      a ,  

      Jian Zhong-Bao

      b ,  
    • Chinese Journal of Polymer Science   Vol. 42, Issue 8, Pages: 1085-1092(2024)

    • DOI：10.1007/s10118-024-3148-y    

      CLC：

    • Published：01 August 2024，

      Published Online：05 June 2024，

      Received：24 March 2024，

      Revised：17 April 2024，

      Accepted：18 April 2024

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  • Hong-Yu Ji, Hong-Liang Mu, Chun-Feng Tang, et al. Concerted Steric and Electronic Strategy in Thermostable Salicylaldiminato Nickel Catalysts for Ethylene (Co)polymerization. [J]. Chinese Journal of Polymer Science 42(8):1085-1092(2024) DOI： 10.1007/s10118-024-3148-y.

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    Abstract

    Olefin polymerization is one of the most important chemical reactions in industry. This work presents a strategy that emphasizes the synergistic meta/para-steric hindrance of N-aryl groups and electronic effects in newly synthesized neutral salicylaldiminato nickel catalysts. These nickel(II) catalysts exhibit exceptional thermostability, ranging from 30 °C to 130 °C, demonstrating enhanced catalytic activities and broadly regulated polyethylene molecular weights (3−341 kg·mol−1) and controlled polymer branch density (2−102 brs/1000C). The preferred catalyst Ni3 with concerted steric and electronic effects enables the production of solid-state semi-crystalline polyethylene materials at temperatures below 90 °C. Notably, Ni3 exhibits an impressive tolerance of 110 °C and can withstand even the challenging polymerization temperature of 130 °C, leading to the production of polyethylene wax and oil. Also, functionalized polyethylene is produced.

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    Keywords

    Polyolefin; Salicylaldiminato ligand; Nickel catalyst; Oil; Wax

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    INTRODUCTION

    Polyolefins, the most extensively produced and utilized synthetic polymer material globally, have a broad scope of applications. This versatility is attributed to their diverse properties, ranging from linear polyethylene (HDPE) and low-density polyethylene (LDPE), to thermoplastic elastomers (TPE), and even waxes and oils. Since Brookhart’s seminal work on nickel (Ni(II)) and palladium (Pd(II)) α-diimine complexes in the 1990s,[

    1,2] late transition metal catalysts for olefin polymerization have garnered significant attention from both academia and industry over the past three decades. This interest is largely due to their remarkable tolerance towards heteroatoms and the intriguing chain-walking feature.[3−23] In industry, most commercial branched polyolefin products usually be prepared based on early-transition metal catalytic copolymerization of ethylene and α-olefins, and the use of late-transition metal catalysts with unique chain walking characteristics to catalyze the homopolymerization of ethylene with a single feedstock for the preparation of polyethylene (PE) with different branching microstructures is very promising. Among these, single-component neutral nickel(II) catalysts based on the salicylaldimine ligand bearing various substituents have been reported (Scheme 1).[24−28]
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    Fig 1  Steric and electronic modifications on the Ni(II) catalysts bearing salicylaldimine ligands and this work.

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    Catalyst modifications of neutral salicylaldiminato nickel(Ni(II)) typically focus on the three substitution sites of R1, R2, and R3, as illustrated in Scheme 1. Grubbs et al. reported the first array of salicylaldiminato Ni(II) complexes that are active for ethylene polymerization with the aid of Ni(COD)2 (COD = 1,5-cyclooctadiene) or B(C6F5)3. A bulky group at the ortho-position of the phenoxy moiety is found to enhance the catalytic activity and molecular weight of the resulting PE (Scheme 1, I). Additionally, this type of neutral Ni(II) exhibited high activity for olefin polymerization in the presence of polar-substituted norbornene or polar additives.[

    29,30] Brookhart’s group modified the anilines of salicylaldimine ligand into a 2,8-diarylnaphthyl substituent (Scheme 1, II), the corresponding catalyst can produce a branched ultrahigh-molecular-weight polyethylene.[31] Mecking et al. explored the remote substituents at the terphenyl 3,5-positions and found a remarkable improvement in the catalytic performance (Scheme 1, III), despite their spatial remoteness from the metal center.[32−38] Besides, elegantly designed cyclophane ligand could provide Ni(II) center a proper steric environment for ethylene polymerization (Scheme 1, IV).[39] Chen et al. investigated the electronic effect of the para-position of the N-aryl group on the ethylene polymerization process (Scheme 1, V).[40] Our group reported an effective shielding on both apical positions of catalysts by a Cs-type arrangement to afford strictly linear UHMWPE devoid of any branches, whose catalytic performance could be tuned by modifying the ortho- (R1) and para- (R) substituents of phenol, among which the electron-donating tBu substituted VI exhibited good thermostability and catalytic activity in ethylene polymerization (Scheme 1, VI).[41−45] Recently, we studied the effect of meta-substituted anilines on α-diimine Ni(II) catalyzed ethylene polymerization, superior thermostability was realized by the installation of a five-membered ring on the meta-position of anilines.[46] Inspired by this, in this contribution, we used a series of different types of meta-/para- substituted anilines of the N-aryl groups (Scheme 1, VII) to design a family of salicylaldiminato Ni(II) catalysts that had never been applied in this catalyst system. We envision a concerted flexible and electronic strategy to improve the catalytic performance of salicylaldiminato Ni(II) catalysts especially for thermostability and catalytic activity. On the one hand, the N-meta-aryl flexible substituents as the steric hindrance are installed into the axial position of the metal active species; on the other hand, oxygen atoms are crucially introduced into the catalyst as electron-donating groups.
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    EXPERIMENTAL

    General Considerations

    All syntheses involving air- and moisture sensitive compounds were carried out using standard Schlenk-type glassware (or in a glove box) 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). The molecular weights (Mw) and molecular weight distributions (Mw/Mn) of polyethylenes were measured by means of gel permeation chromatography (GPC) on a PL-GPC 220-type high-temperature chromatograph equipped with three PL-gel 10 μm Mixed-B LS type columns at 150 °C. Melting point temperature (Tm) of polyethylenes were measured through DSC analyses, which were carried out on a TA Q2000 DSC Instrument under nitrogen atmosphere at heating and cooling rates of 10 °C/min (temperature range: −80 °C to 160 °C). Elemental analysis was performed at the National Analytical Research Centre of CIAC.

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    General Procedure for Ethylene Polymerization

    In a typical experiment, a 350 mL Chemglass pressure reactor connected with a high-pressure gas line was firstly dried at 110 °C under vacuum for at least 1 h. The reactor was then adjusted to the desired polymerization temperature. Toluene (98 mL) was added to the reactor under N2 atmosphere, then the desired amount of Ni(II) catalyst in 2 mL of toluene was injected into the polymerization system via syringe. With a rapid stirring, the reactor was pressurized and maintained at required of ethylene pressure. After specific time, the pressure reactor was vented and the polymerization was quenched via the addition of 200 mL of EtOH. The obtained polymer was dried in a vacuum oven to constant weight.

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    General Procedure for the Copolymerization of Ethylene with Polar Monomers

    In a typical experiment, a 150 mL glass pressure reactor connected with a high-pressure gas line was dried at 110 °C under vacuum for at least 1 h and adjusted to the desired polymerization temperature. Next, 23 mL of toluene and the desired amount of 5-hexenyl acetate was added to the reactor under N2 atmosphere, then the desired amount of Ni catalyst in 2 mL of toluene was injected into the polymerization system via syringe. With a rapid stirring, the reactor was pressurized and maintained at required of ethylene pressure. After specific time, the pressure reactor was vented and the polymerization was quenched via the addition of 100 mL of EtOH. The obtained polymer was dried in a vacuum oven to constant weight.

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    RESULTS AND DISCUSSION

    Synthesis of Ligands and Catalysts

    The synthesis routes for ligands L1−L3 and the neutral salicylaldiminato Ni(II) catalysts based on the unique anilines groups are shown in Scheme 2. A series of large steric aromatic amines were synthesized based on 5-aminopindan, 5,6,7,8-tetrahydro-2-naphthylamine and 3,4-(methylenedioxy)aniline, in which the N-meta-aryl of N1 and N2 were flexible cyclopentyl and cyclohexyl structures, and the N-meta-aryl of N3 was dioxopentyl, which possesses concerted steric and electronic effects. These arylamines were reacted with Lucas reagent to introduce diphenylmethyl group in high yields. The resultant arylamine and 3-(9-anthracyl)-2-hydroxybenzaldehyde[

    47] were condensed into 20 mL methanol by adding a catalytic amount of p-toluenesulfonic acid (PTSA) through a condensation reaction, yielding the corresponding ligands L1−L3. Nickel complexes Ni1−Ni3 were readily obtained by the reaction of corresponding ligands with 1 equivalent of NiMe2(tmeda) (tmeda = N,N,N',N'-tetramethylethylenediamine) in the presence of excess pyridine in toluene (Scheme 2). Besides, Ref-Ni was also synthesized as a contrast. The occurrence of singlets at −0.14 ppm to −0.21 ppm for Ni–CH3 resonances in 1H-NMR spectra confirmed the formation of new Ni-Me, which was also supported by 13C-NMR spectroscopy. These obtained ligands and Ni(II) complexes were fully characterized by 1H- and 13C-NMR spectroscopies and elemental analysis (see the electronic supplementary information, ESI, for detailed information).
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    Fig 2  Synthesis of neutral salicylaldiminato ligands and the corresponding nickel complexes.

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    Nickel Catalyzed Ethylene Homo-polymerization

    In the absence of any activating cocatalysts, these neutral single-component Ni(II) catalysts were assessed for ethylene polymerization at various temperatures and pressures. Notably, a significant disparity can be observed in the catalytic properties of Ni1–Ni3 (Table 1 and Fig. 1). Comprehensive parameters including activity, polymer molecular weight (MW), polymer dispersity index (Mw/Mn), and branching density (brs) were considered. Catalysts Ni1−Ni2 in this work exhibited comparable catalytic activities to Ref-Ni in ethylene polymerization, and Ni3 bearing the dioxolane group stood out with diverse polymer production.

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    Table 1  Effect of temperature on ethylene polymerization with Ni(II) catalysts. a
    EntryCat.T (°C)p (MPa)Yield (g)Act. b (105)Mw c (104)Mw/Mn cBrs dTm e (°C)
    1 Ni1 30 0.8 0.78 3.1 34.1 1.59 5 130.0
    2 Ni1 50 0.8 2.99 12.0 10.5 1.87 9 122.4
    3 Ni1 70 0.8 1.21 4.8 3.0 1.73 20 110.8
    4 Ni2 30 0.8 1.21 4.8 17.2 1.65 3 129.3
    5 Ni2 50 0.8 1.68 6.7 8.4 1.85 12 120.3
    6 Ni2 70 0.8 5.00 20.0 2.7 1.84 28 105.6
    7 Ni2 90 0.8 1.15 4.6 1.3 1.88 48 88.7
    8 Ni3 30 0.8 0.71 2.8 18.6 1.52 2 130.1
    9 Ni3 50 0.8 4.10 16.4 16.1 2.28 9 121.3
    10 Ni3 70 0.8 5.20 20.8 4.0 2.19 26 112.0
    11 Ni3 90 0.8 3.91 15.6 2.5 3.00 39 100.5
    12 Ni3 110 0.8 2.92 11.7 0.8 2.02 64 73.3 (wax)
    13 Ni3 130 0.8 1.79 7.2 0.3 2.23 102 − (oil)
    14 Ni3 70 0.2 2.52 10.1 1.1 2.06 78 86.7
    15 Ni3 70 0.4 2.90 11.6 3.0 2.16 55 106.6
    16 Ni3 70 2.0 8.80 35.2 5.9 2.60 20 116.7
    17 Ref-Ni 30 0.8 0.98 3.9 31.8 1.42 2 130.3
    18 Ref-Ni 50 0.8 1.20 4.8 28.8 1.71 4 127.3
    19 Ref-Ni 70 0.8 2.89 11.6 10.4 2.24 16 111.2
    20 Ref-Ni 90 0.8 1.50 6.0 5.0 1.81 22 108.5

    a Reaction conditions: Ni catalyst (5 µmol), toluene (100 mL), polymerization time (0.5 h), all entries are based on at least two runs, unless noted otherwise; b Activity is in unit of g·mol−1·h−1; c Determined by GPC in 1,2,4-trichlorobenzene at 150 °C and in unit of g·mol−1; d Brs = Number of branches per 1000C, as determined by 1H-NMR spectroscopy; e Determined by DSC (second heating).

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    Fig 1  (a) Plots of activity, (b) molecular weight and (c) branching density versus reaction temperature and ethylene pressure with Ni(II) catalysts; The effect of ethylene pressure (d) on Ni3 catalytic performance.

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    In general, Ni1−Ni3 exhibited different catalytic activities, (Fig. 1a) with the polymer molecular weights ranging from 13 kg·mol−1 to 341 kg·mol−1. Under identical conditions, the molecular weight (Mw) of the polymer (Fig. 1b) decreases as the polymerization temperature increases due to an increased ratio of chain transfer relative to chain propagation at high temperatures. In addition, the branching densities of the obtained polymers (Table 1, entries 1−13) increased with elevated polymerization temperature (Fig. 1c). This was primarily attributed to the increased ratio of the chain walking relative to chain growth as polymerization temperature increased.[

    42,45] The polymer branching degree was similar between Ni1 and Ni2 from 30 °C to 90 °C (3/1000C−48/1000C, Table 1, entries 1−7), yielding semi-crystalline polyethylenes with melting temperatures ranging from 88.7 °C to 130.0 °C, which was slightly higher than when using the control catalyst Ref-Ni (2/1000C−22/1000C, Tm=108.5−130.3 °C, entries 17−20). Notably, Ni2 achieved a maximum catalytic activity at 70 °C as 20×105 g·mol−1·h−1, followed by a decrease in catalytic activity with increasing temperature (Fig. 1a). Compared to Ref-Ni, the superior activities of Ni1 at 50 °C and Ni2 at 70 °C suggest a higher chain propagation rate, which could be attributed to the introduction of a ring group on the meta-position of anilines (Table 1, entries 2 versus 18; entries 6 versus 19).[46] This confirmed the effectiveness of the steric effect of meta-substituent of N-aryl groups of catalysts on ethylene polymerization.
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    In terms of catalytic activity and thermostability, Ni3 outperformed the other nickel catalysts, demonstrating the efficacy of the concerted flexible and electronic strategy. Notably, the activity of Ni3 surpassed that of Ni2, particularly at 90 °C. Despite the temperature rising to 110 °C, the activity of Ni3 retained at 106 g·mol−1·h−1 (Fig. 1a). In this temperature, we obtained a wax-like polyethylene with a reduced molecular weight and increased branching degree, which exhibited a broad and weak melting peak (Fig. 2). Further elevating the temperature to 130 °C enabled the production of polyethylene oil with an impressively higher polymer branching of 102/1000C (Table 1, entries 8−13). Abstractly, the molecular weights of the polymers produced by Ni3 and its activities decreased more gradually with increasing temperature compared to those prepared by catalysts Ni1−Ni2. However, the molecular weights of polymers produced by Ni3 were lower than those produced by other catalysts, potentially due to higher chain transfer rate. As ethylene pressure increased, the polymerization activity significantly rose, reaching the highest value at 2.0 MPa as 35.2×105 g·mol−1·h−1 (Fig. 1d). The obtained polymer molecular weight also increased with the rise of ethylene pressure, attributed to the increase in chain propagation rate at high ethylene pressure. In this case, the branching densities of the obtained polymers also decreased with elevated ethylene pressure (Table 1, entries 10 and 14−16).

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    Fig 2  (a) DSC curves and (b, c) photographs of the representative polyethylene samples with diverse microstructures: (i) Table 1, entry 10, (ii) Table 1, entry 12, (iii) Table 1, entry 13.

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    The molecular weight of the polymers was pivotal in the formation of polyethylene wax and oil.[

    45] The trend of the polymer’s molecular weight is clearly shown in Fig. 1(b). Notably, at elevated temperatures of 110 and 130 °C, low molecular weight polyethylene wax and oil can be readily produced using catalyst Ni3 with high activities ranging from 7.2×105−11.7×105 g·mol−1·h−1. This outstanding thermostability underscores the significance of the concerted flexible cycloalkyl-based and electronic strategy. Current state-of-the-art lubricant base oil production utilizes α-olefin polymerization facilitated by metallocene or non-metallocene systems.[48] The utilization of a single-component catalyst and ethylene as the exclusive monomer presents an intriguing alternative route.
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    In addition to polymer molecular weight, branch pattern was also crucial in the formation of polyethylene wax and oil.[

    45] The 13C-NMR spectra of the wax and oil produced by Ni3 at temperatures of 110 and 130 °C were thoroughly analyzed to elucidate the polymer branching patterns.[45] As illustrated in Fig. 3 and Table 2, the highly branched wax and oil suggest extensive chain-walking during polymerization. The microstructural differences between wax and oil are significant, with an increase in branching density (from 67/1000C to 95/1000C, as shown in Table 2). In terms of wax, methyl branching was the predominant branching pattern, accounting for approximately half of the total branches (calculated by 13C-NMR), second to which were butyl and longer branches; ethyl branches and sec-butyl branches were also observed. Key resonances of methyl chain branch units were 19.96 ppm and long chain branch units were 14.37 ppm. As for oil, along with lower polymer molecular weight, a decrease in the number of methyl branching was observed, and an increase of long-chain and sec-branching. This suggests increased chain walking and chain transfer during polymerization. Long chain branching contributes to a softer mechanical property, ultimately resulting in oil. These polyethylene wax and oil have a wide range of internal double bonds and a small amount of terminal double bonds (see Fig. S36 in ESI).
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    Fig 3  13C-NMR spectra (500 MHz, CDCl3, 25 °C) of the polymers generated by complexe Ni3 from Table1, entries 12 and 13.

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    Table 2  Analyses of branching number and pattern of polyethylene wax and oil generated by Ni3. a
    EntryPolymer typeT (°C)Mw (104)Mw/MnBrs b (13C)Branching distribution b
    C1C2C3C4+Sec-
    1 Wax 110 0.75 2.02 67 0.52(35) 0.10(7) 0.06(5) 0.23(15) 0.08(5)
    2 Oil 130 0.33 2.23 95 0.42(40) 0.12(11) 0.06(6) 0.27(25) 0.14(13)

    a Determined using 13C-NMR spectroscopy; b Calculated from 13C-NMR, C1 = methyl branch, C2 = ethyl branch, C3 = propyl branch, C4+ = butyl and longer branches. Data in bracket: the exact number of branches per 1000C.

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    Ethylene Copolymerization with Polar Monomers

    The copolymerization of ethylene (E) and 5-hexenyl acetate (HAc) could produce functionalized polyethylene.[

    49] These neutral, single-component Ni1−Ni3 catalysts are evaluated, in the copolymerization of ethylene with 5-hexenyl acetate (HAc) (Table 3). Under pressure-reactor conditions, exposure of each catalyst to 0.8 MPa of ethylene (E) and 0.1 mol/L HAc at 70 °C results in the generation of E-HAc copolymers (Table 3, entries 1−3). Notably, Ni3 exhibits not only the highest activity but also the highest comonomer incorporation (XM=0.18 mol%) and the highest molecular weight (Mw=0.91×104 g·mol−1) compared to Ni1−Ni2 (Table 3, entry 3). This suggests that Ni3 possesses superior thermostability. As the increase of temperature, both activity and HAc incorporation were elevated using Ni3, the highest incorporation of HAc (0.23 mol%) could be achieved at 90 °C.[49]
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    Table 3  Copolymerization of ethylene and 5-hexenyl acetate (HAc) with Ni(II) catalysts. a
    EntryCat.T (°C)Comon. bAct. c (103)XM e (mol%)Mw d (104)Mw/Mn dBrs eTm f (°C)
    1 Ni1 70 HAc (0.1 mol/L) 0.5 0.11 − g − g 25 − g
    2 Ni2 70 HAc (0.1 mol/L) 1.0 0.11 0.72 2.30 20 112.9
    3 Ni3 70 HAc (0.1 mol/L) 1.5 0.18 0.91 2.18 19 115.3
    4 Ni3 90 HAc (0.1 mol/L) 7.0 0.23 1.34 2.17 38 98.2

    a Reaction conditions: Ni(II) catalyst (10 μmol), toluene (25 mL), ethylene (0.8 MPa), polymerization time (120 min), all entries are based on at least two runs, unless noted otherwise; b mol·L−1; c Activity is in unit of g·mol−1·h−1; d Determined by GPC in 1,2,4-trichlorobenzene at 150 °C and in unit of g·mol−1; e Determined by 1H-NMR spectroscopy. Brs = Number of branches per 1000C, XM =incorporation ratio of the polar monomer; f Determined by DSC (second heating); g Not determined.

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    CONCLUSIONS

    In summary, we demonstrated a concerted flexible steric and electronic strategy to address the issue of activity and thermostability of salicylaldiminato Ni(II) catalyst. Hereby, a new family of neutral Ni(II) catalysts based on meta-/para-substituents in the N-aryl groups are rationally designed and prepared. They have been applied to ethylene polymerization and ethylene/5-hexenyl acetate copolymerization. In comparison to the control Ni(II) catalyst, Ni3 bearing concerted flexible steric and electronic effect exhibits enhanced catalytic activities, which is able to produce diverse polyethylene materials at elevated polymerization temperatures, even reaching 130 °C. This is a notable accomplishment in the salicylaldiminato Ni(II) catalyst system. As a result, it is capable of producing a variety of polyethylenes, including semi-crystalline PE, PE wax, PE oil, and functionalized PE.

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