Nickel catalysts with asymmetric steric hindrance for ethylene polymerization

Wanlu Tian; Fuzhou Wang; Chen Zou

doi:10.52396/JUSTC-2024-0001

JUSTC > 2024 > 54(6): 0607. > DOI: 10.52396/JUSTC-2024-0001 CSTR: 32290.14.JUSTC-2024-0001

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Open Access JUSTC Chemistry Article 14 July 2024

Nickel catalysts with asymmetric steric hindrance for ethylene polymerization

Wanlu Tian^{1, 2},
Fuzhou Wang^2, ,,
Chen Zou^1, ,

1.
Hefei National Research Center for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China
2.
Institutes of Physical Science and Information Technology, Anhui University, Hefei 230601, China

Cite this: JUSTC, 2024, 54(6): 0607

https://doi.org/10.52396/JUSTC-2024-0001

CSTR: 32290.14.JUSTC-2024-0001

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Author Bio:
Wanlu Tian is currently a graduate student in the Institutes of Physical Science and Information Technology, Anhui University. His research mainly focuses on the synthesis and characterization of polyolefins from new types of later transition metal catalysts

Fuzhou Wang is currently an Associate Professor in the Institutes of Physical Science and Information Technology, Anhui University. He received his M.S. degree from Northwest Normal University in 2013. He carried out his doctoral work in polymer chemistry at Hiroshima University under the direction of Prof. Takeshi Shiono from 2013 to 2017. In 2018, he joined Anhui University as an Associate Professor. His current research mainly focuses on olefin polymerization by late transition metal catalysts

Chen Zou is currently an Associate Professor at the University of Science and Technology of China, mainly engaged in research on polyolefin catalysts and functionalized polyolefin materials. He graduated with a Bachelor's degree from Qingdao University of Science and Technology in 2012 and a doctoral degree from the University of Science and Technology of China in 2016
Corresponding author:
Fuzhou Wang, E-mail: wangfuzhou@ahu.edu.cn

Chen Zou, E-mail: chen1215@ustc.edu.cn
Received Date: January 21, 2024
Accepted Date: March 04, 2024
Available Online: July 14, 2024

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Abstract

Abstract

α-Diimide catalysts have attracted widespread attention due to their unique chain walking characteristics. A series of α-diimide nickel/palladium catalysts with different electronic effects and steric hindrances were designed and synthesized for olefin polymerization. In this work, we synthesized a series of asymmetric α-diimide nickel complexes with different steric hindrances and used them for ethylene polymerization. These nickel catalysts have high ethylene polymerization activity, up to 6.51×10⁶ g·mol⁻¹·h⁻¹, and the prepared polyethylene has a moderate melting point and high molecular weight (up to 38.2 × 10⁴ g·mol⁻¹), with a branching density distribution between 7 and 94 branches per 1000 carbons. More importantly, the polyethylene prepared by these catalysts exhibits excellent tensile properties, with strain and stress reaching 800% and 30 MPa, respectively.

Graphical Abstract

Synthesis of branched polyethylene by ethylene polymerization catalyzed by nickel catalysts with asymmetric steric hindrance.

Abstract

α-Diimide catalysts have attracted widespread attention due to their unique chain walking characteristics. A series of α-diimide nickel/palladium catalysts with different electronic effects and steric hindrances were designed and synthesized for olefin polymerization. In this work, we synthesized a series of asymmetric α-diimide nickel complexes with different steric hindrances and used them for ethylene polymerization. These nickel catalysts have high ethylene polymerization activity, up to 6.51×10⁶ g·mol⁻¹·h⁻¹, and the prepared polyethylene has a moderate melting point and high molecular weight (up to 38.2 × 10⁴ g·mol⁻¹), with a branching density distribution between 7 and 94 branches per 1000 carbons. More importantly, the polyethylene prepared by these catalysts exhibits excellent tensile properties, with strain and stress reaching 800% and 30 MPa, respectively.
Public Summary
- A series of novel α-diimine nickel complexes with asymmetric steric hindrance were synthesized.
- The nickel catalysts can withstand 90 °C and exhibit a high polymerization activity of 6.51 × 10⁶ g·mol⁻¹·h⁻¹.
- The nickel catalysts can be used to prepare high-molecular-weight polyethylene with an adjustable branching density distribution between 7 and 94 branches per 1000 carbons.
- The polyethylene materials prepared from these α-diimine nickel catalysts exhibit excellent tensile properties.

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1. Introduction

Since the early 1930s, the first polyethylene product (low-density polyethylene) was generated by free radical polymerization^[1]. More importantly, in the 1950s, Nata et al.^[2] and Ziegler et al.^[3] used coordination polymerization catalysts to achieve olefin polymerization under mild conditions, thereby promoting the vigorous development of the polyolefin industry. Over seventy years later, polyolefins were widely used in food packaging, healthcare, machinery, automotive manufacturing, and other applications due to their excellent chemical corrosion resistance and mechanical properties^[4].

The transition metal catalysts required for coordinating polymerization to prepare polyolefins have attracted widespread attention from both industry and academia with the continuous development of the polyolefin industry^[5]. Among them, late transition metal catalysts such as nickel and palladium have been widely studied in the copolymerization of olefins and polar monomers due to their high tolerance to polar groups and low hydrophilicity^[6]. For example, these nickel/palladium catalysts include [N,N] catalysts^[7], [N,O] catalysts^[8,9], [P,O] catalysts^[10–12], etc. Among them, α-diimine nickel/palladium catalysts (Fig. 1a) was first reported by Johnson et al.^[13] in 1995 and is known for its unique chain walking mechanism; these catalysts can be utilized to synthesize polyolefins with branched chain structures and diverse polymer topologies.

Figure 1. Representative diimide catalysts (a–e) and novel structures used in this work (f).

DownLoad: Full-Size Img PowerPoint

Since then, domestic and foreign scholars have extensively designed and researched this type of highly distinctive [N,N]-type catalyst by designing and synthesizing a series of novel α-diimine nickel/palladium catalysts with varying degrees of electronic effects and steric hindrance^[14–17]. For example, one important progress was the introduction of a diisopropylphenyl substituent, which significantly increased the axial steric hindrance of the metal center of the catalysts, thereby suppressing β-H elimination and improving the stability of the catalyst, allowing it to maintain high activity at 100 °C (Fig. 1b)^[18]. Dai et al.^[19] also studied the electronic effect of metal center coordination based on this large steric hindrance α-diimine palladium catalyst, further regulating the polymerization performance of the catalyst. In addition, Brookhart, Daugulis, and Coates et al.^[20] synthesized a “sandwich” type with increased axial steric hindrance using an 8-(p-tolyl) naphthalene-1-amine α-diimine nickel catalyst (Fig. 1c), which can prepare polyethylene with a high branching density (up to 150 branches per 1000 carbons) in ethylene polymerization.

By further adjusting the electronic effects and steric hindrance of the complex center, Wang et al.^[21] synthesized a chiral α-diimine nickel catalyst with paraphenylethyl compounds that exhibited high ethylene polymerization activity (up to 2.85 × 10⁶ g·mol⁻¹·h⁻¹·bar⁻¹) and produced polyethylene with a branching density of up to 117 branches per 1000 carbons at high temperatures. In addition, Coates et al.^[22–24] introduced similar chiral structures into α-diimine nickel catalysts and used them for the polymerization of propylene and 1-butene to improve the regularity of the polymer. The steric hindrance of these nickel catalysts was significantly different from that of classical 2,6-diisopropyl nickel catalysts and 2,6-diisopropylphenyl nickel catalysts, with only half of the steric hindrance present. On the basis of these works, we designed new nickel catalysts with asymmetric steric hindrance and studied the polymerization performance and physical properties of the generated polyethylene in detail.

2. Results and discussion

2.1 Synthesis and characterization

According to Scheme 1, aniline derivatives were prepared from precursors and p-anisidine by Friedel-Crafts alkylation. Subsequently, acetic acid was used as a catalyst, and ligand L1−L3 in high yield was easily prepared by the condensation of two equivalent aniline derivatives and one equivalent of 1,2-acenaphthenedione (Scheme 1). The ligands were reacted with equimolar amounts of (DME)NiBr₂ to prepare the corresponding α-diimine nickel complex Ni1–Ni3, which was a reddish-brown solid in almost quantitative yield. These complexes were characterized by elemental analysis, ¹H NMR and ¹³C NMR. The purity and identity of the two ortho-phenyl-substituted nickel complexes were determined by elemental analysis and XRD.

1. Synthetic route to nickel complexes Ni1−Ni3.

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Single crystals of complexes Ni1 and Ni2 were prepared by treating toluene solution with n-pentane at 0 °C on the top layer (Figs. 1 and 2, Tables S1 and S2). X-ray crystallographic analysis showed that the nickel dibromide complexes Ni1 and Ni2 (Figs. 2 and 3) crystallized symmetrically at two nickel centers with a twisted tetrahedral geometry at the center of the bromine-bridged dimer, where each nickel atom is connected to two bridged bromine atoms and an α-diimine ligand. The two nickel atoms in complexes Ni1 and Ni2 are connected by a quaternary ring.

Figure 2. Crystal structure of Ni1. The selected bond lengths (Å) and angles (°) were as follows: Br2–Ni1 = 2.3442(13), Br1–Ni1 = 2.3577(13), Ni1–N1 = 2.043(5), Ni1–N2 = 0.055(6), Br2–Ni1–Br1 = 110.57(5), N1–Ni1–N2 = 83.0(2), N1–Ni1–Br2 = 97.94(17), N1–Ni1–Br1 = 134.85(18), and N2–Ni1–Br1 = 99.18(16).

DownLoad: Full-Size Img PowerPoint

Figure 3. Crystal structure of Ni2. The selected bond lengths (Å) and angles (°) are as follows: Br1–Ni1 = 2.3451(9), Ni1–Br2 = 2.3256(9), Ni1–N2 = 2.020(4), Ni1–N1 = 2.020(4), Br2–Ni1–Br1 = 122.07(4), N2–Ni1–Br1 = 115.23(10), N2–Ni1–Br2 = 107.94(10), N1–Ni1–Br1 = 101.99(10), N1–Ni1–Br2 = 120.38(10), and N1–Ni1–N2 = 82.98(15).

DownLoad: Full-Size Img PowerPoint

2.2 Ethylene polymerization

Under fixed aggregation conditions and with the assistance of the AlEt₂Cl catalyst, the effect of the Ni1‒Ni3 catalysts on ethylene polymerization was investigated by altering the temperature (0–90 °C) while maintaining a [Al]/[Ni] ratio of 500 (Table 1). As shown in Table 1, Ni1 exhibited a high activity of up to 6.51 × 10⁶ g·mol⁻¹·h⁻¹ and produced polyethylene with a high molecular weight of up to 38.2 × 10⁴ g·mol⁻¹ (Table 1, Entries 1–4). For Ni1, as the polymerization temperature increased, the polymerization activity, molecular weight and melting point changed. The polymerization activity reached its maximum value at 30 °C, while the molecular weight of the polymer decreased with increasing temperature (from 38.2 × 10⁴ g·mol⁻¹ to 16.6 × 10⁴ g·mol⁻¹), the melting point of the polyethylene decreased with increasing polymerization temperature (from 128.2 to 114.0 °C), and the branching density increased to 41 branches per 1000 carbons. Compared with Ni1, Ni2 was obtained by replacing 2-phenylpropan-2-yl with 1-phenylethyl, and the steric hindrance of the nickel metal center decreased due to the reduction in the number of methyl groups. Compared with those of Ni1, the polymerization activity of Ni2 and the molecular weight of the prepared polyethylene slightly decreased, while the branching density increased to 61 branches per 1000 carbons (Table 1, Entries 5–8). Compared to Ni1 and Ni2, Ni3 obtained by completely removing the substituent at this position significantly reduced the ethylene polymerization activity, especially at high temperatures (90 °C), where it almost became inactive. The molecular weight of the polymer was reduced by more than half compared to that of Ni1 and Ni2 (with a maximum value of only 16.2 × 10⁴ g·mol⁻¹). However, due to the sharp decrease in the steric hindrance of Ni3, chain transfer and chain walking accelerated, and the branching density of polyethylene increased to 94 branches per 1000 carbons (Table 1, Entries 9–12). Furthermore, we investigated the temperature-dependent polymerization activity of the Ni1 and Ni2 catalysts at 90 °C (Fig. 4), and the results showed that the Ni1 and Ni2 catalysts could maintain their polymerization activity for up to 40 min at 90 °C, with Ni1 exhibiting the best performance.

Table 1. Ethylene homopolymerization catalyzed by nickel catalyst Ni1−Ni3^a.

Entry	Catalyst	T(°C)	Yield^b (g)	Activity^b (10⁶)	M_n^c(10⁴)	PDI^c	Branch^d	T_m^e(°C)
1	Ni1	0	2.65	3.18	38.2	1.8	7	128.2
2	Ni1	30	5.43	6.51	33.6	2.1	21	118.0
3	Ni1	60	2.52	3.02	21.9	2.0	40	114.4
4	Ni1	90	2.01	2.41	16.6	2.1	41	114.0
5	Ni2	0	2.35	2.82	37.6	1.8	15	120.2
6	Ni2	30	4.11	4.93	29.7	1.9	26	117.0
7	Ni2	60	2.76	3.31	17.3	2.1	46	113.6
8	Ni2	90	0.90	1.08	14.8	2.1	61	80.9
9	Ni3	0	1.10	1.32	16.2	2.3	36	115.1
10	Ni3	30	1.94	2.33	12.8	2.6	51	106.1
11	Ni3	60	0.80	0.96	11.9	3.1	72	69.1
12	Ni3	90	0.02	0.02	11.5	3.2	94	–
^a 1 μmol of catalyst in CH₂Cl₂ (2 mL), [Al]/[Ni] = 500. V_n_-heptane = 20 mL, t_{polymerization} = 10 min, P_ethylene = 8 atm. ^b Activity is in units of 10⁶ g·mol⁻¹·h⁻¹. ^c Determined by Gel Permeation Chromatography (GPC) in 1,2,4-trichlorobenzene at 150 °C. ^d Branches per 1000 carbons, determined by ¹H NMR. ^e Determined by differential scanning calorimetry.

| Show Table

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Figure 4. The polymerization performance of catalysts. (a) Polymer weight comparisons of generated polyethylene at 0 °C, 30 °C, 60 °C and 90 °C. (b) Molecular weight comparisons of generated polyethylene at 0 °C, 30 °C, 60 °C and 90 °C. (c) Branching density comparisons of generated polyethylene at 0 °C, 30 °C, 60 °C and 90 °C. (d) Time-dependent studies (polymer yields vs polymerization time) at 90 °C.

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2.3 Mechanical properties of synthesized polyethylene

Subsequently, as shown in Fig. 5, we investigated the tensile properties of polyethylene prepared with these nickel catalysts and found that the stress distribution of these polyethylene materials was between 15 and 30 MPa and that the strain distribution was between 400% and 800%. These polyethylene materials exhibit excellent tensile properties, which may be attributed to their high molecular weight (M_n greater than 17.3 × 10⁴ g·mol⁻¹). For the polyethylene samples prepared from Ni1 and Ni2, the sample with the highest stress was obtained at 30 °C, while the polyethylene sample obtained at 0 °C had the lowest stress, which may be related to its branching density.

Figure 5. Tensile curve of polyethylene prepared with nickel catalysts.

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3. Conclusions

We synthesized a series of novel α-diimine nickel complexes with asymmetric steric hindrance and studied their catalytic performance for ethylene polymerization. The polymerization activity of these catalysts can reach up to 6.51 × 10⁶ g·mol⁻¹·h⁻¹ and can be used to prepare high-molecular-weight polyethylene with adjustable branching density distributions between 7 and 94 branches per 1000 carbons. More importantly, the polyethylene prepared from these α-diimine nickel catalysts exhibited excellent tensile properties.

Supporting information

The supporting information for this article can be found online at https://doi.org/10.52396/JUSTC-2024-0001. The supporting information includes 34 figures. Experimental details, figures, and characterization data for the nickel catalysts and polymers are provided in the supporting information.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (52203016), the USTC Research Funds of the Double First-Class Initiative (YD9990002018), the Overseas Students Innovation and Entrepreneurship Support Program Project of Anhui Province (2021LCX022), the Key R&D Projects in Anhui Province (2022i01020012), the Natural Science Foundation of Hefei (2022039), and the Excellent Research and Innovation Team Project of Anhui Province (2022AH010001).

Conflict of interest

The authors declare that they have no conflict of interest.

A series of novel α-diimine nickel complexes with asymmetric steric hindrance were synthesized. The nickel catalysts can withstand 90 °C and exhibit a high polymerization activity of 6.51 × 10⁶ g·mol⁻¹·h⁻¹. The nickel catalysts can be used to prepare high-molecular-weight polyethylene with an adjustable branching density distribution between 7 and 94 branches per 1000 carbons. The polyethylene materials prepared from these α-diimine nickel catalysts exhibit excellent tensile properties.

References (24)

References

[1]	Ittel S D, Johnson L K, Brookhart M. Late-metal catalysts for ethylene homo- and copolymerization. Chem. Rev., 2000, 100 (4): 1169–1204. DOI: 10.1021/cr9804644
[2]	Natta G, Pino P, Corradini P, et al. Crystalline high polymers of α-olefins. J. Am. Chem. Soc., 1955, 77 (6): 1708–1710. DOI: 10.1021/ja01611a109
[3]	Ziegler K, Holzkamp E, Breil H, et al. Polymerisation von Äthylen und anderen Olefinen. Angew. Chem., 1955, 67 (16): 426. DOI: 10.1002/ange.19550671610
[4]	Galli P, Vecellio G. Technology: driving force behind innovation and growth of polyolefins. Prog. Polym. Sci., 2001, 26 (8): 1287–1336. DOI: 10.1016/S0079-6700(01)00029-6
[5]	Mu H, Pan L, Song D, et al. Neutral nickel catalysts for olefin homo- and copolymerization: relationships between catalyst structures and catalytic properties. Chem. Rev., 2015, 115 (22): 12091–12137. DOI: 10.1021/cr500370f
[6]	Nakamura A, Anselment T M J, Claverie J, et al. Ortho-phosphinobenzenesulfonate: a superb ligand for palladium-catalyzed coordination-insertion copolymerization of polar vinyl monomers. Acc. Chem. Res., 2013, 46 (7): 1438–1449. DOI: 10.1021/ar300256h
[7]	Sun Y, Chi M J, Bashir M S, et al. Influence of intramolecular π–π and H-bonding interactions on pyrazolylimine nickel-catalyzed ethylene polymerization and co-polymerization. New J. Chem., 2021, 45 (30): 13280–13285. DOI: 10.1039/D1NJ02437J
[8]	Younkin T R, Connor E F, Henderson J I, et al. Neutral, single-component nickel (II) polyolefin catalysts that tolerate heteroatoms. Science, 2000, 287 (5452): 460–462. DOI: 10.1126/science.287.5452.460
[9]	Zou C, Dai S Y, Chen C L. Ethylene polymerization and copolymerization using nickel 2-iminopyridine- N-oxide catalysts: modulation of polymer molecular weights and molecular-weight distributions. Macromolecules, 2018, 51 (1): 49–56. DOI: 10.1021/acs.macromol.7b02156
[10]	Wang Q, Wang W B, Qu W C, et al. Ethylene homo and copolymerization by phosphorus-benzoquinone based homogeneous and heterogeneous nickel catalysts. J. Polym. Sci., 2023, 61 (2): 115–122. DOI: 10.1002/pol.20220381
[11]	Klabunde U, Itten S D. Nickel catalysis for ethylene homo- and co-polymerization. J. Mol. Catal., 1987, 41 (1/2): 123–134. DOI: 10.1016/0304-5102(87)80023-8
[12]	Wang W B, Chen M, Pang W M, et al. Palladium-catalyzed synthesis of norbornene-based polar-functionalized polyolefin elastomers. Macromolecules, 2021, 54 (7): 3197–3203. DOI: 10.1021/acs.macromol.1c00201
[13]	Johnson L K, Killian C M, Brookhart M. New Pd(II)- and Ni(II)-based catalysts for polymerization of ethylene and α-olefins. J. Am. Chem. Soc., 1995, 117 (23): 6414–6415. DOI: 10.1021/ja00128a054
[14]	Hu X Q, Kang X H, Jian Z B. Suppression of chain transfer at high temperature in catalytic olefin polymerization. Angew. Chem. Int. Ed., 2022, 61 (33): e202207363 DOI: 10.1002/anie.202207363
[15]	Zheng H D, Li Y W, Du W B, et al. Unprecedented square-planar α-diimine dibromonickel complexes and their ethylene polymerizations modulated by Ni–phenyl interactions. Macromolecules, 2022, 55 (9): 3533–3540 DOI: 10.1021/acs.macromol.2c00360
[16]	Lu W Q, Ding B H, Zou W P, et al. Direct synthesis of polyethylene thermoplastic elastomers with high molecular weight and excellent elastic recovery via a hybrid steric bulky strategy. Eur. Poly. J., 2023, 201 (11): 112577 DOI: 10.1016/j.eurpolymj.2023.112577
[17]	Ji H Y, Zhang Y X, Chi Y, et al. Concerted flexible and steric strategy in α-diimine nickel and palladium mediated insertion (Co-)Polymerization. Polymer, 2024, 290: 126591. DOI: 10.1016/j.polymer.2023.126591
[18]	Rhinehart J L, Brown L A, Long B K. A robust Ni(II) α-diimine catalyst for high temperature ethylene polymerization. J. Am. Chem. Soc., 2013, 135 (44): 16316–16319. DOI: 10.1021/ja408905t
[19]	Dai S Y, Sui X L, Chen C L. Highly robust palladium(II) α–diimine catalysts for slow-chain-walking polymerization of ethylene and copolymerization with methyl acrylate. Angew. Chem. Int. Ed., 2015, 54 (34): 9948–9953. DOI: 10.1002/anie.201503708
[20]	Padilla-Vélez O, O’Connor K S, LaPointe A M, et al. Switchable living nickel(II) α-diimine catalyst for ethylene polymerisation. Chem. Commun., 2019, 55 (53): 7607–7610. DOI: 10.1039/C9CC03154E
[21]	Wang F Z, Yuan J C, Li Q S, et al. New nickel(II) diimine complexes bearing phenyl and sec-phenethyl groups: synthesis, characterization and ethylene polymerization behaviour. Appl. Organomet. Chem., 2014, 28 (7): 477–483. DOI: 10.1002/aoc.3151
[22]	Rose J M, Deplace F, Lynd N A, et al. C₂-symmetric Ni(II) α-diimines featuring cumyl-derived ligands: synthesis of improved elastomeric regioblock polypropylenes. Macromolecules, 2008, 41 (24): 9548–9555. DOI: 10.1021/ma8019943
[23]	Cherian A E, Rose J M, Lobkovsky E B, et al. A C₂-symmetric, living α-diimine Ni(II) catalyst: regioblock copolymers from propylene. J. Am. Chem. Soc., 2005, 127 (40): 13770–13771. DOI: 10.1021/ja0540021
[24]	Vaccarello D N, O’Connor K S, Iacono P, et al. Synthesis of semicrystalline polyolefin materials: precision methyl branching via stereoretentive chain walking. J. Am. Chem. Soc., 2018, 140 (20): 6208–6211. DOI: 10.1021/jacs.8b02963

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Figure 1. Representative diimide catalysts (a–e) and novel structures used in this work (f).

1. Synthetic route to nickel complexes Ni1−Ni3.

Figure 5. Tensile curve of polyethylene prepared with nickel catalysts.

References

[1]	Ittel S D, Johnson L K, Brookhart M. Late-metal catalysts for ethylene homo- and copolymerization. Chem. Rev., 2000, 100 (4): 1169–1204. DOI: 10.1021/cr9804644
[2]	Natta G, Pino P, Corradini P, et al. Crystalline high polymers of α-olefins. J. Am. Chem. Soc., 1955, 77 (6): 1708–1710. DOI: 10.1021/ja01611a109
[3]	Ziegler K, Holzkamp E, Breil H, et al. Polymerisation von Äthylen und anderen Olefinen. Angew. Chem., 1955, 67 (16): 426. DOI: 10.1002/ange.19550671610
[4]	Galli P, Vecellio G. Technology: driving force behind innovation and growth of polyolefins. Prog. Polym. Sci., 2001, 26 (8): 1287–1336. DOI: 10.1016/S0079-6700(01)00029-6
[5]	Mu H, Pan L, Song D, et al. Neutral nickel catalysts for olefin homo- and copolymerization: relationships between catalyst structures and catalytic properties. Chem. Rev., 2015, 115 (22): 12091–12137. DOI: 10.1021/cr500370f
[6]	Nakamura A, Anselment T M J, Claverie J, et al. Ortho-phosphinobenzenesulfonate: a superb ligand for palladium-catalyzed coordination-insertion copolymerization of polar vinyl monomers. Acc. Chem. Res., 2013, 46 (7): 1438–1449. DOI: 10.1021/ar300256h
[7]	Sun Y, Chi M J, Bashir M S, et al. Influence of intramolecular π–π and H-bonding interactions on pyrazolylimine nickel-catalyzed ethylene polymerization and co-polymerization. New J. Chem., 2021, 45 (30): 13280–13285. DOI: 10.1039/D1NJ02437J
[8]	Younkin T R, Connor E F, Henderson J I, et al. Neutral, single-component nickel (II) polyolefin catalysts that tolerate heteroatoms. Science, 2000, 287 (5452): 460–462. DOI: 10.1126/science.287.5452.460
[9]	Zou C, Dai S Y, Chen C L. Ethylene polymerization and copolymerization using nickel 2-iminopyridine- N-oxide catalysts: modulation of polymer molecular weights and molecular-weight distributions. Macromolecules, 2018, 51 (1): 49–56. DOI: 10.1021/acs.macromol.7b02156
[10]	Wang Q, Wang W B, Qu W C, et al. Ethylene homo and copolymerization by phosphorus-benzoquinone based homogeneous and heterogeneous nickel catalysts. J. Polym. Sci., 2023, 61 (2): 115–122. DOI: 10.1002/pol.20220381
[11]	Klabunde U, Itten S D. Nickel catalysis for ethylene homo- and co-polymerization. J. Mol. Catal., 1987, 41 (1/2): 123–134. DOI: 10.1016/0304-5102(87)80023-8
[12]	Wang W B, Chen M, Pang W M, et al. Palladium-catalyzed synthesis of norbornene-based polar-functionalized polyolefin elastomers. Macromolecules, 2021, 54 (7): 3197–3203. DOI: 10.1021/acs.macromol.1c00201
[13]	Johnson L K, Killian C M, Brookhart M. New Pd(II)- and Ni(II)-based catalysts for polymerization of ethylene and α-olefins. J. Am. Chem. Soc., 1995, 117 (23): 6414–6415. DOI: 10.1021/ja00128a054
[14]	Hu X Q, Kang X H, Jian Z B. Suppression of chain transfer at high temperature in catalytic olefin polymerization. Angew. Chem. Int. Ed., 2022, 61 (33): e202207363 DOI: 10.1002/anie.202207363
[15]	Zheng H D, Li Y W, Du W B, et al. Unprecedented square-planar α-diimine dibromonickel complexes and their ethylene polymerizations modulated by Ni–phenyl interactions. Macromolecules, 2022, 55 (9): 3533–3540 DOI: 10.1021/acs.macromol.2c00360
[16]	Lu W Q, Ding B H, Zou W P, et al. Direct synthesis of polyethylene thermoplastic elastomers with high molecular weight and excellent elastic recovery via a hybrid steric bulky strategy. Eur. Poly. J., 2023, 201 (11): 112577 DOI: 10.1016/j.eurpolymj.2023.112577
[17]	Ji H Y, Zhang Y X, Chi Y, et al. Concerted flexible and steric strategy in α-diimine nickel and palladium mediated insertion (Co-)Polymerization. Polymer, 2024, 290: 126591. DOI: 10.1016/j.polymer.2023.126591
[18]	Rhinehart J L, Brown L A, Long B K. A robust Ni(II) α-diimine catalyst for high temperature ethylene polymerization. J. Am. Chem. Soc., 2013, 135 (44): 16316–16319. DOI: 10.1021/ja408905t
[19]	Dai S Y, Sui X L, Chen C L. Highly robust palladium(II) α–diimine catalysts for slow-chain-walking polymerization of ethylene and copolymerization with methyl acrylate. Angew. Chem. Int. Ed., 2015, 54 (34): 9948–9953. DOI: 10.1002/anie.201503708
[20]	Padilla-Vélez O, O’Connor K S, LaPointe A M, et al. Switchable living nickel(II) α-diimine catalyst for ethylene polymerisation. Chem. Commun., 2019, 55 (53): 7607–7610. DOI: 10.1039/C9CC03154E
[21]	Wang F Z, Yuan J C, Li Q S, et al. New nickel(II) diimine complexes bearing phenyl and sec-phenethyl groups: synthesis, characterization and ethylene polymerization behaviour. Appl. Organomet. Chem., 2014, 28 (7): 477–483. DOI: 10.1002/aoc.3151
[22]	Rose J M, Deplace F, Lynd N A, et al. C₂-symmetric Ni(II) α-diimines featuring cumyl-derived ligands: synthesis of improved elastomeric regioblock polypropylenes. Macromolecules, 2008, 41 (24): 9548–9555. DOI: 10.1021/ma8019943
[23]	Cherian A E, Rose J M, Lobkovsky E B, et al. A C₂-symmetric, living α-diimine Ni(II) catalyst: regioblock copolymers from propylene. J. Am. Chem. Soc., 2005, 127 (40): 13770–13771. DOI: 10.1021/ja0540021
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Nickel catalysts with asymmetric steric hindrance for ethylene polymerization

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