Applications of metal N-heterocyclic carbene complexes in olefin polymerizations

Abstract

Polyolefins play an important role in modern society and are applied as both commodity plastics and high-performance polymers. Nowadays, they are widely used in large-scale industrial production and considered one among the most important polymers in the world. Since the 1950s, the exploration of Ziegler–Natta catalysts has not just brought continuous breakthroughs in the industrialization of polyolefins, but has also promoted the development of research in organometallic chemistry. Obviously, a key and long-standing focus in the polyolefin industry is the investigation of new metal catalysts that possess customizable catalytic performances. Over the past few decades, metal complexes with N-heterocyclic carbene (NHC) ligands have achieved great improvements and found vast applications in diverse research fields, and definitely, they have been also demonstrated to show excellent catalytic performances in olefin polymerization. Numerous reviews have been published on the catalytic performances of NHC-chelated metal complexes; however, a comprehensive overview of them with the central metals across the periodic table in olefin polymerization is currently lacking. This review aims to bridge this gap by providing a comprehensive summary of NHC catalysts and their corresponding structure–activity relationships in the polymerization and copolymerization of different olefins.

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Ting Feng

Ting Feng was born in Gansu, China. She received her master's degree from Northwest University (Supervisor: Prof. Ying-Feng Han). She is currently a doctoral student under the supervision of Prof. Ying-Feng Han at Northwest University (China).

Le Zhang

Le Zhang was born in Shaanxi, China. He received his bachelor's degree at Yan'an University (2016). He obtained his PhD (2023) under the supervision of Prof. Ying-Feng Han at Northwest University (China). Currently, he is a postdoctoral fellow at Northwest University (China).

Shang-Xiao Wu

Shang-Xiao Wu was born in Shanxi, China. He received his BE degree (2020) in Chemical Engineering from Taiyuan University of Science and Technology. He is a master's student under the supervision of Prof. Ying-Feng Han at Northwest University (China).

Xiaochao Shi

Xiaochao Shi received his BSc and MSc in Applied Chemistry from Dalian University of Technology in 2006 and 2009, respectively. In 2013, he received his PhD in Organic Chemistry at Fudan University under the supervision of Prof. Guo-Xin Jin. During 2013–2016, he did postdoctoral research in RIKEN under the supervision of Prof. Zhaomin Hou in Japan. In 2017, he joined the School of Materials Science and Engineering at Shanghai University as a professor.

Ying-Feng Han

Ying-Feng Han obtained his PhD (2009) in chemistry from Fudan University, China. The same year, he joined Fudan University as a lecturer and was promoted to associate professor in 2011. He was a postdoctoral fellow at the University of Münster, Germany (Alexander von Humboldt Fellowship). In 2016, he moved to Northwest University and accepted a position as a full professor (Qinling Professor) of chemistry. His main research interests are carbene chemistry, photochemistry, supramolecular chemistry and AI chemistry.

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1. Introduction

Polyolefins, such as polyethylenes (PEs) and polypropylenes (PPs), are commercially synthetic polymers worldwide due to their cost-effectiveness and excellent properties.¹ The majority of these polymers are synthesized via the strategy of transition-metal-catalyzed olefin polymerization. Ever since Karl Ziegler and Giulio Natta discovered titanium-based olefin polymerization catalysts in the early 1950s, an extensive range of transition metal complexes with various central metals and chelating ligand frameworks have been developed as catalysts for olefin polymerization. These advancements have significantly contributed to the development of metal catalysis, organometallic chemistry, and polymer science, which remain the most exciting research fields nowadays. Notably, group 4 metallocene catalysts, rare-earth metal half-sandwich alkyl catalysts, and late transition metal catalysts have exhibited outstanding catalytic performances for olefin copolymerization.² Given the significance of metal catalysts in the field of polyolefins, researchers from both academia and industry are enthusiastic about designing and synthesizing novel efficient metal catalysts, as well as achieving precise synthesis of polyolefin products.^3,4 On the other hand, the research and application of N-heterocyclic carbenes (NHCs) have become a hot topic in chemistry since Arduengo and colleagues first isolated and characterized the free and stable NHCs.⁵ Various NHCs with distinct structures and their corresponding metal complexes have been synthesized and reported, leading to significant breakthroughs in a wide range of catalytic reactions. Generally, phosphine ligands are among the most important class of ligands to stabilize the metal complexes because of the alterability of their electronic and steric properties in a systematic and predictable way. It has been proved that NHCs have similar chemical properties to phosphine ligands and can partially replace phosphine ligands in catalytic reactions. Furthermore, the strong σ-donating ability of NHCs allows for the formation of more stable metal NHC complexes compared to their phosphine-based counterparts.^6,7 This effectively addresses the deactivation issue commonly observed with phosphine-based metal complexes during catalytic processes.⁸ Consequently, NHCs have emerged as versatile ancillary ligands for constructing metal complexes, which have proved to be effective homogeneous catalysts for various reactions including olefin metathesis, coupling reactions, ring-opening translocation polymerization, and olefin coordination polymerization.^8,9 In general, as a soft carbon-donor ligand, NHCs are more prone to coordinate late-transition metals to form thermally and chemically stable complexes than the early transition metals in high oxidation states because of the hard–soft mismatch. This poses a challenge for the applications of NHCs in olefin polymerization, considering that complexes with early-transition metal centers are the predominant industrial catalyst candidates for such reactions. On the other hand, the covalent tethering of coordinating functional groups to NHCs offers a practical approach for enhancing the interactions of NHC ligands with the central metals, especially the early transition metals. This has stimulated the development of numerous metal complexes with hetero-atom-tethered NHC ligands as excellent catalyst candidates for olefin polymerization.

This review focuses on the literature published since 1998 regarding various well-defined metal NHC catalysts used in the polymerization of olefins, particularly α-olefins, styrene, norbornene, and others. The synthesis of NHC-ligated complexes and the structure–activity relationship in NHC-complex-catalyzed olefin polymerization are discussed in a comparative manner, and the catalytic data, including the structures of complexes, co-catalysts, monomers, activities and microstructures of resultant polymers and so on, are summarized in Table 1. Notably, the catalyst activities are classified in this review as high (1000–100), moderate (100–10), low (10–1), and very low (<1) for α-olefins, but for norbornene polymerization, the categories are high (>10 000), moderate (10 000–1000), low (1000–100), and very low (<100) (in unit: kg mol⁻¹ (M) h⁻¹). For a more comprehensive understanding of the coordination chemistry of metal NHC complexes and their reactivity in diverse reactions, it is highly recommended to refer to other excellent overviews.^10–13

Table 1 The catalytic performances of the catalysts in olefin polymerizations investigated in this study

Complexes	Monomers	Co-catalyst	Activity (10^3 g/molM^−1 h^−1)	M n (10^4 g mol^−1)	M w/Mn	Key features in polymerization	Ref.
Complexes with monodentate NHC ligands
	α-Olefin	[Ph3C][B(C6F5)4]	8–2520	0.8–63.1	1.7–3.0	Atactic poly(α-olefin), tunable insertion ratio of 1,5-hexadiene	14
	1-Hexene/1,5-hexadiene	[Ph3C][B(C6F5)4]	72–216	2.2–4.3	1.7–2.6
	o-Methoxystyrene derivatives	[Ph3C][B(C6F5)4]/Al^iBu3	6.9–2160	0.5–455.5	1.37–2.24	Syndiotactic (up to rrrr > 99%)	15
	Sulfur-functionalized α-olefin	[Ph3C][B(C6F5)4]		0.32–10.3	1.53–11.6	Syndiotactic (rrrr up to 0.95), crystalline materials	16
	α-Olefin	[Ph3C][B(C6F5)4]	1–285	2.8–21.8	1.72–2.45	Atactic poly(α-olefin), pendant vinyl groups in the copolymers (22.1%)	17
	1-Hexene/1,7-octadiene	[Ph3C][B(C6F5)4]		2.9–4.9	2.06–3.12
	Ethylene/propylene	Et3Al2Cl3	7.2–110.0	Up to 1612 (Mw)		Randomly distributed monomer	18 and 19
	Ethylene (6 atm)	MAO or [Ph3C][B(C6F5)4]/Al^iBu3	3–4590	28.0–341	1.8–2.7	High catalytic activity	20
	Ethylene/1-hexene	d-MAO	66–2470	4.10–23.5	1.9–5.0	Efficient 1-hexene incorporation
	Ethylene	MAO	29–67			Supported, high stability (300 minutes)	21
	Ethylene (2 atm)	MAO	Low			Dual peaks in GPC curve	22
	Ethylene (1 atm)	MMAO	12.9–15.1			1-Butene produced, fast β-H elimination	23
	Styrene	MAO		0.94–16.5	2.0–4.0	Atactic polymer	24
	Norbornene	MAO				Trace polymer	25
	1,3-Butadiene			4.54	2.9	Poly(cis-1,4-butadiene) and atactic polystyrene	26 and 27
	Styrene			8.0	2.5
	Norbornene and derivates	AgSbF6 or AgBF4		0.45–13.3	1.25–2.89	Catalytic activity was highly dependent on the counter anion	28
	Norbornene		24.1–328.0	12–41 (Mw)	2.4–3.3	Low catalytic activity	29
	5-Norbornene-2-methyl acetate	Li[B(C6F5)4]	2–123	5.8–16.6	1.73–2.53	Low catalytic activity	30
	5-Alkylidene-2-norbornenes	Na[B(3,5-(CF3)2C6H5)4]	110–140 000	1.3–68.7	1.4–4.4	Extremely high activity, performed in an atmosphere of air and in wet solvents	31
	Substituted nadimides	AgSbF6 or AgBF4		19.6–188.9		Proper combination of a cocatalyst and a Pd precatalyst	32
	Norbornene and derivates	Na[B(3,5-(CF3)2C6H5)4]		0.30–0.68	1.23–1.35	Redox control	33
	Ethylene (2 atm)	MAO	1–75			Linear polyethylene generated	34
	Ethylene (10 bar), propylene, 1-butene	MAO, AlEtCl2	2.7, 13.4 (ethylene)			Predominant dimerization	35–38
	Ethylene (10 bar)	MAO, AlEtCl2	0.2–42.8 (ethylene)			Predominant dimerization	38 and 39
	Styrene derivates			0.86–18.2	1.7–3.4	Atactic polystyrenes, co-catalyst-free	40
Complexes with a bidentate NHC ligands (bidentate NHC ligand with a neutral functionality)
	Ethylene (6.89 atm)	MAO, Et2AlCl	0.318–1.871				41
	Ethylene (10 atm)	MAO, AlEtCl2	0.15–2.39 (gM^−1)			Mainly polymerization in all the cases	42
	Ethylene (10 atm)	MAO, AlEtCl2	1.67–8.79			MAO as cocatalyst: oligomerization AlEtCl2 as cocatalyst: polymerization	42
	Ethylene (1 atm)	MAO	2–4			Poor activity	43
	Ethylene (5 atm)					Low activity in ethylene polymerization	44
	Ethylene (5 atm)					77% polyethylene and 23% soluble oligomers with rather low activity	44
	Ethylene	MAO	40–140			Linear polyethylene	45
	Norbornene	MMAO or Na[B(3,5-(CF3)2C6H5)4]	32.0–350 (Pd)			Low catalytic activity, C7 linkage in the polynorbornene	46
	Norbornene	MAO	1300–26 000	111–420	2.3–3.5	High catalytic activity for norbornene polymerization and high catalytic activity for ethylene polymerization	47
	Ethylene (1 atm)	MAO	330	6.7	12.8
	Norbornene	MAO	5200–77 000	—	—	High catalytic activity	48
	Ethylene (2.5 atm) and styrene	AgPF6				Predominant dimerization	49
	Ethylene					Predominant dimerization	50
	Norbornene	MAO	500–3000			Moderate catalytic activity	51
	Ethylene (10 atm)	EtAlCl2	13.3–15.6			Predominant dimerization	39
	Ethylene (30 atm)		29–152	0.65–1.3	2.0–2.3	Low incorporation ratio of polar monomers	52
	Ethylene (30 atm) /MA derivates		1.7–13	0.48–0.94	2.1–2.3
	Ethylene (30 atm)		Trace			Very low catalytic activation	52
	Norbornene	MAO	996–1700	24–82 (Mv)		Moderate catalytic activation	25
	Norbornene	MAO	303			Low catalytic activation	25
Complexes with bidentate NHC ligands (bidentate NHC ligand with an anionic functionality)
	Isoprene	[Ph3C][B(C6F5)4]/Al^iBu3		2.03–3.81	1.36–1.40	3,4-Regulated (99%) and syndiotactically enriched microstructure	53 and 54
	Isoprene	[Ph3C][B(C6F5)4]/Al^iBu3		3.73–39.4	1.05–1.38
	Ethylene (1 atm)/α-olefin		760–4120	0.59–5.52 (Mw)	1.2–2.9	Varied comonomer incorporation (2.1%–38.7%)	55
	Ethylene (1–6 atm) /styrene		5–508	0.6–18.9	1.26–2.41	Pseudo-random microstructures	56
	Ethylene (1–6 atm)/polar styrene derivates		7–362	0.47–3.55	1.49–2.55	Pseudo-alternating microstructures	57
	Ethylene (4 atm)/butadiene		150–1640	2.18–20.2	1.81–2.74	trans-1,4 regularity and alternating sequence	58
	Ethylene (8–40 atm)	MMAO-3A	35.5–636 (gM^−1)			Moderate catalytic activity, polyethylene and no oligomerization	59
	Styrene	NaBPh4		2.7	1.89	Low catalytic activity, styrene polymerization at 80 °C	60
	Ethylene or (6 atm)	MAO	4.1–29	63–370 (Mw)	1.6–3.4	Low catalytic activity, linear polyethylene, isotactic polypropylene together with an atactic fraction	61
	Propylene		0.029–0.038	38–73 (Mw)	1.7–5.7
	Ethylene (6 atm)	MAO	0.3–7.3	64–377 (Mw)	2.3–3.4		62
	Propylene		0.3–31	31–175 (Mw)	2.3–3.5
	Ethylene (3–27.2 atm)	MAO	16–259	0.446–82.4	8.24–85.6	Extremely high molecular weight polyethylene (Mw)	63 and 64
	Styrene	MAO	0.48–1.3	1.01–1.33	2.4–5.5	Highly syndiotactic polystyrene	64
	Ethylene (4 atm) or ethylene/norbornene or ethylene/cyclopentene	MAO	25–210	>600		Extremely high-molecular weight (co-)polymers	65
	Ethylene (15 atm)		0.8–56	0.10–0.71	1.95–2.5	Polymerizing ethylene in the absence of co-catalysts, linear polymer	66
	Ethylene (15 atm)	ZnEt2	1.5–17.5	0.126–0.211	2.4–20.68	Highly linear, low molecular-weight polyethylene	67
	Norbornene	MAO	Up to 100			Low catalytic activity	68
	Norbornene	MAO	110–14 500	29–211 (Mw)	1.5–3.3	Moderate to high catalytic activity	69–71
	Norbornene	MAO	2690–49 970			High catalytic activity	72 and 73
	Norbornene	MAO	23 640			High catalytic activity	72
	Norbornene	MAO	23 290–23 960			High catalytic activity	72
	Norbornene	MAO	7800–49 500			High catalytic activity	74
	Norbornene	MAO	27 230			High catalytic activity	74
	Norbornene	MAO	26 440			High catalytic activity	74
	Norbornene	MAO	19 280			High catalytic activity	75
	Norbornene	MAO, Et2AlCl, Et3Al, [Ph3C][B(C6F5)4]	490–49 030			High catalytic activity	75
	Norbornene	MAO	9900–32 170			High catalytic activity	75
	Norbornene	MAO	1100–58 420			High catalytic activity	76
	Norbornene	MAO	399–2965			Moderate catalytic activity	76
	Norbornene	MAO	4450–101 340			Moderate to high catalytic activity	77
		[Ph3C][B(C6F5)4]	1270–42 350			High catalytic activity
	Norbornene	MAO	1900–42 800			High thermal stability	78
		Et2AlCl	900–8500			Low to high catalytic activity
	Norbornene	MAO	4000–54 800			High thermal stability
	Norbornene	Et2AlCl	1500–13 300			Moderate to high catalytic activity
	Norbornene	MAO	12 000–60 000			High thermal stability
		Et2AlCl	7400–29 000			Moderate to high catalytic activity
	Norbornene	MAO or Et2AlCl	2880–49 000			Moderate to high catalytic activity	79
	Norbornene/polar derivates	MAO or Et2AlCl	290–2080	3.66–8.64	1.63–2.47	Moderate catalytic activity and high incorporation of polar monomers
	Ethylene (4 MPa), propylene and 1-butene		3.6–1310	0.21–8.0	1.8–3.3	High thermal stability of catalyst, moderate to high catalytic activity	80
	Ethylene (4 MPa)/polar allyl monomers or propylene/polar allyl monomers		0–10.8	0.23–2.3	2.0–5.6	Successful insertion of polar monomer
	Ethylene (1–2 MPa)/1,1-disubstituted ethylenes		0.16–2.3	0.08–2.7	1.7–2.9	Successful incorporation of polar comonomers	81 and 82
	Ethylene (3.0 MPa)/3-ethylidene-6-vinyltetrahydro-2H-pyran-2-one		0.6–48	0.07–3.2	1.7–4.7	Copolymer of ethylene/CO2/1,3-butadiene	83
	Ethylene (2.0 MPa)/norbornenes functionalized with polar groups		2.0	2.2	1.4	High incorporation of polar comonomers	84
	Ethylene (4.0 MPa)		240–1190	0.26–8.4	2.0–2.7	Linear polyethylene and successful incorporation of polar comonomers	85
	Ethylene (4.0 MPa)/polar allyl monomers		0–2.1	0.063–1.4	2.1–3.9
	Ethylene (4.0 MPa) and propylene		9.6–347	0.31–23.7	1.3–1.9	Moderate catalytic activity, successful incorporation of polar comonomers	86
	Ethylene (4 MPa)/polar allyl monomers		1.8–9.5	0.22–2.7	1.3–2.4
	Ethylene (4.0 MPa)		3.6, 5.2	0.18, 0.25	3.6, 2.0	Branched polyethylene	87
	Ethylene (4.0 MPa)		4.1–5.3	0.36–0.7	1.7–3.1	Branched polyethylene	87
	Ethylene (4.0 MPa)/methyl acrylate			0.09–1.17	1.9–3.2	Successful incorporation of polar comonomers	88
	Ethylene (4.0 MPa)			16.2–127.0	0.032–0.07	Oligomer in all cases, telechelic microstructure of co-oligomer	89
	Ethylene (4.0 MPa)/methyl acrylate			2.8	0.035
Complexes with tridentate NHC ligands (tridentate NHC ligand with neutral functionality)
	Ethylene (10 atm)	MAO	1.7–9.2 (g Ni)			Oligomer in all cases	90
	Ethylene (10 atm)	Et2AlCl	12.7–14.6 (g Ni)			Oligomer in all cases	90
	Ethylene (1–5 atm)	MAO	15–40 440			High catalytic activity (Cr) for oligomerization, high molecular weight for polymer (Ti, V)	91 and 92
	Ethylene (up to 30 atm)	MAO				No activity for oligomerization and polymerization	93
	Ethylene (1 atm)	MAO	17–35			Linear polyethylene	94
	Isoprene	[Ph3C][B(C6F5)4]		1.84–49.2	1.03–1.07	Highly 3,4-regular polyisoprene	95
	Isoprene/ε-caprolactone	[Ph3C][B(C6F5)4]		4.9–10.2	1.12–1.16
	2-Phenyl-1,3-butadiene	[Ph3C][B(C6F5)4]		1.84–7.08	1.17–1.66	Highly 3,4-regular	96
	Isoprene	[Ph3C][B(C6F5)4]/Al^iR3		3.9–95.6	1.40–3.87	Highly cis-1,4-regular	97 and 98
Complexes with tridentate NHC ligands (tridentate NHC ligand with monoanionic functionality)
	Styrene	NaBPh4		1.76	1.97	Low activity	99
	Norbornene	Me2AlCl/Me2AlCl/MAO/B(C6F5)3	3.67–3210 (Ni)	5.19–25.89 (Mw)	1.72–2.80	Low to moderate catalytic activity	70, 73 and 100
			4074–274 000 (Pd)	3.8–16.4	1.64–2.16	Moderate to high catalytic activity
	Norbornene	Me2AlCl/Me2AlCl/MAO/B(C6F5)3	0.41–2280 (Ni)	1.61–36.69 (Mw)	1.41–2.05	Low to moderate catalytic activity	70 and 73
			4074–59 990 (Pd)	8.6–12.5	1.66–1.71	Moderate to high catalytic activity
	Norbornene	Me2AlCl	286 000–274 000			High catalytic activity	100
	Norbornene/norbornene with polar functionality		18.2–86.1	1.57–3.22	1.36–2.15	Low catalytic activity, high incorporation ratio of polar monomer
	Norbornene	MAO	1120–6570	22.4–102.5 (Mw)	2.3–2.8	Moderate catalytic activity	101
Complexes with tridentate NHC ligands (tridentate NHC ligand with dianionic functionality)
	Ethylene (3.5–9 atm)	MMAO	9.71–290	0.13–3.98		Moderate catalytic ability, linear polyethylene with low molecular weight	102 and 103
	Ethylene	[Ph3C][B(C6F5)4]	125			Moderate catalytic ability	104
	1-Hexene	[Ph3C][B(C6F5)4]				Oligo(1-hexenes) formed	105
	1-Hexene	[Ph3C][B(C6F5)4]				Atactic oligomer	106
Complexes with tetradentate NHC ligands
	Norbornene	MAO	810–1340	18.4–25.1 (Mw)	10.3–11.5	Moderate catalytic activity	101

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2. Complexes with monodentate NHC ligands

The N-heterocyclic carbene ligated Sc trialkyl complexes [(2,6-C₆H₃R₂NCH)₂CSc(CH₂SiMe₃)₃] 1 (Fig. 1, left) were synthesized by the reaction of 1.0 equiv. of [Sc(CH₂SiMe₃)₃(THF)₂] with the corresponding NHC ligands.¹⁴ Upon activation with 2.0 equiv. of [Ph₃C][B(C₆F₅)₄], 1 exhibited remarkably high activities for 1-hexene homopolymerization (up to 2.52 × 10⁶ g mol⁻¹ h⁻¹), generating the polymers with moderate to high molecular weights. The bis(2,6-dimethyl)-substituted complex was also shown to be an efficient catalyst in the copolymerization of 1-hexene with 1,5-hexadiene, yielding random atactic copolymers after addition of 2.0 equiv. of [Ph₃C][B(C₆F₅)₄]. 1/[Ph₃C] [B(C₆F₅)₄] also exhibited good activities in the polymerization of 1-octene or 1-decene. Moreover, in the presence of 2.0 equiv. of [Ph₃C][B(C₆F₅)₄] and excess AlⁱBu₃, bis(2,6-diisopropylphenyl)-substituted Sc dication active species displayed excellent activity (2.2 × 10⁶ g mol⁻¹ h⁻¹) in the polymerization of polar o-methoxystyrene and its silyloxy- or fluorine-substituted derivatives, yielding ultrahigh molecular weight syndiotactic poly(oMOS) derivatives (up to rrrr > 0.99).¹⁵ The density functional theory (DFT) calculation studies provided evidence that the double bonds and heteroatoms of oMOS pass through the σ,π-coordination mode, associated with high oxyphilic Sc center with two empty coordination sites, which reduces the activation energy of monomer insertion. The 1/[Ph₃C] [B(C₆F₅)₄] binary system has been found to be capable of catalyzing the polymerization of thioether-substituted olefin monomers. This resulted in the formation of syndiotactic sulfur-functionalized poly(α-olefin)s (rrrr up to 0.95), which formed crystalline materials with T_m = 91 °C and T_d = 378 °C. The presence of electron-donating groups greatly influenced the crystallinity of the polymer. Furthermore, factors such as R substituents, spacer length of the sulfur atom and C=C double bond, and the stereoregularity of the polymer also impacted its crystalline structure significantly.¹⁶

Fig. 1 Monodentate NHC-ligated rare-earth metal complexes.

In another study, Pan et al. investigated the catalytic activities of bis(oxazoline)-derived NHC lanthanide complexes 2 (Fig. 1, right) in the polymerization of α-olefins. By activation with 2.0 equiv. of [Ph₃C][B(C₆F₅)₄], the Sc complex showed high activities (2.85 × 10⁵ g mol⁻¹ h⁻¹) towards 1-hexene polymerization. The resulting polymers had molecular weights of 2.8–8.7 × 10⁴ g mol⁻¹ and 2.18 × 10⁵ g mol⁻¹ at room temperature and −20 °C, respectively. However, the Y and Lu analogues were inert for α-olefin polymerization. The Sc-based catalytic system 2/2[Ph₃C][B(C₆F₅)₄] was capable of catalysing the copolymerization of 1-hexene with 1,7-octadiene, giving random copolymers with diene incorporation levels varying from 30% to 70% (depending on the co-monomer feed) with about 20% pendant vinyl groups.¹⁷

The treatment of equimolar NHCs with VOCl₃ in toluene at room temperature smoothly afforded the corresponding NHC V complexes 3 (Fig. 2).^18,19 Upon activation with Et₃Al₂Cl₃, these complexes served as efficient catalysts for the copolymerization of ethylene with propylene. The resulting copolymers displayed narrow polydispersity and randomly distributed monomer units. Interestingly, the catalytic activity remained unaffected at increased ratios of cocatalysts, indicating the stability of the active species. The 2,6-ⁱPr₂C₆H₃ substituted complex/Et₃Al₂Cl₃ system displayed the highest activity of 1.1 × 10⁵ g mol⁻¹ h⁻¹, and this catalytic system yielded ultra-high-molecular-weight ethylene–propylene copolymers (M_w = 1.612 × 10⁶ g mol⁻¹) in a living manner. The stabilization and long lifetime of active species in this catalytic system were attributed to the isopropyl substituent in the NHC ligand with bulky and electron-rich phenyl rings.

Fig. 2 Vanadium complexes with monodentate NHC ligands.

Half-titanocene NHC complexes 4 (Fig. 3, left), which contained anionic NHCs with a weakly coordinating borate B(C₆F₅)₃ group, were synthesized and employed as pre-catalysts for ethylene polymerization. In the presence of AlⁱBu₃/[Ph₃C] [B(C₆F₅)₄] as the cocatalyst, the ^tBu-substituted complex 4 exhibited high catalytic activity (4.59 × 10⁶ g mol⁻¹ h⁻¹) for ethylene polymerization. The polymers had high molecular weights and a unimodal molecular weight distribution. The dimethyl complex 4, when used in the copolymerization of ethylene and 1-hexene with a small amount of MAO as a cocatalyst, displayed efficient incorporation ability for 1-hexene. This complex is considered one of the most active catalysts for ethylene copolymerization with 1-hexene and showing considerable incorporation.²⁰

Fig. 3 Half-sandwich titanium and chromium NHC complexes.

In 2000, an international patent reported the utilization of a metal NHC complex in olefin polymerization.²¹ The cyclopentadienyl Cr^II carbene complex 5 (Fig. 3, middle) was activated by the addition of MAO, and the catalytic activities of 5/MAO and the corresponding supported cases on SiO₂, MgCl₂/SiO₂ and polystyrene/divinylbenzene were systematically studied. 5/MAO exhibited an activity of 2.9 × 10⁴ g mol⁻¹ bar⁻¹ h⁻¹, while the activity of the supported catalyst for ethylene polymerization increased to 6.7 × 10⁴ g mol⁻¹ bar⁻¹ h⁻¹. Importantly, the supported catalytic systems maintained high activity and stability for at least about 300 minutes. In the same year, Jolly and colleagues reported a Cr^III carbene analogue 6 (Fig. 3, right).²² However, the catalytic activity of 6/MAO was rather low in ethylene polymerization, partially attributed to its low solubility in the polymerization medium, toluene. The resulting polymer was highly branched and melted in two steps, consistent with the low- and high-molecular-weight fractions of the polymer as indicated in GPC measurement. This suggested the presence of more than one active species during ethylene polymerization.

In theory, the soft NHC ligand is much more suitable for stabilizing late transition metals than early transition metals. Shen et al. reported the synthesis of Ni based half-sandwich indene-carbene complexes 7 (Fig. 4, left) via imidazolium salt metathesis and indene elimination. The authors revealed that the distance of the Ni–C_carbene bond in the solid structure of 7 was shorter than that of the Ni–P bond in (Ind)-(PPh₃)NiCl, indicating that the σ-donor ability of 1,3-diisopropylimidazol-2-ylidene is stronger than that of PPh₃. Upon activation with modified MAO (MMAO), complex 7 exhibited moderate activity and high selectivity in catalyzing ethylene dimerization into 1-butene.²³ This suggested that β-H elimination via the Ni–Bu intermediate was too fast for chain propagation. Subsequently, a series of cyclopentadienyl Ni carbene analogues 8 (Fig. 4, middle) were synthesized and their catalytic activity for styrene polymerization was tested by Pietrzykowski and co-workers.²⁴ Upon the addition of MAO as the cocatalyst, all Ni complexes 8 efficiently catalyzed styrene polymerization (maximum TON was 4450), producing atactic polystyrenes. The study highlighted that the catalytic activity and polystyrene yield were contingent on the number and stability of cationic Ni centers following treatment with MAO.

Fig. 4 Half-sandwich nickel and iridium carbene complexes.

Jin and co-workers reported the pentamethylcyclopentadiene Ir NHC complex 9 (Fig. 4, right).²⁵ However, in the presence of MAO as the cocatalyst, complex 9 only yielded trace amounts of polymer in the polymerization of norbornene.

Cámpora and coworkers reported the synthesis of single NHC ligand and the H₂O/CH₃CN stabilized cationic allyl-Ni complexes 10 (Fig. 5, top left) through protonating aryloxide-containing precursors.²⁶ The 2,6-ⁱPr₂C₆H₃ substituted Ni complex with an H₂O ligand 10 exhibited moderate catalytic activity in the polymerization of 1,3-butadiene (TOF = 400 h⁻¹) and styrene (TOF = 323 h⁻¹), producing poly(cis-1,4-butadiene) and atactic polystyrene. In comparison with the ⁱPr-substituted 10, the Me-analogue displayed much lower catalytic activity in 1,3-butadiene polymerization (TOF = 19 h⁻¹) but a relatively higher catalytic activity in the selective oligomerization of styrene (TOF = 748 h⁻¹), yielding the head-to-tail dimer. Similarly, single NHC allyl-Pd complexes 10 were synthesized and applied in norbornene polymerization.²⁷ After treatment with AgSbF₆ and LiB(C₆F₅)₄, the Pd complexes showed high activity in catalyzing polymerization of norbornene and polar norbornene derivatives (the TOF up to 7.5 × 10⁵ h⁻¹), even in water as a solvent. Subsequently, Chung and co-workers reported a series of similar NHC allyl-Pd complexes and studied their catalytic activity for the copolymerization of norbornene and ester-functionalized norbornene derivatives.²⁸ A high catalytic activity was observed in all cases, and the SbF₆⁻ counterion-based complexes showed much higher activities than their BF₄⁻ – analogues. The polymerization of ester-functionalized norbornenes was slower than that of norbornene, and the catalytic activity was dependent on the counter anion, reaction solvent, and temperature.

Fig. 5 Nickel and palladium complexes with mixed allyl and NHC ligands.

As an analogue to the allyl complex, the η³-benzylnickel NHC complexes 11 (Fig. 5, top right) were also successfully prepared, and their catalytic activities for ethylene and norbornene polymerization were investigated.²⁹ In the absence of any activators, 11 exhibited excellent catalytic activity in norbornene homopolymerization, while the highest activity was obtained in the presence of a triflate-coordinated imidazolylidene complex. The catalyst possessed considerable thermal stability, and its activity increased steadily as the temperature increased, reaching 6.5 times higher activity at 130 °C than that at 25 °C. Using a η³-substituted allyl group, the corresponding NHC Pd complexes 12 (Fig. 5, bottom left) were employed as air-stable active catalysts for the polymerization of 5-norbornene-2-methyl acetate, after activation with LiB(C₆F₅)₄.³⁰ The catalytic activity and stability increased with the increase of steric hindrance of the allyl substituents, and the maximum activity reached1.23 × 10⁵ g mol⁻¹ h⁻¹.

Bermeshev and co-workers conducted a study on a series of NHC Pd complexes 13 (Fig. 5, bottom right), by investigating their catalytic activity in selectively catalyzing the addition polymerization of norbornene derivatives, specifically 5-methylene-2-norbornene and 5-ethylidene-2-norbornene. Upon activation with a borate, these complexes showed special selective polymerization performance, involving only endocyclic norbornene double bonds while completely retaining the exocyclic double bond.³¹ Similar to other allyl-Pd analogues, these catalytic systems exhibited robustness and excellent thermal stability. The smallest five-membered NHC and/or smaller steric hindrance on nitrogen atoms in NHC showed the highest activity (up to 1.4 × 10⁸ g mol⁻¹ h⁻¹). Remarkably, the polymerization reactions could be conducted in air and using wet solvents with very low Pd-catalyst loadings, as low as 50 ppb. Moreover, the authors successfully achieved the vinyl-addition polymerization of polar norbornene with imide-, ether- and cellosolve groups. This resulted in the production of soluble high-molecular-weight polymers featuring high glass-transition temperatures and good thermal stability.³² The study revealed that both the pre-catalyst and cocatalyst played crucial roles in catalytic performances, and high-yield polymers were obtained when AgSbF₆ and AgBF₄ were used as cocatalysts. Interestingly, NHC Pd complexes in conjunction with only cocatalysts were also capable of producing high-molecular-weight polymers.

Chen and co-workers prepared redox-active naphthoquinone NHC Pd complexes 14 (Fig. 6),³³ which could undergo reduction and re-oxidation by CoCp₂ and [FeCp₂][BAF] (BAF = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate), respectively. In the presence of Na[BAF], the neutral Pd complexes served as effective catalysts in the copolymerization of norbornene, 5-norbornene-2-yl acetate and 1-chloro-1-octyne. However, the reduced analogous catalyst systems exhibited very low activity in these polymerizations. Notably, the addition of the oxidizing agent [FeCp₂][BAF] partly restored the monomer conversion rate of the corresponding reduced catalyst systems, demonstrating a redox-controlled property. The electronic effect of the NHC moiety in ligand was identified as the responsible factor for this redox-controlled phenomenon.

Fig. 6 Palladium complexes with an allyl and a monodentate NHC ligands.

Erker et al. synthesized a serial of bis(NHC)-ligated group 4 metal Zr halide complexes 15 (Fig. 7, left). X-ray diffraction revealed that the carbene ligands are trans-coordinated at the Zr center, and DFT calculations indicated that the NHC ligands played the role of σ-donor in these complexes. Upon activation with a large excess of MAO in toluene, all the complexes 15 exhibited moderate activity (7–7.5 × 10³ g mol⁻¹ bar⁻¹ h⁻¹) in ethylene polymerization, yielding linear polyethylene.³⁴

Fig. 7 Zirconium and nickel complexes with two monodentate NHC ligands.

Dicarbene Ni complexes 16 (Fig. 7, middle) with alkyl N-substituents were employed to catalyze the dimerization of propylene and 1-butene in toluene or imidazolium-based ionic liquids.³⁵ While the complexes were inactive to sparingly active in toluene, highly active species were formed in ionic liquids with good stability. The observed results were attributed to the stabilization by the imidazolium-based ionic liquid on the catalyst, as the complexes rapidly decomposed through the reducing elimination of imidazolium cations in toluene. However, a patent demonstrated that high activities of Me-N-substituted complex 16 could be obtained in hydrocarbon solvents when AlEtCl₂ was used as the cocatalyst in 1-butene dimerization.³⁶ When the N-substituents were aryl groups, the active species formed by the activation of 16 with MAO were also stable enough in toluene to catalyze the dimerization of ethylene, even though the 2,6-ⁱPr₂C₆H₃ substituted 16/MAO catalyst system was very sluggish.³⁷ Braunstein and Wesolek described a series of bis-NHC Ni halide complexes 17 (Fig. 7, right) with ether- and thioether-groups. Due to the presence of two asymmetric NHC ligands, all the complexes featured syn/trans isomerization, and X-ray diffraction showed that no intra- or intermolecular interaction existed between the Ni center and the (thio)ether group.^38,39 With AlEtCl₂ as a cocatalyst, all the complexes were active for ethylene oligomerization, with productivities reaching up to 1.81 × 10⁴ gC₂H₄ gNi⁻¹ h⁻¹. Due to the inherent non-coordination of pendant (thio)ether groups, the steric effect of the N-substituent or the R group attached to the ether or thioether at the NHC ligand was the key factor exerting influence on the catalytic activity.

Cui and co-workers reported the synthesis of the NHC-stabilized rare-earth phosphide Yb complex 18 (Fig. 8) through the reaction of [(THF)₄Yb(PPh₂)₂] with three equivalents of 1,3,4,5-tetramethylimidazol-2-ylidene. Complex 18 exhibited activity in the polymerization of styrene in toluene or THF, yielding atactic polymers. Notably, the polymerization of styrene occurred even more rapidly in a solvent-free environment. When the monomers for polymerization were 1-chloro-4-vinylbenzene and 1-methoxy-4-vinylbenzene, the yields were 43% and 22% within 20 hours, respectively.⁴⁰

Fig. 8 Ytterbium complex with three monodentate NHC ligands.

3. Complexes with bidentate NHC ligands

3.1 Bidentate NHC ligand containing neutral functionality

Theopold and co-workers prepared several Cr^III complexes 19 (Fig. 9, left).⁴¹ Upon activation with MAO or excess Et₂AlCl, these complexes catalyzed the polymerization of ethylene with low activity (1.87 × 10³ g mol⁻¹ bar⁻¹ h⁻¹) and produced the polymer with a broad molecular weight distribution. Detailed studies revealed that the reaction of mononuclear complexes 19 with commonly used chloride abstraction reagents generated dinuclear, chloride-bridged alkyl cations, suggesting that a binuclear polymerization mechanism could not be ruled out. The bis-carbene Cr^III complexes 19 were easily reduced to the Cr^II analogues, which were inactive for ethylene polymerization under identical conditions. Braunstein and Danopoulos prepared phosphanyl-substituted NHC Cr^II/Cr^III complexes 20 (Fig. 9, middle) and 21 (Fig. 9, right).⁴² The treatment of phosphanyl- and diphosphanyl-substituted NHC ligands with either Cr^II or Cr^III chlorido precursors formed Cr^II complexes 20, which could be further transformed to the benzyl Cr^III counterpart 21 by reacting with Mg(benzyl)₂(THF)₂. Upon activation with MAO, the Cr^III complexes were active in ethylene oligomerization, displaying catalytic performances superior to those of the Cr^II precursors. Notably, the oligomers generated by the 2,6-ⁱPr₂C₆H₃ substituted Cr^II complex 20 were almost exclusively 1-hexene and 1-butene when the reaction was initiated at 30 °C. When AlEtCl₂ was used as a cocatalyst, all the Cr complexes predominantly produced polyethylene.

Fig. 9 Bis-carbene and phosphine-functionalized NHC chromium complexes.

A patent reported imine-functionalized NHC Cr, Fe, Co, and Pd complexes 22 (Fig. 10, top left),⁴³ which were tested for ethylene polymerization after treatment with MAO. The activities of Cr complexes were very low, and the Co complex showed activity as low as 2 × 10³ g mol⁻¹ bar⁻¹ h⁻¹, while the Fe complex was inactive in ethylene polymerization. The highest activity of the Pd complex was 4 × 10³ g mol⁻¹ bar⁻¹ h⁻¹ with a bulky 2,6-diisopropylphenyl substituent in the NHC ligand. McGuinness et al. reported the bidentate carbene-pyridine Cr complex 23 (Fig. 10, top right) and carbene-thiophene Cr complex 24 (Fig. 10, bottom left) with the aim of preparing ethylene oligomer.⁴⁴ When activated with cocatalysts, 23 did not produce any ethylene oligomers but rather bits of polyethylene with low activity (TON = 710), while 24 produced 77% polyethylene and 23% soluble oligomers with a TON 1030.

Fig. 10 Metal complexes with neutral N or S-functionalized NHC ligands.

Aryl-substituted imine NHC group 4 and 6 transition metal complexes 25 (Fig. 10, bottom right) were synthesized by Lavoie and co-workers. The Ti and Zr complexes were found to be active for ethylene polymerization at room temperature after the activation of MAO, with the Zr complex exhibiting higher activity and a productivity of 1.4 × 10⁵ g mol⁻¹ h⁻¹.⁴⁵ Analogous imine-NHC Pd complexes 25 with bulky substituents on both the imine and the NHC groups were tested for norbornene polymerization.⁴⁶ After activation with MMAO, the Pd dichloride complexes and the corresponding methyl-Pd complexes exhibited low activities for the addition polymerization of norbornene. Treatment of the methyl-Pd complexes with Na[BAF] in CH₃CN yielded cationic Pd complexes, which also smoothly polymerized norbornene. Moreover, the polymerization experiments demonstrated that bulky substituents on both the imine and NHC sides could suppress the reductive elimination of methyl-Pd active species.

Since the pyridine motif was frequently utilized to construct a chelating ligand, various pyridine modified NHC ligands were also synthesized, and their corresponding transition metal complexes were widely studied in olefin polymerization. Jin and coworkers reported the picolyl-functionalized ligand and its corresponding Ni and Pd complexes. X-ray single-crystal analyses revealed that two NHC ligands chelate the Ni center metal ions in 26 (Fig. 11, top left), forming a square planar configuration,⁴⁷ while the Pd complexes 27 (Fig. 11, top right) contain one NHC ligand.⁴⁸ After activation with MAO, the Ni complex 26 catalyzed ethylene polymerization with a moderate activity of 3.3 × 10⁵ g mol⁻¹ h⁻¹, producing linear polyethylene. The 26/MAO catalytic system also promoted norbornene polymerization with a high activity (TOF = 2.44 × 10⁴ h⁻¹).⁴⁷ Similarly, the Pd complex 27 exhibited ultra-high catalytic activities (up to 7.7 × 10⁷ g mol⁻¹ h⁻¹) for the addition polymerization of norbornene after treatment with MAO.⁴⁸ By modifying the steric effects of the C,N-bidentate pyridine-functionalized NHC ligand, Albrecht and co-workers found that the catalytic activities of Pd complexes 28 (Fig. 11, bottom left) could be easily regulated for olefin polymerization.⁴⁹ Interestingly, the flexible methylene-linked pyridine NHC Pd complex was inactive in ethylene polymerization. Compared to complex 27 with moderate catalytic activity, the bulky substituent on the NHC side was supposed to be the main reason that retarded the coordination and insertion of ethylene. When pyridine was directly linked to the NHC motif, the corresponding Pd complexes catalyzed the oligomerization of ethylene in the case of H-group and the dimerization of ethylene in the case of mesityl group on the pyridine ring, respectively. Moreover, all three complexes could catalyze the dimerization of styrene. The same authors later prepared cationic pyridine NHC Pd complexes 29 (Fig. 11, bottom right) that contained bulky substituents on the pyridine motif.⁵⁰ Ethylene polymerization experiments demonstrated that the large steric substituent of the NHC motif promoted β-H elimination and gave the product as mainly dimers or oligomers.

Fig. 11 Pyridine-functionalized NHC nickel and palladium complexes.

The cis dihalido-bis-NHC Ni complex 30 (Fig. 12, left) with amine substitution was synthesized and tested for olefin polymerization.⁵¹ Both the transformation of ligand lithium and silver-NHC transmetalating methods gave complex 30 with an uncoordinated amine motif. It was inactive in ethylene polymerization but showed moderate catalytic activity (0.5–3.0 × 10⁶ g mol⁻¹ h⁻¹) towards the addition polymerization of norbornene when MAO was used as the cocatalyst. The neutral NHC ligand with a pendant-NMe₂ donor group and the corresponding Ni complexes 31 (Fig. 12, right) were also studied in the catalysis of ethylene oligomerization. In the presence of MAO as a cocatalyst, the complexes 31 exhibited good activity for ethylene oligomerization, with the products mainly being C₄ and C₆ fractions.³⁹ The amine-chelated Ni complex was observed to possibly stabilize and increase the active species’ lifetime.

Fig. 12 Nickel complexes with a neutral amine-functionalized NHC ligand.

Bidentate NHC phosphine oxide and imidazo[1,5-a]pyridine-NHC phosphine oxide Pd complexes 32 (Fig. 13, left) and 33 (Fig. 13, right) were studied for ethylene polymerization and ethylene/polar monomer copolymerization by Nozaki and co-workers.⁵² Complexes 32 exhibited very poor catalytic activity and produced ethylene oligomers. Complexes 33 showed a catalytic activity of up to 1.52 × 10⁵ g mol⁻¹ h⁻¹ for ethylene polymerization at 50 °C, despite with rapid decomposition within 1 hour. Moreover, the complexes 33 were also active in catalyzing the copolymerization of ethylene with polar monomers such as methyl acrylate, allyl acetate, and allyl chloride, albeit with low catalytic activity.

Fig. 13 NHC phosphine oxide palladium complexes.

Pyridine and picolyl-functionalized pentamethylcyclopentadiene NHC Ir complexes 34 (Fig. 14, left) and 35 (Fig. 14, right) were used to catalyze the addition polymerization of norbornene.²⁵ In the presence of cocatalyst MAO, complex 34 exhibited a higher catalytic activity of 1.7 × 10⁶ g mol⁻¹ h⁻¹ and complex 35 showed moderate catalytic activity (3.03 × 10⁵ g mol⁻¹ h⁻¹). Detailed studies indicated that the hemilabile coordinated pyridine nitrogen could easily cleave off to form an empty site around the central metal, which was essential for the coordination of monomers.

Fig. 14 Half-sandwich iridium complexes with pyridine-substituted NHC ligands.

3.2 Bidentate NHC ligand containing anionic functionality

Theoretically, the introduction of one or more linked anionic functionalities is much more beneficial for stabilizing the metal centers than the cases of NHC carbene solely or neutral functionalities involved because the bond energies of ionic bonds are usually higher than that of coordination bonds. Indenyl and fluorenyl functionalized NHC lanthanide complexes 36 (Fig. 15, left) and 37 (Fig. 15, middle) were reported by Cui and co-workers.^53–58 Complex 36 was the first example of NHC-stabilized monomeric rare-earth metal bis(alkyl) complex. Upon treatment with [Ph₃C][B(C₆F₅)₄] and AlⁱBu₃, all the Sc complexes were inactive for isoprene polymerization, while the complexes with other metal centers exhibited moderate activities, yielding highly 3,4-regular polyisoprene. Generally, the fluorenyl NHC-stabilized rare-earth metal (Y, Lu and Ho) complexes 37 showed higher 3,4-regio-selectivity in isoprene polymerization than their indenyl analogues 36 because the relatively bulkier fluorenyl ligand was more beneficial for regularizing the insertion of isoprene in a determined fashion.^53,54 Moreover, the fluorenyl NHC-stabilized complexes 37 were also used in the copolymerization of ethylene with 1-hexene or 1-octene. Compared to the inactivity of Dy and Er complexes, the Sc complex showed high activity (4.12 × 10⁶ g mol⁻¹ h⁻¹ atm⁻¹) towards the copolymerization of ethylene and 1-hexene with a high comonomer insertion ratio (up to 20.2% 1-hexene insertion) after treatment with AlⁱBu₃ and [Ph₃C][B(C₆F₅)₄]. In addition, this Sc ternary system catalyzed the copolymerization of ethylene and 1-octene with a good catalytic activity (up to 3.64 × 10⁶ g mol⁻¹ h⁻¹ atm⁻¹). By changing the feeding ratio of 1-octene, the contents of the poly(1-octene) sequence in the polymers ranged from 2.1% to 38.7%.⁵⁵ Also, the Sc complex 37 gave an adjustable ratio of the polystyrene sequence (16.1 mol% to 43.2 mol%) in the copolymerization of ethylene and styrene derivatives, giving the products with a pseudo-random distribution.⁵⁶ Amazingly, the Sc complex 37 displayed moderate activity towards the direct copolymerization of ethylene with a range of polar and nonpolar styrenes. The N-methyl substituted fluorenyl NHC-Sc complex showed the highest copolymerization activity of 3.19 × 10⁵ g mol⁻¹ h⁻¹, offering pseudo-alternating or perfect alternating copolymers regardless of the polar comonomers used.⁵⁷ Meanwhile, this Sc complex also showed a high catalytic activity (1.64 × 10⁶ g mol⁻¹ h⁻¹) towards the copolymerization of ethylene and 1,4-butadiene, producing ethylene/1,4-butadiene alternating copolymers with high molecular weights and trans-poly(1,4-PBD)-dominant microstructure. The vulcanized E/BD copolymer rubber had excellent physical and mechanical performance, with some properties even surpassing the commercialized natural or styrene–butadiene rubber.⁵⁸

Fig. 15 Indenyl and fluorenyl NHC-stabilized rare-earth metal and chromium complexes.

Hanton and Danopoulos et al. described a family of Cr^III and Cr^II complexes bearing analogous dimethylindenyl-functionalized NHC ligands. In particular, the Cr^II complexes could convert to dimeric complexes when placed in THF for an extended period of time. In the presence of MMAO, all the Cr^III complexes 38 (Fig. 15, right) showed moderate activity for ethylene polymerization.⁵⁹ Mechanistic studies demonstrated the cationic nature of the active species during the polymerization.

Before the development of indenyl and fluorenyl-functionalized NHC lanthanide complexes 36 and 37, Shen et al. reported the Ind-NHC ligated Ni complex 39 (Fig. 16). Their study showed that 39 was much more thermally stable than the corresponding complex bearing the indenyl and free NHC ligands. This suggested the special advantage of indenyl-functionalized NHC ligands in stabilizing the metal center. After treatment with NaBPh₄, complex 39 showed excellent activity for styrene polymerization.⁶⁰ More importantly, 39 also exhibited higher catalytic activity than its analogue bearing the indenyl and free NHC ligands, highlighting the role of the indenyl-functionalized NHC ligand in stabilizing the Ni-based active species during polymerization.

Fig. 16 Indenyl NHC-ligated nickel complex.

The alkoxide-functionalized NHC Zr complexes 40 (Fig. 17, top left) and 41 (Fig. 17, top right) have been reported to catalyze the polymerization of ethylene and propylene.^61,62 Upon the activation of MAO, the cyclopentenyl-containing complex 40 produced linear polyethylene with a high molecular weight and broad polydispersity (M_w/M_n > 2, bimodal). The same catalytic system could also generate highly isotactic polypropylene and an atactic fraction. DFT study of the stability of the catalyst's stereoisomer indicated the presence of more than one active species during the polymerization. In the catalytic systems of cyclohexenyl-containing Zr/MAO, the activity increased steadily with an increasing molar ratio of Al/Zr from 500–3000. However, for the ethenyl alkoxide Zr complex 41, the activity reached a plateau when the molar ratio of Al/Zr was 1000. Nonetheless, the molecular weight of the generated polyethylene always increased in all cases with an increasing Al/Zr ratio, suggesting that β-H elimination had been prevented. Also, more than one active species were formed during the 41-catalyzed ethylene polymerization, similar to the case of 40. Grubbs et al. reported novel Ti and Zr complexes 42 (Fig. 17, bottom left) bearing bidentate phenoxide-functionalized NHC ligands. In the presence of MAO, the Ti complexes 42 were efficient catalysts for ethylene polymerization and its copolymerization with 1-octene and norbornene, generating high-molecular-weight products. However, the analogous Zr catalytic system only gave low-molecular-weight polymers.⁶³ The same Ti complex was also found to serve as an efficient catalyst for the copolymerization of styrene, yielding the copolymers with highly syndiotactic polystyrene sequence.⁶⁴

Fig. 17 Alkoxide-functionalized NHC titanium and zirconium complexes.

Buchmeiser and coworkers reported Ti complexes 43 (Fig. 17, bottom right) with mixed ligands, which contained a bidentate O-chelating NHC and aryloxo ligands.⁶⁵ Due to the subtle balance of steric and electronic effects within the complex structures, Ti complexes 43 efficiently suppressed β-H elimination in the homopolymerization of ethylene and its copolymerization with cyclic olefins, generating high-molecular-weight copolymers.

A family of special enolate-NHC ligands and their ligated neutral Ni complexes 44 (Fig. 18, left) have been reported by Waymouth and co-workers. In the absence of cocatalysts, 44 catalyzed ethylene polymerization with rapid deactivation via β-H elimination to form an unstable Ni–H species, producing a linear polyethylene with low molecular weight. The neutral complexes 44 also showed activity for propylene oligomerization to give low-molecular-weight atactic oligomers.⁶⁶ In a subsequent study, similar enolate-NHC allyl complexes 45 (Fig. 18, right) were prepared and tested for ethylene polymerization.⁶⁷ Different from the neutral Ni analogues 44, the complexes 45 were completely inactive without cocatalysts, and the nitrobenzene-substituted complex showed low activity for ethylene polymerization with the activation of ZnEt₂, producing highly linear but low-molecular-weight polymers. It should be noticed that the nitrobenzene-substituted 45/ZnEt₂ catalytic system had a much longer lifetime than complex 44 for ethylene polymerization, indicating the good stability of the active species during 45-catalyzed polymerization.

Fig. 18 Enolate-NHC ligated nickel complexes.

Zhang et al. reported an aryloxy-substituted NHC ligand, and the corresponding Ni complex 46 (Fig. 19, left) showed low activity (10⁵ g mol⁻¹ h⁻¹) towards the addition polymerization of norbornene with MAO as a cocatalyst. Compared to its phosphine analogues, 46 was much less sensitive to air and moisture due to the NHC's strong donor properties.⁶⁸ Wang and Yan et al. prepared several air- and moisture-stable Ni complexes 47 (Fig. 19, right) bearing o-hydroxyaryl NHC ligands.^69–71 X-ray diffraction studies revealed two aryloxide-NHC ligands located in a cis arrangement around the Ni ion. After treatment with MAO, these Ni complexes exhibited moderate to high activities (up to 14.5 × 10⁶ g mol⁻¹ h⁻¹) in the addition polymerization of norbornene, which were much more active than 46 (10⁵ g mol⁻¹ h⁻¹) with a flexible ligand framework. Interestingly, the steric effect of N-substituents at the NHCs slightly influenced the catalytic activities of 47. When R = -CH₂Py, complex 47 showed a low activity in norbornene polymerization because of its poor thermal stability on activation with either Et₂AlCl or Me₂AlCl.

Fig. 19 Nickel complexes bearing aryloxy-substituted NHC ligands.

Later, the corresponding bis(aryloxide-NHC) Pd complexes were synthesized by Wang et al. When R = Me, ⁱPr and CH₂Ph, trans isomers 48 (Fig. 20, left) were obtained while cis isomers 50 (Fig. 20, right) were isolated for R = Ph and mesityl. But for R = ^tBu, a cis isomer 49 (Fig. 20, middle) with a normal and an abnormal NHC ligand was obtained.^72,73 When MAO was used as the cocatalyst, these bis(aryloxide-NHC) Pd complexes (48, 49 and 50) exhibited excellent catalytic activities (up to 49.97 × 10⁶ g mol⁻¹ h⁻¹) in the addition polymerization of norbornene. Like the case of Ni complexes 47, the N-substituents at the NHCs and the configurations of the complexes 48, 49 and 50 had slight influence on the activity.

Fig. 20 Bis(aryloxide-NHC) palladium complexes.

By changing the Pd reactants, Wang et al. synthesized and isolated the mono(aryloxide-NHC) palladacycle complexes (51, 52, 53, Fig. 21) successfully.⁷⁴ Similar to its bis(aryloxide-NHC) analogue 49, complex 52 also bonded to an abnormal NHC ligand when the N-functional group on the NHC was tertbutyl. The N-chelated and o-aryloxide NHC ligated Pd complexes 51 showed excellent catalytic activities in the addition polymerization of norbornene with MAO as the cocatalyst, and the highest activity was 4.95 × 10⁷ g mol⁻¹ h⁻¹. Noticeably, complex 52 (2.72 × 10⁷ g mol⁻¹ h⁻¹) also exhibited high catalytic activity for norbornene polymerization upon the activation of MAO. Meanwhile, the N-chelated C(sp²)–Pd NHC complex 53 displayed slightly lower catalytic activity than 51 under the same polymerization conditions, perhaps because of the higher reactivity of C(sp³)–Pd bond compared to the C(sp²)–Pd bond.

Fig. 21 Mono(aryloxide-NHC) palladacycle complexes.

Wang and co-workers further synthesized the analogous σ,π-cycloalkenyl Pd complexes 54–56 (Fig. 22).⁷⁵ Note that complexes 56 were generated with a free hydroxyl group of the phenol when the N-functional group on the NHC was bulky tert-butyl, by using the similar procedures as for complexes 54 and 55. Although complexes 54 and 55 had similar structures with neutral single-component catalysts, however, the complexes had no catalytic activity for the polymerization of norbornene in the absence of cocatalysts. By treatment with MAO, complexes 54 and 55 showed excellent catalytic activities (the highest was 4.9 × 10⁷ g mol⁻¹ h⁻¹) for norbornene polymerization. Meanwhile, the 56/MAO system also displayed comparable catalytic activity (3.2 × 10⁷ g mol⁻¹ h⁻¹), and it was supposed that the active species might be a “Pd” cation without the o-aryloxide-substituted NHC ligand.

Fig. 22 Mono(aryloxide-NHC) cycloalkenyl palladium complexes.

The power of phosphine-sulfonate ligated Ni/Pd complexes for ethylene copolymerization stimulated Wang and coworkers to prepare a family of analogous NHC-arenesulfonate ligands and their corresponding Pd complexes. In the presence of MAO, the N-chelated C(sp³), NHC-sulfonate palladacycles 57 (Fig. 23, left) showed higher catalytic activities (up to 5.84 × 10⁷ g mol⁻¹ h⁻¹) than the corresponding C(sp²) complex 58 (up to 2.97 × 10⁶ g mol⁻¹ h⁻¹; Fig. 23, middle) in the addition polymerization of norbornene. Complex 58 possessed high thermal stability at 100 °C, and the activities increased with temperature increase.⁷⁶ The lutidine-coordinated analogues 59 (Fig. 23, right) were then prepared, and after treatment with MAO or [Ph₃C][B(C₆F₅)₄], all the complexes 59 showed high catalytic activities in norbornene polymerization (up to 1.01 × 10⁸ g mol⁻¹ h⁻¹).⁷⁷ For complexes 57–59, the steric hindrance effect of N-substituents of NHC ligands had a slight influence on the catalytic activities.

Fig. 23 NHC-sulfonate palladium complexes.

Another kind of NHC-arenesulfonate ligands, namely imidazo[1,5-a]pyridine-sulfonate, were prepared, and their palladacycle complexes 60–62 (Fig. 24) served as excellent catalyst precursors for norbornene polymerization.⁷⁸ In the presence of MAO or Et₂AlCl as a cocatalyst, palladacycle complexes 60–62 exhibited high activities (up to 6.0 × 10⁷ g mol⁻¹ h⁻¹) towards norbornene polymerization. High thermal stability of 60–62/MAO was observed, and the catalytic activity reached the maximum at 100 °C. Various aspects, such as the palladacycle structure, the coordination atom type and the substituent of the ligand, significantly affected the polymerization activity. Later, Wang and coworkers reported the allyl Ni and Pd complexes 63 (Fig. 24, bottom right) chelating similar imidazo[1,5-a]pyridine-sulfonate ligands.⁷⁹ In the presence of Me₂AlCl or MAO, the Ni and Pd complexes displayed high activities (4.5 × 10⁷ g mol⁻¹ h⁻¹) and high thermal stability in norbornene polymerization. Moreover, the Pd complexes could also catalyze the copolymerization of norbornene with butyl vinyl ether (BVE), 10-undecenoate (UA), and methyl acrylate (MA) with high insertion content of polar monomers. However, the corresponding Ni catalysts did not catalyze the copolymerization of norbornene and ester monomers efficiently.

Fig. 24 NHC-arenesulfonate nickel and palladium complexes.

By a rational design to suppress β-H elimination, Nozaki et al. prepared novel tethered NHC ligands (imidazo[1,5-a]quinolin-9-olate-1-ylidene (IzQO)) and their corresponding Pd complexes 64 (Fig. 25, left), which were successfully applied as catalyst precursors for olefin polymerization and olefin/polar monomer copolymerization.⁸⁰ This study proved that the rigid skeleton and coplanar configuration of the NHC plane with the metal plane were essential for Pd/IzQO complex 64 as efficient catalysts to produce polymers with considerable catalyst durability. The complexes 64 could promote the homopolymerization of ethylene, propylene, and 1-butene, giving high-molecular-weight linear polyethylene, and moderate-molecular-weight polypropylene and poly(1-butene), respectively. The applied high polymerization temperature (80–120 °C) proved the high thermal stability of 64 as polymerization catalysts. Also, 64 successfully realized the copolymerization of ethylene or propylene with polar monomers, such as allyl carboxylate copolymers and methyl acrylate, and so on.

Fig. 25 IzQO and aImPy NHC-phenolate nickel and palladium complexes.

As powerful catalysts, 64 were further utilized to polymerize diverse monomers. Using 64 as catalysts, Nozaki et al. successfully copolymerized ethylene with polar-group-functional 1,1-disubstituted ethylenes, yielding the copolymers in which polar units were non-sequentially incorporated into the main chain for the first time. An analysis of the microstructure of resultant copolymers revealed the poly(methyl methacrylate) (PMMA) sequences mainly in the 1,2-insertion mode and the insensitivity towards steric hindrance of the Pd /IzQO system. Detailed investigation suggested that copolymerization indeed proceeded via a coordination–insertion mechanism.^81,82 Nozaki and co-workers further studied the activity of Pd/IzQO complexes 64 in the copolymerization of ethylene with 3-ethylidene-6-vinyltetrahydro-2H-pyran-2-one (EVP), which acted as a CO₂/butadiene-derived lactone.⁸³ The coordination/insertion copolymerization of ethylene and EVP proceeded slowly, generating divergent unsaturated lactone-functionalized polyethylenes in the main chain with a considerable EVP incorporation ratio (2.0 mol%) and moderate copolymer molecular weight (M_n = 6000). Michael addition or aminolysis were successfully accomplished to further modify the resultant copolymers. This study provided a novel method to access various polyethylene materials made from ethylene, CO₂, and 1,3-butadiene. The complex 64 could also catalyze the copolymerization of ethylene with methyl 5-norbornene-2-carboxylate, giving a copolymer with a 7.9 mol% incorporation ratio of methyl 5-norbornene-2-carboxylate (endo-/exo-mixture). Similar catalytic performance of 64 in the copolymerization of propylene with polar norbornenes was observed.⁸⁴

Due to the success of Pd/IzQO complexes in olefin copolymerization and the urgent requirement for replacing precious metals in catalysis with readily available metals, the IzQO-ligated Ni complexes 65 (Fig. 25, middle) were successfully synthesized and applied for ethylene polymerization at 50–100 °C without a cocatalyst.⁸⁵ Addition of Ni(cod)₂ was beneficial for increasing the catalytic activity of 65 and the molecular weight of resultant polyethylenes. The copolymerization of ethylene and allyl acetate could also be realized in the catalysis of 65 and produced a copolymer with a low molecular weight, which was increased after the addition of Ni(cod)₂ in the catalytic system. It is worth noting that the Ni complexes 65 exhibited relatively lower catalytic activity than their corresponding Pd/IzQO complexes in ethylene/allyl acetate copolymerization under similar conditions. Bulkier substituents on the N atom were highly beneficial for increasing the molecular weight of ethylene/allyl acetate copolymers, but the insertion rate of the polar monomer decreased significantly.

Abnormal imidazo-[1,5-a]pyridine(aImPy)-based NHC-phenolate ligands with a tuneable electronic environment and a rigid tricyclic framework were synthesized, and their Pd complexes 66 (Fig. 25, right) showed good efficiency in catalyzing the homopolymerization of ethylene/propylene and copolymerization of ethylene/polar monomers.⁸⁶ High-molecular-weight (M_n = 2.37 × 10⁵ g mol⁻¹) linear polyethylene and regio-defect-free moderate-molecular-weights polypropylene were produced. Good tolerance to polar groups enabled these catalytic systems to copolymerize ethylene and polar monomers with good activity, resulting in high incorporation ratios (up to 3.0%) of polar monomers. In addition, MMA was also successfully incorporated into the main chain of polyethylene. DFT calculation studies indicated that the energy of ethylene insertion into a Pd–alkyl bond by aImPy-Pd catalyst 66 was lower than that by a Pd catalyst 64 bearing the corresponding normal NHC ligand.

The saturated tetrahydroimidazo[1,5-a]quinolin-9-olate-1-ylidene (SIzQO) NHC-phenolate Pd complexes 67 (Fig. 26, left) and 68 (Fig. 26, middle) were also prepared and applied in the polymerization of ethylene.⁸⁷ They catalyzed the polymerization of ethylene in the range of 100–120 °C with activities of 3.6–5.3 × 10³ g mol⁻¹ h⁻¹. Compared with previously reported Pd/IzQO complexes 64, 68 had catalytic activities lower by more than 100 times in ethylene polymerization. To tune the catalytic property, Nozaki and co-worker synthesized the Pd/IzQO complex 69 (Fig. 26, right) bearing oligoethylene glycol moieties bound to the quinolinolate part.⁸⁸ Comparing with the reaction without the salt, the addition of LiBAr^F₄ increased the activity of polymerization and decreased the molecular weight with a slight increase in MA incorporation. This was attributed to the fact that the addition of Lewis acid decreased the electron donation ability of the oxygen atom towards Pd ion. Also, the addition of Lewis acid accelerated the chain transfer reaction during polymerization. Notably, the counteranion of lithium also exerted an obvious influence on the catalyst performance: LiB(C₆F₅)₄ showed a similar polymerization result to LiBAr^F₄, while the LiPF₆ and LiSbF₆ suppressed the MA incorporation.

Fig. 26 SIzQO and IzQO NHC-phenolate palladium complexes.

Nozaki and co-workers also prepared the NHC-sulfonamide Pd complexes 70 (Fig. 27) and tested their catalytic activities in ethylene oligomerization and ethylene/polar monomer copolymerization.⁸⁹ Complex 70 with a bulky 2,4,6-Me₃C₆H₂-substituent exhibited high activity for ethylene oligomerization up to 1.27 × 10⁵ g mol⁻¹ h⁻¹. After the induction period of oligomerization, the catalyst slowly deactivated during the subsequent oligomerization process. The copolymerization of ethylene and MA catalyzed by 70 also gave a co-oligomer, which was shown to have the telechelic microstructure of the resultant polymer with an MA-moiety at the chain ends.

Fig. 27 NHC-sulfonamide palladium complexes.

4. Complexes with tridentate NHC ligands

4.1 Tridentate NHC ligand containing neutral functionality

Tridentate NHC pincer ligands with mixed N-imine and N′-amine substituents have been synthesized, and their corresponding Cr/Ni complexes 71 (Fig. 28, left) and 72 (Fig. 28, right) served as efficient catalysts for ethylene oligomerization by the activation of suitable cocatalysts.⁹⁰ The C₂ linked Cr complex 71 in ethylene oligomerization showed moderately activity after treatment with MAO, and the longer C₃ linked complex 71 showed higher productivity (up to 9.2 × 10³ gC₂H₄ gNi⁻¹ h⁻¹) and a very high selectivity for α-olefins from C₄ to C₈. Moreover, the analogous Ni complexes 72 showed a productivity of up to 1.46 × 10⁴ gC₂H₄ gNi⁻¹ h⁻¹, and the generated oligomer consisted of 65% butenes and 30% hexenes.

Fig. 28 Mixed N-imine and N′-amine substituented NHC chromium and nickel complexes.

Bis(carbene)pyridine transition metal complexes 73 (Fig. 29, left) have been studied in ethylene oligomerization and polymerization by Gibson and co-workers.^91,92 After treatment with MAO, the 2,6-ⁱPr₂C₆H₃ substituted Cr^III complexes 73 showed a high activity (4.044 × 10⁷ g mol⁻¹ bar⁻¹ h⁻¹) for ethylene oligomerization, generating α-olefins as the main product with the 80% to 93% selectivity. Increased steric bulk of ligand increased the K value (K = k_prop/(k_prop + k_{ch transfer}) = mol of C_n+2/mol of C_n) of the resultant oligomer.⁹¹ In combination with MAO, bis(carbene)pyridine Ti and V complexes gave rise to highly active ethylene polymerization catalysts, and the V complexes (up to 1.446 × 10⁶ g mol⁻¹ bar⁻¹ h⁻¹) exhibited relatively higher catalytic activity than Ti analogues (up to 7.91 × 10⁵ g mol⁻¹ bar⁻¹ h⁻¹). Both Ti and V catalytic systems produced high-molecular-weight polyethylene with no significant branching. The bis(carbene)pyridine Fe^II, Fe^III and Co complexes were also prepared and tested for ethylene oligomerization/polymerization. However, all these complexes were inactive when treated with MAO. The study revealed that the Fe and Co complexes might decompose via reductive elimination of an alkylimidazolium cation by MAO. This suggested that the bis(carbene)pyridine ligand was less suitable for Fe and Co olefin polymerization catalysts compared to the earlier transition-metal analogues.⁹² The ethylene oligomerization mechanism with bis(carbene)pyridine Cr complexes has been investigated and suggested that this system produced α-olefins via an extended metallacycle mechanism. When the metallacycle mechanism was applied in a bis(carbene)thiophene Cr complex 74/MAO (Fig. 29, right) catalytic system, no catalytic activity for ethylene oligomerization was observed even at ethylene pressures up to 30 bar.⁴⁴ Bis(carbene)pyridine Cr complexes 73 also efficiently catalyzed α-olefin oligomerization after activation by MAO, leading mainly to head-to-tail dimers with vinylidene unsaturation. First-order in Cr and zero-order in 1-octene concentration were found in the kinetics investigation of 1-octene dimerization, and the most likely possibility involving metallacycles mechanism was also determined.⁹³

Fig. 29 Bis(carbene)pyridine/thiophene metal complexes.

Aryl-substituted imine NHC ligands that could serve as tridentate chelating ligands and their corresponding 1,3-bis(imino)-NHC Cr, Fe, Co complexes 75 (Fig. 30) were prepared by Lavoie and co-workers. X-ray diffraction and NMR spectra showed that the 1,3-bis(imino)-NHC ligands coordinate the central metal ions via a bidentate mode through the C_carbene and one of the two N_iminic atoms in these complexes. After treatment with MAO, the Cr complexes showed moderate activities for ethylene polymerization, giving a linear polymer. However, neither the Fe nor the Co complexes were active for ethylene polymerization. The decrease of electron-donating or the increase of π-accepting ability on the ligand would increase the catalytic activity of the Cr complexes in olefin polymerization.⁹⁴

Fig. 30 Bis(imino)-NHC chromium, iron, cobalt complexes.

4.2 Tridentate NHC ligand containing monoanionic functionality

Cui et al. successfully synthesized the amidino-functionalized NHC ligated lanthanide bis(alkyl) complex 76 (Fig. 31, left). In the presence of an organoborate, the Lu complex showed excellent catalytic activity for isoprene polymerization, producing 3,4-regioregular (99.3%) polymer with livingness. Additionally, the Lu-polyisoprene active species could trigger the polar ε-caprolactone ring-opening polymerization, yielding poly(3,4-isoprene)-b-polycaprolactone diblock copolymers with steerable molecular weight (ranging from 4.9 × 10⁴ to 10.2 × 10⁴) and narrow polydispersity.⁹⁵ Activated by [Ph₃C][B(C₆F₅)₄], complexes 76 also catalyzed the polymerization of 2-phenyl-1,3-butadiene with high activities and 3,4-selectivities (96.9%). The resulting polymers represented novel plastic polydienes with high glass-transition temperatures. Both polymerization temperature and solvent polarity impacted the 3,4-selectivity, which decreased accordingly with decreasing solvent polarity and increasing polymerization temperature.⁹⁶

Fig. 31 Rare-earth metal complexes bearing a NHC ligand with a monoanionic functionality.

The CCC-pincer 2,6-xylenyl bis(carbene)-ligated lanthanide dibromides 77 (Fig. 31, right) were obtained via dropwise addition of ⁿBuLi to a solution of imidazolium dibromide pro-ligands and LnCl₃.^97,98 In the presence of AlR₃ (R = Me, Et, ⁱBu) and [Ph₃C][B(C₆F₅)₄], Y, Nd, Gd, Dy, and Ho complexes catalyzed isoprene polymerization with high activity and cis-1,4 selectivity (99.6%, 25 °C), while Sc, Sm, La, Tm, and Lu complexes were inactive. Notably, these catalytic systems exhibited high thermostability, catalytic activity and high cis-1,4 selectivity (97.4%) even at 80 °C. In situ NMR spectra suggested that the Y hydrido aluminate cation was probably the active species for isoprene polymerization.

The Ni complex 78 (Fig. 32, top left), chelating a salicylaldimine-functionalized tridentate NHC ligand, was successfully synthesized using strategies involving both metal transformation reaction and nickelocene elimination reaction. X-ray analysis revealed a twisted square-planar geometric structure for the complex.⁹⁹ Following the treatment with NaBPh₄ at 80 °C, the system exhibited good activity in styrene polymerization. Wang and co-workers reported tridentate phenoxide-functionalized NHC Ni/Pd complexes 79 (Fig. 32, top right) and 80 (Fig. 32, bottom left).^70,73,100 Upon treatment with MAO or Et₂AlCl, these Pd complexes displayed excellent catalytic activities (up to 5.99 × 10⁷ g mol⁻¹ h⁻¹) in the addition polymerization of norbornene. Notably, the tridentate Pd complexes 79 and 80 exhibited higher activities compared to the bidentate bis(aryloxide-NHC) Pd analogue 50 in the presence of MAO. Interestingly, polymers produced by 79 or 80/Et₂AlCl were soluble in CHCl₃, and the polymerization initiated by these Pd complexes followed a vinyl-type addition mechanism.⁷³ The corresponding Ni complexes 79 and 80 were further investigated for norbornene polymerization and copolymerization of norbornene with 1-octene upon activation with suitable cocatalysts.⁷⁰ When activated with either Me₂AlCl or Et₂AlCl, these NHC Ni complexes exhibited low activities in norbornene polymerization. However, after treatment with B(C₆F₅)₃, these Ni complexes showed excellent activities in the homopolymerization of norbornene (up to 3.21 × 10⁶ g mol⁻¹ h⁻¹) and its copolymerization with 1-octene at 100 °C, achieving high 1-octene insertion ratios (9%–17%).

Fig. 32 Phenoxide-functionalized NHC nickel/palladium complexes.

Following the strategy of appending a dimethylaminoethyl sidearm, similar tridentate Pd/IzQO complexes 81 (Fig. 32, bottom right) were developed. These complexes exhibited high activities (up to 2.74 × 10⁸ g mol⁻¹ h⁻¹) in the homo- and copolymerization of norbornene with n-butyl vinyl ether (BVE) and methyl 10-undecanoate (UA) in the presence of Me₂AlCl.¹⁰⁰ The rigid annulated backbone increased the activities of 81 compared to their non-annulated analogues 79, and 81 also showed high thermal stability (up to 140 °C) during polymerization. The electron-withdrawing groups on the annulated backbone positively impacted the copolymerization activity, and the produced copolymers possessed moderate molecular weight, high comonomer incorporation, and good thermal stability.

Treatment of the corresponding tridentate imidazolium ligand with Ni acetate resulted in the formation of complex 82 (Fig. 33). X-ray diffraction analysis revealed that Ni²⁺ is four-coordinated, with one O and one N atom far away from the metal center on two ligands.¹⁰¹ Chiral NHC–Ni complexes 82 showed good activities for norbornene polymerization (10⁶ g mol⁻¹ h⁻¹) upon treatment with MAO. The activity received an increase with increasing the Al/Ni ratios (ranging from 1500–2500) and polymerization temperatures (20–80 °C).

Fig. 33 Structure of a nickel complex.

4.3 Tridentate NHC ligand containing dianionic functionality

Kawaguchi and colleagues reported bis(aryloxido)-functionalized NHC Ti complexes 83 (Fig. 34, top left), and X-ray diffraction revealed that the tridentate ligand coordinates to the metal center with two mutually cis chlorine atoms. In the presence of MMAO, 83 showed activity as high as 2.9 × 10⁵ g mol⁻¹ bar⁻¹ h⁻¹ for the polymerization of ethylene.¹⁰² Meanwhile, the activity of analogous Ti complexes reached up to 9.7 × 10⁴ g mol⁻¹ bar⁻¹ h⁻¹ for ethylene polymerization upon activation with MAO, producing linear polymers with low molecular weight.¹⁰³ Aminolysis reactions of the ligand with M(NMe₂)₄ (M = Zr, Hf) and subsequent alkyl transformation gave rise to complexes 84 (Fig. 34, top right), as reported by Fryzuk and co-workers. Activation of 84 with [Ph₃C][B(C₆F₅)₄] yielded cationic complexes that exhibited slight activity for 1-hexene polymerization. However, the cationic Zr complex showed moderate catalytic activity for ethylene polymerization.¹⁰⁴

Fig. 34 Bis(aryloxido)- and bis(arylamine)-functionalized NHC titanium, zirconium and hafnium complexes.

The protonolysis reaction between bisphenol-functionalized tridentate NHC complexes 85 (Fig. 34, bottom left) and [HNMe₂Ph][B(C₆F₅)₄] led to quantitative formation of corresponding anilinium benzyl cations. These cations were active in 1-hexene polymerization. In contrast to the low activity of the Hf-based system, the Zr-based 85 system exhibited moderate activity for 1-hexene oligomerization, generating highly regio-regular poly(1-hexene) containing mainly trimer with vinylene end groups.¹⁰⁵ The highly regioselective 2,1-insertion of 1-hexene process was attributed to the coordinated NMe₂Ph group, which enhanced the steric hindrance at the Zr center. A similar bisphenol-functionalized tridentate benzimidazolylidene NHC Zr complex 86 (Fig. 34, bottom right) was also reported.¹⁰⁶ After the addition of [Ph₃C][B(C₆F₅)₄], 86 served as an active catalyst for α-olefin transformations that could be “tuned” from oligomerization to polymerization or inactivity by adding appropriate ligand. In this study, ligand binding seemed to increase catalytic activity, which was attributed to the inherently essential role of ligand in accelerating the initiation of 1-hexene polymerization.

5. Complexes with tetradentate NHC ligands

Zi and Song et al. reported the tetradentate chiral bis-imidazolium ligand precursor and its corresponding complex 87 (Fig. 35) by treating the ligand precursor with 1.0 equiv. of Ni(OAc)₂ in the presence of K₂CO₃ and NaBPh₄.¹⁰¹ Upon the activation of MAO, 87 was proved to be an efficient catalyst for norbornene polymerization with good activity under mild conditions, producing polymers with high molecular weight and broad dispersity. It was speculated that the steric hindrance around the metal center and the solid bonding between the NHC ligands and Ni atom hampered the generation of active species.

Fig. 35 Structure of nickel complex with a tetradentate ligand.

6. Conclusions

While numerous moderately to highly active NHC-based metal catalysts have been developed for the polymerization of olefins in recent decades, they continue to be considered as a promising and interesting topic in catalytic olefin polymerization. This review summarizes such a category of metal catalysts with various NHC ligands and central metals across the periodic table to date. The structure–activity relationships during the polymerization are discussed in detail for all catalytic systems, mainly focusing on the steric and electronic effects of ligands and the inherence of central metals. In general, NHC-ligated late-transition-metal complexes exhibit robust and thermostable characteristics, showcasing impressive catalytic activities in the polymerization of olefins and their copolymerization with polar monomers. In particular, the unprecedented success of NHC-based metal catalysts, in which the NHC-plane is structurally coplanar to the metal plane, can give valuable hints to design more efficient catalysts for olefin polymerization as well as other catalytic applications. It is foreseeable that, in the future, feasible synthetic methodologies and novel NHC ligands will be introduced to prepare well-defined metal complexes, especially those involving early transition metals, with attractive catalytic properties. Exploiting new efficient metal catalysts ligating novel rationally designed NHC ligands for olefin polymerization apparently represents one of the most important directions worth moving. NHC-ligated early-transition metal complexes with high stability during the polymerization are highly expected, and their success in producing polymers with designable microstructures (molecular weight, molecular weight distribution and tacticity and so on) is also a difficult issue that needs to be perfectly resolved. On the other hand, undoubtedly, the copolymerization of ethylene or propylene and polar monomers catalysed by, especially, newly designed NHC-ligated late-transition metal complexes with satisfactory catalytic performances is another hot and continuous issue in this field. The review also covers polymer properties, such as molecular weight, regulation of polymer chain branching, comonomer incorporation (polar and non-polar), tacticity and so on, along with structural illustration of the catalysts. Apparently, the copolymerization of different monomers will provide new polymers, and thus, reasonable design and synthesis of new monomers that are polymerized by the NHC-ligated metal catalytic systems represent another anticipated strategy and valuable direction to prepare new useful polymers. Novel polymeric materials with special microstructures and physical properties will be undoubtedly prepared by utilizing new NHC-based catalysts in the future to satisfy the applications in the fields of manufacturing, construction and so on.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Natural Science Fund for Distinguished Young Scholars of China (No. 22025107), the Shaanxi Fundamental Science Research Project for Chemistry & Biology (No. 22JHZ003), the National Youth Top-notch Talent Support Program of China, the Key International Scientific and Technological Cooperation and Exchange Project of Shaanxi Province (No. 2023-GHZD-15), Xi'an Key Laboratory of Functional Supramolecular Structure and Materials, and the FM&EM International Joint Laboratory of Northwest University.

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