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Review

Homogeneous Non-Metallocene Group 4 Metals Ligated with [N,N] Bidentate Ligand(s) for Olefin Polymerization

by
Zhao Wen
,
Changjiang Wu
*,
Jian Chen
,
Shuzhang Qu
,
Xinwei Li
and
Wei Wang
*
SINOPEC (Beijing) Research Institute of Chemical Industry Co., Ltd., No. 14 Beisanhuan Donglu, Chao Yang District, Beijing 100013, China
*
Authors to whom correspondence should be addressed.
Polymers 2024, 16(3), 406; https://doi.org/10.3390/polym16030406
Submission received: 2 January 2024 / Revised: 19 January 2024 / Accepted: 26 January 2024 / Published: 1 February 2024
(This article belongs to the Section Polymer Chemistry)
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Abstract

:
The development of catalysts has significantly advanced the progress of polyolefin materials. In particular, group 4 (Ti, Zr, Hf) non-metallocene catalysts ligated with [N,N] bidentate ligand(s) have garnered increasing attention in the field of olefin polymerization due to their structurally stability and exceptional polymerization behaviors. Ligands containing nitrogen donors are diverse and at the core of many highly active catalysts. They mainly include amidine, guanidinato, diamine, and various N-heterocyclic ligands, which can be used to obtain a series of new polyolefin materials, such as ultrahigh molecular weight polyethylene (UHWMPE), olefin copolymers (ethylene/norbornene and ethylene/α-olefin) with high incorporations, and high isotactic or syndiotactic polypropylene after coordination with group 4 metals and activation by cocatalysts. Herein, we focus on the advancements and applications of this field over the past two decades, and introduce the catalyst precursors with [N,N] ligand(s), involving the effects of ligand structure, cocatalyst selection, and polymerization conditions on the catalytic activity and polymer properties.

1. Introduction

Polymer materials are increasingly widely used in people’s daily lives. Polyolefins have become an indispensable part of the polymer field, and now they are also the most used and diverse polymer materials [1,2]. Polyolefins have the characteristics of excellent mechanical strength, chemical inertness, recyclability, and electrical insulation. This makes them very cost-effective. Polyolefins are mainly used in economic pillar industries, such as packages, construction, automobiles, sports, electronics, and agriculture. In 2021, global plastic production exceeded 390 million tons, among which polyolefin dominated by polyethylene and polypropylene accounted for 46.2%. The aforementioned statements suffice to substantiate its significance. In the field of polyolefins, particularly in relation to high-end materials such as UHMWPE, polyolefin elastomer, and cycloolefin copolymer, these materials exhibit enhanced economic value and utility. However, the synthesis of such advanced materials and the exploration of novel alternatives heavily rely on advancements in high-performance catalyst research.
It has been over sixty years since Ziegler and Natta discovered the polymerization catalysts and first obtained high-density polyethylene (HDPE) and isotactic polypropylene (iPP). During this time, different types of catalysts such as Phillips chromium-based catalysts, metallocene catalysts, and non-metallocene catalysts made significant breakthroughs in the field of polymer catalytic synthesis [3,4,5,6]. In the 1980s, the milestone progress of metallocene catalysts had a significant impact on polymer synthesis and catalytic chemistry, due to the discovery of methylaluminoxane (MAO), a high-efficiency cocatalyst, which led to a revolution in polyolefin industry. The most significant difference between metallocene catalysts and Z-N catalysts is the single-site characteristic of the former. However, there are some problems existing with the use of metallocene catalysts in olefin polymerization: (i) the high cost of synthesis; (ii) the high cost of the large dosage of MAO to scavenge impurities to maintain high activity; (iii) at the same time, the patent protection of the catalyst structure is too comprehensive, making it difficult to achieve innovation in metallocene structures. Therefore, researchers turned their attention to non-metallocene catalysts that can also form a single active site in the catalytic process by replacing cyclopentadienyl with other organic groups [7].
Non-metallocene catalyst precursors, in a narrow sense, are defined as the organometallic compounds of group 4 metals (Ti, Zr, and Hf) without a cyclopentadienyl group. These catalysts are one of the hot topics in the research into olefin coordination polymerization because of their diverse ligand designs and strong copolymerization ability. The development of non-metallocene catalysts was initiated in the mid-1990s. In 1995, Schaverien et al. [8] reported that biphenyloxide titanium complex A (complexes A, B, C and D see Figure 1) showed a relatively high activity for olefin polymerization. In 1996, McConville et al. [9] reported a chelated diamide titanium complex B, which can polymerize α-olefin at room temperature. In 1999, Gibson et al. [10] reported titanium and zirconium complexes C with a ferrocenyldithiolate ligand leading to medium to low activity toward ethylene polymerization. Although new non-metallocene catalysts had been developed, their activity was always lower than the typical metallocene catalyst Cp2ZrCl2 [11]. In 2000, Fujita group, Mitsui Chemical Company, reported a non-metallocene olefin catalyst with phenoxy-imine as bidentate ligand and zirconium as the metal center, the so-called “FI catalyst” D exhibiting very high activity (51.9 × 106 g-PE/mol-Zr·h) twenty times that of Cp2ZrCl2. By adjusting the polymerization conditions and changing the ligand structure, the activity can be further increased to more than two orders of magnitude greater than Cp2ZrCl2 [7,12]. By using 13C NMR spectroscopy, they revealed that the syndiotacticity of polypropylene arose from a chain-end control mechanism, indicating that the propylene monomer included chain propagation by 2,1-insertion [13]. The syndiospecificity can be attributed to a rapid site-inversion relative to chain propagation if a catalytically active species possesses an octahedral geometry with a trans-O and cis-N arrangement. In addition, when changing the structure of ligands (monoamidinate and pyridine-amido ligands) or replacing the substituents (hydrogen on the imine carbon is replaced by the large steric hindrance substituent [14]), the mechanism of polymerization is changed, and finally iPP is obtained. So, practically all possible combinations of regiochemistries and stereochemistries have been observed in propylene polymerization with non-metallocene catalysts [15].
Non-metallocene catalysts have been extensively researched and elaborated upon by numerous scholars up to this point. For example, in 2003, Gibson et al. [16] reviewed the application and development of non-metallocene catalysts in olefin polymerization according to the classification of the coordination metal center, and the importance of high-throughput screening (HTS) for the study of catalysts was proposed. In 2011, Kim et al. [17] reviewed the synthesis of non-metallocene catalysts based on N-heterocyclic ligands and their activity in olefin oligomerization and polymerization over the previous 25 years. In 2020, Sun et al. [18] reviewed the advancements of non-metallocene catalysts coordinated with N-ligated for (co)polymerization of ethylene. However, many reviews lack detailed reports on the types and applications of bidentate non-metallocene catalysts coordinated with group 4 metals using N-donors. Therefore, our focus is on summarizing ligands with [N,N] and the structures of non-metallocene catalysts featuring group 4 metals (Ti, Zr, and Hf) as central metal atoms. We emphasize the activity and applications of catalytic systems with different structures in olefin (co)polymerization while also discussing the future development trends of [N,N]-bidentate non-metallocene catalysts.

2. [N,N]-Ligands Metal Complexes

[N,N]-ligands metal complexes are obtained by coordinating or directly forming covalent bond connections between two N-donors on the ligand group and group 4 metals (Ti, Zr, and Hf). These complexes are the most abundant and widely used bidentate non-metallocene catalysts. According to their different ligand structures, [N,N]-ligands are composed mainly of amidine, guanidinato, diamine, and various N-heterocyclic ligands.

2.1. Amidinate Ligands Metal Complexes

In the amidinate ligands, the anionic moiety [R-C(NSiMe3)2]− is a four-electron donor, promoting higher electrophilicity at the metal center. Within the amidinate-based ligands, a variety of benzamidinate group 4-containing complexes have been described. Benzamidine ligand is considered to be spatially equivalent to cyclopentadienyl (Cp) and its derivatives, which lead the polymerization of various olefin monomers to be carried out. Zambelli et al. [19,20] reported a series of monoamidinate titanium complexes 1a–1g (complexes 1–4 see Figure 2), which can be activated by MAO and used as precatalysts in propylene polymerization at low temperatures (−60 °C). In comparison to 1a, 1b, 1d, and 1g, the 1c/MAO catalytic system has a larger yield and higher Mn. The stereochemical triad compositions and the amounts of vicinal methyls of 1e and 1f showed that these products were prevailingly isotactic and more regioregular than others. Detailed polymerization data are shown in Table 1. These complexes/MAO catalysts systems can achieve stereospecific polymerization of 1,3-butadiene and styrene. The monoamidinate ligand complexes, however, are unstable, leading to the deactivation of metal centers during high-temperature olefin polymerization. Eisen et al. [21,22,23,24] reported the C2-symmetric bis(benzamidinate) dichloride or dialkyl complexes 2a–2m of group 4 metals. Among them, complexes 2a–2d, activated with MAO, can yield different species and properties of polypropylene, for example, the 2a/MAO catalytic system can obtain atactic polypropylene at atmospheric pressure in toluene, 2d/MAO can obtain elastomer with polypropylene, and the 2d/MAO catalytic system had the highest activity (28.5 × 105 g-PP/mol-Ti·h). A cationic bis-amidinate dialkyl complex 2f, resulting from ligand dissociation of the tri-amidinate complex and its migration to aluminum in MAO, produced elastomeric polypropylene, as illustrated in Scheme 1. The 2a, 2b, and 2f/MAO catalytic systems can effectively polymerize 1,5-hexadiene, with the highest catalytic activity from 2f/MAO (11.0 × 104 g-polymer/mol-Zr·h). Complexes 2g–2m with different aryl substituents had moderate activity, and due to the ligand dissociation, the molecular weight distribution (MWD) of polypropylene became wider. For a more intuitive comparison, the polymerization data of mono-amidinate and bis(amidinate) complexes are shown in Table 2. Zhou et al. [25] reported that asymmetric aliphatic benzamidine zirconium complex 3 can be used for ethylene polymerization, exhibiting a high activity of 54.4 × 104 g-PE/mol-Zr·h. This method of combining asymmetric aromatic and chiral amidine ligands provides ideas for the design of new ligands. In addition, novel structural catalysts can also be obtained by bridging with heteroatoms. Liu et al. [26] reported that alkyl-ended silicon-bridged ansa-bis(amidinate) complexes 4a–4d could also be used for ethylene polymerization after MAO activation, but their activity was relatively lower than 3/MAO. This may be because the bridge structure affected the original steric hindrance of the catalyst. It was found that in mono-amidinate ligand complexes, syndiotactic polypropylene and polystyrene were obtained, and catalytic activity was increased due to the perfluorophenyl substituent of the amidinate ligand increasing the electrophilicity of the active species. In comparison, bis-amidinate ligand complexes could obtain poly(methyl-1,3-cyclopentane) and isotactic, atactic, and elastomeric polypropylene, and complexes with oxygen-containing ligands showed higher activities than complexes with phenyl or alkyl benzene rings, though the activity was moderate. Higher activity and better stereoregularity were achieved by performing the polymerization in CH2Cl2, which had higher polarity compared with toluene. Additionally, the bis(benzamidinate) zirconium complex with a methyl group in the para-position of the benzene ring can obtain highly isotactic polypropylene with high activity, but, when titanium was used as the metal center without any substituents on the benzene ring, both activity and isotacticity decreased, resulting in the formation of elastomeric polypropylene. In the zirconium complexes, the more open metal center could possibly allow for more facile propylene insertion.

2.2. Guanidinato Ligand Metal Complexes

Compared with amidino ligands, guanidine ligands also have strong π-electron donating abilities and can ligate with central metal as neutral, mono, and multiple anionic forms. So, the complexes with guanidinato ligands have potential for coordination and organometallic chemistry and could be used as alternatives to unique cyclopentadienyl species in order to form suitable catalytic sites for the polymerization of α-olefins. Liu et al. [28,29,30] reported non-symmetric guanidinato zirconium and hafnium complexes 5a–5f (complexes 5 and 6 see Figure 3). Among them, complexes 5a–5e showed moderate activity and high molecular weight (5d/MAO activity was 29.2 × 103 g-PE/mol-Hf·h, Mη = 3.5 × 105) in ethylene polymerization. This can be attributed to the strong donor substituents (more basic) and guanidinato ligand increasing the electron density of the metal. Furthermore, it was found that the molecular weight of PE increased with the increase in polymerization temperature. When the alkyl substituent in the ortho position of phenyl connected to the amino, the triguanidinato complex was no longer produced, and the asymmetric chlorine-bridged guanidinato binuclear zirconium complex 5f, which was a high-activity ethylene polymerization catalyst system (5f/MAO, activity was 64.8 × 104 g-PE/mol-Zr·h), was obtained by selecting the appropriate Al/Zr (molar ratio of co-catalyst to complex). The increase in the number of cation centers after the activation of the binuclear complex facilitated polymerization, thereby enhancing its catalytic activity. Under similar conditions, comparing the activity of various catalysts, it was found that the activity of the non-symmetric guanidinato complexes was not significantly affected by the change of substituents or central metals. However, when a second monoanionic ligand was introduced to the monoanionic guanidinato ligand, the behaviors of the complex changed significantly. Kempe et al. [31] reported relevant complexes 6a–6e. The guanidinato titanium dichloride complexes 6b–6e were prepared by simple salt-elimination reactions from the titanium precursor 6a and the Li salt of the ligands. Among them, complex 6b was completely inactive, whereas other complexes with bulky monoanionic bidentate N-ligands had high activity of ethylene polymerization. The addition of different bulky ligands led to different polymerization products, for example, complex 6d, which was activated by dry methylaluminoxane (d-MAO) to obtain UHMWPE (Mw = 24.88 × 105 g/mol) with high activity (88.0 × 105 g-PE/mol-Ti·h). In view of the properties of the amidino and guanidine ligands of the monoanion we could speculate that the many bulky monoanionic non-cyclopentadienyl ligands might be useful candidates to further develop nonbridged, mixed-ligand group 4 metal complexes combining high polymerization activity and unusual selectivity patterns.

2.3. Diamine Ligand Metal Complexes

Diamine-based non-metallocene catalysts have a small number of electrons, high electrophilicity, and higher catalytic activity. In this review, ligands with β-diiminato, α-diiminato, and amino-containing complexes (amino groups directly attached to imido, or between N, P, Si, and ferrocene), are collectively classified as diamine ligand complexes.
β-diiminato ligands appeared early in the field of olefin polymerization, have a wide range of applications, and have diverse ligand structures. Xie et al. [32] reported a series of novel mono β-diiminato titanium complexes 7a–7c (complexes 7–11 see Figure 4), and the significant electronic effects of fluoro-substituents on the olefin polymerization activity of mono β-diiminato titanium complexes were identified. Titanium complex 7a with fluorine-containing β-diiminato ligands, on activation with modified methylaluminoxane (MMAO), had very high activity for polymerization of ethylene (76.8 × 105 g-PE/mol-Ti·h) and higher activity for copolymerization of ethylene/1-henxene (26.1 × 106 g-copolymer/mol-Ti·h). During copolymerization, the activity increased with the increase in the initial concentration of 1-hexene, which showed the positive “comonomer effect”. The high activity of such o-fluoro-substituted complexes was due to the non-covalent interaction between o-fluoro atoms and ethylene monomers that contributed to the coordination of ethylene with the central metal [33], thereby improving catalytic activity. In addition to mono β-diiminato ligands, Sun et al. [34] reported asymmetric bis(β-diiminato) zirconium 8a and 8b, which can also be used for ethylene polymerization, and under the condition of MAO activation and appropriate increase of ethylene pressure (10 atm), the activity of 8a/MAO was 10.0 × 105 g-PE/mol-Zr·h. Comparing mono and bis β-diiminato ligands, the activity of 8a/MAO is not as good as that of the 7a/MMAO catalytic system, but a high molecular weight of PE can also be obtained (Mw = 26.0 × 105 g/mol). This is due to the increase of steric hindrance around the active metal center leading to β elimination and the decrease of chain transfer reaction rate. According to this theory, when there are substituents with larger steric hindrance on the N donor, ultrahigh molecular weight polymers will be obtained. Tokitoh et al. [35] reported β-diiminato complexes 9a–9d with extremely bulky substituents, in which Tbt-substituted (Tbt = 2,4,6-[(Me3Si)2CH]3C6H2) complex 9a, activated by Al(i-Bu)3/[PhNHMe2][B(C6F5)4], had high activity in ethylene polymerization (65.0 × 105 g-PE/mol-Ti·h) and ethylene/1-hexene polymerization (51.0 × 105 g-polymer/mol-Ti·h). Inspired by co-activation of cocatalysts, corresponding precatalysts can also have the same innovation. Zhang et al. [36] successfully prepared medium-molecular-weight branched PE by using a novel binary tandem catalytic system composed of 2,6-bis(imino)pyridyl iron complex and mono β-diiminato titanium complex 7a. After adjusting the molar ratio of Fe and Ti (Fe:Ti = 4:1), the activity of 7a/MMAO was up to 52.3 × 105 g-PE/mol-Ti·h. Chen et al. [37] reported complex 10, having high activity in ethylene polymerization (22.4 × 104 g-PE/mol-Zr·h) after activation by MAO, which was obtained by the neutral cyclo-1,3-diazasilane heterocyclic ring coordinated with the zirconium center, different from the traditional coordination mode of β-diiminato (a plausible mechanism for the formation of 10 is shown in Scheme 2). The formation of 10 in the reaction of the ligand with ZrCl4 was presumably an intermolecular interaction of the weak acceptor, zirconium, with the anionic imido nitrogen leading to the cleavage of a Si–C bond and the formation of a new Si–N bond with concomitant elimination of MeLi. The (co)polymerization results of β-diiminato complexes are shown in Table 3. It was found that the metal center had a wide open coordination environment in mono-/bis(β-diminiato) complexes, which caused a little steric hindrance for the coordination of the comonomer and metal center, thus obtaining high catalytic activity.
The structure of α-diimine ligands is similar to that of the β-diiminato ligand, and part of α-diimide catalyst is obtained by alkylation of a neutral ligand with a C=N bond; this method can replace the elimination of salts, amines, and alkanes to synthesize catalysts. Mashima et al. [38] reported α-diimine complexes 11a–11f with moderate polymerization activity for 1-hexene and vinylcyclohexane and copolymerization of ethylene and 1-hexene, in which the 11b/[Ph3C][B(C6F5)4] catalytic system could obtain narrow molecular-weight distribution and high-molecular-weight polymer (Mw = 4.51 × 105, MWD = 1.5). Comparing α-diimide and β-diiminato complexes, the former have been the subject of fewer reports and studies. This is due to the weak structural stability of the five-membered ring formed by α-diimide complexes with metal centers; the raw materials for the synthesis of ligands are relatively scarce and α-diimide titanium complexes could not be formed because of the small ionic radius of titanium.
Amino-containing complexes exhibit exceptional performance and possess a unique structural arrangement. In comparison to β-diiminato complexes, these compounds require a lengthier synthetic pathway. However, their innovative structures broaden the scope of non-metallocene catalysts, with some demonstrating high activity in olefin polymerization. Zhang et al. [39] reported titanium and zirconium complexes 12a–12d (complexes 12–19 see Figure 5) with aminoiminophosphorane ligands and speculated that at higher monomer concentrations (ethylene pressure of 0.5 MPa), the coordination between the active species and monomer will be more effective and the steric congestion could be negligible. Bildstein et al. [40] reported complexes 13a and 13b with [N,N]-iminohydruoxamato ligands. The 13a/MAO catalytic system had moderate activity in ethylene polymerization (30.7 × 104 g-PE/mol-Ti·h). The structural stability of these catalysts was weak, which hindered olefin coordination. Zhou et al. [41] reported titanium imido complexes 14a–14c with dianionic ligands of triazapentadienyl derivatives. The 14a/MAO catalytic system had a high catalytic activity (56.6 × 103 g-PE/mol-Ti·h), and the complex structure was more stable than 13a and 13b. Sun et al. [42] reported a series of complexes 15a–15d of μ2,η1-N-[(N,N-dimethylamino)dimethylsilyl]-2,6-diisopropylanilido. The silane-bridged pseudotetrahedral complex 15a and chloro-bridged binuclear tetrahedral complexes 15b–15d, activated by MAO, exhibited high activity for ethylene polymerization (15c/MAO catalytic system with the highest activity up to 17.3 × 105 g-PE/mol-Zr·h), and obtained PEs with broad MWD (4.2–65). Complex 15a obtained polymer with the narrow MWD and relatively low Mw, attributing to the two ligands occupying spaces around zirconium influencing the slow coordination of ethylene, and binuclear complexes 15b–15d seem to form less stable active species. Although activity was higher, multiple active sites were formed, leading to PEs with wider MWD. In addition, complexes 15a and 15b could be used in copolymerization of ethylene and 1-hexene, and 15b/MAO had high activity (10.3 × 105 g-copolymer/mol-Zr·h). Liu et al. [43] reported an amidino–amino binuclear zirconium complex 16, the formation of which was due to the presence of trimethylsilane that extended the skeleton of the ligand and formed a more stable seven-ring coordination. Meanwhile, 16/MAO exhibited high catalytic activity (48.6 × 104 g-PE/mol-Zr·h), a high molecular weight, and broad distribution (Mw = 9.5 × 105g/mol, MWD = 10.08). This is closely associated with the enhancement of zirconium electrophilicity and the presence of binuclear species. Upon examining the aforementioned structures of these complexes, it can be observed that coordination complexes with double ligands exhibit improved stability and lead to polymers with a narrow MWD, while coordination complexes with single ligands demonstrate high activity and form polymers with a relatively wide MWD. Additionally, binuclear complexes result in polymers with an even wider MWD. These enable the design of the catalyst structure to be further optimized based on the desired properties of the polymer product, thereby enhancing efficiency in designing a rational catalyst structure.
Buchmeiser et al. [44] reported novel zirconium complexes 17a and 17b with a 6-[2-(diethylboryl)phenyl]pyrid-2-ylamido motif. Complexes 17a and 17b had high catalytic activity and could obtain ethylene/cyclopentene (E/CPE) copolymer (75.0 × 105 g-copolymer/mol-Zr·h) and high-molecular-weight linear HDPE (activity = 18.8 × 104 g-PE/mol-Zr·h, 1.8 × 106 g/mol ≤ Mn ≤ 4.0 × 106 g/mol). The coordination between CPE and metals facilitated alkyl transfer to the aluminum atoms of MAO, resulting in the copolymer lacking vinyl end groups. In addition, a comparison between complex 17a and 17b revealed that the presence of a bulky substituent in close proximity to the metal center resulted in ultra-high molecular weight at 90 °C. This was due to the large substituent group effectively impeding both α- and β-agostic interactions and preventing any of the β-hydride elimination reactions, which made chain propagation continue. Sun et al. [45] reported hydrazonide complexes 18a and 18b. These complexes were highly sensitive to air and moisture due solely to using less bulky hydrazonide ligands. After MMAO activation, complex 18a showed moderate activity (36.0 × 103 g-PE/mol-Zr·h) and produced UHMWPE with narrow distribution (Mw = 2.05 × 106 g/mol, MWD = 1.49). Arnold et al. [46,47] described ferrocene-containing olefin polymerization zirconium complexes 19a and 19b; after [Ph3C][B(C6F5)4](TB) activation, 19a/TB could be used in the polymerization of 1-hexene and yielded poly-1-hexene with molecular weights of 20,000 and narrow MWD (1.3–1.4). This was due to the CH activation of a mesityl methyl group in this catalytic system, which may lead to deactivation. After changing the substituents on the amido, the 19b/TB catalytic system had low activity for ethylene polymerization (10.2 × 104 g-PE/mol-Zr·h). While ferrocene-containing complexes had less activity for olefin polymerization, the increased stability facilitates the investigation of their structure and reactivity. The polymerization results of amino-containing complexes are shown in Table 4. It was found that the novel amino-containing complexes, particularly 17a and 18a, exhibited relatively moderate activity; however, their distinctive ligand structure enabled the synthesis of UHMWPE with MWD below 10 through homopolymerization of ethylene.

2.4. N-Heterocyclic Amido Ligand Metal Complexes

N-heterocyclic ligands play a crucial role in [N,N]-based non-metallocene catalysts, serving as essential components. The metal complexes exhibit diverse structures and steric properties owing to the presence of different nitrogen heterocyclic coordination, thereby influencing their catalytic activity in olefin polymerization. According to the different nitrogen heterocycles, they are mainly divided into pyrrolide-, pyridine-, pyrazolato-, thiazole-, imidazole-, and quinoline-amido ligands.
Mashima et al. [48] reported benzyl zirconium complexes 20a–20e (complexes 20–24 see Figure 6) of 2-(N-aryliminomethyl)pyrrolyl ligand. The 20b/MMAO catalytic system had very high activity of ethylene polymerization at 0 °C (10.8 × 105 g-PE/mol-Zr·h), whereas 20c–20e, which had the di-/tribenzyl zirconium center supported by one (iminomethyl)pyrrolyl ligand, exhibited 102 less catalytic activity. The results showed that the activity of the catalyst system could be improved by alkylation of the imino moiety of the ligand in the pyrrolide-imine ligand. The group 4 metal centers coordinated with bis(pyrrolide-imine) ligand, which were developed by Fujita’s team in the early 2000s, were often referred to as PI catalysts. In comparison to FI catalysts, the catalytically active cationic species originating from PI catalysts exhibited higher electrophilicity and a sterically open nature, enabling strong electron interaction with olefin monomers and influencing the activity of the catalyst. As a consequence, Fujita et al. reported [49,50,51] different structures of pyrrolide-imine complexes 21a–21f. PI catalysts had high activity for ethylene polymerization, among which the 21a/MAO catalytic system, with an adamantyl substituent, had the highest activity (22.9 × 106 g-PE/mol-Zr·h) after exchanging the substituent on the imine with cyclohexyl. The 21c/MAO catalytic system had high activity (42.7 × 105 g-polymer/mol-Ti·h) for the copolymerization of ethylene and norbornene (NBE); the copolymers had narrow MWD (1.28) and different kinds of block copolymers were obtained. The block copolymers were synthesized using a sequential-addition polymerization procedure (Figure 7). Additionally, in the 21c/MAO catalytic system, the Mn value increased proportionally with polymerization time (1 min, Mn = 74 × 103; 3 min, Mn = 179 × 103; 5 min, Mn = 285 × 103; 10 min, Mn = 521 × 103), and the narrow Mw/Mn value was retained (Mw/Mn 1.07–1.16), suggesting that the system was indeed living. Calculation via DFT confirmed that when ethylene was connected to the metal center in the copolymerization, the next inserted molecule was more inclined to NB because the coordination energy was −17.05 kg/mol for NBE and −8.72 kg/mol for ethylene, so this living copolymerization was highly controllable. This highly controllable living copolymerization based on the active species of PI catalysts possessed high affinity and high incorporation ability for NBE and PI catalysts therefore had an electronic attractive interaction (Figure 8). In addition, 21c/MAO had high catalytic activity for ethylene/propylene copolymerization (13.1 × 105 g-polymer/mol-Ti·h) and ethylene/1-hexene copolymerization (45.8 × 105 g-polymer/mol-Ti·h).
Based on these advantages of pyrrolide-imino ligands, a series of derivatives of pyrrolide-imino ligands can be obtained by modifying the structure of the ligand, which could also be utilized in olefin polymerization when coordinated with group 4 metals with excellent properties. Sun et al. [52] reported that bis(imino-indolide) titanium complexes 22a–22e, activated by MAO, could be used in ethylene polymerization and E/NBE copolymerization to obtain polymers of high molecular weight, and found that titanium complexes with electron-withdrawing groups in ligands had higher catalytic activity than complexes with electron-donating groups (detail polymerization data are shown in Table 5). Ohta et al. [53] reported di(methylindolyl)-phenylmethane titanium complex 23. The ethylene polymerization activity of the 23/MMAO catalyst system was low (16.3 × 103 g-PE/mol-Ti·h), but pretreatment of 23 with ClSiMe3 followed by activation with MMAO improved the activity up to 15.4 × 104 g-PE/mol-Ti·h. ClSiMe3 served as a chlorinating reagent, and the substitution of the chloride ligand with an alkyl ligand via MMAO occurred at a faster rate than that of the amido ligand, resulting in improved ethylene polymerization activity. In addition to indolide, pyrrolide-imine derivatives also have pyrrolide-benzoxazole ligands. You et al. [54] reported the titanium catalysts 24a–24d supported by pyrrolide-benzoxazole ligands and their application in ethylene polymerization. After activation with MAO, all complexes were able to obtain high-molecular-weight PE (96.10 × 105 g/mol) with high activity, in which the maximum activity was 39.8 × 104 g-PE/mol-Ti·h when there was chlorine substitution in the benzene ring of the ligand framework. Because the electron-withdrawing ability of chlorine increases the electrophilicity of the metal center, it thus promotes the coordination of ethylene.
Pyridine-amido ligands have relatively simple synthesis pathways and easily regulated structures. Eisen et al. [55] reported complex 25 (complexes 25–33 see Figure 9), which had good stability and moderate catalytic activity for propylene polymerization (13.1 × 103 g-PP/mol-Ti·h). After activation with MAO, the catalytic system could obtain elastomeric polypropylene. Busico et al. [56] reported complex 26 with very high activity (18.0 × 107 g-PP/mol-Hf·h) for propylene polymerization, which had bulky steric hindrance substituents on the amido and pyridine rings, and the isotactic polypropylene produced by complex 26 had high values of Mw (above 700 kDa) with narrow MWD. Liu et al. [57] reported bis(pyridine-amido) binuclear titanium complex 27, which formed a more stable flat structure, providing more space for coordination of the ethylene monomer. The 27/MAO catalytic system had high catalytic activity (12.4 × 104 g-PE/mol-Ti·h), but because of the binuclear arrangement, the product of HDPE had a wide MWD (10.7). Compared with complex 27, Zhou et al. [58] reported that zirconium complex 28 with tris(aminopyridinato) ligand had higher activity (14.7 × 104 g-PE/mol-Zr·h). The high Lewis acidity of the metal center combined with the strong basicity of the amide and pyridine ligands as well as the chelating strained η2-coordination of the amino–pyridine ligand contributed to the increased activity of complex 28. It was found that the presence of a mono- or multi-aminopyridine ligand complex enables the stabilization of the metal center through the lone nitrogen pair, resulting in diverse complex structures exhibiting high activity for olefin polymerization.
The primary research focus for pyrazolato-based metal complexes is metal complexes with tripyrazolato-based [N,N,N]-tridentate ligands, which have been extensively studied over the years. Cuenca et al. [59] reported that [N,N]-bidentate pyrazolato zirconium complex 29 had low catalytic activity (38.0 × 103 g-PE/mol-Zr·h) for ethylene polymerization and the MWD of the polymer was greater than 57.
Sun et al. [60] reported imidazole-amine hafnium complexes 30a–b and thiazole-amine hafnium complexes 31a–31e for isospecific polymerization of propylene. They found that of all the catalysts screened, complex 30a exhibited the best combination of activity and stereospecificity. At 130 °C, 30a still had high activity (78.0 × 106 g-PE/mol-Hf·h). This could be explained by the steric bulk of the aryl substituent at the chiral center at the bridge position; the Hf-N bond length in imidazole-amine hafnium was slightly shorter than thiazole, which led to the higher basicity of the imidazole amine, and the five-membered benzofurazan ring meant that orthometalation may have been more likely to occur, which lessened the unfavorable steric interactions. After the activation of TMA(trimethylaluminum)/[PhNMe2H]+[B(C6F5)4]−, complexes 31a–b had very low activity, and the resulting polypropylene was not highly isotactic. In contrast to 31a and 31b, 31c–31e had high activity, yielding polymer with high Mw and higher crystallinity. Among them, 31d/TMA/[PhNMe2H]+[B(C6F5)4]− showed the highest activity (40.9 × 107 g-PE/mol-Hf·h) and Mw (1320 k). The Hf heterocyclic amido complexes exhibit significant potential and value in olefin polymerization, particularly when modified with diverse heteroaryl groups. These modifications enhance their high temperature resistance, thereby enabling the exploration of copolymerization of higher α-olefins.
Amidoquinoline ligand metal complexes also have excellent thermostability; however, their high synthesis costs limit their application in olefin polymerization. Fontaine et al. [61] reported a novel synthetic pathway to amidoquinoline olefin polymerization catalysts 32a–32c, which involved the utilization of more cost-effective and readily available starting materials. The synthesis of the new route to an aminoquinoline ligand is shown in Scheme 3. After activation with [HNMe(C18H37)2]+[B(C6F5)4]−, complexes 32a–32c can be applied to ethylene/1-octene copolymerization. At 140 °C, when the molar ratio of 1-octene to ethylene was 2:1, complex 32a showed high catalytic activity (46.7 × 106 g-copolymer/mol-Hf·h) and incorporation of 1-octene (13.3 mol%) and produced high-molecular-weight copolymer (Mw = 4.61 × 105 g/mol). However, the polymerization activity and molecular weight of complex 32b with zirconium were significantly lower than those of 32a. Additionally, under the same conditions, titanium complex 32c exhibited complete inactivity. Li et al. [62] reported group 4 metal complexes 33a–33d with an amido-trihydroquinoline ligand that had excellent thermal stability in ethylene polymerization and ethylene/1-octene copolymerization. Complex 33a with bulk substituents, activated by [Ph3C][B(C6F5)4], had high activity (88.8 × 106 g-PE/mol-Hf·h) in ethylene polymerization, and the catalytic systems with different metal centers had their own suitable cocatalysts. Hf complexes produced polymers with higher molecular weights (Mw above 106 g/mol) in comparison to Zr and Ti complexes by using [Ph3C][B(C6F5)4] as a cocatalyst, while Zr complexes produced polymers with high molecular weight with B(C6F5)3 as the cocatalyst (Mw = 93.1 × 104 g/mol). In addition, complexes 33a had high 1-octene incorporation (43.5 mol%) in the copolymerization of ethylene and 1-octene, which was higher than complex 32a. The complexes containing aminoquinoline or its derivative ligands were found to be generally suitable for high-temperature (co)polymerization, primarily due to the enhanced rigidity of the ligand resulting from the presence of aromatic rings. To better illustrate the significant impact of complex structure on catalytic activity and product performance, the (co)polymerization results of some N-heterocyclic ligand complexes with pyridine, pyrazolato, imidazole, quinoline, and trihydroquinoline are shown in Table 6.

3. Conclusions and Outlook

Non-metallocene catalysts have been a research focus in the field of olefin polymerization catalysts for the past two decades. Among them, nitrogen-donor-containing bidentate ligands play an important role, especially for [N,N] ligand(s). [N,N] ligand(s) metal complexes, as a series of non-metallocene catalysts with great variety and a wide range of applications, have progressed the development of more new polyolefin materials, such as UHWMPE, olefin copolymers (E/NBE and ethylene/α-olefin) with high insertion rates, and highly isotactic or syndiotactic polypropylene. The catalytic systems mentioned in this paper generally have high catalytic activity for olefin polymerization, but there are also extremes, such as thiazole-amine hafnium complex 31d with extremely high catalytic activity and pyrazolato zirconium complex 29 with very low catalytic activity. The synthetic raw materials of [N,N] bidentate ligands containing N-heterocyclic pyridine amine and quinoline amine are readily accessible at low cost, and the corresponding metal complexes can be easily synthesized. The structure of ligands determines the performance of catalytic systems. In [N,N] ligands, different metal complexes have various properties. The titanium complexes bis(benzamidinate) (2d), β-diiminato (9a), and amino-containing (13a) have higher activity than zirconium complexes that have the same ligand(s), while guanidinato binuclear (5f) and pyrrolide-imine zirconium (21a) have higher activity than other metal complexes. The thiazole–, imidazole–, and pyridine–amine ligands containing N-heterocyclic groups are relatively rare [N,N] bidentate ligands coordinated with hafnium, which exhibit superior properties compared with titanium or zirconium complexes. However, the performance of catalysts is not only affected by metal. In most of the complexes, the steric hindrance and electronic effect of the catalyst system can be changed by modifying the substituents on the ligand skeleton, influencing the activity and thermal stability of the system. However, in non-symmetric guanidinato complexes, the modified ligands have little effect on the catalytic system. The presence of a single metal center is commonly observed in [N,N] non-metallocene catalysts, and binuclear or multinuclear systems are also present, although they may lead to the appearance of multiple active sites, resulting in a wider molecular weight distribution within the polymer. Additionally, the performance of catalysts is also influenced by the choice of solvent and co-catalyst, as well as modifications to the polymerization conditions.
The design of novel catalyst structures and further exploration into catalytic mechanisms will enhance the potential application of [N,N] bidentate non-metallocene catalysts in a wider range of fields. From oligomers to ultrahigh-molecular-weight polymers and atactic to highly isotactic polymers, complexes with different structures can obtain products with different monomer incorporation, molecular weight, and distribution. It is crucial to identify the coordination number between the ligand(s) and metal center and investigate the electronic and steric hindrance on the ligand framework, as well as to screen complex structures with diverse properties. At present, bidentate non-metallocene catalysts exhibit superior activity, thermostability, and adjustability of ligand structure compared with traditional catalysts. Catalysts with exceptional performance have been successfully employed in both industrial and commercial applications. However, when compared with the entirety of non-metallocene catalyst systems, the potential for further enhancement remains. The development of new ligands remains the primary focus as a future direction for advancement, and based on the research into early non-metallocene catalysts, combination with other types of catalysts is also a direction of development.

Author Contributions

Z.W.: literature collection and paper draft writing; C.W.: project administration; J.C., S.Q. and X.L.: literature collection; W.W.: funding acquirement and project administration. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Natural Science Foundation of China (NSFC, U22B6012, U23B6011).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

All authors were employed by the company SINOPEC (Beijing) Research Institute of Chemical Industry Co., Ltd. (BRICI). All authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Polymers 16 00406 g001
Figure 1. The structure of complexes (A–D).
Figure 1. The structure of complexes (A–D).
Polymers 16 00406 g001
Polymers 16 00406 g002
Figure 2. The structure of complexes 1a–1j, 2a–2m, 3, 4a–4d.
Figure 2. The structure of complexes 1a–1j, 2a–2m, 3, 4a–4d.
Polymers 16 00406 g002
Polymers 16 00406 sch001
Scheme 1. The reaction of the tri-amidinate complex’s migration to aluminum in MAO.
Scheme 1. The reaction of the tri-amidinate complex’s migration to aluminum in MAO.
Polymers 16 00406 sch001
Polymers 16 00406 g003
Figure 3. The structure of complexes 5a–5f, 6a–6e (L1–L5).
Figure 3. The structure of complexes 5a–5f, 6a–6e (L1–L5).
Polymers 16 00406 g003
Polymers 16 00406 g004
Figure 4. The structure of complexes 7a–7c, 8a–8b, 9a–9d, 10, 11a–11f.
Figure 4. The structure of complexes 7a–7c, 8a–8b, 9a–9d, 10, 11a–11f.
Polymers 16 00406 g004
Polymers 16 00406 sch002
Scheme 2. Plausible mechanism for the formation of complex 10.
Scheme 2. Plausible mechanism for the formation of complex 10.
Polymers 16 00406 sch002
Polymers 16 00406 g005
Figure 5. The structure of complexes 12a–12d, 13a, 13b, 14a–14c, 15a–15d, 16, 17a, 17b, 18a, 18b, 19a, 19b.
Figure 5. The structure of complexes 12a–12d, 13a, 13b, 14a–14c, 15a–15d, 16, 17a, 17b, 18a, 18b, 19a, 19b.
Polymers 16 00406 g005
Polymers 16 00406 g006
Figure 6. The structure of complexes 20a–20e, 21a–21j, 22a–22e, 23, 24a–24d.
Figure 6. The structure of complexes 20a–20e, 21a–21j, 22a–22e, 23, 24a–24d.
Polymers 16 00406 g006
Polymers 16 00406 g007
Figure 7. The synthesis of the block copolymer.
Figure 7. The synthesis of the block copolymer.
Polymers 16 00406 g007
Polymers 16 00406 g008
Figure 8. The electronic attractive interaction of PI catalysts.
Figure 8. The electronic attractive interaction of PI catalysts.
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Polymers 16 00406 g009
Figure 9. The structure of complexes 25, 26, 27, 28, 29, 30a, 30b, 31a–31e, 32a–32c, 33a–33d.
Figure 9. The structure of complexes 25, 26, 27, 28, 29, 30a, 30b, 31a–31e, 32a–32c, 33a–33d.
Polymers 16 00406 g009
Polymers 16 00406 sch003
Scheme 3. The synthesis of a new route to an aminoquinoline ligand.
Scheme 3. The synthesis of a new route to an aminoquinoline ligand.
Polymers 16 00406 sch003
Table 1. Propylene polymerization results of monoamidinate titanium complexes at −60 °C.
Table 1. Propylene polymerization results of monoamidinate titanium complexes at −60 °C.
ComplexSolvent[Propene] (mol·L−1)[mm] (%)[mr] (%)[rr] (%)Vicinal Methyl Fraction (%)Time (h)Yield
(g)		Mw (×104)Mw/MnRef.
1aToluene5.9542532.5160.401.21.76[19]
1bToluene5.9441552.3170.63 [19]
1cToluene5.9543523.2171.101.51.71[19]
1dToluene5.9740534.8160.32 [19]
1eToluene5.93932291.8160.05 [19]
1fToluene5.94139201.7160.05 [19]
1gToluene5.9940512.5160.30 [19]
Table 2. Polymerization dates of monoamidinate and bis(benzamidinate) complexes a.
Table 2. Polymerization dates of monoamidinate and bis(benzamidinate) complexes a.
Monomer bComplexAmount
(μmol)		Al:MActivity cTemp.
(°C)		[mmmm]
%		Time (h)Mw (×104)Mw/MnRef.
P1a2020050.0 × 101−60 161.201.76[19]
P1c2020012.9 × 102−60 171.501.71[19]
P1c5020031.3 × 10320 2 [19]
P1a5020030.0 × 10320 2 [19]
S1a5020020.0 × 10250 1 [19]
BD1c5020013.3 × 102−20 3 [19]
P2a7.425011.0 × 1042586.0 44.001.69[23]
P d2a7.4100079.0 × 1042598.0 8.301.42[23]
P2d20100028.5 × 1052517.0 31.012.48[21]
HD2c7.940068.0 × 10225 1.352.62[27]
HD2f7.280011.0 × 10450 0.471.50[27]
P2g15.5100019.0 × 1022563.534.766.80[24]
P2k15.5100036.0 × 1022571.03379.322.33[24]
P2l15.5100013.7 × 1032566.131.542.57[24]
P2m15.5100093.0 × 1022565.035.125.12[24]
a The solvent in all of the polymerization was toluene. b P = propylene, S = styrene, BD = 1,3-butadiene, HD = 1,5-hexadiene. c g-polymer per mol-M per h. d solvent was CH2Cl2.
Table 3. The polymerization results of β-diiminato complexes a.
Table 3. The polymerization results of β-diiminato complexes a.
MonomerComplexAmount (μmol)Al:MPressure (atm)Temp. (°C)Time
(min)		Activity bMwMWDH/% cRef.
E7a12000130576.8 × 10550.27 × 1022.30 [32]
E7c520001306088.0 × 10213.13 × 1046.53 [32]
E/H d7a12000130526.1 × 10615.48 × 1031.8232.3[32]
E/H d7c520001306013.2 × 10321.13 × 10413.357.4[32]
E8a5100010703010.0 × 10526.00 × 1054.10 [34]
E8b5100010701011.0 × 10579.00 × 1047.20 [34]
E9a0.14006402065.0 × 105 [35]
E9d0.14006402039.0 × 105 [35]
E/H e9a0.14006402051.0 × 105 [35]
E1055001503022.4 × 10412.40 × 1056.70 [37]
a The solvent for all of the polymerization was toluene. b g-polymer per mol-M per h. c Hexene contents in copolymer. d E = ethylene, H = 1-hexene (5 mL). e E = ethylene, H = 1-hexene (60 μL).
Table 4. The polymerization results of amino-containing complexes a.
Table 4. The polymerization results of amino-containing complexes a.
MonomerComplexAmount (μmol)Al:MPressure (atm)Temp. (°C)Time
(min)		Activity bMw
(g/mol)		MWDRef.
E13a2.7excess40704530.7 × 104 [40]
E13b3.6excess40701511.2 × 104 [40]
E14a5100010703056.6 × 103 [41]
E14b5100010703045.0 × 103 [41]
E15a5100010703069.2 × 10425.30 × 1044.20[42]
E15b5100010703097.6 × 10441.10 × 10411.00[42]
E/H c15b5100010703010.3 × 10533.40 × 10426.00[42]
E15c5100010703017.3 × 10533.70 × 1048.70[42]
E165100010503048.6 × 10495.10 × 10410.08[43]
E17a120004906018.8 × 10440.00 × 1057[44]
E17b120004906018.5 × 10490.00 × 1048[44]
E/CPE d17b120004506075.0 × 10550.00 × 1032.5[44]
E18a2100010303036.0 × 10320.50 × 1051.49[45]
H e19a47f 30 20.00 × 1031.30~1.40[46]
E19b46f 510.2 × 104 [47]
a The polymerization solvent of complex 19b was C6H5Cl, and the other polymerization solvents were toluene. b g-polymer per mol-M per h. c E = ethylene, H = 1-hexene (6 mL). d E = ethylene, CPE = cyclopentene (40 vol%) CPE content in the copolymer was 3.5 mol%. e H = 1-hexene. f Co-catalyst was [Ph3C][B(C6F5)4].
Table 5. Polymerization data of complexes 22a–22e a.
Table 5. Polymerization data of complexes 22a–22e a.
M bComplexAmount (μmol)MAO (equiv.)Pressure (atm)Temp. (°C)Time
(min)		Activity cMw
(kg/mol)		MWDRef.
E22a550010803040.0 × 10317608,1[52]
E22b550010803052.0 × 10328007.3[52]
E22c550010803056.0 × 10319805.6[52]
E22d550010803096.0 × 1036607.7[52]
E22e550010803094.0 × 1035607.6[52]
E/NBE d22d7.550010801020.0 × 105906.0[52]
E/NBE e22d7.55005801017.0 × 105554.2[52]
a The solvent in all of the polymerization was toluene. b E = ethylene, NBE = norbornene. c g-polymer/mol-Ti·h. d Norbornene = 1.8 mol/L, norbornene content = 31.3 mol%. e Norbornene = 1.8 mol/L, norbornene content = 40.8 mol%.
Table 6. The polymerization results of some N-heterocyclic ligand complexes a.
Table 6. The polymerization results of some N-heterocyclic ligand complexes a.
MonomerComplexAmount (μmol)Al:MPressure (atm)Temp. (°C)Time
(min)		Activity bMw
(g/mol)		MWDRef.
P25 600 2560013.1 × 10311.34 × 1043.60[55]
P260.58 c6.9903.318.0 × 107d3.20[56]
E275200010603012.4 × 10420.30 × 10410.70[57]
E285200010503014.7 × 10429.60 × 10460.00[58]
E291010004251538.0 × 10325.30 × 1044.20[59]
P30a 130 78.0 × 106 [60]
P30b e6.8110 66.0 × 106d [60]
P31b e6.8110 24.6 × 105d13.2[60]
P31d e6.875 40.9 × 107d [60]
E/O f32a0.610 g19.61401046.7 × 10646.04 × 1043.38 h[61]
E/O f32b2.510 g19.61401061.0 × 10528.52 × 1043.68 h[61]
E33a5i5100288.8 × 10514.63 × 1053.80[62]
E33c5i5100248.0 × 10563.00 × 1042.20[62]
E33d5i5100230.0 × 10561.40 × 1044.50[62]
E33c5j5100213.8 × 10693.10 × 1043.70[62]
E/O f33a5j5100238.6 × 10633.80 × 1042.50 h[62]
a The polymerization solvent was toluene. b g-polymer per mol-M per h. c Co-catalysts were Al(iBu)2H and [Me2PhNH][B(C6F5)4] (1.1 equiv). d Mw = 710 kDa (complex 26), Mw = 250 k (complex 30b), Mw = 220 k (complex 31b), Mw = 1320 k (complex 31d). e Co-catalysts were TMA and [PhNMe2H]+[B(C6F5)4]. f E = ethylene, O = 1-octene (300 g) in complexes 32a and 32b, 1-octene (3 M) in complex 33a. g Precatalyst: [HNMe(C18H37)2][B(C6F5)4]:MMAO = 1:1.2:10. h 1-octene incorporation = 13.3 mol% (complex 32a), 1-octene incorporation = 10.4 mol% (complex 32b), 1-octene incorporation = 43.5 mol% (complex 33a). i Co-catalyst is [Ph3C][B(C6F5)4] (1.1 equiv). j Co-catalyst is [B(C6F5)3] (1.1 equiv).
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Wen, Z.; Wu, C.; Chen, J.; Qu, S.; Li, X.; Wang, W. Homogeneous Non-Metallocene Group 4 Metals Ligated with [N,N] Bidentate Ligand(s) for Olefin Polymerization. Polymers 2024, 16, 406. https://doi.org/10.3390/polym16030406

AMA Style

Wen Z, Wu C, Chen J, Qu S, Li X, Wang W. Homogeneous Non-Metallocene Group 4 Metals Ligated with [N,N] Bidentate Ligand(s) for Olefin Polymerization. Polymers. 2024; 16(3):406. https://doi.org/10.3390/polym16030406

Chicago/Turabian Style

Wen, Zhao, Changjiang Wu, Jian Chen, Shuzhang Qu, Xinwei Li, and Wei Wang. 2024. "Homogeneous Non-Metallocene Group 4 Metals Ligated with [N,N] Bidentate Ligand(s) for Olefin Polymerization" Polymers 16, no. 3: 406. https://doi.org/10.3390/polym16030406

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