Next Article in Journal
Hybrid Molecules with Purine and Pyrimidine Derivatives for Antitumor Therapy: News, Perspectives, and Future Directions
Previous Article in Journal
Systematic Investigations of the Huperzine A—Producing Endophytic Fungi of Huperzia serrata in China and Fermentation Optimization Using OSMAC Strategy
Previous Article in Special Issue
One-Pot Syntheses and Characterization of Group VI Carbonyl NHC Coordination Compounds
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Molecules
Volume 30
Issue 13
10.3390/molecules30132706
molecules-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Trifluoromethoxy- and Fluorobenzhydryl-Tuned Nickel Catalysts for Polyethylene Elastomers

by
Ming Liu
1,
Min Sun
1,
Yanping Ma
2,*,
Yizhou Wang
2,
Mingfeng Li
1,* and
Wen-Hua Sun
2,*
1
State Key Laboratory of Petroleum Molecular & Process Engineering, SINOPEC Research Institute of Petroleum Processing Co., Ltd., Beijing 100083, China
2
Key Laboratory of Engineering Plastics, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry Chinese Academy of Sciences, Beijing 100190, China
*
Authors to whom correspondence should be addressed.
Molecules 2025, 30(13), 2706; https://doi.org/10.3390/molecules30132706
Submission received: 17 May 2025 / Revised: 4 June 2025 / Accepted: 18 June 2025 / Published: 23 June 2025
(This article belongs to the Special Issue Featured Papers in Organometallic Chemistry—2nd Edition)
Browse Figures
Versions Notes

Abstract

A series of para-trifluoromethoxy-substituted and fluorobenzhydryl-functionalized 1,2-bis(imine)acenaphthene ligands: 1-[2,6-{(4-F-C6H4)2CH}2-4-F3COC6H2N]-2-(ArN)C2C10H6 (Ar = 2,6-Me2C6H3 L1, 2,6-Et2C6H3 L2, 2,6-iPr2C6H3 L3, 2,4,6-Me3C6H2 L4, 2,6-Et2-4-MeC6H2 L5), were synthesized and used to generate their corresponding nickel(II) bromide complexes (Ni1–Ni5). Elemental analysis, 19F NMR, and FT-IR spectroscopy were employed to characterize these five nickel complexes. Single-crystal X-ray diffraction of Ni2 and Ni4 confirmed distorted tetrahedral geometries. Upon activation with either EtAlCl2 (ethylaluminum dichloride) or EASC (ethyl aluminum sesquichloride), these complexes showed exceptional high activities (up to 22.0 × 106 g PE mol−1 (Ni) h−1) and remarkable thermal stability (4.82 × 106 g PE mol−1(Ni) h−1 at 80 °C) towards ethylene polymerization. The resulting polyethylenes are highly branched, with the type and extent of branches tunable by temperature, solvent, and co-catalyst choice. Moreover, these polymers demonstrated excellent tensile strength (σb up to 20.7 MPa) and elastic recovery (up to 58%), characteristic of thermoplastic elastomers (TPEs). These results highlight the dual role of trifluoromethoxy and fluorobenzhydryl groups in enhancing catalytic performance and polymer properties.

1. Introduction

Thermoplastic olefin elastomers (TPEs) have emerged as indispensable polymeric materials due to their excellent mechanical properties, weather resistance, and processability. Their inherent compatibility with polyolefins and cost-effectiveness in enhancing low-temperature toughness have driven widespread applications across automotive, construction, footwear, and consumer goods [1,2,3]. Conventionally synthesized through ethylene/α-olefin copolymerization (e.g., 1-butene, 1-hexene), TPE production relies on advanced catalytic technologies such as Dow Chemical (Midland, MI, USA) Insite™ solution polymerization [4], ExxonMobil’s Exxpol [5,6], and SABIC SK (Ulsan, South Korea) Nexlene™ processes [5]. However, the pursuit of more efficient catalytic systems capable of precise polymer architecture control remains a critical challenge in polyolefin chemistry.
A pivotal breakthrough occurred in the 1990s when Brookhart et al. discovered that α-diimine Ni/Pd complexes could serve as single-center catalysts for the homopolymerization of ethylene, generating branched polyethylene (PE) [7,8] via a chain-walking mechanism [9,10,11,12]. While this innovation sparked intensive research into late-transition metal catalysts [13,14,15,16,17,18,19,20,21,22,23,24], practical implementation was hindered by inherent limitations: insufficient thermal stability and low activity at industrially relevant temperatures. Recent advancements in iron-based α-olefin polymerization catalysts [25] have revitalized interest in addressing these challenges through innovative ligand design.
The catalytic performance of α-diimine nickel (II) complexes (A, Chart 1) is profoundly governed by structural modifications to the ligand framework, particularly through adjustments to the backbone geometry and N-aryl substituents. Strategic incorporation of bulky groups (such as benzhydryl and its derivatives, or cycloalkyl groups) [26,27,28,29,30,31,32] and para-substituents possessing varied electronic characteristics (-Me, -tBu, -OMe, -Cl, -F, -NO2) enables precise modulation of both steric protection and electronic environment at the metal center. Bulky substituents can effectively improve the thermal stability of catalysts by suppressing deactivation pathways at higher temperature [28,29,33,34], while these para-substituents influence the activities of catalysts by influencing the electron density of the metal center.
For instance, our group demonstrated that 2,6-benzhydryl-substituted asymmetric α-diimine-nickel(II) complexes (B, Chart 1) generate high-molecular-weight, branched PE with activities correlating to the substituent electronic effects (Me, t-Bu, OMe, Cl, F, NO2) [35,36,37,38] at the para-positions of the N-aryl. Notably, catalysts bearing electron-withdrawing substituents tend to exhibit higher activity. When we introduced the strongly electron-withdrawing -OCF3 group into the catalyst framework (C, Chart 1), the net positive charge at the nickel center was increased, boosting its activity up to 15.1 × 106 g PE mol−1 (Ni) h−1. Remarkably, replacing conventional toluene with n-hexane as the reaction solvent further enhanced both catalytic performance and thermal stability [39].
Fluorine substitution has emerged as a powerful strategy for improving catalytic performance. Incorporating distal fluorine atoms into benzhydryl group (D, Chart 1) (Me [40], t-Bu [41], OMe [42], CH(p-FPh2) [43], F [44], NO2 [45]) markedly improved the thermal stability relative to non-fluorinated analogs (B, Chart 1). For example, DNO2, containing strong electron-withdrawing nitro and fluorine substituents, exhibited superior catalytic activity and thermal stability compared to BNO2 complex [38,45]. The DtBu/EASC catalytic system maintained high catalytic activity at 90 °C, and the resulting highly branched PE exhibited typical thermoplastic elastomer (TPE) characteristics [41].
Building on these insights, to synergistically boost both catalytic activity and thermal stability, we devised a dual-functional modification strategy herein: simultaneous installation of a strongly electron-withdrawing trifluoromethoxy group at the N-aryl para-position and distal fluorine atoms into the benzhydryl groups (E, Chart 1). The impact of these modifications on ethylene polymerization were systematically explored by exploring variables such as the ligand structure, co-catalyst choice, solvent (toluene vs. n-hexane), polymerization temperature, running time, and ethylene pressure. Exceptional performance was disclosed at industrially relevant temperatures (up to 80 °C), producing highly branched PE with tunable architectures. Comprehensive characterization (1H/13C/19F NMR, FT-IR, GPC, DSC, XRD, etc.) was used to confirm structures of the ligands, complexes, and PE.

2. Results and Discussion

2.1. Synthesis and Characterization of the Ligands and Complexes

The N,N′-diimineacenaphthenes L1–L5 1-[2,6-{(4-FC6H4)2CH}2-4-F3COC6H2N]-2- (ArN)C2C10H6 (Ar = 2,6-Me2C6H3 (L1), 2,6-Et2C6H3 (L2), 2,6-iPr2C6H3 (L3), 2,4,6-Me3C6H2 (L4), 2,6-Et2-4-MeC6H2 (L5)), were prepared via a two-step process (Scheme 1).
Firstly, 2-[2,6-bis(4,4′-difluorobenzhydryl)-4-(trifluoromethoxy)phenylimino]-acenaphthylen-1-one, was synthesized in reasonable yield by the acid catalyzed condensation reaction of acenaphthylene-1,2-dione with one molar equivalent of 2,6-bis(4,4′-difluorobenzhydryl)-4-(trifluoromethoxy)aniline [40,41,42,43,44,45]. This imine-ketone then reacted with the corresponding aniline to afford L1–L5 in moderate yield (22.5–38.0%) (Scheme 1). All ligands were characterized by 1H/13C/19F NMR (Figures S1–S15), FT-IR spectroscopy, and elemental analysis, confirming their structural integrity. Nickel(II) bromide complexes Ni1–Ni5 were generated in good yields (85–92%) by treatment of L1–L5 with (DME)NiBr2 (DME = 1,2-dimethoxyethane) in a mixed solvent system composed of dichloromethane and ethanol at room temperature (Scheme 1). All five complexes were characterized by elemental analysis, FT-IR and 1H/19F NMR spectroscopy (Figures S16–S25). Moreover, the molecular structures of Ni2 and Ni4 were confirmed by single-crystal X-ray diffraction analysis (see Table S1: Crystal data and structure refinement for Ni2 and Ni4).
Single crystals of Ni2 and Ni4 of suitable quality for the X-ray determinations were grown by layering diethyl ether into their dichloromethane solutions at ambient temperature. The molecular structures of Ni2 and Ni4 (Figure 1) revealed distorted tetrahedral geometries with the nickel center coordinated by two bromide ligands and the two nitrogen donors from the N,N′-chelating bis(imine)acenaphthene, as is similar to those reported previously [40,44].
Key bond parameters including bond lengths and angles were listed in Table 1. The N1-Ni1-N2 bite angle of 82.73(12)° in Ni2 is close to 82.6(3)° in Ni4, as is the same for Br1-Ni-Br2 bite angles (126.05(3)° (Ni2) vs. 126.56(7)° (Ni4)). Nickel-nitrogen bond lengths of Ni1–N1 [ 2.037(3) Å (Ni2), 2.041(7) Å (Ni4)] are slightly longer than those of Ni1-N2 [2.022(3) Å (Ni2), 2.011(7) Å (Ni4)], reflecting subtle steric variations between the aryl substituents. The C=Nimine bond lengths for both complexes range from 1.261(11) to 1.290(4) Å, which are typical for this functional group [24]. Each imine unit is positioned nearly co-planar with the adjacent acenaphthene unit. The plane of the N-aryl group linked to N1 is almost perpendicular to the acenaphthene plane, with dihedral angles of 89.37° for Ni2 and 89.04° for Ni4. On the other hand, due to the smaller steric hindrance, axial rotation is easier for the second N-aryl group (aryl = 2,6-Et2C6H3 Ni2; 2,4,6-Me3C6H2 Ni4), showing some variation [87.09° for Ni2, 87.41° for Ni4]. Notably, the F3C–O bond has double bond character with distance values of 1.342(6) Å for Ni2, 1.23(5) Å for Ni4, suggesting partial ionic resonance contribution from the limiting form ArO+=CF2F− [46]. Meanwhile, there are no significant intermolecular contacts.
19F NMR spectra of L1–L5 (Figure S11–S15) and Ni1–Ni5 (Figure S21–S25) corroborate ligand coordination. Taking Ni1 as an example, the spectra were compared with that of the free ligand L1. For L1, two distinct signals at δ −116.12 and −116.35 arise from restricted rotation of the ortho-aryl groups leading to inequivalent CH(4-FC6H4)a(4-FC6H4)b groups. Upon complexation (Ni1), these signals split further (δ −115.73 and −116.07), consistent with previous studies [41]. The para-trifluoromethoxy resonance shifts upfield slightly from δ −58 in L1 to δ −57 in Ni1, reflecting electronic modulation at the nickel center [39]. FT-IR spectra confirm effective coordination of the two imine nitrogen atoms to the nickel center via redshifted νC = N absorptions (ligands: 1672–1635 cm−1; complexes: 1649–1618 cm−1).

2.2. Ethylene Polymerization

2.2.1. Co-Catalyst Screening with Ni1 for Ethylene Polymerization

The nickel-based catalyst precursor Ni1 was evaluated in ethylene polymerization using five alkylaluminum reagents co-catalysts: methylaluminoxane (MAO), modified methylaluminoxane (MMAO), ethylaluminum dichloride (EtAlCl2), ethylaluminum sesquichloride (EASC), and diethylaluminum chloride (Et2AlCl)]. Polymerizations were conducted in toluene with ethylene pressure of 10 atm at 30 °C for 30 min, with the results summarized in Table 2.
All alkylaluminum-activated Ni1 catalysts exhibited high ethylene polymerization activity, producing PEs with high molecular weight (~105 g mol−1) and low melting temperatures (Tm < 100 °C) indicative of a branched polymer microstructure. The molecular weight distributions were unimodal in the range of 2.48–4.03, indicating single site active species. With regard to the three chloroalkylaluminums, EtAlCl2 exhibits the highest activity, presumably attributed to its stronger Lewis acidity, enabling more efficient activation of the metal center [47]. This phenomenon is contrasts to previously reported findings [48]. Among these co-catalysts screened, EtAlCl2 and EASC exhibited the highest catalytic activities, reaching 13.2 × 106 g PE mol−1 (Ni) h−1 and 8.78 × 106 g PE mol−1 (Ni) h−1, respectively. These values represent an enhancement compared to MAO and MMAO under identical conditions. Due to their superior performance, EtAlCl2 and EASC were selected for subsequent optimization of polymerization conditions (such as Al/Ni ratio, polymerization temperature, reaction time, and ethylene pressure) and evaluation with other catalyst precursors.

2.2.2. Optimizing Ethylene Polymerization Parameters Using Ni1–Ni5/EtAlCl2

In this section, we systematically investigated the effects of Al:Ni molar ratio, polymerization temperature, reaction time, ethylene pressure, and ligand structure on ethylene polymerization using Ni1–Ni5 precatalysts activated by EtAlCl2 (Table 3).
First, we systematically evaluated the polymerization behavior across reaction temperatures ranging from 30 °C to 80 °C, under fixed conditions: 10 atm ethylene pressure, Al/Ni = 300, 30 min (runs 1–6, Table 3). The results revealed that catalytic activity reached its peak at 40 °C (14.2 × 106 g PE mol−1 (Ni) h−1), followed by a decrease at higher temperatures attributed to partial deactivation of active species and reduced ethylene solubility [49,50]. Despite this, the activity value of 2.76 × 106 g PE mol−1 (Ni) h−1 at 80 °C remained notably high, underscoring the positive influence of bulky substituents and the distal F group on thermal stability [39,40,41]. Concurrently, the polymer molecular weight gradually decreased from 5.96 × 105 g mol−1 to 2.26 × 105 g mol−1 with increasing the temperature from 30 to 80 °C, due to accelerated chain transfer rate [45,51,52,53]. Notably, over this temperature span, all polymers displayed narrow dispersity (Mw/Mn range: 1.73–3.55) and a unimodal distribution (Figure S26), confirming a single active species participated in the polymerization process. The polymer’s melting point (Tm) inversely correlated with the reaction temperature, likely due to reduced molecular weight and increased branching density at elevated temperatures, leading to reduced crystallinity. These results suggest that the high temperatures promote chain walking isomerization, enhancing branching [54].
Next, we fixed the polymerization temperature at 40°C and varied the Al:Ni molar ratio from 100:1 to 300:1 to assess the influence of co-catalyst concentration (runs 2, 7–10, Table 3). As the Al:Ni molar ratio increased, catalytic activity initially rose, reaching a peak of 17.2 × 106 g PE mol−1 (Ni) h−1 at 200:1. Beyond this optical ratio, the activity declined, indicating that excess EtAlCl2 concentration may facilitate chain transfer from the Ni center to the Al center, resulting in chain termination [55].
Further analysis of reaction time dependence (runs 2, 11–14, Table 3) revealed a distinct inverse relationship between time and activity. The highest activity was achieved within the first 5 min (33.2 × 106 g PE mol−1 (Ni) h−1), followed by a gradual decline, suggesting rapid initial activation of the Ni center by EtAlCl2, and progressive deactivation of active species over time [41]. Nonetheless, catalytic activity remained robust at 60 min [11.8 × 106 g PE mol−1 (Ni) h−1], highlighting the stability of the residual active species and their prolonged catalytic lifetime. Meanwhile, the PE molecular weight increased steadily from 2.78 × 105 g mol−1 to 5.08 × 105 g mol−1, indicating that although some active sites deactivated, others retained activity, enabling continued chain growth over time. When polymerization pressures were lowered (10 atm, 5 atm, 1 atm), both catalytic activity and polymer molecular weight were found to diminish, likely due to the reduced ethylene concentration at lower pressure [38,44].
Under optimized conditions (Al/Ni = 200, 40 °C, 30 min, 10 atm), ligand modifications in Ni1–Ni5 significantly impacted catalytic performance. All complexes exhibited high ethylene polymerization catalytic activity [6.74–17.2 × 106 g PE mol−1 (Ni) h−1], producing high molecular weight PE (Mw: 3.57–6.68 × 105 g mol−1) with low melting points (Tm: 51.1–63.8 °C), indicating a high degree of branching. The catalytic activity followed the order: Ni1 > Ni4 > Ni5 > Ni2 > Ni3, indicating that catalytic systems with smaller substituents demonstrate superior activity, by minimizing steric hindrance to ethylene coordination and insertion at the active site [45]. Conversely, Ni3 bearing bulkier substituents, exhibited the lowest activity by yield the PE with highest molecular weight (6.68 × 105 g mol−1, run 18, Table 3), suggesting that bulky substituents protect active species and mitigate chain termination.

2.2.3. Optimizing Ethylene Polymerization Parameters Using Ni1–Ni5/EASC

Similarly, we evaluated ethylene polymerization performance of Ni1–Ni5 complexes using EASC as the co-catalyst, with experimental results listed in Table 4. The Ni1–Ni5/EASC system also exhibited efficient catalytic activity, yielding high molecular weight PE with narrow distribution and low melting points, indicative of branched polymer microstructures.
As the temperature rises from 30 °C to 80 °C (run 1–6, Table 4), the catalytic activity of Ni1/EASC initially increases to a maximum of 21.4 × 106 g PE mol−1(Ni) h−1 at 40 °C, followed by a gradual decline, while polymer molecular weight steadily decreased—a pattern mirroring the Ni1/EtAlCl2 system (Figure 2). Notably, Ni1/EASC showed 1.5-fold higher peak activity than Ni1/EtAlCl2 [21.4 vs. 14.2 × 106 g (PE)·mol−1(Ni)·h−1] and retained superior thermal stability at elevated temperatures. At 80 °C, Ni1/EASC maintained 75% higher activity [4.82 × 106 vs. 2.76 × 106 g (PE)·mol−1(Ni)·h−1], underscoring the beneficial influence of EASC as co-catalyst on the thermal stability of the catalytic system. Moreover, due to enhanced chain transfer rate at higher temperatures, the molecular weight of PEs falls as temperature increases: At 80 °C, Mw drops by 54% for Ni1/EASC and by 63% for Ni1/EtAlCl2 compared with their values at 30 °C (Figure 2b).
Next, we set the reaction temperature to 40 °C and varied the Al:Ni molar ratio from 250:1 to 450:1 to examine the impact of co-catalyst concentration on polymerization performance (runs 2, 7–10, Table 4). The activity peaked at 350:1 [22.0 × 106 g PE mol−1 (Ni) h−1], higher than the optical ratio for EtAlCl2 (200:1), attributed to the distinct properties of the two alkylaluminum reagents in EASC. Notably, this activity value surpasses most reported for N,N′-nickel catalysts under similar reaction conditions [24,35−45], except for one cycloalkyl substituted analog [22]. At an Al/Ni ratio of 450:1, the PE’s relative molecular weight reached its maximum value of 7.16 × 105 g mol−1.
Under optimal conditions, Ni1/EASC exhibits significantly faster initial activation (33.2 × 106 vs. 19.6 × 106 g PE mol−1(Ni) h−1 5 min) but accelerated deactivation (60% activity loss at 60 min) compared to Ni1/EtAlCl2 (40% loss) (Figure 3a). Despite this, Ni1/EASC consistently yielded PE with higher molecular weight (runs 8, 11–14, Table 4) than Ni1/EtAlCl2 (e.g., 7.78 × 105 vs. 5.08 × 105 g mol−1 at 60 min; Figure 3b), underscoring its superior propagation efficiency under similar conditions. Likewise, reducing ethylene pressure diminishes both activity and polymer molecular weight in the Ni1/EASC system (runs 8, 15–16, Table 4), mirroring trends observed for Ni1/EtAlCl2.
Having identified the optimal reaction conditions for Ni1/EASC (Al:Ni = 350:1, 30 °C, 30 min, 10 atm ethylene), we further investigated the influence of ligand structural variations on polymerization performance (runs 2, 17–20, Table 4). The activity of the nickel complexes decreased in the following order: Ni1 > Ni5 > Ni2 ≈ Ni4 > Ni3. While the reasons behind these activity variations remain unclear, it is possible that different counteranions generated during catalyst activation affect the solubility of the complexes. Additionally, the electron-donating methyl group at the para-position of the N-aryl substituent appears to negatively affect polymer molecular weight, a phenomenon previously reported [45].
Building on prior findings that solvents critically influence polymerization reactions [39], we explored the impact of n-hexane versus toluene on Ni1/EASC (Table 5).
Using identical optimal reaction conditions (Al:Ni = 350:1, 40 °C, 30 min, 10 atm ethylene), replacing toluene with n-hexane noticeably reduced catalytic activity [hexane: 2.46 × 106 vs. toluene: 22.0 × 106 g PE mol−1 (Ni) h−1] but increased polymer molecular weight [hexane: 8.24 × 105 vs. toluene: 6.15 × 105 g mol−1]. Further optimization the polymerization using hexane as the solvent showed catalytic activity peaked at 13.7 × 106 g PE mol−1 (Ni) h−1 when Al:Ni = 550:1 at 40 °C, though still lower than the maximum activity observed in toluene. The cause of this behavior remains uncertain, it may be due to solvent-induced changes in catalyst solubility, solvent polarity, ligand electronic effects, and the ethylene coordination environment. Additionally, polymers synthesized in n-hexane exhibited higher melting points (e.g., 116.8 °C in n-hexane vs. 59.5 °C in toluene, Al/Ni = 350, T = 40 °C), suggesting reduced branching (vide infra).

2.3. Systematic Investigation of Substituent Effects on Catalytic Performance

2.3.1. Role of the para-Trifluoromethoxy (-OCF3) Group

To clarify the impact of the para-trifluoromethoxy (-OCF₃) group on catalytic activity and molecular weight of the polymer, we compared Ni1 with two reported asymmetric bis(arylimino)acenaphthene-nickel(II) bromide complexes (D, Chart 1) bearing para-OMe [42] and para-NO2 [45] substituents (Figure 4). All polymerization reactions shown in the figure were carried out under identical conditions (EASC co-catalyst, toluene, 30 °C, and 10 atm ethylene).
From Figure 4, it is clear that introducing an -OCF3 group at the para-position significantly enhances catalytic activity: Ni1 achieved 22.0 × 106 g PE mol−1 (Ni) h−1 at 40 °C, nearly three times that of its para-methoxy (-OMe) analog. This improvement is due to the strong electron-withdrawing nature of the para-OCF3, which likely polarizes the nickel center, facilitating ethylene coordination and insertion. In contrast, the para-nitro (-NO2) analog exhibited reduced activity attributed to stronger electron-withdrawing of -NO2, which impedes ethylene insertion [45]. Regarding PE molecular weight, all three nickel complexes in Figure 4 produced high molecular weight PEs (105 g mol−1), suggesting that the ortho-bulky substituents help promote chain growth while effectively suppressing chain-transfer reaction [37]. Among them, the complex with the -OCF3 group generated the highest molecular weight PE at 6.15 × 105 g mol−1, emphasizing the promoting effect of this strongly electron-withdrawing group on ethylene insertion [45].

2.3.2. Effects of the Distal Fluorine Atom

To isolate the influence of the distal fluorine substituent, we compared Ni1 with its non-fluorinated analog C [39] (Chart 1) under identical conditions (Figure 5). We found that Ni1 bearing the distal fluorine exhibits markedly enhanced catalytic activity and remains highly efficient for ethylene polymerization at the industrially relevant temperature (70 °C), presumably due to the enhanced Lewis acidity of the active metal species, leading to more efficient ethylene coordination and insertion. This demonstrates the positive role of the fluorine atom in boosting the catalyst’s thermal stability and suggests excellent prospects for industrial application. In terms of polymer molecular weight, introduction of the distal F atom causes a slight decrease in the resulting PE’s molecular weight, which may attribute to participant of the distal F atoms in chain-terminating interactions via weak C–H···F interactions with the growing polymer chain or solvent. This observation is consistent with previous study [56]. However, due to the bulky ortho substituent and the strongly electron-withdrawing para-trifluoromethoxy group, the obtained PE still maintains Mw on the order of 105 g mol−1.
In summary, by employing a dual-modification strategy with introducing trifluoromethoxy group and distal fluorine substituents, we have dramatically enhanced the activity and thermal stability of the pre-catalyst, enabling efficient ethylene polymerization at industrially relevant temperatures to yield high-molecular-weight PEs.

2.4. Branching Properties of the PEs

All PEs synthesized in this study have melting temperatures (Tm) below 120 °C, characteristic of branched PE (Table 2, Table 3, Table 4 and Table 5). To quantify the effects of temperature, solvent, co-catalyst type and concentration on branching density and topology, 13C NMR spectroscopy (measured at 100 °C, using C6D4Cl2 as solvent) was used to analyze selected PE samples (Figure 6 and Figures S27–S31). From these spectra, characteristic signals corresponding to distinct carbon environments in the polymer chain and branches were identified, with results shown in Table 6.
A comparative analysis of PE samples synthesized with EtAlCl2 (PE-40EADC/300/T, PE-60EADC/300/T) and EASC PE-40EASC/300/T, and PE-60EASC/300/T revealed that increasing polymerization temperature elevated the degree of branching, while reducing the methyl branch content and promoting other types of branches. This indicates that higher temperatures favor chain walking, which leads to the formation of longer and more complex branched structures [57].
Table 6. Branching analysis of selected PE samples a.
Table 6. Branching analysis of selected PE samples a.
PE Sample bBranches/1000 C’sMethyl Branches/%Ethyl Branches/%Propyl Branches/%Butyl Branches/%Amyl Branches/%Longer Branches/%
PE-40EADC/300/T96.577.05.215.503.522.596.17
PE-60EADC/300/T12870.27.296.925.233.047.31
PE-40EASC/300/T90.580.85.635.183.711.073.56
PE-60EASC/300/T10779.45.465.263.261.625.03
PE-40EASC/450/T10277.35.605.753.103.035.21
PE-40EASC/450/H52.683.04.833.701.811.555.10
a Data determined from the 13C NMR spectrum using approaches described by G. B. Galland [58]; b Naming example: PE-40EADC/300/T: 40 °C, EtAlCl2, Al:Ni = 300, toluene.
Compared to EtAlCl2, PE produced with EASC as a co-catalyst exhibited a lower degree of branching and lower long-chain content. In terms of the co-catalyst concentration, as Al:Ni increased from 300 to 450, the degree of branching in the resulting polymer increased (90.5 to 102 branches/1000 C), which explains why PE-40EASC/450/T has a lower melting point (56.7 vs. 58.7 °C) despite a higher molecular weight (7.16 vs. 6.41 × 105 g mol−1; Table 4 run 10 vs. Run 2). Clearly, the choice of solvent affects the degree of branching: under the same conditions, samples obtained in toluene exhibited higher branching (102 branches/1000 C) than those obtained in hexane (52.6 branches/1000 C). This is contrasts to previously reported findings [39], and it seems that the introduction of distal fluorine substituent may perturb solvent-polymer interactions or chain-transfer pathways, though the underlying cause of this phenomenon requires further investigation.

2.5. Mechanical Properties of the PE

To explore the mechanical properties of these branched polymers, five samples—PE-40EADC/300/T (run 2, Table 3), PE-40EASC/300/T (run 2, Table 4), PE-60EASC/300/T (run 4, Table 4), PE-40EASC/450/T (run 10, Table 4), and PE-40EASC/450/H (run 2, Table 5) were subjected to monotonic tensile stress–strain testing. Five specimens of each sample were tested, and the average results were calculated to ensure statistical reliability. The tensile test data are shown in Table 7, and the stress–strain curves and hysteresis loops are provided in Figure 7.
We found that these polymers exhibited superior tensile strength compared to those previously reported using nickel-catalysted systems [19,20,35,39,41,50]. In terms of the effects of polymerization conditions, PE-40EADC/300/T (activated with EtAlCl2) and PE-40EASC/300/T (activated with EASC) showed comparable tensile strengths, but PE-40EADC/300/T, which had a higher degree of branching (96.5 vs. 90.5 branches/1000 C), demonstrated a higher elongation at break (585% vs. 482%). For samples obtained at higher temperatures, crystallinity decreased to 17.0%, resulting in a reduction in tensile strength to 13.7 MPa, while elongation at break increased to 572%. On the other hand, when n-hexane was used as the solvent instead of toluene, PE-40EASC/450/H exhibited improvements in both tensile strength (20.7 MPa) and elongation at break (439%) compared to PE-40EASC/450/T. These results indicate that polymer molecular weight, crystallinity, and branching structure significantly influence the tensile properties of PE.
Finally, to assess the elastic recovery performance of the polymers, dynamic mechanical analysis (DMA) was used to perform hysteresis tests on PE-60EASC/300/T. The tests were conducted at 30 °C, with each cycle repeated 10 times, as shown in Figure 7b. The elastic recovery rate (SR) was calculated using the standard formula SR = 100(εa − εr)/εa, where εa is the applied strain, and εr is the strain after 10 cycles at zero load. Notably, after 10 cycles, the sample retained good elastic recovery performance with an SR of 58.2%, which can be attributed to the high degree of branching and the relatively high content of long branches in the PE [35]. In summary, these PE samples demonstrated excellent tensile strength, elongation at break, and elastic recovery, making them potential substitutes for commercial thermoplastic elastomers.

3. Materials and Methods

3.1. Synthesis of 2-(2,6-Bis(bis(4-fluorophenyl)methyl)-4-trifluoromethoxy)acenaphthylen-1-one

To a mixture of 2,6-bis(4,4′-difluorobenzhydryl)-4-(trifluoromethoxy)aniline (4.65 g, 8 mmol), acenaphthylene-1,2-dione (1.60 g, 8.8 mmol), and a catalytic amount of p-toluenesulfonic acid (1.25 g), toluene (30 mL) was added. The resulting solution was refluxed for 5.5 h. After the reaction, all volatiles were removed under reduced pressure, and the residue was purified by alumina column chromatography (500/8 petroleum ether/ethyl acetate). The product was obtained as a yellow powder (3.76 g, 63.0%). 1H NMR (400 MHz, CDCl3, TMS): δ 8.13–8.08 (m, 2H), 7.86 (d, J = 8.4 Hz, 1H), 7.78 (t, J = 7.8 Hz, 1H), 7.10 (t, J = 7.8 Hz, 1H), 6.95 (d, J = 6.8 Hz, 9H), 6.81 (s, 2H), 6.79–6.75 (m, 4H), 6.31 (t, J = 8.4 Hz, 3H), 5.99 (d, J = 6.8 Hz, 1H), 5.40 (s, 2H). 13C NMR (100MHz, CDCl3, TMS): δ 188.98, 162.98, 162.88, 162.07, 160.44, 159.62, 146.49, 145.81, 142.54, 137.24, 137.12, 136.38, 136.35, 133.82, 132.66, 132.52, 131.27, 131.20, 130.93, 130.86, 130.72, 130.64, 130.56, 130.07, 129.99, 129.53, 129.47, 128.48, 128.14, 127.04, 126.19, 123.34, 122.27, 122.12, 121.53, 120.85, 119.17, 115.88, 115.67, 115.49, 115.28, 115.20, 114.99, 50.51.

3.2. Synthesis of Ligands (L1–L5)

L1. To a mixture of 2-(2,6-bis(4,4′-difluorobenzhydryl)-4-trifluoromethoxy) acenaphthylen-1-one (1.49 g, 2.0 mmol), 2,6-dimethylaniline (0.36 g, 3.0 mmol), and a catalytic amount of p-toluenesulfonic acid (0.37 g), toluene (30 mL) was added. The resulting solution was refluxed for 6 h. After the reaction, all volatiles were removed under reduced pressure, and the residue was purified by alumina column chromatography (500/8 petroleum ether/ethyl acetate). The product was obtained as a yellow powder (0.49 g, 27.6%). 1H NMR (400 MHz, CDCl3, TMS): δ 7.82 (d, J = 8.4 Hz, 1H, An-H), 7.75 (d, J = 8.0 Hz, 1H, An-H), 7.33 (t, J = 7.6 Hz, 1H, An-H), 7.19 (d, J = 7.6 Hz, 2H, Ar-H), 7.13–7.09 (m, 1H, Ar-H), 7.06 (t, J = 7.6 Hz, 1H, An-H), 7.02–6.94 (m, 8H, Ar-H), 6.87–6.82 (m, 6H, Ar-H), 6.59 (d, J = 7.2 Hz, 1H, An-H), 6.32 (t, J = 8.4 Hz, 1H, Ar-H), 5.98 (d, J = 7.2 Hz, 1H, An-H), 5.59 (s, 2H, -CHPh2), 2.19 (s, 6H, -CH3). 13C NMR (100MHz, CDCl3, TMS): δ 164.2, 162.9, 162.0, 161.1, 160.4, 159.6, 148.8, 147.4, 145.4, 139.9, 137.5, 137.5, 136.5, 136.5, 134.2, 131.1, 131.0, 130.7, 130.6, 130.1, 129.3, 129.2, 128.5, 128.3, 128.1, 127.9, 126.7, 124.5, 124.1, 123.6, 122.4, 121.8, 120.9, 119.2, 115.4, 115.2, 115.1, 114.9, 50.9, 50.6, 18.1. 19F NMR (470 MHz, CDCl3, TMS): δ −58.17, −116.12, −116.35. FT-IR (cm−1): 3074 (w), 3043 (w), 2977 (w), 2941 (w), 1662 (m, vC = N), 1635 (w, vC = N), 1697 (m), 1506 (s), 1465 (w), 1440 (m), 1257 (w), 1217 (w), 1159 (w), 1095 (w), 1040 (w), 1012 (w), 928 (w), 875 (w), 828 (m), 802 (w), 769 (m). Anal. calcd for C53H35F7N2O·H2O (848.86): C, 74.99; H, 4.16; N, 3.30. Found: C, 75.24; H, 4.10; N, 3.11%.
L2. Using a typical procedure as described for the synthesis of L1, L2 was prepared as a yellow powder (0.51 g, 29.1%). 1H NMR (400 MHz, CDCl3, TMS): δ 7.80 (d, J = 8.0 Hz, 1H), 7.71 (d, J = 8.4 Hz, 1H), 7.31 (t, J = 7.8 Hz, 1H), 7.25–7.20 (m, 3H), 7.02–6.94 (m, 9H), 6.86–6.83 (m, 6H), 6.58 (d, J = 7.2 Hz, 1H), 6.30 (t, J = 8.6 Hz, 4H), 5.88 (d, J = 7.2 Hz, 1H), 5.60 (s, 2H), 2.70–2.61 (m, 2H), 2.54–2.45 (m, 2H), 1.17 (t, J = 7.4 Hz, 6H). 13C NMR (100MHz, CDCl3, TMS): δ 162.9, 162.0, 161.2, 160.4, 159.6, 147.9, 147.5, 139.9, 137.8, 136.5, 136.4, 134.2, 131.0, 130.7, 130.6, 130.4, 130.00, 129.2, 129.0, 128.3, 127.8, 126.6, 126.4, 124.5, 123.7, 122.9, 121.0, 115.4, 115.2, 115.1, 114, 9, 50.6, 24.5, 14.4. 19F NMR (470 MHz, CDCl3, TMS): δ −58.18, −116.13, −116.33. FT-IR (cm−1): 3053 (w), 2966 (w), 2937 (w), 2878 (w), 1672 (w, vC = N), 1654 (w, vC = N), 1598 (w), 1506 (s), 1437 (m), 1253 (w), 1224 (w), 1159 (w), 1095 (w), 1042 (w), 1014 (w), 923 (w), 875 (w), 828 (w), 802(w), 782 (w). Anal. calcd for C55H39F7N2O·H2O (876.92): C, 75.33; H, 4.48; N, 3.19. Found: C, 75.65; H, 4.32; N, 2.87%.
L3. Using a typical procedure as described for the synthesis of L1, L3 was prepared as a yellow powder (0.44 g, 24.1%). 1H NMR (400 MHz, CDCl3, TMS): δ 7.78 (d, J = 8.4 Hz, 1H), 7.68 (d, J = 8.4 Hz, 1H), 7.33–7.29 (m, 4H, An-H), 7.02–6.93 (m, 9H), 6.85–6.82 (m, 6H), 6.48 (d, J = 7.2 Hz, 1H), 6.27 (t, J = 8.6 Hz, 4H), 5.79 (d, J = 7.2 Hz, 1H), 5.61 (s, 2H), 3.14–3.04 (m, 2H), 1.29 (d, J = 6.8 Hz, 6H), 1.01 (d, J = 6.8 Hz, 6H). 13C NMR (100MHz, CDCl3, TMS): δ 164.4, 162.9, 162.0, 161.8, 160.4, 159.6, 147.6, 146.7, 145.5, 139.9, 137.9, 136.4, 135.4, 134.3, 131.0, 130.8, 130.7, 130.0, 129.3, 128.9, 128.2, 127.7, 127.5, 126.6, 124.9, 123.8, 123.4, 121.1, 115.4, 115.2, 115.0, 50.6, 28.7, 24.2, 23.7. 19F NMR (470 MHz, CDCl3, TMS): δ −58.19, −116.12, −116.28. FT-IR (cm−1): 3053 (w), 2965 (w), 2932 (w), 2876 (w), 1671 (w, vC = N), 1649 (w, vC = N), 1598 (w), 1506 (s), 1436 (w), 1392 (w), 1361 (w), 1333 (w), 1300 (w), 1228 (w), 1165 (w), 1095 (w), 1041 (w), 1014 (w), 923 (w), 876 (w), 830 (m), 785 (w), 761 (w). Anal. calcd for C57H43F7N2O·H2O (904.97): C, 75.65; H, 4.79; N, 3.10. Found: C, 75.37; H, 5.06; N, 3.44%.
L4. Using a typical procedure as described for the synthesis of L1, L4 was prepared as a yellow powder (0.66 g, 38.0%). 1H NMR (400 MHz, CDCl3, TMS): δ 7.81 (d, J = 8.0 Hz, 1H), 7.74 (d, J = 8.4 Hz, 1H), 7.34 (t, J = 7.8 Hz, 1H), 7.07–6.93 (m, 11H), 6.86–6.82 (m, 6H), 6.66 (d, J = 7.2 Hz, 1H), 6.32 (t, J = 8.6 Hz, 4H), 5.98 (d, J = 6.8 Hz, 1H), 5.58 (s, 2H), 2.39 (s, 3H), 2.15 (s, 6H). 13C NMR (100MHz, CDCl3, TMS): δ 164.2, 162.9, 162.1, 161.2, 160.5, 159.6, 147.5, 146.4, 145.4, 139.9, 137.6, 136.6, 134.2, 133.4, 131.1, 131.0, 130.8, 130.7, 130.1, 129.2, 128.5, 128.1, 126.6, 124.3, 123.5, 122.4, 121.8, 120.9, 119.3, 115.4, 115.2, 115.1, 114.9, 50.9, 50.7, 20.9, 18.0. 19F NMR (470 MHz, CDCl3, TMS): δ −58.17, −116.16, −116.39. FT-IR (cm−1): 3043 (w), 2975 (w), 2912 (w), 1668 (w, vC = N), 1642 (w, vC = N), 1599 (w), 1506 (m), 1482 (w), 1440 (w), 1256 (w), 1224 (w), 1155 (w), 1096 (w), 1035 (w), 1013 (w), 925 (w), 831 (m), 778 (m), 732 (w). Anal. calcd for C54H37F7N2O·H2O (862.89): C, 75.17; H, 4.32; N, 3.25. Found: C, 75.56; H, 4.73; N, 3.09%.
L5. Using a typical procedure as described for the synthesis of L1, L5 was prepared as a yellow powder (0.40 g, 22.5%). 1H NMR (400 MHz, CDCl3, TMS): δ 7.79 (d, J = 8.4 Hz, 1H), 7.70 (d, J = 8.4 Hz, 1H), 7.32 (t, J = 7.6 Hz, 1H), 7.04 (s, 2H), 7.02–6.94 (m, 9H), 6.86–6.82 (m, 6H), 6.64 (d, J = 7.2 Hz, 1H), 6.30 (t, J = 8.4 Hz, 4H), 5.87 (d, J = 7.2 Hz, 1H), 5.60 (s, 2H), 2.65–2.56 (m, 2H), 2.51–2.42 (m, 5H), 1.14 (t, J = 7.4 Hz, 6H). 13C NMR (100MHz, CDCl3, TMS): δ 164.2, 162.9, 162.1, 161.2, 160.4, 159.6, 147.5, 146.3, 145.4, 139.9, 137.6, 136.6, 134.2, 133.4, 131.1, 131.0, 130.7, 130.6, 130.1, 129.2, 129.1, 128.4, 128.0, 126.6, 124.3, 123.5, 122.4, 121.0, 115.4, 115.2, 115.0, 114.8, 50.8, 50.6, 20.9,18.0. 19F NMR (470 MHz, CDCl3, TMS): δ −58.18, −116.17, −116.38. FT-IR (cm−1): 3048 (w), 2965 (w), 2918 (w), 2854 (w), 1661 (w, vC = N), 1640 (w, vC = N), 1598 (w), 1505 (m), 1463 (w), 1438 (w), 1256 (w), 1217 (w), 1152 (w), 1093 (w), 1042 (w), 1013 (w), 927 (w), 864 (w), 829 (w), 786 (w). Anal. calcd for C56H41F7N2O·H2O (890.95): C, 75.49; H, 4.64; N, 3.14. Found: C, 75.12; H, 4.26; N, 3.54%.

3.3. Synthesis of Nickel Complexes (Ni1–Ni5)

Ni1. Under a nitrogen atmosphere, L1 (0.10 g, 0.12 mmol), NiBr2(DME) (0.04 g, 0.12 mmol), dichloromethane (6 mL), and ethanol (4 mL) were stirred at ambient temperature for 12 h. The solution was concentrated, and diethyl ether (25 mL) was added to precipitate the product. It was collected by filtration and washed with excess diethyl ether (3 × 15 mL) affording Ni1 as a red powder (0.12 g, 92.1% yield). 1H NMR (400 MHz, CD2Cl2, TMS): δ 28.48 (s, 6H, −CH3), 25.39 (s, 2H, Ar-Hm), 25.06 (s, 1H, An-H), 21.81 (s, 2H, Ar-Hm), 18.74 (s, 1H, An-H), 16.90 (s, 1H, An-H), 16.06 (s, 1H, An-H), 11.91 (broad, 0.81H, −CHPh2F2), 8.16 (s, 4H, Ar-H), 7.07 (s, 4H, Ar-H), 6.31 (s, 1H, An-H), 5.46 (s, 4H, Ar-H), 5.05 (s, 1H, An-H), 1.25 (s, 4H, Ar-H), -16.17 (s, 1H, Ar-Hp). 19F NMR (470 MHz, CDCl3, TMS): δ −57.12, −115.73, −116.07. FT-IR (cm−1): 3071 (w), 3036 (w), 2972 (w), 2906 (w), 1649 (w, vC = N), 1623 (w, vC = N), 1601 (w), 1585 (w), 1506 (s), 1444 (w), 1421 (w), 1262 (w), 1221 (w), 1158 (m), 1096 (w), 1051 (w), 1014 (w), 959 (w), 878 (w), 834 (m), 773 (m). Anal. calcd for C53H35Br2F7N2NiO (1067.37): C, 59.64; H, 3.31; N, 2.62. Found: C, 60.01; H, 3.17; N, 2.35%.
Ni2. Using a typical procedure as described for the synthesis of Ni1, Ni2 was also afforded as a red powder (0.11 g, 86.2% yield). 1H NMR (400 MHz, CDCl3, TMS): δ 28.89 (s, 2H, −CH2CH3), 27.44 (s, 2H, −CH2CH3), 25.69 (s, 1H, An-H), 25.51 (s, 2H, Ar-Hm), 22.31 (s, 2H, Ar-Hm), 19.48 (s, 1H, An-H), 17.36 (s, 1H, An-H), 16.52 (s, 1H, An-H), 13.21 (broad, 1.2H, −CHPh2F2), 8.20 (s, 4H, Ar-H), 7.07 (s, 4H, Ar-H), 6.09 (s, 1H, An-H), 5.54 (s, 4H, Ar-H), 4.97 (s, 1H, An-H), 1.79 (s, 2H, Ar-H), 1.22 (s, 1H, Ar-H), 0.81 (s, 7H, Ar-H, −CH2CH3), −16.87 (s, 1H, Ar-Hp). 19F NMR (470 MHz, CDCl3, TMS): δ −57.09, −115.67, −116.16. FT-IR (cm−1): 3069 (w), 3046 (w), 2978 (w), 2942 (w), 1649 (w, vC = N), 1621 (w, vC = N), 1603 (w), 1583 (w), 1505 (s), 1440 (w), 1421 (w), 1293 (w), 1256 (w), 1217 (w), 1157 (w), 1104 (w), 1050 (w), 1014 (w), 958 (w), 877 (w), 831 (m), 771 (w), 729 (w). Anal. calcd for C55H39Br2F7N2NiO (1095.42): C, 60.31; H, 3.59; N, 2.56. Found: C, 60.11; H, 3.46; N, 2.78%.
Ni3. Using a typical procedure as described for the synthesis of Ni1, Ni3 was also afforded as a red powder (0.11 g, 84.9% yield). 1H NMR (400 MHz, CDCl3, TMS): δ 26.25 (s, 1H, An-H), 25.16 (s, 2H, Ar-Hm), 22.75 (s, 2H, Ar-Hm), 2.10 (s, 1H, An-H), 17.89 (s, 1H, An-H), 16.72 (s, 1H, An-H), 13.80 (broad, 2H, −CHPh2F2), 8.14 (s, 4H, Ar-H), 7.02 (s, 4H, Ar-H), 6.85 (s, 1H, An-H), 5.68 (s, 6H, Ar-H), 4.97 (s, 1H, An-H), 1.87 (s, 7H, Ar-H, −CH(CH3)2), 1.69 (s, 6H, −CH(CH3)2), 1.30 (s, 1H, Ar-H), 1.03 (s, 2H, −CH(CH3)2), −16.41 (s, 1H, Ar-Hp). 19F NMR (470 MHz, CDCl3, TMS): δ −57.19, −115.91, −116.09. FT-IR (cm−1): 3072 (w), 3038 (w), 2969 (w), 2913 (w), 1645 (w, vC = N), 1620 (w, vC = N), 1601 (w), 1582 (w), 1506 (s), 1442 (w), 1421 (w), 1390 (w), 1364 (w), 1328 (w), 1298 (w), 1261 (w), 1220 (w), 1159 (w), 1097 (w), 1050 (w), 1014 (w), 958 (w), 876 (w), 832 (m), 772 (w), 734 (w). Anal. calcd for C57H43Br2F7N2NiO (1123.47): C, 60.94; H, 3.86; N, 2.49. Found: C, 60.73; H, 3.52; N, 2.57%.
Ni4. Using a typical procedure as described for the synthesis of Ni1, Ni4 was also afforded as a red powder (0.11 g, 87.8% yield). 1H NMR (400 MHz, CDCl3, TMS): δ 34.47 (s, 3H, Ar–p–CH3), 29.31 (s, 6H, -CH3), 25.92 (s, 1H, An-H), 25.31 (s, 2H, Ar-Hm), 22.07 (s, 2H, Ar-Hm), 19.14 (s, 1H, An-H), 17.23 (s, 1H, An-H), 16.40 (s, 1H, An-H), 12.71 (broad, 0.99H, −CHPh2F2), 8.18 (s, 4H, Ar-H), 7.07 (s, 4H, Ar-H), 6.20 (s, 1H, An-H), 5.47 (s, 4H, Ar-H), 4.92 (s, 1H, An-H), 1.26 (s, 2H, Ar-H), 1.07 (s, 2H, Ar-H). 19F NMR (470 MHz, CDCl3, TMS): δ −57.15, −115.78, −116.10. FT-IR (cm−1): 3069 (w), 2991 (w), 2911 (w), 1643 (w, vC = N), 1624 (w, vC = N), 1602 (w), 1583 (w), 1505 (s), 1443 (w), 1269 (m), 1217 (m), 1191 (w), 1155 (m), 1096 (w), 1048 (w), 1015 (w), 962 (w), 877 (w), 832 (m), 777 (w). Anal. calcd for C54H37Br2F7N2NiO (1081.39): C, 59.98; H, 3.45; N, 2.59. Found: C, 60.26; H, 3.58; N, 2.27%.
Ni5. Using a typical procedure as described for the synthesis of Ni1, Ni5 was also afforded as a red powder (0.12 g, 90.5% yield). 1H NMR (400 MHz, CDCl3, TMS): δ 34.75 (s, 3H, Ar–p–CH3), 28.61 (s, 2H, −CH2CH3), 27.53 (s, 2H, −CH2CH3), 26.25 (s, 1H, An-H), 25.19 (s, 2H, Ar-Hm), 22.29 (s, 2H, Ar-Hm), 19.29 (s, 1H, An-H), 17.30 (s, 1H, An-H), 16.54 (s, 1H, An-H), 13.02 (broad, 1.2H, −CHPh2F2), 8.24 (s, 4H, Ar-H), 7.08 (s, 4H, Ar-H), 6.16 (s, 1H, An-H), 5.53 (s, 4H, Ar-H), 4.90 (s, 1H, An-H), 1.22 (s, 3H, Ar-H), 0.87 (s, 7H, Ar-H, −CH2CH3). 19F NMR (470 MHz, CDCl3, TMS): δ −58.16, −115.86, −116.09. FT-IR (cm−1): 3079 (w), 2975 (w), 2935 (w), 2873 (w), 1644 (w, vC = N), 1618 (w, vC = N), 1602 (w), 1583 (w), 1504 (s), 1444 (w), 1383 (w), 1296 (w), 1252 (w), 1220 (w), 1174 (w), 1157 (w), 1098 (w), 1054 (w), 1015 (w), 960 (w), 868 (w), 831 (m), 773 (m), 731 (w). Anal. calcd for C56H41Br2F7N2NiO (1109.45): C, 60.63; H, 3.73; N, 2.53. Found: C, 60.48; H, 3.61; N, 2.88%.

4. Conclusions

In this study, a novel series of asymmetric 1,2-bis(imine)acenaphthene nickel(II) complexes (Ni1–Ni5) featuring para-trifluoromethoxy and distal-fluoro-benzhydryl groups, was developed. These dual-functionalized catalysts achieved exceptional ethylene polymerization activity (up to 22.0 × 106 g PE mol−1(Ni) h−1) and outstanding thermal stability, maintaining 4.82 × 106 g PE mol−1(Ni) h−1 at 80 °C. The synergistic combination of electron-withdrawing trifluoromethoxy and sterically shielding fluorobenzhydryl groups enhanced both the electrophilicity of the nickel center and resistance to thermal deactivaton, enabling efficient ethylene polymerization under industrially relevant conditions. By systematically modulating ligand architecture and co-catalyst, solvent (toluene vs. n-hexane), and polymerization parameters (temperature, time, and ethylene pressure), we achieved precise control over PE branching density and molecular weight. The strong electron-withdrawing para-trifluoromethoxy group facilitates ethylene insertion, thereby increasing the molecular weight of the resultant polymer. Furthermore, the bulky ortho-substituent promotes chain growth and chain walking while effectively suppressing chain transfer reactions, leading to the formation of polyethylenes with high molecular weight and high branching density. The resulting PEs displayed highly branching levels alongside robust mechanical properties, including tensile strengths up to 20.7 MPa and 58% elastic recovery. Comprehensive characterization of ligands, complexes, and polymers revealed structure–property relationships critical for advancing TPEs production technologies. This work underscores the potential of rationally designed nickel catalysts for high-temperature ethylene polymerization processes, offering a versatile platform to tailor polyolefin architectures for advanced elastomeric applications, where the integration of fluorine-based substituents elevates catalytic efficiency and enables scalable production of high-performance thermoplastic elastomers (TPEs) with tunable mechanical properties.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules30132706/s1. General Considerations [48], Table S1: Crystal data and structure refinement for Ni2 and Ni4 [59,60,61]; Figure S1–S15: 1H/13C/19F NMR spectra of L1–L5; Figure S16–S25: 1H/19F NMR spectra of Ni1–Ni5; Figure S26: GPC traces of the PE’s obtained using Ni1/EtAlCl2; Figure S27–S31: 13C NMR spectra of the selected PEs. References [59,60,61] are cited in the Supplementary Materials.

Author Contributions

Methodology, investigation, writing—original draft preparation, M.L. (Ming Liu); data curation, M.L. (Ming Liu) and M.S.; GPC analysis, writing—review and editing Y.M.; funding acquisition, M.L. (Mingfeng Li); X-Ray analysis, Y.W.; supervision, M.L. (Mingfeng Li) and W.-H.S.; resources, project administration, W.-H.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by SINOPEC Research Institute of Petroleum Processing Co., Ltd. No. 36800000-23-ZC0699-0122 and the Strategic Priority Research Program of the Chinese Academy of Sciences, No. XDC0270102.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

CCDC reference numbers 2451477 and 2451478 contain supplementary crystallographic information for Ni2 and Ni4 (see Supplementary Materials), and the data presented in this study are available in this article.

Conflicts of Interest

The authors declare that this study received funding from SINOPEC Research Institute of Petroleum Processing Co., Ltd. and the Strategic Priority Research Program of the Chinese Academy of Sciences. The funder was not involved in the study design, collection, analysis, interpretation of data, the writing of this article or the decision to submit it for publication.

References

  1. Braunecker, W.A.; Matyjaszewski, K. Controlled/living radical polymerization: Features, developments, and perspectives. Prog. Polym. Sci. 2007, 32, 93–146. [Google Scholar] [CrossRef]
  2. Zanchin, G.; Leone, G. Polyolefin Thermoplastic Elastomers from Polymerization Catalysis: Advantages, Pitfalls and Future Challenges. Prog. Polym. Sci. 2021, 113, 101342. [Google Scholar] [CrossRef]
  3. Adhikari, B.; De, D.; Maiti, S. Reclamation and recycling of waste rubber. Prog. Polym. Sci. 2000, 25, 909. [Google Scholar] [CrossRef]
  4. Dow Chemical Company. Olefin Solution Polymerization. WO1997036942A1, 9 October 1997. [Google Scholar]
  5. Chatterjee, A.M. Handbook of Industrial Polyethylene and Technology Definitive Guide to Manufacturing, Properties, Processing, Applications and Markets; John Wiley & Sons Inc.: Hoboken, NJ, USA, 2017; pp. 124–125. [Google Scholar]
  6. Exxon Chemical Patents Inc. Process and Catalyst for Polyolefin Density and Molecular Weight Control. US, 5324800A, 28 June 1994. [Google Scholar]
  7. Johnson, L.K.; Killian, C.M.; Brookhart, M. New Pd(II)- and Ni(II)-Based Catalysts for Polymerization of Ethylene and α-Olefins. J. Am. Chem. Soc. 1995, 117, 6414–6415. [Google Scholar] [CrossRef]
  8. Killian, C.M.; Tempel, D.J.; Johnson, L.K.; Brookhart, M. Living Polymerization of α-Olefins Using NiII-α-Diimine Catalysts. Synthesis of New Block Polymers Based on α-Olefins. J. Am. Chem. Soc. 1996, 118, 11664–11665. [Google Scholar] [CrossRef]
  9. Leatherman, M.D.; Svejda, S.A.; Johnson, L.K.; Brookhart, M. Mechanistic Studies of Nickel(II) Alkyl Agostic Cations and Alkyl Ethylene Complexes: Investigations of Chain Propagation and Isomerization in (α-diimine)Ni(II)-Catalyzed Ethylene Polymerization. J. Am. Chem. Soc. 2003, 125, 3068–3290. [Google Scholar] [CrossRef]
  10. Michalak, A.; Ziegler, T. Polymerization of Ethylene Catalyzed by a Nickel(+2) Anilinotropone-Based Catalyst: DFT and Stochastic Studies on the Elementary Reactions and the Mechanism of Polyethylene Branching. Organometallics 2003, 22, 2069–2079. [Google Scholar] [CrossRef]
  11. Falivene, L.; Wiedemann, T.; Göttker-Schnetmann, I.; Caporaso, L.; Cavallo, L.; Mecking, S. Control of Chain Walking by Weak Neighboring Group Interactions in Unsymmetrical Catalysts. J. Am. Chem. Soc. 2018, 140, 1305–1312. [Google Scholar] [CrossRef]
  12. O’Connor, K.S.; Lamb, J.R.; Vaidya, T.; Keresztes, I.; Klimovica, K.; LaPointe, A.M.; Daugulis, O.; Coates, G.W. Understanding the Insertion Pathways and Chain Walking Mechanisms of α-Diimine Nickel Catalysts for α-Olefin Polymerization: A 13C NMR Spectroscopic Investigation. Macromolecules 2017, 50, 7010–7027. [Google Scholar] [CrossRef]
  13. Gates, D.P.; Svejda, S.A.; Oňate, E.; Killian, C.M.; Johnson, L.K.; White, P.S.; Brookhart, M. Synthesis of Branched Polyethylene Using (α-Diimine)nickel(II) Catalysts: Influence of Temperature, Ethylene Pressure, and Ligand Structure on Polymer Properties. Macromolecules 2000, 33, 2320–2334. [Google Scholar] [CrossRef]
  14. Wegner, M.M.; Ott, A.K.; Rieger, B. Gas Phase Polymerization of Ethylene with Supported α-Diimine Nickel(II) Catalysts. Macromolecules 2010, 43, 3624–3633. [Google Scholar] [CrossRef]
  15. Camacho, D.H.; Salo, E.V.; Ziller, J.W.; Guan, Z. Cyclophane-Based Highly Active Late-Transition-Metal Catalysts for Ethylene Polymerization. Angew. Chem. Int. Ed. 2004, 43, 1821–1825. [Google Scholar] [CrossRef] [PubMed]
  16. Gao, H.; Hu, H.; Zhu, F.; Wu, Q. A thermally robust amine–imine nickel catalyst precursor for living polymerization of ethylene above room temperature. Chem. Commun. 2012, 48, 3312–3314. [Google Scholar] [CrossRef] [PubMed]
  17. Li, L.; Jeon, M.; Kim, S.Y. Synthesis, characterization and ethylene polymerisation of 9,10-phenanthrenequinone-based nickel(II)- α-diimine complexes. J. Mol. Catal. A Chem. 2009, 303, 110–116. [Google Scholar] [CrossRef]
  18. Carlini, C.; Macinai, A.; Masi, F.; Galletti, A.M.R.; Santi, R.; Sbrana, G.; Sommazzi, A. Ethylene Polymerization by Bis(salicylaldiminate)nickel(II)/Aluminoxane Catalysts. J. Polym. Sci. Part A Polym. Chem. 2004, 42, 2534–2542. [Google Scholar] [CrossRef]
  19. Popeney, C.S.; Guan, Z. Ligand Electronic Effects on Late Transition Metal Polymerization Catalysts. Organometallics 2005, 24, 1145–1155. [Google Scholar] [CrossRef]
  20. Popeney, C.S.; Guan, Z. Effect of Ligand Electronics on the Stability and Chain Transfer Rates of Substituted Pd(II) α-Diimine Catalysts. Macromolecules 2010, 43, 4091–4097. [Google Scholar] [CrossRef]
  21. Lanza, G.; Fragala, I.L.; Marks, T.J. Ligand Substituent, Anion, and Solvation Effects on Ion Pair Structure, Thermodynamic Stability, and Structural Mobility in “Constrained Geometry” Olefin Polymerization Catalysts: An Ab Initio Quantum Chemical Investigation. J. Am. Chem. Soc. 2000, 122, 12764–12777. [Google Scholar] [CrossRef]
  22. Saeed, H.; Mahmood, Q.; Yuan, R.; Wang, Y.; Zou, S.; Tahir, K.F.; Ma, Y.; Liang, T.; Sun, W.-H. High-performance polyethylene elastomers using a hybrid steric approach in α-diimine nickel precatalysts. Polym. Chem. 2024, 15, 1437–1452. [Google Scholar] [CrossRef]
  23. Kuhn, P.; S-emeril, D.; Jeunesse, C.; Matt, D.; Neuburger, M.; Mota, A. Ethylene Oligomerisation and Polymerisation with Nickel Phosphanylenolates Bearing Electron-Withdrawing Substituents: Structure–Reactivity Relationships. Chem. Eur. J. 2006, 12, 5210–5219. [Google Scholar] [CrossRef]
  24. Hu, Z.; Ren, G.; Mahmood, Q.; Yu, Z.; Wang, Y.; Tahir, K.F.; Zou, S.; Liang, T.; Sun, W.-H. Thermostability promotion of α-diimine nickel precatalysts tailored with 2,6-bis(bis(4-fluorophenyl)methyl)-3,4,5-trimethoxyaniline for PE elastomer synthesis. New J. Chem. 2024, 48, 12174–12187. [Google Scholar] [CrossRef]
  25. Sun, W.-H.; Jie, S.; Zhang, S.; Zhang, W.; Song, Y.; Ma, H.; Chen, J.; Wedeking, K.; Fröhlich, R. Iron Complexes Bearing 2-Imino-1,10-phenanthrolinyl Ligands as Highly Active Catalysts for Ethylene Oligomerization. Organometallics 2006, 25, 666–677. [Google Scholar] [CrossRef]
  26. Ittel, S.D.; Johnson, L.K.; Brookhart, M. Late-Metal Catalysts for Ethylene Homo- and Copolymerization. Chem. Rev. 2000, 100, 1169–1203. [Google Scholar] [CrossRef]
  27. Bianchini, C.; Giambastiani, G.; Luconi, L.; Meli, A. Olefin oligomerization, homopolymerization and copolymerization by late transition metals supported by (imino)pyridine ligands. Coord. Chem. Rev. 2010, 254, 431–455. [Google Scholar] [CrossRef]
  28. Wang, Z.; Liu, Q.; Solan, G.A.; Sun, W.-H. Recent advances in Ni-mediated ethylene chain growth: Nimine-donor ligand effects on catalytic activity, thermal stability and oligo-/polymer structure. Coord. Chem. Rev. 2017, 350, 68–83. [Google Scholar] [CrossRef]
  29. Mahmood, Q.; Hu, Z.; Ren, G.; Sun, W.-H. Recent Advances in Nickel Catalysis for Thermoplastic Polyethylene Elastomers: Synthesis Strategies, Properties, and Future Perspectives. Coord Chem. Rev. 2025, 541, 216833. [Google Scholar] [CrossRef]
  30. Suo, H.; Solan, G.A.; Ma, Y.; Sun, W.-H. Developments in compartmentalized bimetallic transition metal ethylene polymerization catalysts. Coord. Chem. Rev. 2018, 372, 101–116. [Google Scholar] [CrossRef]
  31. Mahmood, Q.; Sun, W.-H. N,N-chelated nickel catalysts for highly branched polyolefin elastomers: A survey. R. Soc. Open Sci. 2018, 5, 180367. [Google Scholar] [CrossRef]
  32. Mahmood, Q.; Sun, W.-H. Late Transition Metal Catalysts (Ni and Pd) for Ethylene Polymerization, Chapter in Encyclopedia of Polymers, Polymeric Materials, and Polymer Technology; Mishra, M.K., Takeuchi, D., Sudo, A., Eds.; CRC Press: Boca Raton, FL, USA, 2025. [Google Scholar] [CrossRef]
  33. Wang, Y.; Gao, R.; Gou, Q.; Lai, J.; Zhang, R.; Li, X.; Guo, Z. Developments in late transition metal catalysts with high thermal stability for ethylene polymerization: A crucial aspect from laboratory to industrialization. Eur. Polym. J. 2022, 181, 111693. [Google Scholar] [CrossRef]
  34. Rhinehart, J.L.; Brown, L.A.; Long, B.K. A Robust Ni(II) α-Diimine Catalyst for High Temperature Ethylene Polymerization. J. Am. Chem. Soc. 2013, 135, 16316–16319. [Google Scholar] [CrossRef]
  35. Wu, R.; Wang, Y.; Zhang, R.; Guo, C.-Y.; Flisak, Z.; Sun, Y.; Sun, W.-H. Finely tuned nickel complexes as highly active catalysts affording branched polyethylene of high molecular weight: 1-(2,6-Dibenzhydryl-4-methoxyphenylimino)-2-(arylimino)acenaphthylenenickel halides. Polymer 2018, 153, 574–586. [Google Scholar] [CrossRef]
  36. Kong, S.; Guo, C.-Y.; Yang, W.; Wang, L.; Sun, W.-H.; Glaser, R. 2,6-Dibenzhydryl-N-(2-phenyliminoacenaphthylenylidene)-4-chloroaniline nickel dihalides: Synthesis, characterization and ethylene polymerization for polyethylenes with high molecular weights. J. Organomet. Chem. 2013, 725, 37–45. [Google Scholar] [CrossRef]
  37. Fan, L.; Du, S.; Guo, C.-Y.; Hao, X.; Sun, W.-H. 1-(2,6-Dibenzhydryl-4-fluorophenylimino)-2-aryliminoacenaphthylylnickel Halides Highly Polymerizing Ethylene for the Polyethylenes with High Branches and Molecular Weights. J. Polym. Sci. Part A Polym. Chem. 2015, 53, 1369–1378. [Google Scholar] [CrossRef]
  38. Mahmood, Q.; Zeng, Y.; Wang, X.; Sun, Y.; Sun, W.-H. Advancing polyethylene properties by incorporating NO2 moiety in 1,2-bis(arylimino)acenaphthylnickel precatalysts: Synthesis, characterization and ethylene polymerization. Dalton Trans. 2017, 46, 6934–6947. [Google Scholar] [CrossRef]
  39. Liu, M.; Zhang, R.; Ma, Y.; Han, M.; Solan, G.A.; Yang, W.; Liang, T.; Sun, W.-H. Trifluoromethoxy-substituted nickel catalysts for producing highly branched polyethylenes: Impact of solvent, activator and N,N′-ligand on polymer properties. Polym. Chem. 2022, 13, 1040–1058. [Google Scholar] [CrossRef]
  40. Du, S.; Kong, S.; Shi, Q.; Mao, J.; Guo, C.; Yi, J.; Liang, T.; Sun, W.-H. Enhancing the Activity and Thermal Stability of Nickel Complex Precatalysts Using 1-[2,6-Bis(bis(4-fluorophenyl)methyl)-4-methylphenylimino]-2-aryliminoacenaphthylene Derivatives. Organometallics 2015, 34, 582–590. [Google Scholar] [CrossRef]
  41. Zhang, Q.; Zhang, R.; Ma, Y.; Solan, G.A.; Liang, T.; Sun, W.-H. Branched polyethylenes attainable using thermally enhanced bis(imino)acenaphthene-nickel catalysts: Exploring the effects of temperature and pressure. Appl. Catal. A 2019, 573, 73–86. [Google Scholar] [CrossRef]
  42. Wu, R.; Wang, Y.; Guo, L.; Guo, C.-Y.; Liang, T.; Sun, W.-H. Highly Branched and High-Molecular-Weight Polyethylenes Produced by 1-[2,6-Bis(bis(4-fluorophenyl)methyl)-4-MeOC6H2N]-2-aryliminoacenaphthylnickel(II) halides. J. Polym. Sci. Part A Polym. Chem. 2019, 57, 130–145. [Google Scholar] [CrossRef]
  43. Wang, Y.; Vignesh, A.; Qu, M.; Wang, Z.; Sun, Y.; Sun, W.-H. Access to polyethylene elastomers via ethylene homo-polymerization using N,N′-nickel(II) catalysts appended with electron withdrawing difluorobenzhydryl group. Eur. Polym. J. 2019, 117, 254–271. [Google Scholar] [CrossRef]
  44. Wang, X.; Fan, L.; Yuan, Y.; Du, S.; Sun, Y.; Solan, G.A.; Guo, C.Y.; Sun, W.-H. Raising the N-aryl fluoride content in unsymmetrical diaryliminoacenaphthylenes as a route to highly active nickel(II) catalysts in ethylene polymerization. Dalton Trans. 2016, 45, 18313–18323. [Google Scholar] [CrossRef]
  45. Zhang, R.; Wang, Z.; Ma, Y.; Solan, G.A.; Sun, Y.; Sun, W.-H. Plastomeric-like polyethylenes achievable using thermally robust N,N′-nickel catalysts appended with electron withdrawing difluorobenzhydryl and nitro groups. Dalton Trans. 2019, 48, 1878–1891. [Google Scholar] [CrossRef] [PubMed]
  46. Leroux, F.R.; Manteau, B.; Vors, J.-P.; Pazenok, S. Trifluoromethyl ethers–synthesis and properties of an unusual substituent. Beilstein J. Org. Chem. 2008, 4, 13. [Google Scholar] [CrossRef] [PubMed]
  47. Wang, L.; Liu, M.; Mahmood, Q.; Yuan, S.; Li, X.; Qin, L.; Zou, S.; Liang, T.; Sun, W.-H. Enhancing the Thermal Stability of α-Diimine Ni(II) Catalysts for ethylene polymerization via Symmetrical axial shielding strategy. Eur. Polym. J. 2023, 194, 112112. [Google Scholar] [CrossRef]
  48. Hosseinzadeh, Z.; Liu, M.; Zhang, Q.; Liang, T.; Solan, G.A.; Ma, Y.; Sun, W.-H. Electronic Tuning of Sterically Encumbered 2-(Arylimino)Pyridine-Nickel Ethylene Polymerization Catalysts by Para-Group Modification. Catalysts 2022, 12, 1520. [Google Scholar] [CrossRef]
  49. Wang, R.; Sui, X.; Pang, W.; Chen, C. Ethylene Polymerization by Xanthene-Bridged Dinuclear α-Diimine NiII Complexes. ChemCatChem 2016, 8, 434–440. [Google Scholar] [CrossRef]
  50. Zhang, Y.; Zhang, Y.; Hu, X.; Wang, C.; Jian, Z. Advances on Controlled Chain Walking and Suppression of Chain Transfer in Catalytic Olefin Polymerization. ACS Catal. 2022, 12, 14304–14320. [Google Scholar] [CrossRef]
  51. Svejda, S.A.; Brookhart, M. Ethylene Oligomerization and Propylene Dimerization Using Cationic (α-Diimine)nickel(II) Catalysts. Organometallics 1999, 18, 65–74. [Google Scholar] [CrossRef]
  52. HelldÖrfer, M.; Milius, W.; Alt, H.G. The influence of the ligand structure on the properties of (α-diimine)nickel catalysts in the polymerization and oligomerization of ethylene. Inorg. Chim. Acta 2003, 351, 34–42. [Google Scholar] [CrossRef]
  53. Bahuleyan, B.K.; Son, G.W.; Park, D.-W.; Ha, C.-S.; Kim, I. Ethylene Polymerization by Sterically and Electronically Modulated Ni(II) a-Diimine Complexes. J. Polym. Sci. Part A Polym. Chem. 2008, 46, 1066–1082. [Google Scholar] [CrossRef]
  54. Ye, Z.B.; Xu, L.X.; Dong, Z.M.; Xiang, P. Designing polyethylenes of complex chain architectures via Pd–diimine-catalyzed ‘‘living’’ ethylene polymerization. Chem. Commun. 2013, 49, 6235–6255. [Google Scholar] [CrossRef]
  55. Britovsek, G.J.P.; Cohen, S.A.; Gibson, V.C.; Meurs, M.V. Iron Catalyzed Polyethylene Chain Growth on Zinc: A Study of the Factors Delineating Chain Transfer versus Catalyzed Chain Growth in Zinc and Related Metal Alkyl Systems. J. Am. Chem. Soc. 2004, 126, 10701–10712. [Google Scholar] [CrossRef] [PubMed]
  56. Hu, X.; Zhang, Y.; Li, B.; Jian, Z. Fluorinated α-Diimine Nickel Mediated Ethylene(Co)Polymerization. Chem.-A Eur. J. 2021, 27, 11935–11942. [Google Scholar] [CrossRef] [PubMed]
  57. Li, S.; Dai, S. 8-Arylnaphthyl substituent retarding chain transfer in insertion polymerization with unsymmetrical α-diimine systems. Polym. Chem. 2020, 11, 7199–7206. [Google Scholar] [CrossRef]
  58. Galland, G.B.; de Souza, R.F.; Mauler, R.S.; Nunes, F.F. 13C NMR determination of the composition of linear low-density polyethylene obtained with [η3-methallyl-nickel-diimine]PF6 complex. Macromolecules 1999, 32, 1620–1625. [Google Scholar] [CrossRef]
  59. Dolomanov, O.V.; Bourhis, L.J.; Gildea, R.J.; Howard, J.A.K.; Puschmann, H. OLEX2: A complete structure solution, refinement and analysis program. J. Appl. Cryst. 2009, 42, 339–341. [Google Scholar] [CrossRef]
  60. Sheldrick, G.M. SHELXT—Integrated space-group and crystalstructure determination. Acta Cryst. 2015, A71, 3–8. [Google Scholar]
  61. Sheldrick, G.M. Crystal structure refinement with SHELXL. Acta Cryst. 2015, C71, 3–8. [Google Scholar]
Molecules 30 02706 ch001
Chart 1. Structural evolution of N,N′-diiminoacenaphthene-nickel(II) halide precatalysts (A–E).
Chart 1. Structural evolution of N,N′-diiminoacenaphthene-nickel(II) halide precatalysts (A–E).
Molecules 30 02706 ch001
Molecules 30 02706 sch001
Scheme 1. Synthesis of L1–L5 and their corresponding nickel(II) bromide complexes Ni1–Ni5.
Scheme 1. Synthesis of L1–L5 and their corresponding nickel(II) bromide complexes Ni1–Ni5.
Molecules 30 02706 sch001
Molecules 30 02706 g001
Figure 1. ORTEP diagram of Ni2 (a) and Ni4 (b) with the thermal ellipsoids shown at the 50% probability level. All hydrogen atoms have been omitted for clarity.
Figure 1. ORTEP diagram of Ni2 (a) and Ni4 (b) with the thermal ellipsoids shown at the 50% probability level. All hydrogen atoms have been omitted for clarity.
Molecules 30 02706 g001
Molecules 30 02706 g002
Figure 2. Temperature-dependent (a) activity and (b) molecular weight trends using Ni1/EASC vs. Ni1/EtAlCl2.
Figure 2. Temperature-dependent (a) activity and (b) molecular weight trends using Ni1/EASC vs. Ni1/EtAlCl2.
Molecules 30 02706 g002
Molecules 30 02706 g003
Figure 3. Time-dependent (a) activity decay and (b) molecular weight evolution using Ni1/EASC vs. Ni1/EtAlCl2.
Figure 3. Time-dependent (a) activity decay and (b) molecular weight evolution using Ni1/EASC vs. Ni1/EtAlCl2.
Molecules 30 02706 g003
Molecules 30 02706 g004
Figure 4. Comparison of activity and Mw of the PE generated by Ni1 (R = OCF3) with precatalyst D (Chart 1, R = OMe [35], NO2 [36]) under identical conditions (EASC, 40 °C, PC2H4 = 10 atm, toluene).
Figure 4. Comparison of activity and Mw of the PE generated by Ni1 (R = OCF3) with precatalyst D (Chart 1, R = OMe [35], NO2 [36]) under identical conditions (EASC, 40 °C, PC2H4 = 10 atm, toluene).
Molecules 30 02706 g004
Molecules 30 02706 g005
Figure 5. Comparison of activity and Mw of the PE generated by Ni1 (R = F) with precatalyst C (Chart 1, R = H [43]) under identical conditions (EASC, 70 °C, Al/Ni = 300, PC2H4 = 10 atm, toluene).
Figure 5. Comparison of activity and Mw of the PE generated by Ni1 (R = F) with precatalyst C (Chart 1, R = H [43]) under identical conditions (EASC, 70 °C, Al/Ni = 300, PC2H4 = 10 atm, toluene).
Molecules 30 02706 g005
Molecules 30 02706 g006
Figure 6. 13C NMR spectrum of PE-40EADC/300/T along with a segment of the assigned polymer backbone; recorded in C6D4Cl2 at 100 °C.
Figure 6. 13C NMR spectrum of PE-40EADC/300/T along with a segment of the assigned polymer backbone; recorded in C6D4Cl2 at 100 °C.
Molecules 30 02706 g006
Molecules 30 02706 g007
Figure 7. (a) Stress–strain curves for PE-40EADC/300/T, PE-40EASC/300/T, PE-60EASC/300/T, PE-40EASC/450/T, and PE-40EASC/450/H obtained during monotonic tensile deformation; (b) Hysteresis loops for PE-60EASC/300/T obtained using ten cycles at 30 °C.
Figure 7. (a) Stress–strain curves for PE-40EADC/300/T, PE-40EASC/300/T, PE-60EASC/300/T, PE-40EASC/450/T, and PE-40EASC/450/H obtained during monotonic tensile deformation; (b) Hysteresis loops for PE-60EASC/300/T obtained using ten cycles at 30 °C.
Molecules 30 02706 g007
Table 1. Key bond parameters for Ni2 and Ni4.
Table 1. Key bond parameters for Ni2 and Ni4.
Ni2Ni4
Bond lengths (Å)
Ni(1)-Br(1)2.3342 (6)2.3253 (16)
Ni(1)-Br(2)2.3300 (7)2.3388 (17)
Ni(1)-N(1)2.037 (3)2.041 (7)
Ni(1)-N(2)2.022 (3)2.011 (7)
N(1)-C(12)1.290 (4)1.261 (11)
N(1)-C(13)1.446 (4)1.448 (10)
N(2)-C(1)1.283 (5)1.289 (11)
N(2)-C(46)1.440 (4)1.450 (11)
O(1)-C(45)CF31.342 (6)1.23 (5)
Bond angles (°)
N(1)-Ni(1)-N(2)82.73 (12)82.6 (3)
Br(1)-Ni(1)-Br(2)126.05 (3)126.56 (7)
N(1)-Ni(1)-Br(1)110.13 (7)109.70 (19)
N(1)-Ni(1)-Br(2)110.20 (7)111.2 (2)
N(2)-Ni(1)-Br(1)110.18 (8)107.7 (2)
N(2)-Ni(1)-Br(2)109.08 (8)110.3 (2)
C(45)-O(1)-C(16)116.3 (4)125 (3)
Table 2. Ethylene polymerization results with Ni1 using various alkylalumimum co-catalysts a.
Table 2. Ethylene polymerization results with Ni1 using various alkylalumimum co-catalysts a.
RunCo-CatalystAl/NiPE (g)Activity bMw cMw/Mn cTm d (°C)
1MAO20003.366.724.513.0981.7
2MMAO20004.348.686.853.5297.0
3EtAlCl23006.5913.25.963.4966.4
4EASC3004.398.787.542.4899.6
5Et2AlCl3003.647.285.804.0380.9
a Conditions: 1.0 μmol Ni1, 100 mL of toluene, 10 atm C2H4, 30 °C, 30 min; b 106 g (PE) mol−1 (Ni) h−1; c Mw: ×105 g mol−1, Mw and Mw/Mn determined by GPC; d Determined by DSC.
Table 3. Ethylene polymerization with Ni1–Ni5/EtAlCl2 a.
Table 3. Ethylene polymerization with Ni1–Ni5/EtAlCl2 a.
RunPrecat.Al/NiT (°C)T (min)PE (g)Activity bMw cMw/Mn cTm d (°C)
1Ni130030306.5913.25.963.4966.4
2Ni130040307.1114.24.783.5561.6
3Ni130050304.779.544.262.7045.5
4Ni130060303.847.683.211.7336.8
5Ni130070302.424.842.642.5135.8
6Ni130080301.382.762.262.7816.8
7Ni110040305.3710.74.182.6158.6
8Ni115040306.3012.64.432.7466.5
9Ni120040308.5917.24.592.2959.4
10Ni125040308.0716.14.883.2354.2
11Ni12004051.6319.62.783.0062.3
12Ni120040154.5718.33.683.0867.3
13Ni1200404510.614.14.862.1656.3
14Ni1200406011.811.85.082.8862.8
15 eNi120040303.587.163.732.5955.4
16 fNi120040300.741.482.331.75- g
17Ni220040305.1610.34.212.7059.0
18Ni320040303.376.746.682.9051.1
19Ni420040307.6315.34.233.0763.8
20Ni520040306.9814.03.572.2258.7
a Conditions: 1.0 μmol pre-catalyst, 100 mL of toluene, 10 atm C2H4; b 106 g (PE) mol−1 (Ni) h−1; c Mw: ×105 g mol−1, Mw and Mw/Mn determined by GPC; d Determined by DSC; e 5 atm C2H4; f 1 atm C2H4; g Broad and weak endotherms, amorphous polymers.
Table 4. Ethylene polymerization with Ni1–Ni5/EASC a.
Table 4. Ethylene polymerization with Ni1–Ni5/EASC a.
RunPrecat.Al/NiT (°C)T (min)PE (g)Activity bMw cMw/Mn cTm d (°C)
1Ni130030304.398.787.542.4899.6
2Ni1300403010.721.46.413.5958.7
3Ni130050308.4116.85.924.6251.9
4Ni130060306.1812.44.712.6451.6
5Ni130070303.046.083.613.9739.8
6Ni130080302.414.823.443.0832.9
7Ni125040309.7419.55.903.6965.2
8Ni1350403011.022.06.153.5059.2
9Ni140040309.8519.77.123.4065.2
10Ni145040308.8317.77.163.8056.7
11Ni13504052.7733.23.593.1279.9
12Ni135040156.4725.95.103.0483.9
13Ni1350404512.817.16.555.0894.4
14Ni1350406013.413.47.783.0972.7
15 eNi135040305.1810.44.292.6272.5
16 fNi135040300.861.723.702.17- g
17Ni235040307.4114.85.103.3281.2
18Ni335040306.2812.65.063.3259.8
19Ni435040307.1114.23.842.8778.2
20Ni535040308.6117.23.473.7468.3
a Conditions: 1.0 μmol pre-catalyst, 100 mL of toluene, 10 atm C2H4; b 106 g (PE) mol−1 (Ni) h−1; c Mw: ×105 g mol−1, Mw and Mw/Mn determined by GPC; d Determined by DSC; e 5 atm C2H4; f 1 atm C2H4; g Broad and weak endotherms, amorphous polymers.
Table 5. Solvent effect on Ni1/EASC performance a.
Table 5. Solvent effect on Ni1/EASC performance a.
RunAl/NiT (°C)T (min)PE (g)Activity bMw cMw/Mn cTm d (°C)
135040301.232.468.243.65116.8
245040305.2210.47.363.60114.7
350040306.1612.36.633.52104.9
455040306.8713.76.093.60110.3
560040303.797.587.633.80119.7
655030304.008.0011.12.33118.4
755050306.0112.03.053.31107.4
a Conditions: 1.0 μmol Ni1, 100 mL of hexane, 10 atm C2H4; b 106 g (PE) mol−1 (Ni) h−1; c Mw: ×105 g mol−1, Mw and Mw/Mn determined by GPC; d Determined by DSC.
Table 7. Stress strain properties of the selected PE samples.
Table 7. Stress strain properties of the selected PE samples.
PE SampleTm (°C) aMw bBranches/1000 C’sΧc a (%)Stress (MPa) cStrain (%) c
PE-40EADC/300/T61.64.7896.523.216.4585
PE-40EASC/300/T58.76.4190.522.315.3482
PE-60EASC/300/T51.64.7110717.013.7572
PE-40EASC/450/T56.77.1610219.516.8237
PE-40EASC/450/H114.77.3652.643.420.7439
a Determined by DSC, Χc = 100 × ΔHf (Tm)/[ΔH°f (T°m)], where ΔH°f (T°m) = 248.3 J g−1; b Mw (×105 g mol−1), determined by GPC; c Determined using a universal testing instrument.
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Liu, M.; Sun, M.; Ma, Y.; Wang, Y.; Li, M.; Sun, W.-H. Trifluoromethoxy- and Fluorobenzhydryl-Tuned Nickel Catalysts for Polyethylene Elastomers. Molecules 2025, 30, 2706. https://doi.org/10.3390/molecules30132706

AMA Style

Liu M, Sun M, Ma Y, Wang Y, Li M, Sun W-H. Trifluoromethoxy- and Fluorobenzhydryl-Tuned Nickel Catalysts for Polyethylene Elastomers. Molecules. 2025; 30(13):2706. https://doi.org/10.3390/molecules30132706

Chicago/Turabian Style

Liu, Ming, Min Sun, Yanping Ma, Yizhou Wang, Mingfeng Li, and Wen-Hua Sun. 2025. "Trifluoromethoxy- and Fluorobenzhydryl-Tuned Nickel Catalysts for Polyethylene Elastomers" Molecules 30, no. 13: 2706. https://doi.org/10.3390/molecules30132706

APA Style

Liu, M., Sun, M., Ma, Y., Wang, Y., Li, M., & Sun, W.-H. (2025). Trifluoromethoxy- and Fluorobenzhydryl-Tuned Nickel Catalysts for Polyethylene Elastomers. Molecules, 30(13), 2706. https://doi.org/10.3390/molecules30132706

Article Metrics

Back to TopTop