2-[2,6-Bis(4,4′-difluorobenzhydryl)-4-methylphenylimino]-3-aryliminobutylnickel complex precatalysts tuning polyethylene elastomers with different molecular weights

Abstract

A series of 2-[2,6-bis(4,4′-difluorobenzhydryl)-4-methylphenylimino]-3-aryliminobutylnickel complexes was synthesized and fully characterized using FT-IR, elemental analysis, and single-crystal X-ray diffraction in the case of Ni1. The structural analysis revealed significant deviation from ideal tetrahedral geometry. When activated with MAO, these complexes demonstrated superior catalytic performance compared to previously reported unsymmetrical 2,3-bis(arylimino)butylnickel analogs. The optimized system, incorporating ortho-difluorobenzhydryl and para-methyl electron-donating groups, achieved an exceptional activity of 26.56 × 10⁶ g(PE) mol⁻¹ (Ni) h⁻¹. The resulting polyethylenes exhibited a broad spectrum of microstructures, ranging from semi-crystalline to nearly amorphous, with unimodal ultra-high molecular weights (M_w: 4.33–26.72 × 10⁵ g mol⁻¹) and tunable branching degrees (62–200/1000 C) achieved through a controlled chain-walking mechanism. The unique balance of molecular weight and crystalline-amorphous regions in these polymers translated to outstanding mechanical properties, including tensile strengths of 1.68–13.42 MPa, elongations at break of 388–529%, and elastic recoveries of 21–73%. Notably, the Ni1/Et₂AlCl catalyst system demonstrated enhanced thermal stability for ethylene polymerization, achieving a higher activity of 2.56 × 10⁶ g(PE) mol⁻¹ (Ni) h⁻¹ at 90 °C compared to 1.94 × 10⁶ g(PE) mol⁻¹ (Ni) h⁻¹ at 70 °C for Ni1/MAO. However, the polymers produced with Ni1/Et₂AlCl exhibited lower molecular weights (2.67–10.90 × 10⁵ g mol⁻¹) and inferior mechanical properties, underscoring the critical role of molecular weight in determining material properties.

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Introduction

Polyethylene elastomers (PEEs),^1–4 being produced via ethylene feedstock, have recently garnered significant attention as useful materials. They can offer a compelling combination of elasticity, chemical resistance, and processability, characteristics that are on par with those of thermoplastic elastomers (TPEs).^5–10 Structurally, PEEs are characterized by their flexible backbones incorporating diverse branching architectures (e.g., short-chain and long-chain branches), and perform the properties similar to those of polyolefin elastomers (POEs), which are commonly produced through the copolymerization of ethylene and α-olefins.^11–22 Beyond facilitating more economical process without costly α-olefins, PEEs with different microstructures could be tailored via the adjustable chain-walking mechanisms by modified α-diimine–nickel complex precatalysts in conjunction with optimal polymerization parameters, ultimately controlling their distinctive elastomeric properties.^23–33

Pioneered by Brookhart group utilizing α-diimine Pd/Ni precatalysts for branched polyethylenes,²³ the concerted effort has been made in extensively modifying such model catalysts, primarily focusing on developing substituents on the N-aryl moieties as well as new model complexes.^2,33 Through finely tuning steric hindrance and electronic effects of ligands for the nickel complexes, the catalytic performances have been gradually improved including better thermostability and higher activities,^2,33 meanwhile the resultant polyethylenes are simultaneously enhanced for useful properties.^1,33

Notably, the unsymmetrical configuration of ligands has significantly enhanced the performances of their nickel complexes,³³ stabilizing active species and controlling resultant polyethylenes with high branching degrees (exceeding 100/1000 C).^34–36 The systematic investigations of bis(arylimino)acenaphthene (A–B, Scheme 1)^34–47 and 2,3-bis(arylimino)butyl (C–D, Scheme 1)^48–51 have been explored by employing bulky substituents with N-aryl moieties, especially eitherbenzhydryl (CHPh₂) or its fluorinated derivatives [CH(p-FPh₂)], guaranteeing higher activity and robust thermostability. For instance, the benzhydryl substituted A_CHPh2 (ref. 37) and C_CHPh2 (ref. 50) maintained high activity as 2.97 × 10⁶ g(PE) mol⁻¹ (Ni) h⁻¹ at 90 °C and 1.70 × 10⁶ g(PE) mol⁻¹ (Ni) h⁻¹ at 100 °C, respectively. Model B and D (Scheme 1), incorporating fluoro-benzhydryl substituents, demonstrated higher activities and controlled the microstructures of resultant polyethylene elastomers (PEEs).^42–47,51 The remote fluorine has considered to effectively reduce the electron density around nickel centre in enhancing the ethylene coordination and boosting its catalytic activity.^46,52

Scheme 1 Unsymmetrical bromonickel catalysts: 1,2-bis(arylimino)acenaphthenes (A–B) and 2,3-bis(arylimino)butane (C–E).

In general, nickel complexes incorporating electron-withdrawing groups (A_F,³⁹ B_F,⁴⁶ C_Cl,⁴⁹ and D,⁵¹ Scheme 1) demonstrated higher activity and yielded polyethylenes with higher molecular weights, their counterparts featuring electron-donating groups (especially in A_t-Bu,³⁶ B_t-Bu,⁴⁴ and C_Me,⁴⁸ Scheme 1) produced polyethylenes with higher branching degrees (e.g., A_Me,³⁴ Scheme 1) and better mechanical properties (A_t-Bu,³⁶ stress: 13.22 MPa, strain: 1002%, Scheme 1).

Additionally, the chlorinated precatalysts C_Cl (ref. 49) and D (ref. 51) (Scheme 1) generated bimodal polyethylenes characterized by a relative lower branching degree—a crucial factor closely associated with the inferior elastomeric properties observed in these branched polyethylenes. Contrastingly the polyethylene elastomer generated by para-methyl analogues (A_Me,³⁴ B_Me,⁴² C_Me,⁴⁸ Scheme 1) demonstrated unimodal and significantly higher branches.

Achieving the high-molecular-weight and high branched polyethylene elastomers by 2,3-bis(arylimino)butylnickel catalysts,^48,50,51 para-fluoro-benzhydryl substituents and para-methyl on the N-aryl moiety has been employed for the series of unsymmetrical 2,3-bis(arylimino)butylnickel complexes (E, Scheme 1). Interestingly, the new system performs high active and results unimodal and ultra-high molecular weights along with various branching degrees. The in-depth exploration, in turn, serves as a fundamental basis for the rational design and development of more efficient and precisely tuning α-diimine nickel precatalysts to prepare a wide range of high-value PEEs.

Results and discussion

Synthesis and characterization of ligands (L1–L5) and nickel complexes (Ni1–Ni5)

A straightforward two-step approach synthesis protocol was employed to prepare a series of 2-[2,6-bis(4,4′-difluorobenzhydryl)-4-methylphenylimino]-3-(2,6-R¹-4-R²-phenyl)iminobutyl ligands (R¹ = Me, R² = H for L1; R¹ = Et, R² = H for L2; R¹ = ⁱPr, R² = H for L3; R¹ = R² = Me for L4; R¹ = Et, R² = Me for L5) (Scheme 2). Initially, the monoketone 3-[2,6-bis(4,4′-difluorobenzhydryl)-4-methylphenylimino]-2-butanone synthesized via a keto-amine condensation reaction between 2,3-butanedione and 2,6-bis(4,4′-difluorobenzhydryl)-4-methylaniline, catalyzed by p-toluenesulfonic acid in dichloromethane (DCM) at room temperature, followed by recrystallization using methanol. Subsequently, the reaction of this monoketone with five anilines at 80 °C in acetic acid (99.5 w%) in the presence of zinc chloride (ZnCl₂) yielded zinc complexes as intermediates. The ZnCl₂ was then removed from the α-diimine zinc chloride intermediates by treatment with a saturated aqueous potassium carbonate solution (K₂CO₃) in DCM, resulting in the formation of the corresponding α-diimine ligands (L1–L5) with high yields (65–78%). The structures of all newly synthesized organic compounds were confirmed using ¹H/¹³C NMR spectroscopy (Fig. S1–S12). Nickel bromide complexes were obtained with high yields (72–90%) by reacting the L1–L5 ligands with NiBr₂(DME) in a 1.06 : 1 molar ratio in dichloromethane at ambient temperature (Scheme 2). FT-IR spectroscopy revealed imine functional groups with stretching frequencies ranging from 1636–1659 cm⁻¹ in the ligands (Fig. S13), which shifted slightly to a lower range of 1636–1640 cm⁻¹ in the corresponding nickel complexes (Fig. S14), indicating the occurrence of efficient coordination between the imine and nickel centers.⁴⁸ The purity and structural integrity of the organic compounds and nickel complexes were validated by carbon, hydrogen, and nitrogen elemental analysis.

Scheme 2 Synthesis route of ligands (L1–L5) and corresponding nickel complexes (Ni1–Ni5).

Moreover, X-ray single crystal diffraction analysis of Ni1 unveiled a tetra-coordinated structure, where the nickel atom is centrally positioned and coordinated by two nitrogen atoms from imine groups and two bromide atoms (Fig. 1). Notably, deviates from a regular tetrahedral geometry, aligning with previous reports on nickel complexes featuring the 2,3-bis(arylimino)butane ligand framework.^48,49,51 The phenyl rings in Ni1, derived from the N-2,6-bis(4,4′-difluorobenzhydryl)-4-methylphenyl and N-2,6-dimethylphenyl groups, are nearly perpendicular to the coordination plane defined by [C(2)–C(3)–N(2)–Ni(1)–N(1)], with angles of 90.0° and 85.8° in Ni1, respectively. These angles are significantly larger than those observed in analogs lacking the remote fluoro substituent (87.3° and 79.6° in C_Me),⁴⁸ indicating that the introduction of remote fluoro group directly influences the open space around the nickel center, potentially impacting catalytic performance. Additionally, the Ni1–N2 bond lengths is slightly longer than the that of Ni1–N1 [2.012(7) Å vs. 2.009(8) Å], suggesting that the coordination of bulky N-aryl moiety with nickel center is slightly weaker compared to the smaller N-aryl unit, likely due to greater steric hindrance exhibited by the bulky N2-aryl moiety. Furthermore, it was observed that the bond angles N1–Ni1–N2 (81.0°) and Br1–Ni1–Br2 (119.24°) in Ni1 are significantly smaller compared to those in the analogous acenaphthene-based complex B_Me (83.1° and 127.1°, respectively, Scheme 1)⁴² indicating the diminished likelihood of chain-transfer reactions due to the crowded coordination environment could facilitate the synthesis of polyethylenes with higher molecular weights.⁴⁹

Fig. 1 ORTEP drawing of Ni1. Thermal ellipsoids are shown at the 30% probability level while the hydrogen atoms have been omitted for clarity. Selected bond lengths (Å): Ni(1)–N(1) 2.009(8), Ni(1)–N(2) 2.012(7), Ni(1)–Br(1) 2.3318(16), Ni(1)–Br(2) 2.3364(15), N(1)–C(2) 1.286(11), N(1)–C(14) 1.450(11), N(2)–C(3) 1.293(11), N(2)–C(5) 1.444(11) and bond angles (°): Br(1)–Ni(1)–Br(2) 119.24(6), N(1)–Ni(1)–Br(1) 118.9(2), N(1)–Ni(1)–Br(2) 106.5(2), N(1)–Ni(1)–N(2) 81.0(3), N(2)–Ni(1)–Br(1) 112.9(2), N(2)–Ni(1)–Br(2) 112.14(19).

Ethylene polymerization

Co-catalyst selection. In the quest to identify the optimal co-catalyst for ethylene polymerization, Ni1 was systematically paired with a range of alkylaluminum reagents, namely methylaluminoxane (MAO), modified methylaluminoxane (MMAO), diethylaluminum chloride (Et₂AlCl), and ethylaluminum sesquichloride (EASC), to comprehensively evaluate its catalytic performance in ethylene polymerization under the specific conditions of 30 °C and a reaction duration of 30 minutes (entries 1–4, Table 1). The detailed result, presented in Table 1, revealed a hierarchy of catalytic activities for the different combinations: MAO (7.04 × 10⁶ g(PE) mol⁻¹ (Ni) h⁻¹) > MMAO (3.47 × 10⁶ g(PE) mol⁻¹ (Ni) h⁻¹) > Et₂AlCl (2.30 × 10⁶ g(PE) mol⁻¹ (Ni) h⁻¹) > EASC (0.33 × 10⁶ g(PE) mol⁻¹ (Ni) h⁻¹). Additionally, Et₂AlCl as a representative alkyl aluminum reagent was chosen to assess the performance of Ni/Et₂AlCl across varying temperature ranges, with the aim of elucidating its temperature-dependent behavior in ethylene polymerization.⁴⁸

Table 1 Ethylene polymerization for cocatalyst selection^a

Entry	Co-catalyst	Al/Ni	Activity^b	Mw^c	Mw/Mn^c	Tm^d
a Conditions: 2 μmol Ni1 and 100 mL toluene, 10 atm C2H4, 30 °C, 30 min.b 10^6 g(PE) mol^−1 (Ni) h^−1.c Mw: 10^5 g mol^−1, Mw and Mw/Mn determined by GPC.d Determined by DSC.
1	MAO	2000	7.04	8.66	2.11	57.11
2	MMAO	2000	3.47	20.69	1.77	95.14
3	EASC	400	0.33	13.49	1.55	55.40
4	Et2AlCl	600	4.27	9.02	3.37	41.18

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Catalytic evaluation of Ni/MAO system for ethylene polymerization. To optimize the polymerization conditions of the Ni1/MAO system and establish a reference standard for screening other nickel complexes, a systematic optimization study was conducted. The results presented in Table 2 unequivocally demonstrate that the catalytic activity of the Ni1/MAO system initially exhibited an upward trend as the Al/Ni molar ratio increased from 1000 : 1 to 2000 : 1, under the conditions of 30 °C and a 30-minute reaction time (entries 1–3, Table 2). A peak catalytic activity of 7.04 × 10⁶ g PE mol⁻¹ (Ni) h⁻¹ was achieved at an Al/Ni ratio of 2000 : 1 (entry 3, Table 2). This phenomenon is likely attributed to the facilitated abstraction process of the halide ligand in the case of high co-catalyst dosage, which facilitates the formation of active species under these circumstances.⁵¹ However, when the Al/Ni ratio was further increased to 3000 : 1, a decline in catalytic activity was observed, dropping to 6.50 × 10⁶ g(PE) mol⁻¹ (Ni) h⁻¹ (entry 5, Table 2). This decrease can be explained by the fact that an excessive amount of MAO promotes chain transfer from the nickel active centers to the aluminum reagent. As a result, chain-termination reactions become more prevalent than chain-propagation reactions.³ Notably, similar effects were also observed on the properties of the resulting polyethylenes. As the Al/Ni ratio increased from 1000 : 1 to 3000 : 1, the molecular weight of polyethylenes decreased from 11.51 × 10⁵ to 4.33 × 10⁵ g mol⁻¹ (Fig. 2).^38,49 Simultaneously, the melting point dropped from 107.41 °C to 48.31 °C and the M_w/M_n narrowed from 3.51 to 1.53. These changes are all a consequence of the increased frequency of chain transfer to aluminum.

Table 2 Catalytic evaluation using Ni1–Ni5/MAO for ethylene polymerization^a

Entry	Precat.	T/°C	t/min	Al/Ni	Activity^b	Mw^c	Mw/Mn^c	Tm^d
a Conditions: 2 μmol Ni1 and 100 mL toluene, 10 atm C2H4.b 10^6 g(PE) mol^−1 (Ni) h^−1.c Mw 10^5 g mol^−1, Mw and Mw/Mn determined by GPC.d Determined by DSC.e 5 atm of ethylene.f 1 atm of ethylene.g Broad and weak endotherms, amorphous-like polyethylenes.
1	Ni1	30	30	1000	2.17	11.51	3.51	107.41
2	Ni1	30	30	1500	4.20	8.12	2.04	95.31
3	Ni1	30	30	2000	7.04	8.66	2.11	57.11
4	Ni1	30	30	2500	6.61	6.91	2.01	56.57
5	Ni1	30	30	3000	6.50	4.33	1.53	48.31
6	Ni1	10	30	2000	0.80	14.95	2.06	110.10
7	Ni1	15	30	2000	5.80	22.34	1.59	120.77
8	Ni1	20	30	2000	9.84	12.33	1.85	88.82
9	Ni1	40	30	2000	5.11	7.47	2.35	37.87
10	Ni1	50	30	2000	4.70	7.30	2.67	33.96
11	Ni1	60	30	2000	3.19	6.12	2.25	21.08
12	Ni1	70	30	2000	1.94	4.33	1.52	—^g
13	Ni1	20	5	2000	26.56	7.72	1.62	50.05
14	Ni1	20	15	2000	12.74	8.30	2.20	95.50
15	Ni1	20	45	2000	7.20	20.91	1.92	103.63
16	Ni1	20	60	2000	5.65	26.72	1.21	106.97
17^e	Ni1	20	30	2000	3.58	8.33	3.97	81.55
18^f	Ni1	20	30	2000	1.10	7.67	1.96	54.70
19	Ni2	20	30	2000	3.65	11.61	1.71	101.64
20	Ni3	20	30	2000	1.34	25.02	1.68	104.45
21	Ni4	20	30	2000	6.46	12.61	2.06	90.92
22	Ni5	20	30	2000	5.87	19.67	2.52	92.86

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Fig. 2 (a) GPC traces and (b) plots of catalytic activity and M_w of the polyethylene produced using Ni1/MAO at different Al/Ni molar ratios (entries 1–5, Table 2).

To thoroughly elucidate the temperature-dependent catalytic performance of the Ni1/MAO system, a series of experiments was conducted within 30-minute time-frame, spanning a temperature range from 10 °C to 70 °C (during polymerization, in the event of a temperature spike, cooling water was used to stabilize the temperature within ±3 °C of the set value within 1–2 minutes) while maintaining a fixed Al/Ni molar ratio of 2000 : 1 (entries 3, 6–12, Table 2). When the temperature was raised from 10 °C to 15 °C, a striking enhancement in catalytic activity was observed, with the activity surging from 0.80 × 10⁶ to 5.80 × 10⁶ g(PE) mol⁻¹ (Ni) h⁻¹ (entries 6 and 7, Table 2). This more than seven-fold increase strongly implies that at lower temperatures, the nickel centers were not fully activated, which hindered efficient initiation and chain propagation. The catalytic activity reached its peak at 20 °C, achieving an outstanding value of 9.84 × 10⁶ g(PE) mol⁻¹ (Ni) h⁻¹ (entry 8, Table 2). However, as the temperature was further increased beyond 20 °C, a sharp decline in activity ensued, plummeting to 1.94 × 10⁶ g(PE) mol⁻¹ (Ni) h⁻¹ at 70 °C. This decrease is likely due to the reduced solubility of ethylene in toluene and the decomposition of active species at higher temperatures.⁴⁶ Meanwhile, both the molecular weight and melting temperature of the polyethylenes generally decreased, with M_w dropping from 12.33 × 10⁵ to 4.33 × 10⁵ g mol⁻¹ and T_m from 88.82 °C to a point where it was no longer detectable suggesting that at elevated temperatures, enhanced chain-transfer and chain-walking reactions occurred simultaneously. Notably, in contrast to the bimodal distribution of polyethylene produced by para-chloro substituted 2,3-bis(arylimino)butylnickel complexes D (Scheme 1),⁵¹ all polyethylene obtained in current Ni1/MAO system exhibited unimodal distributions, confirming the retention of single-site catalytic characteristics (Fig. 3).

Fig. 3 (a) GPC traces and (b) plots of catalytic activity and M_w of the polyethylene produced using Ni1/MAO at different run temperatures (entries 8, 6–12, Table 2).

With the polymerization temperature fixed at 20 °C and the Al/Ni molar ratio at 2000 : 1, the polymerization time of the Ni1/MAO system was explored from 5 to 60 minutes (entries 8, 13–16, Table 2). At the outset, a 5-minute reaction duration unleashed an astonishing catalytic activity of 26.56 × 10⁶ g(PE) mol⁻¹ (Ni) h⁻¹ indicating a rapid burst of activity fueled by the swift generation of active nickel species. As the reaction time was extended to 15, 45, and ultimately 60 minutes, a gradual decline in activity from 12.74 × 10⁶ to 5.65 × 10⁶ g(PE) mol⁻¹ (Ni) h⁻¹ occurred due to the progressive deactivation of active center.³⁶ Remarkably, even after 60 minutes, the current system retained a substantial level of activity, underscoring its significantly longevity. Meanwhile, the molecular weight of the resulting polyethylene steadily increased from 7.72 × 10⁵ to 26.72 × 10⁵ g mol⁻¹ (Fig. 4), confirming the continuous growth of polymer chains over time.

Fig. 4 (a) GPC traces and (b) plots of catalytic activity and M_w of the polyethylene produced using Ni1/MAO at different run times (entries 8, 13–16, Table 2).

A systematic investigation into the effects of ethylene pressure, which was varied from 1 to 10 atm, revealed a profound pressure-dependent modulation of both catalytic activity and the properties of the resulting polyethylenes. Catalytic activity exhibited a striking increase, rising from 1.10 × 10⁶ g(PE) mol⁻¹ (Ni) h⁻¹ at 1 atm (entry 18, Table 2) to 3.58 × 10⁶ g(PE) mol⁻¹ (Ni) h⁻¹ at 5 atm (entry 17, Table 2) and further to 9.84 × 10⁶ g(PE) mol⁻¹ (Ni) h⁻¹ at 10 atm (entry 8, Table 2). This represents a three-fold and nine-fold enhancement under higher ethylene pressure, respectively (entries 8, 17 vs. entry 18, Table 2). This significant boost in activity could be attributed to the increasing concentration of ethylene molecules in the toluene under higher ethylene pressure, which effectively promotes the coordination and insertion process of ethylene.^44,47 Simultaneously, the molecular weight of the resulting polyethylenes followed an upward trend, escalating from 7.67 × 10⁵ to 12.33 × 10⁵ g mol⁻¹ due to the suppression of chain-transfer reactions and the promotion of chain propagation under higher ethylene pressures. Additionally, the melting point value of the polyethylenes constantly increased from 54.70 °C to 88.82 °C, demonstrating a positive correlation with the applied ethylene pressure. This suggests that higher ethylene pressure could facilitate the formation of more ordered crystalline domains within the polymer matrix.

After establishing the most effective reaction conditions for Ni1/MAO system, the remaining four nickel pre-catalysts (Ni2–Ni5) were evaluated under similar condition to discern the influence of ligand framework on catalytic performance (entries 8, 19–22, Table 2). All nickel catalysts demonstrated commendable activity, with values ranging from 1.34 × 10⁶ to 9.84 × 10⁶ g(PE) mol⁻¹ (Ni) h⁻¹. Additionally, varying the N-aryl moiety of the nickel complexes was found to fine-tune the activity and polyethylene properties.^37,46 Specifically, the activity order was Ni1 [2,6-di(Me)] > Ni4 [2,4,6-tri(Me)] > Ni2 [2,6-di(Et)] > Ni5 [2,6-di(Et)-4-Me]> Ni3 [2,6-di(ⁱPr)], revealing that bulkier ortho-substituents could impede the coordination/insertion of ethylene, thereby reducing the polymerization activity. Conversely, the M_w of the resulting polyethylene followed a different order: Ni3 [2,6-di(ⁱPr)] > Ni5 [2,6-di(Et)-4-Me] > Ni4 [2,4,6-tri(Me)] > Ni1 [2,6-di(Me)] > Ni2 [2,6-di(Et)] (Fig. 5) reflecting that the introduction of bulky ortho-substituent and para-methyl substituents is beneficial for protecting or stabilizing the active nickel center, thereby suppressing chain transfer reactions and enabling the formation of longer polymer chains. Consequently, Ni3, which contains the bulkiest ortho-isopropyl group, showed the lowest activity but produced polyethylenes with the highest molecular weight.

Fig. 5 (a) GPC traces and (b) plots of the catalytic activity and M_w of the polyethylene produced using Ni1–Ni5 on activation with MAO (entries 8, 19–22, Table 2).

Catalytic evaluation of Ni/Et₂AlCl system for ethylene polymerization. Temperature and co-catalyst type are pivotal factors that exert a profound influence on the regulation of polyethylene microstructures. Consequently, the secondary active co-catalyst Et₂AlCl was selected to assess its performance across a range of temperatures (entries 1–7, Table 3). As illustrated in Fig. 6, a marked decrease in the values of catalytic activity, molecular weight (M_w), and melting temperature (T_m) as the temperature increased (entries 3, 6–12, Table 2). The polyethylenes synthesized at lower temperatures (20–40 °C) displayed bimodal characteristics, which were distinct from those obtained in Ni1/MAO system. This disparity is likely attributable to the distinct alkylation behaviors of MAO and Et₂AlCl. MAO, with its steric hindrance, facilitates the formation of single-site active species through a more accessible orientation. In contrast, Et₂AlCl allows for the generation of cationic active species with diverse conformations due to the flexibility of the C2–C3 bond.⁴⁹ Conversely elevating temperatures (50–90 °C),⁵⁰ the polyethylenes exhibited a unimodal and narrow molecular weight distribution implying that the increased thermal energy prompts the catalyst active centers to become more homogeneous, resulting in a narrower molecular weight distribution. Notably, when comparing high-temperature experiments, the decline in activity of the Ni1/Et₂AlCl system (2.67 × 10⁶ g(PE) mol⁻¹ (Ni) h⁻¹ at 90 °C) was significantly lower than that of the Ni1/MAO system (1.94 × 10⁶ g(PE) mol⁻¹ (Ni) h⁻¹ at 70 °C), indicating the superior thermal stability of the former system.

Table 3 Catalytic evaluation at different temperatures using Ni1/Et₂AlCl for ethylene polymerization^a

Entry	T/°C	Activity^b	Mw^c	Mw/Mn^c	Tm^d
a Conditions: 2 μmol Ni1 and 100 mL toluene, 10 atm C2H4, 30 min, Al/Ni = 600.b 10^6 g(PE) mol^−1 (Ni) h^−1.c Mw: 10^5 g mol^−1, Mw and Mw/Mn determined by GPC.d Determined by DSC.e Broad and weak endotherms, amorphous-like polyethylenes.
1	20	4.67	10.90	6.81	55.40
2	30	4.27	9.02	3.38	41.18
3	40	4.07	6.57	2.29	39.62
4	50	3.82	5.29	2.19	15.94
5	70	3.20	5.15	1.72	—^e
6	80	2.92	4.64	2.23	—^e
7	90	2.56	2.67	1.21	—^e

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Fig. 6 (a) GPC traces and (b) plots of catalytic activity and M_w of the polyethylene produced using Ni1/Et₂AlCl at different run temperatures (Table 3).

Catalytic comparison with previous reports. For the sake of comparison, the catalytic activities and molecular weights (M_w) of polyethylenes synthesized by a series of previously reported and newly prepared nickel complexes bearing the N-2,6-dimethylphenyl, B–E, under same polymerization conditions are presented in Fig. 7. The bar chart unequivocally illustrates that, under identical polymerization conditions, the catalytic activities of these nickel complexes follow the sequence: B_Me (ref. 42) > E (current work) > D_Cl (ref. 51) > C_Me (ref. 48) > C_Cl (ref. 49). Conversely, the M_w values of the resulting polyethylenes exhibit a distinct pattern: C_Cl (ref. 49) > E > D_Cl (ref. 51) > B_Me (ref. 42) > C_Me.⁴⁸ This trend suggests that the two para-methyl substituted nickel complexes (B_Me and E) demonstrate superior activity within this series, attributable to the enhanced solubility of the nickel complexes in toluene implying that the incorporation of an ortho-difluorobenzhydryl group along with a para-methyl group synergistically enhances catalytic activity. In contrast, complex E yielded significantly higher molecular weight values compared to B_Me, underscoring the profound influence of the ligand backbone on the coordination environment. This, in turn, affects the coordination/insertion of ethylene and the chain transfer process during polymerization. In addition, when compared to the para-methyl substituted C_Me complex, the para-chloro substituted C_Cl complexes produced polyethylene with a relatively higher molecular weight and melting point. This is likely attributable to the electron-withdrawing effect of the chloro group, which increases the molecular weight and reduces branching degree of the resulting polyethylenes.³ However, when remote fluoro groups were introduced into the benzhydryl moiety of the C_Cl complex, the resulting D_Cl complex yielded polyethylenes with a significantly lower molecular weight, demonstrating that an excessive electron-withdrawing influence exerted on the nickel center can destabilize the active catalytic site, thereby reducing the molecular weight of polyethylenes produced. In contrast, the combination of an ortho-difluorobenzhydryl group and a para-methyl substituent (E) led to a substantial increase in the molecular weight of the polyethylenes, albeit at the expense of a significantly reduced melting point. This phenomenon is presumably due to the moderate electron-withdrawing effect provided by the remote fluoro groups, which is beneficial for stabilizing the nickel active species during polymerization.³ Moreover, the polyethylene synthesized by catalyst E (Scheme 1) displayed the lowest melting temperature, approximately 57.11 °C, accompanied by a broad peak indicative of its amorphous state. This amorphous nature arises from the high degree of branching introduced by the chain-walking mechanism.³⁶ To summarize, the catalyst E proposed in the current study achieves a favorable balance of excellent catalytic activity, a reasonable molecular weight, and high branching. This combination is advantageous for optimizing the mechanical properties of polyethylenes and opens up new avenues for their application.

Fig. 7 A comparative analysis of the catalytic performance among a series of α-diimine nickel complexes, featuring either the 1,2-bis(arylimino)acenaphthene or the 2,3-bis(arylimino)butane ligand backbone, was conducted under identical polymerization conditions (Al/Ni ratios of 2000 : 1, 30 °C, 30 min, 10 atm).

Microstructure of the polyethylenes. To systematically evaluate the branching architecture of polyethylenes, five representative samples namely PE-MAO-20, PE-MAO-30, and PE-MAO-70 (entries 8, 3, and 12 in Table 2) and PE-Et₂AlCl-20 and PE-Et₂AlCl-70 (entries 2 and 5 in Table 3), were prepared using the Ni1/MAO and Ni1/Et₂AlCl systems and subsequently subjected to high-temperature ¹³C NMR spectroscopic analysis. The high-temperature ¹³C NMR spectra were acquired in deuterated 1,1,2,2-tetrachloroethane-d₂ at 110 °C, and the resulting spectral data are depicted in Fig. 8 and S15–S18. By utilizing established literature-based assignments, the branching content and types within the polyethylene samples were precisely determined and analyzed.⁵³ A distinct temperature-dependent trend was observed in both Ni1/MAO (62–200/1000 C) and Ni1/Et₂AlCl (126–201/1000 C) system. Elevated polymerization temperatures facilitated the formation of polyethylenes with a higher degree of branching, owing to the increased likelihood of chain walking. Concurrently, in the Ni1/MAO system, an increase in the branching degree from 126/1000 C to 201/1000 C (Table 4) resulted in a significant reduction in the melting temperature (T_m) from 88.82 °C to a point where it was no longer detectable. This was accompanied by a concurrent decline in crystallinity from 57.91% to a fully amorphous state.⁴⁸ Notably, the PE-MAO-70 sample exhibited a significantly high branching degree up to 200/1000 C highlighting that the Ni1/MAO system exhibited unique catalytic behavior under elevated temperatures (entry 12, Table 2). A comparative analysis of polyethylenes synthesized via the Ni1/MAO and Ni1/Et₂AlCl systems further unveils distinct trends with PE-Et₂AlCl-20 sample exhibiting a higher branching degree, a lower melting temperature and reduced crystallinity compared to the PE-MAO-20 sample (126/1000 C, 55.40 °C, 32.50% vs. 62/1000 C, 88.82 °C, 57.91%, respectively). However, when subjected to a high polymerization temperature of 70 °C, the PE-Et₂AlCl-70 and PE-MAO-70 samples demonstrate comparable branching degree (approximately 200/1000 C) and a similar amorphous state. The varying microstructures of the polyethylenes, resulting from different polymerization conditions, ultimately exert a significant influence on their mechanical properties, a topic that will be explored in subsequent sections.

Fig. 8 High temperature ¹³C NMR spectrum of PE-MAO-50 sample (entry 10, Table 2).

Table 4 Branching analysis of the PE samples obtained using Ni1/MAO and Ni1/Et₂AlCl at different temperatures

PE sample	Branches/1000C	Branching composition (%)						Tm^a (°C)	Xc^a (%)
		Me	Et	Pr	Bu	Amyl	Longer branch
a Determined by DSC;
PE-MAO-20	62	77.7	2.9	2.1	8.8	1.9	6.6	88.82	57.91
PE-MAO-50	133	79.3	4.4	4.1	6.3	2.3	3.6	33.96	13.68
PE-MAO-70	200	78.3	1.8	4.5	5.7	2.6	7.0	—	—
PE-Et2AlCl-20	126	72.4	3.3	5.0	7.0	8.0	4.3	55.44	32.50
PE-Et2AlCl-70	201	75.3	4.2	3.9	8.6	2.6	5.4	—	—

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Mechanical properties of the polyethylenes. To evaluate the mechanical properties of the branched polyethylenes synthesized, six samples were selected, generated using Ni1/MAO and Ni1/Et₂AlCl systems at varying reaction temperatures (20 °C, 50 °C and 70 °C). Each sample underwent both a tensile stress–strain test using a universal testing machine and a stress–strain recovery test via dynamic mechanical analysis (DMA). The comprehensive results are presented in Table 5. Initially, monotonic tensile stress–strain measurements were conducted at room temperature, with each test involving three specimens to ensure data consistency. The stress–strain curves are depicted in Fig. 9a and 10a. The molecular weight and branching degree of polyethylenes were closely linked to their mechanical properties.^36,37 Specifically, PE-MAO-20 exhibits a relatively low branching degree (62/1000 C), high molecular weight (12.33 × 10⁵ g mol⁻¹) and high crystallinity (57.91%), resulting in high tensile strength (σ_b = 13.42 MPa) and high elongation at break (ε_b = 388%). As the temperature increased to 50 °C and 70 °C, the branching degree rose from 133/1000 C to 200/1000 C, disrupting the regular packing of polymer chains and sharply reducing melting temperature and crystallinity. Consequently, the strength of polyethylenes decreases from 5.48 MPa to 1.68 MPa.⁴⁸ As the crystallinity of polyethylenes decreases from 13.68% to approach an amorphous-like state, the elongation at break increases from 431% to 529% (Table 5) indicating that the amorphous regions can undergo molecular chain slippage and rearrangement when subjected to stress, thereby absorbing more energy and exhibiting higher toughness. For the Ni1/Et₂AlCl-based samples, PE-Et₂AlCl-20 showed lower initial tensile strength (3.77 MPa vs. 13.43 MPa) and elongation (239% vs. 388%) compared to PE-MAO-20, consistent with its lower molecular weight (10.90 × 10⁵ g mol⁻¹ vs. 12.33 × 10⁵ g mol⁻¹) and lower crystallinity (32.50% vs. 57.91%). Further increasing the temperature led to a continuous reduction in mechanical properties (Fig. 10a), with PE-Et₂AlCl-70 having the lowest tensile strength (1.03 MPa).

Table 5 Selected properties of the PE samples obtained using Ni1 by MAO/Et₂AlCl with various run temperatures

PE sample	T (°C)	Mw^a	Tm^b	Xc^b (%)	σb^c (MPa)	εb^c (%)	SR^d (%)
a Determined by GPC, values ×10^5 g mol^−1.b Determined by DSC; c Determined by using a universal tester.d Strain recovery values (SR) were calculated by using the standard formula SR = 100(εa − εr)/εa, where εa is the applied strain and εr is the strain in the cycle at 0 loads after 10 cycles.e Broad and weak endotherms, amorphous-like polyethylenes.
PE-MAO-20	20	12.33	88.82	57.91	13.42	388	40
PE-MAO-50	50	7.30	33.96	13.68	5.48	431	73
PE-MAO-70	70	4.33	—^e	—^e	1.68	529	21
PE-Et2AlCl-20	20	10.90	55.40	32.50	3.77	239	59
PE-Et2AlCl-50	50	5.29	15.94	6.56	1.46	177	65
PE-Et2AlCl-70	70	5.19	—^e	—^e	1.03	267	42

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Fig. 9 (a) Stress–strain curves and (b–d) recovery tests for the selected polyethylene samples, PE-MAO-20, PE-MAO-50, and PE-MAO-70.

Fig. 10 (a) Stress–strain curves and (b–d) recovery tests for the selected polyethylene samples, PE-Et₂AlCl-20, PE-Et₂AlCl-50, and PE-Et₂AlCl-70.

To further investigate the elastomeric properties of these polyethylenes, stress–strain recovery tests were conducted using dynamic mechanical analysis (DMA), and the results are presented in Fig. 9b–d and 10b–d. These tests were typically performed at 30 °C, with each cycle being repeated up to 10 times. After the first cycle, the stress–strain hysteresis loops of all samples exhibited consistent recovery levels indicating that all the polyethylenes display the characteristics of polyethylene elastomers.⁴¹ Based on the DSC and high temperature ¹³C NMR results presented earlier, it is evident that as the temperature increased, the polyethylenes with different branching degrees transition from a semi-crystalline state to a fully amorphous state. Consequently, PE-MAO-20, with high crystallinity (X_c = 57.91%), exhibited a limited stress–strain recovery value of merely 40% due to its relatively low branching degree (62/1000 C) and insufficient soft segments. In contrast, PE-MAO-70 with an extremely high branching degree (200/1000 C), becomes a fully amorphous and soft material with fewer hard segments, exhibiting a low stress–strain recovery value of 21%.⁴⁸ For the sample PE-MAO-50, which had a moderate branching degree (133/1000 C) and crystallinity (13.68%), a balance between hard and soft segments is achieved, resulting in the highest stress–strain recovery value of up to 73%, a value comparable to those obtained by previously reported unsymmetrical α-diimine nickel precatalysts.⁴⁸ Similarly, comparable stress–strain recovery values were observed for the Ni1/Et₂AlCl system (SR for PE-Et₂AlCl-20, 59%; SR for PE-Et₂AlCl-50, 65%; SR for PE-Et₂AlCl-70, 42%, Table 5). In summary, an ideal branching degree and sufficient crystallinity are essential for obtaining optimal elastomeric properties, as these factors facilitate the formation of balanced hard and soft regions.

Conclusions

In this study, a series of unsymmetrical 2-[2,6-bis(4,4′-difluorobenzhydryl)-4-methylphenylimino]-3-arylimino-butylnickel precatalysts has been successfully synthesized and comprehensively characterized. These nickel precatalysts demonstrated remarkable catalytic performance during ethylene polymerization, characterized by high activity, excellent thermal stability, and the ability to tailor the properties of the resulting polyethylenes. Notably, the current system maintained a high level of catalytic activity at high temperature, which reached up to 1.94 × 10⁶ g(PE) mol⁻¹ (Ni) h⁻¹ at 70 °C in Ni/MAO system and 2.56 × 10⁶ g(PE) mol⁻¹ (Ni) h⁻¹ even at 90 °C in Ni/Et₂AlCl system. Furthermore, the polymerization temperature emerged as a crucial factor influencing the microstructure and physical properties of the produced polyethylenes. As the reaction temperature rose, there was a significant increase in the branching degree, ranging from 62 to 201 branches/1000 C. Concurrently, the crystallinity decreased, transitioning the polyethylenes from a semi-crystalline state to a completely amorphous state. The synergistic interplay among molecular weight, controlled branching degree, and crystallinity, which struck a balance between hard and soft segments, enabled the polyethylene synthesized in this work to exhibit both high mechanical strength (up to 13.42 MPa) and an elastic recovery of 73%. Consequently, it shows great promise as a material for advanced polyethylene elastomer applications.

Experimental

Synthesis of monoketone and ligands (L1–L5)

3-[2,6-Bis(4,4′-difluorobenzhydryl)-4-methylphenylimino]-2-butanone. In a 300 mL solution of dichloromethane (DCM), a mixture was prepared by combining 2,6-bis(4,4′-difluorobenzhydryl)-4-methylaniline (10.24 g, 20.0 mmol), 2,3-butanedione (1.72 g, 20.0 mmol), and a catalytic amount of p-toluenesulfonic acid (0.57 g, 3.0 mmol). The resulting mixture was vigorously stirred at room temperature for a duration of 4 hours. Subsequently, the solvent was removed under reduced pressure through evaporation. The crude product obtained was then subjected to purification by recrystallization from methanol.⁵⁴ This purification process yielded a bright yellow solid (8.82 g, 76%). ¹H NMR (400 MHz, CDCl₃, TMS): δ 6.69–6.94 (m, 16H, Ar–H), 6.60 (s, 2H, Ar–H), 5.06 (s, 2H, Ar–CH(Ph)₂), 2.30 (s, 3H, O=C–CH₃), 2.17 (s, 3H, Ar–CH₃), 0.86 (s, 3H, N=C–CH₃). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 199.3 (O=C–CH₃), 168.8 (N=C–CH₃), 162.8, 160.4, 144.3, 138.7, 138.3, 137.9, 131.1, 131.0, 130.8, 130.7, 128.9, 115.8, 115.7, 115.3, 115.1, 50.8, 25.0, 21.4, 14.9. FT-IR (cm⁻¹): 2956 (w), 2902 (w), 2854 (w), 1702 (s, v_C=O), 1653 (m), 1602 (w), 1504 (s), 1455 (m), 1417 (m), 1353 (m), 1157 (s), 1115 (m), 1096 (m), 1015 (m), 867 (w), 834 (s), 768 (m), 733 (m), 710 (w), 662 (w). Anal. calcd for C₃₇H₂₉F₄NO (579.64): C,76.67; H, 5.04; N, 2.42%. Found; C, 76.75; H, 5.39; N, 2.63%.

2-[2,6-Bis(4,4′-difluorobenzhydryl)-4-methylphenylimino]-3-(2,6-R¹-4-R²-phenyl)imino-butyl ligands (L1–L5)

L1, R¹ = Me, R² = H. A 25 mL round-bottom flask, equipped with a magnetic stir bar, was charged with zinc(II) chloride (0.50 g, 1.5 mmol), 2,6-dimethylaniline (0.18 g, 1.5 mmol), 3-[2,6-bis(4,4′-difluorobenzhydryl)-4-methylphenylimino]-2-butanone (0.87 g, 1.5 mmol), and 1 mL of acetic acid. The reaction mixture was then heated to 80 °C and stirred magnetically for 4 hours. After cooling the mixture to room temperature, 10 mL of diethyl ether was added. This addition led to the formation of a yellow precipitate (zinc(II) chloride complex), which was subsequently isolated by filtration. The intermediate zinc(II) chloride complex was dissolved in dichloromethane (50 mL). A saturated aqueous solution of potassium carbonate was then added to the DCM solution and the resulting mixture was stirred at room temperature for 1.5 hours.^55–57 The organic layer was separated using a separatory funnel, washed three times with deionized water and then dried over anhydrous magnesium sulfate. After removing the solvent by rotary evaporation, the crude product was recrystallized from n-hexane, yielding L1 as a bright yellow powder (0.80 g, 78%). ¹H NMR (400 MHz, CDCl₃, TMS): δ 7.06–7.01 (m, 6H, Ar–H), 6.97–6.91 (m, 13H, Ar–H), 6.60 (s, 2H, Ar–H), 5.18 (s, 2H, Ar–CH(p-FPh)₂), 2.18 (s, 3H, Ar–CH₃), 1.98 (s, 6H, Ar–CH₃), 1.79 (s, 3H, N=C–CH₃), 1.01 (s, 3H, N=C–CH₃). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 170.0 (N=C–CH₃), 167.5 (N=C–CH₃),162.8, 162.8, 160.4, 160.3, 145.5, 139.1, 138.2, 132.4, 131.5, 131.1, 130.9, 128.8, 128.1, 124.4, 123.5, 115.7, 115.5, 115.2, 115.0, 50.9, 21.4, 18.0, 16.4, 15.9. FT-IR (cm⁻¹): 2919 (w), 2885 (w), 1645 (m, v_C=N), 1601 (m), 1478 (s), 1474 (m), 1449 (w), 1415 (w), 1361 (m), 1304 (w), 1215 (s), 1205 (s), 1201 (w), 1156 (m), 1121 (m), 1095 (m), 1042 (w), 1014 (w), 937 (w), 875 (w), 832 (s), 795 (m), 767 (m), 733 (m), 688 (w), 656 (w). Anal. calcd for C₄₅H₃₈F₄N₂ (682.81): C, 79.16; H, 5.61; N, 4.10%. Found; C, 79.33; H, 5.82; N, 4.56%.

L2, R¹ = Et, R² = H. Adopting a synthetic strategy akin to that utilized for the synthesis of L1, the substitution of 2,6-dimethylaniline with 2,6-diethylaniline led to the formation of L2 as a yellow solid (0.78 g, 73%). ¹H NMR (400 MHz, CDCl₃, TMS): δ 7.11–7.09 (d, J = 8.0 Hz, 2H, Ar–H), 7.06–7.02 (m, 5H, Ar–H), 6.99–6.93 (m, 12H, Ar–H), 6.61 (s, 2H, Ar–H), 5.20 (s, 2H, Ar–CH(p-FPh)₂), 2.34–2.23 (m, 4H, Ar–CH₂CH₃), 2.19 (s, 3H, Ar–CH₃), 1.82 (s, 3H, N=C–CH₃), 1.17 (s, J = 8.0 Hz, 6H, Py–CH₂CH₃), 1.03 (s, 3H, N=C–CH₃). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 170.0 (N=C–CH₃), 167.5 (N=C–CH₃), 162.9, 162.8, 160.4, 160.3, 147.3, 145.5, 139.2, 138.2, 132.4, 131.4, 131.2, 130.9, 130.8, 130.6, 130.2, 128.9, 126.1, 123.4, 115.7, 115.5, 115.3, 115.0, 50.9, 24.6, 21.4, 16.4, 13.8. FT-IR (cm⁻¹): 2965 (w), 2929 (w), 2901 (w), 1643 (m, v_C=N), 1601 (m), 1458 (s), 1448 (m), 1413 (w), 1360 (m), 1299 (w), 1222 (s), 1198 (w), 1097 (m), 1014 (m), 965 (w), 933 (w), 874 (w), 831 (s), 798 (m), 759 (w), 734 (m), 685 (w), 656 (w). Anal. calcd for C₄₇H₄₂F₄N₂ (710.86): C,79.41; H, 5.96; N, 3.94%. Found; C, 79.10; H, 5.68; N, 4.31%.

L3, R¹ = ⁱPr, R² = H. Adopting a synthetic strategy akin to that utilized for the synthesis of L1, the substitution of 2,6-dimethylaniline with 2,6-diisopropylaniline led to the formation of L3 as a yellow solid (0.78 g, 73%). ¹H NMR (400 MHz, CDCl₃, TMS): δ 7.16–6.95 (m, 19H, Ar–H), 6.60 (s, 2H, Ar–H), 5.19 (s, 2H, Ar–CH(p-FPh)₂), 2.59–2.52 (m, 2H, Ar–CH(CH₃)₂), 2.18 (s, 3H, Ar–CH₃), 1.47 (s, 3H, N=C–CH₃), 1.22 (d, J = 8.0 Hz, 6H, Ar–CH(CH₃)₂), 1.17 (d, J = 8.0 Hz, 6H, Ar–CH(CH₃)₂), 1.01 (s, 3H, N=C–CH₃). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 170.1 (N=C–CH₃), 167.7 (N=C–CH₃), 162.9, 162.8, 160.4, 160.3, 146.0, 145.5, 139.2, 138.2, 134.9, 132.4, 131.4, 131.1, 131.0, 130.9, 130.8, 128.9, 124.1, 123.2, 115.7, 115.5, 115.3, 115.0, 50.9, 28.4, 23.4, 23.2, 21.4, 16.7, 16.5. FT-IR (cm⁻¹): 2962 (w), 2919 (w), 2900 (w), 1639 (m, v_C=N), 1602 (m), 1480 (s), 1460 (w), 1434 (w), 1359 (m), 1325 (m), 1304 (w), 1221 (s), 1190 (w), 1157 (m), 1098 (m), 1047 (w), 1015 (m), 957 (w), 935 (w), 876 (w), 832 (s), 785 (m), 764 (m), 736 (w), 688 (w), 656 (w). Anal. calcd for C₄₉H₄₆F₄N₂ (738.91): C, 79.65; H, 6.28; N, 3.79%. Found; C, 80.01; H, 6.60; N, 3.29%.

L4, R¹ = Me, R² = Me. Adopting a synthetic strategy akin to that utilized for the synthesis of L1, the substitution of 2,6-dimethylaniline with 2,4,6-trimethylaniline led to the formation of L4 as a yellow solid (0.75 g, 72%). ¹H NMR (400 MHz, CDCl₃, TMS): δ 7.05–6.88 (m, 18H, Ar–H), 6.61 (s, 2H, Ar–H), 5.19 (s, 2H, Ar–CH(p-FPh)₂), 2.29 (s, 3H, Ar–CH₃), 2.19 (s, 3H, Ar–CH₃), 1.95 (s, 6H, Ar–CH₃), 1.80 (s, 3H, N=C–CH₃), 1.01 (s, 3H, N=C–CH₃). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 170.1 (N=C–CH₃), 167.7 (N=C–CH₃), 162.9, 162.8, 160.4, 160.3, 145.9, 145.5, 139.1, 138.2, 138.1, 132.7, 132.3, 131.5, 131.2, 130.9, 130.8, 128.9, 128.8, 50.9, 21.4, 20.8, 18.0, 16.4, 15.8. FT-IR (cm⁻¹): 2919 (w), 2909 (w), 2890 (w), 1659 (m, v_C=N), 1635 (m), 1602 (m), 1501 (s), 1474 (m), 1455 (m), 1421 (w), 1361 (m), 1302 (w), 1220 (s), 1158 (s), 1125 (m), 1099 (m), 1044 (w), 1013 (m), 964 (w), 933 (w), 832 (s), 799 (m), 723 (m), 679 (w). Anal. calcd for C₄₆H₄₀F₄N₂ (696.83): C, 79.29; H, 5.79; N, 4.02%. Found; C, 78.89; H, 6.08; N, 4.32%.

L5, R¹ = Et, R² = Me. Adopting a synthetic strategy akin to that utilized for the synthesis of L1, the substitution of 2,6-dimethylaniline with 2,6-diethyl-4-methylaniline led to the formation of L5 as a yellow solid (0.79 g, 73%). ¹H NMR (400 MHz, CDCl₃, TMS): δ 7.05–6.91 (m, 18H, Ar–H), 6.61 (s, 2H, Ar–H), 5.20 (s, 2H, Ar–CH(p-FPh)₂), 2.32 (s, 3H, Ar–CH₃), 2.26–2.23 (m, 4H, Ar–CH₂CH₃), 2.19 (s, 3H, Ar–CH₃), 1.82 (s, 3H, N=C–CH₃), 1.16 (t, J = 4 Hz, 6H, Py–CH₂CH₃), 1.02 (s, 3H, N=C–CH₃). ¹³C NMR (100 MHz, CDCl₃, TMS): δ 170.1 (N=C–CH₃), 167.7 (N=C–CH₃), 162.9, 162.8, 160.4, 160.3, 145.5, 144.9, 139.2, 139.1, 138.2, 138.1, 133.0, 132.3, 131.5, 131.2, 131.1, 130.9, 130.8, 130.2, 128.9, 126.9, 126.9, 115.7, 115.5, 115.2, 115.0, 50.9, 24.6, 21.4, 21.1, 16.4, 16.3. FT-IR (cm⁻¹): 2964 (w), 2928 (w), 2901 (w), 1636 (m, v_C=N), 1602 (m), 1474 (s), 1455 (m), 1416 (w), 1359 (m), 1302 (w), 1221 (s), 1157 (s), 1097 (m), 1042 (w), 1016 (m), 936 (w), 867 (w), 832 (s), 798 (m), 751 (w), 726 (m), 688 (w), 655 (w). Anal. calcd for C₄₈H₄₄F₄N₂ (724.89): C, 79.53; H, 6.12; N, 3.86%. Found; C, 80.11; H, 6.43; N, 3.80%.

Synthesis of nickel complexes (Ni1–Ni5)

Ni1. Within a Schlenk flask, L1 (0.11 g, 0.16 mmol) and (DME)NiBr₂ (0.05 g, 0.15 mmol) were combined with 10 mL of dichloromethane (DCM). The reaction mixture was vigorously stirred at room temperature for a duration of 12 hours. Following this, 10 mL of anhydrous diethyl ether was introduced to the mixture, prompting the precipitation of the complex. The resulting solid was then washed with diethyl ether and subsequently dried under reduced pressure. This process yielded Ni1 as a brick-red powder (0.13 g, 90%). FT-IR (cm⁻¹): 2978 (w), 2965 (w), 2908 (w), 1636 (m, v_C=N), 1601 (m), 1569 (m), 1478 (s), 1455 (m), 1412 (w), 1378 (m), 1301 (w), 1221 (s), 1157 (s), 1097 (m), 1040 (w), 1013 (m), 985 (w), 870 (w), 836 (w), 771 (m), 724 (m), 679 (w). Anal calcd for C₄₅H₃₈Br₂F₄N₂Ni (901.31): C, 59.97; H, 4.25; N, 3.11%. Found; C, 59.69; H, 4.52; N, 3.30%.

Ni2. Adopting a synthetic strategy akin to that utilized for the synthesis of Ni1, the substitution of L1 with L2 led to the formation of Ni2 as a brick-red powder (0.12 g, 81%). FT-IR (cm⁻¹): 2977 (w), 2960 (w), 2920 (w), 1638 (m, v_C=N), 1602 (m), 1571 (m), 1504 (s), 1445 (m), 1412 (w), 1377 (m), 1333 (w), 1301 (w), 1224 (s), 1127 (s), 1089 (m), 1013 (m), 981 (w), 914 (w), 836 (s), 793 (m), 724 (m), 668 (w). Anal calcd for C₄₇H₄₂Br₂F₄N₂Ni (929.36): C, 60.74; H, 4.56; N, 3.01%. Found; C, 60.55; H, 4.25; N, 3.19%.

Ni3. Adopting a synthetic strategy akin to that utilized for the synthesis of Ni1, the substitution of L1 with L3 led to the formation of Ni3 as a brick-red powder (0.11 g, 72%). FT-IR (cm⁻¹): 2965 (w), 2927 (w), 2890 (w), 1639 (m, v_C=N), 1602 (m), 1508 (s), 1457 (m), 1411 (w), 1380 (m), 1325 (w), 1302 (w), 1225 (s), 1158 (s), 1129 (w), 1098 (m), 1057 (m), 1015 (m), 981 (w), 936 (w), 867 (w), 831 (s), 790 (m), 726 (m), 680 (w). Anal calcd for C₄₉H₄₆Br₂F₄N₂Ni (957.42): C, 61.47; H, 4.84; N, 2.93%. Found; C, 61.21; H, 4.52; N, 3.19%.

Ni4. Adopting a synthetic strategy akin to that utilized for the synthesis of Ni1, the substitution of L1 with L4 led to the formation of Ni4 as a brick-red powder (0.12 g, 82%). FT-IR (cm⁻¹): 2978 (w), 2922 (w), 2900 (w), 1640 (m, v_C=N), 1601 (m), 1569 (m), 1507 (s), 1455 (m), 1411 (w), 1376 (m), 1301 (w), 1220 (s), 1157 (s), 1120 (w), 1098 (m), 1040 (m), 1013 (m), 938 (w), 835 (s), 788 (m), 724 (m), 679 (w). Anal calcd for C₄₆H₄₀Br₂F₄N₂Ni (915.34): C, 60.36; H, 4.40; N, 3.06%. Found; C, 60.26; H, 5.00; N, 3.19%.

Ni5. Adopting a synthetic strategy akin to that utilized for the synthesis of Ni1, the substitution of L1 with L5 led to the formation of Ni5 as a brick-red powder (0.11 g, 73%). FT-IR (cm⁻¹): 2976 (w), 2927 (w), 2868 (w), 1638 (m, v_C=N), 1601 (m), 1569 (m), 1478 (s),1456 (m), 1413 (w), 1377 (m), 1337 (w), 1303 (w), 1217 (s), 1157 (s), 1124 (w), 1098 (m), 1013 (m), 985 (w), 943 (w), 862 (w), 836 (s), 787 (m), 724 (m), 675 (w). Anal calcd for C₄₈H₄₄Br₂F₄N₂Ni (943.39): C, 61.11; H, 4.70; N, 2.97%. Found; C, 61.41; H, 5.10; N, 3.24%.

Author contributions

Dongzhi Zhu, Dedong Jia, and Yanping Ma: methodology, investigation, data curation, original draft preparation, and editing; Qiuyue Zhang: writing, review and editing; Wen-Hua Sun: conceptualization, resources, supervision, funding acquisition, review, and editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The synthetic procedures for the ligand and nickel complexes are presented in the text along with the analytic data, and related NMR spectra are presented in the SI. The molecular structures and selected bond angles and bond lengths of Ni1 is presented in the text; the CCDC number, crystal data, and structure refinements are shown in the SI. The detailed data in regard to the catalytic performances of all new nickel complexes appear in the manuscript along with GPC curves and high temperature ¹³C NMR of representative polyethylenes. More detailed high temperature ¹³C NMR spectra of polyethylenes appear in the SI. All raw data, such as NMR, GPC, DSC and FT-IR can be obtained upon request from the authors.

CCDC 2452147 contains the supplementary crystallographic data for this paper.⁵⁸

General considerations, general procedure for ethylene polymerization, X-ray crystallographic studies, ¹H/¹³C NMR spectra of monoketone and ligand L1–L5, high temperature ¹³C NMR of polyethylene samples, crystal data and structure refinement for Ni1, and references. CCDC 2452147 for Ni1 complex. For ESI and crystallographic data in CIF or other electronic format. See DOI: https://doi.org/10.1039/d5ra04501k.

Acknowledgements

We gratefully acknowledge the financial support by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDC0270101), the Guangxi Natural Science Foundation Program (2025GXNSFBA069499) and the Scientific Research Foundation of Guangxi Minzu University (2023KJQD46).

Notes and references

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