The advancement of olefin polymerization technologies has been fundamentally driven by continuous innovations in organometallic catalysts. Following Ziegler and Natta’s groundbreaking discovery of Ti-based catalysts in the 1950s, the field has witnessed successive generations of catalytic systems ranging from Phillips catalysts to metallocene catalysts and post-metallocene catalysts [
1–
7]. These breakthroughs in catalyst technology have notably propelled the development of the global polyolefin industry [
8].
Olefin polymerization employs two principal methodologies: homogeneous solution processes and heterogeneous gas/slurry-phase systems (
Fig. 1). Homogeneous catalysis, typically mediated by solvent-soluble molecular catalysts such as metallocenes, excels in producing low-crystallinity plastomers and elastomers [
9,
10]. This method is not optimal for semi-crystalline polyolefins such as high-density polyethylene (HDPE) and isotactic polypropylene (
iPP), where these polymers are insoluble in the reaction medium. To prevent reactor fouling and ensure smooth operations, morphologically uniform polymer particles are required, thereby enabling a continuous process [
11,
12]. As such, it becomes pertinent to prepare immobilized catalysts by combining the catalyst with inorganic or organic supports, through covalent [
13–
15], ionic [
16–
20], or hydrogen bonding [
21]. However, immobilization of catalysts often negatively affected their performance especially for catalytic activity [
22,
23]. Particularly for late transition metal catalysts (
e.g., α-diimine Ni(Ⅱ) systems) that exhibit chain-walking behavior [
24–
37], immobilization on either silica or other inorganic support often suppresses the characteristic branching density, diminishing the unique elastomeric properties of resultant polyethylenes [
38,
39].
Recently, in terms of the important role of multi-nuclear catalysts in enhancing olefin polymerization, polymeric catalysts have garnered interest as an innovative catalyst design approach [
40–
46]. Unlike traditional multi-nuclear catalysts that always require multi-step, tedious, and difficult synthesis, polymeric catalysts offered a more straightforward synthesis, facilitating the transition from mono-nuclear to multi-nuclear catalyst through polymerizable groups such as the common unsaturated double bonds. Representative advances included poly(norbornene-
graft-styrene)-supported Ti and Zr phenoxy-imine catalysts, demonstrating one order of magnitude higher activity over monomeric analogues in ethylene polymerization [
47], and styrene-derived polymeric α-diimine Pd(Ⅱ) catalysts, achieving improved tolerance to polar comonomers, leading to higher activities as well as higher molecular weights in ethylene–methyl acrylate copolymerization [
46].
In this study, we report a strategy of oligomeric Ni(Ⅱ) catalysts for chain walking ethylene polymerization. Through rational introduction of polymerizable norbornene moiety into the milestone α-diimine Ni(Ⅱ) catalyst, we achieved a control over catalyst oligomerization
via ring-opening metathesis polymerization (ROMP) (
Fig. 2a). Through the control of the degree of polymerization (DP) of the oligomeric catalysts, two highly active Ni(Ⅱ) catalysts were prepared. Notably, compared to their monomeric nickel counterpart, the oligomeric variants exhibited remarkable improvements in both catalytic activity and polymer molecular weight. By using a 20 L polymerization reactor, a kilogram scale of polyethylene (PE) elastomer was successfully produced in an activity level of 10
7 g mol
-1 h
-1. Notably, these polyethylenes were highly branched and showcased amorphous properties and excellent transparency: hallmark attributes of elastomers.
The synthetic route for the oligomeric nickel catalysts for chain walking olefin polymerization in this study was delineated in
Fig. 2a. The polymerizable ligand was designed by installing a ring-opening metathesis polymerization (ROMP) type cyclic olefin unit into the α-diimine ligand, which was synthesized by the reaction of a norbornene-modified diketone [
48] and 2,6-diisopropyl aniline (detailed synthetic procedures and characterizations were provided in Supporting information). Subsequent coordination of the ligand with stoichiometric NiBr
2(DME) (DME = 1,2-dimethoxyethane) yielded the monomeric catalyst Ni
1 (
Fig. 2b). Oligomerization of Ni
1 was then achieved
via G2 (the 2
nd generation Grubbs catalyst) mediated ROMP, producing well-defined oligomeric catalysts Ni
3 and Ni
5 with degrees of polymerization (DP) of 3 and 5, respectively. Compared with conventional multi-nuclear catalysts that require sophisticated structural design and intricate synthesis procedures, the strategy of oligomeric nickel catalysts offered a more efficient synthesis process to construct a multi-nuclear structure. For instance, the production of Ni
5 can be efficiently scaled up to 50 g in a laboratory (Supporting information).
Comprehensive structural characterization of the ligand and catalysts was performed using NMR (nuclear magnetic resonance) spectroscopy, MS (mass spectrometry), and elemental analysis (Supporting information). A distinct chromatic evolution was observed among these ligand and catalysts: Solutions transitioned progressively from yellow (free ligand) to orange (monomeric Ni
1), and ultimately to deep red/maroon for oligomeric Ni
3 and Ni
5 (
Fig. 2c). This transition was correlated with characteristic UV–vis spectral changes (
Fig. 2d), where an absorption peak emerged at 540 nm from the ligand to Ni
1. This peak corresponded to green light, resulting in a shift in the solution’s color from yellow to orange, which can be attributed to the pronounced metal-to-ligand charge transfer (MLCT) absorptions in the visible range of 500–600 nm [
49]. Both Ni
3 and Ni
5 exhibited absorption within this range with heightened intensity, in which Ni
5 with the higher DP demonstrated the most intense absorption, accounting for the gradual deepening of the solution color.
Under the activation of methylaluminoxane (MAO), Ni
1, Ni
3, and Ni
5 were employed in ethylene polymerization (
Table 1). In general, the catalytic performance of these three catalysts demonstrated similar trends across varying reaction conditions, producing highly branched PE (
Fig. 3). As the reaction temperature increased from 30 ℃ to 90 ℃, a decrease in both catalytic activity and polymer molecular weight was observed, along with an increase in polymer branching density. These observations were consistent with previous reports [
14]. Higher temperatures could cause partial deactivation of the catalyst and lower solubility of ethylene. The former reduced the active species, while the latter led to a slower chain growth rate. Both factors contributed to a decrease in catalytic activity. The reduction in polymer molecular weight and increase in branching density can be attributed to a higher propensity for β-H elimination at a higher temperature, thereby accelerating chain transfer and chain walking [
50,
51]. As ethylene pressure elevated from 1 bar to 4 bar and 8 bar, both catalytic activity and polymer molecular weight were enhanced (
Table 1, entries 11, 13 and 14). Concurrently, the polymer branching density witnessed a gradual decline. This can be attributed to the rise in ethylene concentration, which resulted in an accelerated chain growth rate [
52]. Notably, replacing toluene with industrially favorable hexane (Table S4 in Supporting information) resulted in comparable polymer molecular weights but enhanced catalytic activity.
It is noteworthy that oligomeric catalysts Ni
3 and Ni
5 demonstrated remarkable enhancement in catalytic performance relative to the monomeric catalyst Ni
1 in terms of thermostability, catalytic activity, and polymer molecular weight (
Fig. 3). Under identical reaction conditions, Ni
5 significantly outperformed Ni
3 and Ni
1 in both catalytic activity and polymer molecular weight (
Figs. 3a and
b). At 30 ℃, Ni
5 exhibited 4.6-fold higher activity (20.2 × 10
6 vs. 4.4 × 10
6 g mol
-1 h
-1) and 3.8-fold greater polymer molecular weight (201
vs. 53 kDa) than Ni
1 (
Table 1, entry 9
vs. 1). Even at 90 ℃, Ni
3 and Ni
5 retained substantial activities (3.2 × 10
6 and 4.5 × 10
6 g mol
-1 h
-1, respectively), while Ni
1 was complete deactivation. The enhanced catalytic performance scaled with the increase of the DP of oligomeric catalysts, as Ni
5 (DP ≈ 5) consistently surpassed Ni
3 (DP ≈ 3) across all tested conditions, indicating a DP-dependent feature (
Table 1, entries 5–12). In addition, the polymer branching densities of PE produced by multi-nuclear catalysts (Ni
3 and Ni
5) were found to be lower than that produced by the monomeric Ni
1. This could be attributed to the increased steric hindrance surrounding the metal centers due to the formation of multi-nuclear architectures, which effectively reduced β-H elimination and chain-walking, thus resulting in a decreased branching density.
Ethylene polymerization using Ni
1, Ni
3, and Ni
5 activated by diethyl aluminum chloride (Et
2AlCl) was also systematically investigated in parallel with MAO (Table S1 and Fig. S1 in Supporting information). Polymerization results showed that Ni
1, Ni
3, and Ni
5 catalysts exhibited up to 2-fold enhanced activities (22.8 × 10
6 vs. 11.4 × 10
6 g mol
-1 h
-1) when activated by Et
2AlCl compared to MAO (Table S1, entry 10
vs.
Table 1, entry 10), while demonstrated a reduction in polymer molecular weights (
Mw = 77.4 kDa
vs. 49.9 kDa). Notably, both catalytic systems displayed analogous temperature- and structure-dependent trends, with oligomeric Ni
3 and Ni
5 demonstrating superior performance in both activity and polymer molecular weight compared to the monomeric Ni
1 catalyst (
Fig. 3 and Fig. S1). This consistent behavior over different activator underscored the advantage of oligomeric catalyst in ethylene polymerization. The synthesized highly branched polyethylene displayed properties akin to conventional elastomers. An exemplary sample (Table S1, entry 9; brs = 72/1000C) achieved a tensile strength of 4.0 MPa with 1100% of elongation at break, exhibiting typically elastic recovery of 72% (
Fig. 4). In contrast, samples with a lower branching density (
Table 1, entry 9; brs = 48/1000C) exhibited enhanced tensile strength (23.8 MPa, Fig. S54 in Supporting information). Moreover, the polymer branching density also influenced materials density (Table S3 in Supporting information). Specifically, the less branched sample (
Table 1, entry 9) exhibited a higher density (0.890 g/cm
3) compared to its highly branched counterpart (Table S1, entry 9; 0.879 g/cm
3).
Scale-up experiments conducted in a 20 L high-pressure reactor (20 bar of ethylene pressure) at hexane confirmed the potentially industrial viability of oligomeric catalysts (
Fig. 5). Ni
5 achieved remarkable productivity of 10
7 g mol
-1 h
-1 (approximately 15 kg
PE/g
Cat), producing 1 kg of branched polyethylene with moderate molecular weight (
Mw = 91.1 kDa) and excellent melt processability (melt flow index, MFI = 3.0 g/10 min at 190 ℃/2.16 kg). This significantly outperformed the sample from
Table 1, entry 9 (MFI = 0.023 g/10 min) with higher molecular weight of
Mw = 488 kDa. Notably, Ni
5 demonstrated consistent activities over five repeated polymerizations, suggesting an outstanding repeatability (
Fig. 5b). In contrast, the monomeric Ni
1 exhibited significantly lower activity (2.4 × 10
6 g mol
-1 h
-1, approximately 3.6 kg
PE/g
Cat) and molecular weight (
Mw = 14.7 kDa) under identical conditions, as detailed in
Table 2.
1 kg of polyethylene prepared by Ni
5 in a 20 L reactor exhibited highly branched architecture with a complicated branching pattern, including methyl (B1), ethyl (B2), propyl (B3), butyl (B4) and longer branches (B4+) (
Fig. 5c).
13C NMR [
53] spectrum revealed that methyl branching at 19.88 ppm was the most branching pattern, contributing up to 70% of the content, followed by longer branches than butyl (B4+, 14.30 ppm) with a content of 13%. This high branching structure endowed the PE with notable properties such as a low melting point (46 ℃), low density (0.850 g/cm
3), excellent ductility (Fig. S54), good solubility, and high transparency due to reduced crystallinity. In detail, the highly branched PE could fully dissolve in the reaction solvent (hexane) without any precipitation, yielding a clear, transparent, and highly viscous solution with polymer content up to 10.4 wt% after polymerization (
Fig. 5a). Also, the polymer film exhibited high transparency, with a transmittance reaching 93.8% at 500 nm (sample thickness: 0.17 mm) (
Fig. 5d). This all-hydrocarbon structure endowed such highly branched polyethylene with good insulation properties, achieving a volume resistivity of 2 × 10
16 Ω/m. Coupled with hydrophobicity, it exhibited a water vapor permeability of 0.03 g/m
2/day, comparable with commercial polyolefin elastomer (Table S3), suggesting potentially practical applications.
In summary, this work presented an oligomeric approach for α-diimine nickel catalyst design, which advances the conventional mono-nuclear nickel catalysts and reforms the multi-nuclear nickel catalysts using a complicated synthesis. By incorporating norbornene-modified ligands followed by ring-opening metathesis polymerization, oligomeric nickel catalysts with varied degree of polymerization (DP) were constructed. Comprehensive evaluation revealed that the oligomeric catalyst strategy significantly governed catalytic performance enhancements: Ni5 demonstrated 4.6-fold higher activity and produced polyethylene with 379% increased molecular weight, relative to the monomeric counterpart Ni1. Notably, these improvements were intensified as the DP of the oligomeric catalyst increased, indicating the effectiveness of the oligomeric catalyst strategy. The resultant highly branched polyethylenes exhibited remarkable elastic recovery (72%), while kilogram-scale synthesis in a 20 L reactor indicated potentially industrial application. In contrast to previous modifications that have majorly relied on tedious ligand substituents, the oligomeric nickel catalyst described in this work has witnessed a key step to enhance the catalytic performance in olefin polymerization. This provides a new avenue for the development of late-transition metal catalysts and beyond. At present, we are working for new polyolefin catalysts by using this oligomeric catalyst strategy.
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