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. 2026 May 23;65(30):e1265929. doi: 10.1002/anie.1265929

Direct Synthesis of Polar Polyolefin Elastomers Enabled by Neutral Nickel‐Catalyzed High Temperature Solution Copolymerization

Xiaoqiang Hu 1,2, Jingmin Chen 1,3, Lei Wang 4, Hongliang Mu 1, Zhongbao Jian 1,3,✉
PMCID: PMC13383161  PMID: 42175829

ABSTRACT

Copolymerization of olefin and polar monomer to produce the desired polar polyolefin materials is a longstanding challenge, which becomes much more difficult when carried out at industrially preferred high temperatures due to both catalyst deactivation and rapid chain transfer reactions. Here, we report for the first time the direct synthesis of high‐molecular‐weight polar polyolefin elastomers (P‐POEs) from high‐temperature solution copolymerization of ethylene and biomass‐derived polar monomer at above 130°C. The key lies in the discovery of highly thermostable α‐ketocarboxamide nickel catalysts, which are not only neutral catalysts that do not require excess alkyl aluminum as an activator/scavenge/protecting agent but also in situ generate desired branches (36.8–70.6/1000C) through chain walking reaction that circumvents the need of α‐olefins. Molecular weights of P‐POEs with ester incorporations of 0.6–8.7 mol% produced at 130°C–170°C, which is also the preferred temperature for the synthesis of commercially non‐polar polyolefin elastomer (POE), reach 59–276 kDa that are 2–9 times higher than those obtained previously at high temperatures. Both high‐molecular‐weights and polar functional characteristics enable P‐POE as attractive materials, endowing them with desired properties, including tensile strength, strain recovery, melt flowability, hydrophilicity, dyeability, adhesion, and compatibilization properties.

Keywords: functional polyolefin, homogeneous catalysis, nickel catalyst, polar polyolefin elastomer, solution polymerization

Exceptionally thermally stable neutral α‐ketocarboxamide nickel catalysts with the unique chain walking ability have been developed, which for the first time enable the direct synthesis of high‐molecular‐weight polar polyolefin elastomers (P‐POEs) with desired tensile strength, strain recovery, melt flowability, hydrophilicity, dyeability, adhesion, and compatibilization properties via high‐temperature solution copolymerization of ethylene and biomass‐derived methyl 10‐undecenoate (130°C–170°C) without the need of additional reagents.

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1. Introduction

Polyolefin elastomer (POE), namely the ethylene/α‐olefin copolymer prepared by early transition metal‐catalyzed high‐temperature solution polymerization, has become an attractive material in modern industry and consumer sectors, spanning automotive components, packaging, construction sealants, and electronic encapsulants [1, 2, 3]. With an increasing imperative for high‐performance and functionalized materials in nascent fields such as advanced manufacturing, renewable energy, and biomedical engineering, polar polyolefin elastomers (P‐POEs) are emerging as a critical area of research. The strategic integration of polar functional groups into the non‐polar polyolefin backbone endows P‐POE with significantly improved compatibility with polar polymers, enhanced adhesion to polar substrates, and augmented filler interactions, thereby expanding the scope of application and adding value [4, 5]. However, the synthesis of P‐POE suitable for a potentially industrial process is substantially difficult [6, 7].

Terpolymerization of ethylene with an α‐olefin and a polar α‐olefin represents an effective approach to prepare P‐POEs (Figure 1a). Classic phosphine‐sulfonate palladium catalysts enabled the production of P‐POEs with excellent mechanical properties; however, the high cost of the palladium catalyst, the requirement for expensive α‐olefins (also cyclic olefins) comonomers, and a relatively low polymerization temperature (80°C) have hindered potential industrial application [5, 8, 9]. A series of para‐SiR3‐modified half‐titanocene catalysts has been developed that enabled highly active terpolymerization of ethylene, α‐olefin, and Al i Bu3‐protected 9‐decen‐1‐ol at a low temperature of 50°C. Also, this system required large amounts of alkyl aluminum as masking agents (105 equivalents) [10]. Recently, Chen et al. proposed a unique silane protection strategy using industrially relevant highly active early transition metal catalysts to achieve the preparation of high‐performance P‐POEs at a low temperature of 30°C. This protocol involved the pre‐protection of monomers and post‐polymer deprotection steps [11].

FIGURE 1.

FIGURE 1

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(a) Terpolymerization of ethylene, α‐olefin, and polar monomers by transition metal catalysts. (b) Copolymerization of ethylene and polar monomers by late transition metal catalysts. (c) This work: high‐temperature solution copolymerization (HTSP) of ethylene and methyl 10‐undecenate by neutral α‐ketocarboxamide nickel catalysts with chain walking polymerization (CWP) ability to produce highly branched, high‐molecular‐weight P‐POEs.

Direct copolymerization of ethylene with polar monomers simplifies the monomer system but presents notable challenges. Early transition metal and rare‐earth metal catalysts typically produce extremely limited branching and thus rely heavily on high incorporation of polar monomer. This requires excessive consumption of polar monomers, thus significantly increasing the cost of the product. Furthermore, even with ample masking agents, polar monomers often poison active sites, leading to low polymer molecular weights [12, 13, 14, 15, 16, 17, 18, 19]. Cui et al. reported a unique boryloxy titanium catalyst capable of producing copolymers with high polar monomer content (at 20°C–100°C), but its utility is hampered by additional pre‐ or post‐deprotection steps and the use of methylaluminoxane (MAO, 6000 equivalents) as a cocatalyst [20]. Late transition metal (Ni and Pd) catalysts enable the formation of branching via the “chain walking” mechanism [21, 22], but often require excess aluminum reagents or are cost‐prohibitive (particularly palladium‐based systems) (Figure 1b). In particular, polymerization temperatures are typically lower than 120°C, as higher temperatures not only deactivate the catalyst but also drastically reduce the copolymer molecular weight (Table S3, Figure S1) [23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37]. Brookhart et al. achieved ethylene/methyl acrylate copolymerization at 120°C and 70 bar using α‐diimine nickel catalysts, yet the reaction required extensive cocatalysts (211 equiv. B(C6F5)3, 105 equiv. of NaBArF) and a prolonged reaction time (90 h), yielding a branched copolymer with low molecular weight and comonomer incorporation (M w = 31 kDa, X M = 0.43 mol%) [38]. Very recently, Wang et al. utilized a sterically hindered α‐diimine nickel catalyst with 1000 equivalents of MAO to copolymerize ethylene and methyl 10‐undecenoate at 120°C, producing a low molecular weight branched copolymer (M w = 6 kDa) with poor comonomer incorporation (X M = 0.11 mol%), highlighting the longstanding challenge of synthesizing high‐molecular‐weight P‐POEs with sufficient polar monomer incorporation under elevated temperatures [39].

A bottleneck in the production of P‐POE arises from the polymerization process, typically requiring low temperatures (≤ 100°C), which deviates from the industrial process for the production of POE. Furthermore, the necessity of complicated procedures, such as extensive alkyl aluminum masking or polar monomer protection/deprotection protocols, substantially increases production cost and impedes industrial scalability. To synthesize P‐POE via industrially preferred high‐temperature solution copolymerization [40, 41, 42], the proposed methodology is guided by six principles: (1) ensuring catalyst thermostability under harsh conditions; (2) enhancing resistance to polar monomer‐induced poisoning; (3) suppressing chain transfer at high temperature to yield high‐molecular‐weight copolymers; (4) developing a catalyst with intrinsic “chain‐walking” capability to reduce reliance on high polar monomer incorporation or α‐olefin comonomers; (5) avoiding masking agents to minimize cost; (6) eliminating monomer protection/deprotection steps for process simplification. To date, an efficient, cost‐effective, and scalable method for producing P‐POE with precisely tailored structures and superior performance remains a formidable challenge.

Herein, we report a class of highly thermostable and neutral α‐ketocarboxamide nickel catalysts that enable the direct copolymerization of ethylene and methyl 10‐undecylenate (UCOOMe) via the high‐temperature solution polymerization method (130°C–170°C). This approach eliminates the need for large amounts of alkyl aluminum as an activator, scavenger, or protecting agent, and generates branched architectures due to their inherent “chain‐walking” mechanism, thereby obviating the requirement for α‐olefin comonomers (Figure 1c). The resulting P‐POEs exhibit high molecular weights, desired branching densities, tunable comonomer incorporations, and excellent physicochemical properties. This facile method provides a viable and efficient pathway for the potentially industrial production of high‐performance P‐POEs.

2. Results and Discussion

2.1. Neutral Nickel Catalyst Design and Synthesis

As a rationale for catalyst design, the N^O‐type α‐ketocarboxamide catalyst design strategically combines an 8‐arylnaphthyl motif with either a diarylmethyl or a 3,5‐disubstituted phenyl moiety, creating a unique “half‐sandwich” spatial configuration by leveraging the synergistic benefits of each component. Starting from 8‐(3,5‐dimethylphenyl)naphthalen‐1‐amine [43, 44], reactions with diarylmethanol using Lucas’ reagent or with 3,5‐disubstituted phenylboronic acids using Suzuki coupling afforded the key 2,4,8‐trisubstituted naphthylamines. Subsequently, condensation of these sterically hindered amines with in situ generated benzoylformyl chloride, catalyzed by pyridine at room temperature for several hours, efficiently provided a library of N^O‐type α‐ketocarboxamide ligands L1–L5 in high yields (85%–95%) (Figure 2).

FIGURE 2.

FIGURE 2

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Synthesis of “half‐sandwich” N^O‐type neutral α‐ketocarboxamide Ni(II) catalysts. The structures of Ni1–Ni5 are shown, in which the solid‐state structure of Ni1 is provided.

Ligands L1–L5 were deprotonated with sodium hydride to form the corresponding sodium salts. Subsequent reaction with 1 equivalent of trans‐[NiPhCl(PMe3)2] afforded neutral N^O‐type α‐ketocarboxamide nickel catalysts (Ni1–Ni5) in excellent yields of 75%–89% with a simple isolation process (Figure 2). All the synthesized ligands and neutral nickel catalysts were fully characterized and identified by means of 1H/13C/31P NMR spectroscopy and elemental analysis (Figures S6–S38).

Furthermore, single‐crystal x‐ray diffraction analyses were performed for Ni1 and Ni3 to elucidate their solid‐state structures (Figures 2 and S113). Crystallographic data revealed that these catalysts possess a stable five‐membered chelate framework, with the ligand coordinating to the nickel center in a bidentate N^O‐binding mode. Notably, the aryl plane at the 8‐position of the naphthalene forms small dihedral angles (22.7° for Ni1, 15.5° for Ni3) with the nickel N^O‐coordination plane, confirming a “half‐sandwich” structure. Strikingly, noncovalent Ni–phenyl π‐interactions were also observed in Ni1 and Ni3. The distances between the nickel atom and two carbon atoms of the 8‐(3,5‐dimethylphenyl) group are shorter (3.3–3.5 Å) than the sum of van der Waals radii for Ni and C atoms, which is indicative of intramolecular Ni–phenyl interactions [45]. These distinctive structural features are likely responsible for their exceptional thermal and air stability, as they remained intact after 72 h of air exposure [46].

2.2. High‐Temperature Solution Polymerization of Ethylene Using Neutral Ni(II) Catalysts

With the typical range of 130°C–150°C for high‐temperature solution olefin polymerization, this study evaluated five α‐ketocarboxamide neutral nickel catalysts (Ni1–Ni5) in ethylene polymerization (Table 1, Entries 1–10). At 130°C and 20 bar, all Ni(II) catalysts displayed high polymerization activity, producing POEs with considerable molecular weights and branching densities (M w = 194–296 kDa; Brs = 34.8–57.1/1000 C). The polymer generated by the 2,4‐bis(diphenylmethyl)‐substituted Ni(II) catalyst (Ni1) exhibited the highest molecular weight and branching density (Table 1, Entry 1). Replacing phenyl with naphthyl groups in Ni2 decreased molecular weight and activity (Table 1, Entries 1 vs. 3), aligning with trends observed in related α‐diimine Ni(II) systems [47]. In contrast, Ni3 bearing 2,4‐bis(3,5‐dimethylphenyl) substituents achieved the highest polymerization activity (4.50 × 105 g·mol−1·h−1), which is attributable to its slightly more accessible spatial environment, which facilitates enhanced ethylene coordination and thus higher catalytic efficiency (Table 1, Entry 5). Introducing trifluoromethyl groups instead of methyl groups in Ni4 comprehensively reduced molecular weight, activity, and branching density, demonstrating the detrimental effect of strong electron‐withdrawing substituents on catalyst stability (Table 1, Entries 5 vs. 7). Further steric enhancement in Ni5 by substituting methyl with phenyl groups, as expected, enhanced chain transfer inhibition but decreased polymerization activity and branching density (Table 1, Entries 5 vs. 9).

TABLE 1.

High‐temperature solution polymerization of ethylene with neutral nickel catalysts at 130°C–190°C. a

Entry Cat. T (°C) Act. (105) b M n (kDa) c M w (kDa) c M w /M n c brs d T m e (°C) ρ f χ c e (%)
1 Ni1 130 2.60 167 296 1.77 57.1 63.4 0.876 6.5
2 Ni1 150 0.83 77 133 1.74 78.9 38.5 0.865 1.0
3 Ni2 130 1.16 111 201 1.81 51.0 69.1 0.881 5.7
4 Ni2 150 0.52 62 119 1.92 70.8 43.8 0.869 1.9
5 Ni3 130 4.50 120 214 1.79 42.5 78.9 0.885 11.4
6 Ni3 150 1.46 56 102 1.82 59.9 58.2 0.880 2.2
7 Ni4 130 1.91 109 194 1.79 34.8 91.7 0.897 18.3
8 Ni4 150 0.89 57 103 1.80 51.3 66.8 0.884 9.0
9 Ni5 130 4.09 143 254 1.78 37.5 87.2 0.892 13.1
10 Ni5 150 2.23 68 125 1.83 55.4 65.2 0.886 6.4
11 g Ni5 170 2.22 60 124 2.06 65.1 61.5 0.875 7.8
12 g Ni5 190 0.77 35 70 1.98 73.0 53.7 0.874 3.6
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aReaction conditions: Ni(II) catalyst (10 µmol), Ni(COD)2 (2.5 equiv.), pressure (20 bar), time (1 h), toluene (130°C), mesitylene (150°C–190°C) (100 mL).

bActivity is in unit of g mol−1 h−1.

cDetermined by GPC in 1,2,4‐trichlorobenzene at 150°C.

dDetermined by 1H NMR spectroscopy, brs = Number of branches per 1000C.

eDetermined by DSC (second heating), χ c (crystallinity) calculated using the theoretical melting enthalpy of 293 J g−1 for 100% crystalline polyethylene.

fDensity of polymer sample is in unit of g cm−3, determined by an electronic analytical density balance (PTX‐FA2105).

gPolymerization pressure (40 bar).

As shown in Table 1 and Figure 3, increasing the polymerization temperature from 130°C to 150°C resulted in a decline in activity and molecular weight for Ni1–Ni5, while substantially increasing branching density (Brs = 51.3–78.9/1000C). This trend aligns with observations in other late‐transition‐metal olefin polymerization catalytic systems and is attributed to an increased rate of chain transfer relative to chain propagation at elevated temperatures [28, 48, 49, 50]. Given that Ni5 maintained the highest catalytic activity at 150°C, its excellent thermal stability motivated further investigation at higher temperatures. Remarkably, at 170°C and 40 bar, Ni5 maintained high activity (2.22 × 105 g·mol−1 h−1) and produced a desired branched high‐molecular‐weight POE (M w = 124 kDa; Brs = 65.1/1000C) with outstanding mechanical properties (Table 1, Entry 11; see below). Upon further increasing the polymerization temperature to 190°C, Ni5 exhibited moderate activity (7.70 × 104 g·mol−1·h−1) and yielded POE with a molecular weight of 70 kDa, showcasing exceptional extensibility (Table 1, Entry 12; see below). Notably, the preparation of POE from ethylene/α‐olefin copolymerization at 170°C–190°C is relatively uncommon even for early‐transition‐metal catalysts, underscoring the superior thermal resistance and competitiveness of this N^O‐type α‐ketocarboxamide nickel catalytic system. Polymers synthesized via high‐temperature solution polymerization by Ni1–Ni5 exhibited densities ranging from 0.865 to 0.897 g cm−3, classifying them as ultra‐low‐density polyethylene (ULDPE) (ENGAGE POE product selection guide). A clear inverse correlation between polymer branching and POE density was observed (Figure 3c).

FIGURE 3.

FIGURE 3

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High‐temperature solution polymerization of ethylene through all Ni(II) catalysts. (a) Compared POE molecular weight and catalytic activity in Ni1–Ni5 mediated ethylene polymerization at 130°C–150°C. (b) Compared branching density of POEs produced by Ni1–Ni5 at 130°C–150°C. (c) Correlation between the POE density and the number of branches per 1000C.

2.3. High‐Temperature Solution Copolymerization of Ethylene and Methyl 10‐Undecenoate Using Neutral Ni(II) Catalysts

Building upon the outstanding performance of these “half‐sandwich” neutral nickel catalysts in high‐temperature solution polymerization of ethylene, Ni1–Ni5 were then evaluated for the demanding high‐temperature solution copolymerization of ethylene with methyl 10‐undecenoate (UCOOMe) (Table 2). Introducing even small amounts of functional groups (< 0.5 mol%) into nonpolar polyolefins significantly enhances end‐use properties, enabling the upgrade of polyolefins for value‐added and advanced applications [51]. Under reaction conditions of 130°C, 20 bar, and 0.2 mol L−1 of UCOOMe, all five Ni(II) catalysts effectively yielded high molecular weight P‐POEs (165–276 kDa) with UCOOMe incorporation ranging from 0.7 to 1.1 mol%, exhibiting moderate to high catalytic activity (Table 2, Entries 1–5). Crucially, a comparison with POEs under otherwise identical conditions revealed only a slight drop in molecular weight in the P‐POEs produced by Ni1–Ni5 (Table 1, Entries 1, 3, 5, 7, 9 vs. Table 2, Entries 1–5). This significant observation underscores the advanced capabilities of “half‐sandwich” α‐ketocarboxamide nickel catalysts, particularly their robust intramolecular Ni–phenyl interactions. Remarkably, Ni1 and Ni5 maintained catalytic activities comparable to those in ethylene homopolymerization, indicating enhanced tolerance to polar functional group poisoning (Table 1, Entries 1 and 9 vs. Table 2, Entries 1 and 5).

TABLE 2.

High‐temperature solution copolymerization of ethylene and methyl 10‐undecenoate with neutral nickel catalysts at 130°C–170°C. a

Entry Cat. T (°C) PM (mol L−1) Act. (105) b M n (kDa) c M w (kDa) c M w/M n c X M d (mol%) brs d T m e (°C) ρ f χ c e (%)
1 Ni1 130 0.2 2.23 159 276 1.73 1.1 56.5 56.5 0.882 5.4
2 Ni2 130 0.2 0.65 94 165 1.76 0.9 50.9 59.6 0.891 5.4
3 Ni3 130 0.2 2.54 116 199 1.71 0.9 43.5 76.3 0.890 11.0
4 Ni4 130 0.2 1.51 100 175 1.75 0.8 36.8 86.3 0.899 11.5
5 Ni5 130 0.2 3.52 134 234 1.75 0.7 37.6 85.1 0.895 12.3
6 Ni1 130 0.4 1.67 139 249 1.80 2.1 55.3 53.9 0.885 4.5
7 Ni1 130 0.6 1.01 113 202 1.79 2.8 53.2 54.8 0.884 2.4
8 Ni1 130 1.0 0.51 96 172 1.80 4.6 56.1 49.8 0.876 1.3
9 Ni1 130 2.0 0.35 60 109 1.84 8.7 44.7 — i — i — i
10 g Ni1 130 0.2 0.51 70 124 1.76 1.6 70.6 40.7 0.871 1.3
11 g Ni5 130 0.2 1.30 78 140 1.79 1.2 44.5 69.3 0.891 6.9
12 h Ni1 150 0.2 0.87 90 168 1.88 0.8 63.3 55.2 0.879 3.1
13 Ni5 150 0.2 0.72 48 86 1.80 1.1 55.5 62.7 0.889 4.5
14 Ni5 170 0.1 0.22 26 59 2.25 0.6 58.0 60.4 — i 5.9
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aReaction conditions: Ni(II) catalyst (10 µmol), Ni(COD)2 (2.5 equiv.), pressure (20 bar), time (1 h), toluene (130°C), mesitylene (150°C–170°C) (100 mL).

bActivity is in unit of g mol−1 h−1.

cDetermined by GPC in 1,2,4‐trichlorobenzene at 150°C.

dThe incorporation of methyl 10‐undecenoate and brs., as determined by 1H NMR spectroscopy, brs = Number of branches per 1000C.

eDetermined by DSC (second heating), χ c (crystallinity) calculated using the theoretical melting enthalpy of 293 J g−1 for 100% crystalline polyethylene.

fDensity of polymer samples is in unit of g cm−3, determined by an electronic analytical density balance (PTX‐FA2105).

gPressure (10 bar).

hPressure (40 bar).

iNot determined.

To improve polar monomer incorporation and synthesize P‐POEs with higher functional group content, further high‐temperature solution copolymerization of ethylene and UCOOMe was carried out using Ni1, the catalyst previously yielding copolymers with the highest molecular weight and branching density. At 130°C and 20 bar, the concentration of UCOOMe was progressively increased from 0.2 to 2.0 mol L−1, ultimately achieving a remarkable UCOOMe incorporation of up to 8.7 mol%. As expected, both the catalytic activity and the copolymer molecular weight decreased progressively with increasing comonomer incorporation (Table 2, Entries 1 and 6–9). Crucially, the resulting high‐functionality P‐POEs retained high molecular weights (> 100 kDa), a critical characteristic for enhancing the mechanical properties of materials in practical applications.

As anticipated, reducing ethylene pressure from 20 to 10 bar led to increased P‐POEs' branching density and UCOOMe incorporation, while simultaneously decreasing the polymer molecular weight by nearly half (Table 2, Entries 1 and 5 vs. 10 and 11). Notably, the copolymerization activities of Ni1 and Ni5 at low pressure showed a marked deviation from the expected first‐order dependence on ethylene pressure, registering values substantially lower than the predicted ones. This significant decrease in activity is likely attributed to difficulties in both the opening of σ‐carbonyl coordination of UCOOMe and the coordination and insertion of ethylene under low pressure. Particularly, both Ni1 and Ni5 yielded P‐POEs with high molecular weights (168 kDa for Ni1; 86 kDa for Ni5) at 150°C (Table 2, Entries 12 and 13). Even at 170°C, Ni5 successfully produced P‐POE with a moderate molecular weight of 59 kDa, demonstrating exceptional thermal stability (Table 2, Entries 14). This comprehensive dataset represents the first successful synthesis of high‐molecular‐weight P‐POEs with appropriate branching densities at above 130°C.

Molecular weight and branching density are two key parameters, which determine polymer mechanical properties. Insufficient molecular weight leads to a significant reduction in tensile strength and elastic recovery, rendering materials soft and highly extensible. Conversely, polymers with low branching density typically exhibit high strength and low elasticity, resembling high‐density polyethylene. α‐Imino ketone nickel catalysts enable the synthesis of high molecular weight polar polyolefins at 110°C, but they generally yield low branching densities (< 20/1000C) [48, 52, 53]. Based on the high‐temperature solution polymerization technique, we compared the molecular weights of P‐POEs prepared in this work with those reported in the literatures (Table S3, Figure S1) [25, 26, 27, 34, 36, 38, 39]. Previous cationic α‐diimine nickel catalysts operating at 120°C met the requirement of branching density but needed excess cocatalysts and produced only low molecular weight copolymers with limited comonomer incorporation (M w = 31 kDa, X M = 0.43 mol%) [38, 39]. In particular, the copolymer properties remained elusive. Thus far, there has been no report on the synthesis of P‐POEs at 130°C and above. In a sharp contrast, this study has synthesized P‐POEs with ester contents ranging from 0.6 to 8.7 mol% at elevated temperatures of 130°C–170°C. The molecular weights reached 59–276 kDa, a 2–9‐fold increase compared to the previously reported 31 kDa at 120°C. Crucially, copolymerization at high temperatures of 130°C–170°C, coupled with enhanced product branching density, significantly improved polymer solubility and reduced solution viscosity. This merit ensures complete solubility of the prepared high molecular weight P‐POEs throughout the polymerization process, facilitating continuous production and effectively preventing reactor and agitator fouling (Figure 4a).

FIGURE 4.

FIGURE 4

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(a) Stacked autoclave photos of reaction process and P‐POE photos. (b) 13C NMR spectrum of the P‐POE sample from Table 2, Entry 7 (with 2.8 mol% of UCOOMe incorporation). (c) Stacked infrared spectroscopy (IR) traces of P‐POEs. (d) Stacked differential scanning calorimetry (DSC) traces of P‐POEs. (e) Correlation between the P‐POEs density and the calibrated branch number per 1000C (calibration formula: branch number = brs + X M*5).

2.4. Structures and Densities of P‐POEs

To confirm the microstructures of these P‐POEs, 1H NMR, representative 13C NMR and infrared spectroscopy (IR) spectra were determined and analyzed (Figures 4b,c and S51–S64). The incorporation of polar comonomer into the polyethylene chain was verified by key characteristic resonances: –COOCH 3 at δ = 3.71 ppm and –CH 2(COOCH3) at δ = 2.34 ppm (1H NMR); and carbonyl (C = O) at δ = 173.87 ppm and –COOCH 3 at δ = 51.15 ppm (13C NMR). Notably, the unsaturated vinyl groups as terminals were not observed in both 1H and 13C NMR spectra, precluding the formation of low molecular weight copolymers with incorporated polar monomer (Figures 4b and S57). Notably, the microstructure of P‐POE exclusively exhibited short‐chain methyl branching (approx. 93%) and ethyl branching (approx. 7%), with methyl branch resonances at δ = 19.88 ppm (13C NMR) and ethyl branch resonances at δ = 11.07 ppm (13C NMR). IR spectroscopy further identified the –COOCH3 group, showing that the C = O signals typically appear at 1735–1745 cm−1 and the C–O–C signals at 1160–1210 cm−1 (Figure 4c). At a given branching density (∼55–57/1000 C), higher UCOOMe content resulted in lower melting points (T m) (63.4°C (0.0 mol%), 56.5°C (1.1 mol%), 53.9°C (2.1 mol%), and 49.8°C (4.6 mol%)). With a higher comonomer incorporation (8.7 mol%), the melting peak vanished, despite a comparatively lower branching density (Figure 4d). Density is a significant physical parameter, which reflects material compactness. The density of P‐POE is doubly regulated by the incorporation of polar monomer and the branching density [9]. For these P‐POEs synthesized via high‐temperature solution copolymerization, densities ranged from 0.871 to 0.899 g cm−3, classifying them as ultra‐low‐density polyethylenes. As anticipated, P‐POE density generally decreased with increasing branching density and UCOOMe incorporation (Figure 4e).

2.5. Comprehensive Properties of P‐POEs

2.5.1. Thermal and Surface Properties of P‐POEs

The thermomechanical and surface properties of the high‐molecular‐weight P‐POEs were characterized by thermogravimetric analysis (TGA) and water contact angle (WCA) measurements. The decomposition temperature (T d,5%) of P‐POEs with varying contents of ester functional groups remained high, ranging from 408°C to 427°C, which is comparable to that of nonpolar POE (418°C) (Figure 5a). This indicates that polar comonomer incorporation did not substantially compromise the polyolefin's thermal stability. As expected, these P‐POEs presented significantly lower WCAs compared to POE, decreasing from 105.5° to 68.9° (Figure 5b), suggesting improved surface wettability.

FIGURE 5.

FIGURE 5

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Comprehensive properties of P‐POEs. (a) Stacked thermogravimetric analysis (TGA) curves of P‐POEs. (b) Stacked water contact angle (WCA) photos of P‐POEs with varied incorporation of UCOOMe. (c) Melt flow index (MFI) of P‐POEs with varying incorporation of UCOOMe and different molecular weights. (d) UV‐vis absorption spectra of the dye extracted from the (co)polymers were measured immediately after extraction and after 72 h of incubation in acetone. (e) Adhesion properties of POE 8660 and POE‐8660/P‐POE blends on two copper strips, respectively. (f) SEM images of fractured surfaces for different blend samples.

2.5.2. Melt Flowability of P‐POEs

Melt Flow Index (MFI) is an important parameter for assessing polymer processability, which is measured in grams per 10 min. MFI values affect efficient and uniform material distribution during molding, extrusion, and other fabrication processes [54]. The initial high‐molecular‐weight POE (296 kDa) exhibited an MFI of 0.179 g/10 min. With increasing UCOOMe incorporation and a concomitant decrease in molecular weight, the MFI steadily increased. A P‐POE with 4.6 mol% of UCOOMe incorporation reached 2.161 g/10 min, representing a 12‐fold enhancement over the POE. Notably, when comparing two samples with identical UCOOMe incorporation but significantly different molecular weights (Table 2, Entries 1 and 13), the lower molecular weight P‐POE achieved an MFI of 5.263 g/10 min (a 25‐fold increase compared to the high‐molecular‐weight P‐POE), demonstrating the significant influence of molecular weight on MFI (Figure 5c, Table S1).

2.5.3. Dyeing Properties of P‐POEs

Dyeing properties of polyolefins, closely linked to their surface characteristics, are of growing academic and industrial interest [4, 11, 55]. In this study, P‐POE and POE powders were blended with Solvent Red 43 dye powder and melt‐pressed at 140°C to fabricate test specimens (samples from Table 1, Entry 1 and Table 2, Entry 7). Initially, samples were immersed in acetone to establish baseline absorbance values. Subsequently, the samples underwent a 72 h hot acetone treatment for simultaneous washing and heating, followed by cooling and re‐determination of absorbance. The UV–vis absorption analysis (Figure 5d) revealed a significant increase in absorbance intensity from the red to the blue curve, indicating poorer dye uptake by POE compared to P‐POE. Overall, the introduction of polar functional groups effectively improves the dyeability of these polyolefin materials.

2.5.4. Adhesion Properties of P‐POEs

POEs demonstrate significant utility in pouch cell packaging for power batteries and optical film lamination [4, 56, 57, 58]. However, commercially available POEs suffer from poor interfacial adhesion with substrates like metal current collectors or optical base films, frequently leading to delamination and interfacial failure. P‐POEs effectively overcome this limitation. For instance, a blend of commercial POE‐8660 and 10 wt% P‐POE (samples from Table 2, Entries 7 and 9) was hot‐pressed between two copper strips at 120°C for 2 h. Subsequent standardized adhesive strength evaluation revealed that the POE‐8660 mixture containing 10 wt% P‐POE (with 8.7 mol% UCOOMe) achieved an adhesive strength 7.3 times higher than POE‐8660 (2.2 vs. 0.3 MPa). This finding suggests that introducing polar functional groups significantly enhances POE's adhesive properties. Notably, a POE‐8660 mixture with 10 wt% of a higher‐molecular‐weight P‐POE achieved a greater adhesive strength, reaching 16.7 times that of POE‐8660 (4.7 MPa vs. 0.3 MPa), despite a lower UCOOMe content (Figure 5e). This observation further highlights the critical role of high molecular weight in P‐POEs.

2.5.5. Compatibilization of P‐POEs

POEs play an important role in the toughening of polyolefins and their composites [4, 5, 11]. Introducing functional groups into nonpolar POEs significantly enhances the compatibility between polyolefin substrates and polar materials. For instance, blending HDPE with polylactide (PLA) can improve toughness and processing, while also conferring biodegradability to the composite. However, the pronounced polarity difference between HDPE and PLA often leads to poor compatibility in direct blends, resulting in inferior mechanical properties and delamination. Scanning electron microscopy (SEM) images clearly show significant phase separation when HDPE is directly blended with PLA (80 wt% HDPE and 20 wt% PLA). In contrast, the addition of 5% P‐POE (sample from Table 2, Entry 7) substantially reduces the phase separation, effectively compatibilizing the immiscible phases and promoting homogeneous dispersion of PLA within the HDPE matrix (Figure 5f). These findings underscore the superior compatibilization capabilities of P‐POE, for blending nonpolar polyolefins with polar additives.

2.5.6. Mechanical Properties of P‐POEs

Tensile strength and elasticity are fundamental parameters for characterizing polymer performance [59, 60, 61]. Tensile testing was employed to evaluate the mechanical properties of POEs and P‐POEs, which are closely correlated with polymer molecular weight, branching density, and the incorporation of polar monomer. As depicted in Figure 6, the obtained POEs and P‐POEs generally exhibited typical elastomeric behavior, characterized by low and broad yielding maxima. P‐POEs synthesized using Ni1 demonstrated tensile strengths ranging from 12.9 to 13.6 MPa and elongations at break of 776%–977% (Figure 6a). Compared to the nonpolar POE (black line in Figure 6a), the introduction of 2.8 mol% UCOOMe (green line in Figure 6a) resulted in a 1.2‐fold increase in elongation at break, concurrently reducing tensile strength by approximately 1.2‐fold. This disparity is primarily attributed to differences in molecular weight and the incorporation of polar monomer. Furthermore, P‐POEs prepared with different catalysts displayed significant mechanical property variations (Figure 6b). The P‐POE synthesized with Ni4 exhibited a tensile strength of 36.1 MPa and an elongation at break of 1186% (green line in Figure 6b). Relative to Ni1 (black line in Figure 6b), the tensile strength was enhanced by 2.8‐fold, and the elongation at break was improved by 1.5‐fold, placing it among the top‐performing polyolefin‐type elastomers.

FIGURE 6.

FIGURE 6

Open in a new tab

Mechanical properties of POEs and P‐POEs. (a) Stress‐strain curves for the (co)polymers synthesized by Ni1 from Table 1 (Entry 1) and Table 2 (Entry 1, with 1.1 mol% of UCOOMe; Entry 6, with 2.1 mol% of UCOOMe; Entry 7, with 2.8 mol% of UCOOMe). (b) Stress‐strain curves for the copolymers synthesized by Ni1‐Ni5 from Table 2 (Entry 1, with 1.1 mol% of UCOOMe; Entry 2, with 0.9 mol% of UCOOMe; Entry 3, with 0.9 mol% of UCOOMe; Entry 4, with 0.8 mol% of UCOOMe; Entry 5, with 0.7 mol% of UCOOMe). (c) Stress‐strain curves of the commercial polyolefin elastomers 8150 and 7447 by Dow Chemical Company and the (co)polymers synthesized by Ni1 and Ni5 from Table 1 (Entries 11 and 12) and Table 2 (Entry 12, with 0.8 mol% of UCOOMe; Entry 13, with 1.1 mol% of UCOOMe). (d–f) Error bar of tensile strengths in (a–c). (g–h) Plots of hysteresis experiments with 10 cycles at a strain of 300% for of (co)polymers from Table 1 (Entry 11) and Table 2 (Entry 8, with 4.6 mol% of UCOOMe). (i) The strain recovery values of the (co)polymers versus the UCOOMe incorporation.

Notably, POEs and P‐POEs prepared by Ni1 and Ni5 at elevated temperatures (150°C–170°C) also exhibited exceptional mechanical properties, achieving tensile strengths of 28.2–31.8 MPa and remarkable elongations at break of 1402%–1536% (Figure 6c). Increasing the temperature to 190°C resulted in an elongation at break of 1822% for the POE, but a drastic decrease in tensile strength from 31.8 to 5.6 MPa (black and red lines in Figure 6c). This performance shift is likely attributed to both excessive branching that disrupts crystalline polyethylene segments and a significantly lower molecular weight, which reduces tensile strength while enhancing elasticity. Furthermore, we attempted to evaluate the tensile properties of commercial polyolefin elastomer (POE) samples 7447 (ethylene/1‐butene copolymer) and 8150 (ethylene/1‐octene copolymer) from Dow Chemical Company (dark green and yellow lines in Figure 6c). Commercial POE products 7447 and 8150 exhibited ultimate tensile strengths of 6.5 and 16.8 MPa, respectively, and ultimate elongations at break of 1954% and 1368%, respectively. Notably, POEs and P‐POEs synthesized at high temperatures using either Ni1 or Ni5 demonstrated outstanding characteristics in both tensile strength and elongation at break.

Cyclic tensile tests confirmed the elastomeric behavior of the POEs and P‐POEs samples, with strain recovery values ranging from 40.8% to 60.0% (Figures 6g–i and S3). Higher incorporation of polar monomers improved elastic recovery in P‐POEs. Specifically, the P‐POE sample containing 4.6 mol% UCOOMe exhibited the best elasticity, achieving a strain recovery of 60.0%. Conventional late‐transition‐metal catalytic systems are incapable of affording polar polyolefins with both high elasticity and high mechanical strength, highlighting one major advantage of the α‐ketocarboxamide nickel catalysts in P‐POE synthesis.

3. Conclusions

In summary, this work reports for the first time the direct synthesis of high‐molecular‐weight polar polyolefin elastomers (P‐POEs) via high‐temperature solution copolymerization of ethylene and biomass‐derived methyl 10‐undecenoate (130–170°C). This is enabled by neutral α‐ketocarboxamide nickel catalysts designed by us, which exhibit exceptional thermal stability, resistance to polar monomer poisoning, and intrinsic chain‐walking capability. Crucially, these catalysts circumvent the need for excess alkyl aluminum activators/scavengers, α‐olefin comonomers, or polar monomer protection/deprotection steps—key limitations of prior systems. The resulting P‐POEs feature tunable ester incorporations (0.6–8.7 mol%), branched architectures (36.8–70.6/1000C), and high molecular weights (59–276 kDa) that are 2–9 times higher than those reported previously. Notably, the high‐temperature polymerization method enhances polymer solubility and reduces solution viscosity, facilitating continuous industrial scalability. The P‐POEs display outstanding comprehensive properties, including excellent tensile strength (up to 36.1 MPa) and elongation at break (up to 1822%), improved hydrophilicity, dyeability, adhesive strength (7.3–16.7‐fold increase vs. commercial POE), and effective compatibilization of immiscible polymer blends. This facile and cost‐effective strategy addresses long‐standing challenges in polar polyolefin synthesis, paving the way for potentially industrial production of high‐performance P‐POEs.

Author Contributions

Xiaoqiang Hu: conceptualization, methodology, data curation, investigation, writing – original draft. Jingmin Chen: methodology, data curation, investigation, formal analysis. Lei Wang: formal analysis, funding acquisition. Hongliang Mu: formal analysis, investigation. Zhongbao Jian: conceptualization, methodology, formal analysis, supervision, funding acquisition, project administration, writing – review and editing.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: anie72842‐sup‐0001‐SuppMat.pdf.

ANIE-65-e1265929-s001.pdf (6MB, pdf)

Acknowledgments

The authors are thankful for financial support from the National Natural Science Foundation of China (Nos. U23B6011 for Z.J. and 22501227 for X.H.), the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDC0250100 for H.M.), the Jilin Provincial Science and Technology Department Program (No. 20260201098GX for Z.J.), and the Wanhua Chemical Group Co., Ltd. Project.

Data Availability Statement

The authors declare that all data supporting the findings of this study are available in the text or the Supporting Information. (1) Supporting Information: (i) Supplementary Tables S1–S3 and Figures S1–S5. (ii) General synthesis and polymerization procedures, as well as characterization and analysis data (NMR, GPC, and DSC) for ligands, nickel catalysts, and (co)polymers (Figures S6–S111). (2) Supporting CIF file for Ni1 and Ni3: The x‐ray crystallographic data for Ni1 and Ni3 have been deposited at the Cambridge Crystallographic Data Center (CCDC) under deposition numbers 2512029 and 2512028, respectively. These data can be obtained free of charge via the website www.ccdc.cam.ac.uk/data_request/cif.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Supporting File: anie72842‐sup‐0001‐SuppMat.pdf.

ANIE-65-e1265929-s001.pdf (6MB, pdf)

Data Availability Statement

The authors declare that all data supporting the findings of this study are available in the text or the Supporting Information. (1) Supporting Information: (i) Supplementary Tables S1–S3 and Figures S1–S5. (ii) General synthesis and polymerization procedures, as well as characterization and analysis data (NMR, GPC, and DSC) for ligands, nickel catalysts, and (co)polymers (Figures S6–S111). (2) Supporting CIF file for Ni1 and Ni3: The x‐ray crystallographic data for Ni1 and Ni3 have been deposited at the Cambridge Crystallographic Data Center (CCDC) under deposition numbers 2512029 and 2512028, respectively. These data can be obtained free of charge via the website www.ccdc.cam.ac.uk/data_request/cif.

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