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Direct synthesis of poly(α-olefin) thermoplastic elastomers via nickel catalyzed chain walking polymerization

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Veröffentlicht/Copyright: 14. Januar 2026
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Pure and Applied Chemistry
Aus der Zeitschrift Pure and Applied Chemistry

Abstract

This study presents the direct synthesis of poly(α-olefin) thermoplastic elastomers (PTEs) via nickel-catalyzed chain walking polymerization of linear α-olefins (LAOs) such as 1-hexene, 1-octene, and 1-decene. A series of axially flexible α-diimine nickel(II) catalysts with bulky cycloalkyl substituents were employed to investigate the effects of polymerization conditions – including temperature, monomer concentration, reaction time, and catalyst structure – on the activity, molecular weight, branching density, and thermal properties of the resulting polymers. The catalysts exhibited moderate to high activity and produced high-molecular-weight (M n up to 683 kg/mol) polymers with tunable branching densities and melting points. Mechanical testing revealed that the resulting PTEs possess excellent elastomeric properties, including high strain at break (strain up to 1487 %) and moderate stress at break (stress up to 18.2 MPa), along with good elastic recovery (SR up to 76 %). This work demonstrates an efficient one-step route to tailor-made PTEs using single LAO monomers, offering a promising alternative to traditional multi-monomer copolymerization systems.

Keywords: α-diimine nickel(II) complexes; α-olefin polymerization; controlled chain walking; IUPAC2025; thermoplastic elastomers

Introduction

Polyolefin elastomers (POEs), a class of thermoplastic elastomers produced through the copolymerization of ethylene and α-olefins (e.g., 1-butene, 1-hexene, or 1-octene) using metallocene or other advanced catalysts, combine the elastic and resilient nature of rubber with the processability of thermoplastics. 1 , 2 , 3 , 4 These materials exhibit outstanding elasticity, impact resistance, and low-temperature performance, alongside superior thermal stability, UV resistance, and compatibility with fillers compared to conventional elastomers. The homogeneous distribution of short-chain branches within their molecular structure allows for the precise tuning of mechanical properties. Owing to their recyclability and energy-efficient processing, POEs serve as a sustainable alternative to vulcanized rubbers and find broad applications in automotive components, packaging, wire and cable insulation, and polymer modification. 5 The first commercial POE was developed by The Dow Chemical Company using a constrained geometry metallocene catalyst (CGC) technology, marketed under the name INSITE (Scheme 1a). 6 , 7 In 2006, the same company introduced olefin block copolymers (OBCs) via chain-shuttling polymerization employing dialkyl zinc as chain-transfer agents (Scheme 1b). 8

Scheme 1: (a) POEs synthesized by Group 4 metallocenes catalysts, (b) POEs synthesized by chain-shutting polymerization, (c) POEs synthesized by α-diimine nickel(II) or palladium(II) catalyzed chain-walking ethylene polymerization, (d) POEs synthesized by α-diimine nickel(II) or palladium(II) catalyzed chain-walking LAO polymerization in this work.
Scheme 1:

(a) POEs synthesized by Group 4 metallocenes catalysts, (b) POEs synthesized by chain-shutting polymerization, (c) POEs synthesized by α-diimine nickel(II) or palladium(II) catalyzed chain-walking ethylene polymerization, (d) POEs synthesized by α-diimine nickel(II) or palladium(II) catalyzed chain-walking LAO polymerization in this work.

Conventional POE synthesis typically relies on multi-step copolymerization processes involving Group 4 metallocenes. In contrast, the direct production of POEs from a single monomer in one step represents an attractive and commercially promising alternative. Late-transition-metal catalysts, such as α-diimine complexes of nickel(II) and palladium(II), enable a unique chain-walking mechanism – a process involving β-hydride elimination followed by reinsertion with opposite regiochemistry. 9 , 10 , 11 , 12 , 13 This mechanism has been extensively exploited in the late-transition-metal-catalyzed homopolymerization of ethylene to produce POEs. 14 Considerable efforts have been devoted to developing axial steric hindrance strategies in α-diimine nickel catalysts for tailoring POE architectures, including unsymmetric designs, axially flexible bulky groups, hybrid steric configurations, and rigid-flexible combined systems. These approaches have facilitated the synthesis of POEs with specialized properties, such as high-temperature resistance, enhanced resilience, and ultra-high molecular weights (Scheme 1c). 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 In comparison, reports on palladium-catalyzed POEs remain limited, mainly due to the challenge in controlling branching density – a parameter largely insensitive to polymerization conditions in α-diimine palladium systems and primarily governed by axial steric modulation. Nonetheless, the ability of palladium catalysts to copolymerize ethylene with polar monomers offers a viable route to polar-functionalized POEs. Our group has previously reported the use of palladium catalysts bearing distal tert-butyl-modified bulky diarylmethyl groups to achieve this goal. 29 Moreover, the synthesis of POEs through the polymerization of a single linear α-olefin (LAO) monomer is still underexplored. While Leone, Bertini, Guo, and co-workers have reported the nickel-catalyzed polymerization of 1-octene to form POEs, 30 , 31 , 32 our group has demonstrated one-step POE synthesis from LAOs including 1-hexene, 1-octene, and 1-decene using sterically hindered diarylmethyl palladium catalysts (Scheme 1d). 33 In the present work, we utilize a class of flexible yet sterically demanding α-diimine nickel catalysts (Fig. 1) to accomplish the direct one-step polymerization of single LAO monomers, yielding high-performance POEs with tailored properties (Scheme 1d).

Fig. 1: Molecular structures of the axially flexible α-diimine nickel catalysts investigated for LAO chain-walking polymerization.
Fig. 1:

Molecular structures of the axially flexible α-diimine nickel catalysts investigated for LAO chain-walking polymerization.

Result and discussions

The influence of temperature, monomer concentration, and polymerization time on Ni4-catalyzed 1-hexene polymerization

Four bulky and flexible cycloalkyl α-diimine nickel complexes (Fig. 1) were selected for the synthesis of polyolefin elastomers via LAO polymerization. All nickel complexes used in this study had been previously reported by our research group. 24 Initially, the axial cyclopentyl-butyl-substituted nickel complex Ni4 was employed for the polymerization of 1-hexene to investigate the effects of polymerization temperature, monomer concentration, and reaction time on the catalytic activity, molecular weight, and other properties of the resulting polyolefin elastomers. When activated with 200 equivalents of Et2AlCl, Ni4 exhibited moderate activity (TOF = 313–1107 h−1) and produced high-molecular-weight (142–244 kg/mol) polyolefins with high branching density and low melting point (Table 1). As the temperature increased, both the polymerization activity (TOF) and the molecular weight of the resulting poly(1-hexene) decreased, while the melting point and branching density remained largely unchanged. This suggests that higher temperatures promote chain transfer but have little effect on the insertion selectivity of 1-hexene or on chain walking.

Table 1:

Polymerization of 1-hexene (C6) using Ni4.a

Ent. Temp. (°C) [M] (mol/L) Time (h) Yield (g) TOFb (h−1) M n c (kg/mol) PDIc B d T m e (°C)
1 30 2 3 1.93 766 216 1.41 86 36.6
2 50 2 3 1.27 504 191 1.59 86 35.6
3 70 2 3 1.05 417 151 1.33 85 36.2
4 30 1 3 0.79 313 142 1.12 80 42.5
5 30 3 3 2.79 1107 210 1.48 84 34.6
6 30 2 0.5 0.45 1071 174 1.16 80 33.7
7 30 2 1 0.86 1024 202 1.33 82 34.6
8 30 2 2 1.45 863 244 1.40 85 33.7

  1. aConditions: 10 μmol Ni4, Et2AlCl = 200 eq., total volume toluene + 1-hexene = 10 mL. bTurnover frequency refers to the moles of substrate converted per mole of catalyst per hour. cMolecular weight was determined by GPC using polystyrene standards. d B = branches per 1000 carbon atoms, determined by 1H NMR analysis, B = 1000 × 2(I CH3)/3(I CH2+CH + I CH3). eDetermined by differential scanning calorimetry (DSC) analysis.

The influence of monomer concentration was subsequently examined. Increasing the monomer concentration led to a significant rise in polymerization activity. The molecular weight of poly(1-hexene) initially increased and then plateaued, whereas the branching density showed a slight increase, resulting in a gradual reduction in melting point as the monomer concentration increased (Table 1). Finally, data collected at different polymerization durations indicated a gradual decline in TOF over time, likely due to the decreasing monomer concentration as the reaction proceeded. The molecular weight of poly(1-hexene) increased initially and then stabilized, with branching density following a similar trend. These results suggest that a balance between chain transfer and chain propagation is established as the reaction advances.

The influence of catalyst and carbon chain length on Ni-catalyzed LAO polymerization

In this study, the effects of longer-chain LAOs, specifically 1-octene (C8) and 1-decene (C10), as well as the structure of nickel-based catalysts, were investigated as key factors in LAO polymerization. With respect to polymerization activity, the influence of LAO chain length varied depending on the catalyst used, with no consistent trend observed (Table 2, Fig. 2). For example, when Ni1 was employed as the catalyst, C8 exhibited the lowest activity, whereas with Ni2, C8 showed the highest activity (Fig. 2a). Overall, catalysts featuring a butyl backbone (Ni3 and Ni4) demonstrated higher TOF than those with an acenaphthyl backbone (Ni1 and Ni2). In contrast, variations in the axial cycloalkyl group did not lead to a clear trend in activity (Fig. 2a). We speculate that these divergent trends may arise from the interplay between the steric bulk of the axial cycloalkyl group and the electronic nature of the catalyst backbone, which could influence monomer coordination and insertion differently depending on the chain length of the LAO, though further mechanistic studies are needed to confirm this hypothesis.

Table 2:

Polymerization of 1-hexene (C6), 1-octene (C8) and 1-decene (C10) by Ni(II) complexes.a

Ent. Cat. LAO Yield (g) TOFb (h−1) M n c (kg/mol) PDIc B d T m e (°C)
1 Ni1 C6 1.44 571 306 1.67 118 −21.2
2 Ni2 C6 1.00 397 174 1.49 119 −22.0
3 Ni3 C6 1.57 623 226 1.24 98 28.1
4 Ni4 C6 1.93 766 216 1.41 86 36.6
5 Ni1 C8 1.47 438 450 1.73 104 −8.9
6 Ni2 C8 2.44 726 151 1.70 99 −6.8
7 Ni3 C8 2.60 774 322 1.23 89 43.2
8 Ni4 C8 2.45 729 257 1.56 78 44.7
9 Ni1 C10 2.49 593 683 1.61 94 8.0
10 Ni2 C10 2.75 655 191 1.55 93 1.4
11 Ni3 C10 3.45 821 435 1.35 81 54.7
12 Ni4 C10 3.53 840 290 1.54 70 54.9

  1. aConditions: 10 μmol Ni catalyst, Et2AlCl = 200 eq., total volume toluene + 1-hexene = 10 mL, 3 h, LAO = 2 mol/L. bTurnover frequency refers to the moles of substrate converted per mole of catalyst per hour. cMolecular weight was determined by GPC using polystyrene standards. d B = branches per 1000 carbon atoms, determined by 1H NMR analysis. eDetermined by DSC. Bolded numbers represent the types of catalysts and monomers, respectively.

Fig. 2: Comparison of the TOF (a), molecular weight (b), branching density and melting temperature (c) of the poly(α-olefin) generated by Ni1-Ni4.
Fig. 2:

Comparison of the TOF (a), molecular weight (b), branching density and melting temperature (c) of the poly(α-olefin) generated by Ni1-Ni4.

When considering molecular weight, the behavior of the catalysts and monomers was more consistent (Fig. 2b). In most cases, increasing the LAO chain length resulted in PLAOs with higher molecular weights. This can be attributed to the greater molecular mass of the longer-chain monomers, which contributes to higher product molecular weights at equivalent degrees of polymerization. Interestingly, the axial cycloalkyl group exerted a more regular and pronounced influence on molecular weight than the catalyst backbone. Specifically, cyclohexyl-substituted catalysts (Ni1 and Ni3) produced PLAOs with higher molecular weights than their cyclopentyl-substituted counterparts (Ni2 and Ni4), with this effect being more evident in acenaphthyl-based systems (Fig. 2b). In contrast, the influence of the backbone showed an opposing trend: the acenaphthyl-based Ni1 yielded higher molecular weight PLAO than the butyl-based Ni3, while the acenaphthyl-based Ni2 gave lower molecular weight PLAO than the butyl-based Ni4 (Fig. 2b). These results suggest that the cyclohexyl group is more effective than cyclopentyl in suppressing chain transfer, and that the backbone effect is modulated by the nature of the axial substituent.

Finally, clear trends were observed in the branching density and corresponding melting point of the resulting PLAOs (Fig. 2c). First, longer-chain LAOs led to polymers with lower branching density and higher melting points. This is mainly due to the similar regioselectivity of the catalysts combined with the formation of longer crystallizable segments when longer-chain monomers are used. 32 , 33 , 34 Notably, both the size of the axial cycloalkyl group and the structure of the catalyst backbone influenced these properties. The butyl backbone and the cyclopentyl axial group each contributed to reduced branching density and elevated melting points (Fig. 2c). These variations directly affected the mechanical properties of the resulting PLAOs. In addition, further analysis of the 13C NMR spectra revealed that the obtained poly(1-hexene) from Table 2, entry 1 primarily contains methyl, butyl, and long-chain branching structures, with methyl branches being the dominant type (Fig. 3). The branching pattern is less diverse compared to that of the corresponding polyethylene. 24

Fig. 3: 13C NMR spectrum of PLAO obtained from Ni1-catalyzed 1-hexene polymerization (Table 2, entry 1).
Fig. 3:

13C NMR spectrum of PLAO obtained from Ni1-catalyzed 1-hexene polymerization (Table 2, entry 1).

Mechanical properties of PLAOs

In terms of melting behavior, the PLAOs produced using Ni1 and Ni2 catalysts exhibited melting points below room temperature, leading to inadequate mechanical performance under ambient conditions. As a result, tensile testing was performed only on PLAOs synthesized with Ni3 and Ni4 catalysts. These materials showed high elongation at break (612–1487 %) alongside moderate tensile strength at break (5.4–18.2 MPa) (Table 3). Notably, when the same catalyst was used, increasing the carbon chain length of the LAO monomer generally resulted in higher tensile strength but lower elongation at break (Fig. 4, Table 3). This trend is likely due to the enhanced crystallinity and higher melting point imparted by longer LAO chains. Further analysis based on the chain straightening ratio (χ L ), calculated from PLAO branching density, revealed that shorter LAO chains promote higher χ L values, helping shorter-chain monomers achieve adequate crystallinity or physical crosslinking density (Table 3). Overall, for a given LAO chain length, Ni4 yielded PLAOs with relatively higher melting points, chain straightening ratios, and Young’s modulus (Table 3). This can be attributed to the smaller steric hindrance of the cyclopentyl-based Ni4 catalyst, which is known in related α-diimine Ni systems to favor higher 2,1-selectivity by reducing repulsion during monomer coordination and enabling less hindered insertion pathways. We also evaluated the strain recovery (SR) behavior of the PLAOs through hysteresis measurements (Fig. 5). The materials exhibited moderate elastic recovery (SR = 30–76 %). In most cases, for a given catalyst, longer LAO chain lengths corresponded to lower SR values (Table 3). Under identical LAO chain length conditions, Ni4 consistently provided better elastic recovery, consistent with its tendency toward higher crystallinity or melting point.

Table 3:

Mechanical properties for different PLAO samples.a

Ent. Cat. LAO M n (kg/mol) T m (°C) χ L (%)b Strain (%)c Stress (MPa)c SR (%)d
1 Ni3 C6 226 28 41 1487 5.4 49
2 Ni3 C8 322 43 29 705 10.1 55
3 Ni3 C10 435 55 19 612 18.2 30
4 Ni4 C6 216 37 48 842 15.1 76
5 Ni4 C8 257 45 38 810 14.0 62
6 Ni4 C10 290 55 30 791 17.3 45

  1. aConditions: performed at 10 mm/min by means of a universal test machine (UTM2502) at room temperature (25 °C). Molecular weight (M n ) and melting point (T m ) data are from Table 2. bMolar ratio of 1,ω-α-olefin enchainment (chain straightening), calculated using the following equation: %1,ω-enchainment (χ L ) = [(1 − (ω − 2)R)/(1 + 2R)] × 100, where R = [CH3]/[CH2]. cStrain and stress at break values. dThe strain recovery (SR) values can be calculated by SR = 100(ε a  − ε r )/ε a , where ε a is the applied strain and ε r is the strain in the cycle at zero load after the 10th cycle. Bolded numbers represent the types of catalysts and monomers, respectively.

Fig. 4: Stress-strain curves for PLAO samples produced by Ni3-Ni4 at 2M (a–f).
Fig. 4:

Stress-strain curves for PLAO samples produced by Ni3-Ni4 at 2M (a–f).

Fig. 5: Hysteresis experiment plots of PLAO samples produced by Ni3-Ni4 at 2M (a–f).
Fig. 5:

Hysteresis experiment plots of PLAO samples produced by Ni3-Ni4 at 2M (a–f).

Conclusions

In summary, this work successfully demonstrates the direct synthesis of high-performance poly(α-olefin) thermoplastic elastomers via nickel-catalyzed chain walking polymerization of single LAO monomers. By systematically varying catalyst structure and polymerization conditions, we achieved precise control over molecular weight, branching density, and thermal properties, ultimately yielding materials with excellent mechanical performance – including high strain at break (612–1487 %), tunable stress resistance (5.4–18.2 MPa), and good elastic recovery (SR up to 76 %). This efficient one-step strategy provides a promising and sustainable alternative to conventional multi-monomer approaches for producing recyclable polyolefin elastomers.

Experimental sections

General method for polymerization of LAOs

The polymerization of 1-hexene, 1-octene, or 1-decene was conducted in a Schlenk reactor using toluene as the solvent. The reactor was first flame-dried under vacuum and purged with nitrogen, repeating this cycle at least three times. Monomer (1-hexene, 1-octene, or 1-decene) and Et2AlCl (200 equiv.), along with a measured amount of toluene, were introduced into the reactor. Subsequently, a solution of the nickel catalyst (10 μmol in 1.0 mL of dichloromethane) was injected to initiate polymerization. The total solvent volume was maintained at 10 mL throughout the reaction. After 3 h, the reaction was quenched with a mixture of 12 M aqueous HCl and ethanol (95:5 v/v). The resulting polymer was collected by filtration, washed thoroughly with ethanol, and dried under vacuum at 40–50 °C for approximately 12 h.

Corresponding author: Shengyu Dai, School of Chemistry and Materials Science, Anhui Normal University, Wuhu, 241002, China, e-mail:
Article note: A collection of articles based on contributions from the 50th IUPAC World Chemistry Congress held from July 14 to 19, 2025, in Kuala Lumpur, Malaysia and organized by the Institut Kimia Malaysia (IKM).

Funding source: Natural Science Foundation of Anhui Province

Award Identifier / Grant number: 2408085MB042

  1. Research ethics: Not applicable.

  2. Informed consent: Not applicable.

  3. Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.

  4. Use of Large Language Models, AI and Machine Learning Tools: None declared.

  5. Conflict of interest: The authors state no conflict of interest.

  6. Research funding: This work has been supported by Natural Science Foundation of Anhui Province (2408085MB042).

  7. Data availability: Not applicable.

  8. Supporting information: Full experimental details for polymers.

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Supplementary Material

This article contains supplementary material (https://doi.org/10.1515/pac-2025-0627).

Received: 2025-09-25
Accepted: 2025-12-09
Published Online: 2026-01-14

© 2026 the author(s), published by De Gruyter, Berlin/Boston

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

https://doi.org/10.1515/pac-2025-0627
Schlagwörter für diesen Artikel
α-diimine nickel(II) complexes; α-olefin polymerization; controlled chain walking; IUPAC2025; thermoplastic elastomers
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