Controlled Chain-Walking Polymerization in the Gas Phase to Ultrahigh Molecular Weight Polyethylene

Abstract

This study presents an unconventional approach for the gas-phase synthesis of ultrahigh molecular weight polyethylene (UHMWPE) using sterically hindered 2,6-bis­(diarylmethyl) α-diimine palladium­(II) catalysts. By leveraging a self-supported chain-walking polymerization mechanism, the catalysts form thin films on reactor walls, enabling efficient ethylene polymerization without solvents. The gas-phase chain-walking mechanism facilitates the migration of palladium species within the porous polymer matrix, enabling them to locate and access optimal sites for ethylene capture and subsequent insertion. The distal substituents on the catalysts were systematically varied to investigate their electronic and steric effects on polymerization activity, molecular weight, and branching density. Under optimized conditions (6 atm ethylene, 25 °C), the catalysts achieved high activity (up to 3.90 × 10⁵ g/(mol·h)) and produced UHMWPE with molecular weights exceeding 1600 kg/mol. Kinetic studies revealed a unique three-stage polymerization process, while branching densities (29–131 branches/1000 C) were tunable via catalyst design. The resulting polyethylene exhibited a porous network morphology and balanced mechanical properties, combining high impact resistance with processability. This work highlights the potential of gas-phase polymerization as an environmentally friendly and cost-effective route to UHMWPE with tailored microstructures.

1. Introduction

Gas-phase olefin polymerization is a process where olefin monomers, such as ethylene and propylene, undergo polymerization in their gaseous state. This method eliminates the need for solvents, simplifying equipment requirements and reducing both investment and production costs. Furthermore, it minimizes environmental impact by avoiding solvent recovery and disposal issues. − Therefore, the production of high-performance polyolefin materials through gas-phase polymerization is of great significance, both from the perspectives of environmental protection and cost savings.

Ultrahigh molecular weight polyethylene (UHMWPE) is a remarkable polymer distinguished by its exceptionally high molecular weight, typically exceeding one million g/mol. This polymer exhibits numerous superior properties, including excellent stability, wear resistance, corrosion resistance, impact resistance, biofouling resistance, and good biocompatibility, leading to its widespread application in various fields such as artificial joints, elevator guide rails, and oil pipelines. − The synthesis of UHMWPE generally involves polymerization processes, where ethylene monomers are polymerized using specific catalysts. Traditional catalysts, such as Ziegler–Natta catalysts, have been widely employed for this purpose. However, recent advancements in catalyst technology have introduced late transition metal catalysts as potential alternatives for synthesizing UHMWPE. , Due to their propensity for β-hydride elimination, late transition metal alkyl complexes often face limitations in enhancing molecular weight due to subsequent chain transfer reactions. − Consequently, the utilization of axially bulky ligands is typically crucial in most late transition metal-catalyzed olefin polymerization systems aimed at producing UHMWPE. For example, in neutral nickel salicylaldiminato systems, catalysts featuring electron-withdrawing 2,6-diaryl and 2,8-diarylnaphthyl groups efficiently catalyze ethylene polymerization to yield UHMWPE. − Similarly, in cationic nickel α-diimine systems, substituents such as 2,6-diaryl, 8-arylnaphthyl, and 2,6-diarylmethyl also effectively suppress chain transfer, resulting in the production of UHMWPE. − Recently, variations of these catalytic systems, such as the neutral β-imino-enol nickel system and cationic α-imino-ketone nickel system, have been developed for the preparation of UHMWPE through ethylene polymerization, marking significant advancements. Furthermore, these late transition metal catalysts exhibit a unique tolerance to polar groups, enabling them to catalyze ethylene polymerization in polar solvents, including unexpected aqueous phases, to produce UHMWPE. Notable examples include certain neutral, bulky nickel salicylaldiminate catalysts and nickel phosphinophenolate catalysts reported by Mecking and others, which have achieved controllable preparation of UHMWPE in polar solvents such as tetrahydrofuran, ether, and even challenging water environments. ,− Conversely, there are relatively few reports on the use of late transition metal catalysts to prepare UHMWPE in the gas phase. Not long ago, we achieved gas-phase polymerization using a series of cationic palladium catalysts with varying steric hindrance. Among them, symmetric bulky diarylmethyl-based catalysts were capable of producing high molecular weight and even ultrahigh molecular weight polyethylene. In this study, we have successfully achieved efficient production of UHMWPE in the gas phase through a self-supported chain-walking polymerization mechanism, utilizing some reported bulky cationic diarylmethyl α-diimine palladium catalysts (Scheme ). This study focuses on the influence of distal substituents of the catalyst on this gas-phase polymerization.

1. Formation of UHMWPE via a Self-Supported Strategy using Chain-Walking Catalysts in Gas-Phase Ethylene Polymerization.

2. Results and Discussion

2.1. Ethylene Gas-Phase Polymerization

Bulky symmetrical diarylmethyl α-diimine Pd­(II) catalysts featuring distal groups with varying electronic and steric effects were selectively employed for gas-phase polymerization (Chart ). With the exception of Pd6, all other catalysts have been reported previously. − Detailed synthesis and characterization of Pd6 can be found in the Supporting Information (Figures S1–S4). Our study focused on examining the influence of these distal groups on the gas-phase polymerization process, especially in terms of chain-walking control. In contrast to conventional supported catalysts for ethylene gas-phase polymerization, this study employed a class of self-supported palladium catalysts with moderate chain-walking capability, which significantly simplified both catalyst preparation and subsequent polymer purification. Specifically, the catalyst was first activated by treatment with two equivalents of sodium tetrakis­(3,5-bis­(trifluoromethyl)­phenyl)­borate (NaBArF) in 2 mL of dichloromethane, followed by spin-coating onto the inner walls of the polymerization flask to form a thin catalyst film (Figure a). Upon ethylene introduction, a network-like polymer film initially formed on the flask walls, with the catalyst gradually diffusing throughout the polymer network via the chain-walking mechanism (Figure b). Prolonged polymerization led to the growth of a porous white polymer aggregate extending into the flask space (Figure c,d).

1. Selected 2,6-Bis­(diarylmethyl) α-Diimine Pd­(II) Catalysts for Ethylene Gas-Phase Polymerization.

1.

Four distinct stages (a–d) of chain-walking gas-phase polymerization: a visual representation.

While the polymer’s adherent, porous network morphology differs from conventional flowable powders, continuous operation could be enabled by reactor adaptations such as (1) in situ detachment: mechanical scrapers or pulsed gas flow to dislodge polymer films from reactor walls; (2) hybrid immobilization: precoating catalysts on removable liners/meshes to combine self-supporting stability with easier product separation; (3) postreactor processing: mild fragmentation (e.g., grinding) to convert aggregates into handleable particles compatible with existing slurry/gas-phase handling systems. Industrial precedents (e.g., fluidized-bed reactors for sticky polymers) demonstrate the feasibility of combining mechanical and gas-phase strategies for similar challenges. Further engineering optimization is needed, but the system’s environmental and cost efficiencies underscore its potential for continuous production.

Kinetic studies revealed an initial low polymerization activity that gradually increased during the early stages (within 2 h), followed by a steady-state period after the formation of the polymer network support (2–8 h), and eventually decreased over extended reaction times (8–12 h) (Figure ). This behavior can be rationalized by considering that the catalyst initially forms clusters on the flask walls, with only surface-active species participating in polymerization (Figure a). As the reaction progresses, more catalyst sites become accessible, leading to increased activity (Figure b). When all catalytic sites are engaged, the system reaches a steady state (Figure c), followed by gradual deactivation of some catalyst species over time.

2.

Yield versus time curve for ethylene gas-phase polymerization catalyzed by Pd1.

All selected catalysts demonstrated high activity in ethylene gas-phase polymerization, producing polyethylene with varying molecular weights and branching densities (Table ). Notably, catalysts Pd1–Pd5 were capable of producing UHMWPE (M _n up to 1624 kg/mol) under relatively high ethylene pressures (Table ). Both the nature of the distal groups on the catalysts and the polymerization conditions significantly influenced the polymerization behavior. Higher ethylene pressures resulted in increased polymerization activity and higher molecular weight products, while simultaneously leading to slightly decreased branching density and correspondingly higher melting points of the resulting polyethylene (Table ). For more detailed mechanistic studies, the Pd3 catalyst system was selected to examine temperature effects on gas-phase polymerization. The data reveal an inverse relationship between temperature and catalytic performance: increasing temperature led to (1) substantially decreased polymerization activity, (2) substantially reduced molecular weights, (3) elevated branching density, and (4) slightly lower melting points (entries 25–26 vs 7, Table ). These observations suggest that chain transfer processes become increasingly favored over chain propagation at higher temperatures in gas-phase polymerization, whereas chain walking behavior remains comparatively less temperature-sensitive.

1. Effects of Ligand Structure and Polymerization Conditions on Ethylene Gas-Phase Polymerization .

ent.	cat.	P (atm)	yield (g)	act.	M n (× 10–4)	PDI	B	T m (°C)
1	Pd1	6	23.42	3.90	119.78	1.92	34	76
2	Pd1	3	13.34	2.22	109.74	1.58	40	73
3	Pd1	1	4.93	0.82	62.40	1.59	40	67
4	Pd2	6	15.29	2.55	162.40	1.59	29	83
5	Pd2	3	9.67	1.61	126.61	1.67	32	79
6	Pd2	1	3.29	0.55	59.55	1.58	34	76
7	Pd3	6	20.21	3.37	143.83	1.75	36	75
8	Pd3	3	12.03	2.01	108.24	1.86	42	70
9	Pd3	1	4.24	0.71	52.31	1.95	42	65
10	Pd4	6	13.22	2.20	133.96	1.78	31	77
11	Pd4	3	7.72	1.29	114.99	1.44	36	72
12	Pd4	1	2.78	0.46	44.32	2.01	39	65
13	Pd5	6	19.75	3.29	117.81	1.73	34	78
14	Pd5	3	11.71	1.95	108.78	2.05	43	71
15	Pd5	1	3.12	0.52	61.03	1.88	47	66
16	Pd6	6	13.09	2.18	95.15	1.57	35	73
17	Pd6	3	6.08	1.01	96.13	1.72	39	70
18	Pd6	1	1.16	0.19	23.32	2.21	42	72
19	Pd7	6	4.61	0.77	49.45	1.90	76
20	Pd7	3	3.83	0.64	38.91	1.69	103
21	Pd7	1	1.34	0.22	20.34	1.75	104
22	Pd8	6	6.91	1.15	55.87	2.03	79
23	Pd8	3	3.03	0.51	39.21	1.95	129
24	Pd8	1	1.13	0.19	22.36	1.87	131
25	Pd3	6	13.86	2.31	78.11	2.01	38	75
26	Pd3	6	5.34	0.89	54.32	2.31	52	74
27	Pd-iPr	6	6.79	1.13	43.01	2.46	92

^aConditions: 5 μmol precatalyst, 2.0 equiv of NaBArF, 12 h, 25 °C.

^bActivity (Act.) = 10⁵ g/(mol·h).

^cMolecular weight was determined by gel permeation chromatography (GPC).

^d B = branches per 1000 carbons, branching numbers were determined using ¹H NMR spectroscopy.

^eMelting temperature determined by differential scanning calorimetry (DSC).

^fNo distinct melting point was observed within the tested temperature range.

^gTemperature = 50 °C.

^hTemperature = 70 °C.

Among all the screened catalysts, Pd1, featuring eight distal methoxy groups at the axial positions, exhibited the highest polymerization activity (entry 1, Table ), while Pd2, containing eight distal methyl groups at the axial positions, produced polyethylene with the highest molecular weight (entry 4, Table ). Notably, the catalysts Pd7 and Pd8, which bear eight distal sterically hindered tert-butyl groups, demonstrated significantly lower polymerization activity (entries 1–18 vs 19–24, Table ). The polyethylene generated by these catalysts had a notably lower molecular weight compared to that produced by other catalysts, but exhibited a significantly higher branching density (entries 1–18 vs 19–24, Table ). The observed phenomena may be attributed to the presence of the axial tert-butyl groups, which likely hindered the diffusion of ethylene molecules to the catalytic center, thereby significantly reducing chain propagation. This resulted in a corresponding decrease in polymerization activity and an increase in chain walking relative to chain growth, leading to an increased branching density. Additionally, the branching density of polyethylene produced via gas-phase polymerization was higher than that of the products obtained in solution under nearly identical polymerization conditions, particularly for the tert-butyl catalysts Pd7 and Pd8 (18–27 vs 29–36/1000 C for Pd1–Pd4; 59–64 vs 76–79/1000 C for Pd7–Pd8). , This suggests that these catalysts expended more effort in the chain walking process during gas-phase polymerization, possibly due to the need to find more suitable positions to contact and capture ethylene molecules. Overall, from the perspective of electronic effects, both axial and aniline-donating substituents (e.g., OMe) contribute to enhanced polymerization activity (SI, Figure S5). In contrast, steric effects imposed by bulky axial substituents (e.g., tBu) hinder ethylene coordination, resulting in a substantial reduction in polymerization activity (SI, Figure S5).

Compared to the previously reported Pd-CHPh ₂ catalyst (ref , Chart a), our systematically modified distal-group catalysts exhibit superior control over chain-walking dynamics and ethylene insertion kinetics. This precise modulation results in enhanced catalytic activity (3.37 vs 2.18 × 10⁵ g/(mol·h)), while maintaining comparable molecular weights and enabling tunable branching densities (29–131 vs 38 branches/1000 C) under comparable polymerization conditions. When benchmarked against conventional Brookhart-type Pd-iPr catalyst (Chart b), our optimized catalyst system demonstrates significantly higher polymerization activity, higher molecular weight, and a markedly lower branching density (entry 4 vs 27, Table ). The excessive branching characteristic of Pd-iPr-derived polyethylene leads to the formation of a hyperbranched polymer network that lacks structural support during polymerization. As evidenced by Figure , this results in catalyst encapsulation within a viscous polymer matrix, which severely restricts ethylene diffusion to active sites and ultimately terminates polymerization. These findings corroborate our previous report that successful gas-phase ethylene polymerization requires careful balancing of chain-walking behavior to produce polyethylene with optimal branching density (20–50 branches/1000 C). Only within this range can the polymer network maintain sufficient structural support to prevent catalyst encapsulation while ensuring adequate ethylene permeability.

2. Schematic Structures of the Two Reference Catalysts (a and b).

3.

Morphological comparison of polyethylene produced by Pd3 (a) vs Brookhart’s Pd-iPr catalyst (b) in gas-phase polymerization.

Regarding the structural differences between our chain-walking-derived UHMWPE (29–43 branches/1000 C; Table ) and conventional Ziegler–Natta (Z–N) UHMWPE (<5 branches/1000 C), while Z–N UHMWPE achieves high crystallinity, tensile strength, and wear resistanceproperties critical for applications like artificial joints and high-performance fibersour moderately branched architecture introduces distinct advantages. The controlled branching density creates amorphous domains that (i) enhance impact resistance through energy dissipation, (ii) improve processability via reduced melt viscosity compared to linear UHMWPE, and (iii) enable functionalization at branch sites for compatibility with polar modifiers or fillers. These characteristics position our material for emerging applications where traditional linear UHMWPE faces limitations, including: (1) protective coatings requiring balanced toughness and corrosion resistance (e.g., pipeline liners), (2) polymer blends with elastomers for vibration-damping materials, and (3) functionalized UHMWPE grafts exploiting branch-site reactivity. Although medical-grade applications may still require linear architectures, our system provides a structurally tunable, scalable route to UHMWPE variants addressing unmet needs in specialty markets. The aforementioned structural differences also lead to a significantly lower melting point in our synthesized UHMWPE compared to commercially available UHMWPE products.

The high-temperature ¹³C NMR analysis of the obtained polyethylene (Table , entry 21) revealed that its branching structure primarily consists of methyl and long-chain branches (LCBs), with a minor proportion of ethyl branches (Figure ). This branching profile differs distinctly from that of commercially available copolymerized branched polyethylenes with single-type branching (e.g., LLDPE, POE). Compared to commercial linear UHMWPE, our branched UHMWPE exhibited lower tensile strength at break but higher strain at break, with no observable yield pointcharacteristics resembling those of commercial POE materials (SI, Figure S6). Further elastic recovery tests demonstrated its moderate elastic recovery capability, showing an SR value of 43%. Currently, industrial POE materials are predominantly synthesized via metallocene-catalyzed high-temperature solution copolymerization of ethylene and α-olefins, with no successful reports of their gas-phase polymerization to date. Therefore, the gas-phase polymerization of POE-like materialsespecially high-molecular-weight POEsdemonstrated in this work represents a significant breakthrough.

4.

Detailed analysis of the ¹³C NMR spectrum of polyethylene obtained by using Pd7 at 1 atm (Table , entry 21).

3. Conclusions

This work demonstrates the successful gas-phase synthesis of UHMWPE using sterically hindered 2,6-bis­(diarylmethyl) α-diimine palladium­(II) catalysts through a self-supported chain-walking mechanism. Systematic variation of distal substituents revealed that electron-donating groups (e.g., OMe) enhanced catalytic activity, while bulky groups (e.g., tBu) increased branching density. Optimal conditions (6 atm ethylene, 25 °C) yielded UHMWPE with molecular weights up to 1624 kg/mol and tunable branching densities (29–131 branches/1000 C). The resulting polyethylene exhibited a porous network morphology and balanced mechanical properties, combining high impact resistance with improved processability compared to linear UHMWPE. This solvent-free approach not only simplifies production and reduces environmental impact but also provides a scalable route to structurally tailored polyolefins, offering potential for applications in specialty materials such as impact-resistant coatings and elastomeric polymers. The study highlights the importance of catalyst design in controlling polymerization kinetics and polymer architecture, advancing the development of sustainable gas-phase polyolefin technologies.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/prechem.5c00067.

• Full experimental details for the synthetic procedures; materials; analytical methods; and NMR, DSC, and GPC curves of polymer samples (PDF)

The authors declare no competing financial interest.