Fluorine-Rich Cobalt Catalysts Keep Making Polyethylene Even at 100 °C

Polyethylene is the most widely produced synthetic polymer in the world, and the catalysts that make it are locked in a constant struggle with heat. Most commercial ethylene polymerization processes, including solution and high-temperature gas-phase operations, run above 100 °C to improve monomer solubility, mass transfer, and reactor productivity. Yet bis(imino)pyridylcobalt precatalysts, a workhorse family for making strictly linear polyethylene, tend to lose activity sharply as temperatures climb past 90 °C. A new study published in Discover Catalysis reports a ligand design that confronts this weakness directly: cobalt complexes decorated with three fluorine atoms and bulky benzhydryl groups on one N-bound phenyl ring remained highly active across a 30–100 °C window, reaching activities on the order of 10⁷ g mol⁻¹ h⁻¹ at 70 °C and still delivering 2.22 × 10⁶ g mol⁻¹ h⁻¹ at 100 °C.

The research team, led by Geng Ren, Qaiser Mahmood, Yizhou Wang, Yanping Ma, Tongling Liang, and Wen-Hua Sun, synthesized a series of unsymmetrical 2-[1-(2,6-dibenzhydryl-3,4,5-trifluorophenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridyl-cobalt(II) chloride complexes, along with one symmetrical analogue. The variable N-aryl unit carried different ortho- and para-alkyl substituents: 2,6-dimethylphenyl (Co1Me), 2,6-diethylphenyl (Co2Et), 2,6-diisopropylphenyl (Co3iPr), 2,4,6-trimethylphenyl (Co4Me), and 2,6-diethyl-4-methylphenyl (Co5Et), with the symmetrical Co6Ph bearing benzhydryl groups on both aryl rings. All complexes were obtained in high yields of at least 80% by reacting the corresponding ligands with cobalt(II) chloride in ethanol and dichloromethane under nitrogen.

Structural confirmation came from a battery of techniques. Ligands were characterized by FT-IR, proton, carbon-13, and fluorine-19 NMR spectroscopy, and elemental analysis, while the cobalt complexes were analyzed by FT-IR and elemental analysis. Single-crystal X-ray diffraction of Co2Et and Co5Et revealed mononuclear, five-coordinate cobalt centers best described as distorted square pyramids, with two imine nitrogens, one pyridyl nitrogen, and two chlorides around the metal. The imine C=N stretching vibrations, seen at 1637–1643 cm⁻¹ in the free ligands, shifted down to 1608–1610 cm⁻¹ upon coordination, reflecting electron donation from the imine nitrogen lone pair to cobalt. Notably, the plane of the bulky trifluorinated, dibenzhydryl-substituted N-aryl group sits nearly perpendicular to the ligand backbone, at 85.2° in Co2Et and 87.1° in Co5Et.

The fluorine atoms appear to do more than tune electronics. Comparative analysis of key bond angles showed that introducing meta- and para-fluorine substituents together on the N-aryl ring contracts the benzhydryl groups toward the metal center relative to previously reported cobalt complexes bearing one or two fluorines. Calculations of the ligand buried volume around cobalt confirmed the enhanced steric crowding. The authors propose that intramolecular repulsion between the substituents on the same phenyl ring pushes the benzhydryl groups closer to the metal, tightening its steric shield, an effect they had earlier observed in diimine–nickel catalysts for ethylene polymerization.

Polymerization testing began with Co2Et and methylaluminoxane (MAO) as cocatalyst, systematically varying temperature, the aluminum-to-cobalt ratio, ethylene pressure, and reaction time. Raising the temperature from 30 to 70 °C at 1 MPa ethylene and Al/Co = 2000 lifted the activity from 3.85 × 10⁶ to 15.43 × 10⁶ g mol⁻¹ h⁻¹, roughly a 300% increase. Above 70 °C the activity declined, attributed to decomposition of the active species and lower ethylene solubility, falling to 2.22 × 10⁶ g mol⁻¹ h⁻¹ at 100 °C, about seven times the 70 °C value lower. Crucially, activity stayed at the 10⁶ level across the entire 30–100 °C range, the hallmark of high thermal stability.

Molecular weights moved in the opposite direction with temperature. As chain transfer increasingly outpaces chain propagation at higher temperatures, the polyethylene molecular weight fell from 566.0 kg mol⁻¹ at 30 °C to 44.9 kg mol⁻¹ at 100 °C, while melting points dropped from 142.4 °C to 133.9 °C. Distributions stayed unimodal and moderate throughout. The optimum Al/Co ratio with MAO proved to be 1500, where activity peaked at 17.33 × 10⁶ g mol⁻¹ h⁻¹; lower ratios left precatalyst incompletely activated, while higher ratios promoted chain transfer to aluminum, cutting molecular weights from 402.3 to 114.5 kg mol⁻¹ across the tested range. Ethylene pressure mattered too: increasing it from 0.1 to 1 MPa improved activity nearly seven-fold, from 2.58 × 10⁶ to 17.33 × 10⁶ g mol⁻¹ h⁻¹, and raised molecular weight from 150.4 to 267.7 kg mol⁻¹, a correlation the authors read as first-order kinetics in monomer.

Time-course experiments revealed a short induction period. At 70 °C and Al/Co = 1500, the highest activity of 37.1 × 10⁶ g mol⁻¹ h⁻¹ appeared at just 5 minutes, after which activity gradually declined, likely from polymer accumulation hindering monomer access and partial catalyst decomposition. Even at 60 minutes the system still delivered 10.73 × 10⁶ g mol⁻¹ h⁻¹, while polymer yield and molecular weight climbed steadily with time, from 6.18 to 21.45 g and from 114.5 to 364.6 kg mol⁻¹ respectively, evidence that the active species survive prolonged operation.

Screening all six complexes under optimized conditions exposed clear structure–activity relationships. Less hindered ortho-substituents favored activity: Co1Me reached 18.28 × 10⁶ g mol⁻¹ h⁻¹ with MAO, about 1.1 times that of Co2Et and 2.2 times that of the crowded Co3iPr. Electron-donating para-methyl groups depressed activity, presumably by lowering the Lewis acidity of the metal and slowing monomer coordination and insertion. The reverse held for molecular weight: bulkier ortho-groups suppress chain transfer, so Co3iPr produced the heaviest polyethylene, with Co2Et giving 267.7 kg mol⁻¹ versus 209.4 kg mol⁻¹ for Co1Me. The symmetrical Co6Ph, choked by six benzhydryl-fluorine substituents, produced only a small amount of polymer with MAO and trace amounts with MMAO. The overall activity order ran Co1Me ~ Co2Et > Co4Me > Co5Et > Co3iPr >> Co6Ph.

Switching the cocatalyst to modified MAO (MMAO) preserved the same qualitative trends but at a cost. With Co2Et/MMAO, the best activity was 7.88 × 10⁶ g mol⁻¹ h⁻¹ at 70 °C, peaking at 11.70 × 10⁶ g mol⁻¹ h⁻¹ at an Al/Co ratio of 1250, and the system still managed 1.1 × 10⁶ g mol⁻¹ h⁻¹ at 100 °C. Across all conditions, MMAO delivered about 67% lower activity and 52% lower molecular weight than MAO, which the authors attribute to MAO’s more efficient generation and stabilization of active cationic species. Under MMAO, Co1Me again led the unsymmetrical series at 12.58 × 10⁶ g mol⁻¹ h⁻¹, and Co3iPr again topped the molecular-weight ranking.

The polyethylene itself proved strictly linear. Samples made at temperatures from 70 to 100 °C and with several complexes showed high melting points between 131.8 and 142.4 °C with sharp endothermic peaks, and proton and carbon-13 NMR spectra displayed only the singlet resonances characteristic of -(CH2)n- repeat units, with no signals from unsaturated chain ends. The absence of such end groups suggests that chain transfer proceeds predominantly through interactions with aluminum species under the applied conditions. Compared with previously reported cobalt precatalysts bearing one or two fluorines, or electron-withdrawing para-groups such as chlorine, nitro, or trifluoromethoxy, the new trifluoro-benzhydryl design pushed activities higher at elevated temperatures while keeping dispersities moderate at 2.1–2.8. The authors frame the strategy of concerted steric and electronic tuning as a template worth exporting to other catalytic systems, though the results remain laboratory-scale polymerization data rather than demonstrated industrial performance.

Subject of Research: Thermally stable multi-fluoro bis(imino)pyridylcobalt precatalysts for high-temperature ethylene polymerization

Article Title: Multi-fluoro incorporated thermally stable bis(imino)pyridylcobalt precatalysts for high-temperature ethylene polymerization

Article References: Ren, G., Mahmood, Q., Wang, Y., Ma, Y., Liang, T., & Sun, W.-H. (2026). Multi-fluoro incorporated thermally stable bis(imino)pyridylcobalt precatalysts for high-temperature ethylene polymerization. Discover Catalysis, 3(1), Article 6. https://doi.org/10.1007/s44344-026-00035-w

Image Credits: AI Generated

DOI: 10.1007/s44344-026-00035-w

Keywords: ethylene polymerization, cobalt catalysts, bis(imino)pyridine, polyethylene, thermal stability, MAO, MMAO, ligand design, fluorine substituents, molecular weight, organometallic chemistry, catalysis