Synthesis of low-molecular weight and branched polyethylenes via ethylene polymerization using 9-(arylimino)-5,6,7,8-tetrahydrocyclohepta-pyridylnickel precatalysts

Targeting pour point depressants of low-molecular weight and branched polyethylenes, a series of 9-[2,4-bis(benzhydryl)-6-R-phenylimino]-5,6,7,8-tetrahydro-cycloheptapyridine-nickel complexes (Ni1-Ni10) were developed as efficient precatalysts. Upon activation with either EASC or MAO, all nickel complex precatalysts exhibited high activity [up to 8.12 × 106 g PE (mol of Ni)-1 h-1] with single-site behavior toward ethylene polymerization, producing low-molecular weight and unimodal polyethylenes. The resultant polyethylenes possessed high branching with predominant methyl groups and longer chains, along with either internal vinylene or vinyl end groups. The activities of these complex precatalysts were heavily rationalized on the basis of the electronic and steric influences of their 6-R-substituents, with bromides following the order of Ni5 (F) > Ni4 (Cl) > Ni1 (Me) > Ni2 (Et) > Ni3 (iPr) and chlorides following the order of Ni10 (F) > Ni9 (Cl) > Ni6 (Me) > Ni7 (Et) > Ni8 (iPr). DFT calculations revealed the crucial role of agostic interactions (-Ni⋯H-C(Ph2)) between the nickel metal and the hydrogen atom of the ortho bulky group in achieving high catalytic activity and intramolecular hydrogen bonding with the fluoride atom in producing low Mw PE wax. Moreover, the organic compounds and nickel complexes were well characterized, including representative complexes Ni3 and Ni4, via single-crystal X-ray diffraction.

Exploring fluoride effects in sterically enhanced cobalt ethylene polymerisation catalysts; a combined experimental and DFT study

The fluoro-substituted 2,6-bis(arylimino)pyridine dichlorocobalt complexes, [2-{CMeN(2,6-(Ph₂CH)₂-3,4-F₂C₆H)}-6-(CMeNAr)C₅H₃N]CoCl₂ (Ar = 2,6-Me₂C₆H₃ Co1, 2,6-Et₂C₆H₃Co2, 2,6-iPr₂C₆H₃Co3, 2,4,6-Me₃C₆H₂Co4, 2,6-Et-4-MeC₆H₂Co5), were synthesized in good yield from the corresponding unsymmetrical N,N,N'-ligands, L1-L5. Besides characterization of Co1-Co5 by FT-IR spectroscopy, ¹⁹F NMR spectroscopy and elemental analysis, the molecular structures of Co2 and Co5 were also determined highlighting the unsymmetrical nature of the terdentate ligand and the pseudo-square pyramidal geometry about the metal center. When either MAO or MMAO were employed as activators, Co1-Co5 were able to achieve a wide range of catalytic activities for ethylene polymerisation. Co5/MAO exhibited the highest activity of the study at 60 °C (7.6 × 10⁶ g PE mol^-1 (Co) h^-1) which decreased to 3.3 × 10⁶ g PE mol^-1 (Co) h^-1 at 80 °C. In addition, it was found that the polymerisation activity increased as the steric hindrance imparted by the ortho groups was enhanced (for MMAO: Co3 > Co5 > Co2 > Co1 > Co4), a finding that was supported by DFT calculations. Furthermore, it was shown that particularly high molecular weight polyethylene could be generated (up to 483.8 kg mol^-1) when using Co5/MMAO at 30 °C, while narrow dispersities (M _w/M _n range: 1.8-4.7) and high linearity (T _m > 131.4 °C) were a feature of all polymers produced. By comparison of Co3 with its non-fluorinated analogue using experimental data and DFT calculations, the substitution of fluorides at the meta- and para-positions was demonstrated to boost catalytic activity and improve thermal stability.

Thermally Stable and Highly Efficient N,N,N-Cobalt Olefin Polymerization Catalysts Affixed with N-2,4-Bis(Dibenzosuberyl)-6-Fluorophenyl Groups

The cobalt(II) chloride N,N,N-pincer complexes, [2-{(2,4-(C15H13)2-6-FC6H2)N=CMe}-6-(ArN=CMe)C5H3N]CoCl2 (Ar = 2,6-Me2C6H3) (Co1), 2,6-Et2C6H3 (Co2), 2,6-i-Pr2C6H3 (Co3), 2,4,6-Me3C6H2 (Co4), 2,6-Et2-4-MeC6H2 (Co5), and [2,6-{(2,4-(C15H13)2-6-FC6H2)N=CMe}2C5H3N]CoCl2 (Co6), each containing at least one N-2,4-bis(dibenzosuberyl)-6-fluorophenyl group, were synthesized in good yield from their corresponding unsymmetrical (L1–L5) and symmetrical bis(imino)pyridines (L6). The molecular structures of Co1 and Co2 spotlighted their distorted square pyramidal geometries (τ5 value range: 0.23–0.29) and variations in steric hindrance offered by the dissimilar N-aryl groups. On activation with either MAO or MMAO, Co1–Co6 all displayed high activities for ethylene polymerization, with levels falling in the order: Co1 > Co4 > Co5 > Co2 > Co3 > Co6. Indeed, the least sterically hindered 2,6-dimethyl Co1 in combination with MAO exhibited a very high activity of 1.15 × 107 g PE mol−1 (Co) h−1 at the operating temperature of 70 °C, which dropped by only 15% at 80 °C and 43% at 90 °C. Vinyl-terminated polyethylenes of high linearity and narrow dispersity were generated by all catalysts, with the most sterically hindered, Co3 and Co6, producing the highest molecular weight polymers [Mw range: 30.26–33.90 kg mol−1 (Co3) and 42.90–43.92 kg mol−1 (Co6)]. In comparison with structurally related cobalt catalysts, it was evident that the presence of the N-2,4-bis(dibenzosuberyl)-6-fluorophenyl groups had a limited effect on catalytic activity but a marked effect on thermal stability.

High molecular weight PE elastomers through 4,4-difluorobenzhydryl substitution in symmetrical α-diimino-nickel ethylene polymerization catalysts

The following family of N,N-diaryl-2,3-dimethyl-1,4-diazabutadienes, ArN[double bond, length as m-dash]C(Me)C(Me)[double bond, length as m-dash]NAr (Ar = 2,6-Me2-4-{CH(4-FC6H4)2}C6H2L1, 2-Me-6-Et-4-{CH(4-FC6H4)2}C6H2L2, 2,4-{CH(4-FC6H4)2}2-6-MeC6H2L3, 2,4-{CH(4-FC6H4)2}2-6-EtC6H2L4, 2,4-{CH(4-FC6H4)2}2-6-iPrC6H2L5), each incorporating para-substituted 4,4-difluorobenzhydryl groups but differing in the ortho-pairing, have been synthesized and used as precursors to their respective nickel(ii) bromide complexes, Ni1-Ni5. Compound characterization has been achieved through a combination of FT-IR, multinuclear NMR spectroscopy (1H, 13C, 19F) and elemental analysis. In addition, L1, Ni1 and Ni5 have been structurally characterized with Ni1 and Ni5 revealing similarly distorted tetrahedral geometries about nickel but with distinct differences in the steric protection offered by the ortho-substituents. All nickel complexes, under suitable activation, showed high activity for ethylene polymerization with a predilection towards forming branched high molecular weight polyethylene with narrow dispersity. Notably the most sterically bulky Ni5, under activation with either EtAlCl2, Et2AlCl or EASC, was exceptionally active (0.9-1.0 × 107 g of PE per (mol of Ni) per h) at an operating temperature of 40 °C. Furthermore, the polyethylene generated displayed molecular weights close to one million g mol-1 (M w range: 829-922 kg mol-1) with high branching densities (86-102/1000 carbons) and a selectivity for short chain branches (% Me = 94.3% (EtAlCl2), 87.2% (Et2AlCl), 87.7% (EASC)). Further analysis of the mechanical properties of the polymers produced at 40 °C and 50 °C using Ni5 highlighted the key role played by crystallinity (X c) and molecular weight (M w) on tensile strength (σ b) and elongation at break (ε b). In addition, stress-strain recovery tests reveal these high molecular weight polymers to exhibit characteristics of thermoplastic elastomers (TPEs).

Fluorinated 2,6-Bis(arylimino)pyridyliron Complexes Targeting Bimodal Dispersive Polyethylene; probing chain termination pathway via a combined experimental and DFT study

The fluorinated 2,6-bis(arylimino)pyridyl iron (II) complexes, [2-[CMeN{2,4-{(4-FC6H4)2CH}2-6-F}]-6-(CMeNAr)C5H3N]FeCl2 (Ar = 2,6-Me2C6H3 Fe1, 2,6-Et2C6H3 Fe2, 2,6-iPr2C6H3 Fe3, 2,4,6-Me3C6H2 Fe4, and 2,6-Et2-4-MeC6H2 Fe5) and [2-[CMeN{2-{(4-FC6H4)2CH}-4-{(C6H5)CHAr’}-6-F}]-6-(CMeN(2,6-iPr2C6H3))C5H3N] FeCl2 (Ar’ = 3-(4-FC6H4)2CH}2-4-NH2-5-FC6H2 Fe6), being verified with...

Bis(imino)-6,7-dihydro-5H-quinoline-cobalt complexes as highly active catalysts for the formation of vinyl-terminated PE waxes; steps towards inhibiting deactivation pathways through targeted ligand design

A set of five related bis(imino)-6,7-dihydro-5H-quinoline-cobalt(ii) complexes, [2-(ArN = CPh)-8-(NAr)-C9H8N]CoCl2 (Ar = 2,6-Me2C6H3Co1, 2,6-Et2C6H3Co2, 2,6-i-Pr2C6H3Co3, 2,4,6-Me3C6H2Co4, 2,6-Et2-4-MeC6H2Co5), have been synthesized in reasonable yield by the template reaction of cobalt(ii) chloride hexahydrate, 2-benzoyl-6,7-dihydro-5H-quinolin-8-one and the corresponding aniline. The molecular structures of Co1 and Co4 highlight both the differences in the two imino-carbon environments (phenyl-capped chain vs. cyclic) and also the steric properties exerted by the bulky N imine-aryl groups. On pre-treatment with either modified methylaluminoxane (MMAO) or methylaluminoxane (MAO), all complexes proved productive catalysts for the polymerization of ethylene. In particular, Co1/MAO was the most active reaching a very high level of 1.62 × 107 g PE per mol (Co) per h over a 30 minute run time. Owing to the presence of the imino-phenyl substituent, Co1-Co5 were able to exhibit good thermal stability by displaying appreciable catalytic activity at temperatures between 50 and 80 °C, generating polyethylenes with narrow dispersities (M w/M n range: 1.66-3.28). In particular, the least sterically bulky precatalysts, Co1 and Co4 formed polyethylene waxes (M w range: 1.94-5.69 kg per mol) with high levels of vinyl unsaturation as confirmed by high temperature 1H/13C NMR spectroscopy and by IR spectroscopy.