Polyethylene Waxes with Short Chain Branching via Steric and Electronic Tuning of an 8-(Arylimino)-5,6,7-trihydroquinoline-nickel Catalyst

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.

Enhancing isoprene polymerization with high activity and adjustable monomer enchainment using cyclooctyl-fused iminopyridine iron precatalysts

In this study, a series of structurally rigid cyclooctyl-fused iminopyridine iron complexes, [L2FeCl][FeCl4] and [2L3Fe][Cl][3FeCl4], was synthesized via a one-pot method and investigated as precatalysts in conjunction with methylaluminoxane for isoprene (Ip) polymerization. Combined characterization through FTIR analysis, elemental analysis and single crystal XRD analysis fully verified the structure of these complexes. The most active iron complex, FeH, exhibited a trisligated nature, with its cation adopting an octahedral geometry around the metal center. In contrast, all the other iron complexes (Fe2Me, Fe2Et, Fe2iPr, Fe3Me, Fe2Et,Me) displayed bisligated configurations, with distorted trigonal bipyramidal geometry of cations. During isoprene polymerization, the extent of steric hindrance of the ligand framework exerted a significant impact on catalytic performance. The FeH precatalyst with less steric hindrance demonstrated excellent performance, producing high molecular weight polyisoprenes with conversions exceeding 99% for 4000 equiv. of monomer. Even at very low catalyst loadings, as low as 0.0025 mol% (Fe/Ip), the polymerization of isoprene could proceed smoothly with an exceptionally high activity of 4.0 × 106 gPI (molFe, h)-1. Moreover, this precatalyst exhibited good thermal stability, maintaining high activity levels (typically 105 gPI (molFe, h)-1) across a broad temperature range from -20 °C to 100 °C. Additionally, by adjusting steric substituents and the reaction temperature, the 1,4/3,4 regioselectivity could be modulated from 9/91 to 69/31 while maintaining a high stereoselectivity of cis-1,4 structures (cis/trans: >99/1).

Structural evolution of iminopyridine support for nickel/palladium catalysts in ethylene (oligo)polymerization

The interest in the late transition metal catalyst based design of new architectures of polyethylene (PE) has continuously been increasing over the last few years. The structure of these catalysts is predominantly important in controlling the morphological and architectural properties of the resulting polyethylene. Particularly, iminopyridine is a versatile bidentate support for Ni and Pd catalysts in ethylene (oligo)polymerization providing a wide variety of products ranging from volatile oligomers to ultra-high molecular weight polyethylene. Extensive structural modifications have been induced in the iminopyridine ligand through steric and electronic substitution, tuning the catalyst behavior in terms of activity and properties of the resulting polymer. Carbocyclic-fused iminopyridine and N-oxide iminopyridine are the new state of the art iminopyridine ligand designs. In this review, we aim to summarize all the developments in mononuclear iminopyridine-nickel and -palladium catalysts for ethylene (oligo)polymerization since the first report published in 1999 to present, focusing on the correlation among the pre-catalyst, co-catalyst type, thermal stability and polymer/oligomer structure. For comparison, the structural variations in the binuclear iminopyridine-nickel catalysts are also described. The detailed comparison of the structural variations in these catalysts with respect to their polymerization performance will give deep understanding in the development of new efficient catalyst designs.