The secondary coordination interaction that differs from ligand steric and electronic modification is an important tool to modulate the olefin polymerization process. In this contribution, by introducing polyethylene glycol units into the seminal α-diimine framework, a new family of “polyethylene glycol sandwich” α-diimine nickel and palladium catalysts was designed and fully synthesized. The nickel catalysts with varying polyethylene glycol units under the activation of an alkyl aluminum reagent drastically enhanced catalytic activities (up to 9 times) and notably were capable of tuning branching densities of the polymer in ethylene polymerization. With the increase of polyethylene glycol units, the microstructures of polyethylene varied from highly branched (106/1000C) to lightly branched (19/1000C). This corresponded to a conversion from amorphous polyethylene to semi-crystalline polyethylene (T_m = 106.7 °C). The probable “ligand–cocatalyst effect” via the chelation of the oxygen atom in polyethylene glycol units with the aluminum atom in the activator was proposed as a unique secondary coordination interaction. The obtained polyethylenes showed very narrow molecular weights (PDI = 1.01–1.59), and even living polymerization of ethylene was possible. In contrast, under the activation of borate, the palladium analogues showed no activities towards ethylene polymerization.

By utilizing an efficient strategy of the combination of the most bulky and the least bulky substituents in the ortho-N-aryl groups of α-diimine ligands, a new class of unsymmetrically monosubstituted (H/CHAr₂) α-diimine nickel complexes Ni-1–5 and palladium complexes Pd-1–5 with trans- and cis-forms was synthesized and comprehensively identified by NMR spectroscopy, single crystal X-ray diffraction, mass spectroscopy, and elemental analysis. In the presence of excess MAO, all these nickel complexes Ni-1–5 were thermally stable (−10–70 °C) and exhibited extremely high activities of 10⁶–10⁷ g mol⁻¹ h⁻¹ for ethylene polymerization. By readily varying the reaction temperature, highly linear polyethylenes (<2/1000C, T_m = 132–140 °C) with moderate molecular weights (M_n: 10⁴ g mol⁻¹) and (highly) branched ethylene oligomers (up to 73/1000C) with a M_n of 10³ g mol⁻¹ could be produced with broad bimodal molecular weight distributions, respectively. More remarkably, chain-end-functionalization of these generated oligoethylenes could be achieved in situ by the Friedel–Crafts addition to the aromatic compounds of the reaction solvent (such as toluene and anisole) and could be also tuned from 0% to 100% conversion with temperature, solvent, and catalyst. As a comparison, under the activation of NaBArF, all these palladium complexes Pd-1–5 displayed good activity (10⁴ g mol⁻¹ h⁻¹) for ethylene polymerization to yield low molecular weight branched ethylene oligomers (52–70/1000C) and were also active for the copolymerization of ethylene and methyl acrylate (MA) to generate ethylene/MA co-oligomers with an ester functional group at the end of branches (0.87–3.65 mol%).

In transition-metal catalyst structures, both the ligand structure and the initiating group are crucial components for olefin polymerization. Compared to numerous studies on tuning the electronic and steric effects of ligands, there is no report on the comprehensive investigation of initiating groups. In this contribution, five different initiating groups including “NiMe”, “NiPh”, “Ni(allyl)”, “Ni(COD)”, and “Ni(acac)/AlEt₂Cl” were designed and installed into sterically bulky phosphino-phenolate nickel complexes Ni1–Ni5, respectively, which were further tested for ethylene (co)polymerization. In ethylene polymerization, the order of activity was Ni1-PPh₃ (NiMe) > Ni2 (NiPh) ≫ Ni3 (Ni(allyl)) = Ni4 (Ni(COD)) = Ni5 (Ni(acac)) at low temperature conditions (30 °C) with Ni1 being the most active group (850 kg mol⁻¹ h⁻¹). By comparison, at high temperatures (50 °C–90 °C), the activity followed the trend of Ni2 > Ni1-PPh₃ > Ni4 ≫ Ni5 > Ni3 with Ni2 exhibiting the highest activity of 6290 kg mol⁻¹ h⁻¹. These results indicated that the choice of initiating groups was important in the polymerization reaction. In addition, Ni1-pyr and Ni2 enabled the copolymerization of ethylene with polar comonomers such as vinyl trimethoxysilane, 6-chloro-1-hexene, and ⁿbutyl allyl ether to give polar functionalized polyethylenes with incorporation of up to 1.28 mol% and high molecular weights (up to 66 kDa).

A class of sterically very bulky aliphatic/aromatic phosphine-sulfonate ligands L1–L4 and the corresponding neutral palladium complexes 1–4 {[κ²(P,O)-(menthyl)ArP(C₆H₄SO₂O)]PdMe(pyr)} (1: Ar = 2-(2′,6′-(MeO)₂C₆H₃)C₆H₄; 2: Ar = 2-(C₆H₅)C₆H₄; 3: Ar = 2-(2′,6′-F₂C₆H₃)C₆H₄; 4: Ar = (MeO)₃C₆H₂) were synthesized and comprehensively identified by NMR spectroscopy, single-crystal X-ray diffraction, and elemental analysis. All of these palladium catalysts were employed in ethylene polymerization, generating highly linear polyethylenes (methyl branches < 1/1000 C) with a high molecular weight (up to 19.4 × 10⁴ g mol⁻¹) in a high activity of up to 3.3 × 10⁶ g mol⁻¹ h⁻¹. More remarkably, in the copolymerization of ethylene and a variety of challenging polar vinyl monomers such as acrylate, acrylic acid, acrylamide, vinyl ether, vinyl acetate, and acrylonitrile, the preferred catalysts 3 and 4 again showed significantly high activity, affording highly linear functionalized polyethylenes with a clearly enhanced molecular weight. Especially for the copolymerization of acrylates with ethylene, both the catalytic activity and the copolymer molecular weight are close to the same order of magnitude for ethylene homopolymerization, which are up to 10⁶ g mol⁻¹ h⁻¹ and 10⁵ g mol⁻¹, respectively.