N,N-Bis(2,4-Dibenzhydryl-6-cycloalkylphenyl)butane-2,3-diimine-Nickel Complexes as Tunable and Effective Catalysts for High-Molecular-Weight PE Elastomers

Four examples of N,N-bis(aryl)butane-2,3-diimine-nickel(II) bromide complexes, [ArN=C(Me)-C(Me)=NAr]NiBr₂ (where Ar = 2-(C₅H₉)-4,6-(CHPh₂)₂C₆H₂ (Ni1), Ar = 2-(C₆H₁₁)-4,6-(CHPh₂)₂C₆H₂ (Ni2), 2-(C₈H₁₅)-4,6-(CHPh₂)₂C₆H₂ (Ni3) and 2-(C₁₂H₂₃)-4,6-(CHPh₂)₂C₆H₂ (Ni4)), disparate in the ring size of the ortho-cycloalkyl substituents, were prepared using a straightforward one-pot synthetic method. The molecular structures of Ni2 and Ni4 highlight the variation in the steric hindrance of the ortho-cyclohexyl and -cyclododecyl rings exerted on the nickel center, respectively. By employing EtAlCl₂, Et₂AlCl or MAO as activators, Ni1-Ni4 displayed moderate to high activity as catalysts for ethylene polymerization, with levels falling in the order Ni2 (cyclohexyl) > Ni1 (cyclopentyl) > Ni4 (cyclododecyl) > Ni3 (cyclooctyl). Notably, cyclohexyl-containing Ni2/MAO reached a peak level of 13.2 × 10⁶ g(PE) of (mol of Ni)^-1 h^-1 at 40 °C, yielding high-molecular-weight (ca. 1 million g mol^-1) and highly branched polyethylene elastomers with generally narrow dispersity. The analysis of polyethylenes with ¹³C NMR spectroscopy revealed branching density between 73 and 104 per 1000 carbon atoms, with the run temperature and the nature of the aluminum activator being influential; selectivity for short-chain methyl branches (81.8% (EtAlCl₂); 81.1% (Et₂AlCl); 82.9% (MAO)) was a notable feature. The mechanical properties of these polyethylene samples measured at either 30 °C or 60 °C were also evaluated and confirmed that crystallinity (X_c) and molecular weight (M_w) were the main factors affecting tensile strength and strain at break (ε_b = 353-861%). In addition, the stress-strain recovery tests indicated that these polyethylenes possessed good elastic recovery (47.4-71.2%), properties that align with thermoplastic elastomers (TPEs).

Unsymmetrical Strategy on α-Diimine Nickel and Palladium Mediated Ethylene (Co)Polymerizations

Among various catalyst design strategies used in the α-diimine nickel(II) and palladium(II) catalyst systems, the unsymmetrical strategy is an effective and widely utilized method. In this contribution, unsymmetrical nickel and palladium α-diimine catalysts (Ipty/ⁱPr-Ni and Ipty/ⁱPr-Pd) derived from the dibenzobarrelene backbone were constructed via the combination of pentiptycenyl and diisopropylphenyl substituents, and investigated toward ethylene (co)polymerization. Both of these catalysts were capable of polymerizing ethylene in a broad temperature range of 0-120 °C, in which Ipty/ⁱPr-Ni could maintain activity in the level of 10⁶ g mol^-1 h^-1 even at 120 °C. The branching densities of polyethylenes generated by both nickel and palladium catalysts could be modulated by the reaction temperature. Compared with symmetrical Ipty-Ni and ⁱPr-Ni, Ipty/ⁱPr-Ni exhibited the highest activity, the highest polymer molecular weight, and the lowest branching density. In addition, Ipty/ⁱPr-Pd could produce copolymers of ethylene and methyl acrylate, with the polar monomer incorporating both on the main chain and the terminal of branches. Remarkably, the ratio of the in-chain and end-chain polar monomer incorporations could be modulated by varying the temperature.

Electronic Tuning of Sterically Encumbered 2-(Arylimino)Pyridine-Nickel Ethylene Polymerization Catalysts by Para-Group Modification

A collection of five related 2-(arylimino)pyridines, 2-{(2,6-(CH(C6H4-p-F)2)2-4- RC6H2)N=CMe}C5H4N, each ortho-substituted with 4,4′-difluorobenzhydryl groups but distinct in the electronic properties of the para-R substituent (R = Me L1, Et L2, i-Pr L3, F L4, OCF3 L5), were prepared and combined with (DME)NiBr2 to form their corresponding LNiBr2 complexes, Ni1–Ni5, in high yields. All the complexes were characterized by FT-IR, 19F NMR spectroscopy and elemental analysis, while Ni5 was additionally the subject of an X-ray determination, revealing a bromide-bridged dimer. The molecular structure of bis-ligated (L4)2NiBr2 (Ni4’) was also determined, the result of ligand reorganization having occurred during attempted crystallization of Ni4. On activation with either EtAlCl2 or MMAO, Ni1–Ni5 exhibited high catalytic activities (up to 4.28 × 106 g of PE (mol of Ni)−1 h−1 using EtAlCl2) and produced highly branched polyethylene exhibiting low molecular weight (Mw range: 2.50–6.18 kg·mol−1) and narrow dispersity (Mw/Mn range: 2.21–2.90). Notably, it was found that the type of para-R group impacted on catalytic performance with Ni5 > Ni4 > Ni3 > Ni1 > Ni2 for both co-catalysts, underlining the positive influence of electron withdrawing substituents. Analysis of the structural composition of the polyethylene by 1H and 13C NMR spectroscopy revealed the existence of vinyl-end groups (–CH=CH2) and high levels of internal unsaturation (–CH=CH–) (ratio of vinylene to vinyl, range: 3.1:1–10.3:1) along with various types of branch (Me, Et, Pr, Bu, 1,4-paired Me, 1,6-paired Me and LCBs). Furthermore, reaction temperature was shown to greatly affect the end group type, branching density, molecular weight and in turn the melting points of the resulting polyethylenes.