<i>N</i>,<i>N</i>-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 <i>N</i>,<i>N</i>-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 (<b>Ni1</b>), Ar = 2-(C₆H₁₁)-4,6-(CHPh₂)₂C₆H₂ (<b>Ni2</b>), 2-(C₈H₁₅)-4,6-(CHPh₂)₂C₆H₂ (<b>Ni3</b>) and 2-(C₁₂H₂₃)-4,6-(CHPh₂)₂C₆H₂ (<b>Ni4</b>)), disparate in the ring size of the <i>ortho</i>-cycloalkyl substituents, were prepared using a straightforward one-pot synthetic method. The molecular structures of <b>Ni2</b> and <b>Ni4</b> highlight the variation in the steric hindrance of the <i>ortho</i>-cyclohexyl and -cyclododecyl rings exerted on the nickel center, respectively. By employing EtAlCl₂, Et₂AlCl or MAO as activators, <b>Ni1</b>-<b>Ni4</b> displayed moderate to high activity as catalysts for ethylene polymerization, with levels falling in the order <b>Ni2</b> (cyclohexyl) > <b>Ni1</b> (cyclopentyl) > <b>Ni4</b> (cyclododecyl) > <b>Ni3</b> (cyclooctyl). Notably, cyclohexyl-containing <b>Ni2</b>/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 (<i>X</i>_c) and molecular weight (<i>M</i>_w) were the main factors affecting tensile strength and strain at break (<i>ε</i>_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).