Bimodal and Highly Branched Polyethylenes Using Dual-Site Nickel Catalysts Supported on an Unsymmetrical BIAN-PI Compartmental Ligand Scaffold

A straightforward two-step synthesis of a new family of electronically distinct para-biphenylene-linked 1,2-bis(imino)acenaphthene/pyridyl-2-imine (BIAN-PI) ligands, 2′-CMeN{1-[2,6-(Ph2CH)2-4-R-C6H2N]-2-[3,3′,5,5′-iPr4-1,1′-(C6H2)2-4-N]C2C10H6}C5H4N (R = Me L1′, iPr L2′, tBu L3′, OMe L4′, OCF3 L5′, Cl L6′, F L7′, NO2 L8′), is disclosed. Their reaction with two equivalents of NiBr2(DME) results in occupation of the two inequivalent N,N′ pockets within the BIAN-PI framework to afford the binuclear nickel(II) bromide complexes, [2′-CMeN{1-[2,6-(Ph2CH)2-4-R-C6H2N]-2-[3,3′,5,5′-iPr4-1,1′-(C6H2)2-4-N]C2C10H6}C5H4N]Ni2Br4 (R = Me Ni21′, iPr Ni22′, tBu Ni23′, OMe Ni24′, OCF3 Ni25′, Cl Ni26′, F Ni27′, NO2 Ni28′). The molecular structures of their aqua adducts, Ni21′(OH2)–Ni28′(OH2), emphasize not only the dissimilar coordination environments but also the variation in steric protection imparted to the two remotely positioned metal centers (Ni···Ni separation: 12.515–12.718 Å). In the presence of AlMe3 or Me2AlCl, these bimetallic nickel catalysts exhibited moderate to high activities for ethylene polymerization (up to 8.5 × 106 g (PE) mol–1 (Ni) h–1 for Ni23′/Me2AlCl at 30 °C) affording polyethylenes with distinctly bimodal distributions that contain both a high molecular weight component and a much lower molecular weight one, with the relative ratios influenced by the run temperature. In all cases, the polyethylenes were highly branched with the branching density strongly affected by both catalyst structure and the type of aluminum-methyl activator employed. Moreover, these bimodal polyethylenes showed superior tensile strength and comparable elastic recovery when compared with the unimodal material produced using a mononuclear comparator, [1-[2,6-(Ph2CH)2-4-NO2–C6H2N]-2-[4′-NH2-3,3′,5,5′-iPr4-1,1′-(C6H2)2-4-N]C2C10H6]NiBr2 (Ni8). Indeed, both types of materials can be considered as promising candidates for use as thermoplastic elastomers. In addition to Ni8, three other mononuclear nickel complexes (Ni9–Ni11) that mimic the distinct coordination environments within Ni28′ have also been prepared and studied as binary catalysts.