Abstract
Ten unsymmetrical N,N'‐bis (imino) acenaphthene‐nickel (II) halide complexes, [1‐[2,6‐{(4‐MeOC6H4)2CH}2–4‐MeC6H2N]‐2‐(ArN)C2C10H6]NiX2, each appended with one N‐2,6‐bis(4,4'‐dimethoxybenzhydryl)‐4‐methylphenyl group, have been synthesized and characterized. The molecular structures of Ni1, Ni3, Ni5 and Ni6 highlight the variation in steric protection afforded by the inequivalent N‐aryl groups; a distorted tetrahedral geometry is conferred about each nickel center. On activation with diethylaluminum chloride (Et2AlCl) or methylaluminoxane (MAO), all complexes showed high activity at 30°C for the polymerization of ethylene with the least bulky bromide precatalysts (Ni1 and Ni4), generally the most productive, forming polyethylenes with narrow dispersities [Mw/Mn: < 3.4 (Et2AlCl), < 4.1 (MAO)] and various levels of branching. Significantly, this level of branching can be influenced by the type of co‐catalyst employed, with Et2AlCl having a predilection towards polymers displaying significantly higher branching contents than with MAO [Tm: 33.0–82.5°C (Et2AlCl) vs. 117.9–119.4°C (MAO)]. On the other hand, the molecular weights of the materials obtained with each co‐catalyst were high and, in some cases, entering the ultra‐high molecular weight range [Mw range: 6.8–12.2 × 105 g mol−1 (Et2AlCl), 7.2–10.9 × 105 g mol−1 (MAO)]. Furthermore, good tensile strength (εb up to 553.5%) and elastic recovery (up to 84%) have been displayed by selected more branched polymers highlighting their elastomeric properties.
Highlights
The propensity of α-diimine-nickel catalysts to promote the formation of polyethylenes with various levels of branching and molecular weight has provided the impetus behind numerous metal-mediated polymerization studies since the first report in the mid 1990’s [1]
The integration of the imine-carbon substituents of the diimine unit into a rigid five-membered ring has seen the development of bis(imino)acenaphthene-type nickel precatalysts (A, Figure 1) that have proved highly active for ethylene polymerization [5]
FT-IR and 1H NMR spectroscopy as well as elemental analysis have been used to characterize Ni(1) distances [2.034(3) Å (Ni1) – Ni10, while crystals of Ni1, Ni3, Ni5 and Ni6 have been used for single crystal X-ray diffraction studies
Summary
The propensity of α-diimine-nickel catalysts to promote the formation of polyethylenes with various levels of branching and molecular weight has provided the impetus behind numerous metal-mediated polymerization studies since the first report in the mid 1990’s [1]. A study is conducted using ten examples of D, differing in the substitution pattern of the second N-aryl group and the halide ligand, to ascertain how precatalyst structure, type/amount of co-catalyst and temperature, affect catalytic activity and in particular the branching and molecular weight of the polymer.
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