Abstract
Imino- and amino-pyridine ligands are versatile scaffolds to probe how ligand sterics influence polymerization outcomes for nickel(II) ethylene polymerization catalysts. Six imino- and one amino-pyridine nickel(II) complexes with varied imino/amino-carbon substitution (R1 = methyl, ethyl, isopropyl or tert-butyl) were synthesized (1-7). Five of these complexes were analyzed crystallographically: dimeric {[2-[(2,6-Me2-C6H3)N=C(R1)]C5H4N]NiBr2}2 (R1 = Et 2; R1 = t-Bu 4), monomeric [2-[(2,6-Me2-C6H3)N=C(R1)]C5H4N]NiBr2NCCH3 (R1 = Me 5; R1 = t-Bu 6), and monomeric [2-[(2,6-Me2-C6H3)NHCH(t-Bu)]C5H4N]NiBr2 (7). Complexes 1-7 are competent ethylene polymerization precatalysts at 0°C, and tert-butyl substituted imino-pyridine precatalysts 4 and 6 and amino-pyridine precatalyst 7 display the best activity, highest polyethylene molecular weight, and narrowest dispersity values of series. 13C-NMR analyses of polyethylene samples reveal a lightly branched morphology with a modest amount of methyl branching; polymer Tm values are consistent with this assessment. Ongoing work includes further exploration of precatalysts 4, 6, and 7 in attempt to maximize polyethylene molecular weight and achieve living polymerization.
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