Ring size enlargement in an ortho-cycloalkyl-substituted bis(imino)pyridine-cobalt ethylene polymerization catalyst and its impact on performance and polymer properties

Four unsymmetrical examples of bis(arylimino)pyridines that each possess an ortho-cycloalkyl substituent of unique ring size, 2-{CMeN(2,4,6-Me3C6H2)}-6-{CMeN(2,4-(CHPh2)2-6-RC6H2)}C5H3N (R = cyclopentyl L1, cyclohexyl L2, cyclooctyl L3, cyclododecyl L4), have been prepared and employed as reactants in the formation of the corresponding cobaltous chloride complexes, (N,N,N′)CoCl2 (Co1–Co4). Structural characterization of Co2 and Co3 spotlights the five-coordinate geometry and the steric disparity imposed on the metal center by the inequivalent N-mesityl and N-2,4-dibenzyhydryl-6-cycloalkylphenyl groups. Co1–Co4 all proved productive precatalysts for ethylene polymerization achieving optimal performance at a temperature of 60°C on activation with either MAO or MMAO. Cyclopentyl-containing Co1 displayed the highest level (up to 10.3 × 106 g PE per mol [Co] per h), whereas its cyclododecyl counterpart Co4 the lowest (down to 0.14 × 106 g PE per mol [Co] per h). Strictly linear polyethylenes were produced with molecular weights spanning the range 22.0–36.0 kg per mol for Co1–Co4/MAO and 22.6–33.6 kg per mol for Co1–Co4/MMAO, with cyclohexyl Co2 producing the highest values with either activator. With the exception of the polymer produced using Co4, all catalysts afforded narrow unimodal distributions (Mw/Mn range: 1.7–2.2) for the polyethylenes in line with single site active species. By contrast, Co4 bearing the largest and conformationally flexible cycloalkyl group formed polyethylenes that were distinctly bimodal, which would imply the presence of two active species. End-group analysis of lower molecular weight polymer samples identified vinyl groups congruent with a termination step involving β-H elimination.