Synthesis of branched polyethylene using (α-diimine)nickel(II) catalysts : influence of temperature, ethylene pressure, and ligand structure on polymer properties

TLDR: In this paper, the effects of structural variations of the diimine ligand on catalyst activities, polymer molecular weights, and polymer microstructure are described. And the results show that as the number and number of ortho substituents increase, the polymer molecular weight, turnover frequency and extent of branching in homopolyethylenes all increase.

Abstract: Detailed investigations of the polymerization of ethylene by (α-diimine)nickel(II) catalysts are reported. Effects of structural variations of the diimine ligand on catalyst activities, polymer molecular weights, and polymer microstructure are described. The precatalysts employed were [{(2,6-RR‘C6H3)−NC(Nap)−C(Nap)N−2,6-RR‘C6H3)}NiBr2] (Nap = 1,8-naphthdiyl) (4a, R = CF3, R‘ = H; 4b, R = CF3, R‘ = CH3; 4c, R = C6F5, R‘ = H, 4c, R = C6F5, R‘ = H; 4d, R = C6F5, R‘ = CH3; 4e, R = CH3, R‘ = H, 4f, R = R‘ = CH3; 4g, R = R‘ = CH(CH3)2), [{(2,6-C6H3(i-Pr)2)−NC(CH2CH2CH2CH2)CN−(2,6-C6H3(i-Pr)2)}NiBr2] (5), and [{(2,6-C6H3(i-Pr)2)−NC(Et)C(Me)N−(2,6-C6H3(i-Pr)2)}NiBr2] (6). Active polymerization catalysts were formed in situ by combination of 4−6 with modified methylaluminoxane. In general, as the bulk and number of ortho substituents increase, polymer molecular weights, turnover frequencies and extent of branching in the homopolyethylenes all increase. Effects of varying ethylene pressure and temperature on polyme...