A study of the structure of poly(hexene-1) prepared by nickel(α-diimine)/MAO catalyst using high resolution NMR spectroscopy

Homopolymerization of hexene-1 with nickel(α-diimine) catalyst forms poly(hexene-1) containing different types of branches and varying number of methylene units in the backbone. Types of branches identified by NMR spectroscopy include, butyl (C4), longer than butyl (>C4) and methyl (isolated and meso and racemic head-to-head). Formation of ethyl (C2) and propyl (C3) branches were not observed. The migration of the nickel during polymerization not only causes the formation of branches other than C4, but also the runs of methylene units in the backbone due to complete 1,6-enchainment (chain running). The formation of regio-irregular methyl branches required an insertion of hexene-1 in the nickel-secondary carbon. Formation of methyl branches and (1, ω) enchainment has also been observed during the polymerization of octene-1, decene-1, tetradecene-1 using the same catalyst system. The extent formation of these subunits in the polymer depends on polymerization temperature. A plausible explanation for the formation of different branches is discussed.