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

Abstract

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.