Randomness in Ziegler–Natta olefin copolymerizations as determined by 13C-NMR spectroscopy—the influence of chain heterogeneity

The chain heterogeneity of a polyethylene resin can be assessed by simple dissolution techniques followed by compositional analyses. This heterogeneity influences common measurements of the randomness of the polymerization process. Specifically, the values for the product of reactivity ratios and the relative monomer dispersities will suggest a less random polymerization process than is actually occurring on a molecular scale. For the system studied, compositional analysis of the parent resins demonstrates the blockiness (non-random) character commonly reported for heterogeneous Ziegler–Natta polymerizations. Compositional analyses of the soluble and insoluble fractions show at least two chain populations with significantly different ethylene/1-hexene molar ratios, each showing a near random incorporation of comonomer. These data are consistent with at least two catalyst sites, each with different comonomer incorporation efficiency.