Effect of Branching and Molecular Weight on Heterogeneous Catalytic Deuterium Exchange in Polyolefins

Abstract

Small-angle neutron scattering (SANS) is a powerful method for probing the structural properties of polymeric materials. Contrast between polymer chains can be obtained by labeling with deuterium, which provides an opportunity for analyzing individual chain behavior in bulk. A transition metal (Pt/Re)-catalyzed reaction in isooctane was used to exchange deuterium for hydrogen in various saturated hydrocarbon polymers, including a commercial polyethylene. We have investigated the role of two forms of molecular heterogeneity on the labeling reaction using narrow dispersity hydrogenated polybutadiene (hPBD) samples with controlled molecular weight and ethyl branch content (short chain branching). These materials were prepared by anionic polymerization, followed by catalytic hydrogenation. A monotonic increase of deuterium labeling from 65% to 84% was observed when molecular weight was increased from 4000 to 216 000. Increasing the molecular weight to 635 000, however, resulted in almost no exchange, which is...