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

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 216â€000. Increasing the molecular weight to 635â€000, however, resulted in almost no exchange, which is possibly due to the existence of a lower critical solution temperature (LCST) in isooctane. A similar trend with molecular weight was found for an isotope-labeled commercial linear low-density polyethylene material with 2.5% butyl branches and molecular weight ranging between 1000 and 1â€000â€000. Variation of ethyl branches from 2 to 50 ethyl branches per 100 backbone carbons in hPBDs reduced the level of exchange from 78% to 34%, with deuterons preferentially entering the pendant methyl groups at higher levels of branching. The materials generated from this isotope exchange reaction proved to be viable materials for SANS, providing consistent single chain statistics through proper analysis strategies, which take into account the inhomogeneous distribution of deuterium along and among individual chains caused by partial labeling and the molecular weight dependence of exchange. These results suggest that for a given chain, isotope exchange occurs on the metal catalyst surface during relatively few adsorption steps.