Mechanism and Application of a Heterogeneous Catalytic Hydrogen-deuterium Exchange Reaction for Polyolefins

Abstract

The mechanism of a heterogeneous catalytic H/D exchange reaction with polyolefins is investigated in this thesis. The model polymers used in this study were hydrogenated polybutadienes (hPBDs), and a metallocene linear low density polyethylene (LLDPE). When mixed at 170 ºC with isooctane, Pt/Re-SiO2 catalyst, and gaseous deuterium, the polyolefins dissolve and undergo H/D exchange reaction at the surface of the catalyst, producing partially deuterium labelled polyolefins. Polymers with varying molecular weight, varying ethyl branch density and narrow molecular weight distribution were synthesized by anionic polymerization of 1,3-butadiene followed by saturation with gaseous hydrogen. The LLDPE polymer with relatively broader molecular weight distribution is a commercial product and was supplied by ExxonMobil Chemical Company. The extent of deuterium labelling is analyzed with density measurement, proton nuclear magnetic resonance spectroscopy (1H-NMR) and Fourier transform infrared (FTIR) spectroscopy. A size exclusion chromatography (SEC) instrument equipped with an IR detector was used to analyze the deuterium concentration within the LLDPE polymer as a function of molecular weight. Small angle neutron scattering (SANS) was conducted for both the pure labelled polyolefins and their blends. The partially labelled LLDPE sample was fractionated according to the molecular weight. The partially labelled fractions were blended with the normal LLDPE to create samples with different molecular weight portions labelled. These labelled blends were uniaxially stretched at room temperature while simultaneously monitored with SANS, providing a method to characterize the single chain alignment process at different stages of polyethylene deformation, as a function of time. In this thesis, several aspects of the isotope exchange reaction were investigated. We first examined the dependence of the isotope exchange on the molecular weight and branch content of the substrate polyolefins. The extent of isotope exchange was found to strongly favor the high molecular weight molecules. High branch concentration hinders the exchange reaction, but has a less impact at low branch content. These observations are best explained by viewing the exchange reaction as an absorption controlled process. The deuterium distribution was found to be inhomogeneous evidenced by both the SEC-IR and SANS results. From SANS results modeling, it was confirmed that mathematical accommodation of the inhomogeneous deuterium distribution is necessary to extract chain statistics. Finally, the in situ tensile-SANS experiments revealed that the single chains develop a high degree of alignment along the stretching direction during the elastic and plastic deformation processes of the LLDPE, and maintain that alignment during the strain hardening regime. A remarkable higher degree of chain alignment was found for the high molecular weight chains, a result of longer chains being able to form more tie chains between lamellae. The results of this work provided a scheme of analyzing commercial polyolefins on the single molecular scale, without the necessity to access the synthesis route of the materials.