Dynamics of Polypropylene Chains at a Model Catalytic Interface Revealed by Quasielastic Neutron Scattering: Can Interfacial Diffusivity Predict Reactivity in Chemical Recycling?

Abstract

Catalytic processes developed for petroleum refining, such as bifunctional hydrocracking, can be adopted to chemically recycle plastic polymers. In these reactions, polymer substrates often exhibit distinct kinetics from small molecules with identical chemical structures, emphasizing that the macromolecular behavior of polymer chains beyond local bonding modulates the reactivity. Here, we report the direct measurement of the dynamics of polymer chains at a model catalytic interface, using quasielastic neutron scattering (QENS). Three polypropylenes (PPs) of varying chain lengths and tacticities were studied. In all cases, polymer–catalyst interaction leads to the loss of bulk melting behavior, and segmental dynamics of PP chains can be described by continuous translational diffusion. A lower activation barrier and less favored entropy change at the transition state were observed at the catalytic interface relative to bulk melts, leading to reduced segmental diffusivity at reaction temperatures. The interfacial dynamics are faster for isotactic PP than for atactic PP. This difference is absent in the melt and rationalizes the higher reactivity of isotactic PP in diffusion-limited hydrocracking. This work pioneers the use of H-sensitive neutron-based techniques to probe the consequential macromolecular behaviors of reacting polymer chains at catalytic interfaces. The integration of such novel fundamental knowledge into bonding-level chemical kinetics can transform the understanding of catalytic polymer recycling reactions into a holistic framework, enabling the accurate explanation and prediction of phenomena not accounted for by traditional kinetic approaches.