Mechanisms Underpinning Heterogeneous Deconstruction of Circular Polymers: Insight from Magnetic Resonance Methodologies

Abstract

Circular plastics thrive on the ability to chemically recycle polymers into reusable monomers, ideally closing the loop from manufacturing to the end of life. Mechanisms for polymer deconstruction are complex, involving diffusion and transport of reagents to reactive sites in a material continuously undergoing chemical transformations. A deeper understanding of the deconstruction phenomena would better inform the molecular basis of circularity. Here, we show how nuclear magnetic resonance (NMR) spectroscopy, relaxometry, and diffusometry enable monitoring of the heterogeneous deconstruction of a model elastomer with acid-cleavable diketoenamine bonds. In chaotropic aqueous HBr, polydiketoenamine (PDK) deconstruction is fast, enabled by macro- and microscale swelling, which facilitates acid penetration and protonation of reaction sites deep within the polymer. We observe a previously unrecognized hydrogen-bond-stabilized amine intermediate that is persistent throughout deconstruction. In kosmotropic aqueous H₂SO₄, PDK deconstruction is notably slower. Here, swelling occurred at a more gradual pace, characterized by low polymer chain mobility, thereby trapping the acid in matrix pores and modifying the activity of the reaction medium under confinement in the process. We find that polymer swelling, chain mobility, and deconstruction kinetics are strongly linked, requiring a multifaceted NMR characterization tool box for in-depth analysis.