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

Circular plastics thrive on the ability to chemically recycle polymers into reusable monomers, ideally closing the loop from manufacturing to end of life. Mechanisms for heterogeneous polymer deconstruction are complex, involving diffusion and transport of reagents to reactive sites in a material continuously undergoing chemical transformations. A deeper understanding of deconstruction phenomena would better inform the molecular basis of circularity in plastics. Here, we show how nuclear magnetic resonance (NMR) spectroscopy, relaxometry, and diffusometry enable monitoring of heterogeneous deconstruction of a model elastomer with acid-cleavable diketoenamine bonds. In chaotropic aqueous HBr, polydiketoenamine (PDK) deconstruction is fast, enabled by macro- and micro-scale swelling in early stages, which facilitates acid penetration and protonation of reaction sites deep within the polymer sample. We observe a previously unrecognized hydrogen-bond stabilized protonated amine intermediate that is persistent throughout deconstruction, and found a strong correlation of its reactivity with swelling and chain kinetics. In kosmotropic aqueous H2SO4, PDK deconstruction is notably slower. Swelling occurred at a more gradual pace, creating a porous polymer matrix, yet polymer chain mobility remained low. Most notably, the kosmotropic character of aqueous H2SO4 resulted in a reaction site that was comparably less active in advancing hydrolysis and deconstruction to shorter oligomer chains, instead trapping acid in matrix pores and modifying the activity of the reaction medium under confinement in the process.