Unravelling crosslinking and molecular structure in complex polyurethanes via advanced multinuclear solid-state NMR

A crucial aspect of creating cutting-edge chemical processes that enhance mechanical strength, stability, sustainability, and circularity is acquiring a more profound comprehension of the atomic-scale structure of native polymers. In this work, we present a multinuclear 13 C and 15 N solid-state NMR combined with fast magic angle spinning (fast-MAS) 1 H NMR at high magnetic field (22.3 T) in characterizing thermoplastic polyurethanes (PUs) chemistry. The study highlights the significance of performing atomic-scale characterization to obtain an accurate chemical picture of the polymers, especially when investigating crosslinking among polymer chains. This is because indirect confirmation of crosslinking, such as through molecular weight gains or losses, can only provide an incomplete picture, due to the complex chemical structure involved. Moreover, given that variations in synthesis parameters, such as the ratio of reactants, can significantly influence the properties of polyurethanes, it is essential to establish structure-function relationships in these polymers. This research notably emphasizes the significance of fast-MAS 1D 1 H NMR at high-field in characterizing multiple polyurethane samples within a brief experimental duration of minutes. The study demonstrates that fast-MAS 1 H NMR facilitates straightforward chemical shift assignment and enables quantitative comparison of chemical compositions among three complex PU samples synthesized with distinct reactant proportions. Finally, this study examines the limitations of 1 H fast-MAS NMR, particularly in the context of characterizing low levels of crosslinking in polyurethanes. Despite the inherent low sensitivity of 15 N nuclei, this research demonstrates that 15 N Cross-Polarization (CPMAS) NMR experiment serves as robust method for the unambiguous detection of allophanates in polyurethanes. • Multinuclear 1 H, 13 C and 15 N NMR at high magnetic field is a reliable and efficient technique for characterizing PUs and similar polymers in their native state • The high sensitivity and resolution of fast-MAS 1 H NMR allows for chemical shift assignment and comparison of different PU samples • Fast-MAS 1 H NMR can significantly reduce experimental time compared to conventional 13 C CPMAS NMR • Fast-MAS 1 H NMR exhibits limitations in complex cases such as crosslinking/allophanate detection, whereas 15 N CPMAS NMR demonstrates greater suitability, albeit with reduced sensitivity