Enhancing Polyhydroxyurethane Properties via the Formation of Dioxaborolane- and Dioxazaborocane Vitrimers

While polyhydroxyurethanes (PHUs) have been offered as "greener" alternatives to conventional polyurethanes (PUs) for ∼15 years, their low molecular weights, high hydrophilicity, and water uptake limit practical utility. In this study, we directly address these limitations by coupling pendant hydroxyl groups in tailored thermoplastic PHUs with 1,4-phenylenediboronic acid to form tough, robust PHU vitrimers, whose enhanced viscoelastic properties and reduced moisture sensitivity significantly differentiate them from existing linear and vitrimeric PHUs. Three families of PHU vitrimers incorporate either dioxaborolane or dioxazaborocane moieties based on aromatic or aliphatic amines. A fundamental comparison of structure-property relationships confirmed that dioxazaborocanes stabilized by aliphatic nitrogen atoms improve thermal and hydrolytic stability, as demonstrated through direct observations via solid-state NMR. In contrast to aromatic nitrogens, aliphatic nitrogens form N→B dative bonds, reducing chain mobility and increasing T _g. This significantly enhances mechanical performance (+65-170% tensile strength, +125-300% break strain, ∼4-11× increase in tensile toughness) vs the parent PHUs and allows the resulting vitrimers to maintain their viscoelastic properties even at elevated humidity levels. Moreover, these N-stabilized dioxazaborocane PHU vitrimers are readily recyclable, both mechanically (up to at least 3×) and chemically (using ethanol/NaOH_aq), without loss of performance.