Phosphaza-bicyclo[2.2.2]octane based diols as monomers for hybrid materials

Most synthetic polymers are constructed from linear or monocyclic building blocks; the controlled use of bicyclic, three-dimensional organic monomers remains synthetically challenging. In contrast, incorporating unstrained inorganic cages like polyhedral oligomeric silsesquioxanes (POSS) and carboranes into polymer backbones is facile, and has been used to make materials with exceptional stability and mechanical properties. But inorganic cages besides these two are minimally explored in polymer chemistry. In this study, a phosphorus-nitrogen (PN) cage was converted via Staudinger reactions with azido alcohols to cage-containing aromatic (PNBP) and aliphatic (PNBH) diols, making monomers analogous to organic diols commonly used in polycondensation. Crystallographic analysis reveals how functional-group identity and conformational freedom influence intermolecular association and solubility. Attempted addition of these diols to diisocyanates does not yield polyurethanes, showing evidence of side-reactions involving an Aza-Wittig pathway instead. On the other hand, polycondensation with Me2SiCl2 yields PN–siloxane polymers featuring cages embedded directly within the main chain. The aromatic PN-siloxane polymer exhibits high rigidity, crystallinity, elevated melting and crystallization temperatures, and enhanced thermal robustness. Its aliphatic analogue exhibits high solubility, flexibility, and a low glass-transition temperature. Thermal and preliminary flame-retardant analyses demonstrate synergistic stabilization from phosphorus, nitrogen, and silicon. These results establish PN-based diols as versatile building blocks for cage-dense hybrid polymers and a platform for next-generation high-performance materials.