Fabrication of healable and flame-retardant polymer films based on phosphonate dynamic covalent crosslinking networks

Phosphoester linkages are attractive not only for flame retardancy but also dynamic covalent networks (DCNs) while preparing self-healing materials. However, thermal activation of these linkages may induce irreversible structural changes by condensation or volatile product evolution. In this study, phosphonate crosslinkages were investigated as an alternative dynamic linkage and directly compared with phosphoester crosslinkages in polymer networks based on poly(2-hydroxyethyl methacrylate) (PHEMA). Phosphoester- and phosphonate-crosslinked films were prepared using 2-methoxy-2-oxo-1,3,2-dioxaphosphorane or 2-methyl-2-oxo-1,3,2-dioxaphosphorane, respectively. Upon thermal treatment, the phosphoester-based DCN exhibited irreversible changes in mechanical properties and mass loss, resulting from the evolution of volatile byproducts during transesterification. Conversely, the phosphonate-based DCN maintained its mechanical properties and catalyst-free dynamic bond exchangeability with an activation energy of 122.0 kJ mol^-1, enabling repeatable thermal healing with approximately 99% recovery of the original tensile toughness. The two networks also exhibited distinct flame-retardant behaviors. Although both networks showed self-extinguishing behavior after flame exposure, the phosphonate-based DCN extinguished much more rapidly and exhibited substantially reduced internal thermal damage. Furthermore, the polymer film containing phosphonate-based DCNs exhibited superior flame-retardant properties owing to the instantaneous flame extinction capabilities by the P-C bonds. These results demonstrate that phosphonate-based dynamic crosslinking provides a promising strategy for designing multifunctional polymer networks that combine structural stability, thermal healing intrinsic flame retardancy.