<p>Designing dynamic covalent networks (DCNs) that can spontaneously undergo bond exchange reactions without catalysts is a promising strategy for producing self-healing, recyclable polymeric materials. In this study, flexible polymer films with phosphoester-based DCNs were prepared by subjecting a solution containing poly(2-hydroxyethyl methacrylate) (poly(HEMA)) and 2-methoxy-2-oxo-1,3,2-dioxaphospholane (MP) to heat-assisted solvent evaporation. The ring-opening esterification of MP with the hydroxy groups of poly(HEMA) resulted in the formation of β-hydroxy groups, which promoted continuous esterification with neighboring groups. The esterification proceeded without any catalysts, affording crosslinked network structures. The chemical structure of the networks was characterized by ¹H–³¹P heteronuclear multiple bond correlation (HMBC) spectroscopy, which revealed the presence of repeating units of phosphoesters and crosslinks. The Young’s modulus of the films was controlled by changing the amount of phosphoester units in the films. When the amount of phosphoester units exceeded 0.7 molar equivalents per HEMA unit, flexible films could be obtained. More interestingly, the flexible films exhibited self-healing, reshaping, and degradable properties. This design approach could be applied to various polymers containing hydroxy groups, providing access to diverse sustainable polymeric materials.</p>