Direct Polymerization of 5-Hydroxymethylfurfural Derivatives via Kabachnik–Fields Reaction and Postpolymerization Modification

5-Hydroxymethylfurfural (HMF) is a versatile biomass-derived platform chemical, but the efficient and mild conversion of its derivatives to high-performance polymers remains a challenge. In this work, we introduce HMF derivatives into Kabachnik–Fields three-component polymerization (KF-3CP) without isocyanide components to prepare a biobased polymer with α-aminophosphonate. The KF-3CP protocol has proven generally applicable to diverse diamine and dialdehyde monomers, affording polymers with high molecular weights (Mw up to 27,800 g/mol), excellent yields (up to 98%), and favorable water solubility. Remarkably, the prepared KF-3CP polymers exhibited nontraditional intrinsic fluorescence that was efficiently quenched by Fe3+ ions, combined with low cytotoxicity and outstanding cell imaging capability. Furthermore, it can undergo postpolymerization modification through the Diels–Alder reaction, thereby imparting excellent mechanical properties. These findings establish the KF-3CP products derived from HMF derivatives as highly promising candidates for various applications in the biomedical and materials fields.