Phosphorus-Containing Acrylate-Based Core–Shell Polymers: Synthesis and Flame-Retardant Properties

To address the flammability issues limiting polymer applications, phosphorus-containing flame retardants have received increasing attention as halogen-free alternatives. In this study, two phosphorus-containing methacrylate monomers, diphenylphosphinyl methacrylate (<b>DPMA</b>, +1 oxidation state, P-Ph bond) and diphenylphosphoryloxy methacrylate (<b>DPOMA</b>, +5 oxidation state, P-O-C bond), were synthesized and selectively incorporated into the shell of core-shell polymers via seeded emulsion polymerization. The resulting particles were uniform, spherical, and low polydispersity (PDI < 0.07). The flame-retardant behavior was strongly affected by the oxidation state of phosphorus. When 70 wt % of each monomer was placed in the shell, <b>PA-DPMA70</b> reduced the peak heat release rate (pk-HRR) by 44% and showed a significant decrease in the fire growth index (FGI) and average effective heat of combustion (av-EHC), indicating a dominant gas-phase inhibition. In contrast, <b>PA-DPOMA70</b> reduced the pk-HRR by 19% but greatly increased the residual char yield, producing a dense and highly graphitized structure with an <i>I</i> _D/<i>I</i> _G ratio of 1.56, which is characteristic of condensed-phase protection. These findings clearly demonstrate that the oxidation state of phosphorus determines whether gas-phase radical quenching or condensed-phase char formation governs the flame-retardant mechanism. In addition to earlier reports on incorporating phosphorus methacrylates into bulk matrices or silicone-acrylic adhesives, this work establishes a shell-specific localization strategy that clarifies the oxidation state mechanism relationship and offers guidance for the rational design of next-generation halogen-free flame-retardant polymers.