Miniemulsion Radical Ring-Opening Polymerization (rROP) of Phenyl Thionophthalide for Degradable Polymer Particle Synthesis: Laplace Pressure Works Key Role for rROP

Radical ring-opening polymerization (rROP) has emerged as a promising strategy for introducing degradable linkages into polymer backbones, particularly through the use of thionolactone derivatives. However, the development of suitable monomers and their application in aqueous heterogeneous systems remain limited. In this study, we report the synthesis of main-chain-degradable polymer particles via miniemulsion rROP using 3-phenylthionophthalide (PTP), a newly designed five-membered thionophthalide monomer. Copolymerization of PTP with n-butyl acrylate (nBA) yielded colloidally stable poly(nBA-co-PTP) nanoparticles with high monomer conversion and complete ring-opening in the miniemulsion system. Comparative studies with structurally related thionophthalide derivatives revealed that the presence of two benzene rings in PTP plays a crucial role not only in promoting efficient radical ring-opening copolymerization but also in enabling effective aminolytic degradation. Furthermore, we demonstrated that the conversion and ring-opening efficiency of PTP were significantly enhanced in miniemulsion rROP compared to solution polymerization. This enhancement was attributed to Laplace pressure within nanometer-sized droplets, which facilitates the ring-opening process. Additional experiments using suspension polymerization and high-pressure reactors confirmed the pressure-dependent nature of rROP, establishing Laplace pressure as a key factor in promoting degradable polymer synthesis. These findings provide new mechanistic insights into pressure-assisted rROP and offer a versatile strategy for designing environmentally friendly polymer particles in aqueous media, contributing to the development of sustainable materials to address microplastic pollution.