Reversible Addition–Fragmentation Chain-Transfer Aqueous Emulsion Polymerization Observed by Transmission Electron Microscopy

Heterogeneous polymerizations account for a significant fraction of global polymer production and underpin the synthesis of latexes used in coatings, adhesives, and advanced materials. Yet, despite their widespread use, emulsion polymerizations remain mechanistically underexplored due to the lack of direct observational tools compatible with liquid-phase environments. In this study, we leverage the combination of liquid-phase transmission electron microscopy (LPTEM) and dry-state transmission electron microscopy (TEM) with dynamic light scattering to visualize the nanoscale evolution of monomer droplets and particles during reversible addition-fragmentation chain-transfer (RAFT) aqueous emulsion polymerization of butyl acrylate. Using a poly(ethylene glycol)-based macro-chain transfer agent to stabilize the growing latex, we capture the progression of the emulsion mechanism, including micellar nucleation, particle growth, and relatively rare higher-order morphologies and morphological transitions in later stages. This visualization provides a mechanistic bridge between kinetic models and particle formation dynamics in emulsion systems. By addressing a key observational barrier, this work establishes a new platform for mechanistically guided design of advanced colloidal materials in solution-phase polymerization systems.