Optimizing Emulsifier-Free Emulsion Polymerizationof (Meth)acrylates with Zwitterionic Monomers: Impact of Radical InitiatorTypes

Colloidal stabilization via ionic monomers in emulsion polymerization has emerged as an effective emulsifier-free approach for waterborne (meth)­acrylic coatings. Zwitterionic monomers (ZMs), such as 2-(methacryloyloxy) ethyl dimethyl-(3-sulfopropyl) ammonium hydroxide (DMAPS), are promising candidates for producing in situ colloidal stabilizers by copolymerizing with (meth)­acrylic monomers for protective coating application. Achieving this requires the careful selection of an initiator, which plays a pivotal role in enabling the copolymerization of water-soluble ZMs with (meth)­acrylates placed in emulsion droplets. This study evaluated thermal and redox initiators with varying radical hydrophilicities to determine their impact on DMAPS copolymerization with methyl methacrylate and <i>n</i>-butyl acrylate and the entry of oligoradicals onto polymer particles for colloidal stabilization. Results showed that a high initial radical flux is essential for generating sufficient stabilizing units. More hydrophobic initiating radicals shorten oligoradical residence time in the aqueous phase, promoting earlier particle entry, enhanced DMAPS incorporation, and reduced formation of water-soluble oligomers, which is detrimental to stability. Coating films from these dispersions exhibited enhanced mechanical strength due to the formation of a DMAPS-rich polymer network with ionic bonding within the (meth)­acrylic polymer matrix. However, the redox reaction byproduct increased water uptake, resembling conventional surfactant effects by creating hydrophilic pockets within the film. Removing soluble species prior to film formation reduced water uptake to below 3% after 2 weeks of immersion of the film in water. Besides being useful tools for suitable initiator selection, these findings provide a pathway for developing durable, high-performance waterborne coatings with reduced environmental impact.