Aqueous RAFT polymerization-induced self-assembly (PISA): amphiphilic macroRAFT self-assembly vs. monomer droplet nucleation (miniemulsion polymerization)

The effect of monomer droplet size on the particle formation mechanism in aqueous heterogeneous polymerization using a hydrophilic macroRAFT agent has been investigated. The specific system explored was the polymerization of styrene using the trithiocarbonate (TTC) based macroRAFT agent poly(acrylic acid-stat-poly(ethylene glycol) methyl ether acrylate)-TTC, with polymerizations conducted in the presence of monomer droplets of different size: (i) magnetic stirring, (ii) handshaking before polymerization, and (iii) ultrasonication before polymerization. A polymerization recipe that generates worm-like morphology under normal polymerization-induced self-assembly (PISA) conditions, i.e. stirring as in (i), was employed. For these particular conditions, high-order morphologies were observed when particle formation occurred via block copolymer micelles (traditional PISA mechanism), regardless of the extent of mixing. Monomer droplet nucleation (miniemulsion mechanism) was predominant only for (iii) in the presence of hexadecane, sodium dodecyl sulfate, and hydrophobic initiator, in which case only spherical nanoparticles were observed. The overall findings suggest that the formation of spherical nanoparticles, as opposed to high order morphologies, becomes predominant as the loci of polymerization change to monomer droplets.