Miniemulsion Polymerization with Arrested Ostwald Ripening Stabilized by Amphiphilic RAFT Copolymers

We describe a method and conditions for the preparation of monodisperse latex particle dispersions using low molecular-weight, amphiphilic styrene-b-acryiic acid macro-RAFT diblock copolymers as sole stabilizers. The macro-RAFT copolymers are soluble in styrene monomer droplets when the acrylic acid is in its neutral, protonated form, but adsorb irreversibly at the interface between the monomer droplets and the aqueous phase to form an insoluble monolayer when conditions favor deprotonation. Droplets thus stabilized exhibited no coarsening by Ostwald ripening over at least several days. Polymerization yielded a one-to-one correspondence between the size of the initial droplets and the resultant latex particles. consistent with ideal miniemulsion polymerization, when the least labile macro-RAFT stabilizer was used. GPC analysis of molecular weights showed that in all cases the polymerization remained under RAFT control. The approach described enables true miniemulsion polymerization to proceed in the absence of both free surfactant and hydrophobic stabilizer.