Amphiphilic Block-Random Copolymer Stabilizers: A "Seeded-Coagulative" Emulsion Polymerization Mechanism

Polystyrene-b-[polystyrene-r-poly(acrylic acid)] block-random copolymers are effective stabilizers of emulsion polymerizations but exhibit unusual dispersion behavior, self-assembling with low aggregation numbers or possibly self-folding into single-chain nanoparticles. In emulsion polymerizations stabilized by these block-random copolymers, the total final particle surface area was directly proportional to the stabilizer concentration and was unaffected by kinetically driven processes such as initiation and particle growth rate; this behavior is not [Stabilizer) (RINI) explained by any known emulsion polymerization mechanism. A "seeded-coagulative" emulsion polymerization mechanism is proposed, in which particles grow by polymerization and continuous coagulation, conserving total surface area. Mathematical equations for this proposed mechanism fit the experimental data well and explain the observed dependence of particle number on stabilizer concentration (N-P similar to [S](3)) and solids content (N-P similar to tau(-2)). The novel emulsion polymerization mechanism is attributed to the unique properties of the stabilizers used.