Radical polymerization in direct mini-emulsion systems

Polymerization in direct mini-emulsions is a relatively new polymerization technique which allows the preparation of submicron latex particles within the range 100 < particle diameter < 500 nm. This process involves the gener ation of a large population of submicron monomer droplets in water (termed the mini-emulsion) by intensive shear force with the aid of an adequate emu lsifier and coemulsifier (or hydrophobe). These stable, homogenized monomer droplets have an extremely large surface area and, therefore, can compete effectively with the monomer-swollen micelles, if present, for the oligomer ic radicals generated in water. Monomer droplets may thus become the predom inant loci for particle nucleation and polymerization. This article present s a review of the current literature in the field of radical polymerization of conventional monomers and surface-active monomers in direct mini-emulsi on systems. Besides a short introduction into some kinetic aspects of radic al polymerization in direct emulsion, mini-emulsion and micro-emulsion syst ems, we mainly focus on the particle nucleation mechanisms and the common a nd different features between the classical emulsion and finer mini-emulsio n polymerization systems. The effects of the type and concentration of init iator, emulsifier, coemulsifier (hydrophobe) and monomer are evaluated. The influence of the complex formation, the close-packed structure, etc. withi n the mini-emulsion droplet surface layer on the colloidal stability, the n ature of the surface film, and the radical entry process are summarized and discussed. These results show that the nature of the coemulsifier (hydroph obe), the weight ratio of emulsifier to coemulsifier, the mechanisms of son ification, the stability of monomer droplets, and the presence or absence o f free micelles play a decisive role in the polymerization process.