Monodisperse polystyrene (PS) particles are widely used as model colloids. Aqucous PS latices are usually prepared by emulsion polymerization. The emulsion polymerization can be performed either with a surfactant or without a surfactant (soap-free or emulsifier-free).
The theory of emulsion polymerization, in which a surfactant is used, was established by Harkins (1) and by Smith and Ewart (2). The main disadvantage of the use of a surfactant is that the emulsifier is adsorbed at the interface of the PS particle and water. The removal of the adsorbed emulsifier can be quite difficult and one can never be sure that all emulsifier molecules are removed from the interface.
These drawbacks led to the development of emulsifier-free emulsion polymerization. Previous work on this subject has been reported by Matsumoto and Ochi (3), Kotera et al. (4,5), and Goodwin et al. ((6,7)).
In these polymerization systems ((6,7)) monomer concentration, initiator concentration, ionic strength, and temperature have been found to be important variables. Under the conditions used it has heen impossible to prepare PS latices with particle sizes larger than (1 \mu \mathrm{m}) in a one-step process.
A seeded-growth procedure seemed to be a reliable way to obtain larger particles (8), but in many sceded-growth experiments new nucleation occurred and bimodal and even broader distributions were found.
Investigations on the one-step surfactant-free emulsion polymerization of styrene to obtain large PS particles (particle size larger than (1 \mu \mathrm{m}) ) have, to our knowledge, never been published thus far.
In the present paper the preparation and characterization of PS particles (up to (3.2 \mu \mathrm{m}) ) made by a one-step emulsion polymerization is described. Variables such as monomer concentration, initiator concentration, ionic sirength, and flow pattern have been found to play a very important role. The particles obtained had a high degree of monodispersity.