Polymerization of Styrene in Ternary Microemulsion Using Cationic Gemini Surfactants

The use of cationic dimeric (“gemini”) surfactants in the oil in water (o/w) microemulsion polymerization of styrene is reported. Gemini surfactants of the alkanediyl-α,ω-bis(dimethylalkylammonium bromide) type (m-s-m) with m being 12 and s being 2, 4, 6, 8, 10, and 12 were used. The phase behavior of the microemulsion is decisively influenced by the spacer length s. All surfactants form single-phase o/w microemulsions with styrene in a temperature range from 25 to 60 °C. For s being 2, only very small stable microemulsion regions were observed, while for s being 4, 6, and 8, the clear and stable regions were gradually increased. The shape of the single-phase regions was very similar to that of the analogous single tail surfactant dodecyltrimethylammonium bromide. For s being 10, the microemulsion region was distinctly extended, while for s being 12, it was decreased again at low temperature, but even more increased at high temperature. Polymerization of the monomeric microemulsions led to spherical latex particles, whose size range could be easily controlled by the monomer/surfactant ratio. A significant dependence of the particle size on the surfactant spacer length was observed. At 25 °C, the particle size was maximum for s being 10, at 60 °C the particle size increased with s. Therefore, the particle size was directly correlated with the size of the single-phase microemulsion region. The molecular weight was maximum at medium spacer length. The experimental results are discussed by taking into account hydrophobic and electrostatic effects and the respective microdroplet structure resulting from the varying interfacial spontaneous curvature due to the different surfactant shapes.