Emulsion cationic polymerization in water Was achieved with rare earth metal triflates [Ln(OTf)(3); Ln = Yb, Sc, Dy, Sm, Gd, and Nd; OTf = OSO2CF3] that are well-known as water-tolerant Lewis acids. Even in the presence of a large amount of surfactants such as dodecylammonium halides, sodium laurylbenzenesulfate, and poly(vinyl methyl ether), Ln(OTf)(3) induced cationic polymerizations of p-methoxystyrene (pMOS) in aqueous media in conjunction with the pMOS-HCl adduct (1) at 30 degrees C. In particular, the use of cationic surfactants such as dodecyltrimethylammonium chloride accelerated the polymerization and gave polymers of controlled molecular weights and narrower molecular weight distributions (MWDs) (M-w/M-n similar to 1.4) than those in the absence of surfactants. The activity of Ln(OTf)(3) reduced with decreasing the Lewis acidity or the ionic radii of the central metal. The 1/Yb(OTf)(3) system also induced the polymerization of p-tert-butoxystyrene (tBOS) in aqueous media. The homopolymerization of tBOS in water was found to be less controllable than that of pMOS. Random copolymerization of pMOS and tBOS also proceeded in a long-lived fashion in the presence of surfactants. The emulsion cationic polymerization most probably proceeds via the carbocationic species generated from the dormant C-Cl polymer terminal in the organic phase and Ln(OTf)(3) entering from the aqueous phase.