Metal triflates or tetrafluoroborates, such as Zn(OTf)(2), Cu(OTf)(2), Sn(OTf)(2), and Zn(BF4)(2) (OTf = -OSO2CF3), were employed as water-tolerant Lewis acids for the emulsion cationic polymerization of p-methoxystyrene (pMOS) in heterogeneous aqueous media. All these metal compounds, in addition to rare earth metal triflates such as Yb(OTf)a already reported by us, induced the polymerization of pMOS in conjunction with the pMOS-HCl adduct (1) (initiator) and dodecyltrimethylammonium chloride (surfactant) at 30 degrees C in water. The pMOS polymerization was controlled, where the molecular weights of the polymer increased with monomer conversion;and the molecular weight distributions were relatively narrow (M-w/M-n similar to 1.4). H-1 NMR analysis of the obtained polymers showed that the chlorine atom originating from 1 was attached to the growing polymer terminal. In addition, the water tolerance of these catalysts was confirmed by F-19 NMR analysis. Thus, the emulsion cationic polymerization proceeds via reversible activation of the C-Cl terminal mediated by the water-tolerant Lewis acid, which enters from the aqueous phase into the organic phase.