The suspension cationic polymerization of isobutyl vinyl ether (IBVE) in aqueous medium could be achieved by using H3PW12O40, AIPW(12)O(40), FePW12O40, K3PW12O40, or Na3PW12O40 as efficient water-tolerant coinitiators in the presence of HCI. The addition reaction of IBVE with H2O occurred to form IBVE-H2O adduct and then subsequent decomposition immediately took place or turned to acetaldehyde diisobutyl acetal (A) in the presence of AIPW(12)O(40), and (A) decomposed rapidly to form 2-isobutanol (B) and acetaldehyde (C). Cationic polymerization of IBVE in aqueous medium was promoted greatly with increasing HCI concentration and proceeded extremely rapidly to get high polymer yield even at low concentration of AIPW(12)O(40) of 0.3 mM. A sufficient amount of HCI was needed to decrease the hydrolysis of initiator IBVE-HCl and to accelerate the polymerization in aqueous medium simultaneously. The yield and molecular weight of poly( IBVE) increased with increasing concentrations of HCI and AIPW(12)O(40) or with decreasing temperature. The isotactic-rich poly( IBVE)s with m diad of around 60%, having Mn of 1200-4500 g mol(-1) and monomodal molecular weight distribution could be obtained via cationic polymerization of IBVE in aqueous medium. This is the first example of cationic polymerization of IBVE in aqueous medium coinitiated by heteropolyacid and its salts. (C) 2012 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 546-556