In the cat;ionic polymerization of isobutyl vinyl ether (IBVE) with binary initiating systems consisting of a protonic acid as an initiator and a Lewis acid as an activator/catalyst, phosphoric acid derivatives [(RO)(2)POOH] coupled with SnCl4 gave highly isotactic poly(IBVE)s, whereas those with a bulky substituent (R), [C4H9CH(C2H5)CH2O](2)POOH (7) and (n-C10H21)(2)POOH (8), led to the highest isotacticity [meso dyad (m) = 86%]. In contrast, isospecificity was lower with IBVE-HCl and CF3COOH under the same conditions. From the effects of the polymerization temperature (-78 to 0 degreesC), it was concluded that the high isospecificity with 7 and 8 was due to an enthalpic factor. (C) 2001 John Wiley & Sons, Inc.