Cationic polymerization of isobutyl vinyl ether using pyrrole was examined with a variety of metal chlorides in the presence of a weak Lewis base. In conjunction with oxophilic acids such as ZrCl4, long-lived species were produced to yield polymers with narrow molecular weight distributions (MWDs) and number-average molecular weight (M-n) values based on the used amounts of pyrrole. Acid-trapping experiments using silyl ketene acetal indicated that pyrrole worked not as an initiator but as a transfer agent. The polymerization started from adventitious water, followed by the reactions between the propagating species and the 2- and 5-positions of pyrrole, accompanied by the generation of HCl. In addition to the propagation from the generated HCl, the produced pyrrole-bonded chain-end structures were also activated by the oxophilic chlorides to generate propagating carbocation via the abstraction of the isobutoxy group. As a result, the number of growing chains increased. Such transfer reactions occurred predominantly in the early stage of the polymerization stemming from the highly nucleophilic nature of pyrrole. Thus, the resulting polymers had expected M-n values and narrow MWDs as if pyrrole worked as an initiator.