The acid-initiated cationic polymerization of isobutyl vinyl ether (IBVE) with thietane as nucleophilic moderator at -40-degrees-C in dichloromethane leads to star-shaped poly(vinyl ether)-poly-thietane block-copolymers. The block-copolymer structure is obtained because the vinyl ether polymerization is terminated when the active species (alpha-alkoxythietanium ion) is attacked at its endocyclic methylene group by a thietane molecule. This reaction is a termination reaction for the vinyl ether polymerization but is an initiation reaction for the thietane polymerization. The star-shaped structure of the block-copolymers is a consequence of the (predominantly) intermolecular termination in the cationic polymerization of thietane. With a bifunctional initiating system the end result is a polymer network consisting of linear poly-IBVE segments interconnected by branched polythietane segments. These findings support the sulfonium ion structure of the active species in the cationic polymerization of vinyl ethers initiated by the acid-sulfide system.