AbstractCationic polymerization of 1,4,6‐trioxaspiro [4,4]‐nonane (1) with (CH3)3O+SbF6−(2) and CH3OSO2CF3(3) initiators has been investigated. Although the observed rates of initiation and propagation are relatively slow, they consist of rapid reversible elementary reactions. In1H‐NMR spectra, a broadening of the monomer signal was observed, indicating a fast exchange between “free” monomer and monomer engaged in the active species. The variety of orthoester bonds were observed in the polymer formed at the early stages of monomer conversion. The final polymer has, however, structure of a linear poly(ester–ether) including two subsequent ester or ether linkages. To account for these new facts, the mechanism of polymerization was proposed, consisting of a rapid reversible opening of one of the rings in the monomer molecule involved in the growing species, followed by the slower opening of the second ring with formation of the ester linkages. It appears that the rings originally present in the chains rearrange into the linear units int