Our investigations of the cationic ring-opening polymerization of oxetane via active chain end (ACE) mechanism have shown that the use of 1,4-dioxane as solvent can prevent intra- and intermolecular transfer reactions (Scheme 1, part a). Using 3-phenoxypropyl-1-oxonia-4-oxacyclohexane hexafluoroantimonate as a model of an initiator capable of yielding fast initiation, polymers with predictable number-average molecular weight (up to 160 000 g/mol), narrow molecular weight distribution (1.18 < M-W/M-n,M-GPC < 1.28) were produced with no cyclic oligomer formation. On the basis of the kinetic data, a mechanism of controlled and living polymerization has been proposed in which the rate of mutual conversion between "strain ACE species" (chain terminated by a tertiary 1-oxoniacyclobutane ion, A 1) and "strain free ACE species" (chain terminated by a tertiary 1-dxonia4-oxacyclohexane ion, T1) does not obey a quasi-steady-state assumption but depends on the rate at which the monomer converts the stable species T1 into a living "propagating" center A1(d[A1]/dt = -d[T1]/dt not equal 0). With BF3 center dot CH3OH (i.e., initiator yielding a slow initiation), a drift of the linear dependence MGpc vs conversion to lower molecular weight were observed together with the production of cyclic oligomers, similar to 10% of the monomer consumed in 1,4-dioxane against similar to 30% in dichloromethane.