The "living" and/or controlled cationic ring-opening bulk copolymerization of oxetane (Ox) with tetrahydropyran (THP) (cyclic ether with no homopolymerizability) at 35 degrees C was examined using ethoxymethyl-1-oxoniacyclohexane hexafluoroantimonate (EMOA) and (BF3 center dot CH3OH)(THP) as fast and slow initiator, respectively, yielding living and nonliving polymers with pseudoperiodic sequences (i.e., each pentamethylene oxide fragment inserted into the polymer is flanked by two trimethylene oxide fragments). Good control over number-average molecular weight (M-n up to 150 000 g mol(-1)) with molecular weight distribution (MWD similar to 1.4-1.5) broader than predicted by the Poison distribution (MWDs > 1 + 1/DPn) was attained using EMOA as initiating system, i.e., C2H5OCH2Cl with 1.1 equiv of AgSbF6 as a stable catalyst and 1.1 equiv of 2,6-di-tert-butylpyridine used as a non-nucleophilic proton trap. With (BF3 center dot CH3OH)(THP), a drift of the linear dependence M-n(GPC) vs M-n(theory) to lower molecular weight was observed together with the production of cyclic oligomers, similar to 3-5% of the Ox consumed in THP against similar to 30% in dichloromethane. Structural and kinetics Studies highlighted a mechanism of chains growth where the rate of mutual conversion between "strain ACE species" (chain terminated by a tertiary 1-oxoniacyclobutane ion, A1) and "Strain-free ACE species" (chain terminated by a tertiary 1-oxoniacyclohexane ion, T1) depends on the rate at which Ox converts the stable species T1 (kind of "dormant" species) into a living "propagating" center A1 (i.e., k(a)(app)[Ox]). The role of the THP solvent associated with the suspension of irreversible and reversible transfer reactions to polymer, when the polymerization is initiated with EMOA, was predicted by our kinetic considerations. The activation-deactivation pseudoequilibrium coefficient (Q(t)) was then calculated in a pure theoretical basis. From the measured apparent rate of Ox (k(Ox)(app)) and THP (k(THP)(app) = k(a(endo))(app)) consumption, Q(t) and reactivity ratio (k(p)/k(d), k(a(endo))/k(a(exo)), and k(s)/k(a(endo))) were calculated, which then allow the determination of the transition rate constant of elementary step reactions that governs the increase of M-n with conversion.