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,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, A1) and "strain free ACE species" (chain terminated by a tertiary 1-oxonia-4-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 * 0). With BF 3 ‚CH 3 OH (i.e., initiator yielding a slow initiation), a drift of the linear dependence M n,GPC vs conversion to lower molecular weight were observed together with the production of cyclic oligomers, ∼10% of the monomer consumed in 1,4-dioxane against ∼ 30% in dichloromethane.

The "living" and/or controlled cationic ring-opening bulk copolymerization of oxetane (Ox) with tetrahydropyran (THP) (cyclic ether with no homopolymerizability) at 35 °C was examined using ethoxymethyl-1-oxoniacyclohexane hexafluoroantimonate (EMOA) and (BF 3 3 CH 3 OH) 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 ∼ 1.4-1.5) broader than predicted by the Poison distribution (MWDs > 1þ1/DP n ) was attained using EMOA as initiating system, i.e., C 2 H 5 OCH 2 Cl with 1.1 equiv of AgSbF 6 as a stable catalyst and 1.1 equiv of 2,6-di-tert-butylpyridine used as a non-nucleophilic proton trap. With (BF 3 3 CH 3 OH) 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, ∼3-5% of the Ox consumed in THP against ∼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 constant 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.

Well-defined polyoxetane with low polydispersivity has been synthesized via a novel living polymerisation process using 3-phenoxypropyl 1,4-dioxanium hexafluoroantimonate (3-PPD) as a model of a living "monomeric polyoxetane" initiator, in 1,4-dioxane at 35 degrees C.