Cationic Monomer-Isomerization Polymerization of Oxetanes Having an Ester Substituent, to Give Poly(orthoester) or Polyether

Abstract

This study shows the cationic monomer-isomerization polymerization of 3-R1-oxetanes (1) having a 3-ester substituent (—CH2OCOR2) through two ring-opening routes (R1 = CH3, Ph; R2 = Pr, Ph). The polymerization of 1 with Lewis acids usually gave poly(orthoester) (3) or polyether (4), together with bicyclic orthoester (2) as the isomerization product. The isomerization took place prior to polymerization, because of the preferable nucleophilic attack of the intermolecular carbonyl oxygen. Subsequently, 2 polymerized in either a single- or double-ring-opening manner depending on temperature. The single-ring-opening polymerization of 2 below room temperature was an equilibrium polymerization through bicyclic oxonium ends leading to 3, and the thermodynamic parameters were estimated according to Dainton's equation. Thus, Corey's orthoester synthesis from ester-substituted oxetanes is regarded as a phenomenon in the equilibrium polymerization only around the ceiling temperature (Tc). Above Tc, 2 brought about the double-ring-opening polymerization through monocyclic diaoxacarbenium ends to afford 4. Relatively bulky R1 and R2 such as the Ph group exerted a large steric effect on both polymerization routes. As a special case arising from this effect, the CF3SO3H-catalyzed reaction of 1 c (R1, R2 = Ph) resulted in an unusual cyclodimerization accompanied by the elimination of benzoic acid, owing to the susceptibility of the neophyl skeleton to cation transfer.