Substituent Effect on the Cationic Monomer-Isomerization Polymerization of (Cyclic Imide)-Substituted Oxetanes with Two Different Ring-Opening Routes

This study reports the effect of substituent on the cationic monomer-isomerization ring-opening polymerization of 3-(R1-methyl)-substituted 3-R2-oxetanes (1), in which R1 is phthalimide, maleimide, succinimide, or glutarimide, and R2 is ethyl, benzyl, phenyl, or isopropyl. The acid-catalyzed polymerization of 1 gave polyacetal (3) or polyether (4), together with an isomeric bicyclic acetal (2). The isomerization of 1 to 2 took place prior to polymerization. Subsequently the polymerization of 2 occurred by either single or double ring opening depending on temperature. The polymerization mechanism is discussed in detail based on the coordination of 1 to Lewis acid and the substituent effect on the polymerization manner. In the double ring-opening polymerization of 2 at 130°C, a carbon–carbon double bond of the lactam ring was indispensable for stabilizing the carboxonium-propagating end. Therefore, 2 carrying a saturated lactam ring did not polymerize in such a manner. Phenyl-substituted oxetane phthalimide was unique in undergoing an unusual cyclodimerization at 130°C, primarily because of the high susceptibility of the neophyl-type carbon skeleton to a cation transfer. On the other hand, most 2 brought about the single ring-opening polymerization below room temperature, regardless of the lactam substituent and the R2 group. This polymerization was an equilibrium polymerization through a bicyclic oxonium-propagating end, and the thermodynamic parameters of polymerization were determined. Thus, 3 was transformed into 4 in one pot, by a combination of the depolymerization of 3 and the repolymerization of 2 above the ceiling temperature. From the structure analysis of 2 it was inferred that the single ring-opening polymerizability arises from dipole-dipole repulsion between the parallel standing lone pairs of two acetalic oxygen atoms in a nearly symmetric bicycle. Therefore, 2 having a somewhat twisted bicycle showed no single ring-opening polymerizability.