(6) Fundamental Chemistry and Recent Development of Ring-Opening Polymerization

This review article deals with ring-opening polymerization, selecting fundamental chemistry as well as recent topics. In addition to usual thermodynamics, where the ring-strain is released to drive the polymerization by providing the enthalpy, an entropy-driven system is presented for macrocyclic monomers. In a recent trend, efficient monomer recovery through the depolymerization is achieved by reducing the ceiling temperature with the aid of a solvent and a catalyst. Lactone, thiolactone, and cyclic carbonate monomers are explored for such purpose. The propagation mechanisms of ionic ring-opening polymerizations are discussed in terms of electrophilic and nucleophilic characters. The propagation takes place between an ionic chain end and a neutral monomer or between a neutral chain end and an ionic monomer. The former is a usual manner termed as an active chain-end mechanism, while the latter is an activated monomer mechanism, which can rather perform a living fashion. There is additionally a system involving the propagation reaction between a neutral chain end and a neutral monomer. Cyclic polymers are efficiently prepared from lactide, lactone, and NCA by the cyclization between the cationic initiation terminal and the anionic propagation terminal. Radical ring-opening polymerization is revived due to giving degradable functional groups such as ester and thioester to vinyl polymer main chains by the copolymerization. Therein, thionolactone is newly employed in addition to a classical monomer of cyclic ketene acetal. Ring-opening metathesis polymerization of cyclic olefines, showing living characters, is a powerful tool for not only basic research but also industrial production.