Controlled Ring-Expansion Polymerization Based on Acyl-Transfer Polymerization of Thiiranes with Aromatic Heterocycles as Initiators

Abstract

This chapter describes the ring-expansion polymerization (REP) of thiiranes using aromatic benzothiazolone derivatives as initiators, including key fundamental addition reactions between thiiranes and active ester species in the presence of quaternary onium halides. This regiospecific addition reaction proceeds via selective intramolecular rearrangement, which results in the insertion of the thiirane-derived thioethylenes into the active ester structures. Such reaction mode enables the design of the controlled polymerization of thiiranes using the active esters as initiators. Since the acyl-group in the initiators repeatedly transfers at the propagating terminus, the polymerization is named “acyl-transfer polymerization”. When cyclic aromatic heterocycles with S-thiocarbamate moieties are used as initiators, the acyl-transfer polymerization proceeds in a ring-expanding fashion to produce cyclic polysulfides. The unique polymerization behavior was observed based on the rigid 5-membered structures of the initiators, while the living-like nature of the REP was confirmed by careful investigation. Successful synthesis of cyclic block copolymers and the characteristic glass transition properties of the obtained cyclic polysulfides are also described.