Stereoselective Ring‐Opening Polymerization of Heterocyclic Monomers

Abstract

Recent progress in stereoselective ring‐opening polymerization of heterocyclic monomers, especially polymerization and copolymerization of epoxides and lactones, is summarized. Zn, Al, Co, and Cr complexes promote stereospecific ring‐opening polymerization of epoxides as well as copolymerization of epoxides with carbon dioxide. Enantioselective polymerization of epoxide is also achieved to afford optically active polyethers. Cr and lanthanide complexes promote stereoselective polymerization of β‐butyrolactone. Stereospecific polymerization of lactide has been achieved by using varieties of metal catalysts, including Al, Ga, In, Mg, Ca, Sn, Zn, Ti, Zr, Hf, Ge, and rare earth metals. Organic catalysts such as carbenes and phosphazene bases are also utilized for the synthesis of stereoregular polylactide. By utilizing those stereoselective polymerizations, more stereogradient and stereoblock copolymers have been synthesized. Polylactides with high degree of stereoregularity show high melting points. One‐to‐one mixtures of isotactic poly( l ‐lactide) and poly( d ‐lactide) or stereoblock copolymer of l ‐lactide and d ‐lactide show higher melting points compared with the polymer of homochiral monomer, due to the formation of stereocomplex.