Bimetallic Epoxide Polymerization Catalysts

Abstract

Atactic poly(propylene oxide) is a commonly produced polyether that is used as the soft segment in polyurethanes as it is amorphous with a T g of approximately [-]70 °C. Isotactic poly(propylene oxide) is a semicrystalline polymer with a T m of 67 °C, however, there was no catalyst capable of producing solely isotactic poly(propylene oxide) from racemic propylene oxide until the Coates group discovered a (salph)CoOAc catalyst for this transformation in 2005. This development was quickly followed by the discovery of a homogenous, bimetallic (salen)Co catalyst for the enantioselective polymerization of epoxides. In this work, the mechanism of this catalyst is elucidated using a combination of experimental and theoretical studies. Key structural features of the catalyst are identified and then used to develop easier to synthesize bimetallic (salalen)Cr catalysts useful for the isoselective polymerization of propylene oxide in the presence of alcohol chain shuttling agents. These polymers feature controlled molecular weights and hydroxy end groups, making them interesting candidates for the synthesis of new polyurethanes. Finally, the polymerization activity of flexibly linked bimetallic (salen)Cr catalysts is studied. These catalysts stereoselectively polymerize propylene oxide to semicrystalline polymers with low polydispersities. Effects of salen stereochemistry and linker lengths on the polymerization rate and polymer tacticity are examined.