CO2/Propylene Oxide Copolymerization with a Bifunctional Catalytic System Composed of Multiple Ammonium Salts and a Salen Cobalt Complex Containing Sulfonate Anions
A (Salen)Co(III) complex tethering four quaternary ammonium salts with covalent bonds is one of the most highly active catalysts for CO2/epoxide copolymerization. In this work, we aimed to prepare similar (Salen)Co(III) complexes to which quaternary ammonium salts are linked via ionic interactions. Thus, multiple ammonium salts containing 2-5 quaternary ammonium salt units and (Salen)Co(III) complexes containing one or two SO3[PhNH3]+ moieties were prepared. A binary catalytic system composed of the prepared multiple ammonium salts and the prepared (Salen)Co(III) complex containing -SO3-[(nBu)4N]+ moieties showed high activity (TOF, 15004500 h1) for CO2/propylene oxide (PO) copolymerization, whereas a combination of the multiple ammonium salts and the conventional (Salen)Co(III) complex not containing -SO3-[(nBu)4N]+ moieties was inactive under the identical polymerization conditions of [PO]/[Co] = 20000. However, a substantial amount of cyclic carbonate was concomitantly generated, especially when a (Salen)Co(III) complex containing two -SO3-[(nBu)4N]+ moieties was used (2530%). This side-reaction could be mitigated by ca. 50% by employing a (Salen)Co(III) complex containing one -SO3-[(nBu)4N]+ moiety. Cyclic carbonate generation can reasonably be ascribed to ammonium salts ([(nBu)4N]+[carbonate]-) not linked to the (Salen)Co(III) complex formed in the binary catalytic system.
