π-π Interaction Promoting Kinetic Resolution Copolymerization of rac-Epoxides and Isocyanates

The very simple, enantiopure Co(III)-Salen complexes are discovered to be highly efficient in kinetic resolution copolymerization of racemic terminal epoxides with isocyanates, wherein π-π stacking interactions between the catalyst and the propagating polymer chain end significantly enhance enantioselectivity. A monometallic enantiopure Co(III)-Salen-DNP (DNP = 2,4-dinitrophenoxide) complex bearing Me and F at the ortho- and para-positions, respectively, delivered the best balance of activity and enantioselectivity in the resolution copolymerization of various racemic epoxides with p-tosyl isocyanate, affording optically active polyurethanes with up to 96% ee and perfectly alternating structures. Density functional theory calculations further revealed that π-π interactions favor the stereoselective ring opening of one enantiomer of racemic epoxide mixtures and govern the high enantioselectivity by positioning the propagating chain end during the specific discrimination step. This study establishes a novel catalyst design strategy for asymmetric polymerization to prepare stereoregular polymers with main-chain chirality.