Structure and activity relationship studies of N-heterocyclic olefin and thiourea/urea catalytic systems: application in ring-opening polymerization of lactones

Abstract

A highly efficient and controllable ring-opening polymerization of lactones (δ-valerolactone, ε-caprolactone and rac-lactide) has been achieved by using N-heterocyclic olefin (NHO) and thiourea/urea (TU/U) catalytic systems. This catalytic system showed high ring-opening activity and stereoselectivity, delivering biodegradable polyesters with high chain-end fidelity, controlled molecular weights and narrow molar mass dispersities. A detailed investigation of the structure–activity relationship was performed by exploring five NHOs and fourteen TUs/Us. For a fixed NHO, when the acidity of TUs/Us decreases, the polymerization mechanism changes from the (thio)urea anion to the neutral cooperative activation mode, and the catalytic activity first increases and then decreases, displaying a highly effective interval. For a given TU or U, as the basicity of NHOs increases, the catalytic performance improves correspondingly. Besides, highly isoselective ROP of rac-LA (P_m = 0.93) at −78 °C has been also achieved, highlighting the versatility of the NHO/TU(U) system. These findings enrich the type of TU/U and base organocatalyst.