Bis-thiourea and macrocyclic polyamines as binary organocatalysts for the ROP of lactide

New binary catalysts formed by 1,1 '-(propane-1,3-diyl)bis(3-(3,5-bis(trifluoromethyl)phenyl)thiourea) and a series of specifically designed flexible polyaza-macrocycles, prepared by a convenient iterative solid-phase synthesis and reduction, have been investigated for the ring opening polymerization (ROP) of l-lactide. These systems, in the presence of benzyl alcohol as initiator, showed high activity, delivering polylactides with high chain-end fidelity and controlled molecular weights with narrow dispersities. A detailed study of the structure-activity relationship for various polyaza-macrocycles, with different sizes and N-side chains, was performed. The best results were exhibited by cyclen derivatives, improving the performances achieved by the well-established triazacyclononane (TACN) co-catalyst. DFT (density functional theory) calculations, performed on each putative polyazamacrocycle/benzyl alcohol complex, assessed both the ring size and the N-alkyl steric hindrance roles on the initiator activation, providing a rationale for the activity scale experimentally observed for macrocyclic polyamines.