Electronic influence of ligand substituents in the ring-opening polymerization of L-Lactide promoted by OSSO-type zirconium complexes

The zirconium complexes (OSSO Cl )ZrO t Bu 2 (1), (OSSO Br )ZrO t Bu 2 (2) and (OSSO tBu )ZrO t Bu 2 (3) bearing OSSO-type ligands (OSSO Cl -H = 1,4-dithiabutanedyl-2,2′-bis(4,6-dichlorophenol); OSSO Br -H = 1,4-dithiabutanedyl-2,2′-bis(4,6-dibromophenol); OSSO tBu -H = 1,4-dithiabutanedyl-2,2′-bis(4,6-di-tert-butyl-phenol)) were prepared and characterized. NMR spectroscopy revealed a rigid octahedral coordination geometry with a facial-facial coordination of the [OSSO] ligand to the metal complex. These compounds were active in the ring-opening polymerization of L-lactide exerting an effective control over the polymerization reaction. Activities correlated with the electron-withdrawing character of the ortho and para groups of the OSSO ligands. Density functional theory (DFT) investigations indicated that substitution with increasingly electron-withdrawing groups favors the monomer coordination and determines an earlier, more reactive transition state.