β ‐Diketonate Complexes of d ‐Metals as Catalysts for the Ring‐Opening Polymerization of rac ‐Lactide

A series of acetylacetonate and dibenzoylmethanate complexes of Mn(II), Fe(II), Fe(III), Co(II), Ni(II), and Cu(II) are investigated as catalysts for the ring‑opening polymerization (ROP) of rac ‑lactide in bulk at 135°C. All complexes demonstrate catalytic activity both in the presence and absence of alcohol. The addition of alcohol enhances monomer conversion and improves control over molecular weight. Kinetic analysis confirmed pseudo‑first‑order reaction rate. Among the β ‐diketonate complexes examined, [Mn(DBM) 2 ] exhibits the highest apparent rate constant. The study of the mechanism of ROP initiation of lactide in the presence of the catalysts under investigation using 1 H NMR and MALDI‐TOF mass spectrometry revealed that alcohol dictates the chain‐end group structure. The presence of alcohol leads to the formation of terminal ester and hydroxyl groups. In turn, its absence results in a terminal deprotonated lactide moiety. Alongside high catalytic activity, the Mn(II) and Fe(III) complexes demonstrated the lowest in vitro cytotoxicity among the β ‐diketonates studied. These complexes represent promising low‐toxic alternatives to tin‐based catalysts for biomedical PLA synthesis.