Kinetics and Mechanism of <scp>l</scp>-Lactide Polymerization by Rare Earth Metal Silylamido Complexes: Effect of Alcohol Addition

Abstract

The ring-opening polymerization of l-lactide initiated by rare earth metal silylamido complexes [Ln(OSSO){N(SiHMe2)2}(THF)] (1−3: Ln = Y, Lu; OSSO = 1, ω-dithiaalkanediyl-bridged bisphenolato) was studied. MALDI-TOF mass spectrometry and 1H NMR spectroscopy suggested that the polymerization proceeded via a conventional coordination−insertion mechanism involving silylamide ligand as the initiating group and the cleavage of acyl−oxygen bond of the monomer. A two-stage linear relationship between ln([LA]0/[LA]t) and the polymerization time was observed for the yttrium complex [Y(pdtbp){N(SiHMe2)2}(THF)] (pdtbp = 1,5-dithiapentanediyl-bis{4,6-di-tert-butylphenolato}, 3). In both stages, the polymerization showed first-order kinetics for the monomer concentration. The first-order dependency of the initiator concentration was only observed when the monomer conversion to PLA was less than 50−60%. The aggregation of the active growing polymer chain into dimeric structure occurred in the second stage. In contrast, the in situ generated alkoxide initiator [Ln(OSSO){N(SiHMe2)2}(THF)]/iPrOH showed a different behavior. When 3 was reacted with 2-propanol in 1:2 ratio, the in situ generated alkoxides initiated the living polymerization of l-lactide. Neither aggregation nor intramolecular transesterification was observed over the entire conversion range. Polylactides with controlled molecular parameters (Mn, end groups) and low polydispersities were formed as a result of fast alkoxide/alcohol exchange.