Polymerization of l,l-Dilactide Initiated by Tin(II) Butoxide

Abstract

The kinetics and mechanism of l,l-dilactide (LA) polymerization, initiated with tin(II) butoxide (Sn(OBu)2) and carried out in THF solvent (from 20 to 80 °C) or in bulk (at 120 °C), were studied. Polarimetric and size exclusion chromatography (SEC) measurements showed that initiation was fast and quantitative; termination and intramolecular transesterification (backbiting) were not observed. According to the 1H NMR and MALDI-TOF spectra analysis, both alkoxide groups in Sn(OBu)2 were converted into poly(l-lactide) (PLA) growing chains, and monomer addition proceeded with the acyl−oxygen bond scission. SEC and osmometric measurements revealed that number-average molar masses (Mn) of PLA chains were equal to the ratio 144.13([LA]0 − [LA])/2[Sn(OBu)2]0 in the range of Mn from 103 up to ≈106. Propagation was first order (internally) in LA; it was also approximately first order in initiator (at least for [Sn(OBu)2]0 > 10-3 mol L-1). The rate constant of propagation (kp, for one macroalkoxide chain) was equal t...