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 1 H 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 ( M n ) of PLA chains were equal to the ratio 144.13([LA] 0 − [LA])/2[Sn(OBu) 2 ] 0 in the range of M n from 10 3 up to ≈10 6 . 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 ( k p, for one macroalkoxide chain) was equal to 0.5 mol -1 L s -1 (THF, 80 °C). Agreement of the kinetic plots determined by SEC and polarimetry indicated that racemization did not take place. The k p / k tr2 = 125 (where k tr2 is the intermolecular transesterification rate constant) was measured at 80 °C, belonging to the highest values from those determined until now.