Polymerization of L,L-dilactide initiated by tin(II) butoxide

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 degrees C) or in bulk rat 120 degrees 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 H-1 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 acyloxygen 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 approximate 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 degrees C). Agreement of the kinetic plots determined by SEC and polarimetry indicated that racemization did not Lake place. The k(p)/k(tr2) = 125 (where k(tr2) is the intermolecular transesterification rate constant) was measured at 80 degrees C: belonging to the highest values from those determined until now.