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 acyloxygen 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 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 (kp, 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 kp/ktr2 ) 125 (where ktr2 is the intermolecular transesterification rate constant) was measured at 80 °C, belonging to the highest values from those determined until now.

Following our previous papers on the mechanism of ε-caprolactone (CL) and l,l-lactide (LA) polymerization initiated by tin(II) octoate [(tin(II) bis(2-ethylhexanoate), (Sn(Oct)2)] in the presence of the hydroxyl-group-containing compounds [H2O, alcohols or hydroxy-carboxylic acids (ROH)] the present work shows that the CL or LA/Sn(Oct)2/primary amine (RNH2) system, in principle, does not differ mechanistically from the CL or LA/Sn(Oct)2/ROH system. 1H NMR, matrix-assisted laser desorption and ionization time-of-flight (MALDI-TOF) mass spectrometric and kinetic studies reveal that, in the first step of polymerization initiated with the Sn(Oct)2/RNH2 mixtures, formation of the α-amido-ω-alcohol (RNH-m-H (R‘OH)) (in which m denotes the repeating unit derived from CL or LA) proceeds. Then, the further steps of polymerization follow with R‘OH as co-initiator. Thus, eventually, the Sn(Oct)2/RNH2-initiated polymerization proceeds by the monomer insertion into the ...-Sn−OR‘ ‘ bond, reversibly formed in the reaction:  ...-SnOct + R‘ ‘OH ⇌ ...-Sn−OR‘ ‘ + OctH, in which R‘ ‘OH is either the low-molar-mass co-initiator (R‘OH) or a macromolecule fitted with the hydroxyl end group (RNH-(m) n -H); OctH stands for 2-ethylhexanoic (octanoic) acid. These interconversions take place throughout the entire polymerization process. The Sn(Oct)2/amino dendrimer (e.g., commercially available DAB−Am-8 and DAB−Am-32, polyamines, fitted with 8 and 32 primary amino groups) system acts in a similar way. Molar masses of the resulting star-shaped polymer, in which polyester chains bear the hydroxyl end groups, are controlled by the monomer and dendrimer concentrations ratio in the feed.

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