Kinetics and Mechanism of Cyclic Esters Polymerization Initiated with Tin(II) Octoate. Polymerization of ε-Caprolactone and l,l-Lactide Co-initiated with Primary Amines

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.