Kinetics and mechanism of cyclic esters polymerization initiated with tin(II) octoate. Polymerization of epsilon-caprolactone and L,L-lactide co-initiated with primary amines

摘要

Following our previous papers on the mechanism of e-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-earboxylic 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. H-1 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 alpha-amido-omega-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 reversible arrow ...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.