Kinetics and mechanism of cyclic esters polymerization initiated with tin(II) octoate. 3. Polymerization of L,L-dilactide

Following our previous papers on the mechanism of cyclic esters polymerization induced by tin(II) octoate (Sn(Oct)(2)) and particularly papers on epsilon-caprolactone (CL), the present work shows that L,L-dilactide/Sn(Oct)(2) does not differ mechanistically from the CL/Sn(Oct)(2) system. Sn atoms bonded through alkoxide groups to macromolecules were also observed by MALDI-TOF mass spectrometry. Formation of the actual initiator from Sn(Oct)(2) and a hydroxy group-containing compound (ROH) was envisaged by kinetic arguments. The appropriate experiments were carried out to show that some "mechanisms" put forward during the past few decades by several research groups were not sufficiently substantiated. Eventually, we conclude that L,L-dilactide/Sn(Oct)(2) polymerization proceeds by simple monomer insertion into the ...-Sn-OR bond, reversibly formed in the reaction ...-SnOct + ROH reversible arrow ...-Sn-OR + OctH, where ROH is either the low molar mass co-initiator tan alcohol, hydroxy acid, or H2O) or a macromolecule fitted with a hydroxy end group. These interconversions take place throughout the whole polymerization process. Sn(Oct)(2) itself does not play an active role in the polymerization.