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 polymerizat ion induced by tin(II) octoate (Sn(Oct)(2)) and particularly papers on epsi lon-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 bo nded through alkoxide groups to macromolecules were also observed by MALDI- TOF mass spectrometry. Formation of the actual initiator from Sn(Oct)(2) an d a hydroxy group-containing compound (ROH) was envisaged by kinetic argume nts. The appropriate experiments were carried out to show that some "mechan isms" put forward during the past few decades by several research groups we re not sufficiently substantiated. Eventually, we conclude that L,L-dilacti de/Sn(Oct)(2) polymerization proceeds by simple monomer insertion into the ...-Sn-OR bond, reversibly formed in the reaction ...-SnOct + ROH reversibl e arrow ...-Sn-OR + OctH, where ROH is either the low molar mass co-initiat or tan alcohol, hydroxy acid, or H2O) or a macromolecule fitted with a hydr oxy end group. These interconversions take place throughout the whole polym erization process. Sn(Oct)(2) itself does not play an active role in the po lymerization.