MORE ABOUT THE POLYMERIZATION OF LACTIDES IN THE PRESENCE OF STANNOUSOCTOATE

The ring-opening polymerization of lactide cyclic monomers in the bulk in the presence of tin(II) 2-ethylhexanoate (stannous octoate or SnOc t(2)) was reexamined under conditions allowing for the end group chara cterization of growing chains by high-resolution H-1-NMR. Data collect ed for low values of the monomer/initiator (M/I) ratio showed that the DL-lactide ring was opened to yield lactyl octoate-terminated short c hains. A cationic-type mechanism involving co-initiation by octanoic a cid was proposed to account for experimental findings. The formation o f a side product, hydroxytin(II) lactate (HTL), was found which appear ed able to initiate lactide polymerization and to yield a high molecul ar weight PLA50 polymer. However the polymerization with stannous octo ate was faster than the HTL one. Anyhow, data suggested that both SnOc t(2) and HTL are likely to act simultaneously as initiators during the polymerization of lactides in the presence of SnOct(2). A complete re action scheme was proposed to account for the presence of the various compounds likely to be formed under these conditions. (C) 1997 John Wi ley & Sons, Inc.