Polylactones 48. SnOct(2)-initiated polymerizations of lactide: A mechanistic study

Sn(II)2-ethylhexanoate (SnOct(2)) was reacted with 2 equiv of benzyl alcoho l at 20 degrees C, and a liberation of octanoic acid in a rapid equilibrati on was found. When the temperature was raised to 180 degrees C in steps of 40 degrees C, esterification of benzyl alcohol and octanoic acid was observ ed up to a conversion of 90%. This esterification was catalyzed by Sn(II) a nd not by the protons of the free octanoic acid. The esterification liberat ed Sn(OH)(2), which finally precipitated in the form of SnO. This precipita te proved to be a good initiator for the polymerization of lactide above 12 0 degrees C. Analogous results were obtained with 1-decanol, triethylene gl ycol monomethyl ether, and neopentane diol. When SnOct(2) was reacted with methyl lactate at 20 degrees C, a chelate complex of one Sn with two lactat e ligands was formed, liberating almost all octanoic acid. At higher temper atures, esterification of octanoic acid with methyl lactate and transesteri fication of the methyl group (yielding methyl octoate) were observed. The l atter esterification was predominant at higher temperatures, and a Sn lacta te (1:1) complex precipitated under all circumstances. This complex proved to be an initiator for polymerizations of L-lactide. Polymerization of L-la ctide initiated with neat SnOct2 at 180 degrees C yielded polylactides hari ng octanoate end groups, and the molecular weights paralleled the monomer/i nitiator ratio.