Polylactones 48. SnOct₂-Initiated Polymerizations of Lactide:  A Mechanistic Study

Sn(II)2−ethylhexanoate (SnOct2) was reacted with 2 equiv of benzyl alcohol at 20 °C, and a liberation of octanoic acid in a rapid equilibration was found. When the temperature was raised to 180 °C in steps of 40 °C, esterification of benzyl alcohol and octanoic acid was observed up to a conversion of 90%. This esterification was catalyzed by Sn(II) and not by the protons of the free octanoic acid. The esterification liberated Sn(OH)2, which finally precipitated in the form of SnO. This precipitate proved to be a good initiator for the polymerization of lactide above 120 °C. Analogous results were obtained with 1-decanol, triethylene glycol monomethyl ether, and neopentane diol. When SnOct2 was reacted with methyl lactate at 20 °C, a chelate complex of one Sn with two lactate ligands was formed, liberating almost all octanoic acid. At higher temperatures, esterification of octanoic acid with methyl lactate and transesterification of the methyl group (yielding methyl octoate) were observed. The latter esterification was predominant at higher temperatures, and a Sn lactate (1:1) complex precipitated under all circumstances. This complex proved to be an initiator for polymerizations of l-lactide. Polymerization of l-lactide initiated with neat SnOct2 at 180 °C yielded polylactides having octanoate end groups, and the molecular weights paralleled the monomer/initiator ratio.