Lewis acid catalyzed polymerization of L-lactide. Kinetics and mechanism of the bulk polymerization

The kinetics of the L-lactide bulk polymerization was studied using tin(II) bis(2-ethylhexanoate) and zinc bis(2,2-dimethyl-3,5-heptanedionato-O,O'). Up to 80% conversion, the rate of polymerization using tin(II) bis(2-ethylhexanoate) is higher than that with the zinc-containing catalyst, while at conversions beyond 80%, the latter catalyst has the higher rate of polymerization. Crystallization of the newly formed polymer has an accelerating effect on the polymerization. The difference in the rate of polymerization at high conversions for the two catalysts is caused by a difference in crystallinity of the newly formed polymer. Contaminants in the catalyst and monomer are the true initiators in these L-lactide polymerizations. Initiation as well as polymerization proceeds through a Lewis acid catalyzed transesterification reaction between an activated lactone and a hydroxyl group.