Polylactones .36. Macrocyclic polymerization of lactides with cyclic Bu(2)Sn initiators derived from 1,2-ethanediol, 2-mercaptoethanol, and 1,2-dimercaptoethane

Three cyclic initiators were prepared from dibutyltin oxide and 1,2-ethanediol, 2-mercaptoethanol, and 1,2-dimercaptoethane. Mass spectra demonstrated in agreement with literature data that the tin compounds obtained from 1,2-ethanediol and 2-mercaptoethanol form dimeric 10-membered rings. Polymerizations of L-lactide and racemic D,L-lactide were conducted in bulk at 120 degrees C. The resulting molecular weights were higher than expected from the monomer/initiator (M/I) ratio and do not parallel it. H-1 NMR spectra also proved that the lactides exclusively react with the Sn-O bonds. Consequently the 2,2-dibutyl-2-stanna-1,3-dithiolane is an extremely poor initiator. H-1 NMR spectra of reaction mixtures showed that the propagation is more rapid than the initiation. The macrocyclic polylactides resulting from the initiation with the ethanediol-based stannadioxolane can be cleaved with 1,2-dimercaptoethane, so that the dibutyltin group is selectively removed. GPC measurements before and after this ring cleavage suggest that the macrocyclic polylactides are mainly monomeric, although the initiator itself is mainly a dimeric (10-membered) ring.