Polylactones 36. Macrocyclic Polymerization of Lactides with Cyclic Bu₂Sn 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 °C. The resulting molecular weights were higher than expected from the monomer/initiator (M/I) ratio and do not parallel it. 1H 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. 1H 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.