Polylactones. 35. Macrocyclic and Stereoselective Polymerization of .beta.-D,L-Butyrolactone with Cyclic Dibutyltin Initiators

2,2-Dibutyl-1,3-dioxa-2-stannanes prepared from dibutyltin oxide and 1,3-propanediol or neopentanediol were used as cyclic initiators for polymerizations of β-D,L-butyrolactone. Because these initiators were not reactive below 50°C, all polymerizations were conducted in bulk at temperatures 250°C. As evidenced by 13 C NMR spectroscopy, the resulting poly(β-D,L-butyrolactone) is preferentially syndiotactic, but the percentage of syndiotactic diads decreases with increasing reaction temperature. 1 H NMR spectra proved that the insertion of lactones occurred at both Sn-O bonds of the cyclic initiators, and thus, the propagation yields automatically macrocyclic esters. The ring size increases with increasing monomer/initiator ratio and higher conversions. Selective ring opening with 1,2-dimercaptoethane produces linear polyesters with shorter elution times in GPC measurements than the corresponding macrocycles of the same molecular weight. Batchwise copolymerizations with e-caprolactone yielded macrocyclic block copolyesters which after ring-opening with dimercaptoethane turned into linear A-B-A triblock copolymers. However, when mixtures of both lactones were copolymerized at 100°C, random copolyesters were obtained.