POLYLACTONES .28. SYNDIOTACTIC POLY(BETA-D,L-HYDROXYBUTYRATE) BY RING-OPENING POLYMERIZATION OF BETA-D,L-BUTYROLACTONE WITH BUTYLTIN METHOXIDES

Predominantly syndiotactic poly(beta-D,L-hydroxybutyric acid) was prepared by ring-opening polymerization of beta-D,L-butyrolactone with butyltin trimethoxide, dibutyltin dimethoxide, and tributyltin methoxide as initiators. Obviously, due to steric hindrance the reactivity of the catalysts decreases in the given order, whereas the stereoselectivity of the polymerizations increases. The highest yields (up to 92 %) were obtained with Bu2Sn(OMe)2 but the molecular weights were relatively low (less-than-or-equal-to 10 000 g/mol) in all cases. HO and CO2CH3 end groups were identified by H-1 and C-13 NMR spectroscopy whereas CO2H end groups were never found. All initiators yielded preferentially syndiotactic polyesters. Variation of the reaction temperature with BU2Sn(OMe)2 and Bu3SnOMe had a strong influence on the stereoselectivity of the polymerization. At 100-degrees-C perfectly atactic amorphous poly(beta-D,L-HBu) was obtained, whereas the crystalline sample prepared at -17-degrees-C showed a ratio of isotactic to syndiotactic diads (i/s ratio) of 0.2/1.0. Fractionation of this sample yielded a crystalline poly(beta-D,L-HBu) with an i/s ratio of 0.1/1.0 and a melting temperature of 143-degrees-C. The WAXS pattern, IR spectrum, and C-13 NMR spectrum of this syndiotactic polyester are quite different from those of the biogenic, isotactic material.