STEREOCHEMISTRY OF THE RING-OPENING POLYMERIZATION OF (S)-BETA-BUTYROLACTONE

The stereochemical course of the ring-opening polymerization of β-butyrolactone, BL, catalyzed by triethylaluminum/water and diethylzinc/water catalysts was studied using (S)-BL as a stereochemical probe. The (S)-BL monomer, which was prepared in five steps from optically pure poly[(R)-β-hydroxybutyrate], P[(R)-HB], produced by bacteria, had an optical purity in excess of 97%. The stereochemical configuration and isomeric purity of the repeating units in the polymers obtained were determined both from their specific optical rotation and by degradation of the polymers to their component methyl β-by-droxybutyrate units. The isomeric purity of these methyl esters was determined from their 200-MHz1H NMR spectra in the presence of a chiral europium shift reagent, and the diad stereochemical sequence distributions of the polymers were determined by 75.4-MHz13C NMR spectroscopy on appropriate samples. From these investigations it was concluded that (1) the mode of ring opening for an AlEt3/H20 (1/1) catalyst prepared in situ involved primarily cleavage of the bond between the β-carbon and oxygen of the lactone (alkyl cleavage) with inversion of configuration equal to or greater than 93%, (2) the mode of ring opening using the preformed ethylaluminoxane (EAO) catalyst proceeded primarily by bond breaking between the carbonyl carbon and oxygen of the lactone (acyl cleavage) with 85% retention of configuration, and (3) with the ZnEt2/H2O (1/0.6) catalyst prepared in situ ring opening occurred by acyl cleavage with retention of configuration in excess of 97%. The stereoisomeric purities of different molecular weight fractions of the polymers prepared with the EAO catalyst were found to differ dramatically, suggesting that the EAO catalyst prepared in the present study contained more than one type of active site, each of which caused the polymerization of BL to proceed by different ring-opening pathways. © 1990, American Chemical Society. All rights reserved.