PREPARATION OF PREDOMINANTLY SYNDIOTACTIC POLY(BETA-HYDROXYBUTYRATE) BY THE TRIBUTYLTIN METHOXIDE CATALYZED RING-OPENING POLYMERIZATION OF RACEMIC BETA-BUTYROLACTONE

In this report, we have documented the ability of tributyltin methoxide, SnBu3OCH3, to catalyze the ring-opening polymerization of beta-butyrolactone, BL, with a preference for syndiotactic (syn) placement. This has therefore resulted in the chemical synthesis of a new stereoisomeric form of poly(beta-hydroxybutyrate), PHB, specifically syn-PHB, which has predominantly alternating R and S stereocenters. The polymerization of racemic BL catalyzed by SnBU30CH3 was carried out at temperatures of 40, 60, 75, and 90-degrees-C. The yields of hexane/ether (1/1) insoluble PHB ranged from 24 to 69%, with the highest yield for the polymerization temperature of 75-degrees-C. An analysis of the stereochemical sequence distribution for the syn-PHB stereoisomers by C-13 NMR allowed the degree of syndiotacticity achieved as a function of the polymerization conditions to be quantitated. Thus, it was determined that the racemic (r) diad fractions were 0.70, 0.66, 0.63, and 0.61 for the polymerization temperatures of 40, 60, 75, and 90-degrees-C, respectively. Therefore, the degree of syndiotacticity decreased for higher polymerization temperatures, as was anticipated; however, the degree of change was unexpectedly small over the 50-degrees-C temperature range investigated. The M(n) values of the syn-PHB samples ranged between approximately 2500 and 5300 and were determined both by H-1 NMR spectroscopy end group analysis (after derivatization of the products with diazomethane) and by VPO. These M. values were approximately 72% of that calculated by the assumption that the number of polymer chains is equivalent to the number of moles of catalyst added. In addition, by analysis using H-1 NMR of the number of methyl ester end groups which were present on the syn-PHB samples before and after diazomethane derivatization, it was determined that the ratio of carboxylic acid to methyl ester end groups ranged from 2.5 to 3.1. These results suggest that lactone polymerization by SnBu3OCH3 is not solely initiated by an insertion mechanism between the tin-methoxide bond. The syn-PHB polymers when isolated by solution precipitation and analyzed by DSC showed three distinct endotherm component peaks with peak temperatures of approximately 47, 62, and 79-degrees-C where the dominant endotherm component was, in all cases, at approximately 62-degrees-C. Dramatic changes in the endothermic melting transitions were observed when the samples were melt annealed as opposed to solution precipitated. The X-ray diffractogram for the 70-syn-PHB sample and the calculated d-spacings were compared to those of natural-origin (R)-PHB. Specific d-spacings measured at 7.52,5.40, 3.83, and 3.11 angstrom for the 70-syn-PHB sample were not observed for (R)-PHB and are evidence for the existence of a different crystalline structure than that determined for an isotactic PHB stereoisomer. FTIR analysis of the 70-syn-PHB sample revealed spectral features which were unique to the conformational order and crystalline structure of this new PHB stereoisomer. Specifically, tbe appearance of a new band. at approximately 1206 cm-1, with a notable increase in the spectral absorption band at approximately 1103 cm-1.