Synthesis, characterization and crystallization behaviour of stereoregular poly-β-hydroxyoctanoate

Abstract

(S)-β-Pentyl-β-propiolactone (( S)-PPL) was prepared in five steps with an optical purity in excess of 97% starting from the optically pure copolymer produced by Pseudomonas oleovorans when grown with n-octanoic acid. The copolymer contained approximately 85 mol% of β-hydroxyoctanoate units; x=4, in the structure shown below. ▪ The remainder of the copolymer was hexanoate, x = 2, and decanoate, x = 6, units. (S)-PPL was polymerized to the opticaly active homopolymer, poly-β-hydroxyoctanoate (PHO) by the ring-opening polymerization of this lactone with aluminoxane and zinc alkyl catalysts. The stereochemical configurations and isomeric purities of the repeating units of the polymers obtained were determined by degrading the polymers to methyl β-hydroxyoctanoate and analysis by 250 MHz 1H n.m.r. spectroscopy of the complexes of these methyl esters with a chiral europium shift reagent. The diad stereochemical sequence distributions of the polymers were determined by 50.3 MHz 13C n.m.r. spectroscopy. The isothermal rates of crystallization were determined by differential scanning calorimetry (d.s.c.) for the bacterial copolymer and for two synthetic PHOs: a racemic P[( R,S)-HO], and an optically active P[(R)-HO]. The synthetic P[( R)-HO] had a higher enthalpy of fusion and a faster crystallization rate than the racemic PHO, which had a higher enthalpy of fusion and a faster crystallization rate than the bacterial PHO. These differences were explained in terms of the stereoregularities and compositions of the different polymers.