Effect of ethylene glycol on the end group structure of poly(3-hydroxybutyrate)

Ralstonia eutropha was cultivated in a culture medium supplemented with ethylene glycol (EG), which is known to act as a chain transfer agent in the production of poly(3-hydroxybutyrate) (PHB). The PHB extracted from the bacterial cells was analyzed by 1H and 31P NMR spectroscopies and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The addition of EG exerted a remarkable influence on the mass of production and molecular weight of PHB, and then was found to result in chain transfer and termination reactions. MALDI-TOF MS of the partially hydrolyzed PHB revealed that either succinate or glutarate combined with the hydroxyl terminals of PHB as polymerization starters. From 31P NMR analysis, the carboxyl groups of the succinyls and glutaryls held in the terminals of the isolated PHB were found to be capped with EG, giving the telechelic polyester with hydroxy functionalities. Based on these results, we propose a plausible mechanism of enzymatic polymerization in the microbial PHB synthesis in the presence of EG.