Characterization of low molecular weight poly(beta-hydroxybutyrate)s from alkaline and acid hydrolysis

In the present paper, low molecular weight poly(beta-hydroxybutyrates) (PHB s), which were obtained by acidic and basic hydrolysis of bacterially produ ced PHB, were characterized by gel permeation chromotography (GPC), H-1, C- 13 and P-31 NMR. The assignments of the resonances of C-13 NMR spectra from unsaturated end and hydroxy end groups were made using heteronuclear multi ple quantum coherence (HMQC) NMR technique. Unsaturation contents and numbe r average molecular weights of the PHBs were determined mainly by H-1 and P -31 NMR. Direct measurement of the molecular weights of PHB has also been c arried out by matrix assisted laser desorption ionization (MALDI). Based on the characterization of these low molecular weight PHBs, the heterogeneous hydrolysis mechanism is discussed and the scission of PHB chains is propos ed to occur at the surfaces and interfaces of lamellar crystals. The unsatu rated end group is formed by dehydration of the chain ends by beta-eliminat ion after ester hydrolysis, which is different from the mechanistic steps o f the formation of unsaturated ends in thermal decomposition of PHB. The co rrelation of thickness of crystal lamellae and the melting points has also been discussed. The plot of the melting point against reciprocal lamellar t hickness for low molecular weight PHB indicates the pronounced effect of th e surface energy of these crystals, which is caused by the large proportion of end groups. (C) 1999 Elsevier Science Ltd. All rights reserved.