MOLECULAR MODELING OF HELICAL AND EXTENDED-CHAIN POLYHYDROXYBUTYRATESAND POLYTETRAMETHYLENE SUCCINATE

Molecular modelling (MM) has been employed to investigate the crystall ine chain conformations of a series of related aliphatic biodegradable polyesters. The bacterial (isotactic) poly (3-hydroxybutyrate) helica l conformation of 2.98 Angstrom advance/repeat unit can be cold-drawn to a nearly extended chain conformation possessing an advance of 4.6 A ngstrom/repeat unit versus 4.8 Angstrom for complete extension. Syndio tactic PHB has an extended helical chain conformation of 3.85 Angstrom per repeat on account of steric interference between substituents of contiguous R and S units. Molecular modelling techniques have confirme d this result and suggest that up to 25% of S configured units spread randomly or in a stereoblock fashion are isomorphous with the R repeat s of isotactic helix segments. From X-ray fibre diffraction data, a lo w energy crystalline chain conformation for elastomeric poly(4-hydroxy butyrate), P4HB, with a pitch of 11.9 Angstrom per 2 repeat units has been proposed. A synthetic biodegradable polyester, poly(tetramethylen e succinate) or Bionolle(R), undergoes a reversible strain-induced cry stal modification in which the fibre identity period increases from 10 .91 Angstrom to 11.97 Angstrom. Plausible conformers are proposed resp ecting these observed pitches. Modelled P4HB and Bionolle(R) chains in the 'all trans' planar zigzag form correspond to a pitch of 12.4 Angs trom, which overestimates the observed fibre repeat values. Analysis o f the crystalline structure of synthetic and bacterial polyesters give s potential insight into their biodegradability, which is controlled b y specific enzyme binding characteristics. (C) 1998 Elsevier Science L td. All rights reserved.