Synthetic poly[(R,S)-beta-hydroxyheptanoate] (PHH) and poly[(R,S)-beta-hydroxynonanoate] (PHN) were prepared from the ring-opening polymerization of (+/-)-beta-heptanolactone and (+/-)-beta-nonanolactone, respectively, using an in situ trimethylaluminum/water catalyst. Molecular weights of about 550 000 and 750 000 for PHH and PHN, respectively, were determined by GPC. Both PHH and PHN are predominantely isotactic "as synthesized"; i.e., the isotacticity diad indices are between 80 and 82%. Amorphous and crystalline regions could be distinguished in the solid-state C-13 NMR spectra for both polymers which indicated lower motional freedom for the backbone carbons compared to those in the side chain. Melting points of approximately 60-degrees-C and glass transition temperatures between -30 and -45-degrees-C are close to those of bacterial analogues. The unit cell dimensions of PHH are a = 5.26 angstrom, b = 31.25 angstrom, and c = 4.55 angstrom, based on an orthorhombic unit cell and a P2(1)2(1)2(1) space group. Compared to poly(beta-hydroxybutyrate), the compressed fiber repeat with maintainance of the 2(1) helix is attributed to the intramolecular interactions of the extended alkane side chains.