The anionic ring-opening polymerizations of (R,S)-beta-butyrolactone and (S)-beta-butyrolactone initiated with tetrabutylammonium salts or carboxylic acids as initiators were investigated. Poly [(R,S)-3-hydroxybutyrate] and poly [(R)-3-hydroxybutyrate], with relatively high molecular weights (170,000 and 35,000, respectively) and low molecular weight distributions, were synthesized. Moreover, biomimetic poly(3-hydroxybutyrate)s with well-defined microstructures and predictable iso-dyad contents were obtained.