The recently discovered ring opening polymerization (ROP) of beta butyrolactone (beta BL) using Cr-III(salphen) as catalyst converts racemic beta-BL to isotactic enriched poly(hydroxybutyrate) (PHB) These achiral complexes arrange themselves in a dimeric sandwich-like structure entrapping the growing polymer chain and the monomer The polymerization mechanism discussed based on previous DFT calculations is supported by kinetic studies in here Furthermore, the influence of different substitution patterns in salphen complexes on polymerization of beta-BL was investigated In addition, effects of different polymerization starters and additives on the broad molecular weight distribution of PHB have been investigated which show the limitations of this catalysis