A series of amido-oxazolinate zinc complexes have been employed for the ring opening polymerization of β-butyrolactone (BBL). Experimental results show that these complexes efficiently catalyze the reactions, yielding cyclic poly(hydroxybutyrate)s (PHBs) with high molecular weights (Mn up to 196 kg/mol) and low dispersity. In contrast, in the presence of alcohol co-catalysts, the zinc-catalyzed ring-opening polymerization (ROP) reactions lead to the formation of linear PHBs end-capped by the alcohol initiator and hydroxylbutyrate. A possible mechanism for cyclic PHBs is proposed, in which the zinc catalysts function as a loose Lewis pair at elevated temperature, followed by a fast propagation through a zwitterionic intermediate. Use of diols such as 1,4-cyclohexane diol and 1,4-benzenedimethanol results in the formation of polyester diols. The thermal properties were studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) to differentiate between the cyclic and linear polyesters
