Cyclic and Linear Polyhydroxylbutyrates from Ring-Opening Polymerization of β-Butyrolactone with Amido-Oxazolinate Zinc Catalysts

A series of amido-oxazolinate zinc complexes have been employed for the ring-opening polymerization of rac-β-butyrolactone (BBL). Experimental results show that these complexes efficiently catalyze the reactions, yielding cyclic poly(3-hydroxybutyrate)s (PHBs) with high molecular weights (Mn up to 197 kg/mol) and relatively low dispersity. In comparison, in the presence of alcohol cocatalysts, the zinc-catalyzed 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-cyclohexanediol and 1,4-benzenedimethanol results in the formation of telechelic polyester diols. The thermal analysis by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) shows higher thermal resistance of cyclic PHBs than linear ones.