Structure and morphology of poly(b-hydroxybutyrate)synthesized by ring-opening polymerization of racemic (R,S)-b-butyrolactone with distannoxane derivatives

Predominantly syndiotactic poly((R,S)-b-hydroxybutyrate)(PHB)was synthesized by ringopening polymerization of racemic b-butyrolactone with distannoxane derivatives as catalysts. We have studied the polymerization of (R,S)-b-BL using distannoxane derivatives as catalysts and the effects of polymerization time on crude yield and molecular weight of the polymers obtained. Then, a more detailed study of the characterization of polymers obtained using hydroxy- and ethoxydistannoxanes was performed. 13CNMR spectroscopy resolved stereosequences in synthetic PHB at the diad level for the carbonyl carbon and at the triad level for the methylene carbon. These analyses show that distannoxane catalysts produce preferentially syndiotactic polyesters (syndiotactic diads fraction from 0.56 to 0.61). Triad stereosequence distribution of PHB samples agrees favourably with the Bernoullian statistical model of chain-end control, where ideally F=4(mm)(rr)/(mr.rm)2=1 for perfect chain-end control. Polymer samples synthesized from distannoxane catalysts are composed of two distinct transition endotherm components with peak temperatures of approximately 42°C and 75°C. The formation of two melting endotherms may be due to the presence of two different crystalline tructures