Ring -opening polymerization of beta-substituted-beta-propiolactones: Synthesis of biodegradable polymers and stereochemistry study

Biodegradable polymers, for biodegradable plastic utility and drug release biomedical applications, are the primary objectives of this thesis research. We have synthesized poly($\beta$-hydroxybutyrate) (PHB), poly($\beta$-malic acid) (PMA) and their copolymers by the ring-opening polymerization reactions of $\beta$-butyrolactone and $\beta$-benzyl malolactonate. A varity of catalysts were used, including the stereoregulating catalysts, ethyl aluminoxane (EAO) and the product of the in-situ preparation of Et$\sb3$Al/H$\sb2$O, as well as the non-stereoselective catalysts, Et$\sb2$Zn/H$\sb2$O and (5,10,15,20-tetraphenylporphinato)aluminum chloride (TPPAlCl). The degree of stereoregulation of the homopolymers was enhanced by the use of a stereoregulating catalyst, followed by product extraction. The resulting insoluble fractions of PHB and PBML are stereoregular crystalline polymers having high molecular weights, with the former compared with naturally occurring P( (R) -HB) in corresponding properties. The stereoregularity of the synthetic polymers was determined by $\sp{13}$C (75.4 MHz) NMR spectroscopy, differential scanning calorimetry (DSC) and FTIR spectroscopy. The yield of polymerization was optimized by using the Et$\sb2$Zn/H$\sb2$O and TPPAlCl catalysts, resulting the amorphous polymers with relatively low molecular weights. The analysis of copolymer tacticity and comonomer sequences by using $\sp{13}$C-NMR and DSC indicates a random distribution of the copolymer sequences. The stereochemical course of the ring-opening polymerization of BL with the Et$\sb3$Al/H$\sb2$O and Et$\sb2$Zn/H$\sb2$O catalysts was studied using (S) -BL as a stereochemical probe. The (S) -$\beta$-BL, which was prepared in five steps from naturally occurring P( (R) -HB), had an optical purity in excess of 97% as measured by $\sp1$H NMR spectroscopy in the presence of a chiral europium shift reagent, Eu(hfc)$\sb3$. The stereochemical configuration and isomeric purity of the repeating units in the polymers obtained were determined both from their specific optical rotation and by degradation of the polymers to their component methyl $\beta$-hydroxybutyrate units. From our investigations it was concluded that the ring-opening reaction can be carried out by different routes in the Al- and/or Zn-water catalysts, and the two enantiomers, P((R) -HB) and P((S) -HB) were obtained as the result of the configuration retention or inversion. These enantiomers of PHB are very useful for the study of the stereochemistry of synthetic poly($\beta$-lactone)s and their biodegradability