Determining stereochemical relationships: Synthesis of poly(lactide) hexad

The polylactides are environmentally benign polymers with applications as biodegradable and bioresorbable materials. The physical properties of polylactides depend on the crystallinity of the polymer, which in turn are determined by the regularity of the distribution of stereocenters in the backbone of the polylactide chain. A powerful tool for determining the regularity of polymers is the use of Nuclear Magnetic Resonance (NMR) in conjunction with well-defined model compounds and modeling. To date, interpretations of the NMR spectrum of polylactide have often been contradictory.;Poly(lactide) hexads of known stereochemistry were synthesized using an iterative procedure and characterized by NMR to firmly establish the NMR assignments. Comparisons of the 13C NMR spectra of various hexads and shorter oligomers allowed assignment of the methine region of the spectra. A simple additivity model that considers the effects of neighboring and next-nearest neighboring stereocenters provides reasonable agreement with the experimental results.