Stereoregularity of Poly(lactic acid) and their Model Compounds as studied by NMR and Quantum Chemical Calculations

Abstract

In order to understand the origin of the tacticity splitting in the NMR spectra of poly(lactic acid), monomer and dimer model compounds were synthesized and their 1H and 13C NMR chemical shifts were observed. Two stable conformations were obtained from Ramachandran map calculated as a function of the internal rotation angles for the monomer model using Gaussian 09 calculations. Four preferred conformations were selected and optimized for each dimer model. The conformations of neighboring residues were energetically interdependent. The 1H and 13C chemical shifts for dimer model compounds were calculated by averaging the occurrence probabilities obtained from the optimized conformational energies and the calculated chemical shift of each conformation. It was confirmed that the solvent effect on the tacticity-dependent relative chemical shifts was small from the NMR experiments of the model compound observed in different solvents, dimethyl sulfoxide, chloroform, and chloroform/carbon tetrachloride (20/80 v/v) mixture. Good agreement between observed and calculated chemical shifts was obtained for the relative chemical shifts of isotactic and syndiotactic 1H and 13C NMR peaks of the dimer model compounds. The observed tacticity splitting of poly(lactic acid) at the diad level was rationalized on the basis of these chemical shift calculations.