NMR analysis and chemical shift calculations of poly(lactic acid) dimer model compounds with different tacticities

In this work, poly(lactic acid) (PLA) dimer model compounds with different tacticities were synthesized and studied in detail by 1 H and 13 C nuclear magnetic resonance (NMR) in three solvents, deuterated chloroform (CDCl 3 )/CCl 4 (20/80 v/v), CDCl 3 and dimethyl sulfoxide -d 6 . All of the peaks in the 1 H and 13 C NMR spectra were assigned with the help of two-dimensional NMR. Although the solvents were different, the tacticity splitting of the dimers showed no significant difference among the solvents. The chemical shifts were calculated and compared with experimental shifts to understand the origin of the tacticity splitting in PLA. Thus, a conformational energy calculation was first performed to determine the energetically stable states in isotactic and syndiotactic dimers by several quantum chemical calculation methods. The 1 H and 13 C chemical shifts were then calculated for each conformation of the model compounds by considering both the conformational energies of the predominant conformation and the chemical shift of each conformation. The observed tacticity splitting of the chemical shifts between isotactic and syndiotactic 1 H and 13 C NMR peaks of the dimers was reproduced particularly well, using the combination of Becke’s three parameter hybrid method for conformational energy calculations and Hartree-Fock for chemical shift calculations. Isotactic and syndiotactic poly(lactic acid) dimer model compounds were synthesized, and studied by 1 H and 13 C NMR in three solvents, CDCl 3 /CCl 4 (20/80 v/v), CDCl 3 and DMSO-d 6 . All the peaks in the 1 H and 13 C NMR spectra were assigned with the help of two-dimensional NMR. The tacticity splitting of the dimers showed no significant difference among the solvents. The chemical shifts were calculated and compared with the experimental shifts to understand the origin of the tacticity splitting in PLA. The observed tacticity splitting of the chemical shifts between isotactic and syndiotactic 1 H and 13 C NMR peaks were reproduced particularly well using the combination of Becke’s three parameter hybrid method for conformational energy calculation and Hartree-Fock for chemical shift calculation.