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

Authors: SUGANUMA, KOTO - Tokyo University Of Agriculture & Technology; HORIUCHI, KEN - Teijin Pharma Limited; MATSUDA, HIRONORI - Teijin Pharma Limited; Cheng, Huai; AOKI, AKIHIRO - Tokyo University Of Agriculture & Technology

Submitted to: Polymer Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/24/2012
Publication Date: 6/13/2012
Citation: Suganuma, K., Horiuchi, K., Matsuda, H., Cheng, H.N., Aoki, A., Asakura, T. 2012. NMR analysis and chemical shift calculations of poly(lactic acid) dimer model compounds with different tacticities. Polymer Journal. 44:838-844.

Interpretive Summary: Poly(lactic acid) (PLA) is a well known polymer derived from agricultural resources, such as sugarcane, corn starch, and tapioca products. Dimers, trimers, and other oligomers of PLA usually occur in concentrated PLA solutions and are useful materials in their own right. They can be used as surfactants, as substitutions for waxes and oils in commercial formulations, as building blocks for functional polymers, and as model compounds for PLA. A major determinant of the physical properties of PLA is tacticity. Tacticity has been shown to influence PLA mechanical properties, thermal stability, Tm, Tg, crystallinity, solution stability, and polymer degradability. Tacticity is usually measured with NMR spectroscopy. A better understanding of the effect of tacticity on NMR spectrum is helpful in order to fully take advantage of the power and utility of the NMR method. In this work PLA dimers were synthesized and analyzed via two-dimensional NMR. Full assignments were made of all the observed peaks. Calculations of the chemical shifts were made with several quantum mechanical methods, taking into account the conformation of the molecules. Satisfactory results for the calculated shifts of different tacticities of PLA dimers were obtained with two different calculation methods. The calculations suggested that the tacticity observed in PLA depends on the electronic environments of 1H and 13C nuclei, their electron correlations, and the conformation of the molecules.

Technical Abstract: In this work, PLA dimer model compounds with different tacticities were synthesized and studied in detail by 1H and 13C NMR in three solvents, CDCl3/CCl4 (20/80 v/v), CDCl3 and DMSO-d6. All the peaks in the 1H and 13C 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 in order to understand the origin of the tacticity splitting in PLA. Thus, the conformational energy calculation was first performed to determine the energetically stable states in isotactic and syndiotactic dimers by several quantum chemical calculation methods. The 1H and 13C chemical shifts were then calculated for each conformation of the model compounds by considering both 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 1H and 13C NMR peaks of the dimers were reproduced particularly well using the combination of Becke's Three Parameter Hybrid Method (B3LYP) for conformational energy calculation and Hartree-Fock (HF) for chemical shift calculation.