Location: Commodity Utilization ResearchTitle: Stereoregularity of poly (lactic acid) and their model compounds as studied by NMR and quantum chemical calculations Author
Submitted to: Macromolecules
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/23/2011
Publication Date: 11/3/2011
Citation: Suganuma, K., Horiuchi, K., Matsuda, H., Cheng, H.N., Aoki, A., Asakura, T. 2011. Stereoregularity of poly (lactic acid) and their model compounds as studied by NMR and quantum chemical calculations. Macromolecules. 44:9247-9253. Interpretive Summary: Poly(lactic acid) (PLA) is a commercial polymer derived from agricultural resources, such as sugarcane, corn starch, and tapioca products. It is biodegradable and finds uses in diverse areas, ranging from biomedical (sutures, stents, dialysis media) to bioplastics (agricultural mulch films, compost bags, and food packaging). The properties of PLA depend on its stereoregularity. Thus, poly-L-lactide has a melting temperature of about 175°C, but a blend of poly-L-lactide and poly-D-lactide has a melting point that is 40-50° higher. The stereoregularity of PLA is usually achieved with NMR spectroscopy; whereas the NMR spectra of PLA have been mostly assigned, the origin of the different peaks due to stereochemical sequences (“tacticity splitting”) is not previously known. It is important to gain a better understanding of the structure/chemical shift correlation in order to maximize the information available from the NMR data. In this work monomer and dimer model compounds were synthesized and their 1H and 13C chemical shifts observed experimentally and also calculated via a quantum chemical method. The good agreement between the observed and the calculated results suggest that the origin of tacticity splitting in PLA dimer is due to both time-averaged conformations and the chemical shifts of the conformations. An attempt has also been made to rationalize the tacticity splitting of PLA polymer at the diad level on the basis of these chemical shift calculations.
Technical Abstract: In order to understand the origin of the tacticity splitting in the NMR spectrum of poly(lactic acid), monomer model compound and dimer model compounds (both isotactic and syndiotactic) were synthesized and their 1H and 13C NMR chemical shifts observed. Two energetically stable conformations were obtained from Ramachandran map calculated as a function of the internal rotation angles for the monomer model using Gaussian 09 quantum chemical calculations. Four preferred conformations were selected and optimized for each isotactic and syndiotactic dimer model. The 1H and 13C chemical shifts for isotactic and syndiotactic dimer model compounds were calculated by averaging the occurrence probabilities obtained from the optimized conformational energies and the calculated chemical shift of each conformation. 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.