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.

NMR Analysis of Poly(Lactic Acid) via Statistical Models

“…Thus, there have been a large number of NMR tacticity studies of PLA (mostly in CDCl 3 ) at the tetrad level for CH protons and carbon, and partially at the hexad level for carbonyl carbon [11,12,13,14,15,16,17,18,19,20,21,22,23,24]. The assignments were assisted with quantum chemical calculations of chemical shifts [25,26]. Recently, pyridine-d 5 has been found to have a notable solvent effect on the NMR spectra of PLA due to the pyridine ring current effect, and as a result, better resolved peaks of the methyl proton have been successfully obtained and partially assigned [27].…”

Graphene Oxide-Driven Design of Strong and Flexible Biopolymer Barrier Films: From Smart Crystallization Control to Affordable Engineering

“…Another important task in expanding the applications of PLLA is to promote the intrinsically low heat deformation temperature (50–60 °C), which is far below that required for carrying daily life hot foods and beverages. , Strategies to address this prominent limitation, to date, have mainly involved design and development of extraordinarily thermostable stereocomplex crystals (SCs) based on PLLA and its stereoisomer, i.e., poly­( d -lactic acid) (PDLA). − Compared to homocrystals (HCs) formed in PLLA or PDLA, SCs carry a set of peculiarly strong hydrogen-bonds between the stereoisomer chains, leading to distinguishably high melting temperatures (∼50 °C higher than that of HCs). − The unique stereocomplex nature not only allows for the significant thermal distinction but also affords elucidation of higher resistance to hydrolytic degradation, providing the possibility to regulate the degradation kinetics of PLLA in a green and direct fashion, as reported by our group. − This inspires us to conceive the design of a versatile stereocomplex system penetrated with the impermeable GO nanosheets, allowing for the appealing integration of sufficient heat shielding with the aid of thermostable SCs with enhanced gas barrier ability by the creation of robust “nano-barrier walls”. In a recent effort, we unveiled the decoration of a trace amount of GO nanosheets (0.05 wt %) by the preferred formation of SCs, not only on the enormous basal planes but also at the ultrathin edges of GO, during the melt crystallization at 165 and 180 °C .…”

Ring Opening Polymerization of L-Lactide With Two Different Zinc(ii) Phenoxy-Imine Complexes as Initiators

“…The spectra show three signals at 16, 68.5 and 169 ppm, attributed to the methyl, methine and carbonyl groups, respectively. [44][45][46][47] D-lactyl units in the polymer chain is manifested by the appearance of lateral side signals in the methine and carbonyl signals. 4 The figures show the carbonyl and the methine regions of the 13 C NMR spectra of the polymers obtained with LLA/Zn = 2500 and 500.…”