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

Abstract

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.

Polylactide/Poly(ω-hydroxytetradecanoic acid) Reactive Blending: A Green Renewable Approach to Improving Polylactide Properties

“…To obtain direct evidence that in situ synthesized PLA- b -PC14 interfacial compatibilizers were formed herein, two-dimensional (2D) NMR experiments ( 1 H– 13 C HSQC and HMBC) were performed. Both 1 H– 13 C HSQC and HMBC have proven to be useful in elucidating the microstructure of block polymers. , In HSQC and HMBC spectra, most sites throughout the interior of the polymers were detected as a single set of positional averaged resonances and are assigned according to literature values. − A few minor peaks were detected in both unreacted and reacted samples and assigned to the PLA hydroxyl end-group unit including 1 H peaks at 4.35 and 1.4 ppm and 13 C peaks at 66.75 and 16.82 ppm for the methine and methyl groups of PLA, respectively. From HMBC, signals due to the carboxyl end unit of PLA are also observed.…”

Poly(l-Lactic Acid)/Pine Wood Bio-Based Composites

“…Numerous studies have been conducted on PLLA/wood blends in order to investigate their molecular structure, crystallization kinetics, thermal, and mechanical properties [ 16 , 17 , 18 , 19 , 20 , 21 , 22 , 34 ]. Researchers also focused on testing different PLLA formulations using solid-state NMR techniques, mainly high-resolution 13 C and 1 H NMR spectra and carbon spin-lattice relaxation times T 1 ( 13 C) [ 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 ]. However, according to the best of our knowledge, very limited research describes molecular dynamics of PLLA-based modifications using the temperature studies of spin-lattice relaxation times ( T 1 ) [ 47 ].…”

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].…”