NMR Analysis of Poly(Lactic Acid) via Statistical Models

Abstract

The physical properties of poly(lactic acid) (PLA) are influenced by its stereoregularity and stereosequence distribution, and its polymer stereochemistry can be effectively studied by NMR spectroscopy. In previously published NMR studies of PLA tacticity, the NMR data were fitted to pair-addition Bernoullian models. In this work, we prepared several PLA samples with a tin catalyst at different L,L-lactide and D,D-lactide ratios. Upon analysis of the tetrad intensities with the pair-addition Bernoullian model, we found substantial deviations between observed and calculated intensities due to the presence of transesterification and racemization during the polymerization processes. We formulated a two-state (pair-addition Bernoullian and single-addition Bernoullian) model, and it gave a better fit to the observed data. The use of the two-state model provides a quantitative measure of the extent of transesterification and racemization, and potentially yields useful information on the polymerization mechanism.

Robust and high barrier thermoplastic starch – PLA blend films using starch-graft-poly(lactic acid) as a compatibilizer

“…Furthermore, the PLA displayed two distinctive peaks at approximately 5.2 and 1.5 ppm, which correspond to the hydrogen atoms attached at the carbon backbone (position a) and the methyl group -CH 3 (position b), respectively. 33,34 The spectra of the synthesized St-PLA revealed additional signals that correspond to the grafted PLA oligomer, as well as the lactic acid monomer, on the AGUs. The emergence of an additional signal at approximately 1.5 ppm that was not observed in starch, and the overlapping signal intensity at about 5.2 ppm, were emanating from the grafted PLA oligomer on the AGU of starch.…”

“…Furthermore, the PLA displayed two distinctive peaks at approximately 5.2 and 1.5 ppm, which correspond to the hydrogen atoms attached at the carbon backbone (position a) and the methyl group –CH 3 (position b), respectively. 33,34…”

Compatibility and Thermal and Structural Properties of Poly(l-lactide)/Poly(l-co-d-lactide) Blends

“…The thermal properties, hydrolysis resistance, morphology, and mechanical properties of all kinds of PLAs have been widely studied. − The glass transition temperature ( T g ), melting temperature ( T m ), and melting enthalpy (Δ H m ) of poly­( l - co - d -lactides) and poly­( l - co -meso-lactides) increase with increasing stereochemical purity when the PLA polymers are prepared by nonstereoselective catalysts. ,,, The ROP of pure l - or d -lactide results in isotactic PLA formation; the ROP of rac-lactide may result in isotactic, heterotactic, and atactic PLA chains and the ROP of meso-lactide may result in syndiotactic, heterotactic, and atactic PLA chains . The final stereosequence distribution of PLA polymers depends upon the catalyst used for the polymerization and upon the stereochemical compositions of the feed lactides. − The many properties of PLA are influenced by its stereoregularity and stereosequence distribution, and its stereochemistry can be determined by 1 H and 13 C NMR spectroscopy. ,− …”

On the Mechanism of Electron Beam Radiation-Induced Modification of Poly(lactic acid) for Applications in Biodegradable Food Packaging

“…PLA has two naturally occurring stereoregular forms, PDLA and PLLA [28]. A great deal of current research is focused on characterizing these forms with techniques such as nuclear magnetic resonance [29]. A racemic mixture of PLA leads to an amorphous polymeric structure [28].…”