The physical properties of poly(lactic acid) (PLA) are largely influenced by its stereoregularity and stereosequence distribution. NMR is regarded as the best method to study polymer stereoregularity, and many NMR studies of the stereosequence distribution of PLA have been previously reported. Most of these NMR analyses were carried out in deuterated chloroform at the tetrad level for CH proton and carbon, and partially at the hexad level for the carbonyl carbon. Thus far, no detailed 1H or 13C NMR assignments of CH3 group have been reported [Fig.1(a)]. In this study, enhancement of the peak separation has been attempted by changing NMR solvents. We found out that pyridine showed a remarkable solvent effect on the NMR spectra (due to ring current effect) and as a result, well-resolved peaks of the methyl proton were successfully obtained [Fig.1(b)]. The peak assignment has been partially made and more work is in progress. Several methods will be employed to enhance our understanding of the NMR spectra of PLA, including the analysis of the relative peak intensities of individual tetrad sequences for PLA samples prepared under different polymerization conditions, monitoring of the peak behavior by changing the ratio of mixed solvents, and two-dimensional NMR experiments.

NMR analysis of the stereoregularity of poly(lactic acid) through the use of selective solvents

255th National Meeting (2018)

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