NMR analysis of low molecular weight poly(lactic acid)s

H-1 and C-13 NMR analyses of L- and D.L-lactic acid oligomers were found considerably improved by recording spectra in DMSO-d(6) instead of CDCl3. Systematic comparison of oligomer spectra led to the identification of a neighboring effect which was shown to be linearly additive. Assignment of all the C-13 NMR chemical shifts belonging to the different constitutive units was realized up to the octamer. Furthermore, the well-identified positions of the resonances of chain end units in the H-1 NMR spectra were used to determine the absolute average degree of polymerization of oligomers issued from polycondensation. Data were also used to study oligomers with covalently modified chain ends. Findings are of great interest for the study of the ultimate degradation stages of poly(lactic acid)s.