At this point, it is necessary to confirm the assignments
of CH3 protons. The PLA chain is known to be nonsymmetrical;
thus, the lactic acid unit of an observed
tetrad sequence of the CH carbon may be different from the
tetrad sequence of the CH proton [7]. In this work, we have
checked the assignments by using the peak intensities in
the PLA (L/D ¼ 97/3) sample. In Fig. 5A, the peak with an
asterisk (*) in the PLA (L/D ¼ 97/3) sample gives 2.7% intensity
relative to the main peak. In Fig. 5B, the isi sequence
for the same sample has an intensity of 2.5% relative to the
main peak. Thus, the peak with the asterisk in the 1H
spectrum of CH3 protons in C5D5N has the same relative intensity as the isi peak in the 13C spectrum of CH carbon in
CDCl3; no other peak has this intensity correlation. It is thus
confirmed that the assignment of the peak with the asterisk
in Fig. 5A is the isi sequence