Microstructure of poly(lactide).: Phase-sensitive HETCOR spectra of poly(meso-lactide), poly(rac-lactide), and atactic poly(lactide)

We report the synthesis and H-1, C-13{H-1}, and phase-sensitive HETCOR NMR spectra of atactic poly(lactide), poly(meso-lactide) and poly(rac-lactide). The new and existing spectroscopic data are discussed in terms of four alternative explanations: (i) Kricheldorfs tetrad assignments with the presumption that the adjacent chiral units exert an asymmetric influence on chemical shifts which is propagated in opposite directions along the polymer chain for H-1 and C-13 nuclei, respectively, (ii) alternative tetrad assignments we have recently proposed, (iii) assignments based on pentad stereosequences, and (iv) assignments based on the observation of triad and pentad stereosequences rather than tetrads. We show that the available data are insufficient to unequivocally distinguish between explanations i or iv.