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 alt ernative explanations: (i) Kricheldorfs tetrad assignments with the presump tion that the adjacent chiral units exert an asymmetric influence on chemic al shifts which is propagated in opposite directions along the polymer chai n for H-1 and C-13 nuclei, respectively, (ii) alternative tetrad assignment s we have recently proposed, (iii) assignments based on pentad stereosequen ces, and (iv) assignments based on the observation of triad and pentad ster eosequences rather than tetrads. We show that the available data are insuff icient to unequivocally distinguish between explanations i or iv.