Unambiguous determination of the ¹³C and ¹H NMR stereosequence assignments of polylactide using high-resolution solution NMR spectroscopy

Polylactide (PLA) is a biodegradable polyester formed by the ring-opening polymerization of lactide, the cyclic dimer of lactic acid. Many of the physical properties of PLA are influenced by the amount and distribution of the R and S stereocenters in the polymer chain. NMR spectroscopy is the most common technique used to determine the stereosequence distribution of the polymer. The correct determination of the stereosequence distribution is contingent upon the assignment of the peaks in the NMR spectrum to specific stereosequences. Recently, alternative assignments to the commonly accepted stereosequence assignments have been proposed. If any of these alternative assignments are found to be correct, it would invalidate the conclusions of much of the previous work in understanding the kinetics of lactide polymerization and determining the stereoselectivity of new catalysts. The most significant problem is reconciling the commonly accepted peak assignments, which were based upon statistical probabilities, with contradictory connectivity data observed in a HETCOR NMR experiment. To describe the directionality in PLA, we have modified the nomenclature used to describe directionality in peptides and proteins. For PLA, the end containing the carboxylic group is referred to as the C terminus, and the end with the hydroxyl group is the O terminus. We had previously proposed that the central pairwise relationship (isotactic or syndiotactic, denoted i or s) in the ¹H NMR spectrum is determined by the stereocenter in the lactic acid unit attached to the O terminus and that in the ¹³C NMR spectrum it is determined by the central pairwise relationship of the stereocenter in the lactic acid unit attached to the C terminus. One- and two-dimensional NMR techniques, in Combination with selective isotopic labeling, were used to show that this relationship is correct and that the commonly accepted assignments are correct. In addition, all of the nondegenerate resonances in the ¹H and ¹³C NMR spectrum of polylactide at the tetrad stereosequence level have been assigned.