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, ...