1H NMR spectroscopy in the analysis and characterization of poly(lactide)

High resolution 500 MHz H-1 NMR spectroscopy is a powerful tool for the analysis and characterization of poly(lactide) (PLA). It accurately provides information about the distribution of a few stereosequences in the polymer whose resonances are well-resolved in the NMR spectrum. Here, the splitting of the methine resonance due to coupling to the methyl protons in the polymer is removed by homonuclear decoupling. C-13 NMR provides complementary stereosequence information, but due to the poor signal-to-noise ratio its accuracy is not comparable to that of H-1 NMR. Through the analysis of the stereosequence distribution in a number of PLA spectra, it was determined that there is a preference for syndiotactic addition during lactide stereo copolymerization. It was shown that the normalized intensity of a few well-resolved resonances in the H-1 spectrum of PLA can be used to quantitatively determine the lactide stereoisomer composition incorporated in the polymer. The change in stereosequence distribution with polymerization can also be conveniently monitored by H-1 NMR. By following the reversible polymerization of D,L-lactide (racemic lactide) in this manner, it was found that the stereospecificity (or reactivity ratio) for syndiotactic addition reduced with increasing polymerization. This increasingly random polymerization is due to the interplay of kinetic and thermodynamic effects. Kinetic effects control the stereochemistry during the early stages of polymerization while equilibrium effects dominate at later stages. The viscosity changes during the melt polymerization additionally influence the stereochemistry.