A quantitative method for determination of lactide composition in poly(lactide) using H-1 NMR

A method has been developed to quantitatively determine the composition of D-lactide and meso-lactide stereoisomer impurities in poly(lactide) containing predominantly L-lactide. In this method, the stereosequence information obtained from a few web-resolved resonances in the H-1 NMR spectrum representing RR and R stereogenic defects is used. The D-lactide and meso-lactide as minor components lead to RE and R stereogenic defects, respectively, which influence the isotactic chain length distribution and hence affect the polymer properties. Analytical equations relating the stereosequence probability to the lactide feed composition are not available due the complicated kinetics involved for the melt polymerization; viz. the preference for syndiotactic lactide addition decreases with reducing residual lactide concentration in the batch process. Hence, empirical correlations were determined by least-squares fit to the predictions for the specific stereosequence probabilities provided by Monte Carlo calculations of a number of lactide stereocopolymerizations. The Monte Carlo calculations simulate the kinetics observed for melt polymerization at 180 degrees C catalyzed by Sn(II) bis(2-ethyltrexanoate) (Sn(II) octoate) in a 1:10 000 catalyst/lactide ratio.