A QUANTITATIVE METHOD FOR DETERMINATION OF LACTIDE COMPOSITION IN POLY(LACTIDE) USING H-1-NMR

A method has been developed to quantitatively determine the compositio n of D-lactide and meso-lactide stereoisomer impurities in poly(lactid e) containing predominantly L-lactide. In this method, the stereoseque nce 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 stereog enic defects, respectively, which influence the isotactic chain length distribution and hence affect the polymer properties. Analytical equa tions relating the stereosequence probability to the lactide feed comp osition are not available due the complicated kinetics involved for th e melt polymerization; viz. the preference for syndiotactic lactide ad dition decreases with reducing residual lactide concentration in the b atch process. Hence, empirical correlations were determined by least-s quares fit to the predictions for the specific stereosequence probabil ities provided by Monte Carlo calculations of a number of lactide ster eocopolymerizations. The Monte Carlo calculations simulate the kinetic s observed for melt polymerization at 180 degrees C catalyzed by Sn(II ) bis(2-ethyltrexanoate) (Sn(II) octoate) in a 1:10 000 catalyst/lacti de ratio.