Stereochemical aspects of lactide stereo-copolymerization investigated by 1H NMR:: A case of changing stereospecificity

Poly(lactide) is synthesized by ring-opening copolymerization of various combinations of L-lactide, D-lactide, and meso-lactide. The influence of the stereochemical differences of the three lactides on the kinetics of melt copolymerization was determined by monitoring the change in the polymer stereosequence distribution as a function of conversion. The living stereo-copolymerization catalyzed by Sn(II) bis2-ethylhexanoate (Sn(II) octoate) in 1:10 000 molar ratio at 180 degrees C was investigated by comparing to the values predicted by a reversible polymerization scheme that achieves equilibrium at similar to 95% conversion. The stereosequence distribution was measured by high-resolution 500 MHz H-1 NMR. A preference for syndiotactic addition was observed and is thought to be due to steric hindrance at the growing site of the polymer. Furthermore, the syndiotactic preference decreased as the polymerization proceeded in a batch process. Similar behavior was observed during lactide polymerization catalyzed by butyl Sn(IV) tris2-ethylhexanoate. The increasingly random addition during the polymerization process is due to the interplay of kinetic and equilibrium effects. Kinetic effects control the stereochemistry during the early stages of the polymerization whereas equilibrium effects dominate at later stages. Viscosity changes in the melt additionally influence the stereospecificity by increasing the residence time of the lactide at the active site. Monte Carlo calculations were used for stereosequence predictions because analytical equations are not available for these complicated kinetics.