Enzymatic degradation of polylactide stereocopolymers with predominant d-lactyl contents

Abstract

Three stereocopolymers, namely PLA 40, PLA 25, and PLA 10, were synthesized by ring opening polymerization of 40/60, 25/75 and 10/90 l-lactide/ d-lactide feeds, respectively. Polylactide (PLA) films with dimensions of 10×10×0.4 mm were prepared by compression molding and machining. Enzymatic degradation of the films was investigated at 37°C in a pH=8.6 Tris–HCl buffer in the presence of proteinase K. It was confirmed that proteinase K preferentially degrades l-lactyl units as opposed to d-lactyl ones. With only 10% of l-lactyl units in its chains, PLA 10 showed a non negligible enzymatic degradation, while degradation of PLA 40 and PLA 25 was much more pronounced. The higher water uptake ratio in the case of PLA 40 and PLA 25 could also have facilitated the enzymatic attack. On the other hand, there appeared to be a relationship between enantiomeric composition, T g and hydrophilicity of PLA polymers. The more regular the polymer chains, the higher the T g and, the less hydrophilic the polymer. The presence of residual monomer can also lower the T g and enhance the hydroplilicity of PLA polymers.