Synthesis, characterization, and hydrolytic degradation of PLA/PEO/PLA triblock copolymers with long poly(L-lactic acid) blocks

A series of triblock PLA/PEO/PLA copolymers were synthesized by polymerization of L-lactide in the presence of PEG2000, a bifunctional OH-terminated poly(ethylene glycol) (<(M)over bar (n)> = 1800) using Zn metal or CaH2 as catalyst. The resulting copolymers were analyzed by various techniques including H-1 and C-13 nuclear magnetic resonance, size-exclusion chromatography, X-ray diffractometry, optical microscopy, and differential scanning calorimetry. NMR spectra showed that Zn and CaH2 catalyzed lactide polymerization under the selected experimental conditions to yield long PLA blocks at both ends of the PEG macroinitiator. The copolymer composition was comparable to that of the feed even after purification by dissolution/precipitation. Hydrolysis of the triblock copolymers conclusively showed that the early stages of ester bond cleavage proceeded at random along the PLA blocks. As degradation advanced, a highly swollen hydrogel layer expanded from the surface of a still compact, partially degraded specimen. According to NMR analysis, this layer was composed of PLA/PEO/PLA copolymers bearing short PLA blocks which resulted from the degradation of parent long blocks. It remained attached to the surface via physical interactions within hydrophobic microdomains composed of clustered PLA segments.