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 polyme rization of L-lactide in the presence of PEG2000, a bifunctional OH-te rminated poly(ethylene glycol) (<(M)over bar (n)> = 1800) using Zn met al or CaH2 as catalyst. The resulting copolymers were analyzed by vari ous techniques including H-1 and C-13 nuclear magnetic resonance, size -exclusion chromatography, X-ray diffractometry, optical microscopy, a nd differential scanning calorimetry. NMR spectra showed that Zn and C aH2 catalyzed lactide polymerization under the selected experimental c onditions to yield long PLA blocks at both ends of the PEG macroinitia tor. The copolymer composition was comparable to that of the feed even after purification by dissolution/precipitation. Hydrolysis of the tr iblock copolymers conclusively showed that the early stages of ester b ond 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 analys is, this layer was composed of PLA/PEO/PLA copolymers bearing short PL A blocks which resulted from the degradation of parent long blocks. It remained attached to the surface via physical interactions within hyd rophobic microdomains composed of clustered PLA segments.