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

PLA/PEO/PLA triblock copolymers bearing short poly(L-lactic acid) blocks, with the number average degree of polymerization of each PLA block <(DP)over bar (PLA)> = 2, 4, 8, and 12, were synthesized by ring opening polymerization of L-lactide initiated by poly(ethylene glycol) in the presence of CaH2. The length of PEO blocks was varied by using parent PEG of different number average degrees of polymerization <(DP)over bar (PEG)> = 141 26, and 49, respectively, according to SEC and MMR. SEC and H-1 and C-13 NMR showed the resulting triblock copolymers did not contain any detectable PLA homopolymer as side product. The use of DMSO-d(6) as solvent for NMR analyses, instead of CDCl3, greatly enhanced the resolution and permitted the distinction of the signals due to the last two constitutive units located at both ends of each PLA block. Data obtained by FTIR spectroscopy and X-ray diffractometry suggested that PEO and PLA blocks were phase separated even for copolymers with very short PLA blocks. Optical microscopy and DSC showed that an increase in the length of PLA blocks led to a decrease in the crystallinity of PEO blocks up to disappearance. Hydrolysis was carried out in DMSO/D2O in the presence of trifluoroacetic acid and monitored by NMR. Data suggested that intrachain and PEO/PLA connecting ester bonds were cleaved at comparable rates in the selected homogeneous medium.