Stereocomplex formation between enantiomeric PLA-PEG-PLA triblock copolymers: characterization and use as protein-delivery microparticulate carriers

Two enantiomeric triblock ABA copolymers composed of poly(L-lactide)-poly(ethylene glycol)-poly(L-lactide) (PLLA-PEG-PLLA) and poly(D-lactide)-poly(ethylene glycol)-poly(D-lactide) (PDLA-PEG-PDLA) were synthesized with two different middle-block PEG chain lengths by ring-opening polymerization of L-lactide and D-lactide in the presence of PEG, respectively. A pair of enantiomeric triblock copolymers were combined to form a stereocomplex by a solvent-casting method. The triblock copolymers and their stereocomplexes were characterized by ¹H- and ¹³C-NMR spectroscopy and gel permeation chromatography. Their crystalline structures and crystalline melting behaviors were analyzed by the wide-angle X-ray diffraction method and differential scanning calorimetry. The stereocomplex formed between a pair of enantiomeric triblock copolymers exhibited a higher crystalline melting temperature with a distinctive 3/1 helical crystalline structure. PLLA-PEG-PLLA and its stereocomplex with PDLA-PEG-PDLA were used to fabricate a series of microspheres encapsulating a model protein drug, bovine serum albumin (BSA). They were prepared by a double-emulsion solvent-evaporation method. The morphological aspects of the microspheres were characterized and BSA release profiles from them were investigated.