Thermoreversible gelation of poly(ethylene oxide) biodegradable polyester block copolymers. II

Poly(ethylene oxide)-b-poly(L-lactic acid) (PEO-PLLA) diblock copolymers we re synthesized via a ring opening polymerization from poly(ethylene oxide) and L-lactide. Stannous octoate was used as a catalyst in a solution polyme rization with toluene as the solvent. Their physicochemical properties were investigated by using infrared spectroscopy, H-1-NMR spectroscopy, gel per meation chromatography, and differential scanning calorimetry, as well as t he observational data of gel-sol transitions in aqueous solutions. Aqueous solutions of PEO-PLLA diblock copolymers changed from a gel phase to a sol phase with increasing temperature when their polymer concentrations are abo ve a critical gel concentration. As the PLLA block length increased, the ge l-sol transition temperature increased. For comparison, diblock copolymers of poly-(ethylene oxide)-b-poly(L-lactic acid-co-glycolic acid) [PEO-P(LLA/ GA)] and poly(ethylene oxide)-b-poly(DL-lactic acid-co-glycolic acid) [PEO- P(DLLA/GA)] were synthesized by the same methods, and their gel-sol transit ion behaviors were also investigated. The gel-sol transition properties of these diblock copolymers are influenced by the hydrophilic/hydrophobic bala nce of the copolymer, block length, hydrophobicity, and stereoregularity of the hydrophobic block of the copolymer. (C) 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2207-2218, 1999.