Nontoxic and biodegradable star-shaped poly(l-lactide-b-ethylene oxide) (star PLLA−PEO) block copolymers were synthesized by the coupling reaction of two reactive precursors, a hydroxy-terminated 3-armed poly(l-lactide) (star PLLA) and α-monocarboxy-ω-monomethoxypoly(ethylene oxide) (CMPEO). The chemical structure and physical properties of the resulting star-block copolymers were characterized. In the dilute aqueous solutions of star-block copolymers, micellization behavior was investigated, and over specific concentration and specific temperature, the unimer to micelle transition occurred. 1,6-Diphenyl-1,3,5-hexatriene (DPH) was solubilized in the micelle core, and the absorbance of DPH at 356 nm increased when micelle was formed. In addition, the effective diameter was highly affected by temperature. With increasing temperature, micelle size decreased dramatically. In high-concentration solution, star-block copolymer showed temperature-sensitive sol−gel transition behavior. Over specific concentration, the whole system cannot flow by the packing of micelles. However, with increasing temperature, packing structure was destroyed by the decrease of the micelle volume due to the dehydration and contraction of the PEO chain, and the system flew. In the sol−gel transition phase diagram obtained by the vial tilting method, the critical gel concentration decreased, the boundary curve shifted to the left, and the gel regions were expanded with increasing the molecular weight of PLLA block and PEO block.

Two types of temperature‐sensitive biodegradable three‐arm and four‐arm star‐shaped poly(DL‐lactic acid‐co‐glycolic acid‐b‐ethylene glycol) (3‐arm and 4‐arm PLGA–PEG) were successfully synthesized via the coupling reaction of 3‐arm and 4‐arm PLGA and α‐monocarboxyl‐ω‐monomethoxypoly(ethylene glycol) (CMPEG). In dilute aqueous solutions, star PLGA–PEGs showed the temperature‐ and concentration‐dependent formation and aggregation of micelles over specific concentration and specific temperature. With increasing the molecular weight and the relative hydrophobicity of hydrophobic PLGA block, critical micelle temperature (CMT) decreased. Aqueous solution of 4‐arm PLGA–PEG started to form micelles at lower temperature and showed sharper temperature‐dependent growth in micelle size. These results are due to the enhanced hydrophobicity of PLGA block. On the other hand, at high concentration, two types of 3‐arm and 4‐arm PLGA–PEG showed sol–gel–sol transition behavior as the temperature was increased. The 3‐arm and 4‐arm PLGA–PEG showed sol–gel transition at higher polymer concentrations (above 24 wt %) than the PEG–PLGA–PEG triblock copolymer. As the molecular weight and the relative hydrophobicity of PLGA block increased, the critical gel concentration (CGC) decreased. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 888–899, 2006