Asymmetric Poly(ethylene glycol) Star Polymers with a Cholic Acid Core and Their Aggregation Properties

Poly(ethylene glycol) (PEG) arms are grafted onto a cholic acid core via anionic polymerization, yielding star-shaped polymers with a unique asymmetric structure with facial amphiphilicity. Well-defined cholic acid-PEG(4) stars (polydispersity index, ca. 1.05) with tunable molar masses (ca. 1000-13000) were obtained and characterized by the use of size exclusion chromatography, MALDI-TOF mass spectrometry, NMR spectroscopy, and thermal analysis. The asymmetric star polymers were found to aggregate differently from cholic acid salt. The critical aggregation concentrations of the star polymers were determined by surface tension measurements, and spherical aggregates of the polymers with different PEG chain lengths were observed by transmission electron microscopy using the freeze-fracture etching technique. The elongated aggregates formed by the sodium salt of cholic acid were also observed. The hydrodynamic diameters of the aggregates were also measured using dynamic light scattering technique. The formation of aggregates makes them interesting systems as potential drug carriers.