Thermally Induced Phase Transition of Carborane-Functionalized Aliphatic Polyester Dendrimers in Aqueous Media

Three recently reported aliphatic polyester dendrimers of generations 3, 4, and 5, having 4, 8, and 16 carborane cages within their interior, respectively, were found to exhibit thermally induced, reversible precipitation in aqueous solution. The cloud-point temperatures for these molecules were observed to be between 40 and 80 degrees C, depending on the dendrimer generation. The three dendrimers investigated have a hydroxyl-to-carborane ratio of 8:1, which provides the ideal balance between the hydrophobic interior and the hydrophilic exterior to enable the thermally induced phase transition to occur. It was found that repeated heating/cooling cycles resulted in a decreasing cloud-point temperature and increased dendrimer solubility. Additionally, the effect of pH on the cloud point was investigated, indicating no significant changes as long as the dendrimers remained stable. Size-exclusion chromatography indicated that dendrimer degradation was occurring at pH above 7.0.