Double-Stage Convergent Approach for the Synthesis of Functionalized Dendritic Aliphatic Polyesters Based on 2,2-Bis(hydroxymethyl)propionic Acid

The fourth generation tridendron dendrimer based on 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) with 48 hydroxyl groups was synthesized in high yields using N,N'-dicyclohexylcarbodiimide (DCC) for the coupling steps. A double-stage convergent approach reduced the number of synthetic and liquid chromatographic steps required in the synthesis and purification of the final dendrimers. The hydroxyl functional dendrimer was subjected to a variety of surface modifications by reaction with different acid chlorides. The acetonide, hydroxy, acetate, n-octanoate, n-palmitoate, and benzoate end-functionalized dendrimers showed large differences in thermal and solution behavior depending on the nature of their end groups. The glass transition temperature varied from −4 °C for the acetate-terminated dendrimer to +57 °C for the hydroxyl-functionalized dendrimer. Dendrimers terminated with long alkyl chains were highly soluble in hexane and dichloromethane and poorly soluble in water and methanol whereas the hydroxy-terminated dendrimer showed the opposite solution behavior. All surface modified dendrimers were amorphous according to differential scanning calorimetry (DSC) except for the n-palmitoate-terminated dendrimer that showed a distinct melting transition at +28 °C in its DSC trace due to the crystallization of the long alkyl chains.