Monodispersed dendritic polyesters with removable chain ends: a versatile approach to globular macromolecules with chemically reversible polarities

A versatile approach to dendritic macromolecules with aromatic polyester inner structure and a readily modified hydrophobic/hydrophilic 'surface' is described. The polyester fragments are prepared by a convergent growth process involving 3,5-bis(benzyloxy)benzoic acid as the 'surface' or chain-ending moiety and trichloroethyl 3,5-dihydroxybenzoate as the monomer unit. The key esterification step is accomplished in high yield using dicyclohexylcarbodiimide and 4-dimethylaminopyridinium toluene-p-sulfonate as condensing agents. The coupling step is followed by activation of the new focal point by removal of the trichloroethyl ester group with zinc-acetic acid. Repetition of this two-step process leads to large dendritic fragments that may be coupled to a polyfunctional core to complete the dendritic macromolecule. The chemistry chosen for this synthesis allows for subsequent selective removal of the numerous benzyl ether chain ends by hydrogenolysis to afford a dendritic macromolecule with phenolic chain ends. Further modification of the chain ends is readily accomplished in processes that effectively transform the initially hydrophobic dendritic molecule into one that is both hydrophilic and water-soluble. These transformations of the 'surface' functionalities are also accompanied by drastic changes in glass transition behaviour.