Unusual macromolecular architectures: the convergent growth approach to dendritic polyesters and novel block copolymers

A versatile approach to dendritic polyesters and their use in the preparation of two different types of novel macromolecular architectures represent dendritic block copolymers is described. The chemistry developed for the synthesis of dendritic polyesters involves generation growth through an esterification step using dicyclohexylcarbodiimide (DCC) and 4-(dimethylamino)pyridinium p-toluenesulfonate (DPTS) followed by activation of the focal point through removal of its trichloroethyl ester group with zinc in acetic acid. Unusual globular block architectures are then obtained by the controlled placement of different ether and ester chemistries in radial or concentric fashion around a central polyfunctional core. Therefore, a dendritic segment-block copolymer is obtained through the attachment of radially alternating dendritic segments incorporating polyester and polyether chemistry to the polyfunctional core. Similarly, a dendritic layer-block is obtained by the concentric alternation of etherand ester-linked layers in the preparation of the dendritic fragments that are finally coupled to the core moiety. The monomer unit used for the preparation of ether-linked fragments was 3,5-dihydroxybenzyl alcohol, while 2,2,2-trichloroethyl 3,5-dihydroxybenzoate was used for the ester-linked fragments. Analysis of the new block copolymers by NMR spectrometry and size-exclusion chromatography suggests that they are pure and monodispersed. In the absence of large differences in the nature of their chain ends, the glass transition temperatures for the various block copolymers appear to be controlled only by the relative proportion of the ether and ester building blocks rather than by the exact geometry.