Preparation of Polymers with Controlled Molecular Architecture. A New Convergent Approach to Dendritic Macromolecules

The novel convergent growth approach to topological macromolecules based on dendritic fragments is described The polyether dendritic fragments are prepared by starting from what will become the periphery of the molecule and progressing inward. In the first step, 2 mol of a benzylic bromide is condensed with the two phenolic groups of the monomer, 3,5-dihydroxybenzyl alcohol, under phase-transfer conditions. After transformation of the benzylic alcohol functionality of the growing molecule into the corresponding bromide, the procedure is repeated with stepwise addition of the monomer followed again by activation of the benzylic site. After several generations of growth, the resulting dendritic wedges, in their benzylic bromide form, can be coupled to a polyfunctional core such as 1,1,1-tris(4'-hydroxyphenyl)ethane to form the final hyperbranched macromolecule. Unique features of the convergent approach include the control over the nature and placement of the groups that are placed at the periphery of the molecule and the fact that each growth step only involves reaction at a single site of the growing macromolecule. The dendrimers can be purified by normal flash chromatography and are fully characterized by use of a combination of spectroscopic and chromatographic techniques. They double their molecular weight at each generation growth step, become progressively denser and more compact, and have a very low polydispersity. The scope and versatility of the “convergent” approach is compared to the more established “divergent” approach to dendritic macromolecules.