Convergent-growth synthesis

J.M.J. Frechet (C. J. Hawker, 1990) replaced the divergent synthesis by a convergent growth of a dendritic polymer. The repeatedly employed monomer, 5-hydroxymethyl-l, 3-benzenediol, was 1,3-O-dibenzylatcd with 3,5-bis(benzyloxy)benzyl bromide. The resulting benzyl alcohol containing 7 benzene rings was converted to the benzyl bromide which was... 

In general, convergent methods produce the most nearly isomolecular dendrimers. This is because the convergent growth process allows purification at each step of the synthesis and therefore no cumulative effects of failed couplings are found [85, 89]. Appropriately purified convergent dendrimers are probably the most precise synthetic macromolecules that exist today. 

The first part of this review deals with the synthesis and characterization of dendritic aliphatic polyesters based on 2,2-bis(dimethylol)propionic acid (bis-MPA) and 1,1,1 -tris(hydroxyphenyl)ethane (THPE), as core molecule. The convergent growth approach described earlier was applied. We also present H NMR self-diffusion studies of dendrimers of first, second, and third generation. 

The chosen ABj monomer for the synthesis of die dendritic aliphatic polyesters Dl, D2, and D3 was bis-MPA. In the convergent growth approach, dendrons of certain generations were initially synthesized. In a final step these dendrons were coupled to the polyfunctional core molecule (see Scheme 2). To get an acceptable overall yield it is important that all reactions such as coupling, protection, and deprotection are selective and proceed in high yields since a laige number of steps are involved in the synthesis of the final dendrimers. 

The convergent growth approach showed to be a useful method in the synthesis of monodisperse dendrimers. The use of bis-MPA, as a building block, resulted in dendrimers with simple H NMR and NMR spectra where different generations... 

Scheme 5.4. Synthesis of dendritic wedges to be used in double stage convergent growth .

Hawker, C.J. Erechet, J.M.J. Control of surface functionality in the synthesis of dendritic macromolecules using the convergent-growth approach. Macromolecules 1990, 23, 4726 729. 

The other accelerated approach for dendrimer synthesis, the orthogonal approach, involves convergent growth with two different monomers. The monomers, an AB2 and a CD2 must be carefully selected such that the focal functionalities of each individual monomer will only react with the periphery of the other monomer (B couples only with C and D only with A) thus removing the need for activation reactions. As... 

---