Divergent Growth

There are two main synthetic rontes used to prepare dendrim (1) divergent growth and (2) convergent growth. 

The method can be illustrated by the preparation of a poly(amido amine), (PAMAM). These are synthesized using either trifnnctional ammonia (NH3) or tetrafunctional ethylene diamine (EDA), NHjCHjCHjNHj, as the core and EDA and methylacrylate (MA) as the branch monomers. The first step is the Michael addition of MA to ammonia or EDA followed by the amidation of each carboxymethyl group using EDA. 

The product (12) can be isolated and pnrified, after which the two-step reaction is repeated to produce the next generation (which will now have six end groups). By repeating these steps, a molecule with up to ten generations can be prepared. 

Although an idealized stracture can be drawn for the dendritic molecule, the greater the amount of growth, the greato the probability of defects. These include incomplete addition reactions, due to steric hindrance, loop formation, retro-Michael reactions, and other side reactions. 

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Scheme 22. a) The divergent growth approach for the preparation of dendritic polyesters. b) The divergent approach for the preparation of dendrimer having 16-hydroxy groups... 

Scheme 23. Polyesters having a pendant double bond prepared by the divergent growth method (adapted from [156])...

Figure 4. Schematic of dendrimer synthetic strategies A. Divergent growth B. Convergent growth ...

Dendritic layer copolymers have also been prepared via the hypercore approach,87 and by divergent growth from the periphery of convergently prepared dendrimers,446 or dendronized polymers.398 This layered dendritic architecture has been used by a number of groups to probe the nature of chirality. 

A fundamental aspect of the divergent-growth approach is the rapid increase in the number of chain-end functional groups. Associated with this is the increase in the number of reactions required to functionalize the chain ends fully. Incomplete reaction of these rapidly increasing terminal groups leads to failure sequences or imperfections in the next generation. These potential difficulties and the lack of control over the number and placement of functionalities at die chain ends led us to reevaluate the synthetic approach to dendritic macromolecules. 

While the divergent-growth approach allowed an entry into the field of dendritic macromolecules and its use continues to produce novel and fascinating structures, the need existed to develop a complementary approach to synthesize dendritic macromolecules. To develop an alternate strategy, we decided to apply the disconnection method in organic... 

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