NOVEL METHODS FOR DENDRIMER SYNTHESIS - Designing Dendrimers - page 37

dendronwithadditionalAB n -building block affords a higher generation dendron.
Finally, these dendrons can be attached to a core possessing multifunctional groups
to form a globular architecture.
A notable advantage of this procedure is the requirement of a small number of
reactions per molecule during coupling and deprotection steps. As the result, the
reactions can be driven to completion with only a slight excess of reagent, in contrast
to the case of divergent method. In addition, product purification after the coupling
step can be performed especially by chromatographic purification because of the little
dissimilarity between the desired dendron and the defect molecules. Thus, the
convergent method can be a defect-free synthetic approach. However, since purifi-
cation by chromatography results in some losses and difficulty in a large-scale
synthesis, the convergent method seems to be difficult to apply industrial use.
In both synthetic methods, a tedious stepwise procedure is required: attaching one
generation to the proceeding one, purifying, followed by changing the functional
groups for the next-stage reaction. Therefore, the development of precise and facile
synthetic methods for dendrimers is becoming one of the significant aspects of current
work in this field.

2.2 ACCELERATED APPROACHES

In response to these problems, many researchers have developed accelerated
approaches.
Frechet and coworkers developed the “double-stage” approach, which implies the
grafting of dendrons to the surface of small dendrimers called “hypercores”
(Figure 2.4) [10]. However, the total number of reactions required for preparing
dendron and hypercore is the same as for the conventional synthesis of the final
dendrimer.

FIGURE 2.4 Double-stage synthesis of dendrimer.