Since the first reported synthesis by Vögtle et al. [1] in 1978, dendrimers have received
increasing interest, well documented by an exponential growth of published
articles [2]. This interest is connected with properties like globular shape
and defined size resulting from low dispersities of these macromolecules with
regular and highly branched three-dimensional architectures.
Dendrimers are synthesized by the repetition of a sequence of reaction steps
leading after each repetition to a dendrimer one generation larger. Two different
synthetic approaches are known depending on the starting point: a divergent
one, building the dendrimer layer-by-layer from a central core to the periphery
and a convergent one, where the dendrimer grows from the periphery toward
the central core. The divergent approach has been used by Vögtle et al. [1], and
later Wörner and Mülhaupt [3] and de Brabander et al. [4] to synthesize
poly(propylene imine) or by Tomalia et al. [5, 6] to synthesize the starburst-polyamidoamine
(PAMAM) dendrimers. The construction process is by the addition
of dendrimer building units to a core molecule possessing one or more reactive
sites, representing the zeroth generation. After addition of the branching units,
each newly created branch is a starting point to which several further sub-branches
may be connected. The number of branches that grow from a branching
point depends on the branching unit. For example, in the case of an A2B branching
unit the number is two, in the case of an A4B branching unit, four. By repeating
this addition process, dendrimers of successive generations are synthesized.
To obtain dendrimers with functional groups on the surface in defined
positions, monomer units carrying the corresponding groups must be added to
the growing dendrimer in the final step. The number of reactive sites, necessary
for the addition of a further layer of branching units, increases exponentially
with the number of generations. Therefore, the synthesis of macromolecules
with molecular masses higher than 10,000 g/mol is possible by using only a few
reaction steps. It is important, however, that all reactive groups react completely,
because separation of product mixtures is in most cases impossible due to the
small differences in properties, like molecular mass, size, or polarity