What Is A Dendrimer?

A concise, but scientific description of what a dendrimer is, how one is created, and the major factors (sterics, geometry, etc) that make them unique and scientifically interesting. (Researched and cited).

Dendrimer synthesis is a relatively new field of polymer chemistry defined by regular, highly branched monomers leading to a monodisperse, tree-like or generational structure. Synthesizing monodisperse polymers demands a high level of synthetic control which is achieved through stepwise reactions, building the dendrimer up one monomer layer, or "generation," at a time. Each dendrimer consists of a multifunctional core molecule with a dendritic wedge attached to each functional site. The core molecule is referred to as "generation 0." Each successive repeat unit along all branches forms the next generation, "generation 1," "generation 2," and so on until the terminating generation.

There are two defined methods of dendrimer synthesis, divergent2 and convergent.3 In the divergent method the molecule is assembled from the core to the periphery; while in the convergent method, the dendrimer is synthesized beginning from the outside and terminating at the core. In either method the synthesis requires a stepwise process, attaching one generation to the last, purifying, and then changing functional groups for the next stage of reaction.

This functional group transformation is necessary to prevent unbridled polymerization. Such polymerization would lead to a highly branched molecule which is not monodisperse -- otherwise known as a hyperbranched polymer. Although there are many uses for hyperbranched polymers, this essay does not investigate them.

In the divergent method, the surface groups initially are unreactive or protected species which are converted to reactive species for the next stage of the reaction. In the convergent approach the opposite holds, as the reactive species must be on the focal point of the dendritic wedge.

Due to steric effects, continuing to react dendrimer repeat units leads to a sphere shaped or globular molecule until steric overcrowding prevents complete reaction at a specific generation and destroys the molecule's monodispersity. The number of possible generations can be increased by using longer spacing units in the branches of the core molecule. The monodispersity and spherical steric expansion of dendrimers leads to a variety of interesting properties.

The steric limitation of dendritic wedge length leads to small molecular sizes, but the density of the globular shape leads to fairly high molecular weights. The spherical shape also provides an interesting study in molecular topology. Dendrimers have two major chemical environments, the surface chemistry due to the functional groups on the termination generation ,which is the surface of the dendritic sphere, and the sphere's interior which is largely shielded from exterior environments due to the spherical shape of the dendrimer structure. The existence of two distinct chemical environments in such a molecule implies many possibilities for dendrimer applications.

Theoretically, hydrophobic/hydrophilic and polar/nonpolar interactions can be varied in the two environments. The existence of voids in the dendrimer interior furthers the possibilities of these two heterogeneous environments playing an important role in dendrimer chemistry. Dendrimer research has confirmed the ability of dendrimers to accept guest molecules in the dendritic voids.

Dendrimers have found actual and potential use as molecular weight and size standards, gene transfection agents, as hosts for the transport of biologically important guests, and as anti-cancer agents, to name but a few. Much of the interest in dendrimers involves their use as catalytic agents, utilizing their high surface functionality and ease of recovery. Dendrimers' globular shape and molecular topology, however, make them highly useful to biological systems as well.

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