Their History and Characteristics

Dendrimers are a new class of polymeric materials discovered in the 1980s that are highly branched, monodisperse macromolecules. These hyperbranched molecules were described by Tomalia et al.38 and named dendrimers. The word dendrimer comes from the Greek dendron, meaning a tree or a branch, and meros meaning part. At the same time, Vogtle and his co-workers39 published the first report of the synthesis of polymers with a dendritic structure that they called cascade molecules. Newkome et al.40 independently reported the synthesis of similar macro-molecules that they called arboros from the Latin word arbor, meaning a tree.

Although linear polymers are highly polydisperse with a wide range of molecular weights because of their automatic assembly, dendrimers are mono dispersed or low-polydispersed that is necessary for homogeneous and reproducible pharmacokinetic properties. Therefore, dendrimers that have well-defined structures and flexible functions are good candidates for drug carriers.

Poly(amidoamine) (PAMAM) dendrimers (Figure 4.1) are the first complete dendrimer family to be synthesized, characterized, and commercialized. Dendrimers have a molecular architecture characterized by regular, dendric branching with radial symmetry. The molecular weights are similar to those of proteins, and their size ranges from 1 to 10 nm with each generation, or layer, of polymer adding 1 nm to the diameter of the molecule.41 Lower generation dendrimers have a highly asymmetric shape and possess more open structures compared with those of a higher generation. As the chains growing from the core molecule become longer and more branched, dendrimers adopt a globular structure. Dendrimers become densely packed as they extend out to the periphery that results in a closed membrane-like structure. When a critical branched state is reached, dendrimers cannot continue to grow because of a lack of space. This is called the starburst effect. In the case of PAMAM dendrimer synthesis, this was observed after the 10th generation that contains 6141 monomer units and has a diameter of about 12.4 nm.41

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