Hyperbranched Molecules
Part I: Properties of Dendrimers

Hyperbranched macromolecules, also called cascade molecules, dendrimers or arborols¹, are repetitively branched tree-Iike structures. They were first discovered by Donald Tomalia and co-workers in the early 1980's.² Since then, extensive research has been done in synthesizing and investigating these unique molecules. Dendrimers³ have a well-defined molecular weight and regular structure which has a great impact on their physical and chemical properties. For example, they have a lower glass transition temperature, a smaller hydrodynamic volume, and a lower viscosity than their linear counterpart having the same chemical composition and molecular weight. They are also fully amorphous, more reactive and more soluble in polar and non-polar solvents than their linear counterpart. The increased solubility/miscibility and higher reactivity is often explained by reduced steric hindrance and by the large number of terminal groups whereas the lower viscosity is caused by the smaller hydrodynamic volume.

The solubility of dendrimers mainly depends on the type of surface group. Dendrimers with hydrophilic terminal groups are highly soluble in polar solvents whereas dendrimers having hydrophobic terminal groups are more soluble in nonpolar solvents. The solubility of dendrimers is often very different from the linear analog. For example, polyester dendrimers are much more soluble in THF than the analogous linear polyester.⁴

The dendrimer structure can be divided into three parts:

• The central core, which is shielded off from the outside by the dendritic wedges. Depending on the type of dendrimer, the core may have very different properties than the surrounding shell.

• An outer shell, which consists of repeatedly branched building blocks having a well-defined dendritic structure. This layer makes up the bulk of the dendrimer.

• A surface layer with a (large) number of terminal functional groups. This layer shields off the interior from the surrounding and may reduce diffusion of solvent molecules into the interior.

Dendrimers are classified by generation, which is the number of repeated branching cycles, and by their chemical structure. The two most important types are

• Starburst dendrimers are based on polyamidoamines starting with a tri- or tetra-functional amine core, typically ammonia or ethylene diamine.² These dendrimers are often abbreviated as PAMAM or Starburst.⁵ This type of dendrimer is synthesized by sequential Michael addition of an α,β-unsaturated ester (acrylate) to an amine.

• Frechet-Type dendrimers are segment-block dendrimers. This type was developed by Hawker and Frechet.⁶ They are usually prepared by reacting the end groups of a (linear) multifunctional oligomer (inner block or core) with two wedges or dendons (outer blocks). The oldes Frechet-type dendrimers are based on polybenzylether dendrons.

Dendrimers often have carboxylic acid or amine as surface groups which increase the solubility of the polyether or polyamide dendrimers in polar solvents and water. They also function as anchoring points for further chemical modifications of the dendrimer surface.

References & Notes

1. Dendron and arbor are the Greek and Latin words for tree.

2. D.A. Tomalia, H. Baker, J.R. Dewald, M. Hall, G. Kallos, S. Martin, J. Roeck, J. Ryder, & P. Smith, Polym. J.  Vol. 17, No. 1, 117-132 (1985)⁸

3. A dendrimer should not be confused with a dendron which describes a dendritic wedge without a core.

4. B. Klajnert and M. Bryszewska, Acta Biochimica Polonica, Vol. 48, No. 1, pp. 199-208 (2001)

5. Starburst is a trademark of the Dow Chemicals Company.

6. C.J. Hawker & J.M.J. Frechet, J. Am. Chem. Soc. 112, 21, 7638-7647 (1990)

7. Donald A. Tomalia, Jean M. J. Frechet, J. Polym. Sci. Part A: Polym. Chem., Vol. 40, 2719 - 2728 (2002)

8. Dendrimers were first developed in a Dow corporate laboratory between 1979-1983.