Novel Topologies—Dendrimers and Hyperbranched Polymers

The poly(propylene imine) system. Both syntheses are of the divergent type. 

In practice, absolute monodispersity is difficult to achieve. Detailed studies by Meijer on the poly(propylene imine) dendrimer show that the fifth generation has an experimental PDI (determined by mass spectrometry) of 1.002, with 23% of the material being the desired fully elaborated dendrimer. This is one of the best studied, most highly optimized dendrimer systems, and so this may be the best that is practically achievable. 

Encapsulation ofaguest molecule in the interior of a dend rimer. 

A number of workers have established that encapsulation can be quite efficient. Meijer has shown that if, after encapsulation, the surface groups of the dendrimer are functionalized with a bulky group, the steric interactions on the surface become so severe that the guest is permanently entrapped in the dendrimer. This is another example of a supramolec-ular container compound of the sort discussed in Chapter 4. Hence, dendrimers have been proposed as receptors for use in sensing and catalysis, due to their interior pockets and tailorability. 

In addition, the huge number of functionalities on the surface of a dendrimer can be exploited. For example, attaching multiple copies of a substrate for an enzyme, or a recognition molecule for an antibody or cell adhesion protein can lead to novel effects. MRI image-enhancement molecules (complexes of Gd ) can decorate the surface of a dendrimer, producing an especially effective contrast agent. Studies thus far suggest that dendrimers are often quite nontoxic, and we can anticipate that more medical applications will appear. 

---