Dendritic macromolecules, potential

Perhaps the most viable short-term use for dendritic macromolecules lies in their use as novel catalytic systems since it offers the possibility to combine the activity of small molecule catalysts with the isolation benefits of crosslinked polymeric systems. These potential advantages are intimately connected with the ability to control the number and nature of the surface functional groups. Unlike linear or crosslinked polymers where catalytic sites may be buried within the random coil structure, all the catalytic sites can be precisely located at the chain ends, or periphery, of the dendrimer. This maximizes the activity of each individual catalytic site and leads to activities approaching small molecule systems. However the well defined and monodisperse size of dendrimers permits their easy separation by ultrafiltration and leads to the recovery of catalyst-free products. The first examples of such dendrimer catalysts have recently been reported... 

Preparation of photo-active and redox-active dendritic macromolecules, which undergo simultaneous muitielectron transfer is also attractive. Considering the potential applications of these multimetallic dendrimers as electron transfer mediators in redox catalysis, photoinduced electron transfer, and molecular electronics, new interesting results can be awaited in the near future. 

Dendritic macromolecules exhibit compact globular structures which lead to their low viscosity in the melt or in solution. Furthermore, dendritic macromolecules are characterized by a very large number of available functional groups, which lead to unprecedented freedom for changing/tuning/tailoring the properties of these multivalent scaffolds via complete or partial derivatization with other chemical moieties. All these features have contributed to multidisciplinary applications of these unique macromolecular structures in recent years 6, 7). The development of efficient synthetic routes in recent years has given rise to a virtually unlimited supply of commercially available dendritic polymers, at very affordable price. The transport properties of hyperbranched and dendritic polymers have recently attracted attention as potentially new barrier and membrane materials 8-9). 

A fundamental aspect of the divergent-growth approach is the rapid increase in the number of chain-end functional groups. Associated with this is the increase in the number of reactions required to functionalize the chain ends fully. Incomplete reaction of these rapidly increasing terminal groups leads to failure sequences or imperfections in the next generation. These potential difficulties and the lack of control over the number and placement of functionalities at die chain ends led us to reevaluate the synthetic approach to dendritic macromolecules. 

These distinguishing features of dendritic macromolecules render these novel materials a reliable alternative to traditional polymers in a wide range of potential applications. The primary focus of this chapter is to cover the range of these potential uses of water-soluble dendrimers - it does not provide comprehensive coverage of these applications but has targeted a number of notable examples of the utilisation of water-soluble hyperbranched macromolecules. 

This chapter will not attempt to present encyclopedic coverage of the field, but it will only provide representative examples of the processes used to access each of the main types of hybrid block architectures, then it will take a somewhat closer look at specific families of amphiphilic or electroactive hybrid dendritic-linear macromolecules with broad potential for practical applications. 

Treatment of a dendritic polyester 5 with a dendritic polyamine 6 is perhaps the simplest example of coupling between two dissimilar cascade macromolecules.178-801 Scheme 9.2 illustrates some potential products (i.e., 7 and 8) available by this general procedure, as well as a continuation of surface amidation to afford polymeric species such as random network 9. As envisioned, it would be difficult to stop this procedure after the formation of a single amide bond, due to the close proximity of adjacent ester—amine groups. 

As far as electron transfer properties directly involving dendrimers are concerned, it can be generally considered that these reactions may be observed whenever the macromolecular structure contains one or more units featuring redox levels at accessible potentials. The first dendrimers prepared were purely organic macromolecules, with no unit appropriate for electron transfer reactions. Later, however, the introduction of metal and organometallic complexes in the dendritic structure opened new possibilities to the chemistry of dendrimers. Indeed, the incorporated metal units exhibit important properties such as absorption and emission of visible light (relevant for the construction of antenna systems see Volume V, Part 1, Chapter 7) and redox levels at accessible potential, which are necessary for electron transfer reactions. Successively, purely organic electroactive units have also been used to functionalize the dendrimers. 

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