5. Applications of Dendrimers
This section is currently under construction There follows some information on catalysis
The idea that dendrimers may be utilised in catalysis and even as biocatalytic mimics has a long history.[1] Although the large voids in their structures, their sheer size, and the advances in their engineering have led to great expectations, there are still big hurdles to overcome. The transient, dynamic nature of the clefts and channels leave a lot to be desired in terms of pre-organisation, and although the theory for the construction of exact dendrimer constructions exists, their reality is one of long syntheses and unproved ideas. However, steps are being taken in the direction of dendrimer catalysis, and the field is growing quickly. Considering the ability of dendrimers to act as hosts,[49,59,64] their biological activity,[33] and known catalysis by branched compounds,[5,2] catalytic dendrimers seem to be a reasonable goal. Most examples of dendrimer catalysts have taken a known catalytic site and attempted to use the dendrimer as an auxiliary for the enhancement of rate or selectivity. Metalloporphyrins surrounded by dendritic wedges have been found to be shape-selective,[63] and expanded ligands dubbed 'dendrizymes'[3] have demonstrated microenvironmental effects (Figure 11) and some enantioselectivity. Sites attached to the surface of dendrimers have also been used with some microenvironmental effects,[4] although it should be noted that other examples of catalytic dendrimers do not have activities any different from their non-dendritic analogues.[72,79,5]
The final example we shall discuss is the microenvironmental effect of the Fréchet polyethers on polymerisations.[6] Dendrimers have been found to have a significant effect on the degrees of polymerisation and polydispersities of the polymers obtained. It is believed that the growing tip of the polymer chain resides inside the cavity of a dendrimer (Figure 12), avoiding termination by reaction with another chain. Monomer molecules diffuse into the dendrimer, allowing the polymerisation to continue. Particular interest in this system lies in the manner in which the dendrimer is utilised - as a supramolecular entity with its own properties, rather than as an interestingly-shaped support for known catalytic sites. This brings us within sight of a future where dendrimers may perform selective and facile catalyses as a result of synergy between their constitution and topology.
