Dendrimers high generation

The rate of a catalytic reaction depends on the rate of diffusion of both substrates and products to and from the catalytic sites. Therefore it is of outmost importance that the catalytically active sites are freely accessible for reactions. Only dendrimers of low generation number can possibly be expected to be suitable carriers for catalytically active sites, especially when these are located in the interior. In high-generation dendrimers with crowded surfaces catalytic activity of an internal site would be prevented. On the other hand, a crowded surface will not only hinder access to an interior ligand site but will also cause steric hindrance between groups attached to it and thus prevent high reactivity of sites at the periphery. 

The robustness and excellent turnover numbers of platinum complexes with terminal alkynes have made it the catalyst of choice for the synthesis of polymers and other macromolecular architechtures. Alkyne hydrosilylation with platinum has also served as a key element in the synthesis of dendrimers. Sequential reaction of an alkyne with HSiMeCl2 and lithiated phenylacetylene afforded the branching unit of a dendrimer synthesis which has been used to afford a large variety of structures at high generation.44,4411 441 ... 

After a variety of very different dendrimers had become available up to high generations, many groups directed their attention more and more to designing and synthesizing functionalized dendrimers, i.e., dendrimers adorned with functionalities that are intended to give them characteristic properties favourable for application purposes.14,141... 

May be used to prepared high generation (i.e. G = 0-12) dendrimers that precede and exceed the de Gennes dense packed state. 

Repeating the sequence shown in Figure 18.8 another five or six times (when congestion at the periphery occurs) is not an easy task without the introduction of Si-O-Si links and very few silane dendrimers of such a high generation have been prepared. The reaction is much more facile when HSi(CH3)Cl2 is used. Equipped with phosphine complexes either in the core or at the periphery the dendrimeric systems are used as catalysts [12], Controlling the state of platinum seems to be a key problem. 

For PAMAM dendrimers, the molecular dimensions were determined by size-exclusion chromatography and the dimensions of PPl dendrimers were determined by SANs data for the high-generation PPl dendrimers are not available. 

The detailed investigation of the photophysical properties of metal-based dendrimers of high generation has clearly evidenced that5a,e (i) downhill (or even isoergonic)... 

---