Synthesis of Dendrimers
The synthesis of dendrimer molecules can be done by different kind of chemical reaction. The advantages of this kind of reaction is that the can control the main parameters of the dendrimer structure ie surface chemistry, size, shape, topology and flexibility. The chemical reaction which is most commonly used for the synthesis of dendrimers can be classified in to traditional and modern techniques. The one of the most common examples of the traditional technique is the Michael reaction or the Williamson ether synthesis. In case of modern techniques,the following chemical reactions are generally used
a. solid-phase synthesis,
b. organo-transition-metal chemistry,
c. organosilicon chemistry, organo-phosphorus chemistry, or other contemporary organic methodologies. The formation dendritic macromolecules can be done through repeated growth reactions. On the basis of the growth reaction dendrimer synthesis can be divided in to different classification. The growth reaction will ultimately synthesize the dendrimer in the form of spherical branch. The branching of the dendrimer can be achieved either by Divergent' Dendrimer Growth or Convergent' Dendrimer Growth
The choice of the growth reaction confirms the methods in which the branching will be formed into the dendrimer. Branching may either be present in the building blocks as is more often the case or it can be created as a function of the growth reaction, as is the case with the poly (amidoamine)s and the poly (propylene imine)s .
'Divergent' Dendrimer Growth
In pat, dendrimer was synthesized be this divergent approach. This is old, conventional and time consuming. This name originates from the way by which dendrimer spread its branch from the core to outward direction.
Figure 1: Schematic of divergent synthesis of dendrimers
Divegent methods initiated the dendrimer synthesis by extending the outward series which is originated from a multifunctional core. The each and every steps of the chemical reaction should be undertaken with special precaution to prvent the synthesis of impurities which is synthesized due trailing generations (some branches are shorter than the others). Such kind of trailing generation may have great influence on the symmetry of the dendrimer
The first synthesized dendrimers by these divergent methods are polyamidoamines (PAMAMs) dendrimers.
'Convergent' Dendrimer Growth
Due to several disadvantages of divergent dendrimer growth the pharmaceutical chemist has developed the 'convergent' approach to synthesize dendrimer. Convergent growth is entirely different from the divergent method as divergent method starts from the core of the dendrimers and ends at the surface of the dendrimers but in convergent method synthesis of dendrimer begins at the surface of the dendrimer, and works inwards by gradually linking surface units together. The major advantages of convergent growth synthesis are that only two simultaneous reactions are required for any generation-adding step. The convergent methodology also suffers from low yields in the synthesis of large structures. The convergent growth method has several advantages:
1. This method is very effective and easier to remove impurities and shorter branches very effectively. As a result .the final dendrimer can be collected as a pure and monodisperse form.
2. In the method of convergent dendrimer synthesis subtle modification of the surface chemistry of dendritic structure is possible by precise placement of functional groups at the periphery of the dendrimer macromolecules. The main disadvantage of this kind of However dendrimer is that they are not as large as those synthesised by divergent methods because crowding due to steric effects along the core is limiting
Figure 2: Schematic of convergent synthesis of dendrimers