Dendrimer
A dendrimer is a regularly branched fully synthetic polymer molecule which resembles the branches of a
tree. The name comes from the Greek "δενδρον"/dendron, meaning "tree." The first dendrimers were
described by Vögtle in 1978[1], by Denkewalter and coworkers at Allied Corporation as polylysine
dendrimers in 1981[2], by Tomalia at Dow Chemical in 1983[3] and in 1985[4], and by Newkome in 1985[5].
In 1990s dendrimers caused an explosion of scientific interest because of their unique molecular
architecture (Fig 1). This resulted in over 5,000 scientific papers and patents published by the end of 2005.
Synthesis
In the synthesis of dendrimers, monomers lead to a monodisperse polymer, tree-like, or generational structure. There are two
defined methods of dendrimer synthesis, divergent synthesis and convergent synthesis. The former assembles the molecule
from the core to the periphery and the latter from the outside to termination at the core.
However, because a repeated reaction which consists of many steps is needed to protect the active site, it is difficult to
synthesize dendrimers even if both methods are used. This is why there are obstacles to the synthesis of large quantities of
dendrimers. Presently, the only kilogram-scale producers of dendrimers is Dentritech [6]
The original Newkome dendrimer or arborol (1985) started by nucleophilic substitution of 1-bromopentane by triethyl
sodiomethanetricarboxylate in dimethylformamide and benzene. The ester groups were then reduced by lithium aluminium
hydride to a triol in a deprotection step. Activation of the chain ends was achieved by converting the alcohol groups to tosylate
groups with tosyl chloride and pyridine. The tosyl group then served as leaving groups in another reaction with the tricarboxylate,
forming generation two.
This sequence can be repeated many times.
Properties and applications
The properties of dendrimers are dominated by the functional groups on the molecular surface. For example, a dendrimer can be
water-soluble when its end-group is a hydrophilic group, like a carboxyl group. It is theoretically possible to design a watersoluble dendrimer with internal hydrophobicity, which would allow it to carry a hydrophobic drug in its interior.
Another property is that the volume of a dendrimer increases when it has a positive charge. If this property can be applied,
dendrimers can be used for drug delivery systems (DDS) that can give medication to the affected part inside a patient's body
directly.
Photonic excited molecules
The inside of a dendrimer has a unique chemical environment because of its high density. From this property, it has been
discovered that azobenzene is photoisomerized by very weak infrared rays when covered by a dendrimer [7]. Through the
discovery of a function that catches light and conveys this energy using excitation of the molecule, attempts have recently been
made to synthesize dendrimers that insert porphyrin, absorb light, and photosynthesize artificially. In addition, the development
of organic electroluminescent devices and their applications has been undertaken by researchers all over the world.
References
1.
^ "Cascade"- and "Nonskid-Chain-like" Syntheses of Molecular Cavity Topologies Egon
Buhleier, Winfried Wehner, Fritz Vögtle Synthesis 1978; 1978: 155-158 DOI:10.1055/s-1978-24702
2.
^ Patent 4,289,872 (published 1981, filed 1979) and 4,410,688 (published 1983, filed
3.
^ Dow patent is 4,507,466 (published 1985, filed 1983)
4.
^ A New Class of Polymers: Starburst-Dendritic Macromolecules D. A. Tomalia, H.
1981)
Baker, J. Dewald, M. Hall, G. Kallos, S. Martin, J. Roeck, J. Ryder and P. Smith Polymer Journal, Vol. 17 (1985) No. 1
pp.117-132 DOI:10.1295/polymj.17.117
5.
^ Micelles. Part 1. Cascade molecules: a new approach to micelles. A [27]-arborol
George R. Newkome, Zhongqi Yao, Gregory R. Baker, Vinod K. Gupta J. Org. Chem.; 1985; 50(11); 2003-2004.
DOI:10.1021/jo00211a052
6.
^ Dentritech Inc., from Midland, Michigan, USA.Dendritech.
7.
^ Dong-Lin Jiang, Takuzou Aida, Nature 388, 454-456 (1997)