Dendrimer

    Dendrimers are repeatedly branched molecules. The huge number of papers on dendritic architectures such as dendrimers, dendronized, hyperbranched and brush-polymers has generated a vast variety of inconsistent terms and definitions making a clear and concise unfolding of this topic highly difficult. The purpose of this section is to provide the vocabulary required for the description of chemical and physical phenomena as well as application aspects associated with the research in the area of dendritic molecules.

Dendritic molecules are repeatedly branched species that are characterized by their structure perfection. The latter is based on the evaluation of both symmetry and polydispersity. The area of dendritic molecules can roughly be divided into the low-molecular weight and the high-molecular weight species. The first category includes dendrimers and dendrons whereas the second encompasses dendronized polymers, hyperbranched polymers, and brush-polymers (also called bottle-brushes).

The name comes from the Greek "δενδρον"/dendron, meaning "tree". Synonymous terms are arborols and cascade-molecules. Dendrimer is an internationally accepted term. Dendrimers and dendrons are repeatedly branched, monodisperse, and usually highly symmetric compounds. There is no apparent difference in defining dendrimer and dendron. A dendron usually contains a single chemically addressable group that is called focal point. Because of the lack of the molar mass distribution high-molar-mass dendrimers and dendrons are macromolecules but not polymers.

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.

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Properties and applications

The properties of dendrimers are dominated by the functional groups on the molecular surface. Dendritic encapsulation of functional molecules allows for the isolation of the active site, a structure that mimics the structure of active sites in biomaterials because dendritic scaffolds separate internal and external functions.^[6][7][8]. 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 water-soluble dendrimer with internal hydrophobicity, which would allow it to carry a hydrophobic drug in its interior. Recently it has been shown that redox-active nanoparticles can be synthesized, placing the redox molecules between the nanoparticle core and the dendritic wedges; despite their isolation, some of the redox molecules (COOH in this case) remained uncoupled, and thus still reactive.^[9]

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^[10].

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 ^[11]. 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.

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. Divergent syntheses assemble the molecule from the core, extending radially to the periphery and in contrast convergent methods start at the surface and proceed inwards, before the attachment of pre-synthesised dendrons to 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 Dendritech ^[12]

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.

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