Dendrimers chemical structure

Y. Xiao, L. Shao, T.-S. Chung, D. A. Schiraldi, Effects of thermal treatments and dendrimers chemical structures on the properties of highly surface cross-linked polyimide films, Ind Eng. Chem. Res., 44, 3059-3067 (2005). 

The molecular characterization of a polymeric material is a crucial step in elucidating the relationship between its properties (e.g., mechanical, thermal), its chemical structure, and its morphology. As a matter of fact, the development of a new product stems invariably from a good knowledge of the above relationships. Characterization of polymers is often a difficult task because polymers display a variety of architectures, including linear, cyclic, and branched chains, dendrimers, and star polymers with different numbers of arms. 

CHART 4.27 Chemical structure of a first-generation dendrimer. 

The development of mass spectroscopic techniques such as matrix assisted laser desorption (MALDI) and electrospray mass spectrometry has allowed the absolute determination of dendrimer perfection [7,8], For divergent dendrimers such as PAMAM and PPI, single flaws in the chemical structure can be measured as a function of generation to genealogically define an unreacted site of or a side reaction producing a loop at a particular generation level. Mass spectromet-ric results on dendrimers, not only demonstrate the extreme sensitivity of the technique, but also demonstrate the uniformity of the molecular mass. The polydispersity index of Mw/Mn for a G6 PAMAM dendrimer can be 1.0006 which is substantially narrower than that of living polymers of the same molecular mass [7],... 

In fact, it has been previously observed that measured diameters of dendrimer molecules by AFM are much larger than the theoretical values, which indicates that the dendrimers spread out and flatten on the surface [25, 26], Three major factors could account for this deformation. First, the unique architecture and chemical structure of PAMAM dendrimers result in macromolecules that are not solid balls, but instead are relatively open and hence soft materials. It is expected that the rigidity will increase substantially with increasing generation number [9]. Therefore, when deposited on solid substrates, they tend to deform to different degrees as a result of the interplay between their inherent rigidity and surface energetics from the interaction between the dendrimer molecules and the mica surface. Secondly, amine-terminated PAMAM dendrimers possess a... 

Dendrimers are not really discrete chemical structures - they are non-descript materials,... 

Fig. 1. Chemical structure of a dendrimer of fourth generation (G4). Starting with a focal group in the center of the molecule, a fractal-like structure is built up by branches which emanate from three-functional groups. The outer ends of the branches are terminated by end-groups [3]. The molecule may be divided into dendrons designating the substructures originating from a branch point. Hence, the dendrimer shown here may be viewed upon as composed of four dendrons emanating from a central ethylenediamine group...

Dendrimers are branched chemical structures which can possess a range of terminal functionalities. Covalent attachment of dendrimers to a support builds a 3D structure along its surface which can subsequently be grafted with ligands which, as a consequence of the larger surface area, can yield higher probe densities. 

Figure 11.3 Chemical structures of the (a) arborol dendrimer and (b) water-soluble, allhydrocarbon dendrimer.

Figure 11.14 (a) Chemical structures of polyfpropylene imine) dendrimers. (b) Illustration of the formation of the hilayer structure (1) injection of the dendrimer in an aqueous solution of pH < 8 (2) protonation of the nitrogen atoms (3) stmctural inversion of the dendrimer (4) self-aggregation of the protonated dendrimers to form multilaminar vesicles containing interdigitated bilayers. 

Figure 11.16 (Left) Chemical structures of G1 and G3 polyfaryl ether) dendrimers. (Right) Notional depiction of generation-dependent dynamic conformational equilibrium of amphiphilic dendrimers in water interconverting between disklike and spherical morphologies.

The structure of cationic lipids and polymers is readily amenable to chemical modification [35, 36] allowing the exploration of a virtually unlimited number of combinations and strategies at the mercy of chemists creative abilities. Various reviews have been focused on cationic lipids, dendrimers and polymers in terms of their chemical structures and their transfection properties [36—41], in an attempt to shed some light on the chemical requirements necessary to mediate gene delivery. The focus of this chapter will be to explore these carriers from a synthetic perspective, with a description of the chemical strategies used for the preparation via synthetic organic chemistry (excluding polymer synthesis) of cationic lipids and dendrimers. 

Fig. 4.12 Examples of different dendritic architectures (a) hyperbranched macromolecule, (b) chemical structure of classic PAMAM dendrimer, repeat unit, and one full branch of generation GI (c) schematic of a backbone for generation G4. (From ref. [106])...

Fig. 1. (a) A chemical structure of a 2.5th generation carboxylic acid-terminated poly(amido amine) (PAMAM) dendrimer. (b) Transmission surface enhanced infrared absorption spectra (SEIRAS) of dendrimer adlayers prepared at 30 min adsorption from aqueous solutions (0.01 wt.%) of a dendrimer at different pHs. Numerical values are pHs of the solutions, (c) Adsorption-desorption profiles as a function of time at different pHs and adlayer thicknesses at adsorption and desorption equilibrium as a function of pH for aqueous solutions (0.1 wt.%) of the dendrimer. The symbols, j and J, in the top figure denote start of adsorption and desorption, respectively. In the bottom figure, filled circle and opened square denote adlayer thicknesses at adsorption and desorption equilibrium, respectively. The dark tie denotes the calculated dendrimer size width. A solid curve is drawn to be visual, (d) Schematic illustration of dendrimers adsorbed at different pHs. Reprinted with permission from Ref. [69], 2006, American Scientific Publishers. 

Since dendrimers consisting of regularly branched structure are controlled by chemical structure, molecular weight, and its distribution as well as molecular size and molecular shape, they are unique macromolecules with many functional properties. 

Fig. 56 Chemical structures of the tolanoid-like dendrimers (R = 0(CH2CH20)3Me)...

Fig. 58 Chemical structure of elongated distyrylbenzene-based dendrimers (R = OC3H7, OCgHis, OC12H25)...

Although it is simple to understand what a dendrimer is just by looking at the beautiful chemical structures drawn in the literature, there is at present no rigorous definition of the term dendrimer . It is generally agreed that monodispersity is an essential characteristic of dendrimers, which are thus sets of molecules all having the same identical constitution (and thus the same molecular weight), differently from hyperbranched polymers [1], which are a mixture of different, even if chemically similar, macromolecules. 

Many recent reviews report on dendrimers in general [1,3, 5-14], or focus on more specific topics like dendrimers containing metals [15-23], or elements of groups 13, 14 and 15 [23-27], or on redox-active dendrimers [16, 19, 28, 29], For reasons of space, it is impossible here to review all the published literature concerning dendrimers involved in electron transfer processes only selected examples will be presented to illustrate the diversity of chemical structures and research issues that can be encountered in this field. Synthetic procedures will not be dealt with the interested reader should refer to the original literature. 

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