Tomalia-type dendrimers

Figure 1.20 Comparison of densities as a function of generation for (A) asymmetrical branch cell in DenkewaIter-type dendrimers, (B) symmetrical branch cell in Tomalia-type dendrimers ([densities calculated from experimental hydrodynamic diameters and theoretical, D.A. Tomalia, M. Hall, D.M. Hedstrand, J. Am. Chem. Soc., 109,1601 (1987))...

Important physical property subtleties were noted within the dendrimer subset. For example, dendrimers possessing asymmetrical branch cells (i.e. Den-kewalter type) exhibit a constant density versus generation relationship (Figure 1.20). This is in sharp contrast to symmetrical branch cell dendrimers (Tomalia-type PAMAM) that exhibit a minimum in density between G = 4 and G = 7 (NH3 core) [48, 96]. This is a transition pattern that is consistent with the observed development of container properties described in Figure 1.21. 

Dendrimers have attracted considerable attention in the polymer field over the past two decades as they have been recognized as the most important macromolecules possessing tunable internal packing density, void volumes, solvent-dependent size, branching dimensions, and surface functionalities. Since the first report of a dendrimer-like molecule in 1978 [33], significant progress has been made in the dendrimer chemistry. A large number of dendrimer compositions (families) and dendrimer surface modifications have been reported. A plethora of applications related to controlled release of pharmaceuticals have been reported. Currently, there are two widely studied dendrimer families, namely the Tomalia-type polyamidoamine (PAMAM) dendrimers and the Frdchet-type polyether dendrimers. PAMAM dendrimers are the first complete dendrimer family to have been synthesized. 

From Frechet, J. M. J. Tomalia, D. A. Dendrimers and Other Dettdrilic Polymers Wiley Chichester New York, 2001. Table 5 PBzE-type dendrimers with peripheral benzyl ether end groups... 

FIGURE 42.7. Abstract of the first full article describing the synthesis of a complete family of Tomalia-type PAMAM dendrimers. 

Tomalia-type poly(amidoamine) (PAMAM) dendrimers and their use as precise, fundamental building blocks to form poly(dendrimers) or so-called starburst polymers. These poly(dendrimers) are now referred to as megamers [35,36] and are described in more detail later. Other pioneers in the dendritic polymer field include Vogtle, New-kome, Frechet, and others. These historical contributions have been reviewed recently [33],... 

Dendrons and dendrimers are the most intensely investigated subset of dendritic polymers. In the past decade, over 6,000 literature references have appeared dealing with this xmique class of structure-controlled polymers. The word dendrimer is derived from the Greek words dendri- (tree branch-like) and meros (part of), and was coined by Tomalia et al. about 20 years ago in the first full paper on poly (amidoamine) (PAMAM) dendrimers [47,72]. Since this early disclosure, over 100 dendrimer compositions (families) and 1,000 dendrimer surface modifications have been reported. The two most widely studied dendrimer families are the Frechet-type polyether compositions and the Tomalia-type PAMAM dendrimers. PAMAM dendil-mers constitute the first dendrimer family to be commercialized, and represent the most extensively characterized and best-understood series at this time [46]. 

As early as 1983, de Gennes and Hervet [95] proposed a simple equation derived from fundamental principles, to predict the dense-packed generation, for Tomalia-type PAM AM dendrimers. It was predicted that at this generation ideal branching can no longer occur since available surface space becomes too limited for the mathematically predicted... 

Parallel studies on Tomalia type PAMAM dendrimers, the Frechet type poly(ether)dendrons, and other dendrimer families have generated an extensive list of unique properties driven by the dendritic state. Figure 42.14 compares several significant physical property differences between the linear and dendritic topologies related to conformations, crystallinity, solubilities, intrinsic viscosities, entanglement, diffusion/mobility, and electronic conductivity. 

C-branched dendritic structures, based on either ether and amide linkages or simply amide connectivity (iii) Tomalia-type PAMAM dendritic structure, differing from the POPAM by the presence of amide bonds, but keeping the amine AB2 as the branching center and (iv) Fr6chet-type dendritic strnctures, based on ether and phenyl groups as AB2 branching center. Description of the different types of covalent dendrimers reported will follow a classification based on the element content, such as only C, C-N, N-O, C-N-0, or with heteroatoms such as P, Si, and so on. Examples of porphyrin-, thiophene-, carbohydrate-, and fullerene-based dendrimers are described separately, since they can be bound to different types of frameworks. 

More recently, non-traditional polymerization strategies have evolved to produce a fourth new major polymer architecmral class, now referred to as dendritic polymers [43]. This new architectural polymer class consists of four major subsets (1) random hyperbranched, (2) dendrigrafts, (3) dendrons and (4) dendrimers. Dendrimers, the most extensively studied subset were discovered by the Tomalia group while in The Dow Chemical Company laboratories (1979) and represent the first example of synthetic, macromolecular dendritic architecture [43,44]. First use of the term dendrimer appeared in preprints for the first SPSJ International Polymer Conference, held in Kyoto, Japan in 1984 [45]. The following year, a full article in Polymer Journal [46] (Fig. 8) described the first preparation of a complete family of Tomalia-type poly(amidoamine) (PAMAM) dendrimers (G = 1-7) and their use as precise, fundamental building blocks to form poly (dendrimers) or so-called starburst polymers. These poly(dendrimers) are now referred to as megamers [47, 48] and are described in more detail later in Sect. 6.4.3. Other pioneers in the dendritic polymer field include Vogtle, Newkome, Frechet, Majoral, and others. These historical contributions have been reviewed recently [52]. ... 

Plots of intrinsic viscosity [t]], density (d), surface area per Z group (A and refractive index as a function of generation clearly show intrinsic maxima or minima at G = 3-5 for this Tomalia-type PAMAM dendrimer series. These data corroborate computer-assisted molecular-simulation predictions [9,180], as weU as extensive photochemical probe experiments reported by Turro et al, and others [55, 105-108, 181],... 

Flory was the first to hypothesize concepts [28,52], which are now recognized to apply to statistical, or random hyperbranched polymers. However, the first purposeful experimental confirmation of dendritic topologies did not produce random hyperbranched polymers but rather the more precise, structure controlled, dendrimer architecture. This work was initiated nearly a decade before the first examples of random hyperbranched4 polymers were confirmed independently in publications by Odian/Tomalia [53] and Webster/Kim [54, 55] in 1988. At that time, Webster/Kim coined the popular term hyperbranched polymers that has been widely used to describe this type of dendritic macromolecules. 

With precise functionalization, it is possible to prepare different types of dendrimer nanocomposite topologies. Balogh and Tomalia have utilized the functionalization of PAMAM dendrimers to create both external and internal... 

In the beginning, the term dendrimer , which was established by Tomalia in 1985 [42,43], described all types of dendritic polymers. Later a distinction based on the relative degree of structural control present in the architecture was drawn. Nowadays, many other types of dendritic architectures are known, even if most of them, however, have not yet been widely investigated and fully characterized. The term dendritic polymer involves four substructures (Fig. 2), namely dendrimers themselves, dendrons, random hyperbranched polymers, and dendrigraft polymers [44, 45],... 

Frechet JMJ, Ihre H, Davey M (2001) Preparation of Frechet-type polyether dcmdrons and aliphatic polyester dendrimers by convergent growth an experimental primer. In Freehet JMJ, Tomalia DA (eds) Dendrimers and other dendritic polymers. Wiley, Chiehesto, pp 569-586... 

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