Starburst

EDA and other alkylene amines react readily with acrylonitrile or acrylate esters. EDA reacts with acrylonitrile to give tetrakis(2-cyanoeth5i)-ethylenediamine which is reduced over Raney nickel to give tetralds(3-anainoprop5i)-ethyl-enediainine (52). With methyl acrylate and EDA under controlled conditions, a new class of starburst dendritic macromolecules forms (53,54). 

The control of the pore size of porous silica gel by the structure of the organic polymer has been accomplished by using the so-called starburst dendrimer as an... 

The sequential growth and branching involved in the preparation of dendrimers had been considered by Flory many years before they were actually prepared. Flory developed a sound understanding of the kind of processes that would occur in the self-polymerization of a molecule of the type ABj most of which have been shown to be correct by the relatively recent experimental studies. In particular, the existence of a limit to growth was predicted. This limit has become known as the starburst limit, and is the reason for the highly monodisperse nature of fully developed dendrimers. 

The starburst point occurs at a well-defined limit for each dendrimer system, and its occurrence is dependent mainly on (a) the functionality of the core, (b) the multiplicity of the branches and (c) the branch length. However, the volume of the core itself, and the length of the monomer branches also have an influence. The end groups may occupy the outer surface of the dendrimer, or the branches may fold inwards, thus distributing the end-groups within the dendrimer. The factors that control this behaviour of the branches are not fully understood, but include the nature of the solvent and the detailed chemistry of the dendrimer branches. 

The first true dendrimers were the polyamidoamines (PAMAMs). They are also known as starburst dendrimers, and the term starburst is a trademark of the Dow Chemical Company, who have commercialized these materials for a range of applications. These dendrimers use ammonia as the core molecule, and this is reacted with methyl acrylate in the presence of methanol, after which ethylenediamine is added. This is shown in Scheme 9.2. 

Tomalia, D.A. et al. Dendritic macromolecules—Synthesis of starburst dendrimers, Macromolecules, 19, 2466-2468, 1986. 

The complexes of anionic starburst dendrimers with calcium ions are considerably stronger for higher generations than for lower generations. The particle... 

The addition of ammonia to excess methyl acrylate (a linear monomer), followed by amidation with excess ethylenediamine afforded the resultant cascade molecule, and thus Tomalia [37] created the commercially available PAMAM starburst series of dendrimers (2, Fig. 2). Related core molecules such as ethylenediamine and aminoalcohols and other functionalizable groups such as thiol moieties were used to prepare similar dendrimers [38]. This methodology is applicable to most primary amines, resulting in a 1 —> 2 branching pattern. Recently, examples of related Si-, [39] P-, [40] and metallo systems [41], which follow this linear monomer protocol have been reported. 

In a more recent paper, Roy et al. reported on the synthesis and biological pro -perties of mannosylated Starburst dendrimers [46]. In addition to the presence of good biological properties in ligand- and inhibitor-tests, these dendrimers were shown to constitute novel biochromatography materials of high affinity for the rapid and easy isolation and purification of carbohydratebinding proteins from crude mixtures. 

In 1994 we published the first chiral dendrimers built from chiral cores and achiral branches [ 1,89], see for instance dendrimer 57 with a core from hydroxy-butanoic acid and diphenyl-acetaldehyde and with twelve nitro-groups at the periphery (Fig. 21). As had already been observed with starburst dendrimers, compound 57 formed stable clathrates with many polar solvent molecules, and it could actually only be isolated and characterized as a complex [2 (57- EtO-Ac (8 H20))]. Because no enantioselective guest-host complex formation could be found, and since compounds of type 57 were poorly soluble, and could thus not be easily handled, we have moved on and developed other systems to investigate how the chirality of the core might be influencing the structure of achiral dendritic elongation units. 

Tomalia, D. A. Genealogically Directed Synthesis Starburst/Cascade Dendrimers and Hyper-branched Structures. 165 (1993). 

Massive stars play an important role in numerous astrophysical contexts that range from the understanding of starburst environments to the chemical evolution in the early Universe. It is therefore crucial that their evolution be fully and consistently understood. A variety of observations of hot stars reveal discrepancies with the standard evolutionary models (see [1] for review) He and N excesses have been observed in O and B main sequence stars and large depletions of B accompanied by N enhancements are seen in B stars and A-F supergiants [2,3,4,5], All of these suggest the presence of excess-mixing, and have led to the development of a new generation of evolutionary models which incorporate rotation (full reviews in [1], [6], [7]). 

D. A. Tomalia, A. M. Naylor, and W. A. Goddard, Starburst dendrimers-molecular level control of size, shape, surface chemistry, topology, and flexibility from atoms to macroscopic matter, Angew. Chem. Int. Ed. Engl., 29 (1990) 138-175. 

D. Page and R. Roy, Synthesis and biological properties of mannosylated starburst Poly(amidoamine) dendrimers, Bioconjug. Chem., 8 (1997) 714—723. 

D. Zanini and R. Roy, Practical synthesis of starburst PAMAM a-thiosialoden-drimers for probing multivalent carbohydrate-lectin binding properties, J. Org. Chem., 63 (1998) 3486-3491. 

Kukowska-Latallo JF, Bielinska AU, Johnson J, Spindler R, Tomalia DA, Baker JR Jr (1996) Efficient transfer of genetic material into mammalian cells using Starburst polyamidoamine dendrimers. Proc Natl Acad Sci USA 93 4897 -902... 

Barth, R.F., Adams, D.M., Soloway, A.H., Alam, F., and Darby, M.V. (1994) Boronated starburst den-drimer-monoclonal antibody immunoconjugates Evaluation as a potential delivery system for neutron capture therapy. Bioconjugate Chem. 5, 58-66. 

Roberts, J.C., Adams, Y.E., Tomalia, D., Mercer-Smith, J.A., and Lavallee, D.K. (1990) Using starburst dendrimers as linker molecules to radiolabel antibodies. Bioconjugate Chem. 1(5), 305-308. 

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