Repetitive synthesis defined

Because of their high molecular weight and their defined structure, dendrimers offer themselves for studying the expression of chirality on a macromolecular level. The construction of configurationally uniform macromolecules is otherwise a complex task but can be achieved more easily with dendrimers because of repetitive synthesis from identical (chiral) building blocks. Comparison of optical rotation values and circular dichroism (CD) spectra should demonstrate what influence there is of the chiral building blocks on the structure of the whole dendrimer. 

The term iterative hbrary deconvolution describes a process consisting of repetitive synthesis, screening, and selection steps, during which all mixture positions of the hbrary (see also Section 4.3.7.3.2.1) are successively defined, resulting in the identification of individual peptides with the desired bioactivify.P At each step of this iterative process, the number of peptides per mixture is reduced by a factor equal to the number of amino adds in the mixture position that was defined at that step. At the final step of the process, individual peptides are synthesized and tested. For a hexapeptide library with two defined and four mixture positions, the iterative deconvolution consists of four cycles of synthesis, screening, and selection (see Table 2).P 1... 

It is appropriate to identify our approach to developing the present review in the context of the Co chapter in CCC(1987). The first-edition chapter on Co featured a focused discussion and tabulation of synthetic methods, and many of these basic methods are still employed in synthesis today. Consequently, to avoid repetition, there will be diminished description here where prior appropriate methods have been provided, and only newer developments featured. The last two decades feature the development of many mixed-donor and sophisticated multidentate and macrocyclic ligands, which found limited coverage in the previous edition, and these will be discussed in more detail herein. Reaction kinetics and mechanism were also described thoroughly in the previous edition. We shall not reiterate this material, since the core mechanisms of many reactions involving Co compounds are now adequately defined. 

The repeated succession of similar reaction sequences for the stepwise synthesis of molecules which consist of repeated building blocks has been termed in literature as repetitive or iterative synthesis. To a molecule with a defined framework having one or more functional groups A (Fig. 1) an identical molecule unit is attached (Iter. 1). Subsequently the functional groups are then reobtained (Iter. 2) so that the reaction sequences can be repeated over and over with the molecule growing by one unit in each reaction cycle (Iter. 1 + 2). This growth... 

The synthesis of the macrocycles 43 (Scheme 9) is an example of repetitive, highly stereoselective Diels-Alder reaction between bis-dienes 41 and bis-dienophiles 42, containing all oxo or methano bridges syn to one another. The consecutive inter- and intramolecular Diels-Alder reactions only succeed at high pressure. Obviously, both reactions are accelerated by pressure. The macrocycles are of interest in supramolecular chemistry (host-guest chemistry) because of their well-defined cavities with different sizes depending on the arene spacer-units. 

Dendrimer synthesis involves a repetitive building of generations through alternating chemistry steps which approximately double the mass and surface functionality with every generation as discussed earlier [1-4, 18], Random (statistical) hyperbranched polymer synthesis involves the self-condensation of multifunctional monomers, usually in a one-pot single series of covalent formation events [31], Random hyperbranched polymers and dendrimers of comparable molecular mass have the same number of branch points and terminal units, and any application requiring only these two characteristics could be satisfied by either architectural type. Since dendrimer synthesis requires many defined synthetic and process purification steps while hyperbranched synthesis may involve a one-pot synthetic step with no purification, the dendrimers will necessarily be a much more expensive material to produce. 

Some special nanomaterials are of great interest due to their unique properties. Dendrimers are versatile, well-defined, nanosized monodispersing macromolecules which are hyperbranched synthesized polymers constructed by repetitive monomer units. They are perfect nanoarchitectures with size from lnm to more than 10nm depending on the synthesis generation. Drugs can be entrapped into the branches... 

A similar well-defined graft copolymer consisting of polystyrene main chain and branches (G-7) can be prepared simply via repetition of copper-catalyzed living radical polymerizations.209 Thus, the synthesis starts with the copolymerization of styrene and />(acetoxymethy 1)styrene or />(methoxymethyl)sty-rene, followed by bromination of the substituent into the benzyl bromide moiety, which then initiates the copper-catalyzed radical polymerization of styrene to give graft polymers with 8—14 branches. 

Figure 12-3 Plots of the degree of polymerization versus the cumulative number of synthetic steps for various repetitive syntheses (a) conventional linear solid-phase synthesis (b) nonlinear straight-chain sequence synthesis (c) dendrimer synthesis (branching multiplicity of three) (d) double exponential den-drimer synthesis (branching multiplicity of three). In all cases, the degree of polymerization is defined as the total number of monomer units per polymer molecule.

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