Polyfunctional systems

The identification of the gel point with the stage in the polymerization at which infinite networks make their appearance is confirmed by the results cited, and the extension of the assumption of random reaction to polyfunctional systems appears to be warranted. 

The kinetic theory can also be used for polyfunctional systems with unequal reactivities of groups and substitution effects, but an explicit solution of the partial differential equation corresponding to Eq. (23) derived for the equireactive system is not possible. One can use, however, the method of moments for derivation of certain averages as was explained in... 

A limitation in the use of pivaloyl esters as protecting groups in a polyfunctional system is the harsh condition required for its cleavage (especially at sterically hindered secondary centers). The 4-acetoxy-2,2-dimethylbutanoyl (ADMB) esters have been proposed as an alternative because they are easily prepared, show similar reactivity in carbohydrate acylations, and are removed under much milder conditions (catalytic quantity of DBU at room temperature) [254]. 

In dithioacetals the proton geminal to the sulfur atoms can be abstracted at low temperature with bases such as Bu"Li. Lithium ion complexing bases such as DABCO, HMPA and TMEDA enhance the process. The resulting anion is a masked acyl carbanion, which enables an assortment of synthetic sequences to be realized via reaction with electrophiles. Thus, a dithioacetal derived from an aldehyde can be further functionalized at the aldehyde carbon with an alkyl halide, followed by thioacetal cleavage to produce a ketone. Dithiane carbanions allow the assemblage of polyfunctional systems in ways complementary to traditional synthetic routes. For instance, the p-hydtoxy ketone systems, conventionally obtained by an aldol process, can now be constructed from different sets of carbon groups. ... 

Numerous organic syntheses, particularly those concerning natural products analogs, involve polyfunctional systems... 

The pH of polyfunctional systems, such as phosphoric acid or sodium carbonate, can be computed rigorously through use of the systematic approach to multiequilibrium problems described in Chapter 11. Solution of the several simultaneous equations that are involved is difficult and time consuming, however. Fortunately, simplifying assumptions can be invoked when the successive equilibrium constants for the acid (or base) differ by a factor of about 10 (or more). With one exception, these assumptions make it possible to compute pH data for titration curves by the techniques we have discussed in earlier chapters. 

This chapter has analyzed the principal methods, structural organization and architecture of a wide range of metallopolymers obtained by incorporation of metal particles into polymers, as well as promising new ones. The multiplicity of architectural structures in such self-organized systems is a reflection of the infinite variety of natural objects coupled with the synthetic possibilities of chemistry. Even the term supramolecular architecture has appeared [104]. Almost all architectural forms (including those obtained by sol-gel methods in thin-layered films) are used in nature for generation of metal particles in biopolymers and their analogues. Primarily, this relates to supramolecular polyfunctional systems such as enzymes, liposomes and cells. The processes of nanocomposite preparation are very similar to biomineralization, biosorption etc. 

The most complicated task in constructing polyfunctional systems is the coordination of the action of the diverse centers and prevention of their autonomous operation. The main approach to resolve this problem is through control of the spatial arrangement of the active centers, which determines the extent of the transfer pathways of reagents between them, regulates diffusional... 

For the study of the solvent effect, comparable equilibrium constants have to be determined in water and in solution made with non-aqueous solvents or solvent mixtures. Potentiometric (usually pH-metric) equilibrium measurements are used for this purpose in polyfunctional systems. The solvent effect makes the application of potentiometry somewhat difficult. The substitution of water by organic solvents results in changes of the autoprotolysis constant of the solvent changing the pH scale. The lower relative permittivity of the system favours association processes which have to be considered, e.g., in the determination of the ionic strength of the solution. Diffusion potentials at the liquid junctions connecting the galvanic cell with the reference electrode may falsify the measured data. 

In addition to the carboxy-group, most natural long-chain acids contain one or more olefinic and/or acetylenic centres and, occasionally, an additional oxygenated function (hydroxy or epoxy). The reactions and interactions of these polyfunctional systems continue to be actively explored. 

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