Template interaction

The latter method, the template method, involves a reaction to produce a transition state similar to the desired product using a template. The template should have a shape similar to the space of the product. The template interacts with the substrate by forming noncovalent bonds such as coordination bonds (Fig. 3). The representative and most successful examples are found in crown ether chemistry. In the chemistry, alkali metals act as templates to create a crown-ether-like transition state with an ethylene glycol substrate by using metal-oxygen coordination bonds. 

Some limitations of this molecular imprinting technique are obvious the template must be available in preparative amounts, it must be soluble in the monomer mixture and it must be stable and unreactive under the conditions of the polymerization. The solvent must be chosen considering the stability of the monomer-template assemblies and it should result in the porous structure necessary for rapid kinetics of the template interaction with the binding sites. If these criteria are satisfied, a robust material capable of selectively rebinding the template can be easily prepared and evaluated in a short period of time. 

There are two general classes of imprinted polymers covalent and noncovalent MlPs. These two categories refer to the types of interactions between the functional monomer and the template in the prepolymerization complex. There are also hybrid MlPs that utilize a combination of covalent and noncovalent interactions in the preparation and rebinding events (Klein et al. 1999). Covalent MlPs utilize reversible covalent interactions to bind the template to the functional monomers. In contrast, noncovalent MlPs rely on weaker noncovalent functional monomer-template interactions. Each type has specific advantages and disadvantages with respect to sensing applications that will be addressed in subsequent sections. 

Also copolymers with non-conventional structure can be obtained by copolymerization in the presence of templates interacting with comonomers. Average length of sequences of units is in such products higher. Changing the template concentration we can influence the reactivity ratios of monomers and the structure of the product. 

The synthesis of 94 is noteworthy, because no templating interactions seem to be required to form the rotaxane in preparative yields The slipping approach performed in the melt seems to offer universal access to otherwise not obtainable rotaxanes. The absence of solvent not only assists this slipping by guaranteeing high concentrations, but also eases the reaction because no desolvation processes need to take place. 

It has already been reported that the weight loss of as-synthesized MMSs depends on the kind of the template used in the synthesis [17]. This is an obvious consequence of the fact that different templates decompose and thermodesorb at different temperatures. However, it was somewhat unexpected that the decomposition/desorption of the same kind of the template may be dramatically influenced by the framework composition of materials [4,10-14]. This can be understood as an influence of the framework structure on the process of Hoffmann elimination of alkylammonium to the corresponding alkene and low molecular weight amine [4,8], This decomposition process leads not only to the elimination of the electrostatic framework-template interactions but also to the formation of decomposition products of lower molecular weight than that of the surfactant. Thus, the framework-surfactant interactions are crucial factors determining the thermogravimetric behavior. 

Besides from the non-acidic SiOH and POH and the acidic HF and LF hydroxyls, the presence of two bands at 3612 and 3530 cm has been related to the presence of organic templates interacting with HF and LF framework hydroxyls respectively. 

We have seen how elegantly transition metals can template the formation of knots, but what about Nature s favourite templating interaction, the hydrogen bond A remarkably efficient molecular trefoil knot synthesis based on this interaction was reported by Vogtle and co-workers, who made a knotane in 20% yield [39]. This amazing route (Fig. 11) was uncovered serendipitously during the synthesis of catenanes. The crystal structure of the compound was the definitive proof for the structure, because neither NMR nor mass spectrometry could tell it apart conclusively from the macrocycles that are also formed. 

Our chief concern in this chapter will be with systems of this latter type. Namely, those that appear to involve a large measure of self-assembly during their formation - and for which some element of the templating interactions used to direct their construction remains within the interlocked final structures. However, it is still pertinent in the present context to survey briefly a selection of both the early and more recent statistical work. 

Also of considerable importance is the mechanism of site formation. At what stage in the polymerisation are the high energy sites formed and stabilised Does the solution structure of the monomer-template assemblies reflect the disposition of functional groups at the binding sites [24] (See Chapter 5 for a further discussion.) Attempts to correlate the association constants determined for the monomer-template interactions in homogeneous solution with the rebinding association... 

For templates interacting only weakly with the functional monomer, recognition is often only seen when using the same solvent used as diluent. Often, however, no selectivity is observed even then. The most common reasons for the lack of selectivity in these cases are sample overloading and/or slow mass transfer [59]. Decreasing the sample load leads to an increase in both retention and selectivity. Furthermore,... 

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