Monomer template adducts

The imprinting process is less clear-cut (monomer-template adduct is labile and not strictly stoichiometric). 

Determination of the Binding Constant K for the Formation of Monomer-Template Adduct... 

Synthesis of monomer-template adducts is unnecessary [which is essential in covalent imprinting]. 

Polymerization of this monomer-template conjugate (or adduct), and... 

It should be noted that spectroscopic titration usually shows the formation of a 1 1 adduct between template and functional monomer, even when several functional monomers interact with several parts of template. In other words, these interactions are almost independent of each other in solutions (n 1 monomer/template complexes are hardly formed therein because of the unfavorable entropy term). Even in these cases, the monomers which are interacting with the template at different positions are covalently bound to each other in the polymerization. These steps should proceed in a stepwise manner. Thus, the polymerization occurs around a site of the template. Independently, polymers are also formed at other sites of the template. Finally, these polymer do-... 

Such monomer interacts with a suitable p-di ketone forming an isolable enamide which effectively mimicks the transition state of the reaction. Copolymerisation of this adduct with the crosslinker and acrylamide followed by template removal yields small microgels (around 20 nm) that are able to catalyse the aldol condensation reaction about 19 times faster than nonimprinted controls. 

Fig. 3.1. A highly schematic representation of the molecular imprinting process. A monomer mixture with ehemical functionality complementary to that of the template is allowed to form solution adducts through the complementary interacting functionalities (reversible covalent or non-covalent interactions). Polymerisation in the presence of a cross-linking agent, followed by removal of the template, leads to the defining of recognition sites of complementary steric and functional topography to the template molecule.

When considering the suitability of a bead production technique for imprinting, it is essential to evaluate the compatibility of the conditions used for polymerisation with those required for complex formation between functional monomers and templates. Where covalent imprinting methods are used, the covalent adducts are often highly stable and need quite harsh conditions to disrupt them. Such adducts could be used in most of the procedures described below with reasonable expectation of success. The same can be said for many metal-chelate complexes, which have stabilities approaching covalently bonded structures. The use of cyclic boronate esters is an exception. This adduct is unstable in water and hence cannot be combined efficiently with aqueous suspension polymerisation. 

Preparation of covalent conjugate or non-covalent adduct between a functional monomer and a template molecule,... 

In step 1, functional monomer and template are connected by a covalent linkage (in covalent imprinting ) or they are placed nearby through non-covalent interactions (in non-covalent imprinting ). In step 2, the structures of these conjugates (or adducts) are frozen in a three-dimensional network of polymers. The functional residues (derived from the functional monomers) are topographically complementary to the template. In step 3, the template molecules are removed from the polymer. Here, the space in the polymer originally occupied by... 

As described above, the molecular imprinting method is of two types, depending on the nature of adducts between functional monomer and template (either covalent or non-covalent). Typical examples of these two kinds of methods are presented in Figs. 2.2 and 2.3. Both have advantages and disadvantages, and thus the choice of the best method strongly depends on various factors (see below). 

Choice of solvents is dependent on the kind of imprinting. In covalent imprinting, many kinds of solvents are employable as long as they satisfactorily dissolve all the components. In non-covalent imprinting, the choice of solvent is more critical to the promotion of the formation of non-covalent adducts between the functional monomer and the template and thus to enhancement of the imprinting efficiency. Chloroform is one of the most widely used solvents, since it satisfactorily dissolves many monomers and templates and hardly suppresses hydrogen... 

In some cases, non-covalent adducts between functional monomer and template are too unstable to be used at higher temperatures, and the polymerization must be carried out at lower temperatures. Under these conditions, the thermal decomposition of initiator cannot be used to initiate the polymerization, and the initiators are decomposed with UV-light irradiation (photo-initiation never requires high temperatures). If the monomers themselves absorb UV light sufficiently, the polymerization is initiated even in the absence of any radical initiators. 

The reaction procedures for non-covalent imprinting are far simpler than those for covalent imprinting. Functional monomers are simply combined with template in the polymerization mixtures and copolymerized with crosslinking agent. The adducts between the functional... 

In aprotic solvents, the carboxylic acid residues in methacrylic acid and acrylic acid form hydrogen bonds with various basic templates and form non-covalent adducts. These two monomers are being widely used in the current polymer industry, and are available on quite a large scale at low cost. Molecular imprinting with the use of hydrogen bonding between methacrylic acid and atrazine (a herbicide) is described in detail in Chapter 6. 

Adduct formation by other non-covalent interactions is also detectable by -NMR. When cholesterol (template) forms an inclusion complex with cyclodextrin (cyclic functional monomer composed of several glucose units), 18 methyl protons of the cholesterol show up-field shift due to the change in its chemical circumstance. The binding constant is 550 M 1. This system provides ordered assemblies of two cyclodextrin molecules, which bind cholesterol cooperatively (see also Example 5.5 in this chapter) [2]. 

Table 5-2 Binding constants for template-monomer adduct formation under polymerization conditions...

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