Template-functional monomer interactions

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. 

A similar approach has been adopted by Whitcombe et al. [15], where NMR chemical shift studies allowed the calculation of dissociation constants and a potential means for predicting the binding capacities of MIPs. The NMR characterization of functional monomer-template interactions has also been applied to the study of the interaction of 2,6-bis(acrylamido) pyridine and barbiturates [16], and of 2-aminopyridine and methacrylic acid [17]. Recent NMR work in our laboratory [18] has involved the determination of template-monomer interactions for a nicotine-methacrylic acid system. Significantly, it was shown in this study that template self-association complexes are present in the prepolymerization mixture and that the extent template self-association is dependent both upon solvent and the presence of monomer. 

More recently, Priego-Capote et al. reported on the production of MIP nanoparticles with monoclonal behaviour by miniemulsion polymerisation [63]. In the synthetic method that they employed, they devised to use a polymerisable surfactant that was also able to act as a functional monomer by interacting with the template (Fig. 4). The crosslinker content was optimised at 81% mol/mol (higher or lower contents leading to unstable emulsions). In this way, the authors were able not only to produce rather small particles (80-120 nm in the dry state) but also to locate the imprinted sites on the outer particle surface. The resulting MIP nanobeads were very effective as pseudostationary phases in the analysis of (/ ,S)-propranolol by CEC. 

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... 

It is of obvious importance that the functional monomers strongly interact with the template prior to polymerisation, since the solution structure of the resulting assemblies presumably defines the subsequently formed binding sites. By stabilising the monomer-template assemblies it is possible to increase the number of imprinted sites. At the same time the number of non-specific binding sites will be minimised, since there will be a reduction in the amount of free, non-associated functional monomer. For any particular template, the following factors that are likely to affect the recognition properties of the site have been identified (Fig. 5.14). 

The quantity and quality of MIP recognition sites that arise out of binding event is a direct function of the nature and extent of the monomer-template interactions present... 

The solvent should be capable of fully solubilizing the monomers and template in one pot. For monomer-template interactions stabilized by polar forces, non-protic solvents of low polarity should be chosen, since they are less likely to compete with the monomers for the template. The functional monomer-template complexes are often based on hydrogen bond interactions. If the solvent is a good hydrogen bond donor or acceptor, it will compete with the monomers and destabilize the complexes. For monomer-template systems stabilized by solvatophoic forces, more polar solvents and higher temperatures may be favorable. 

Based on the occurring interactions during the functional monomer-template complex formation and template rebinding, three approaches regarding molecular imprinting are reported ... 

The porogen (solvent) is finally responsible for the formation of pores within the polymer network (Fig. 7.3). Under the conditions of a typical dispersion polymerization, the polymer precipitates as it forms, stiU containing high amounts of porogen within the pores. For a successful imprinting, the template should be soluble in the porogen (sometimes the mixture has to be heated to assure this), but the porogen should not compete with the functional monomer for interaction with the template. If this interaction is, for example, mainly to be based on... 

Fig. 6-10. Influence of the number of basic interaction sites of the template versus the separation factor measured in chromatography for the corresponding racemate. The templates were imprinted using MAA as functional monomer by thermochemical initiation at 60/90/120 °C (24 h at each temperature) and using acetonitrile as porogen. (From Sellergren et al. [15].)...

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