Covalent imprinting

The use of a p-vinylphenyl boronate as functional monomer to be covalently linked with a diol-template [2] is demonstrated in Fig. 2. Following polymerization in the presence of a cross-linker, the template has to be extracted from the polymer network. This requires breaking the covalent bond. During the application of covalently imprinted materials, the target molecules have to reform such bonds in order to be retained. Both making and breaking the bonds is at best a time-consuming process. 

Scheme 15.2 Covalent imprinting of a-phenyl-D-mannopyranoside in a divinyl benzene/4-vinylphenylboronic acid matrix, via the formation of covalent boionic ester linkages between the 4-vinylphenylboronic acid and the carbohydrate. Adapted from Wulff, Vesper, et al. (1977). Copyright 1977 Wiley InterScience.

Khasawneh MA, Vallano PT, Remcho VT. Affinity screening by packed capillary high performance liquid chromatography using molecular imprinted sorhents. II. Covalent imprinted polymers. J Chromatogr A 2001 922 87-97. 

Most MIPs show a heterogeneous distribution of binding sites and can be considered as polyclonal in their nature. In non-covalent imprinting, the amorphous material contains binding sites which are not identical because they may have different cross-linking density or accessibility. Moreover, the monomer (M) and the template molecule (A) may form complexes of different stoichiometry (MnA) in the pre-polymerization mixture [5]... 

In theory, the use of stoichiometric non-covalent or covalent imprinting yields a homogeneous binding site population ( monoclonality ) after template removal from the material [24]. 

Both of these MIP preparation procedures have their advantages and limitations [20, 21]. For instance, the size of the analyte molecule is not a discriminating criterion in covalent imprinting since the template selectively determines the interaction sites. In contrast, non-covalent imprinting has the advantage of being simpler since an additional synthetic step is not required to introduce the template into the polymer matrix. Moreover, removal of the template via extraction with the suitable solvent solution is simple and mostly complete for the non-covalent imprinting. 

There are two processes by which the bulk imprinted polymers are formed covalent imprinting and noncovalent imprinting. In the former, the template molecule is first covalently functionalized with the monomer, and then copolymerized with the pure monomer. After that the covalent bond is broken and the template molecule is removed by extraction. In order to facilitate the extraction step, a so-called porogenic solvent is used. It effectively swells the polymer matrix. 

One of the first examples of the covalent imprinting of bulk silica was described by Katz and Davis.58 In this study, an imprint molecule containing an aromatic core and protected 3-aminopropyltrimethoxysilane groups was synthesized. When hydrolyzed and condensed with TEOS followed by removal of the aromatic core, a shape-selective base catalyst was prepared.58 Katz and Defreese also used a similar approach to create proline-amide imprints in bulk microporous silica.59 Additional... 

Figure 20.4 Simplified diagram of covalent imprinting in silica.

For covalent imprinting, the template molecule is copolymerized into the polymer network with the monomer and crosslinker. Instead of extracting... 

The polymeric adsorbents are usually prepared by variations of two-phase suspension processes. These refer to systems where microdroplets of monomers and solvent are converted into solid beads upon polymerisation. In the case where the monomers are not water soluble, as in the case of styrene-based polymers and many methacrylate-based polymers, the monomers, a solvent and a droplet stabiliser are suspended as droplets by stirring in water and then polymerised (o/w suspension polymerisation). The particle size and dispersity can be influenced by the stirring speed and the type of stabiliser. So far, only a few examples of the preparation of imprinted polymers in bead format have been described [4-8] and these are thoroughly reviewed in Chapter 12. In non-covalent imprinting, the main limitation to the use of these techniques is that the imprinting method often requires the use of polar partly water soluble monomers or templates in combination with less polar water insoluble components. Use of the o/w suspension method... 

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