Template polymerisation imprinting

Mosbach et al. also attempted to use metal-coordination interaction to prepare polymeric adsorbents for selective recognition of proteins [27]. Using the monomer (3), they polymerised the protein-monomer complex on methacrylate-derived silica particles in the presence of ribonuclease A as the template protein. After removing the template, the imprinted functionalised silica particles were used as packing... 

However, most of these difficulties should not arise in the case of template-monomer complexes formed with very high association constants. Consequently a number of laboratories, including our own, are currently involved in the design of new polymerisable monomers which can form strong complexes with some of the common structural motifs possessed by many template molecules [5-8], (See also Chapter 4 for further examples.) The use of transition metal co-ordination complexes as templates in imprinting is another particularly promising approach (see Chapter 6 for more detail). 

Zn(II) was employed as a print molecule because of its strong interaction with the bifunctional monomer, DDDPA. Divinylbenzene, L-glutamic acid dioleylester ribitol and toluene were used as matrix-forming monomer, emulsion stabiliser and diluent, respectively. After polymerisation, the print molecules were removed from the resin, upon which selective recognition sites were formed. The schematic illustration of surface template polymerisation with DDDPA is shown in Scheme 9.8. The Zn(II)-imprinted resins were ground into particles, whose volume-averaged diameters were ca. 40 pm. The yield was ca. 80%. 

Burow and Minoura performed a similar kind of investigation to prepare protein imprinted polymers [48]. They used methacrylate modified silica particles as the carrier matrix on which imprinted sites were created. Using acrylic acid as the functional monomer and A,TV -1,2-diethylene bisacrylamide as the cross-linker, template polymerisation was carried out in the presence of glucose oxidase. This approach led to formation of a thin layer of cross-linked polymer film on the silica surface. After removing the template protein, substrate selectivity of the polymer was tested. Preferential affinity of the polymer for its template suggests the formation of substrate-selective binding sites in the polymer matrix. 

An alternative approach to effect chiral discrimination is to use the technique of molecular imprinting, the subject of this book. This technique, sometimes also referred to as template polymerisation, results in synthetic polymers of predetermined selectivity. Receptor-like binding sites are tailor-made in situ by the copolymerisation of cross-linkers and functional monomers, which are interacting with... 

Chiral synthetic polymer phases can be classified into three types. In one type, a polymer matrix is formed in the presence of an optically pure compound to moleculady imprint the polymer matrix (Fig. 10) (107,108). Subsequent to the polymerisation, the chiral template is removed, leaving the polymer matrix... 

A good imprinting effect also relies on the existence of stable assemblies between the template molecule and the functional monomers in the course of the polymerisation process therefore, any preparation method for beaded MIPs must preserve such template-monomer assemblies. 

Another possible way of overcoming the limitations posed by the presence of water in the suspension polymerisation process is to substitute the continuous water phase with alternative solvents that could still act as dispersing medium for the monomer mixture but better preserve noncovalent interactions in the template-monomer assembly. For example, liquid fluorocarbons are chemically inert and do not affect interactions which are used in noncovalent imprinting. Use of such solvents for the preparation of MIP microbeads has been demonstrated already in 1996 by Mayes and Mosbach [16,17]. A range of MIPs were prepared using Boc-l-phenylalanin as the template, MAA as the functional monomer and different kinds and amounts of crosslinkers and porogenic solvents. The resulting MIP microbeads... 

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. 

In the following years, many imprinted polymers were prepared by two- or multi-step swelling and polymerisation method using mostly EDMA as a crosslinker and various functional monomers interacting with the chosen template... 

The thickness of the imprinted polymer shell can be also tuned in the range 10—40 nm by changing the relative amounts of functionalised silica nanoparticles and polymer shell precursors. The resulting core-shell particles exhibit enhanced capacity of rebinding the TNT template over 2,4-dinitrotoluene in comparison to particles prepared by precipitation polymerisation. Nevertheless, this strategy, although leading to impressive results, cannot be easily applied to other templates and monomers. 

The performance of the molecularly imprinted monolith in terms of molecular recognition and flow-through properties depends on several factors, especially the density and the porosity of the polymer. In order to obtain a monolith with high selectivity and high permeability, some preparation conditions must be optimised, in particular the composition of the prepolymerisation mixture including the amount of template, the type and amount of functional monomer, crosslinker, porogenic solvent and the initiator, and the polymerisation conditions such as initiation process and polymerisation time. 

Compared to the in situ polymerisation of a monolith, the grafting approach does not need re-optimisation of the protocol in order to obtain an appropriate porosity and flow properties for the monolith when monomer or template is changed. Moreover, the properties of the core materials are generally preserved and the imprint generated on the surface of the materials only requires a minimum amount of template and provides well-accessible recognition sites. 

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