Porogen molecules

The thermal initiator, present in the polymerization mixture, decomposes at a certain temperature accompanied by disposal of radicals that initiate the polymerization reaction of monomer as well as cross-linking molecules in solution. After becoming insoluble in the employed polymerization mixture (strongly dependent on the nature of porogenic solvent and on the degree of cross-linking), the polymer nuclei precipitate. 

Generally, two different procedures have been adopted for preparation of MIPs. They involve either covalent or non-covalent complex formation of a template and complementary monomers with apt functional groups. [19]. Co-polymerization of this complex with a cross-linking monomer in a porogenic solvent solution, followed by removal of the template, results in formation of the porous polymer material with recognition sites complementary in size and shape to molecules of the target compound that can next be determined as an analyte. 

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. 

In the noncovalent approach, the monomer is self-assembled around the tern-plating molecule and then again copolymerized with the additional monomer. The template is then removed by using a porogenic solvent. 

The print molecule (e.g. clenbuterol) is dissolved in a porogen (e.g. acetonitrile) together with either one or two monomers (e.g. methacrylic acid and 4-vinylpyridine). This allows non-covalent complexes to form... 

Conventionally, MlPs are obtained by bulk co-polymerization from a mixture consisting of a functional monomer, cross-linker, chiral template, and a porogenic solvent mixture. Nowadays, imprinting via non-covalent template binding is preferred over the covalent mode and involves three major steps (see Fig. 9.9). (i) Functional monomers (e.g. methacrylic acid, MAA) and a cross-linker (e.g. ethyleneglycol dimethacrylate, EDMA) assemble around the enantiomeric print molecule, e.g. (S)-phenylalanine anilide (1), driven by non-covalent intermolecular interactions, e.g. ionic interactions, hydrogen bonding, dipole-dipole interaction. Tr-rt-interaction. (ii) By thermally or photochemi-... 

Not only the rigidity is crucial to the efficiency of MIPs, but also the accessibility as many recognition sites as possible should be accessible for rebinding. The material should therefore be porous. This is realised by dissolving monomers, cross-linkers and print molecules in a porogenic solvent prior to polymerisation. The effect of the solvent on the polymer morphology can be monitored by measuring physical parameters such as surface area, pore diameter and pore volume. 

Fig. 21.3. Schematic figure of iton-covalent imprinting of polyurethane with an organic solvent as template and porogen. Highly robust MIPs are best prepared over night at room temperature. Template removal is achieved by evaporation or dissolution. Due to the ultra-thin layers the print molecules are often removed completely.

Low-weight organic molecules, such as volatile organic compounds (VOCs) [25], e.g. hydrocarbons without functionalities or anaesthetics, can be used as print molecules for non-covalent MIPs. If the print molecule is a suitable organic solvent, the print molecule itself is the porogen during the polymerisation process. Enhanced imprinting effects are promoted by n-n interactions between aromatic moieties in monomers and analytes, such as polycyclic aromatic hydrocarbons (PAHs) or aromatic VOCs (xylene or toluene, for example). 

Fig. 21.13. QCM responses to coatings polymerised from different porogens (acting as print molecules) with/without embedded calix[6]arene in brackets. The sensor response of 70 nm layers to 500 ppm of xylene isomers is shown. Compared to the layer polymerised in chloroform, the pronounced imprint effect for xylene can be seen. o-Xylene has the lowest volatility of the xylene isomers and therefore is equally detected in all layers.

For small, uncharged, solute molecules the strength of intermolecular non-covalent interactions is dependent upon the local environment which is, in turn, defined by the properties of the solvent [4, 33, 34]. In general terms, the stability of the pre-polymerisation complex is favoured in non-po-lar solvents. Therefore solubility in potential porogens must be considered when assessing the suitability of a compound for molecular imprinting. For polar template molecules, there is usually a trade off between imprint specificity and solubility. If the solubility of the template is such that a polar solvent is required, then the complex will be less stable and the imprint less specific [17]. 

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