## 3 Overview of Molecularly Imprinted Polymers as Catalysts and Support for Catalysts
### Types of Monomers  
As already mentioned, the monomers are small molecules that can undergo polymerization, thereby contributing to the constitutional unit of the essential structure of macromolecules [67]. The choice of monomers is crucial for the performance of MIPs, and there are basically four types of monomers: those that have acidic, basic, neutral, and cross-linking characteristics. The acidic monomers typically contain functional groups that can release protons (H\({}^{+}\)) in aqueous solutions, hence making them acidic. In contrast, the basic monomers contain functional groups that can accept protons (H\({}^{+}\)) in aqueous solutions, making them basic, whereas the neutral monomers do not significantly alter the pH of the solution. The last type of monomer (cross-linking) has functional groups that can form covalent bonds with other monomers during polymerization, thereby creating a three-dimensional network within the polymer. Table 1 shows some examples of these four classes of monomers. Figure 2: Schematic representation of the molecular imprinting process. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Characteristics** & **Monomer Chemical Structure** & **Name** & \begin{tabular}{c} **Advantages/** \\ **Disadvantages** \\ \end{tabular}  & **Reference** \\ \hline Acidic & & \begin{tabular}{c} \\ **cyclic acid** \\ \end{tabular}  (AA) & \begin{tabular}{c} Strong hydrogen bonding \\ capacity; may be unstable \\ under extreme \\ pH conditions. \\ \end{tabular}  & [68] \\ \hline Acidic & & \begin{tabular}{c} p-vinylbenzoic acid \\ \end{tabular}  (VBA) & \ \begin{tabular}{c} High selectivity; limited \\ solubility may \\ restrict applications. \\ \end{tabular}  & [68] \\ \hline
\begin{tabular}{c} **O** \\  
The careful selection of monomers for the fabrication of MIPs is crucial for ensuring the effectiveness and specificity of the final polymer. Firstly, the affinity between the monomers and the template molecule is essential. The monomers must interact strongly and specifically with the template molecule through non-covalent interactions, such as hydrogen bonding, \(\pi\)-\(\pi\) interactions, or ionic interactions [78]. Besides affinity, the compatibility of the monomers with the chosen polymerization method is another crucial criterion. Different polymerization methods, such as bulk, precipitation, or emulsion polymerization, require specific characteristics of the monomers to ensure efficient polymerization and the formation of a stable polymer matrix. In addition, the stability of the resulting polymer is also a determining factor in the selection of monomers. The monomers must be chosen to confer the necessary resistance to the final MIP to withstand the conditions of use, which may include changes in pH, temperature, and the presence of aggressive solvents. Cross-linking monomers like ethylene glycol dimethacrylate (EGDMA), divinylbenzene (DVB), and trimethylolpropane trimethacrylate (TRIM) are commonly used for providing structural integrity and stability. Another important criterion when choosing monomers is their solubility, since the monomers should be soluble in the chosen porogenic solvent to ensure uniform polymerization. Porogenic solvents like toluene, acetonitrile, dimethyl sulfoxide (DMSO), and methanol play a critical role in controlling the morphology and porosity of the final MIP, influencing its performance in recognizing and binding the target molecule. Thus, the careful monomer selection process can guarantee the creation of MIPs with high specificity, stability, and efficacy in the intended applications [79; 80]. MIPs have several advantages compared to other design materials. MIPs have extremely high chemical and thermal stabilities, and high affinity and specificity for target analytes. MIPs can be reused, are stable upon storage, and maintain their recognition capacity for many years [77; 81]. The synthesis of MIPs presents several challenges, particularly when aiming to create efficient MIPs without prior knowledge of the ideal solvent and monomer.