# Methacrylate Polymer Monoliths for Separation Applications
## 2 Methacrylate Polymer Monoliths
### Methacrylate Monolith Preparation Methods and Compounds  
The preparation of macroporous methacrylate based monoliths was first reported in the literature in the 1990s [3; 74]. In these methods, the free radical polymerization reaction is carried out in the presence of a precipitant (a solvent with poor solubility for the polymer), which results in a porous polymer network or monolithic structure. There are two main methods of inducing free radical polymerization reactions which have been reported to yield monolithic structures, thermally initiated reactions and photo-initiated polymerization. Equation 1 shows the general reaction scheme. \[\text{Monomer}+\text{Crosslinking Agent}\frac{\text{Initiator}}{\Delta \text{ or hv}}\text{Polymer} \tag{1}\]  
The general procedure for preparing a polymer monolith involves adding the chosen monomer units, porogen, initiator, cross-linker, and, if applicable, functional groups, into a capillary or column (for example silica, stainless steel, polyimide), which is then sealed at both ends. Then, the tube is either heated or exposed to light of the appropriate wavelength. Naturally, for the photo-initiated polymerization to work, the tube must be transparent to the chosen wavelength. The polymerization is then allowed to proceed until the polymer monolith of appropriate porosity and surface area is obtained. The unreacted starting materials, cross-linker and porogen are then thoroughly flushed from the column or capillary with a suitable solvent. The most commonly used initiator is Azo Bis-Isobutyronitrile (AIBN), generally present in concentrations of 1% (w/w), relative to the monomer and crosslinking agents. Reacting different monomers together in varying ratios can lead to differences in monolithic structure and separation mechanisms. Porogenic solvents cause no chemical change to the monolithic phase and are used in the polymerization reaction to induce pores in the resulting polymer monolith. By varying the amounts of the porogen relative to the monomer content, the surface area and porosity can be controlled. These properties affect the types and sizes of molecules, which can be separated on the polymer monolith. Only a small number of porogenic mixtures are used and are a crucial consideration in the synthetic design of the polymerization. Generally, various mixtures of cyclohexanol (**5**) and dodecanol (**6**), 1,4 butanediol (**7**), dimethyl sulfoxide (**10**) and tetra hydrofuran (THF) (**11**), are utilized as the porogenic solvent, see Figure 3 for chemical structures. Polymerization reaction time is another critical factor in the preparation of methacrylate monoliths, as with other polymer-based materials. [PERSON] noted, early on in the research into methacrylate polymer monoliths, that increasing the polymerization time (from 1 h to 22 h) resulted in a dramatic reduction of the surface area and porosity of the polymer (500 m\({}^{2}\)/g and 3.8 mL/g to 120 m\({}^{2}\)/g and 1.1 mL/g, respectively) [74]. Interestingly, the nature of the thermally initiated polymerization reaction (bulk _vs._ suspension) was also reported, in the same contribution, to have a marked effect on the subsequent porosity without necessarily affecting the surface area or pore volume to the same extent. Photo-initiated polymerizations of methacrylate monomers were reported later in the 1990s by [PERSON] _et al._[75] using UV lamps (365 nm) for 60 min. These yielded monoliths with significantly lower surface areas (11-165 m\({}^{2}\)/g) and macro-pores. They have also, more recently, been performed using LED sources with 600 nm [76] and 660 nm [10] wavelengths. Some common cross-linker compounds are shown in Figure 4. The cross-linking is via the vinyl bonds present in all of these compounds. Crosslinking agents are responsible for the amount of swelling of the monolith during the polymerization step, and therefore can also affect the void volume of the monolith. The swelling is dependent on the vinyl character of the cross linker, _i.e._, the mono-vinyl _vs._ di-vinyl content.