# Methacrylate Polymer Monoliths for Separation Applications
## 1 Introduction  
Monolithic materials are increasingly utilized in the separation of both chemical and biological molecules. The term monolith in separation science refers to a single discrete piece of porous material used for the separation or pre-concentration of analytes. This is achieved by passing the analyte within a fluid across a monolith material, which has specific macropores, mesopores and surface chemistry. Pores are defined according to the International Union of Pure and Applied Chemists (IUPAC) in the following way: micropores < 2 nm, mesopores a size range of 2-50 nm, while macropores > 50 nm [1]. Over the years, numerous compounds have been investigated as candidates for the efficient and highly-resolved separation of both chemical and biological species. After this introduction to monolith materials, this paper presents a detailed overview of methacrylate-based polymer monoliths and their applications. The ability for monoliths to be used as separation media has been known since the 1950s, when porous gels were reported by [PERSON] and [PERSON] [2]. Later, in the 1970s, the forerunners of modern polymer monoliths were fabricated as porous polyurethane foams. Modern monolith materials which were more readily embraced by the chromatography community first appeared in the 1990s due to the seminal work by [PERSON], [PERSON], and others [34]. Such work has been the subject of numerous reviews, and has encompassed a diverse range of materials including, for example carbonaceous monoliths,silicate based materials, inorganic monoliths, and organic polymers, such as vinyl esters, methacrylates, polystyrenes, ethylene glycols, and acrylonitrile-butadiene-styrene (ABS) [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19]. This list does not cover the wide range of functional groups which has been investigated, in order to tailor the monolith stationary phase to a particular analyte or separation mechanism [8; 9; 10; 13; 15; 20; 21; 22; 23; 24]. A critical feature of polymer monoliths is that they possess a large number of smaller pores, which furnishes them with a higher surface area, which is desirable for increased separation efficiencies. Silica monoliths achieve good separation efficiencies by virtue of their high surface area, which can be 10's-100's m\({}^{2}\)/g, depending on the preparation conditions. Higher surface areas and greater numbers of smaller pores lead to an increase in potential interaction sites for the analytes in the mobile phase, therefore, in theory, leading to better separation efficiencies. While smaller pores are important for separation, the presence of larger pores is important as these allow for sufficient flow of the mobile phase through the monolith in order to avoid high back pressures, which would inhibit separation and potentially cause structural defects in the monolith, High Performance Liquid Chromatography (HPLC) pump, or problems with tubing connections to the HPLC instrument. Monolith columns can thereby exhibit low flow resistance compared to packed bead columns allowing separations at higher flow rates, which is attractive from the point of view of scaling up separation processes. The monomer units of polymer monoliths can be functionalized prior to the polymerization step or functionalized _in situ_ after the polymerization step. Functionalization is discussed in more detail in Section 2.4. Due to these features, polymer monoliths display efficient separations of larger molecules such as proteins. Of the diverse range of monolith materials, methacrylate polymers have received more attention than others. This can be attributed to their relatively ease of preparation, functionalization [9], and that they can be fabricated with a high degree of macroporosity. The necessity of efficiently obtaining and identifying pure samples of DNA, plasmids, nucleic acids or other biomarkers, as well as pure chemical species, such as pharmaceutical products, has been the driving force behind much of the research into methacrylate monolith development.