Porous polymer monoliths for small molecule separations: advancements and limitations

Abstract

Porous polymer monoliths are considered to be one of the major breakthroughs in separation science. These materials are well known to be best suited for the separation of large molecules, specifically proteins, an observation most often explained by convective mass transfer and the absence of small pores in the polymer scaffold. However, this conception is not sufficient to explain the performance of small molecules. This review focuses in particular on the preparation of (macro)porous polymer monoliths by simple free-radical processes and the key events in their formation. There is special focus on the fluid transport properties in the heterogeneous macropore space (flow dispersion) and on the transport of small molecules in the swollen, and sometimes permanently porous, globule-scale polymer matrix. For small molecule applications in liquid chromatography, it is consistently found in the literature that the major limit for the application of macroporous polymer monoliths lies not in the optimization of surface area and/or modification of the material and microscopic morphological properties only, but in the improvement of mass transfer properties. In this review we discuss the effect of resistance to mass transfer arising from the nanoscale gel porosity. Gel porosity induces stagnant mass transfer zones in chromatographic processes, which hamper mass transfer efficiency and have a detrimental effect on macroscopic chromatographic dispersion under equilibrium (isocratic) elution conditions. The inherent inhomogeneity of polymer networks derived from free-radical cross-linking polymerization, and hence the absence of a rigid (meso)porous pore space, represents a major challenge for the preparation of efficient polymeric materials for the separation of small molecules.

Chromatographic efficiency comparison of polyhedral oligomeric silsesquioxanes-containing hybrid monoliths via photo- and thermally-initiated free-radical polymerization in capillary liquid chromatography for small molecules

“…In the thermal polymerization, chain growth and branching are initiated by free radicals, which originate from the thermal decomposition of AIBN and are continuously released. Then the formation of microgel particles is continuously taking place, followed by clustering into globules with less individualized texture [22][23][24][25]. In the photopolymerization, a great number of free radicals are generated in a short time and distributed homogeneously in the polymerization solution.…”

Preparation and evaluation of hydrophilic C18 monolithic sorbents for enhanced polar compound retention in liquid chromatography and solid phase extraction

“…The column efficiency obtained was relatively low when compared to 5 m particle packed column or silica-based monolith. This is a common limitation of polymer monoliths for small molecule separations in isocratic mode because of lack of small pores [32].…”

Preparation and evaluation of poly(alkyl methacrylate-co-methacrylic acid-co-ethylene dimethacrylate) monolithic columns for separating polar small molecules by capillary liquid chromatography

“…A number of review articles have been published to discuss the chemistry of monolith polymerization [16,17] as well as the development and applications [18][19][20][21] of polymer-based monoliths. In previous studies, various polymer-based monolithic stationary phases were successfully utilized to separate large biomolecules such as proteins [4,6,21,22] and/or small molecules [5,8,23,24].…”