Mechanosynthesis: an environmentally friendly strategy to produce hyper-crosslinked polymers from biosourced precursors
Abstract
Hyper-crosslinked polymers (HCPs) are an emerging type of polymers with high specific surface area, porosity, and a stable and rigid structure, as a consequence of the type of linkages and the degree of entanglement. Due to these characteristics, they have been applied in many different areas with outstanding outcomes, such as catalysts, gas adsorption, hazardous compounds removal, etc. HCPs have been typically prepared via solvent-based Scholl coupling and Friedel-Crafts reactions, implying the latter the use of an external crosslinker. These protocols imply long reaction times and mild temperatures. The present work is focused on the use of mechanosynthesis (MS) technology to obtain HCPs with the same favourable properties, but significantly decreasing the environmental impact of chemical- and solvent-based protocols. MS consists of a simple process, in which a cylindrical ball mill, the reactor, is filled with spherical balls of determined diameters, and which rotates at high speeds (around 400-800 rpm). This provokes collisions which provide the energy necessary to produce chemical reactions within, with no need for high temperature or solvents. The present work analyses the different outcomes already obtained in the literature about the formation of HCPs via MS, via Friedel Crafts, Scholl coupling and less common procedures. An analysis of the twelve principles of green chemistry related to MS and compared to solvent-based technologies will be presented. In addition, the main applications will be analysed to provide an overview of the potential of these HCPs prepared with the MS technique. In an even greener approach, the use of biosourced materials to formulate HCPs will be discussed. Finally, the last section about the use of residual fractions to produce HCPs via MS will also be analysed. With this overview, the authors aim to present the information necessary to encourage the research community to explore the use and potential applications of HCPs, and the use of greener alternatives to obtain them.