Intensification of Polymerization Processes by Reactive Extrusion

The unique geometry of screw extruders allows handling very viscous media; thus, they can be used as continuous reactors to conduct polymerization reactions without using any solvents. This is called reactive extrusion polymerization. It is an intensified process as it maximizes reaction rates upon maximizing the concentrations of monomers and an initiator/catalyst (if there is any). It also allows conducting polymerization reactions at higher temperatures, without having to take measures to prevent solvents from evaporating or to work under pressure. However, screw extruders are not suitable for the early stage of polymerization in which the viscosity of the polymerization system may not be high enough for the rotation of the screws to generate a steady flow. A combination of continuously stirred tank reactors and a twin-screw extruder or several in series is preferred for the polymerization process. This review shows how reactive extrusion may intensify different types of polymerization processes under the constraint of no solvents, such as free-radical polymerization, anionic polymerization, ring-opening polymerization, and polycondensation, on one hand; and how challenges are met, on the other hand.