Extruders are the working horse in the polymer processing industry to manufacture final polymeric materials. More recently the interest to employ them as chemical reactors has grown. Reactive extrusion (REX) is already successfully applied for bulk polymerization, and polymer modification such as radical grafting, crosslinking and controlled degradation. The main advantages are (i) the ability to cover highly viscous reaction media both for synthesis and depolymerization (chemical recycling), (ii) excellent mixing and heat transfer, and (iii) the possibility to directly obtain a final product by combining compounding and synthesis in one equipment. Here, the goal is to develop a generic fundamental multi-scale modeling tool that allows for process intensification and functionality control for REX on all length scales (molecular, micro, meso and macro). Currently, the study and design of REX processes is limited as focus is on one scale. The model will be gradually constructed and applied for four industrially relevant REX processes with different levels of complexity, including sustainability control. The starting point is a micro-scale modeling tool for radical grafting, which will be largely extended to a broader range of chemistries, including depolymerization, with fundamental meso-scale mass transfer phenomena, and a more detailed reactor model covering aspects of non-perfect mixing and screw design.