Progress toward sustainable polymer technologies with ball-mill grinding

The ball-mill grinding (BMG) of polymers has a long history, starting with Staudinger showing in the 1930s that polystyrene undergoes chain scission upon ball milling. However, BMG has significantly expanded from being used solely for polymer degradation to a synthetic tool for a range of applications only in the last decade. Now, BMG has emerged as a promising mechanochemistry technique for several critically important polymer technologies, such as recycling and upcycling, and often provides novel or enhanced mechanochemical reactivity. As a solid-state technique in which solvents are often minimized or eliminated, BMG provides a greener and more sustainable route to various applications. Also, in contrast to many other mechanochemistry techniques that are commonly employed with polymers, BMG has the potential to be scaled to industrially relevant levels. In our review, we provide an extended and deep overview of the phenomena that occur when polymers are subjected to BMG and show how these phenomena can be exploited for various applications. We treat particularly technologies that, especially in the context of our current plastic pollution crisis, are relevant to trending topics in the field of polymer science, such as polymer degradation, chemical recycling, recycling, and upcycling. Other important topics covered in this review include the mechanical activation of responsive polymers, by the use of mechanophores or by exploiting the reactivity of the reactive intermediates generated during chain scission, and polymer-assisted grinding, where polymers serve as additives or reagents to aid in mechanochemical syntheses or other processes.