Polymers Thermoplastics can be made stiffer and more durable by cross-linking them, but this makes it more difficult to reprocess and recycle them. Rottger et al. show that the transesterification of boronic esters grafted onto common polymers such as polyethylene and polystyrene improves their shape stability and chemical resistance. However, unlike traditionally cross-linked materials, their polymers can still be extruded or injection-molded. This is because the covalent cross-links can undergo rapid exchange reactions, which allows the material to flow at high temperatures while still retaining a cross-linked structure. Science , this issue p. [62][1] [1]: /lookup/doi/10.1126/science.aah5281