Polymers have revolutionised fields from aerospace and wind energy to food packaging and adhesives. When their end of life is reached, though, some are more challenging than others to deal with sustainably. Thermoplastic polymers like those used in plastic bottles are flexible and elastic (they have plasticity), and they are easily recyclable. Thermoset polymers become ‘set’ in place when heated and moulded – great for applications requiring strength, rigidity and structural integrity but a barrier to recycling. Covalent adaptable networks (CAN), polymer networks with reversible covalent crosslinks, could be the solution. The EU-funded CiMaC project is engineering novel CAN precursors that could overcome barriers and enhance control of CAN properties.