Reprocessing Cross-linked Polyurethanes by Catalyzing Carbamate Exchange
The reprocessing or recycling of cross-linked polymers by incorporating dynamic covalent cross- links has the potential to increase their usable lifetimes and reduce their environmental impact. Polyurethanes (PUs) are the largest class of cross-linked polymers; however, their direct recycling into similar value materials is not well-developed. We demonstrate that several Lewis acid catalysts mediate urethane exchange selectively and under mild conditions. Incorporating these catalysts into cross-linked polyether and polyester PUs with structures similar to commercial PU thermosets gives cross-linked materials that can completely relax stress in 100 seconds at temperatures as low as 140 oC. The dynamic polymers were reprocessed via compression molding to provide materials with similar cross-linking densities to as-synthesized materials. Because these systems are based on commercially available PU monomers and inexpensive Lewis acid catalysts, we anticipate that these findings will enable the recycling of traditional thermosetting PUs.