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 among the largest class of cross-linked polymers, making up 31% of the thermosetting materials market in 2012; however, their direct recycling into similar value materials is not well developed. We demonstrate that several Lewis acid catalysts mediate urethane exchange, likely via a dissociative mechanism, 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 s at temperatures as low as 140 degrees C. 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.