Supramolecular Cross-Linking of Hydrocarbon Polymers for the Formation of High-Performance and Recyclable Materials

Thanks to their excellent thermomechanical properties, cross-linked polymers are widely used for a myriad of applications. However, due to the presence of covalent cross-links, these polymers are usually considered nonrecyclable. For hydrocarbon polymers, this problem is particularly acute because, due to the lack of functional groups, the few known cross-linking strategies are based on highly stable covalent bonds. To address this problem, we propose here a strategy to physically cross-link a polymer by adding a supramolecular host, which can complex two pendant alkyl groups of a macromolecular chain. We demonstrate that dimethoxypillar[5]arene (DM-P[5]A) possesses this unique ability. Taking advantage of this phenomenon, poly(1-decene) (PD) is conveniently cross-linked upon the addition of DM-P[5]A, thus transforming a viscous liquid into an elastomer. Furthermore, reversible cross-linking is also efficient in the solid state: for example, high-T_g poly(5-hexyl-2-norbornene) (PNBE) becomes cross-linked when containing DM-P[5]A, resulting in polymers with greater mechanical properties. Remarkably, DM-P[5]A can be simply washed out with a suitable solvent, allowing the recovery of the pristine polymer free of cross-links. This very straightforward strategy thus allows any polymer containing pendant alkyl groups to be reversibly cross-linked, thus essentially solving the problem of recyclability of cross-linked polymers.