Multiresponsive Reversible Polymer Networks Based on Hydrogen Bonding and Metal Coordination

Abstract

Side-chain-functionalized polymers containing hydrogen bonding and metal coordination sites have been synthesized using ring-opening metathesis polymerization. These polymers were cross-linked reversibly either selectively by using hydrogen bonding or metal coordination or simultaneously using both interactions through the addition of small molecule cross-linking agents. The hydrogen bonding motifs utilized for reversible cross-linking are based on cyanuric acid residues hydrogen bonded to 2,4-diaminotriazine-based cross-linking agents. The metal coordination motifs are based on palladated SCS pincer complexes coordinated to bispyridine cross-linking agents. By controlling the reversible cross-linking strategy, we were able to modulate (1) the rheology of the polymer networks from a free-flowing liquid to a highly elastic gel and vice versa and vary the dynamic moduli over 10 orders of magnitude and (2) the responsiveness of the networks to external stimuli such as temperature and ligand displacement agents. The hydrogen bonded cross-linking resulted in polymer networks that were thermally reversible whereas the metal coordinated cross-linked networks mainly showed chemoresponsive behavior. Since both interactions are fully orthogonal to each other, we successfully cross-linked the polymer using both interactions to obtain multiresponsive networks that exhibited both thermal and chemoresponsiveness. We were also able to selectively de-cross-link the hydrogen bonded cross-links of the multifunctionalized networks through competitive interactions at room temperature via the addition of a monotopic end-capping agent without affecting the metal coordinated cross-links. In contrast, the metal coordination could be de-cross-linked completely using a ligand displacement agent such as triphenylphosphine again without affecting the hydrogen bonded cross-links.