Metal-Ligand Complexes as Dynamic Sacrificial Bonds in Elastic Polymers

The chemical modification of technical rubbers to fine-tune their properties is a subject of current interest. Here, we report a facile approach for the straightforward introduction of non-covalent cross-links based on metal-ligand complexes into styrene-butadiene rubber (SBR). This was achieved by grafting the multidentate 2,6-bis(1′-methylbenzimidazolyl)-pyridine (Mebip) ligand to the polymers' vinylic double bonds via a thiol-ene reaction. The addition of zinc trifluoromethanesulfonate [Zn(OTf)2] leads to the formation of Zn-Mebip complexes, whose concentration can be readily varied. The non-covalent interactions greatly increase the SBR's toughness, strain at break, and tensile strength, without significantly impacting the elasticity. Moreover, the beneficial influence of metal-ligand complexes extends to conventionally vulcanized SBR and filled rubber compounds. The characterization of different materials under significant oscillatory stresses shows that the non-covalent interactions introduce an additional mode of energy dissipation, even in the presence of extensive covalent cross-linking and different reinforcing fillers. The data suggest that metal-ligand complexes can act as dynamic sacrificial bonds that reinforce the rubbers under conditions of high demand, while corresponding materials featuring only covalent cross-links fail. Considering that the interaction strength and dynamics of the metal-ligand complexes can be readily modified via the choice of the ligand and the metal salt and that their concentration can be easily varied, the approach should allow one to adapt the properties of SBRs and other rubbers containing unsaturated bonds over a wide range.