Metal–Ligand Complexes as Dynamic Sacrificial Bonds in Elastic Polymers

Abstract

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 noncovalent cross-links based on metal−ligand complexes into styrenebutadiene 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.

Phase Separation and Gelation in Solutions and Blends of Heteroassociative Polymers

“…However, in many systems, the stickers heteroassociate, forming A–B linkages, either between different sections of a heteropolymer or between different chains in a mixture of associating polymers. Examples of heterocomplementary associations include hydrogen bonding, , ionic bonds between charges of opposite sign, , metal–ligand coordination, − host–guest interactions, , cation−π stacking, or Lewis acid–base pairs . The diversity of heterotypic associations provides a wide range of association strengths (or affinities), which broadens the design space for new reversible networks. − …”

A Facile Strategy to Fabricate Tough and Adhesive Elastomers by In Situ Formation of Coordination Complexes as Physical Crosslinks

“…[26] Improved structure homogeneity can be achieved by directly mixing ligandcontaining polymers with metallic ions in organic solvents, followed by solvent evaporation to obtain tough metallosupramolecular elastomers. [27,28] For instance, curcumin-containing linear poly(dimethylsiloxane) were synthesized and dissolved in a tetrahydrofuran solution of Eu(CF 3 SO 3 ) 3 to form curcumin-Eu 3+ coordination complexes. [27] The mechanical properties of thus obtained elastomer can be well tailored by regulating the ratio of curcumin motif to Eu 3+ ions.…”

Phase Separation and Gelation in Solutions and Blends of Hetero-Associative Polymers

“…[41][42][43][44] However, in many systems, the stickers hetero-associate, forming A − B linkages, either between different sections of a heteropolymer, or between different chains in a mixture of associating polymers. Examples of hetero-complementary associations include hydrogen bonding, 45,46 ionic bonds between charges of opposite sign, 47,48 metal-ligand coordination, [49][50][51][52][53] host-guest interactions, 54,55 cation-π stacking, 56 or Lewis acid-base pairs. 57 The diversity of heterotypic associations provides a wide range of association strengths (or affinities), which broadens the design space for new reversible networks.…”