We report here the nonlinear rheological properties of metallo-supramolecular networks formed by the reversible cross-linking of semi-dilute unentangled solutions of poly(4-vinylpyridine) (PVP) in dimethyl sulfoxide (DMSO). The reversible cross-linkers are bis-Pd(II) or bis-Pt(II) complexes that coordinate to the pyridine functional groups on the PVP. Under steady shear, shear thickening is observed above a critical shear rate, and that critical shear rate is experimentally correlated with the lifetime of the metal-ligand bond. The onset and magnitude of the shear thickening depend on the amount of cross-linkers added. In contrast to the behavior observed in most transient networks, the time scale of network relaxation is found to increase during shear thickening. The primary mechanism of shear thickening is ascribed to the shear-induced transformation of intrachain cross-linking to interchain cross-linking, rather than nonlinear high tension along polymer chains that are stretched beyond the Gaussian range.

We report here the formation and dynamic mechanical properties of metallo-supramolecular networks formed by mixtures of bis-Pd(II) and Pt(II) cross-linkers with poly(4-vinylpyridine) in DMSO. Precise control over the dynamic mechanical properties of the bulk materials is achieved through the dissociation kinetics of the metal-ligand coordination bond responsible for cross-linking, the density of the cross-links, and the relative amounts of multiple cross-links. In networks formed from multiple types of cross-linkers, discrete contributions from each type of cross-linker are evident in the bulk mechanical properties, rather than an average of the contributing species. The heterogeneous rheology is consistent with simple models, to the extent that complex viscoelastic responses can be rationally engineered.

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The thermodynamics of pyridine coordination in 1,4-phenylene bridged binuclear palladium and platinum organometallic complexes [1,4-(MOTf) 2 -{C 6 (CH 2 NR 2 ) 4 -2,3,5,6}] (11, M = Pd, Pt; R = CH 3 , C 2 H 5 , R 2 = -(CH 2 ) 5 -) are measured by 1 H NMR in DMSO-d 6 . The coordination of substituted pyridines by bimetallic complexes 11 or 12 in DMSO is found to proceed via two effectively independent metal-ligand binding events, and the association constants for pyridine coordination and rate constants for pyridine exchange are nearly identical to those measured previously on monometallic analogs. A linear free energy relationship between the association constant for pyridine coordination and the inductive Hammett constant of the pyridine substituent is observed, and the sensitivity (ρ = −1.7 ~ −2.1) in DMSO depends only slightly on metal (Pd vs. Pt) and spectator ligand (pincer dialkylamine vs. triarylphosphine). The association constant for a particular pyridine ligand, however, varies by roughly three orders of magnitude across the series of metal complexes. The effective independence of the two coordination sites and the range of available thermodynamic and kinetic behaviors of the coordination guide the use of these versatile building blocks in metallosupramolecular applications.

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