Specific metal-ligand coordination between bis-Pd(II) and Pt(II) organometallic cross-linkers and poly(4-vinylpyridine) in DMSO defines a three-dimensional associative polymer network. Frequency-dependent dynamic mechanical moduli of a series of four different bulk materials, measured across several decades of oscillatory strain rates, are found to be quantitatively related through the pyridine exchange rates measured on model Pd(II) and Pt(II) complexes. Importantly, the mechanism of ligand exchange in the networks is found to be the same solvent-assisted pathway observed in the model complexes, and so the bulk mechanical properties are determined by relaxations that occur when the cross-links are dissociated from the polymer backbone. It is how often the cross-links dissociate, independently of how long they remain dissociated, that determines the bulk mechanical properties. The quantitative relationship between bulk materials properties and the kinetics and mechanisms observed in model compounds holds promise for the rational, molecular design of materials with tailored mechanical properties.

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.

The desire to control rationally, through small-molecule synthesis, the properties of bulk materials has led to supramolecular approaches to, for example, polymers, [1,2] strong and weak organogels, [3] amphiphilic assemblies, [4] and liquid crystals.[5] In these materials, specific and well-defined interactions between molecules contribute to bulk material properties on two levels: structure and dynamics. Although the dynamic nature of the defining interactions is often the attribute that distinguishes supramolecular materials from their covalent counterparts, [6] direct mechanistic studies of the molecular contributions to the dynamic properties of materials are less abundant than structural studies.[7] Herein, we report that a simple structural variation in the networks of supramolecular sols and organogels provides a direct and quantitative measure of the relationship between molecular dynamics and macroscopic rheological properties. The approach is akin to a macromolecular "kinetic isotope effect", in that kinetic contributions to rate-determining processes are revealed by the differences in two isostructural systems. We find that it is the dynamics of molecular crosslinks, much more so than their thermodynamics, which are specifically and quantitatively responsible for the bulk viscoelastic properties of the supramolecular networks.The system under consideration is poly(4-vinylpyridine) (PVP) that is cross-linked by bis(M ii -pincer) compounds 1 (see Figure 2) derived from the work of van Koten [8] (M = Pd or Pt, Figure 1). [9] We recently reported that simple steric effects in the alkylamino ligands of 1 and related compounds change the rate of ligand exchange by approximately two orders of magnitude while leaving the thermodynamics of association effectively constant. [10] This independent control of dynamics is particularly significant; cross-linkers 1 a and 1 b (Figure 2) are structurally identical components within the network, and so their similar thermodynamics ensure that the extent and nature of cross-linking is essentially the same in the samples prepared from them. By extending the family of cross-linkers to the Pt II -pincer molecules 1 c and 1 d (Figure 2) we are able to probe the effect of a wide range of molecular dynamics.The addition of 2 % (by functional group) 1 b to a 100 mg mL À1 DMSO solution of PVP (Aldrich, M r = 60 000) gives rise to a clear, thick, deep yellow solution whose viscosity is approximately 2000-times greater than that of PVP alone (33 Pa s vs. 0.016 Pa s). Two control experiments confirm that the viscosity increase is due to coordinative cross-linking of the PVP by 1 b. First, the viscosity does not increase upon the addition of the same quantity of monomeric À .

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.