No abstract

Self-assembly provides an attractive route to functional organic materials, with properties and hence performance depending sensitively on the organization of the molecular building blocks. Molecular organization is a direct consequence of the pathways involved in the supramolecular assembly process, which is more amenable to detailed study when using one-dimensional systems. In the case of protein fibrils, formation and growth have been attributed to complex aggregation pathways that go beyond traditional concepts of homogeneous and secondary nucleation events. The self-assembly of synthetic supramolecular polymers has also been studied and even modulated, but our quantitative understanding of the processes involved remains limited. Here we report time-resolved observations of the formation of supramolecular polymers from π-conjugated oligomers. Our kinetic experiments show the presence of a kinetically favoured metastable assembly that forms quickly but then transforms into the thermodynamically favoured form. Quantitative insight into the kinetic experiments was obtained from kinetic model calculations, which revealed two parallel and competing pathways leading to assemblies with opposite helicity. These insights prompt us to use a chiral tartaric acid as an auxiliary to change the thermodynamic preference of the assembly process. We find that we can force aggregation completely down the kinetically favoured pathway so that, on removal of the auxiliary, we obtain only metastable assemblies.

To study the supramolecular polymerisation mechanisms of a self-assembling system, concentration- and temperature-dependent measurements can both be used to probe the transition from the molecular dissolved state to the aggregated state. In this report, both methods are evaluated to determine their effectiveness in identifying and quantifying the self-assembly mechanism for isodesmic and cooperative self-assembling systems. It was found that for a rapid and unambiguous determination of the self-assembly mechanism and its thermodynamic parameters, temperature-dependent measurements are more appropriate. These studies allow the acquisition of a large data set leading to an accurate determination of the self-assembly mechanism and quantification of the different thermodynamic parameters that describe the supramolecular polymerisation. For a comprehensive characterisation, additional concentration-dependent measurements can be performed.

Enzymes are a source of inspiration for chemists attempting to create versatile synthetic catalysts. In order to arrive at a polymeric chain carrying catalytic units separated spatially, it is a prerequisite to fold these polymers in water into well-defined compartmentalized architectures thus creating a catalytic core. Herein, we report the synthesis, physical properties, and catalytic activity of a water-soluble segmented terpolymer in which a helical structure in the apolar core is created around a ruthenium-based catalyst. The supramolecular chirality of this catalytic system is the result of the self-assembly of benzene-1,3,5-tricarboxamide side chains, while the catalyst arises from the sequential ruthenium-catalyzed living radical polymerization of the different monomers followed by ligand exchange. The polymers exhibit a two-state folding process and show transfer hydrogenation in water.

Most polymers consist of long molecular chains made up of many units connected by covalent bonds -but supramolecular polymers are different. The strikingly dynamic properties of these materials arise from the reversible bonds that hold their chains together, and open up the prospect of many new applications. Do we need another class of polymer?In short, yes. Conventional polymers have excellent properties as materials, but when they melt they become highly viscous -the result of entanglement of their macromolecules. High temperatures and pressures are typically required to provide a melt of sufficiently low viscosity for processing, and this limits their applications. But supramolecular polymers combine good material properties with lowviscosity melts that are easy to handle. Some supramolecular polymers also have remarkable characteristics unique to their class, such as the ability to self-heal fractures in their structure. What exactly are supramolecular polymers?They are polymeric arrays of monomer units, held together by reversible and highly directional secondary interactions -that is, noncovalent bonds, such as hydrogen bonds. The resulting materials therefore maintain their polymeric properties in solution. The directions and strengths of the interactions are precisely tuned so that the array of molecules behaves as a polymer (that is, it behaves in a way that can be described by the theories of polymer physics). The high reversibility of the non-covalent bonds ensures that supramolecular polymers are always formed under conditions of thermodynamic equilibrium, and hence the lengths of the chains are directly related to the strength of the non-covalent bond, the concentration of the monomer and the temperature. How do they differ from conventional polymers?Most, if not all, of the material properties of conventional polymers stem from the covalent nature of their spaghetti-like molecules, although secondary interactions between the molecules are also often involved. For example, hydrogen bonding in nylon affects the properties of the polymer's crystals. Supramolecular polymers, on the other hand, make use of reversible interactions in the main chain. Hence, all the mechanical properties of supramolecular polymers are a direct result of secondary interactions, in particular the strength, reversibility and directionality of these interactions. What are the origins of supramolecular polymers?Supramolecular interactions between certain molecules have long been recognized. In most cases, only short chains of up to about 10 units are created, whereas useful supramolecular polymers require at least 100 to 1,000 monomers to be connected at any one time. The key to creating longer chains is to strengthen the interactions between the monomers; put more accurately, the average lifetime of the bonds must be increased. But the window of opportunity is small -if the secondary interactions are too strong, the polymers lose their dynamic flexibility, and their bulk properties become more like those of covalent polymers. ...