Supramolecular polymers: Chain growth in control.

Supramolecular polymers are a class of materials characterized by a repetition of monomer units held together by non-covalent forces such as metal coordination, π–π interactions and hydrogen bonding1–4. Compared with their covalent counterparts the unique properties of these polymers have attracted considerable attention, for instance, the self-healing ability that arises from the thermodynamic compensation for the formation of defects in materials3,5. Yet our understanding of the supramolecular polymerization process remains limited. In most cases, supramolecular polymerization is governed by thermodynamically controlled growth, arising from the fact that a majority of non-covalent interactions are weak and kinetically labile6. For this reason, the polymerization reaction of small molecules typically involves a step-growth mechanism in which large quantities of the precursors are consumed early in the reaction to form oligomers that can further react with each other7. At ambient conditions, such events can be extremely complex, and typically result in the formation of macromolecular assemblies with a broad molecular weight distribution. Writing in Science, Daigo Miyajima, Takuzo Aida and colleagues now report small molecules designed to undergo non-covalent chain-growth polymerization and give aggregates with controlled chain length, polydispersity and stereochemical structure8. In chain-growth polymerization, monomers become part of the growing polymer chain one at a time; the growth is driven by an initiator molecule and normally only occurs at one end of the polymer chain. In that respect, the key mechanism of chain-growth polymerization is reminiscent of a process called translation in protein synthesis. During translation, ribosomes and transfer RNA (tRNA) work together to decode the sequence of a messenger RNA (mRNA) molecule and produce proteins9. The ribosome connects one amino acid at a time and builds a long, sequence-controlled amino acid chain (polypeptide) that eventually becomes part of a protein. Despite considerable recent efforts, however, the design of a synthetic supramolecular system that undergoes chain-growth polymerization has remained challenging. Miyajima, Aida and co-workers have now succeeded in devising such a system by using bowl-shaped corannulene derivatives as monomers that disfavour self-polymerization, yet can be coaxed into it under heating or in the presence of an initiator. In this design, C5-symmetric non-planar corannulene molecules M are functionalized with five amide-appended thioalkyl side chains (Fig. 1a). At ambient temperature, in low-polarity solutions, the monomer M does not spontaneously self-assemble into supramolecular polymers10. Instead, it adopts a metastable cage-like closed conformation owing to strong intramolecular hydrogen bonding interactions between the NH and CO moieties of the five side-chain amide groups (Fig. 1b, Route 1). Under heating, however, M does self-assemble into onedimensional supramolecular aggregates, which continue to grow when a fresh feed of M is added. This shows that M does indeed support polymerization, which inspired the researchers to devise an initiator I — an N-methylated derivative of M — that facilitates it. Compound I adopts an open-cage conformation, as its N-methylated moieties cannot engage in hydrogen bonding. SUPRAMOLECULAR POLYMERS