AsianScientist (Feb. 13, 2015) – In nature, supramolecular complexes—chain-like structures that are composed of many small units linked mainly by weak non-covalent bonds—are assembled and disassembled in a precisely controlled way. Researchers from the RIKEN Center for Emergent Matter Science, led by Professor Takuzo Aida, have demonstrated a new method for artificially building and dismantling supramolecular polymers in a tightly controlled and selective way.
Their results, published in Science, use the methods of traditional polymer chemistry but take advantage of the monomer elements’ tendency to self-organize. This opens the avenue of precision supramolecular engineering, allowing the synthesis of polymers new properties and applications.
The work began from a serendipitous observation in an earlier experiment of a molecule containing a corranulene core seemed to exist in a stable state but could self-assemble into a polymer upon the addition of an initiator. Importantly, the researchers found that the assembly took place not through a step-growth but rather through a chain-growth process where growth takes place by the addition of new units at only one end of a chain.
They found that monomers added to the solvent would join the chain as long as the initiator was present, allowing the size of the chains to be precisely controlled by varying the proportion of monomer and initiator. The process could be performed at room temperature and pressure, simply by adding a given mixture of monomers and initiator to the solvent. The chains could also be easily disassembled.
Importantly, the group discovered that they were able to create polymers that are of the same chirality (either left- or right-handed) by adding an initiator with the specific chirality to the solvent. This is important because there are cases where chirality is crucial. For example, a drug of one chirality will be biologically useful, whereas the opposite enantiomer will be useless or, in the worst case, harmful.
A pure material will typically have quite different properties from a racemic mixture which contains equal amounts of each of the enantiomers. This finding was also important because it allowed the scientists to exert control over the final geometry of the chain.
Dr. Daigo Miyajima, who along with Aida is corresponding author, says, “What is exciting is that, based on lessons from polymer chemistry, we were able to build these macromolecular polymers in a chain-growth rather than step-growth fashion. This has made it possible to control the length of chains, the sequence of molecules, and even the stereochemical structure, and through this we hope to contribute to progress in precision macromolecular engineering.”
Looking to the future, Aida adds, “Supramolecular polymers can have semiconducting properties if properly designed, so this method could be used to reduce the size of transistors. In addition, we found that the polymers can be easily disassembled, with almost 100 percent yield, so materials produced through this method will be completely recyclable—an important property for creating a sustainable society.”
The article can be found at: Kang et al. (2015) A Rational Strategy for the Realization of Chain-growth Supramolecular Polymerization.
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