Polymers and chemistry: where do we go from here?
Nicholas Field
Over the past hundred years, humans have become utterly reliant upon synthetic polymers. As first proposed by Hermann Staudinger in 1920, they are considered conventionally to be macromolecules, consisting of many monomers covalently bonded in long, polymeric chains. 1 The material properties of such polymers are dependent on the nature of outer groups attached to the central chain, and on polymer morphology and interactions. Crystalline polymers have ordered chains and tend to be rigid and opaque, whereas amorphous polymers have tangled chains and tend to be more flexible and transparent. Thermoset plastics have crystalline structures with cross-linked chains, whereas thermoplastics, which are not cross-linked, assume amorphous or semi-crystalline structure, depending on temperature. These covalent polymers have been joined more recently by supramolecular polymers, comprising assemblies of monomers held in place by non-covalent, reversible and highly directional secondary interactions. Supramolecular polymer chemistry advanced rapidly after the description by E.W. Meijer and colleagues in 1997 of strong dimerization between ureidopyrimidinone (UPy) groups. This occurs through the formation of long-lived, self-complementary arrays of four hydrogen bonds, allowing stable supramolecular polymers to be formed for the first time. 2 Through fine-tuning the direction and strength of these interactions, the resulting material can be made to behave similarly to conventional polymers. However, the reversibility of non-covalent bonding results in self-assembly under thermodynamic equilibrium, with chain length and mechanical properties determined by the strength of interactions, the concentration of monomers, and temperature. This results in some peculiar properties which are the subject of current research. One promising field concerns the development of supramolecular polymers that undergo conformational changes through host-guest interactions with metal ions. A variety of applications are possible. For example, monomers comprising thiopene and imidazophenazine, which fluoresces under UV light, self-assemble into supramolecular polymers through intermolecular pi-pi stacking. These interactions are disrupted by the presence of Ag+ ions, causing a reduction in charge transfer between monomers that reduces intensity of UV fluorescence and changes visible absorption. This property provides a sensitive, specific indicator for the presence of Ag+ ions. 3 Such sensitive indicators might be particularly useful when very low-level contamination is hazardous, for instance, in the nuclear industry. In this scenario, however, conformational changes would ideally result in capture of harmful ions. To this end, a mixture of melamine and trimesic acid in aqueous solution has been shown to self- assemble into a hydrogen-bonded supramolecular organic framework that selectively removes
1 Staudinger, H. (1920) ‘Über Polymerisation’, Berichte der deutschen chemischen Gesellschaft 53: 1073 – 85. 2 Sijbesma, R. et al . (1997) ‘Reversible polymers formed from self -complementary monomers using quadruple hydrogen bonding’, Science 278: 1601 – 4. 3 Shi, H. et al . (2017) ‘A novel self -assembled supramolecular sensor based on thiophene-functionalized imidazophenazine for dual-channel detection of Ag + in an aqueous solution’, RSC Advances 7: 53439 – 44.
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