Chemists in the US have created a short chain of interlinked cyclic molecules, the first step towards an entirely new, flexible form of polymer.

Fraser Stoddart and colleagues at the University of California, Los Angeles have made self-assembling [3]catenanes - three macrocyclic molecules interlocked to form a short chain. The final aim is to create polycatenanes, which we expect to have very different properties to today's polymers, said Stoddart.

'An analogy between a steel rod (classical polymer) and a steel chain (mechanically interlocked polymer) is instructive,' said Stoddart. 'Both are extremely durable and strong. The chain, however, is flexible and "soft", able to adapt itself to a much greater variety of applications while maintaining its structural integrity and strength.'

Stoddart's [3]catenane consists of a central, electron-accepting cyclobis(paraquat-p-phenylene) macrocycle, which under the reaction conditions breaks open to allow two electron-donating crown ether macrocycles to thread on, before re-closing to form the chain. The iodide-catalysed reaction proceeds under thermodynamic control - that is, the interlocking reaction is a reversible, equilibrium process. But the equilibrium mixture strongly favours the chain product, due to favourable bonding interactions between the rings.

'This elegant chemistry proves that multicomponent systems based on these robust templates can be assembled in impressive yields, certainly high enough to entertain the possibility of truly polycatenated materials,' said James Wisner, who works with catenanes at the University of Western Ontario, Canada. 'The extension of dynamic covalent chemistry methods to these systems opens many new opportunities for the high yield construction of complex interlocked molecules and molecular devices.' 

The product [3]catenane is isolated in 91 per cent yield - 'about 9 per cent away from where we need to be to make high molecular weight polymers,' said Stoddart. 'But we hope there will be a deal of extra stability to be had from many bonding interactions [when making longer chains]. As we reach out to larger, more complex structures, we have a right to expect that bigger doesn't necessarily mean harder, if we get the design right.'

James Mitchell Crow