Lords of the rings

Many polymer systems can be induced to depolymerise into cyclic oligomers. For years this was of theoretical interest only, but now chemists are harnessing its potential. Simon Hadlington reports

Since the early days of synthetic polymer chemistry it has been known that an equilibrium can exist between the polymer chain and oligomers in the form of macrocyclic rings. High concentrations favour formation of the polymer, while dilute conditions favour formation of the cyclic oligomers. 

Until recently this has been largely of theoretical interest. But now chemists are starting to realise that this phenomenon can be harnessed. The interconversion between the polymer and the oligomeric rings could provide a way of recycling polymers; open up new methods for processing polymers; give rise to novel microfabrication techniques; and provide routes to rapid-throughput syntheses of new copolymer systems. 

Philip Hodge from the University of Manchester, Howard Colquhoun from the University of Reading and David Williams from Imperial College, London, have just shown how one high-performance polymer can be depolymerised into a family of macrocycles that can subsequently regenerate the polymer at high molar mass.1 This demonstrates for the first time, the researchers claim, that the chemical recovery and recycling of high-performance aromatic polymers is in principle entirely feasible. 

The researchers used Radel-R, an industrially important aromatic poly(ether sulfone) manufactured by Solvay Advanced Polymers. Radel-R is used, for example, in surgical instrumentation because of its ability to tolerate high-temperature sterilisation. By dissolving the polymer in dimethylacetamide to a concentration of about 1 per cent (W/V) and heating it in the presence of a caesium fluoride catalyst, the average molar mass was shown progressively to diminish, eventually producing a mixture comprising almost entirely macrocyclic oligomers. These were subsequently isolated and heated in the presence of a variety of initiators in order to effect the repolymerisation. 

This finding is significant because it shows that it might be relatively straightforward to recycle expensive polymers of this type. Waste polymer could be ’cyclo-depolymerised’, the macrocycles purified and then reconstituted back to polymer. The researchers have also demonstrated that this approach works with many other polymer types, including polyesters, unsaturated polyolefins, polyamides and polyurethanes. 

In a subsequent study2 the team has shown that it is possible to take the mixture of macrocycles obtained from depolymerising another high-performance thermoplastic and include them in the conventional polymer production process. In other words it is not necessary to process the macrocycles further; they can simply be obtained from used polymer and recycled to the appropriate point of manufacture of new material. 

The flip-side of macrocyclic depolymerisation is to use macrocyclic oligomers as a useful starting point for synthesising polymers. Simply heating the molten mixture in the presence of a suitable catalyst will cause the linkages within the cyclic species to re-shuffle, eventually forming the desired polymer chain. This provides a potentially valuable way of creating polymer structures in situ. 

Hodge’s team has shown, for example, that olefinic macrocycles can simply be painted on to a glass surface and heated. Polymerisation subsequently occurs, producing a film of polymer coating over the glass. 

Similarly, the team has experimented with using macrocyclic oligomers to microfabricate polymer structures. Conventionally, polymers are melted and injected into moulds. However, the melt can be viscous, which limits the size and complexity of structures that can be fabricated in this way; the melt cannot penetrate the mould fully if the voids in the mould are too fine. Molten cyclic oligomers, on the other hand, are considerably more fluid, and the researchers have been able to use alumina microfilters as moulds. 

By introducing molten macrocycles into the pores and heating to higher temperatures (if necessary in the presence of a catalyst) Colquhoun and Hodge have found that it is possible either to coat the internal surfaces of the pores with polymer or completely fill the pores. When the alumina is dissolved, what remains are either microtubules or a mass of solid fibres that are less than half a ?m in diameter (unpublished results). 

Another useful application of the reversible ring-to-chain phenomenon is in producing combinatorial libraries, both of rings and of chains. Hodge’s team at Manchester has shown that it is possible to take parent linear polyesters and depolymerise them in dilute solution in the presence of a transesterification catalyst to form high yields of crown ether-ester analogues.3 One system, for example, produced a soluble combinatorial library of such macrocycles consisting of 30 different 18- or 27-membered rings. 

Similarly, it is possible to use macrocycles as the starting point for synthesising libraries of copolymers. For example, two different types of macrocycle, consisting of distinct repeat units, can be added in varying proportions to reaction chambers on a multi-well plate. In the presence of an appropriate catalyst and heat, the oligomers will undergo ring opening and polymerisation, producing a range of copolymers depending on the initial proportions of oligomer in the mixture. These can then be rapidly screened for desirable properties. 

This type of high-throughput synthesis and screening could provide a much more rapid and efficient way of generating potentially useful polymer systems than using conventional synthetic routes. 

Source: Chemistry in Britain

Acknowledgements

Simon Hadlington