54 | December 2021 www.facs.website 0% 20% 40% 60% 80% 100% 2020 2016 2012 2007 2003 2020 2016 2012 2007 2003 2020 2016 2012 2007 2003 positive negative unsure Usefulness Environmental friendliness Resource efficiency Figure 8: Impression of plastics surveyed by the Japan Plastics Industry Federation (JPIF).41 nanospaces, even into those whose radii number in the angstroms (Figure 6). Th i s phenomenon opens door s to the previously non-existent concept of ‘preci s ion pol ymer separat ion’7,8. For example, polymers which dif fer only by terminal structure exper ience less and less influence from the terminals as their chains become longer (i.e. as the terminal becomes a smaller component relative to the whole chain) and so existing methods have proven incapable of recognizing and separating them based on this difference. Using MOFs, our group has succeeded in discriminating between polymer chains which possess molecular weights in the tens of thousands of Daltons, and differ only by their terminal structure38. Further, we have carried out liquid chromatographic separation using prototype columns loaded with MOF particles37,39. The result was a shocking level of recognition, with retention times differing enough to discriminate in response to changes as small as one atom on a long polymer chain. As this method disentangles polymer chains into l inear arrays along the MOF pores, it enables structural recognition without neglecting any features wi thin the chain, some of which might otherwise be shrouded by the outer layers of an aggregate structure. Various types of polymer separation have been realized based not only on molecular weight37, but topological differences such as cyclic-versus-linear40, or even monomer composition within a copolymer, making this a macromolecular recognition-separation system of unparal leled precision (Figure 7). Developing “urban oil wells” Our new approach of polymer separation can provide structurally controlled polymers without the use of tedious synthetic techniques. These could then be used as enhanced plastics in applications such as electrical devices and medicine, bringing prosperity to our everyday lives. However, looking at the results of a survey by the Japan Plastics Industry Federation (JPIF)41, we find that for 20 years consumers’ impressions of plastics’ “usefulness” has remained consistently high, and that most are already satisfied with the ‘work’ plastic does for us (Figure 8). A point that’s cause for concern, however, is the “Environmental friendliness” and “Resource ef ficiency” categories, where public opinion has abruptly fallen in recent years (Figure 8)41. One major reason for this is a raised awareness of the pollution problems plastic litter causes, and people continue to celebrate switches toward bioplastics and biodegradable materials in response to this. Such materials do not actually solve the problem in marine environments, however, and the United Nations Environment Programme (UNEP) has stated as much – biodegradation occurs in conditions that rarely ever occur in our oceans, and concern has been raised that hiding such a fundamental problem behind an ‘eco-friendly’ image may actually worsen the situation42. Returning to the survey (Figure 8), and the fact that nearly 100% of respondents were satisfied with plastics’ performance41 – one way to read this would be that what we have already is completely sufficient as-is. From that standpoint, it would be best to let a perfectly satisfactory plastic do its job again and again, so our society must consider having plastics which fulfilled their role once do so multiple times by way of recycling. The current reality, however, is that in Japan roughly 60% of waste plastic resources end up undergoing combustion (in solid fuel, electricity generation, and other applications) by a process called ‘thermal recycling’, with the range of products reused as actual plastics (‘material recycl ing’) restricted mainly to those made of PET43. The reason renewal of other plastics remains stagnant is that they form various mixtures, complexes, and composites from which pure polymers cannot be effectively separated43. So here too, we see that current technology’s inability to separate desired polymers out of a mixture is connected directly to a societal problem. We are confident our MOF method can perform this separation, having witnessed its capabilities in lab-scale separations of simple polymer mixtures. However, realworld application will bring new technological challenges in both the processing of unpredictable mixtures and the upscaling of this process, and even if it proves to be possible in principle, there would be no meaning in doing so if the process takes an excessive amount of energy or creates an extreme environmental burden of its own. Complex socioeconomic factors will also have to be accounted for. But at the end of the line, could we not consider the plastics welling up in the streets a high-quality “urban oil well”? We dream of a future for society where plastic no longer piles up, but is used in cycles just as water and air are. ◆
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