Research Highlight 2 - A Re-Designed Polyethylene with Closed-Loop-Life Cycles.
Nearly 360 million tons of plastics are produced per annum, ~50% of them account for packaging applications – that mainly includes polymers such as polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET). Most of them are thrown away after a single use application, which then ends up in landfill, oceans and in the open environment. Due to their persistence in the environment, we are facing huge environmental consequences. Recycling of these polymers is necessary for the transition to a circular economy and achieve a sustainable society [1]. Currently mechanical recycling (grinding the product–melting–pelletizing) is widely employed in the industry. However, these polymers are prone to chain scission during reprocessing resulting in poor mechanical properties, hence their reuse is limited to downgrade applications. An attractive strategy is chemical recycling – that is, depolymerization of polymer into its starting monomers and then repolymerization to produce polymer with virgin-quality material properties. However, depolymerization of polyolefins (PE & PP) requires high energy intensive process due to their inert C-C backbone structure and thus difficult to convert it into starting monomers in quantitative yield [2-4].
Prof. Stefan Mecking and his colleagues from University of Konstanz, Germany, have recently developed ‘polyethylene-like polymer’ containing ‘ester and carbonate break points’ – which allows chemical depolymerization of the polymer into its starting monomers. The recovered monomers can be repolymerized to form virgin-quality polymer, and the process can be repeated multiple times with full retention of material properties, forming a ‘Closed-Loop Life Cycle’ of Circular Economy. Their breakthrough is published in the journal ‘Nature’ [5].
The key innovation applied by the authors is the use of long-chain aliphatic (C18) building block, 1,18-octadecanedioic acid, which gives polyethylene-like polymer structure. It can be sourced from renewable palm oil/microalgae (via self-metathesis of oleic acid and then hydrogenation) and is commercially available. The authors were modified it (esterification & reduction) to install dimethyl-ester and hydroxyl end functional groups on the monomer (1,18-dimethyl octadecanedioate and 1,18-octadecane diol respectively, see Monomer Synthesis). Upon polycondensation of these monomers, 1,18-octadecane diol with 1,18-dimethyl octadecanedioate or 1,18-octadecane diol with diethyl carbonate under catalytic conditions, lead to form ester or carbonate in-chain functional groups – like connecting points for long aliphatic polyethylene-like chain, which also serve as ‘break points’ for the polymer (see PE-18,18 & PC-18).
The main challenge for the authors was to synthesize a high molecular weight polymer, especially in case of PC-18, to achieve similar mechanical properties as polyethylene. Thus, they chose ‘diethyl carbonate (DEC)’ as carbonate source to synthesize PC-18. The utilization of DEC enabled the molecular weight build-up of the polymer via an additional reaction pathway – via decarboxylation of diethyl carbonate end groups in the oligomer – which produces more reactive hydroxyl groups by the elimination of CO2 and ethylene, that can participate in transesterification with DEC. The authors have mentioned in the article that such reaction is not amenable with the commonly used carbonate source such as ‘dimethyl carbonate’.
Both PE-18,18 and PC-18 showed similar thermal, mechanical and crystalline property as commercial polyethylene (HDPE – high density polyethylene). Also, they can be processed like commercial HDPE via injection moulding and extrusion, but also via 3D printing technique. The real-world application is demonstrated in the article by adding colourants/carbon fibre additives – for example, production of filaments that matches commercial quality and dimensions (via extrusion) and 3D printing of smartphone protective cover.
The polymer–monomer–polymer closed loop recycling of HDPE-like polymer can be achieved by solvolysis, for example, by simply heating the polymer in methanol or basic ethanol (at 120 ºC) – which completely depolymerized the polymer into its starting monomers with almost 98 mol% recovery. The recovered monomers were then utilized for repolymerization (96% recovery from polymer – polymer), and the resulting polymer showed similar material performance as pristine. This is not the case with mechanical recycling of commercial HDPE – which loses its degree of crystallinity after each recycling loop [6,7]. The depolymerization strategy demonstrated by the authors is highly efficient, and is suitable for mixed plastic waste stream. The authors were able to selectively depolymerize PE-18,18 and PC-18 into its starting monomers from a mixed coloured plastic waste stream containing PP, HDPE, and even from PET – a polymer is also susceptible to solvolysis, but may need harsher conditions than applied by the authors. During the whole depolymerization process, the other plastic waste pieces (PP, HDPE & PET) remained unaffected, undissolved, and were able to easily separate from the depolymerized solution. Thus, this newly re-designed HDPE has all the potential to replace commercial HDPE and mitigate plastic pollution.
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[5]. Häußler, M.; Eck, M.; Rothauer, D.; Mecking, S. Closed-loop recycling of polyethylene-like materials. Nature 2021, 590, 423-427.
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