Reactive bonds for closed-loop chemical processing of polyethylene mimics

Installing chemically labile bonds onto polyethylene (PE) chains holds promise for making the recalcitrant polymer susceptible to chain scission. In a recent issue of Nature, Mecking and colleagues reported that ester- carbonate-containing PE polymers possess uncompromised thermomechanical performance strong potential closed-loop chemical processing. About 100 million tons are produced globally every year. As one most common plastics derived from fossil fuels, is widely used in containers, pipes, bags, almost everything. Recycling or upcycling post-consumer more sustainable strategy than incineration, landfills, leakage into environment. The notoriously inert C–C on its backbone make highly difficult degrade. There generally three methods processing (Figure 1A).1Martín A.J. Mondelli C. Jaydev S.D. Pérez-Ramírez J. Catalytic plastic waste rise.Chem. 2021; 7https://doi.org/10.1016/j.chempr.2020.12.006Abstract Full Text PDF PubMed Scopus (28) Google Scholar,2Vollmer I. Jenks M.J.F. Roelands M.C.P. White R.J. van Harmelen T. de Wild P. der Laan G.P. Meirer F. Keurentjes J.T.F. Weckhuysen B.M. Beyond mechanical recycling: giving new life waste.Angew. Chem. Int. Ed. 2020; 59: 15402-15423Crossref (181) Scholar First, thermochemical depolymerization pyrolysis enables scission generate gas, liquid fuel, wax, carbon depending temperature catalyst.3Kunwar B. Moser B.R. Chandrasekaran S.R. Rajagopalan N. Sharma B.K. thermal low value high density plastic.Energy. 2016; 111: 884-892Crossref (48) process energy intensive with poor selectivity yield monomer production. It can recover only <10% ethylene. Second, enzymatic oxidation hydrolysis have been as milder approach fragmenting chains.4Mohanan Montazer Z. P.K. Levin D.B. Microbial degradation synthetic plastics.Front. Microbiol. 11: 580709Crossref (50) Alkane hydroxylases key enzymes involved aerobic alkanes through bacterial extracellular mechanisms. Currently, identities mechanisms enzyme-substrate interactions mostly unknown. Also, cost potentially high, incomplete might produce microplastics. A third higher-value applications.5Chen L. Malollari K.G. Uliana A. Sanchez D. Messersmith P.B. Hartwig J.F. Selective, catalytic oxidations C–H polyethylenes functional materials enhanced adhesion.Chem. 7: 137-145Abstract (17) Scholar,6Coates G.W. Getzler Y.D.Y.L. Chemical recycling an ideal, circular economy.Nat. Rev. Mater. 5: 501-516Crossref (167) fundamental challenge selective functionalization bonds. Metal-catalyzed insertion, peroxide-initiated grafting, carbene many other developed introduce polar groups side PE.7Williamson J.B. Lewis S.E. Johnson 3rd, R.R. Manning I.M. Leibfarth F.A. C-H Functionalization Commodity Polymers.Angew. 2019; 58: 8654-8668Crossref (63) still tremendous challenges ahead, such scalability, cost, physicochemical performance, before commodity applications become possible. Breakthroughs clearly defined utilities needed. Long-chain aliphatic polycondensates class small fraction heteroatoms (e.g., O N) otherwise PE-like chains, providing important perspective attractive route alternative.8Song Zhu Yuan Zhou Zhang Y. Wang Tang Ultra-strong long-chain polyamide elastomers programmable supramolecular oriented crystalline microstructures.Nat. Commun. 10: 1315Crossref (55) Scholar,9Stempfle Ortmann S. bridge gap between semicrystalline polyolefins traditional polycondensates.Chem. 116: 4597-4641Crossref (137) This relies engineering low-density ester, amide, acetal, carbonate, phosphoester) chains. These relatively reactive serve breaking points transformation back monomers oligomers. However, prior efforts not led desirable properties essential consumers, degree crystallinity, modulus, tensile strength, ductility. Thus, this has able take off. Now, writing coworkers similar those high-density (HDPE) behavior,10Häußler M. Eck Rothauer Closed-loop polyethylene-like materials.Nature. 590: 423-427Crossref (56) shown Figure 1B. They started 1,18-octadecanedioic acid, product olefin metathesis refining natural plant oils. diacid was converted C18 diol dimethyl ester. reacted excess diethyl carbonate give polycarbonate PC-18 molecular weight up 88,000 Da. reaction unique given ethyl end offer additional pathway build weight, critical condensation polymerization. Highly OH were generated during decomposition release dioxide contrast, prepared lower 20,000 parallel, ester polymerized via titanium-alkoxide-catalyzed transesterification polyester PES-18,18 80,000 Interestingly, these products gave surface tension, wetting, ductility, crystallinity identical HDPE. solid-state structure dominated by crystallization aligned hydrocarbon segments. mimicries extraordinary, representing promising replace PE. How would weak affect process? Tensile molding objects be completely depolymerized methanol ethanol at elevated temperatures (120°C–180°C) even without use base KOH. Upon cooling, filtered further purified single recrystallization 98% purity over 99%. Using recycled diol, authors 72,000 demonstrated astonishing degradability mixing it commercial PE, polypropylene, terephthalate (PET) process. Complete achieved while mixture stayed intact. On hand, melt blended HDPE subjected methanolysis 120°C. obtained 1:1 ratio matrix remained unaltered. mixed repolymerized afford par virgin polyester. Lastly, evaluated standard Filaments superior dimensional accuracy fabricated after compounding colorants fibers. filaments manufactured 3D printing. Printed showed flexibility repetitive bending, comparatively better polylactic-acid- PET-based counterparts shape recovery. Although melting transitions below 100°C, improved heat-stable (PC-48) diethylcarbonate 1,48-tetraoctacontane. PC-48 shows narrow transition 113°C, which 3D-printed cups withstand boiling water repetitively. work demonstrates simple step-growth mechanism toward high-molecular-weight polyesters polycarbonates fascinating mimic but recyclability superior. With being said, largely depends availability precursors. there few reports scale-up synthesis starting feedstock (1,18-octadecanedioic acid), overall production scale known, could significant hurdle. addition, diols depolymerization, reasonable factor considering feasibility again cost. To competitive, future life-cycle analysis needed validation.