Chemically recyclable polyacetals to deliver useful thermoplastics

In a recent issue of Science, high-molecular-weight polyacetals prepared by Coates and co-workers resolved many standing issues associated with polymers designed for chemical recycling to monomers. These materials simultaneously display tensile strengths similar commodity polyolefins, high thermal stability, efficient depolymerization strong acid catalyst. Main textThe scale the global plastic-waste problem is staggering. Since commercial production started in mid-twentieth century, humankind has collectively consumed then disposed three-quarters all polymer resins ever produced. Specifically, cumulative amount postconsumer plastic waste generated estimated have reached almost 7,000 million metric tons 2017, majority which been dumped landfills or permeated into environment.1Geyer R. Production, use, fate synthetic polymers.in: Letcher Trevor M. Plastic Waste Recycling. Academic Press, 2020: 13-32https://doi.org/10.1016/b978-0-12-817880-5.00002-5Crossref Scopus (100) Google Scholar data highlight two conflicting historical trends: growing technological dependence on products dire inattention ecological impact driven discarded plastics. The balance clearly unsustainable, particularly if end-of-life disposal continues same upward trend.2Lau W.W.Y. Shiran Y. Bailey R.M. Cook E. Stuchtey M.R. Koskella J. Velis C.A. Godfrey L. Boucher Murphy M.B. et al.Evaluating scenarios toward zero pollution.Science. 2020; 369: 1455-1461https://doi.org/10.1126/science.aba9475Crossref PubMed response, surge find sustainable more circular alternatives throughout entire value chain never greater. Mechanical (or downcycling) be go-to method recirculate primarily from single-use packaging materials. However, low penetration this market (representing 18% management 2017)1Geyer emphasizes challenges faced reused feedstocks, can hardly compete cost physical performance virgin plastics.3Schyns Z.O.G. Shaver M.P. Recycling Packaging Plastics: A Review.Macromol. Rapid Commun. 2021; 42: e2000415https://doi.org/10.1002/marc.202000415Crossref (183) ScholarChemical recycling, involves breakdown bonds chain, entails an attractive approach extract could mitigate problems accumulation.4Vollmer 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 Recycling: Giving New Life Waste.Angew. Chem. Int. Ed. Engl. 59: 15402-15423https://doi.org/10.1002/anie.201915651Crossref (306) For instance, valorization upcycling) seeks generate small molecules oligomers applications that differ those envisioned original plastic.5Martín A.J. Mondelli C. Jaydev S.D. Pérez-Ramírez Catalytic processing rise.Chem. 7: 1487-1533https://doi.org/10.1016/j.chempr.2020.12.006Abstract Full Text PDF (67) On other hand, monomer (CRM) generates back monomers reutilized manufacture virtually no differences appearance properties. energetic barriers are well suited provide such high-value-added chemicals good selectivity efficiency. turn, these thermodynamic kinetic conditions met accessible ceiling temperatures (Tc)—that is, whose polymerization reaction reaches equilibrium between consumption regeneration at moderate temperatures. This enables under mild conditions, often triggered use appropriate Polymers obtained ring-opening (ROP) moderately strained cyclic fall category, because net gain loss forward backward reactions hovers near zero. systems CRM face big challenges, mainly achieving conversions producing adequate stability.Inspiration develop new platforms does not just require pursuing convenient conditions. materials’ attributes should also aim contest, surpass, most common plastics they expect eventually replace them. Because polyethylene (PE) polypropylene (PP), largest share today, incredibly valuable target replacement. matching their inertness remarkable mechanical performance, while addressing viable routes, represent stake climb chemists.Heteroatom-containing polymers, as polyesters, polycarbonates, polyamides positioned forefront efforts due vast array available reactivities form break main-chain linkages properties.6Cywar Rorrer N.A. Hoyt C.B. Beckham G.T. Chen E.Y.-X. Biol.-based performance-advantaged properties.Nat. Rev. Mater. https://doi.org/10.1038/s41578-021-00363-3Crossref (42) Indeed, advances shown how oxygenated close delivering polyolefin-like showcasing ingenious routes.7Häußler Eck Rothauer D. Mecking S. Closed-loop polyethylene-like materials.Nature. 590: 423-427https://doi.org/10.1038/s41586-020-03149-9Crossref (120) Scholar,8Shi Li Z.-C. Caporaso Cavallo Falivene Hybrid design unifying polymerizability, recyclability, properties.Chem. 670-685https://doi.org/10.1016/j.chempr.2021.02.003Abstract (31) Still, important considerations remain unsolved, adoption low-cost raw potential adaptation scalability current industrial processes.In team led G.W. revealed polyacetals, well-known class includes polyoxomethylene, fact capable answering long-standing issues.9Abel B.A. Snyder R.L. Chemically recyclable thermoplastics reversible-deactivation acetals.Science. 373: 783-789https://doi.org/10.1126/science.abh0626Crossref (68) By introducing newly developed process transform acetals authors untapped overlooked challenge commonly used PE PP (Figure 1). It turns out pure-enough stability (degradation occurs above 335°C), still demand. Importantly, starting (formaldehyde various diols) commercially cheap even traced bio-based sources, paving way economically sustainably plastics.Cationic (CROP) 1,3-dioxolane (DXL), five-membered ring acetal, traditionally struggled reliably produce poly(1,3-dioxolane) (PDXL). oxophilicity Lewis catalysts reactivity propagating carbocation ends results sluggish rates unwanted termination pathways, respectively. Exposing exceptional material properties hidden much higher molecular weights, where ductility toughness displayed, required specialized methods. Acknowledging recombination exposed oxonium oxocarbenium species hindered desirable CROP control, hypothesized selective catalyst initiator system promotes (RD-CROP) would promote longer-lived ends. key breakthrough was accomplished tweaking dormant halomethyl ether species. neutral exchange active cationic during course polymerization, providing fast yet controlled growth.Indeed, discovered carefully selected combination metal halide (InBr3), (bromomethyl methyl ether, MOMBr), proton trap (2,6-di-tert-butyl pyridine) achieved DXL (>80%) PDXL room temperature. With suppression activation RD-CROP demonstrated living characteristics, including control weight based conversion successful extension after sequential additions. amenable 7- 8-membered acetals, bi-cyclic analog, impressive number average weights (Mn) 101,000 234,000 g mol?1 matter minutes. At regimes unprecedently strength (33.3–40.4 MPa) elongation (640%–720%) rival isotactic high-density polyethylene. Such thermoplastics, longer weak brittle, molded shapes resemble useful applications.The sub-ambient Tc 13°C ([DXL]0 = 4.0 M) contrast remarkably resistance degradation isolated (>330°C), indicating uncatalyzed kinetically prevented. Once formulated mixed catalytic amounts Brønsted acid, (98%) transformed simple distillation 140°C ambient pressure. recovery ideal; it infrastructure energy inputs, coupled mixtures containing additives, resulting pure effectively PDXL.However, few need technology jump large-scale adoption. First, elevating melting transition (Tm 58°C) water will open up range durability when harsher operating Other features gas permeability UV equally determine possible consumer functionalities. Second, greener lower-cost retain activities make desirable. Lastly, understanding composition toxicity any hydrolysable uttermost importance trace biological were its ecosystems.In end, our crisis demands overhaul brought us unsustainable state. Exceling technical functions without considering external harms insufficient respond changes. Instead, performances future, guided context economy, must take account sustainability defining factor.10Zimmerman J.B. Anastas P.T. Erythropel H.C. Leitner W. Designing green chemistry future.Science. 367: 397-400https://doi.org/10.1126/science.aay3060Crossref (347) Addressing becoming critical part process, undergo constitute piece puzzle. ever-exciting quest, re-examining modern lenses, revisited deliver gratifying surprises. Chemical stability. Inspiration chemists. Heteroatom-containing processes. Cationic growth. applications. PDXL. ecosystems. C.K.W. founder chief scientific officer Econic Technologies.