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