Mono-material product design with bio-based, circular, and biodegradable polymers

Abstract

The traditional multi-material product design of plastic products significantly complicates both existing and emerging recycling processes. Here, we discuss the concept of mono-material product design, which is based on circular and biodegradable polymers made of a single monomer, delivering tailorable properties via molecular or macromolecular engineering without changing its chemical makeup or composition. The traditional multi-material product design of plastic products significantly complicates both existing and emerging recycling processes. Here, we discuss the concept of mono-material product design, which is based on circular and biodegradable polymers made of a single monomer, delivering tailorable properties via molecular or macromolecular engineering without changing its chemical makeup or composition. Everyday plastic products such as those designed for packaging, while seemingly simple, are highly engineered, multi-component materials. Namely, when a plastic material requires specific properties that cannot be achieved by a single polymer, a common product design strategy is to use different polymers or the same polymer modified via copolymerization or functionalization and/or add other additives to bring about desired properties. While this approach delivers desired lifetime performance, it also substantially complicates recycling. Using packaging as an example, to keep food fresh, protect products, and brand goods, a combination of 3 to 12 polymers and other additives are usually required to form multi-layered or multi-component products.1Butler T.I. Morris B.A. Multilayer Flexible Packaging. Elsevier, 2016Google Scholar For example, multi-layered packaging products, such as a condiment sachet packet, could comprise a low-density polyethylene (LDPE) inner layer for sealability, a functional oxygen barrier layer of poly(ethylene-co-vinyl alcohol) (EVOH), a poly(ethylene-co-vinyl acetate) adhesive tie layer, and an outer strength layer of high-density polyethylene (HDPE) or poly(ethylene terephthalate) (PET), all assembled through coextrusion or lamination.2Anukiruthika T. Sethupathy P. Wilson A. Kashampur K. Moses J.A. Anandharamakrishnan C. Multilayer Packaging: Advances in Preparation Techniques and Emerging Food Applications.Compr. Rev. Food Sci. Food Saf. 2020; 19: 1156-1186Crossref PubMed Scopus (94) Google Scholar While these types of packaging materials are clearly useful, recycling these multi-material products through conventional mechanical recycling represents a major technical challenge because these materials typically create heterogeneous and immiscible blends. While there are potential ways to compatibilize multi-material blends, such as using dynamically crosslinked block copolymers to unify immiscible phases3Clarke R.W. Sandmeier T. Franklin K.A. Reich D. Zhang X. Vengallur N. Patra T.K. Tannenbaum R.J. Adhikari S. Kumar S.K. et al.Dynamic crosslinking compatibilizes immiscible mixed plastics.Nature. 2023; 616: 731-739Crossref PubMed Scopus (1) Google Scholar or to separate the layers of these materials using selective solvents to recycle them individually,4Walker T.W. Frelka N. Shen Z. Chew A.K. Banick J. Grey S. Kim M.S. Dumesic J.A. Van Lehn R.C. Huber G.W. Recycling of multilayer plastic packaging materials by solvent-targeted recovery and precipitation.Sci. Adv. 2020; 6eaba7599Crossref Scopus (105) Google Scholar these technologies remain nascent and are challenged by the diversity of materials used in packaging today.5Horodytska O. Valdés F.J. Fullana A. Plastic Flexible Films Waste Management – A State of Art Review.Waste Manag. 2018; 77: 413-425Crossref PubMed Scopus (226) Google Scholar As an alternative approach, herein we propose that the creation of multi-functional materials can potentially be realized using a mono-material design based on single, compatible chemical composition. Future sustainable product design would ideally be based on (1) tailorable materials that are derived from (2) a single biologically derived monomer6Cywar R.M. Rorrer N.A. Hoyt C.B. Beckham G.T. Chen E.Y.-X. Bio-Based Polymers with Performance-Advantaged Properties.Nat. Rev. Mater. 2021; 7: 83-103Crossref Scopus (108) Google Scholar and are (3) chemically circular with a closed-loop end-of-life7Shi C. Reilly L.T. Phani Kumar V.S. Coile M.W. Nicholson S.R. Broadbelt L.J. Beckham G.T. Chen E.Y.-X. Design Principles for Intrinsically Circular Polymers with Tunable Properties.Chem. 2021; 7: 2896-2912Abstract Full Text Full Text PDF Scopus (0) Google Scholar,8Coates G.W. Getzler Y.D.Y.L. Chemical Recycling to Monomer for an Ideal, Circular Polymer Economy.Nat. Rev. Mater. 2020; 5: 501-516Crossref Scopus (424) Google Scholar,9Hong M. Chen E.Y.-X. Chemically recyclable polymers: a circular economy approach to sustainability.Green Chem. 2017; 19: 3692-3706Crossref Google Scholar and/or (4) biodegradable.10Westlie A.H. Quinn E.C. Parker C.R. Chen E.Y.-X. Synthetic Biodegradable Polyhydroxyalkanoates (PHAs): Recent Advances and Future Challenges.Prog. Polym. Sci. 2022; 134101608Crossref Scopus (8) Google Scholar First, tailorable materials here mean that a broad range of material properties can be made readily accessible from a single monomeric building block (see Box 1). Second, using a single bio-based monomer greatly reduces synthetic complexity of materials, increases circularity (i.e., reduces waste) by enabling more facile recycling, and includes biogenic carbon that can potentially reduce the environmental impact of synthesis.11Zheng J. Suh S. Strategies to Reduce the Global Carbon Footprint of Plastics.Nat. Clim. Chang. 2019; 9: 374-378Crossref Scopus (458) Google Scholar Third, the materials used in these products will have the capacity to depolymerize completely back to monomer, which makes them chemically circular and creates a closed-loop end-of-life that is not available for multi-material products today. Finally, while this design takes a “Recycle First” approach, if leakage into the natural environment occurs these materials should be able to biodegrade, thus reducing the potential for environmental pollution. The key challenge for recyclable mono-material product design is how to make one polymer behave like many different materials (i.e., tailorable properties) without changing its overall chemical makeup, via either macromolecular engineering of polymer’s stereomicrostructures and architectures or molecular engineering of monomer’s structures and functions so that the same monomer can be polymerized differently (by different mechanisms) into multiple polymers of different properties, namely single-monomer-sourced polymers. In this commentary, we describe three approaches to meet this challenge: (1) design of multi-functional hybrid monomers (see Box 1) to selectively produce different types of polymers that exhibit tailorable properties for various product components but that all can be recycled back to the same monomer in a closed loop; (2) polymer stereomicrostructure engineering, which is tuning the stereo-configuration of a polymer backbone and in turn tuning polymer properties while retaining the same chemical makeup, to transform a single-monomer-based polyester to behave much like all common polymer types; and (3) macromolecular topological/architectural engineering (see Box 1) to tune material properties. The following highlighted examples of the above three approaches show that it is possible that polymers derived from a single monomer can be designed to form materials that span strong and rigid thermoplastics, flexible plastics, stretchable elastomers, tough thermoplastic elastomers, crystalline fibers, or even adhesives. Within a traditional monomer framework, it is challenging to optimize, concurrently, contrasting polymerizability/depolymerizability and recyclability/performance properties of polymers. To address this challenge, we recently developed the hybrid monomer (see Box 1) design12Shi C. Reilly L.T. Chen E.Y.-X. Hybrid Monomer Design Synergizing Property Trade-offs in Developing Polymers for Circularity and Performance.Angew. Chem. Int. Ed. 2023; (e202301850)Google Scholar that hybridizes parent monomer pairs of contrasting, mismatching, or matching properties into offspring monomers that not only unify the above-described conflicting properties but also radically alter the resultant polymer properties far beyond the limits of what either parent homopolymers or their copolymers can normally achieve. In particular, an olefin/lactone bifunctional hybrid monomer based on γ-butyrolactone (γ-BL), which is a low-ceiling-temperature (LCT) monomer toward ring-opening polymerization (ROP) to form polyester, and cyclohexene, which is an LCT monomer toward ring-opening metathesis polymerization (ROMP) to afford poly(cyclic olefin), achieves the unique orthogonal (de)polymerization to establish “one monomer–two polymers–one monomer” closed-loop circularity framework (Figure 1A).13Shi C. Clarke R.W. McGraw M.L. Chen E.Y.-X. Closing the “One Monomer-Two Polymers-One Monomer” Loop via Orthogonal (De)Polymerization of a Lactone/Olefin Hybrid.J. Am. Chem. Soc. 2022; 144: 2264-2275Crossref PubMed Scopus (26) Google Scholar The resulting polyolefin and polyester from the catalyzed ROMP and ROP process, respectively, have distinctively different thermal and mechanical properties that can be applied for different application needs. More significantly, combining the two classes of polymers covalently as a polyolefin-polyester copolymer or as a physical mixture still yields a fully recyclable system that can be easily broken down back to the same single hybrid monomer, thus closing the catalyzed chemical recycling to monomer (CRM) loop for their homopolymers, copolymers, and mixture. This bifunctional hybrid monomer system resembles another bifunctional monomer α-methylene-γ-butyrolactone (MBL), which is a naturally occurring and biomass-sourced hybrid monomer containing both a highly reactive exocyclic C=C bond and a highly stable five-membered γ-BL (Figure 1B). Regulated simply by polymerization temperature, MBL can undergo orthogonal polymerization to produce either poly(2-methylene-4-hydroxybutyrate) (P4HB=) via the kinetically favored ROP pathway or acrylic polymer PMBL via the thermodynamically favored vinyl-addition polymerization (VAP) pathway.14Tang X. Hong M. Falivene L. Caporaso L. Cavallo L. Chen E.Y.-X. The Quest for Converting Biorenewable Bifunctional α-Methylene-γ-Butyrolactone into Degradable and Recyclable Polyester: Controlling Vinyl-Addition/Ring-Opening/Cross-Linking Pathways.J. Am. Chem. Soc. 2016; 138: 14326-14337Crossref PubMed Scopus (109) Google Scholar Both polymers can be broken down back into the same monomer via CRM but exhibit markedly different performance properties, offering another example of a closed one monomer–multiple polymers–one monomer loop. Both polyester (P4HB=)14Tang X. Hong M. Falivene L. Caporaso L. Cavallo L. Chen E.Y.-X. The Quest for Converting Biorenewable Bifunctional α-Methylene-γ-Butyrolactone into Degradable and Recyclable Polyester: Controlling Vinyl-Addition/Ring-Opening/Cross-Linking Pathways.J. Am. Chem. Soc. 2016; 138: 14326-14337Crossref PubMed Scopus (109) Google Scholar and acrylic polymer PMBL15Gilsdorf R.A. Nicki M.A. Chen E.Y.-X. High Chemical Recyclability of Vinyl Lactone Acrylic Bioplastics.Polym. Chem. 2020; 11: 4942-4950Crossref Google Scholar have been shown to exhibit closed-loop chemical circularity. Worth noting here is that P4HB= is the vinyl-functionalized biodegradable P4HB (the pendent vinyl group of which provides facile access to property modifications for P4HB), while PMBL exhibits a high glass-transition temperature (Tg) of 195°C, which is ∼90°C higher than that of petroleum-based poly(methyl methacrylate) (PMMA) and is more solvent resistant than PMMA as well. Overall, the orthogonality in both (de)polymerization processes has been achieved, offering another example of a closed one monomer–multiple polymers–one monomer loop. Polyhydroxyalkanoates (PHAs) are a class of polyesters that are bioderivable and biodegradable.10Westlie A.H. Quinn E.C. Parker C.R. Chen E.Y.-X. Synthetic Biodegradable Polyhydroxyalkanoates (PHAs): Recent Advances and Future Challenges.Prog. Polym. Sci. 2022; 134101608Crossref Scopus (8) Google Scholar Poly(3-hydroxybutyrate) (P3HB) is the most common PHA and has been the subject of much academic and commercial attention since the mid-21st century. In the backbone of this natural polyester lies a pendent methyl group, which is found in naturally occurring PHAs in the absolute (R)-configuration, making it perfectly isotactic (it). This backbone configuration and sequence of configurations in space, or stereomicrostructure, can be engineered to deliver tailorable properties such as mechanical strength, rigidity, flexibility, or barrier performance from PHAs while maintaining the chemical composition. Using a bio-based, eight-membered dimethyl diolide (8DLMe) platform, we have shown that we can provide tailorable properties of strong and rigid it-P3HB16Tang X. Chen E.Y.-X. Chemical Synthesis of Perfectly Isotactic and High Melting Bacterial Poly(3-Hydroxybutyrate) from Bio-Sourced Racemic Cyclic Diolide.Nat. Commun. 2018; 9: 2345Crossref PubMed Scopus (85) Google Scholar (which is a highly crystalline material and exhibits high barriers toward water vapor and oxygen, essential for food packaging design),17Sangroniz A. Zhu J.-B. Tang X. Etxeberria A. Chen E.Y.-X. Sardon H. Packaging Materials with Desired Mechanical and Barrier Properties and Full Chemical Recyclability.Nat. Commun. 2019; 10: 3559Crossref PubMed Scopus (175) Google Scholar stereosequenced P3HB (which can be made ductile and tough),18Tang X. Westlie A.H. Watson E.M. Chen E.Y.-X. Stereosequenced Crystalline Polyhydroxyalkanoates from Diastereomeric Monomer Mixtures.Science. 2019; 366: 754-758Crossref PubMed Scopus (86) Google Scholar and tough and optically clear syndio-rich (sr) P3HB.19Quinn E.C. Westlie A.H. Sangroniz A. Caputo M.R. Xu S. Zhang Z. Urgun-Demirtas M. Müller A.J. Chen E.Y.-X. Installing Controlled Stereo-Defects Yields Semicrystalline and Biodegradable Poly(3-Hydroxybutyrate) with High Toughness and Optical Clarity.J. Am. Chem. Soc. 2023; 145: 5795-5802Crossref PubMed Scopus (1) Google Scholar All P3HB materials are derived from the same monomer, 8DLMe, but can be synthesized through simple modulations of the P3HB stereomicrostructure through choice of diastereomers and catalysts (Figure 2). Traditionally, this is not the case with other PHA monomers such as ß-butyrolactone, the simplest lactone that can be polymerized to P3HB, which has only one stereocenter leading to limited diversity of stereomicrostructures. As an example, in our recent work on sr-P3HB,19Quinn E.C. Westlie A.H. Sangroniz A. Caputo M.R. Xu S. Zhang Z. Urgun-Demirtas M. Müller A.J. Chen E.Y.-X. Installing Controlled Stereo-Defects Yields Semicrystalline and Biodegradable Poly(3-Hydroxybutyrate) with High Toughness and Optical Clarity.J. Am. Chem. Soc. 2023; 145: 5795-5802Crossref PubMed Scopus (1) Google Scholar we showed that engineering the stereomicrostructure by introducing controlled stereoerrors along the polymer chain is what accounts for the drastic changes in performance properties from a strong and rigid thermoplastic, it and st-P3HB, to a tough thermoplastic, sr-P3HB. Our ongoing work shows that the principle learned through this work can be readily applied to create a diverse portfolio of PHAs that also include flexible elastomers and adhesives. Hence, this microstructural engineering strategy will allow us to create essentially all the materials necessary to mimic conventional multi-material products (e.g., in packaging) where all the materials have the same chemical composition in the backbone, thereby greatly simplifying potential chemical and mechanical recycling processes. One of the most attractive features of P3HB is its biodegradability in ambient, unmanaged environments (in soil bio- or synthetic it-P3HB takes 105–145 days to reach 90% degradation19Quinn E.C. Westlie A.H. Sangroniz A. Caputo M.R. Xu S. Zhang Z. Urgun-Demirtas M. Müller A.J. Chen E.Y.-X. Installing Controlled Stereo-Defects Yields Semicrystalline and Biodegradable Poly(3-Hydroxybutyrate) with High Toughness and Optical Clarity.J. Am. Chem. Soc. 2023; 145: 5795-5802Crossref PubMed Scopus (1) Google Scholar). While our approach to redesigning plastic products such as packaging takes a “Recycle First” stance, designing for biodegradability is important, as plastic waste can be leaked into the natural environment and cause adverse environmental and health effects. We envision these mono-material products from a single monomer could be recycled first and biodegraded in case of “emergency” (i.e., leaked into the natural environment) thus creating a “back-up plan” of biodegradation. The sr-P3HB we recently reported demonstrated melt-processability (i.e., it is mechanically recyclable)19Quinn E.C. Westlie A.H. Sangroniz A. Caputo M.R. Xu S. Zhang Z. Urgun-Demirtas M. Müller A.J. Chen E.Y.-X. Installing Controlled Stereo-Defects Yields Semicrystalline and Biodegradable Poly(3-Hydroxybutyrate) with High Toughness and Optical Clarity.J. Am. Chem. Soc. 2023; 145: 5795-5802Crossref PubMed Scopus (1) Google Scholar and biodegradability in fresh water and soil. Further engineering of the P3HB stereomicrostructure is now within sight to create a circular mono-material that we propose can significantly reduce the environmental impact of multi-material product waste. Overall, advanced sustainable features of this P3HB-based mono-material product design include: (1) one monomer, multiple P3HB stereomicrostructures for different properties; (2) tailorable application properties by microstructure engineering; and (3) an equivalent chemical composition for all P3HB products. It is well known that changing a polymer’s architecture (e.g., linear vs. branched or crosslinked) or topology (e.g., linear vs. cyclic), see Box 1, renders materials with drastically different properties. Hence, regulating a polymer’s architecture or topology can be utilized to further modulate the properties of the materials exemplified here, especially P3HB, and widen the design space of circular, biodegradable multi-component products, made from a single monomer. For example, star-shaped P3HB was shown to possess lower shear viscosity than its linear counterparts.20Ebrahimi T. Hatzikiriakos S.G. Mehrkhodavandi P. Synthesis and Rheological Characterization of Star-Shaped and Linear Poly(Hydroxybutyrate).Macromolecules. 2015; 48: 6672-6681Crossref Scopus (18) Google Scholar Combining topological and architectural engineering with the stereomicrostructural engineering described above will allow us to create a wider design space for P3HB, furthering the mono-material product design based on biodegradable P3HB. The traditional multi-material product design significantly complicates both existing and emerging polymer recycling processes. As an alternative, the mono-material product design described here is based on chemically circular and/or biodegradable polymer materials made of a single monomer. This approach employs either a designer multi-functional monomer that selectively produces different types of polymers with tailorable properties that all can be recycled back to the same monomer in a closed loop or stereomicrostructural engineering of a polymer to transform a single-monomer PHA into a suite of tailorable materials that can behave like all common polymer types, without changing the chemical composition of the building blocks. Many future opportunities still remain, including: (1) more economically competitive synthesis of designer monomers at-scale from abundantly available biogenic feedstocks; (2) establishment of closed-loop CRM, especially for P3HB and other PHAs; (3) “drop-in” coextrusion or lamination processing solutions of these materials made of single monomers; and (4) complete techno-economic analyses and life cycle assessments of novel mono-material replacements.7Shi C. Reilly L.T. Phani Kumar V.S. Coile M.W. Nicholson S.R. Broadbelt L.J. Beckham G.T. Chen E.Y.-X. Design Principles for Intrinsically Circular Polymers with Tunable Properties.Chem. 2021; 7: 2896-2912Abstract Full Text Full Text PDF Scopus (0) Google Scholar The challenge in the designer monomers can ultimately be met by coupling of bio/chemocatalytic processes to make monomer synthesis more selective and energy efficient and by recovering the monomers via the CRM process, which also highlights the need to close the chemical loop for P3HB and other PHAs. Chemically circular and biodegradable polymers are likely more sustainable materials than fossil carbon-based plastics, but they may not exhibit all the properties that conventional plastics exhibit, rendering them at least partially incompatible with existing manufacturing processes. Polymer processing will provide additional, important means to tailor properties of mono-material plastics. Hence, to realize commercialization of these mono-material processes, the community needs to work with existing infrastructure to tune their methods to make sure that such materials can be successfully processed into functionable products. Using the design strategies described herein and working together with the chemical and industrial communities, mono-material products made from bio-sourced, recyclable, and biodegradable polymers can be realized to provide more circular and sustainable alternatives to today’s multi-material-based products. Funding was provided by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Materials and Manufacturing Technologies Office (AMMTO), and Bioenergy Technologies Office (BETO). This work was performed as part of the Bio-Optimized Technologies to keep Thermoplastics out of Landfills and the Environment (BOTTLE) Consortium and was supported by AMMTO and BETO under contract DE-AC36-08GO28308 with the National Renewable Energy Laboratory (NREL), operated by Alliance for Sustainable Energy, LLC. The BOTTLE Consortium includes members from Colorado State University. The authors declare no competing interests.Box 1GlossaryMonomer: Monomers are the building blocks used to create polymers. Typically, a monomer has one functionality, such as a double bond or a heterocyclic ring, that is transformed in the polymerization reaction to covalently link monomer molecules together, becoming the polymer’s repeating unit. Hybrid monomers combine favorable but conflicting polymerization and depolymerization properties possessed by contrasting or complementary functionalities brought about from two different monomer subunits to create a hybridized monomer structure that can synergize conflicting properties and thus overcome polymerizability/depolymerizability and performance/recyclability trade-offs.Polymer: a polymer is a macromolecule composed of covalently bonded monomers in the form of repeating units in a long polymer chain. Polymers made from one monomer are called homopolymers, whereas copolymers are synthesized using two or more monomers to create polymers with combined properties characteristic of homopolymers or properties in-between homopolymers. Synthetic, industrial polymers are classified into five major types, including plastics, elastomers, fibers, coatings, and adhesives. Hence, plastics and polymers should not be used interchangeably, as plastics represent only one (major) type of polymer.Polymer architecture and topology: Polymer architecture refers to how monomer units are connected in one, two, and three dimensions, corresponding to linear, branched, and crosslinked (network) structures. For copolymers, architecture also includes monomer sequences, resulting in formation of different copolymers, such as random, block, alternating, and graft copolymers. Polymer topology refers to three-dimensional shapes and relationships of how individual chains are arranged within a whole polymer. Common polymer topologies include rings, stars, dendrimers, combs, ladders, rotaxanes, and catenanes. Changes in polymer architecture and topology can lead to materials with sharply different properties.Sustainability in plastic product design: When redesigning plastic products for sustainability, several factors need to be considered in the entire life cycle of the material. Complete life cycle assessments (LCAs) need to be conducted early and often in the product design and compared to conventional, petroleum-based plastics to ensure an overall environmental benefit of any new plastic material introduced to the market. “Sustainability” for plastic products includes multiple factors beyond recyclability and biodegradability, and overall LCA metrics such as water/energy consumption and greenhouse gas (GHG) emissions of the entire life cycle (i.e., raw material extraction → monomer → polymer → use → disposal → recycling) are essential. Monomer: Monomers are the building blocks used to create polymers. Typically, a monomer has one functionality, such as a double bond or a heterocyclic ring, that is transformed in the polymerization reaction to covalently link monomer molecules together, becoming the polymer’s repeating unit. Hybrid monomers combine favorable but conflicting polymerization and depolymerization properties possessed by contrasting or complementary functionalities brought about from two different monomer subunits to create a hybridized monomer structure that can synergize conflicting properties and thus overcome polymerizability/depolymerizability and performance/recyclability trade-offs. Polymer: a polymer is a macromolecule composed of covalently bonded monomers in the form of repeating units in a long polymer chain. Polymers made from one monomer are called homopolymers, whereas copolymers are synthesized using two or more monomers to create polymers with combined properties characteristic of homopolymers or properties in-between homopolymers. Synthetic, industrial polymers are classified into five major types, including plastics, elastomers, fibers, coatings, and adhesives. Hence, plastics and polymers should not be used interchangeably, as plastics represent only one (major) type of polymer. Polymer architecture and topology: Polymer architecture refers to how monomer units are connected in one, two, and three dimensions, corresponding to linear, branched, and crosslinked (network) structures. For copolymers, architecture also includes monomer sequences, resulting in formation of different copolymers, such as random, block, alternating, and graft copolymers. Polymer topology refers to three-dimensional shapes and relationships of how individual chains are arranged within a whole polymer. Common polymer topologies include rings, stars, dendrimers, combs, ladders, rotaxanes, and catenanes. Changes in polymer architecture and topology can lead to materials with sharply different properties. Sustainability in plastic product design: When redesigning plastic products for sustainability, several factors need to be considered in the entire life cycle of the material. Complete life cycle assessments (LCAs) need to be conducted early and often in the product design and compared to conventional, petroleum-based plastics to ensure an overall environmental benefit of any new plastic material introduced to the market. “Sustainability” for plastic products includes multiple factors beyond recyclability and biodegradability, and overall LCA metrics such as water/energy consumption and greenhouse gas (GHG) emissions of the entire life cycle (i.e., raw material extraction → monomer → polymer → use → disposal → recycling) are essential.