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Polyhydroxyalkanoates (PHAs) have attracted increasing interest as sustainable plastics because of their biorenewability and biodegradability in the ambient environment. However, current semicrystalline PHAs face three long-standing challenges to broad commercial implementation and application: lack of melt processability, mechanical brittleness, and unrealized recyclability, the last of which is essential for achieving a circular plastics economy. Here we report a synthetic PHA platform that addresses the origin of thermal instability by eliminating α-hydrogens in the PHA repeat units and thus precluding facile cis-elimination during thermal degradation. This simple α,α-disubstitution in PHAs enhances the thermal stability so substantially that the PHAs become melt-processable. Synergistically, this structural modification also endows the PHAs with the mechanical toughness, intrinsic crystallinity, and closed-loop chemical recyclability.

Geminal disubstitution of cyclic monomers is an effective strategy to enhance the chemical recyclability of their polymers, but it is utilized for that purpose alone and often at the expense of performance properties. Here we present synergistic use of gem-α,α-disubstitution of available at-scale, bio-based δ-valerolactones to yield gem-dialkyl-substituted valerolactones ([Formula: see text]), which generate polymers that solve not only the poor chemical recyclability but also the low melting temperature and mechanical performance of the parent poly(δ-valerolactone); the gem-disubstituted polyesters ([Formula: see text]) therefore not only exhibit complete chemical recyclability but also thermal, mechanical and transport properties that rival or exceed those of polyethylene. Through a fundamental structure-property study that reveals intriguing impacts of the alkyl chain length on materials performance of [Formula: see text], this work establishes a simple circular, high-performance polyester platform based on [Formula: see text] and highlights the importance of synergistic utilization of gem-disubstitution for enhancing both chemical recyclability and materials performance of sustainable polyesters.

Aliphatic polyamides, or nylons, are typically highly crystalline and thermally robust polymers used in high-performance applications. Nylon 6, a high-ceiling-temperature (HCT) polyamide from ε-caprolactam, lacks expedient chemical recyclability, while low-ceiling temperature (LCT) nylon 4 from pyrrolidone exhibits complete chemical recyclability, but it is thermally unstable and not melt-processable. Here, we introduce a hybrid nylon, nylon 4/6, based on a bicyclic lactam composed of both HCT ε-caprolactam and LCT pyrrolidone motifs in a hybridized offspring structure. Hybrid nylon 4/6 overcomes trade-offs in (de)polymerizability and performance properties of the parent nylons, exhibiting both excellent polymerization and facile depolymerization characteristics. This stereoregular polyamide forms nanocrystalline domains, allowing optical clarity and high thermal stability, however, without displaying a melting transition before decomposition. Of a series of statistical copolymers comprising nylon 4/6 and nylon 4, a 50/50 copolymer achieves the greatest synergy in both reactivity and polymer properties of each homopolymer, offering an amorphous nylon with favorable properties, including optical clarity, a high glass transition temperature, melt processability, and full chemical recyclability.

Two well-known low-ceiling-temperature (LCT) monomers, γ-butyrolactone (γ-BL) toward ring-opening polymerization (ROP) to polyester and cyclohexene toward ring-opening metathesis polymerization (ROMP) to poly(cyclic olefin), are notoriously "nonpolymerizable". Here we present a strategy to render not only polymerizability of both the γ-BL and cyclohexene sites, orthogonally, but also complete and orthogonal depolymerization, through creating an LCT/LCT hybrid, bicyclic lactone/olefin (BiL

Tetrahydropyran (THP) is a five-carbon heterocyclic ether that is non-carcinogenic, non-peroxide forming, biodegradable, and economically competitive with tetrahydrofuran (THF) as a solvent.

Technologically important thermosets face a long-standing end-of-life (EoL) problem of non-reprocessability, a more sustainable solution of which has resolved to nascent vitrimers that can merge the robust material properties of thermosets and the reprocessability of thermoplastics. However, the lifecycle of vitrimers is still finite, as they often suffer from significant deterioration of mechanical performance following multiple reprocessing cycles, analogous to mechanical recycling, and they often show undesired creep under working conditions. To address these two key limitations, we have developed a cross-linked semi-crystalline polythioester with both dynamic covalent bonds and intrinsic crystallinity and chemical recyclability, affording a vitrimeric system that exhibits not only reprocessability and crystallinity-restricted creep but also complete chemical recyclability to initial monomer by catalyzed depolymerization in solution or bulk. Therefore, reported herein is an "infinite" vitrimer system that is empowered with a facile closed-loop EoL option once serial reprocessing deteriorates performance and the material can no longer meet the application requirements. Specifically, the polythioester vitrimer was constructed by copolymerization of a bicyclic thioester with a bis-dithiolane, producing dynamically cross-linked polythioesters with excellent property tunability, from amorphous to semi-crystalline states and melting transition temperatures from 91 to 178 °C.

Summary This perspective discusses a set of design principles for next-generation kinetically trapped, intrinsically circular polymers (iCPs) that are inherently, selectively, and expediently depolymerizable to their monomer state once their kinetic barriers of deconstruction are overcome, thereby enabling not only the ideal shortest chemical circularity but also tunable performance properties. After describing four elements of the design principles—thermodynamics and kinetics, strategies to overcome trade-offs and unify conflicting properties, predictive modeling, and supply-chain life-cycle assessment and techno-economic analysis, which are illustrated with state-of-the-art examples—it concludes with presenting key challenges and opportunities for sustainable development of iCPs.

Bio-based compounds with unique chemical functionality can be obtained through selective transformations of plant and other non-fossil, biogenic feedstocks for the development of new polymers to displace those produced from fossil carbon feedstocks. Although substantial efforts have been invested to produce bio-based polymers that are chemically identical to and directly replace those from petroleum, a long-pursued goal is to synthesize new, sustainable, bio-based polymers that either functionally replace or exhibit performance advantages relative to incumbent polymers. Owing to anthropogenic climate change and the environmental consequences of global plastics pollution, the need to realize a bio-based materials economy at scale is critical. To that end, in this Review we describe the concept of performance-advantaged, bio-based polymers (PBPs), highlighting examples wherein superior performance is facilitated by the inherent chemical functionality of bio-based feedstocks. We focus on PBPs with C–O and C–N inter-unit chemical bonds, as these are often readily accessible from bio-based feedstocks, which are heteroatom-rich relative to petroleum-derived feedstocks. Finally, we outline guiding principles and challenges to aid progress in the development of PBPs. Bio-based polymers that exhibit superior performance relative to petroleum-based incumbents can encourage industry adoption and offset fossil carbon use. This Review introduces performance-advantaged, bio-based polymers and highlights examples wherein superior performance is facilitated by the inherent chemical functionality of bio-based feedstocks.

Biobased poly(γ-methyl-α-methylene-γ-butyrolactone) (PMMBL), an acrylic polymer bearing a cyclic lactone ring, has attracted increasing interest because it not only is biorenewable but also exhibits superior properties to petroleum-based linear analog poly(methyl methacrylate) (PMMA). However, such property enhancement has been limited to resistance to heat and solvent, and mechanically both types of polymers are equally brittle. Here we report the expeditious synthesis of well-defined PMMBL-based ABA tri-block copolymers (tri-BCPs)—enabled by dual-initiating and living frustrated Lewis pairs (FLPs)—which are thermoplastic elastomers showing much superior mechanical properties, especially at high working temperatures (80–130 °C), to those of PMMA-based tri-BCPs. The FLPs consist of a bulky organoaluminum Lewis acid and a series of newly designed bis(imino)phosphine superbases bridged by an alkyl linker, which promote living polymerization of MMBL. Uniquely, such bisphosphine superbases initiate the chain growth from both P-sites concurrently, enabling the accelerated synthesis of tri-BCPs in a one-pot, two-step procedure. The results from mechanistic studies, including the single crystal structure of the dually initiated active species, detailed polymerizations, and kinetic studies confirm the livingness of the polymerization and support the proposed polymerization mechanism featuring the dual initiation and subsequent chain growth from both P-sites of the superbase di-initiator., Biobased poly(γ-methyl-α-methylene-γ-butyrolactone) (PMMBL) has attracted interest because it is biorenewable and exhibits superior properties to petroleum-based linear analog poly(methyl methacrylate) (PMMA). Here the authors report the synthesis of well-defined PMMBL-based ABA tri-block copolymers, enabled by dual-initiating and living frustrated Lewis pairs, which have superior mechanical properties compared to those of PMMA-based tri-BCPs.

Reversing the extraordinary growth in the production and accumulation of primary plastics in the environment will require collaboration across research disciplines and scales—from the chemical building blocks to product life cycles, producer and consumer incentives, and waste management. After a recent Cell Press LabLinks symposium on the topic, this Voices asks the event participants: what are the major research priorities for moving toward a circular economy for plastics?

Summary Intrinsically recyclable polymers represent a circular economy approach to address plastics problems. However, the design of such circular polymers is challenged by unyielding trade-offs between the monomer’s polymerizability and the polymer’s depolymerizability and performance properties. Here, we introduce a hybrid monomer design strategy that synergistically couples a high ceiling temperature (HCT) sub-structure for high polymerizability and performance properties with a low ceiling temperature (LCT) sub-structure for high depolymerizability and recyclability within the same monomer structure. Thus, structural hybridization between HCT e-caprolactone and LCT γ-butyrolactone led to an offspring [3.2.1]bicyclic lactone, which exhibits both high polymerizability and depolymerizability, otherwise conflicting properties in a typical monomer. The resulting polymer becomes a high-performance material, and thermal transition temperatures are ∼200°C higher and tensile modulus 10× higher than its parent polymers. These results demonstrate that the HCT/LCT hybrid monomer strategy is a powerful approach for designing circular polymers where conflicting properties must be exploited and unified.

Precision synthesis of cyclic polymers with predictable molecular weight and low dispersity is a challenging task, particularly concerning cyclic polar vinyl polymers through a rapid chain-growth mechanism and without high dilution. Harder yet is the precision synthesis of cyclic block copolymers (cBCPs), ideally from comonomer mixtures. Here we report that Lewis pair polymerization (LPP) capable of thermodynamically and kinetically compounded sequence control successfully addressed this longstanding challenge. Thus, LPP of acrylate/methacrylate mixtures under ambient temperature and normal concentration conditions rapidly and selectively affords well-defined cBCPs with high molecular weight (Mn = 247 kg/mol) and low dispersity (Đ = 1.04) in one step. Such cBCPs have been characterized by multiple techniques, including direct structural observation by imaging.

The catalyzed chemical synthesis of polyhydroxyalkanoates (PHAs) via stereoselective ring-opening polymerization (ROP) of eight-membered cyclic diolides (8DLR, R denotes the two substituents on the...

Ten years have passed since the conception of what was termed Lewis pair polymerization (LPP) that employs Lewis acid and base in pairs to not only activate monomers but also effect chain initiatio...

The ability to synthesize well-defined block copolymers (BCPs) from one-pot comonomer mixtures has powerful chemical and practical implications. However, controlling sequences between highly reactive, homologous comonomers such as acrylates during polymerization is challenging. Here we present a Lewis pair polymerization strategy that uniquely utilizes preferential Lewis acid coordination to differentiate between comonomers, distinctive kinetics, and compounded thermodynamic and kinetic differentiation to precisely control sequences and suppress tapering and misincorporation errors, thus achieving well-defined and resolved di- or tri-BCPs of acrylates.

Bacterial polyhydroxyalkanoates (PHAs) are a unique class of biodegradable polymers because of their biodegradability in ambient environments and structural diversity enabled by side-chain groups. However, the biosynthesis of PHAs is slow and expensive, limiting their broader applications as commodity plastics. To overcome such limitation, the catalyzed chemical synthesis of bacterial PHAs has been developed, using the metal-catalyzed stereoselective ring-opening (co)polymerization of racemic cyclic diolides (rac-8DLR , R=alkyl group). In this combined experimental and computational study, polymerization kinetics, stereocontrol, copolymerization characteristics, and the properties of the resulting PHAs have been examined. Most notably, stereoselective copolymerizations of rac-8DLMe with rac-8DLR (R=Et, Bu) have yielded high-molecular-weight, crystalline isotactic PHA copolymers that are hard, ductile, and tough plastics, and exhibit polyolefin-like thermal and mechanical properties.

Preparation and storage of biorenewable, monomeric indenone, much less the polymerization to a well-defined, high-molecular-weight polymer, is challenging because of antiaromaticity-driven radical autopolymerization. Herein, we report the successful preparation and subsequent Lewis pair polymerization (LPP) of indenone without autopolymerization side reactions using Lewis pairs consisting of sterically encumbered Lewis acid (LA) catalysts, such as B(C6F5)3 and bis(2,6-tert-butyl-4-methylphenoxy)methylaluminum and Lewis base initiators such as silyl ketene acetal and N-heterocyclic olefin nucleophiles. Thus, for the first time, the LPP enabled the synthesis of polyindenone (Pin) with high number-average molecular weight (Mn = 1.72 × 105 g mol–1) and low dispersity (D = 1.13). Observed correlations between the steric bulk of the LA catalyst and diastereoselectivity (57–75%) created the opportunity to model and investigate the relationships between β-substituted monomer motifs, catalyst steric accessibility...

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In typical cyclic polymer synthesis via ring-closure, chain growth and cyclization events are competing with each other, thus affording cyclic polymers with uncontrolled molecular weight or ring size and high dispersity. Here we uncover a mechanism by which Lewis pair polymerization (LPP) operates on polar vinyl monomers that allows the control of where and when cyclization takes place, thereby achieving spatial and temporal control to afford precision cyclic vinyl polymers or block copolymers with predictable molecular weight and low dispersity (≈1.03). A combined experimental and theoretical study demonstrates that cyclization occurs only after all monomers have been consumed (when) via conjugate addition of the propagating chain end to the specific site of the initiating chain end (where), allowing the cyclic polymer formation steps to be regulated and executed with precision in space and time.

Chemically recyclable polymers have attracted increasing attention since they are promising materials in a circular economy, but such polymers appropriate for packaging applications are scarce. Here a combined thermal, mechanical, and transport (permeability and sorption) study is presented of a circular polymer system based on biobased trans-hexahydrophthalide which, upon polymerization, can lead to amorphous, homochiral crystalline, and nanocrystalline stereocomplex materials. This study uncovers their largely different transport properties of the same polymer but with different stereochemical arrangements and synergistic or conflicting effects of crystallinity on transport properties versus thermal and mechanical properties. Overall, the homocrystalline chiral polymer shows the best performance with an outstanding barrier character to gases and vapors, outperforming commercial poly(ethylene terephthalate) and polyethylene. The results presented herein show that it is possible to modify the crystalline structure of the same polymer to tune the mechanical and transport properties and generate multiple materials of different barrier characters.

This work utilizes frustrated Lewis pairs consisting of tethered bis-organophosphorus superbases and a bulky organoaluminum to furnish the highly efficient synthesis of well-defined triblock copolymers via one-step block copolymerization of lignin-based syringyl methacrylate and n-butyl acrylate, through di-initiation and compounded sequence control. The resulting thermoplastic elastomers (TPEs) exhibit microphase separation and much superior mechanical properties (elongation at break up to 2091 %, tensile strength up to 11.5 MPa, and elastic recovery up to 95 % after 10 cycles) to those of methyl methacrylate-based TPEs. More impressively, lignin-based tri-BCPs can maintain TPEs properties up to 180 °C, exhibit high transparency and nearly 100 % UV shield, suggesting potential applications in temperature-resistant and optical devices.

Catalytic hydrosilylation of carbon dioxide has emerged as a promising approach for carbon dioxide utilization. It allows the reductive transformation of carbon dioxide into value-added products at the levels of formate, formaldehyde, methanol, and methane. Tremendous progress has been made in the area of carbon dioxide hydrosilylation since the first reports in 1981. This focus review describes recent advances in the design and catalytic performance of leading catalyst systems, including transition-metal, main-group, and transition-metal/main-group and main-group/main-group tandem catalysts. Emphasis is placed on discussions of key mechanistic features of these systems and efforts towards the development of more selective, efficient, and sustainable carbon dioxide hydrosilylation processes.

Doubly stranded pairs of isotactic (it-) chains of poly(methyl methacrylate) (PMMA) coiled inside of the helical cavity of syndiotactic (st-) PMMA chains generate a crystalline triple-helix stereoc...

Plastics have become indispensable in modern life and the material of choice in packaging applications, but they have also caused increasing plastic waste accumulation in oceans and landfills. Although there have been continuous efforts to develop biodegradable plastics, the mechanical and/or transport properties of these materials still need to be significantly improved to be suitable for replacing conventional plastic packaging materials. Here we report a class of biorenewable and degradable plastics, based on copolymers of γ-butyrolactone and its ring-fused derivative, with competitive permeability and elongation at break compared to commodity polymers and superior mechanical and transport properties to those of most promising biobased plastics. Importantly, these materials are designed with full chemical recyclability built into their performance with desired mechanical and barrier properties, thus representing a circular economy approach to plastic packaging materials., Biodegradable plastics usually have poorer mechanical or transport properties compared to conventional plastics. Here the authors show a class of biorenewable and chemically recyclable plastics based on copolymers of γ-butyrolactone and its ring-fused derivative, with competitive properties compared to conventional plastics.

Current practices in the generation and disposal of synthetic polymers are largely unsustainable, causing severe worldwide polymer pollution and enormous materials value loss. To address these dire environmental and economic issues, several research fronts aim to develop sustainable polymers with closed-loop life cycles.

Frustrated Lewis pairs (FLPs) of Lewis acid (LA) B(C6F5)3 and Lewis base hydrosilane [SiH] have been utilized to promote controlled polymerization of a challenging β-substituted Michael acceptor, methyl crotonate (MC), devoid of chain transfer side reactions. Mechanistic studies show that chain initiation involves LA-catalyzed 1,4-hydrosilylation of MC with [SiH] via FLP-type activation, generating a silyl ketene acetal nucleophile that participates in chain propagation via classic LA activation of monomer and a bimolecular conjugate addition mechanism. The role of the LA is conflicting in the two different catalytic cycles: the FLP activation in chain initiation requires LA-substrate (monomer) dissociation (or weak interaction) while the classic LA activation in chain propagation demands LA-monomer association (or strong interaction).

Organocatalyzed cross-coupling of biomass platform chemicals furfural and 5-hydroxymethylfurfual has been utilized to prepare a pair of constitutional isomers of difuranic C11 diols. Polycondensation of the diols with three diacyl chlorides and polyaddition with two diisocyanates produce linear polyesters and polyurethanes, respectively, with furan rings residing in both the polymer backbone and pendent positions. One monomer isomer has a steric advantage, leading to higher molecular weight polymers. Thermal and mechanical properties of the resulting polymers can be tuned in a broad range by varying the monomer pairs. In addition, a thermally reversible cross-linked network has been realized by applying the Diels–Alder reaction between a bis-maleimide cross-linker and the furan rings located at both the backbone and pendent positions. This property, coupled with formation of a significant amount (up to 34%) of stable carbonaceous materials when heating the difuranic polymers to 700 °C, demonstrates some p...

Summary Plastics, used in countless consumer products that our daily lives depend on, have become indispensable materials essential for modern life and the global economy. At the same time, currently unsustainable practices in the production and disposal of plastics continue to deplete our finite natural resources and create severe worldwide environmental consequences. In the search for feasible solutions to these issues, significant recent advances have been made in developing chemically recyclable plastics, which allow for recovery of the building-block chemicals via depolymerization, for repolymerization to virgin-quality plastics, or for creative repurposing into value-added materials. Among such recyclable plastics, polyesters derived from renewable cyclic esters possess real potential to meet these challenges. Hence, this review highlights the plastics derived from common four-, five-, six-, seven-, and eight-membered cyclic esters by covering synthetic strategies, material properties, and, particularly, chemical recyclability. Such studies have culminated a recent discovery of infinitely recyclable plastics with properties of common plastics.

Current methods for hydrogenation of bio-derived itaconic acid (IA) lead to a mixture of isomeric lactone products. Transition-metal nanoparticles (TM-NPs), in situ-generated through thermolysis of TM(0) (Ru, Fe, W, Cr) carbonyls, in particular Ru-NPs, were found to catalyze regioselective hydrogenation of IA by syngas (2 H2 /CO) into γ-isovalerolactone (GiVL) in approximately 70 % isolated yield. Key sustainability features of this new route include: a one-pot direct transformation of bio-renewable IA into value-added GiVL selectively, use of inexpensive and renewable syngas in aqueous solution, and development of a supported recyclable NP catalyst system, Al2 O3 -Ru-NPs.

Organocatalyzed ring-opening polymerization (O-ROP) of a six-five bicyclic lactone, 4,5-trans-cyclohexyl-fused γ-butyrolactone (4,5-T6GBL), can be topologically selective or living at room temperature, depending on catalyst structure. A screening of (thio)urea [(T)U] and organic base pairs revealed unique trends in reactivity for this monomer as well as the most active catalyst pairs, which were employed as received commercially to produce relatively high molecular weight (Mn up to 106 kDa), low dispersity (Đ = 1.04) linear poly(4,5-T6GBL) in a living fashion. The ROP using a hybrid organic/inorganic pair of TU/KOMe in neat conditions led to poly(4,5-T6GBL) with even higher molecular weight (Mn = 215 kDa, Đ = 1.04). In comparison to the metal-catalyzed system, (T)U-base pairs exhibited competitive kinetics and reached higher monomer conversions, and their reactions can be performed in air. In addition, the resulting polymers required less purification to produce materials with higher onset decomposition temperature. (T)U-base pairs were selective towards linear polymerization only, whereas triazabicyclodecene can catalyze both polymerization and (quantitative) depolymerization processes, depending on reaction conditions. Cyclic polymers with Mn = 41–72 kDa were selectively formed via N-heterocyclic carbene-mediated zwitterionic O-ROP.

The unprecedented growth and socioeconomic impacts of polyolefins clearly outline a major success story in the world of polymer science. Polyolefins revolutionizes industries such as health care, construction, and food packaging. Despite the benefits of polyolefins, there is a rising concern for the environment due to high production volume (i.e., fossil fuel consumption), often short usage time, and problems related to waste management and accumulation in the natural environment. Creating a circular economy for polyolefins through effective recycling technologies has the potential to decrease the environmental impact of these materials. This perspective discusses polyolefins and their impact, existing and emerging recycling/upcycling solutions, and recycle-by-design alternatives that are challenging the status quo.