Closed-loop recyclable polymers: from monomer and polymer design to the polymerization-depolymerization cycle

The extensive utilization of plastic, as a symbol of modern technological society, has consumed enormous amounts of finite and non-renewable fossil resources and produced huge amounts of plastic wastes in the land or ocean, and thus recycling and reuse of the plastic wastes have great ecological and economic benefits. Closed-loop recyclable polymers with inherent recyclability can be readily depolymerized into monomers with high selectivity and purity and repolymerized into polymers with the same performance. They are deemed to be the next generation of recyclable polymers and have captured great and increasing attention from academia and industry. Herein, we provide an overview of readily closed-loop recyclable polymers based on monomer and polymer design and no-other-reactant-involved reversible ring-opening and addition polymerization reactions. The state-of-the-art of circular polymers is separately summarized and discussed based on different monomers, including lactones, thiolactones, cyclic carbonates, hindered olefins, cycloolefins, thermally labile olefin comonomers, cyclic disulfides, cyclic (dithio) acetals, lactams, Diels-Alder addition monomers, Michael addition monomers, anhydride-secondary amide monomers, and cyclic anhydride-aldehyde monomers, and polymers with activatable end groups. The polymerization and depolymerization mechanisms are clearly disclosed, and the evolution of the monomer structure, the polymerization and depolymerization conditions, the corresponding polymerization yield, molecular weight, performance of the polymers, monomer recovery, and depolymerization equipment are also systematically summarized and discussed. Furthermore, the challenges and future prospects are also highlighted.

Designed to Degrade: Tailoring Polyesters for Circularity

A powerful toolbox is needed to turn the linear plastic economy into circular. Development of materials designed for mechanical recycling, chemical recycling, and/or biodegradation in targeted end-of-life environment are all necessary puzzle pieces in this process. Polyesters, with reversible ester bonds, are already forerunners in plastic circularity: poly(ethylene terephthalate) (PET) is the most recycled plastic material suitable for mechanical and chemical recycling, while common aliphatic polyesters are biodegradable under favorable conditions, such as industrial compost. However, this circular design needs to be further tailored for different end-of-life options to enable chemical recycling under greener conditions and/or rapid enough biodegradation even under less favorable environmental conditions. Here, we discuss molecular design of the polyester chain targeting enhancement of circularity by incorporation of more easily hydrolyzable ester bonds, additional dynamic bonds, or degradation catalyzing functional groups as part of the polyester chain. The utilization of polyester circularity to design replacement materials for current volume plastics is also reviewed as well as embedment of green catalysts, such as enzymes in biodegradable polyester matrices to facilitate the degradation process.

Dynamic Chemistry Toolbox for Advanced Sustainable Materials

Developing dynamic chemistry for polymeric materials offers chemical solutions to solve key problems associated with current plastics. Mechanical performance and dynamic function are equally important in material design because the former determines the application scope and the latter enables chemical recycling and hence sustainability. However, it is a long-term challenge to balance the subtle trade-off between mechanical robustness and dynamic properties in a single material. The rise of dynamic chemistry, including supramolecular and dynamic covalent chemistry, provides many opportunities and versatile molecular tools for designing constitutionally dynamic materials that can adapt, repair, and recycle. Facing the growing social need for developing advanced sustainable materials without compromising properties, recent progress showing how the toolbox of dynamic chemistry can be explored to enable high-performance sustainable materials by molecular engineering strategies is discussed here. The state of the art and recent milestones are summarized and discussed, followed by an outlook toward future opportunities and challenges present in this field.

Modified magnesium oxide/silver nanoparticles reinforced poly (butylene succinate-co-terephthalate) composite biofilms for food packaging application

MgO/Ag nanoparticles (NPs) were surface-modified with titanate coupling agent titaniumtriisostearoylisopropoxide (NDZ-130). A new antibacterial biofilm for food packaging was synthesized by combining the modified MgO/Ag NPs with poly (butylene succinate-co-terephthalate) (PBST). The modification improved the compatibility between the MgO/Ag NPs and the PBST matrix. The effects of the modified MgO/Ag NPs on biofilm mechanical, barrier, thermal, antibacterial and food preservation properties were evaluated. Compared with the PBST/MgO/Ag composite film, the modified PBST/MgO/Ag composite film showed an increase in tensile strength (TS) of 8.71% and elongation at break (EB) of 16.66%, additionally decreasing water vapor permeability (WVP) by 42.86%. The composite film also exhibited over 95% inhibition of Staphylococcus aureus and Escherichia coli. The modified PBST/MgO/Ag composite film avoided microbial contamination and preserved cherry tomatoes while maintaining moisture and firmness for six days. All results indicated that the prepared biofilms have a high potential for use as food packaging films.

Overcoming the limitations of Kolbe coupling with waveform-controlled electrosynthesis

The Kolbe reaction forms carbon-carbon bonds through electrochemical decarboxylative coupling. Despite more than a century of study, the reaction has seen limited applications owing to extremely poor chemoselectivity and reliance on precious metal electrodes. In this work, we present a simple solution to this long-standing challenge: Switching the potential waveform from classical direct current to rapid alternating polarity renders various functional groups compatible and enables the reaction on sustainable carbon-based electrodes (amorphous carbon). This breakthrough enabled access to valuable molecules that range from useful unnatural amino acids to promising polymer building blocks from readily available carboxylic acids, including biomass-derived acids. Preliminary mechanistic studies implicate the role of waveform in modulating the local pH around the electrodes and the crucial role of acetone as an unconventional reaction solvent for Kolbe reaction.

Fully Bio-based Poly(ketal-ester)s by Ring-opening Polymerization of a Bicylcic Lactone from Glycerol and Levulinic Acid

A fully renewable bio-based bicyclic lactone containing a five-membered cyclic ketal moiety, 7-methyl-3,8,10-trioxabicyclo[5.2.1]decan-4-one (TOD), was synthesized through a two-step acid-catalyzed process from glycerol and levulinic acid. The ring-opening polymerization (ROP) of TOD at 30°C with benzyl alcohol (BnOH) as the initiator and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as the catalyst can afford high molar mass PTOD with a cis-2.4-disubstitued 2-methyl 1,3-dioxolane moiety in its repeating unit. PTOD is an amorphous polymer with a glass transition temperature (T_g ) of 13°C. It can be hydrolyzed into structurally defined small molecules under acidic or basic conditions by the selective cleavage of either the cyclic ketal or the ester linkage respectively. The TBD-catalyzed copolymerization of L-lactide (L-LA) and TOD at -20°C was investigated. It was confirmed that L-LA polymerized quickly with racemization to form PLA, followed by a slow incorporation of TOD into the formed PLA chains via transesterification. By varying the feed ratios of L-LA to TOD, a series of random copolymers (PLA-co-PTOD) with different TOD incorporation ratios and tunable T_g s were obtained. Under acidic conditions, PLA-co-PTOD degrades much faster than PLA via the selective cleavage of the cyclic ketal linkages. This work provides insights for the development of more sustainable and acid-accelerated degradable alternatives to aliphatic polyesters.

Polymerization, Stimuli-induced Depolymerization, and Precipitation-driven Macrocyclization in a Nitroaldol Reaction System

Dynamic covalent polymers of different topology have been synthesized from an aromatic dialdehyde and α,ω-dinitroalkanes via the nitroaldol reaction. All dinitroalkanes yielded dynamers with the dialdehyde, where the length of the dinitroalkane chain played a vital role in determining the structure of the final products. For longer dinitroalkanes, linear dynamers were produced, where the degree of polymerization reached a plateau at higher feed concentrations. In the reactions involving 1,4-dinitrobutane and 1,5-dinitropentane, specific macrocycles were formed through depolymerization of the linear chains, further driven by precipitation. At lower temperature, the same systemic self-sorting effect was also observed for the 1,6-dinitrohexane-based dynamers. Moreover, the dynamers showed a clear adaptive behavior, displaying depolymerization and rearrangement of the dynamer chains in response to alternative building blocks as external stimuli.

Degradable polyprodrugs: design and therapeutic efficiency

Prodrugs are developed to increase the therapeutic properties of drugs and reduce their side effects. Polyprodrugs emerged as highly efficient prodrugs produced by the polymerization of one or several drug monomers. Polyprodrugs can be gradually degraded to release therapeutic agents. The complete degradation of polyprodrugs is an important factor to guarantee the successful disposal of the drug delivery system from the body. The degradation of polyprodrugs and release rate of the drugs can be controlled by the type of covalent bonds linking the monomer drug units in the polymer structure. Therefore, various types of polyprodrugs have been developed based on polyesters, polyanhydrides, polycarbonates, polyurethanes, polyamides, polyketals, polymetallodrugs, polyphosphazenes, and polyimines. Furthermore, the presence of stimuli-responsive groups, such as redox-responsive linkages (disulfide, boronate ester, metal-complex, and oxalate), pH-responsive linkages (ester, imine, hydrazone, acetal, orthoester, P-O and P-N), light-responsive (metal-complex, o-nitrophenyl groups) and enzyme-responsive linkages (ester, peptides) allow for a selective degradation of the polymer backbone in targeted tumors. We envision that new strategies providing a more efficient synergistic therapy will be developed by combining polyprodrugs with gene delivery segments and targeting moieties.

Temperature/pH-responsive carmofur-loaded nanogels rapidly prepared via one-pot laser-induced emulsion polymerization

Tumor microenvironment-responsive nanogels loading antitumor drugs can improve the chemotherapy efficiency due to their suitable size, great hydrophilicity, excellent biocompatibility, and sensitivity to specific stimulation. Herein, a simple and effective strategy of one-pot laser-induced emulsion polymerization at 532 nm was developed to prepare carmofur-loaded nanogels based on biocompatible and temperature/pH-sensitive monomers including polyethylene glycol diacrylate (PEGDA), N-vinylcaprolactam (NVCL), and 2-(dimethylamino) ethyl methacrylate (DMAEMA). The nanogels loading carmofur with dual-stimuli responsive drug release properties were rapidly obtained under laser irradiation (beam diameter 2.5 mm, laser power 60 mW) for only 100 s. These nanogels exhibited an average hydrodynamic diameter of 195.9 nm and a low polydispersity index of 0.115. The effect of monomer ratio on the size, morphology, double-bond conversion, and thermo/pH-sensitivity of nanogels was investigated. The cumulative carmofur release from nanogels at pH 5.0 within 48 h was nearly three times that at pH 7.4, while the release amount at 42 °C was twice that at 25 °C, showing the controlled and sustainable release with the change of pH and temperature. The in vitro release kinetics of carmofur was in accord with first-order release model.

Chemically Recyclable Poly(β-thioether ester)s Based on Rigid Spirocyclic Ketal Diols Derived from Citric Acid

Incorporating rigid cyclic acetal and ketal units into polymer structures is an important strategy toward recyclable high-performance materials from renewable resources. In the present work, citric acid, a widely used platform chemical derived from biomass, has been efficiently converted into di- and tricyclic diketones. Ketalization with glycerol or trimethylolpropane afforded rigid spirodiols, which were obtained as complex mixtures of isomers. After a comprehensive NMR analysis, the spirodiols were converted into the respective di(meth)acrylates and utilized in thiol–ene polymerizations in combination with different dithiols. The resulting poly(β-thioether ester ketal)s were thermally stable up to 300 °C and showed glass-transition temperatures in a range of −7 to 40 °C, depending on monomer composition. The polymers were stable in aqueous acids and bases, but in a mixture of 1 M aqueous HCl and acetone, the ketal functional groups were cleanly hydrolyzed, opening the pathway for potential chemical recycling of these materials. We envision that these novel bioderived spirodiols have a great potential to become valuable and versatile bio-based building blocks for several different kinds of polymer materials.

Ring opening copolymerization of δ-valerolactone with 2-methyl-1,3-dioxane-4-one towards poly(3-hydroxypropionate-co-5-hydroxyvalerate) copolyesters

A straightforward strategy of ring-opening copolymerization (ROCOP) toward the synthetic copolyesters is gaining increasing interest. In present work, a novel series of poly(3-hydroxypropionate-co-5-hydroxyvalerate) P(3HP-co-5HV) copolyesters are obtained through ROCOP of...

Chemically recyclable thermoplastics from reversible-deactivation polymerization of cyclic acetals

Identifying plastics capable of chemical recycling to monomer (CRM) is the foremost challenge in creating a sustainable circular plastic economy. Polyacetals are promising candidates for CRM but lack useful tensile strengths owing to the low molecular weights produced using current uncontrolled cationic ring-opening polymerization (CROP) methods. Here, we present reversible-deactivation CROP of cyclic acetals using a commercial halomethyl ether initiator and an indium(III) bromide catalyst. Using this method, we synthesize poly(1,3-dioxolane) (PDXL), which demonstrates tensile strength comparable to some commodity polyolefins. Depolymerization of PDXL using strong acid catalysts returns monomer in near-quantitative yield and even proceeds from a commodity plastic waste mixture. Our efficient polymerization method affords a tough thermoplastic that can undergo selective depolymerization to monomer.

Amine-Catalyzed Chain Polymerization of Ethyl Glyoxylate from Alcohol and Thiol Initiators

Polyacetals have significant potential as degradable polymers, but aldehyde polymerizations are generally difficult to control. Here we show that polymerization of ethyl glyoxylate can be initiated from alcohols or thiols by activation with triethylamine to afford poly(ethyl glyoxylate) with controllable molecular weights and relatively low dispersities (Đ = 1.3-1.4), as evidenced by MALDI-TOF mass spectrometry. Stabilization against depolymerization by chain-capping with benzyl chloroformate was found to proceed without side reactions observed from chain-capping with tolyl isocyanate. The use of the stronger base DBU leads to competing side reactions that limit polymer molecular weight.

Degradable Spirocyclic Polyacetal-Based Core-Amphiphilic Assemblies for Encapsulation and Release of Hydrophobic Cargo

Polymeric nanomaterials that degrade in acidic environments have gained considerable attention in nanomedicine for intracellular drug delivery and cancer therapy. Among various acid-degradable linkages, spirocyclic acetals have rarely been used to fabricate such vehicles. In addition to acid sensitivity, they benefit from conformational rigidity that is otherwise not attainable by their non-spirocyclic analogs. Herein, amphiphilic spirocyclic polyacetals are synthesized by Cu-catalyzed alkyne-azide "click" polymerization. Unlike conventional block copolymers, which often form core-shell structures, these polymers self-assemble to form core amphiphilic assemblies capable of encapsulating Nile red as a hydrophobic model drug. In vitro experiments show that while release from these materials can occur at neutral pH with preservation of their integrity, acidic pH accelerates efficient cargo release and leads to the complete degradation of assemblies. Moreover, cellular assays reveal that these materials are fully cytocompatible, interact with the plasma membrane, and can be internalized by cells, rendering them as potential candidates for cancer therapy and/or drug delivery.

Synthesis and properties of l-lactide/1,3-dioxolane copolymers: preparation of polyesters with enhanced acid sensitivity

Herein, we report the first example of cationic ring-opening copolymerization of 5-membered cyclic acetal (1,3-dioxolane (DXL)) with l-lactide (LA) to afford polylactide containing acetal units.

Synthesis and melt-spinning of partly bio-based thermoplastic poly(cycloacetal-urethane)s toward sustainable textiles

Partly bio-based thermoplastic poly(cycloacetal-urethane)s synthesized and melt-spun into textile fibres that can be potentially chemically recycled.

Degradability, thermal stability, and high thermal properties in spiro polycycloacetals partially derived from lignin

Spiro polycycloacetals were synthesized from vanillin and syringaldehyde, along with high-performance co-monomers, exhibiting high glass transition temperatures and thermal stabilities, and rapid rates of hydrolysis in acidic solutions.

Catalyst-Free Metathesis of Cyclic Acetals and Spirocyclic Acetal Covalent Adaptable Networks

Due to the exchangeability of dynamic covalent bonds in the covalent adaptable networks (CANs) at elevated temperature, they possess recyclability while still maintaining many of the superior properties of thermosets. The exploration of dynamic covalent chemistry is of great significance to the expansion of CANs library and hence the sustainable development of thermosets. In this work, we discovered that, in absence of catalyst, the direct metathesis of the cyclic acetals proceeds while the acyclic acetals cannot. The metathesis kinetics of the cyclic acetals were fully revealed with model compounds. For the CANs demonstration, a series of cross-linked spirocyclic acetal polymers with excellent reprocessability, high thermal stability, and high refractivity were prepared via thiol-ene click polymerization. We envisage that the uncovering of the catalyst-free metathesis of cyclic acetals will enrich the dynamic chemistry of acetals and greatly promote the development of acetal-based CANs and their potential applications in optical devices.

Alternating Ring-Opening Metathesis Polymerization Provides Easy Access to Functional and Fully Degradable Polymers

Polymers with hydrolyzable groups in their backbones have numerous potential applications in biomedicine, lithography, energy storage and electronics. In this study, acetal and ester functionalities were incorporated into the backbones of copolymers by means of alternating ring-opening metathesis polymerization catalyzed by third-generation Grubbs ruthenium catalyst. Specifically, combining large-ring (7-10 atoms) cyclic acetal or lactone monomers with bicyclo[4.2.0]oct-1(8)-ene-8-carboxamide monomers provided perfectly alternating copolymers with acetal or ester functionality in the backbones and low to moderate molecular weight distribution (Đ M = 1.2-1.6). Copolymers containing ester and acetal backbones hydrolyzed to significant extent under basic condition (pH 13) and acidic conditions (pH ≤ 5) respectively to yield the expected by-products within 30 hours at moderate temperature. Unlike the copolymer with all-carbon backbone, copolymers with heteroatom-containing backbone exhibited viscoelastic behavior with crossover frequency which decreases as the size of the R group on the acetal increases. In contrast, the glass transition temperature (T g) decreases as the size of the R group decreases. The rate of hydrolysis of the acetal copolymers was also dependent on the R group. Thus, ruthenium-catalyzed alternating ring-opening metathesis copolymerization provides heterofunctional copolymers whose degradation rates, glass transition temperatures, and viscoelastic moduli can be controlled.

100th Anniversary of Macromolecular Science Viewpoint: Polymerization of Cumulated Bonds: Isocyanates, Allenes, and Ketenes as Monomers

Polymer chemistry offers exciting opportunities to tailor the properties of soft materials through control of the composition of the polymers and their interaction with each other, additives, and surfaces. Ongoing advances in the synthesis of polymeric materials demonstrate the drive for materials with tailored properties for enhanced performance in the next generation of materials and devices. One class of small molecules that can serve as monomers in chain growth polymerization are cumulated double bonds of the general form X═Y═Z. The three most common classes of these molecules are isocyanates (N═C═O), allenes (C═C═C), and ketenes (C═C═O), each of which has been explored as monomers under a variety of conditions. The orthogonality of the two pi bonds of the cumulated double bonds (i.e., lack of conjugation) enables the formation of different polymer backbones from a single monomer, provided the regioreactivity is controlled. This Viewpoint outlines the use of these three cumulated double bonds as monomers, illustrating success and current limitations to established polymerization methods. We then provide an outlook to the future of cumulated double bonds as monomers for the generation of tailored polymer compositions.

Hydrolysis and Solvolysis as Benign Routes for the End-of-Life Management of Thermoset Polymer Waste

The production of thermoset polymers is increasing globally owing to their advantageous properties, particularly when applied as composite materials. Though these materials are traditionally used in more durable, longer-lasting applications, ultimately, they become waste at the end of their usable lifetimes. Current recycling practices are not applicable to traditional thermoset waste, owing to their network structures and lack of processability. Recently, researchers have been developing thermoset polymers with the right functionalities to be chemically degraded under relatively benign conditions postuse, providing a route to future management of thermoset waste. This review presents thermosets containing hydrolytically or solvolytically cleavable bonds, such as esters and acetals. Hydrolysis and solvolysis mechanisms are discussed, and various factors that influence the degradation rates are examined. Degradable thermosets with impressive mechanical, thermal, and adhesion behavior are discussed, illustrating that the design of material end-of-life need not limit material performance.

Levulinic acid: a sustainable platform chemical for novel polymer architectures

Levulinic acid is a multipurpose platform chemical that is currently used in a wide variety of applications.

Amphiphilic poly(esteracetal)s as dual pH- and enzyme-responsive micellar immunodrug delivery systems

Amphiphilic poly(esteracetal) micelles encapsulate potent immune modulatory drugs, but fall apart and release them upon dual pH or enzymatic stimuli.

Cellulose-Derived Functional Polyacetal by Cationic Ring-Opening Polymerization of Levoglucosenyl Methyl Ether

The unsaturated bicyclic acetal levoglucosenyl methyl ether was readily obtained from sustainable feedstock (cellulose) and polymerized by cationic ring-opening polymerization to produce a semicrystalline thermoplastic unsaturated polyacetal with relatively high apparent molar mass (up to ca. 36 kg mol-1 ) and decent dispersity (ca. 1.4). The double bonds along the chain can undergo hydrogenation and thiol-ene reactions as well as crosslinking, thus making this polyacetal potentially interesting as a reactive functional material.

Desilylation-Triggered Degradable Silylacetal Polymers Synthesized via Controlled Cationic Copolymerization of Trimethylsilyl Vinyl Ether and Cyclic Acetals

Silylacetal was demonstrated to function as a promising cleavable moiety for preparing polymers degradable via desilylation under diverse, mild conditions. The silylacetal moieties were installed in the main chain of the polymers via the controlled cationic copolymerization of trimethylsilyl vinyl ether (TMSVE) and a cyclic acetal under appropriately designed conditions. Importantly, desilylation reactions of the silylacetal units occurred under weak acid, base, or fluoride ion conditions, which triggered the degradation of the polymer via the spontaneous cleavage of the unstable hemiacetal moieties generated by the desilylation. Moreover, silylacetal moieties were successfully incorporated at the desired positions in the main chain via the addition of a small portion of TMSVE during the controlled cationic copolymerization of a vinyl ether and cyclic acetal. The strategy devised in this study will allow the design of elaborate polymers that undergo degradation triggered by various stimuli.

Designing Biobased Recyclable Polymers for Plastics

Several concurrent developments are shaping the future of plastics. A transition to a sustainable plastics system requires not only a shift to fossil-free feedstock and energy to produce the carbon-neutral building blocks for polymers used in plastics, but also a rational design of the polymers with both desired material properties for functionality and features facilitating their recyclability. Biotechnology has an important role in producing polymer building blocks from renewable feedstocks, and also shows potential for recycling of polymers. Here, we present strategies for improving the performance and recyclability of the polymers, for enhancing degradability to monomers, and for improving chemical recyclability by designing polymers with different chemical functionalities.

Ring-Opening Metathesis Polymerization of Biomass-Derived Levoglucosenol

The readily available cellulose-derived bicyclic compound levoglucosenol was polymerized through ring-opening metathesis polymerization (ROMP) to yield polylevoglucosenol as a novel type of biomass-derived thermoplastic polyacetal, which, unlike polysaccharides, contains cyclic as well as linear segments in its main chain. High-molar-mass polyacetals with apparent weight-average molar masses of up to 100 kg mol^-1 and dispersities of approximately 2 were produced despite the non-living/controlled character of the polymerization due to irreversible deactivation or termination of the catalyst/active chain ends. The resulting highly functionalized polyacetals are glassy in bulk with a glass transition temperature of around 100 °C. In analogy to polysaccharides, polylevoglucosenol degrades slowly in an acidic environment.

Orthogonally functionalizable polyacetals: a versatile platform for the design of acid sensitive amphiphilic copolymers

Dually functionalized amphiphilic copolyacetals as rational approach to the development of pH-responsive site-specific drug delivery systems.

Polyaddition of vinyl ethers and phthalaldehydes via successive cyclotrimerization reactions: selective model reactions and synthesis of acid-degradable linear poly(cyclic acetal)s

A polyaddition via the cyclotrimerization of one vinyl monomer and two conjugated dialdehydes proceeded using a Lewis acid catalyst, yielding a polymer with cyclic acetal structures in the main chain.

Application of functional diols derived from pentaerythritol as chain extenders in the synthesis of novel thermoplastic polyester-urethane elastomers

Functional thermoplastic poly(ester-urethane)s (TPEUs) reported herein offer a wide range of thermal, mechanical and degradation properties which can be fine-tuned through a selection of post-polymerisation reactions.