Circular Polymer Research Articles

Reprocessable and Recyclable Materials for 3D Printing via Reversible Thia-Michael Reactions.

The development of chemically recyclable polymers for sustainable 3D printing is crucial to reducing plastic waste and advancing towards a circular polymer economy. Here, we introduce a new class of polythioenones (PCTE) synthesized via Michael addition-elimination ring-opening polymerization (MAEROP) of cyclic thioenone (CTE) monomers. The designed monomers are straightforward to synthesize, scalable and highly modular, and the resulting polymers display mechanical performance superior to commodity polyolefins such as polyethylene and polypropylene. The material was successfully employed in 3D printing using fused-filament fabrication (FFF), showcasing excellent printability and mechanical recyclability. Notably, PCTE-Ph retains its tensile strength and thermal stability after multiple mechanical recycling cycles. Furthermore, PCTE-Ph can be depolymerized back to its original monomer with a 90% yield, allowing for repolymerization and establishing a successful closed-loop life cycle, making it a sustainable alternative for additive manufacturing applications.

Prediction and reliability analysis of ultimate axial strength for outer circular CFRP-strengthened CFST columns with CTGAN and hybrid MFO-ET model

This study develops a novel hybrid machine learning model to estimate the ultimate axial strength and conduct a reliability analysis for outer circular carbon fiber-reinforced polymer (CFRP)-strengthened concrete-filled steel tube (CFST) columns. The experimental datasets are collected and enriched using the conditional tabular generative adversarial network (CTGAN). The column length, the steel properties (cross-section diameter, thickness, and yield strength), the CFRP properties (thickness, tensile strength, and elastic modulus), and concrete strength are selected as input variables to develop the Extra Trees (ET) model hybridized with Moth-Flame Optimization (MFO) algorithm for the ultimate axial strength estimation. The results reveal that the CTGAN can efficiently capture the actual data distribution of CFRP-strengthened CFST columns and the developed hybrid MFO-ET model can accurately predict the ultimate axial strength with a high accuracy (R2 of 0.985, A10 of 0.867, RMSE of 182.810 kN, and MAE of 124.534 kN) based on the synthetic database. In addition, compared with the best empirical model, the MFO-ET model increases the R2 by (6.78% and 13.48%) and A10 by (108.19% and 122.88%) and reduces the RMSE by (68.19% and 66.24%) and MAE by (71.33% and 68.48%) based on real and synthetic databases, respectively. Notably, a reliability analysis is performed to evaluate the safety of the developed MFO-ET model using Monte Carlo Simulation (MCS). Finally, a web application tool is created to make the developed MFO-ET model easier for users to design practical applications.

High‐Performance Dynamic Photo‐Responsive Polymers With Superior Closed‐Loop Recyclability

AbstractChemical recycling to monomer (CRM) provides an ideal solution for reducing plastic waste and achieving a circular polymer economy. Cyclopentene (CP) and its derivatives are representative molecules that exemplify an efficient CRM process under mild conditions. However, most CP‐based polymers possess limited thermal and mechanical properties, constraining their potential applications. Herein, new CP‐based polymers with superior thermo‐mechanical properties and autonomous functions are proposed by integrating anthracene as a dynamic functional motif. Anthracenes not only serve as supramolecular cross‐linkages through strong π–π interactions but also form stronger chemical ones under UVA light irradiation, enhancing thermo‐mechanical properties. Furthermore, damage‐reporting, self‐healing, and local shape reprogramming capabilities are accomplished by utilizing the reversible nature of photodimerized anthracene. A mild and orthogonal depolymerization condition for the CP‐based polymers facilitates efficient monomer recovery even from mixed plastic wastes. This work presents a simple yet effective strategy to expand the usability of chemically recyclable polymers and contribute to a closed‐loop polymer economy.

Ambient cationic ring-opening polymerization of Dibenzo[c,e]oxepine-5(7H)-thione (DOT): Thermal and nucleophile-initiated depolymerization

With rising concern over plastic waste accumulation worldwide, the quantitative depolymerization of polymers into small molecule building blocks offers avenues toward a circular polymer economy. But a remaining challenge consists in tuning the degradation behaviour of polymers to ensure stability during use and efficient recyclability after use. Herein, the thionolactone dibenzo[c,e]oxepine-5(7H)-thione (DOT) is shown to undergo cationic ring-opening polymerization (CROP) under ambient conditions without the need for inert atmosphere or dry solvents. Involving S–O isomerization, the polymerization gave polythioesters in near-quantitative conversions with tuneable SEC-measured molar masses from 1.3–50 kg/mol and dispersities between 1.5 and 2.0. The polythioesters could be degraded with an excess of amine, with substoichiometric amounts of thiolate (which was shown to involve depolymerization from a thiolate ω-end group), or thermally. The latter two conditions produced the thiolactone dibenzo[c,e]thiepine-5(7H)-one (DTO). While anionic ring-opening polymerization (the common route to polythioesters) gives thiol end groups, the CROP presented herein provided end-capped polymers. Interestingly, the choice of initiator (and resulting end cap) was shown to have a drastic influence on the thermal stability. While a boron trifluoride-initiated polymer showed only 6 % decomposition when heated to 140 °C without solvent, a comparable methyl triflate-initiated polymer underwent 35 % degradation to DTO when heated to the same temperature overnight.

Chemical Recycling of Poly(cyclohexene carbonate)s via Synergistic Catalysis.

Chemical recycling of polymers to the corresponding monomers offers a valuable solution to address the current plastics crisis for creating an ideal and circular polymer economy. Here, we present a bimetallic synergistic depolymerization of the widely studied CO2-based polycarbonates, poly(cyclohexene carbonate)s, to epoxide monomers efficiently. The bimetallic CrIII-complex-mediated highly selective depolymerization and repolymerization was achieved via the regulation of reaction temperature, thus closing the circular loop of poly(cyclohexene carbonate)s in situ. Mechanistic investigation has revealed that the formation of epoxides undergoes a direct chain-end unzipping process. A bimetallic catalysis involving a nucleophilic attack of the metal-alkoxide species toward the methine carbon atom bound with an adjacent carbonyl that is activated by the other metal center features a lower energy barrier in DFT calculations, which promotes the epoxide extrusion.

On the Road to Circular Polymer Brushes: Challenges and Prospects.

Polymer brushes are unique surface coatings that have been of high interest in research for the past decades due to their covalent tethering to surfaces and the broad spectrum of polymers that can be grafted to or grafted from various surfaces. Modification of surfaces with brushes may provide lubricious and/or antifouling properties, and they can also potentially be used in many application fields due to their high responsiveness toward certain stimuli. Generally, polymer brushes are long-lasting coatings, while their end-of-life has to date largely been neglected. Therefore, it is important to consider additional design methodologies to produce circular brushes, which will degrade after a certain period of time such that surfaces can be reused, and the potentially obtained monomers may be used again to synthesize new brushes. In this Perspective, we aim to tackle and understand the challenges to translate the knowledge on degradation and chemical recycling of bulk polymers toward circular polymer brushes. We summarized the recent developments on (bio)degradable polymer brushes and the challenges that are to be tackled toward their potential implementation as circular coatings.

Avantium focuses on maximizing the potential of its FDCA and PEF technology

On December 13, Avantium N.V., a company in renewable and circular polymer materials, gave an update on its progress, strategy and plans during a Capital Markets Day it held for institutional and retail investors.

Chemically recyclable nanofiltration membranes fabricated from two circular polymer classes of the same monomer origin

Nanofiltration is widely used in various industries to separate solutes from solvents. To foster a circular economy, establishing a closed-loop lifecycle for the membrane materials is highly important. In this study, we fabricated recyclable nanofiltration membranes from chemically recyclable polymers —polyester P(BiL=)ROP and poly(cyclic olefin) P(BiL=)ROMP— using γ-butyrolactone as a green solvent. These two polymers, of two different polymer classes, were obtained from a single monomer, which could be recycled back to the same monomer, exhibiting the unique “one monomer–two polymers–one monomer” closed-loop chemical circularity. The effect of physical treatment, such as annealing, hot-pressing, and air exposure on the morphological characteristics and performance of the nanofiltration membranes was investigated. We revealed the interplay between membrane pore size, thickness, density and the molecular sieving performance of the nanofiltration membranes. Solute rejections were mainly governed by the membrane pore size. However, solvent flux was mainly governed by the membrane density that determines the free volume interconnectivity. The membranes exhibited a tunable molecular weight cutoff between 553 and 777 g mol−1 and methanol permeance between 5.9 and 9.8 L m–2 h–1 bar−1. The membranes exhibited excellent long-term nanofiltration stability over 1 week. The combination of the green solvent used for membrane fabrication and the circular life cycle of the polymer membrane brings one step closer to closing the circularity loop of membrane technology.

Molecular Dynamics Simulation of a Feather-Boa Model of a Bacterial Chromosome.

The chromosome of a bacterium consists of a mega-base pair-long circular DNA, which self-organizes within the micron-sized bacterial cell volume, compacting itself by three orders of magnitude. Unlike eukaryotic chromosomes, it lacks a nuclear membrane and freely floats in the cytosol confined by the cell membrane. It is believed that strong confinement, cross-linking by associated proteins, and molecular crowding all contribute to determine chromosome size and morphology. Modelling the chromosome simply as a circular polymer decorated with closed side loops in a cylindrical confining volume has been shown to already recapture some of the salient properties observed experimentally. Here we describe how a computer simulation can be set up to study structure and dynamics of bacterial chromosomes using this model.

The thermodynamics and kinetics of depolymerization: what makes vinyl monomer regeneration feasible?

Depolymerization is potentially a highly advantageous method of recycling plastic waste which could move the world closer towards a truly circular polymer economy. However, depolymerization remains challenging for many polymers with all-carbon backbones. Fundamental understanding and consideration of both the kinetics and thermodynamics are essential in order to develop effective new depolymerization systems that could overcome this problem, as the feasibility of monomer generation can be drastically altered by tuning the reaction conditions. This perspective explores the underlying thermodynamics and kinetics governing radical depolymerization of addition polymers by revisiting pioneering work started in the mid-20th century and demonstrates its connection to exciting recent advances which report depolymerization reaching near-quantitative monomer regeneration at much lower temperatures than seen previously. Recent catalytic approaches to monomer regeneration are also explored, highlighting that this nascent chemistry could potentially revolutionize depolymerization-based polymer recycling in the future.

Improving plastic pyrolysis oil quality via an electrochemical process for polymer recycling: a review

In this review we discuss the application of electrochemical hydrogenation for pyrolysis oil upgrading, thus facilitating a circular polymer economy and low-carbon fuel production.

Envisioning a BHET Economy: Adding Value to PET Waste

Poly(ethylene terephthalate), the fifth most produced polymer, generates significant waste annually. This increased waste production has spurred interest in chemical and mechanical pathways for recycling. The shift from laboratory settings to larger-scale implementation creates opportunities to explore the value and recovery of recycling products. Derived from the glycolysis of PET, bis(2-hydroxyethyl) terephthalate (BHET) exhibits versatility as a depolymerization product and valuable monomer. BHET exhibits versatility and finds application across diverse industries such as resins, coatings, foams, and tissue scaffolds. Incorporating BHET, which is a chemical recycling product, supports higher recycling rates and contributes to a more sustainable approach to generating materials. This review illuminates the opportunities for BHET as a valuable feedstock for a more circular polymer materials economy.

Recycled aggregate concrete-filled steel tube columns with small-diameter circular FRP-confined sea-sand concrete cores: Conceptual and experimental investigations

Recycled aggregate concrete (RAC) prepared from waste concrete can achieve the green recycling of resources; however, its inferior material properties hinder its applications. Sea-sand concrete (SSC) can help alleviate the resource shortage of fresh water and river sands but is prone to steel corrosion. To address these challenges, this study developed a new RAC-filled steel tube column with a small-diameter circular fibre-reinforced polymer (FRP)-confined SSC core. This paper presents and discusses the results of the axial compression tests conducted on RAC-filled steel tube columns, with and without small-diameter circular FRP-confined SSC cores. The study investigated the effects of the configuration of the FRP-confined SSC core, diameter-to-thickness ratio of the FRP tubes, compressive strength of RAC and SSC, and cross-sectional shape of the column. The results revealed that using small-diameter circular FRP-confined SSC cores to partially replace RAC was effective in improving the compressive performance of RAC-filled steel tube columns. The increase in the number of FRP tubes, in-filled concrete strength, and confinement stiffness of the steel and FRP tubes contributed to improvements in the structural strength and ductility, resulting in progressive ladder-shaped declines in the post-peak curve. Finally, a reasonable load–capacity model is presented for the proposed columns.

Ionic-Liquid-Mediated Deconstruction of Polymers for Advanced Recycling and Upcycling.

Ionic liquids (ILs) are a promising medium to assist in the advanced (chemical and biological) recycling of polymers, owing to their tunable catalytic activity, tailorable chemical functionality, low vapor pressures, and thermal stability. These unique physicochemical properties, combined with ILs' capacity to solubilize plastics waste and biopolymers, offer routes to deconstruct polymers at reduced temperatures (and lower energy inputs) versus conventional bulk and solvent-based methods, while also minimizing unwanted side reactions. In this Viewpoint, we discuss the use of ILs as catalysts and mediators in advanced recycling, with an emphasis on chemical recycling, by examining the interplay between IL chemistry and deconstruction thermodynamics, deconstruction kinetics, IL recovery, and product recovery. We also consider several potential environmental benefits and concerns associated with employing ILs for advanced recycling over bulk- or solvent-mediated deconstruction techniques, such as reduced chemical escape by volatilization, decreased energy demands, toxicity, and environmental persistence. By analyzing IL-mediated polymer deconstruction across a breadth of macromolecular systems, we identify recent innovations, current challenges, and future opportunities in IL application toward circular polymer economies.

Machine learning-based model for the ultimate strength of circular concrete-filled fiber-reinforced polymer–steel composite tube columns

This study introduces a machine learning (ML)-based model for predicting the ultimate strength of circular concrete-filled fiber-reinforced polymer (FRP)–steel composite tube (CFSCT) columns. The support vector regression (SVR), back propagation neural network (BPNN), and random forest (RF) ML algorithms were trained and tested based on an extensive dataset containing 305 data samples of circular CFSCT columns under axial loads from existing literature. Seven parameters were selected as input variables, and the ultimate load of the circular CFSCT columns under an axial load was selected as the output variable to develop the ML-based models. The ML-based models were evaluated by comparing them with four existing empirical models previously developed by researchers. The results indicate that the ML-based models are good alternatives to existing empirical models, considering their superior prediction accuracy and applicability. The coefficient of determination (R2) of the SVR model was 0.992, demonstrating better performance than the BPNN and RF models with R2 of 0.984 and 0.982, respectively. However, only the SVR and BPNN model are considered applicable for the ultimate load prediction since the RF model exhibited an overfitting problem. Moreover, the results of the comparison between the ML-based and empirical models were visualized to illustrate the superiority of the ML-based models over existing empirical models in prediction accuracy and dispersion. Finally, sensitivity and parametric analyses were performed to analyze the influence of the input variables on the output variable. The results indicate that concrete compressive strength, steel tube thickness, and FRP thickness positively affect the ultimate strength of CFSCT columns. This study can provide a reference for the design of CFSCT columns.

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

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

Mechanochemical Degradation and Recycling of Synthetic Polymers.

The accumulation of plastic waste, due to lack of recycling, has led to serious environmental pollution. Although mechanical recycling can alleviate this issue, it inevitably reduces the molecular weight and weakens the mechanical properties of materials and is not suitable for mixed materials. Chemical recycling, on the other hand, breaks the polymer into monomers or small-molecule constituents, allowing for the preparation of materials of quality comparable to that of the virgin polymers and can be applied to mixed materials. Mechanochemical degradation and recycling leverages the advantages of mechanical techniques, such as scalability and efficient energy use, to achieve chemical recycling. We summarize recent progress in mechanochemical degradation and recycling of synthetic polymers, including both commercial polymers and those designed for more efficient mechanochemical degradation. We also point out the limitations of mechanochemical degradation and present our perspectives on how the challenges can be mitigated for a circular polymer economy.

Mechanochemical Degradation and Recycling of Synthetic Polymers

AbstractThe accumulation of plastic waste, due to lack of recycling, has led to serious environmental pollution. Although mechanical recycling can alleviate this issue, it inevitably reduces the molecular weight and weakens the mechanical properties of materials and is not suitable for mixed materials. Chemical recycling, on the other hand, breaks the polymer into monomers or small‐molecule constituents, allowing for the preparation of materials of quality comparable to that of the virgin polymers and can be applied to mixed materials. Mechanochemical degradation and recycling leverages the advantages of mechanical techniques, such as scalability and efficient energy use, to achieve chemical recycling. We summarize recent progress in mechanochemical degradation and recycling of synthetic polymers, including both commercial polymers and those designed for more efficient mechanochemical degradation. We also point out the limitations of mechanochemical degradation and present our perspectives on how the challenges can be mitigated for a circular polymer economy.

Ring-Opening Polymerization of a Seven-Membered Lactone toward a Biocompatible, Degradable, and Recyclable Semi-aromatic Polyester

The widespread use of polymer materials and their improper disposal have resulted in significant environmental pollution and wastage of resources. Therefore, the design of chemically recyclable, environmentally compatible, and applicable polymers represents a new approach for a green and circular polymer economy, and it has become a major focus worldwide. Herein, we efficiently synthesized a seven-membered ether lactone, 3,4-dihydro-2H-benzo[b][1,4]dioxepin-2-one (BDXO), through a one-step reaction and successfully realized its controlled ring-opening polymerization (ROP). The ROP of BDXO in bulk and solvent conditions catalyzed by Sn(Oct)2 was investigated systematically, and the thermodynamic parameters obtained were ΔSP0 = −30.90 J·mol–1·K–1 and ΔHP0 = −14.52 kJ·mol–1. The PBDXO polymer exhibits excellent thermal properties, applicable mechanical properties, excellent biocompatibility, and complete hydrolytic degradability. Notably, not only was the direct chemical recycling of PBDXO to BDXO monomer realized with high efficiency through bulk thermal depolymerization with a yield greater than 92% and a purity of up to 96% but also the regenerated PBDXO with almost the same structure and properties as the initial PBDXO was obtained through repolymerization of the recycled BDXO, establishing a polymer–monomer–polymer closed-loop life cycle.

Self-powered difunctional sensors based on sliding contact-electrification and tribovoltaic effects for pneumatic monitoring and controlling

Pneumatic elements such as position and velocity sensors play a vital role in pneumatic monitoring and servo control, which have great demands for integration, multifunction and self-powering. Here we propose an integrated triboelectric nanogenerator (TENG) and tribovoltaic nanogenerator (TVNG) in air cylinder as difunctional pneumatic sensor for simultaneous position and velocity monitoring. Based on the sliding contact-electrification between a semicircular PEDOT: PSS film and a circular PTFE polymer, the output voltage of the freestanding-mode TENG can characterize the rotation angle with a resolution superior of 0.02°. While based on the tribovoltaic effect by the friction between an annular PEDOT: PSS film and a circular aluminum electrode, the output current of TVNG can characterize the rotation angular velocity with a detection limit as low as 1 r/min. With the difunctional sensors, the pneumatic monitoring and high precision servo control of the rotary cylinder is realized within the overshoot of 2% and the control error of 1.5%. This work has provided an active sensing strategy for pneumatic monitoring and demonstrated the great prospects of triboelectric sensor in intelligent pneumatic systems.