The thermodynamics of ring-opening polymerization (ROP) are central when predicting the chemical recyclability of aliphatic polyesters and polycarbonates. Conceptually, the enthalpy of polymerization, DH p, is widely understood as a measure of ring-strain for a given monomer. However, the ring-strain is commonly larger than DH p, generating the question of how the release of ring-strain energy during ring-opening transforms. In this work, we propose that DH p is the sum of the energy released by the ring-strain ðDH ring-strainÞ and the energy absorbed by the polymer conformations ðDH confÞ. Owing to the similar ring-strain, but vastly different DH p values, d-valerolactone, d-caprolactone, d-octalactone, and d-decalactone were used as model compounds to evaluate the energy cost of polymer conformational freedom. Polymer conformation, measured by ¹³C NMR, DSC, and molecular dynamics, results are in good agreement with the hypothesis and can explain previous literature observations i.e. positive DH p for systems with ring-strain, substituent effects, and solvent effects, that are difficult to understand when only using the analogy of ring-strain and DH p. We believe that our results provide a deeper understanding of the underlying thermodynamics and their interpretation in ROP.

The starch-derived isohexides, with their unique structures of two fused tetrahydrofuran rings in a cis conformation, have been exploited to prepare covalent adaptable networks (CANs) and to tailor and understand their structure–property relationships, in pursuit of replacing oil-based thermosets. Here, dynamicity was achieved through vinylogous urethane chemistry, rigidity via the use of the starch-derived isomeric building blocks isosorbide, isomannide, and isoidide, and flexibility through the amines utilized. Similar to what is known for thermoplastics, depending on the isomer chosen, thermal stability and mechanical properties could be tailored to some extent. The distance between cross-links was ruled by the amines employed, and when this distance was long enough to allow sufficient chain mobility, stereochemical effects on mechanical performance were observed. The CAN structures all display thermoset properties, and as a consequence of the incorporated dynamic bonds, they were mechanically reprocessable. Based on the CANs structural design, i.e., isohexide isomer and amine structure, tensile strengths (σ<inf>b</inf>) ranging from 1.57 to 19.1 MPa, glass transition temperatures (T<inf>g</inf>) ranging from 20 to 114 °C, and thermal stabilities (T<inf>d,5%</inf>) between 200 and 305 °C were achievable. Mechanical reprocessing was proven, and no mechanical performance decay was observed after two reprocessing cycles. This provides important information on the structure–property relationship of CANs from starch-derived building blocks, and consequently, how material properties can be tailored depending on the targeted application.

Sustainable technologies have enabled the production of degradable single-use plastics (SUPs) for various applications. However, environmentally friendly, porous disposable absorbents still lack the competitive functionality of synthetic options. In this work, we report the continuous extrusion of fully biopolymer-based porous absorbents derived from integrated proteins and lignocellulosic residues, all sourced from biomass waste. The results show that the saline absorption capacity of the extruded materials increases 1.5 times compared to the reference solely by including oat husk, a lignocellulosic byproduct from the food industry. The absorption was further improved 2 times by including a delignification step on the oat husk and wheat bran, demonstrating the importance of the biomass’s chemistry in increasing the material’s absorption. Here, the addition of 20 wt % of Keratin fibers from food waste increases the material’s absorbency to 6.5 g/g, with the ability to retain 2 g/g of the saline solution in its structure, which is also the highest reported value for extruded protein-based formulations so far. This work advances the development of porous absorbent materials with competitive performance, utilizing industrial methods and upcycling undervalued biomass waste into sustainable consumer products. Introducing porous biopolymer-based materials as alternatives to synthetic counterparts used in the hygiene and sanitary industries ensures the return of safe molecules to nature, paving the way for microplastic-free, single-use, porous absorbents.

Recycling of polymer composites by melt-extrusion has been limited to reinforced thermoplastics. Here, we demonstrate several melt-extrusion recycling cycles for glass fiber-reinforced elastomers cross-linked with a reversible cross-linker (SS). In comparison, reinforced elastomers lacking reversible cross-linking (CC) exhibited clogging during successive melt-extrusion recycling. Furthermore, commercially available starting materials, such as maleated polypropylene (PPgMA), maleated ethylene propylene rubber (EPRgMA), and short-cut glass fibers, were utilized. We showed that when tetrasulfide functionalized glass fibers (SGF) were embedded in SS cross-linked composites (CompSS-SGF), disulfide exchange reactions were further activated. This was manifested in the high tensile strength of 10 MPa and 220% elongation, which were remarkably higher than the 6 MPa and 17% elongation of CC cross-linked composites (CompCC-SGF). Moreover, the higher interactions of CompSS-SGF contributed to at least a 25% increase in tensile strength and storage modulus and a 36% increase in creep resistance compared to composite counterparts prepared with as-received glass fibers (CompSS-AGF) or clean glass fibers (CompSS-CGF). Additionally, the disulfide exchanges in CompSS-SGF possibly contributed to a approximate to 10% higher tensile strength after recycling. This simple up-scalable approach opens new avenues for the development of fiber-reinforced cross-linked elastomers with facile processability, higher dimensional stability, and up-scalable recycling processes.

Aliphatic polyesters synthesized via ring-opening polymerization (ROP) have properties competitive to incumbent plastic (PE, PP), while simultaneously opening up for chemical recycling to monomer (CRM). However, not all aliphatic polyesters are prone to undergo CRM, and the ability to shift the equilibrium between polymer and monomer is tightly associated with the initial monomer structure. The standard strategy to measure CRM is to evaluate the change in free energy during polymerization (triangle GROP). However, triangle GROP is only one-dimensional by assessing the equilibrium between initial monomer and polymer. But under active catalytic conditions, the depolymerization of polymers can lead to formation of larger rings, such as dimers, trimers, tetramers, and so on, via the ring-chain equilibrium (RCE), meaning that the real thermodynamic recycling landscape is multi-dimensional. This work introduces a multi-dimensional chemical recycling to all rings (CRR) via a highly active catalytic system to reach RCE. Thermodynamically triangle GRCE is completely different from triangle GROP. Using triangle GRCE instead of triangle GROP allows us to achieve CRR for polymers notoriously difficult to achieve CRM for, as exemplified within by CRR for poly(epsilon-caprolactone), poly(pentadecalactone), and mixed polymer systems. Overall, this work provides a new general concept of closing the material loop.

Covalent adaptable networks (CANs) are being developed as future replacements for thermosets as they can retain the high mechanical and chemical robustness inherent to thermosets but also integrate the possibility of reprocessing after material use. Here, covalent adaptable polyimine-based networks were designed with methoxy and allyloxy-substituted divanillin as a core component together with long flexible aliphatic fatty acid-based amines and a short rigid chain triamine, yielding CANs with a high renewable content. The designed series of CANs with reversible imine functionality allowed for fast stress relaxation and tailorability of the thermomechanical properties, as a result of the ratio between long flexible and short rigid amines, with tensile strength (σ_b) ranging 1.07-18.7 MPa and glass transition temperatures ranging 16-61 °C. The CANs were subsequently successfully reprocessed up to three times without determinantal structure alterations and retained mechanical performance. The CANs were also successfully chemically recycled under acidic conditions, where the starting divanillin monomer was recovered and utilized for the synthesis of a recycled CAN with similar thermal and mechanical properties. This promising class of thermosets bearing sustainable dynamic functionalities opens a window of opportunity for the progressive replacement of fossil-based thermosets.

Developing crosslinked elastomers that are easily produced and easily recyclable is complex, usually requiring a compromise between performance and recyclability. However, combining reversible exchange reactions together with phase separation phenomena appears as a promising approach. Herein, a simple and up‐scalable extrusion process is proposed, involving commercial maleated ethylene propylene rubber (EPRgMA), maleated polypropylene (PPgMA), and a suitable crosslinker. It is shown that a crosslinker enabling disulfide exchange reactions can provide local and long‐range rearrangements required for extrusion, yielding a robust crosslinked blend (BlendSS) with strength of 15 MPa and an impressive elongation of 1000%. Moreover, the presence of the disulfide crosslinker provided the required fast exchanges for three repetitive recycling cycles by extrusion with close to 80% retention of initial properties. In comparison, the use of a crosslinker without the capability to establish reversible reactions (BlendCC), yielded crosslinked blends of marginal compatibility, strength of 4 MPa and only 40% elongation. The absence of reversible reactions restricted chain rearrangements and consecutive recycling is only possible by compression molding. The recycled blends presented even lower compatibility, elasticity and thermomechanical performance, demonstrating that the proper design of interfacial interactions between PPgMA and EPRgMA can build a bridge between processability, performance, and high‐throughput recyclability.

Defined macromolecular architecture using anionic ring-opening copolymerization (ROcP) of lactones and cyclic carbonates offers facile routes toward copolymers with unique material properties, ranging from thermoplastic to elastomeric. However, monomers with a slow ROP rate are hampered by competing backbiting reactions, scrambling the macromolecular sequence, and, thereby, a loss of material properties occurs. We here solve this issue by controlling the rate of backbiting. Through our approach, we show how block structures previously inaccessible can be synthesized from monomers with vastly different ROP rates, covering small lactones and even including macrolactones. This control can also be extended beyond block structures to include random and gradient architectures by tuning monomer concentration to the relative ROP and backbiting rate.

Fiber reinforced thermosetting composites are essential materials for present and future technologies. However, their lack of reprocessability puts them far behind in achieving current sustainability targets. The recent development of covalent adaptable networks represents a promising approach to mitigate this. Here, mechanical recycling of short-cut aramid fiber reinforced epoxy composites was evaluated with a special focus on the impact of fiber surface functionality. The introduction of aramid fibers with four different surface chemistries to a covalent adaptable network was shown to influence the mechanical properties before and after reprocessing. Epoxy functionalized fibers enabled 52 % and 36 % retention of the original tensile stress and elongation at break after two recycling cycles. In comparison, the unmodified reference samples could only retain 19 % and 12 % of the original tensile stress and elongation at break. Fiber functionalization was thus proven to be a key piece in the development of more sustainable solutions for short-cut fiber reinforced polymer composites (SCFRPC).

Communications Chemistry is pleased to introduce a Collection of articles focused on organomediated polymerization. Here, the Guest Editors highlight the themes within and look towards the future of this research field.