Closed-loop recycling of plastics poses many challenges from degradation reactions during service and repeated processing to unknown additives and harsh conditions required for chemical recycling. Recycling of crosslinked materials is even more challenging. Our recent work focused on molecular design able to promote closed-loop circularity [1]. Polyesters are well suited for mechanical and chemical recycling, still in practice they are often downcycled. To overcome this, we designed a group of aromatic polyesters with high thermal stability and amorphous nature aiming at wide processing window to prevent degradation during reprocessing [2]. Six subsequent injection molding and washing cycles were demonstrated without significant property change. Furthermore, due to accessibility of the designed ester-group, chemical recycling under mild conditions and repolymerization to products with identical properties to original material were demonstrated. In another example, we produced phase separated crosslinked blends from commercially available maleated ethylene propylene rubber (EPRgMA), maleated polypropylene (PPgMA) and a disulfide containing crosslinker by up-scalable melt-extrusion [3]. The combination of phase separation and reversible bonds provided excellent properties (elongation ~1000% and tensile stress at break 15 MPa) and recyclability. After three repeated extrusion cycles 80% of original properties were retained. In comparison, EPRgMA/PPgMA blend crosslinked with analogous crosslinker without disulfide bond had stress at break 4 MPa and elongation of only 40%. It was also not possible to perform three repeated extrusion cycles in the absence of the dynamic disulfide bond. Our results clearly demonstrate how rational molecular design can bridge performance and high-throughput recyclability through commercially viable processes.

Polymers have penetrated almost every aspect of our daily lives with applications ranging from packaging and healthcare to automotive parts and construction materials. With a market share of over 60%, polyolefins emerge as the most widely used synthetic polymers. Consequently, the global use of polyolefins contributes to large volumes of plastic waste and a growing concern about how to effectively manage pollution and polymers accumulating in the environment. Although reutilizing polyolefins seems to be a logic choice, their recycling level remains disappointingly low. This is mainly due to the lack of large-scale availability of efficient and inexpensive compatibilizers for mixed polyolefin waste, typically consisting of HDPE and iPP that despite their similar chemical hydrocarbon structure are immiscible.
Here we describe an unconventional approach of using poly-pentadecalactone, a straightforward and simple to produce aliphatic polyester, as a compatibilizer for iPP/HDPE blends, especially the brittle iPP-rich ones [1]. The surprisingly effective compatibilizer transforms brittle iPP/HDPE blends into unexpectedly tough materials that even outperform the reference HDPE and iPP materials. This simple approach creates opportunities for upcycling poly-mer waste to valuable products.

Acknowledgement: special thanks go to SABIC for funding this scientific work.

The increasing demand for sustainable and recyclable materials has driven the development of chemically recyclable polymer networks.[1-4] Polycyanurate structures (1,3,5-triazine) have recently gained significant attention due to their intrinsic rigidity and emerging recyclability, making them highly valuable in polymer design.[5] In this study, bio-derived polycyanurate networks were synthesized and investigated for their closed-loop recycling potential. The polymer networks were fabricated via photopolymerization using thiol compounds, yielding robust thermosetting materials with tunable mechanical and thermal properties. Characterization studies demonstrated that the resulting polymers exhibited exceptional thermal stability and mechanical strength, comparable to conventional high-performance polymer systems. A key highlight of this work is the efficient monomer recovery achieved through mild depolymerization conditions. The recycled monomers were successfully reintroduced into the polymerization process, leading to the regeneration of polymeric thermosets with properties equivalent to the original networks. This demonstrates the feasibility of producing fully recyclable, high-performance thermosets without compromising material integrity (Figure 1). This study underscores the potential of bio-derived polycyanurate networks in achieving both recyclability and material circularity. The integration of mild depolymerization strategies and efficient monomer recovery offers a promising approach for developing next-generation sustainable thermosets, paving the way for more environmentally friendly polymeric materials.

Growing demands for plastics and limited amounts of natural resources are currently some of the greatest challenges. The two main strategies to overcome these dangerous issues are the use of renewable bio-based feedstocks and rigorous recycling towards a circular economy. [1]
Unfortunately, the recyclability of many established polymers is difficult because degradability was no priority in the material development. This is where the concept of recycling on demand (ROD) comes into play. The aim is to design next-generation polymer materials, that combine good material properties with convenient recycling abilities. [2]

While established recycling processes for polycondensates focus on solvolysis to monomer, this work targets oligomers as desired degradation products. Therefore, selectively cleavable bonds are implemented into the polyesters to degrade them by application of certain triggers as catalysts or elevated temperatures.

Poly(ester-co-acetal)s were synthesized in a solution-based process utilizing organo-catalysis in a sustainable approach. OH-terminated oligoesters were bridged by acid-labile acetal groups, yielding polymeric materials which provide excellent recyclability. [3,4]
Formation and cleavage of the acetal bonds was verified by nuclear magnetic resonance spectroscopy. At the same time the effective polymerization and degradation was confirmed by size exclusion chromatography. Additionally, the OH number of the degraded materials was determined to ensure good stoichiometry for effective repolymerization. These combined efforts resulted in an impressive proof of concept for the proposed ROD principle.
Based on the obtained results, further optimizations towards sustainable and on demand recyclable poly(ester-co-acetal)s are intended. Therefore, the application of less and greener solvents and catalysts will be investigated.

New polymers, designed for end-of-life and efficiently formed from renewable carbon, are key to transition to a more sustainable, circular plastics economy. Promising strategies towards polymers that are chemically recyclable to the monomer (CRM) include ring-opening polymerization (ROP) of cyclic monomers.[1–3] While recent developments have been exciting, reported monomers typically lack a reasonable production route from biobased materials. We recently developed a synthesis route towards renewable, tricyclic oxanorbornene lactones, produced from readily available bioderived furans.[4]
Herein, we demonstrate that these monomers undergo exceptionally rapid and controlled ROP.[5] The polyester was formed in low dispersity and with controllable molecular weight (up to 76.8 kg/mol), and exhibits a high Tg of 120 °C. The orthogonal olefin and lactone functionalities offer access to a wide range of promising materials, as showcased by post-polymerization modification (PPM) by hydrogenation of the olefin, which increased polymer thermal stability by over 100 °C. Next to rapid hydrolytic degradation and solvolysis, the polyester is shown to cleanly undergo CRM, in line with its favorable ceiling temperature (Tc) of 73 °C. PPM hydrogenation lowers the Tc by 94 °C, showcasing that successful (de)polymerization is strongly affected by subtle changes in the monomer structure. The computed ΔH° of ring-opening of γ-butyrolactone-based monomers provided a model to predict Tc of related monomers, and the structural parameters from computed as well as monomer crystal structures offered insights into descriptors that determine the observed high (de)polymerizability, which is key to establishing structure-property relations and helpful for designing the next generation of recyclable plastics.

Thermosetting plastics are essential in industry and daily life due to their superior thermal and mechanical properties, but their non-recyclability and dependence on petroleum resources pose environmental concerns. Developing biobased thermosets with efficient chemical recycling capabilities is thus crucial for advancing a circular plastic economy and reducing environmental impact. Here we present a sustainable and scalable synthesis pathway for recyclable acetal thermosets through the polycondensation of bio-based aldehydes and diols, utilizing oxalic acid as a catalyst which can be easily removed during thermal post-curing. A vanillin-based aldehyde and different diols were reacted in a solvent-free process to produce acetal thermosets. The resultant thermosets demonstrated excellent mechanical properties, and chemical stability in a wide range of solvents. More importantly, the thermosets were efficiently depolymerized under mild acidic conditions into their original monomers, which were easily isolated in high yield and purity. These monomers were subsequently reused to synthesize new, identical polymers, achieving closed-loop recycling (Figure 1). This research addresses the environmental challenges posed by traditional thermosets, offering a practical solution for sustainable polymer recycling without compromising material performance. The solvent-free approach further enhances its industrial viability, making it a promising step toward a more sustainable synthetic strategy to produce closed-loop recyclable thermosets.