The accumulation of plastic waste in the environment is not only an urgent environmental issue but also a waste of resources. Developing chemically recyclable plastics is considered a viable approach to addressing the growing plastic crisis, but the synthesis of cyclic polyolefins remains challenging.

Here, Researcher Jian Zhongbao from the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, demonstrates a ring strain matching concept for the tailored synthesis of cyclic polyolefins. By designing a well-defined 16-membered unsaturated lactone (Ester-16), copolymerization with cyclooctene (COE) followed by complete hydrogenation produces a class of chemically recyclable HDPE-like materials with high molecular weight. These HDPE-like materials possess the bulk properties required of HDPE. They can be degraded into well-defined and easily separable macromonomers, particularly AB-type telechelic PE. The molecular weight of the macromonomers can be precisely tailored as needed and depends solely on the [COE]/[Ester-16] feed ratio. The recovered macromonomers can be readily repolymerized into high molecular weight HDPE-like materials. This macromonomer-polymer-macromonomer lifecycle (as opposed to a monomer-polymer-monomer lifecycle) can be repeated through a simple separation process with significant yields. Overall, this work offers an opportunity to enhance the sustainability of plastics. The related research results were published in the latest issue of "Nature Sustainability" under the title "Tailored synthesis of circular polyolefins."

【Cyclic HDPE Synthesis for Sustainable Utilization】

Since HDPE and other polyolefins account for a large portion of plastic waste, finding methods to produce "circular" versions (that can be chemically depolymerized and repolymerized) represents an urgent sustainability goal. Path I - biomass-derived building blocks (e.g., A2+B2 type monomers) to create polyesters somewhat similar to polyethylene. Path II - using ethylene polymerization or copolymerization with certain functional monomers to introduce cleavable bonds. This study - a new ROMP-based strategy involving cyclooctene (COE) and 16-membered unsaturated lactones to produce materials similar to HDPE, which are then hydrogenated to obtain a saturated backbone. The resulting polymers can degrade into well-defined telechelic PE macromonomers under mild depolymerization conditions. In the context of Figure 1, the authors emphasize the importance of achieving high molecular weight while embedding degradable "weak links." The ultimate goal is to maintain the desirable mechanical properties of HDPE—particularly its high tensile strength and crystallinity—while also giving it the ability to degrade into predictable, separable fragments after chemical treatment.

Figure 1. Circular HDPE synthesis for sustainable utilization

【Application of Unsaturated Lactones in ROMP】

In-depth study of chemistry, the author reports the design and synthesis of a novel 16-membered unsaturated lactone (referred to as "Ester-16") as a key comonomer in ROMP. Previous attempts to embed ester units into polyethylene chains via ROMP often encountered issues such as mismatched ring sizes, side reactions, or difficulty in forming high molecular weight polymers. The authors hypothesized that the "ring strain" in cyclooctene and unsaturated lactones must be carefully matched for their ring-opening polymerization to perform well. Density functional theory (DFT) calculations showed that the ring strain of Ester-16 is comparable to that of cyclooctene. By comparing the average Laplacian bond order (a measure of ring strain) of different lactones (e.g., seven-membered, nine-membered, or seventeen-membered analogs), the authors demonstrated that the sixteen-membered lactone they prepared (actually a cyclic dimer of an eight-membered precursor) best matches COE in ring-opening metathesis. The figure also illustrates the route to produce Ester-16 through simple 1,9-diene with ester substituents via ring-closing metathesis. Details of Ester-16 include: high isolated yield after purification (about 90%). Identification was carried out by NMR, including 1H NMR, 13C NMR, and NOESY experiments, confirming a 1:1 ratio of two isomers, attributed to "head-head/tail-tail" versus "head-tail" orientation during the ring-closing process. X-ray crystallography further confirmed a trans conformation in the solid state. In summary, these results confirm the feasibility of using Ester-16 as a comonomer in ROMP for regularly embedding ester bonds in future polyethylene chains.

Figure 2. Unsaturated lactones applied in ROMP

【Comprehensive Performance of Biodegradable Polyethylene】

After synthesizing unsaturated precursor polymers (via ROMP of COE and Ester-16) and fully hydrogenating them, the authors obtained a new class of degradable polyethylene (dPE). Differential scanning calorimetry (DSC) shows that the new dPE retains melting transitions in the range of 109–130°C, especially for dPE with lower ester content (1.0–4.2 mol% ester units). Compared to standard HDPE, crystallinity (X𝑐) and melting enthalpy (Δ𝐻𝑚) are slightly reduced due to the slight disruption of the lattice by the addition of ester groups. Nevertheless, a melting point above 120°C is still within the practical range for many plastic applications. Wide-angle X-ray diffraction (WXRD) shows that these dPEs exhibit characteristic crystalline peaks typical of polyethylene. Even with only a small fraction of ester bonds, the polymer chains still arrange into largely polyethylene-type lattices. Thermal degradation (T𝑑): These degradable polyethylenes degrade at around 385-415°C, slightly lower than pure HDPE (443°C). This moderate change still allows the new material to remain stable under normal processing conditions. High molecular weight dPE with ester content below ~2 mol% has a tensile strength of about 30–39 MPa and an elongation at break exceeding 1000%. This is comparable to, and in some cases even better than, commercial HDPE. As the ester content increases, the elongation at break remains high, but the tensile strength may decrease slightly due to greater disruption of the PE crystalline regions. Due to the polar ester bonds in the chain, the water contact angle of these dPEs is lower than that of standard HDPE, decreasing from ~113° to ~90° (and even lower if additional polar functional groups are introduced). GPC and NMR studies before and after water treatment confirm that no significant hydrolysis or chain scission occurs under these conditions. Therefore, despite having ester functional groups on the main chain, the polymer can still resist normal water contact.

Figure 3. Comprehensive Performance of Degradable Polyethylene

Custom synthesis of cyclic polyethylene

The author precisely controls the monomer feed ratio of COE and Ester-16, thereby adjusting the position (and total number) of ester bonds in the polymer chain, thus "customizing" the molecular weight and properties of the final polymer. The end product of the hydrogenation step is widely known as "dPE" (degradable polyethylene). Since each Ester-16 unit inserts two ester bonds with each ring-opening, the average spacing of ester groups on the chain is determined by the selected [COE]/[Ester-16] ratio. The correlation between the feed ratio (e.g., m:n of 50:1, 20:1, or 5:1) and the measured average molecular weight (Mn) of the resulting dPE. These correlations match very well with theoretical values, as each ring-opening event of the lactone introduces a predictable segment. Depolymerization ("Depolym") experiments carried out under mild conditions (KOH in methanol, moderate temperature) break down the ester bonds to produce well-defined macromonomers. The authors emphasize that these macromonomers are AB-type telechelic PEs, meaning that each chain now has one hydroxyl-terminated end and another end terminated with a carboxyl or ester group. Overall, one can systematically "program" the repeat units of the polymer, effectively determining the distance between cleavable bonds (in terms of polyethylene segments). This addresses the long-standing challenge of producing degradable polyolefins that still possess sufficient crystallinity and mechanical integrity for typical HDPE applications.

Figure 4. Custom synthesis of cyclo-polyethylene

"macromonomer-polymer-macromonomer" cyclic polyethylene

The system does not recycle the original lactone monomers (which may be more difficult and often leads to incomplete recovery), but rather produces macromolecular monomers during depolymerization. These macromolecular monomers can then undergo step-growth polycondensation—without the need for additional comonomers—to regenerate high molecular weight polymers. In principle, this cycle can be repeated multiple times. Large samples of hydrogenated ROMP products with defined ester spacings, when treated with KOH in methanol under mild conditions, cleave each ester bond, releasing low molecular weight AB-type telechelic PE in nearly quantitative yields. This fraction can be collected simply by filtration, leaving behind small molecule byproducts that can be easily removed. Catalytic step-growth polymerization (e.g., using Ti(OnBu)4 at high temperatures (180°C) to polymerize the telechelic macromolecular monomer) successfully re-forms high molecular weight polymers with crystallinity and tensile strength within the same range as the original material. Repeating the process validates that the cycle can continue without a significant decrease in yield or major side reactions. The mechanical and thermal properties after recycling: step-growth derived high-density polyethylene-like polymers ("dPE-RP") exhibit: tensile strength of about 31–32 MPa, elongation at break exceeding 1,800–1,900%, and melting temperatures around 133°C for samples with low ester content. These properties are comparable to those of the original materials from the ROMP/hydrogenation process, confirming the feasibility of multiple polymerization-depolymerization cycles.

Figure 5. "Macromonomer-Polymer-Macromonomer" Cyclic Polyethylene

【Summary】

This article demonstrates a ring-strain matching strategy, synthesizing well-defined, chemically recyclable, high molecular weight HDPE-like materials with balanced thermomechanical properties and degradability by directly copolymerizing unique 16-membered unsaturated lactones with cyclooctene via ROMP followed by hydrogenation. This avoids cumbersome multi-step reactions. The key structure of long –CH2CH2– units with precise number-average molecular weight and uniformly distributed ester bonds endows the HDPE-like material with competitive thermal and mechanical properties, as well as improved surface properties compared to HDPE. This further enables the complete degradation of HDPE-like materials into macromonomers under mild conditions. These recovered macromonomers have outstanding advantages, including tailored molecular weight, telechelic PE structures similar to AB, and easy separation through simple filtration. They are directly repolymerized into high molecular weight HDPE analogs with comparable thermomechanical properties to HDPE at high yields without the need for additional A2 or B2 bifunctional monomers. The reproduced HDPE analogs can again depolymerize into macromonomers at high yields. These characteristics enable the simple separation and recycling of telechelic PE or degradable PE from mixed plastics of HDPE and iPP. Thus, the authors conceptualize a customized closed-loop macromonomer-polymer-macromonomer system after polymer-depolymerization cycles. This work paves the way for custom synthesis of cyclic polyolefins with desired end-of-life chemical circularity. This approach may have broad implications in the field of plastic recycling.