Mechanical stress is ubiquitous in materials, and it is well accepted that stress causes material wear and failure—which, at the molecular level, results from force-induced bond breakage. Understanding the mechanical behavior of materials at the molecular level requires a quantitative relationship between macroscopic stress and bond-breaking force, a connection that remains largely unexplored. Here we report that the macroscopic stress and the bond-breaking force are quantitatively connected through the kinetics of mechanically activated retro-Diels–Alder reaction of an anthracene–maleimide adduct mechanophore, which is embedded within the crosslink of a double-network elastomer. We find that the force required for bond breakage is largely insensitive to the strain applied to the elastomer but increases linearly with the logarithm of the strain rate. These findings provide insights into the mechanical behavior of polymeric materials and offer valuable guidance for the design of mechanically responsive materials.

Designing thermoset materials with dynamic cross-links is an important strategy to mitigate rising global carbon dioxide emission levels. The development of polymers from sustainable feedstocks, with efficient manufacturing methods, for high-value applications, and with circular end-of-use solutions is essential for advancing material technologies. One approach involves exploiting carbon dioxide itself as a feedstock to create high-performance, sustainable materials by enchaining 50 mol % CO2 via copolymerization with epoxides to yield polycarbonates. This work describes the synthesis, end-functionalization, and curing of poly(propylene carbonate) (PPC) and poly(cyclohexene carbonate) (PCHC) into beta-hydroxy ester vitrimers. These vitrimers demonstrate the ability to be mechanically reprocessed up to 3 times with retention of the material's properties through dynamic transesterification exchange reactions. The polycarbonate vitrimers with gel fractions exceeding 90% exhibit high tensile strength (>50 MPa) and Young's modulus (>2 GPa), achieved by varying the repeat unit structure in the polymer backbone from the low T-g PPC to the more rigid high T-g PCHC structures. Owing to an entropically favorable chain backbiting mechanism, the network chains can be cleaved and depolymerized into cyclic small molecules. In the case of PCHC, this process enables repolymerization back to polycarbonates with 69 wt % CO2 retention through life cycles. The promising mechanical performance and recyclability of these CO2-based polycarbonate vitrimers indicate their potential for sustainable, high-performance materials, paving the way for future innovations in circular polymer technologies and carbon capture utilization.

While oxidative cleavage has been a well-known strategy to degrade unsaturated polymers, most processes require harsh conditions and/or expensive oxidizing agents. Using O2 to degrade polymers is highly desirable, but no reported process is well controlled for the chemical recycling of polymers. Here, we report a photo-mediated oxidative degradation process for unsaturated polymers under O2 using an earth-abundant Mn catalyst, and the process is demonstrated with polybutadiene, polydicyclopentadiene, and dehydrogenated polyethylene. Nonactivated internal alkenes in these polymers can be effectively cleaved without elevated temperature or pressure. The oxidation process generates acetal as the main functionality, which can be used for further recycling. As a proof of concept, the oligomers with acetal end groups, resulting from the oxidation of polybutadiene, are shown to undergo transacetalization with polyols to form a polymer network. The oxidation process demonstrated here holds promise for the recycling of hydrocarbon polymers under mild conditions in a cost-effective fashion.

The consumption of synthetic polymers has ballooned; so has the amount of post-consumer waste generated. The current polymer economy, however, is largely linear with most of the post-consumer waste being either landfilled or incinerated. The lack of recycling, together with the sizable carbon footprint of the polymer industry, has led to major negative environmental impacts. Over the past few years, chemical recycling technologies have gained significant traction as a possible technological route to tackle these challenges. In this regard, olefin metathesis, with its versatility and ease of operation, has emerged as an attractive tool. Here, we discuss the developments in olefin-metathesis-based chemical recycling technologies, including the development of new materials and the application of olefin metathesis to the recycling of commercial materials. We delve into structure-reactivity relationships in the context of polymerization-depolymerization behavior, how experimental conditions influence deconstruction outcomes, and the reaction pathways underlying these approaches. We also look at the current hurdles in adopting these technologies and relevant future directions for the field.

Chemical recycling to monomer (CRM) is a promising route for transitioning to a circular polymer economy. To develop new CRM systems with useful properties, it is important to understand the effects of monomer structure on polymerization/depolymerization behavior. In earlier work, this group demonstrated chemically recyclable polymers prepared by ring-opening metathesis polymerization of trans-cyclobutane fused cyclooctenes (tCBCO). Here, it is investigated how different substituents on cyclobutane impact the thermodynamics and thermal properties of tCBCO polymers. Introducing additional substituents to a cis-diester functionalized tCBCO is found to favor the conversion of polymerization; increased polymerization conversion is also observed when the cis-diester is isomerized into its trans counterpart. The effects of these structural features on the thermal properties are also studied. These findings can provide important insights into designing next-generation CRM polymers.

The composition, sequence, length, and type of glycosidic linkages of polysaccharides profoundly affect their biological and physical properties. However, investigation of the structure-function relationship of polysaccharides is hampered by accessing well-defined polysaccharides in sufficient quantities. Here, we report a chemical approach to precision polysaccharides with native glycosidic linkages via living cationic ring-opening polymerization of 1,6-anhydrosugars. We synthesized well-defined polysaccharides with tunable molecular weight, low dispersity, and excellent regio- and stereoselectivity using a boron trifluoride etherate catalyst and glycosyl fluoride initiators. Computational studies revealed that the reaction propagated through the monomer alpha-addition to the oxocarbenium and was controlled by the reversible deactivation of the propagating oxocarbenium to form the glycosyl fluoride dormant species. Our method afforded a facile and scalable pathway to multiple biologically relevant precision polysaccharides, including D-glucan, D-mannan, and an unusual L-glucan. We demonstrated that catalytic depolymerization of precision polysaccharides efficiently regenerated monomers, suggesting their utility as a class of chemically recyclable materials with tailored thermal and mechanical properties.

Helices are unique structures that play important roles in biomacromolecules and chiral catalysis. The mechanochemical unfolding of helical structures has attracted the attention of chemists in the past few years. However, it is limited to a few cases which investigated how the mechanochemical reactivity is impacted by helical configurations. No synthetic helical mechanophore is reported. Herein, a Zn (II) bidipyrrin (BDPR-Zn) double helix is designed as a potential mechanophore. A cyclic olefin containing a doubly strapped BDPR-Zn is prepared and used for ring-opening metathesis polymerization. The corresponding polymer is subjected to pulsed ultrasonication for mechanochemical testing. The sonication results reveal the mechanochemical inertness of BDPR-Zn unit, which is further supported by force-coupled simulation. Although no obvious activation is observed, our preliminary results on BDPR-Zn unit could inspire further rational designs on force-induced helix unfolding.

Previously, PTX/Pluronic F127 nanocrystals were prepared in our laboratory using the stabilization of nanocrystals (SNC) method. For PTX nanocrystals, dosages could be increased to yield improved antitumor activity over Taxol® without incidence of acute toxicity. The objectives of this current study are to further understand the structure and stability of PTX nanocrystals. More Pluronic F127 surfactant was added in the formulation to attempt to further stabilize the nanocrystals against thermal induced aggregation. However, this resulted in formation of micelles that worsened the stability of nanocrystals. The F127 desorption experiment suggested different surfactant adsorption affinity to nanocrystal surface below and above the CMC. Below the CMC monomers bound to nanocrystal surface with high affinity, but above the CMC low affinity surfactant aggregates readily left the surface upon dilution. At higher temperature the tendency of F127 micellization is enhanced due to drastically lower CMC. Consequently, at 37°C there was F127 desorption even for nanocrystals prepared with low amounts of F127 (1:5 (w/w) PTX/F127). To improve the stability of nanocrystals, re-nanonization by incubation–sonication procedure was used to disrupt the preferred crystal growth pattern of PTX. Furthermore, we have demonstrated that a higher heating temperature (45°C vs. 37°C) used in the incubation–sonication procedure was able to provide even better nanocrystal stability for long periods of incubation time.

Degradable vinyl polymers by radical ring-opening polymerization have become a promising solution to the challenges caused by the widespread use of non-degradable vinyl plastics. However, achieving even distribution of labile functional groups in the backbone of degradable vinyl polymers remains challenging. Herein, we report a photocatalytic approach to truly random degradable vinyl copolymers with tunable main-chain composition via radical ring-opening cascade copolymerization (rROCCP). The rROCCP of the macrocyclic allylic sulfone and acrylates or acrylamides mediated by visible light at ambient temperature achieved near-unity reactivity ratios of both comonomers over the entire range of the comonomer compositions and afforded truly random vinyl copolymers with degradable units evenly distributed in the polymer backbone. Experimental and computational evidence revealed an unusual reversible inhibition of chain propagation by in situ generated sulfur dioxide, which was successfully overcome by reducing the solubility of sulfur dioxide in the reaction mixture. This study provided a powerful approach to truly random degradable vinyl copolymers with tunable main-chain labile functionalities and comparable thermal and mechanical properties to traditional non-degradable vinyl polymers.

The growing consumption of synthetic polymers and the accumulation of polymer waste have led to a pressing need for new routes to sustainable materials. Achieving a closed-loop polymer economy via chemical recycling to monomer (CRM) is one such promising route. Our group recently reported a new CRM system based on polymers prepared by ring-opening metathesis polymerization (ROMP) of trans- cyclobutane fused cyclooctene (tCBCO) monomers. This system offers several key advantages, including the ease of polymerization at ambient temperatures, quantitative depolymerization to monomers under mild conditions, and a broad range of functionalities and thermomechanical properties. Here, we outline detailed protocols for the preparation of tCBCO-based monomers and their corresponding polymers, including the preparation of elastic polymer networks and compression molding of linear thermoplastic polymers. We also outline the preparation of high ring strain E- alkene tCBCO monomers and their living polymerization. Finally, the procedures for the depolymerization of linear polymers and polymer networks are also demonstrated.

Chemically recyclable semi-fluorinated polymers are demonstrated for the first time, and the hydrophobicity, self-assembly, and post-polymerization functionalization of these polymers are explored.

A promising solution to address the challenges in plastics sustainability is to replace current polymers with chemically recyclable ones that can depolymerise into their constituent monomers for circular use of materials. Despite the progress, few depolymerisable polymers exhibit the excellent thermal stability and strong mechanical properties of traditional polymers. Here we report a series of chemically recyclable polymers that show excellent thermal stability (decomposition temperature > 370 ºC) and tunable mechanical properties. The polymers are formed via ring-opening metathesis polymerisation of cyclooctene with a trans-cyclobutane installed at the 5,6-positions. The additional ring converts the non-depolymerisable polycyclooctene into a depolymerisable polymer by reducing the ring strain energy in the monomer (from 8.2 kcal/mol in unsubstituted cyclooctene to 4.9 kcal/mol in the fused ring). The fused-ring monomer enables a broad scope of functionalities to be incorporated, providing access to chemically recyclable elastomers and plastics that show promise as next-generation sustainable materials.