Chemical Synthesis of CO2-Based Polymers with Enhanced Thermal Stability and Unexpected Recyclability from Biosourced Monomers

Controlled Carbon Dioxide Terpolymerizations to Deliver Toughened yet Recyclable Thermoplastics

Using CO2 polycarbonates as engineering thermoplastics has been limited by their mechanical performances, particularly their brittleness. Poly(cyclohexene carbonate) (PCHC) has a high tensile strength (40 MPa) but is very brittle (elongation at break <3%), which limits both its processing and applications. Here, well-defined, high molar mass CO2 terpolymers are prepared from cyclohexene oxide (CHO), cyclopentene oxide (CPO), and CO2 by using a Zn(II)Mg(II) catalyst. In the catalysis, CHO and CPO show reactivity ratios of 1.53 and 0.08 with CO2, respectively; as such, the terpolymers have gradient structures. The poly(cyclohexene carbonate)-grad-poly(cyclopentene carbonate) (PCHC-grad-PCPC) have high molar masses (86 < Mn < 164 kg mol-1, ĐM < 1.22) and good thermal stability (Td > 250 °C). All the polymers are amorphous with a single, high glass transition temperature (96 < Tg < 108 °C). The polymer entanglement molar masses, determined using dynamic mechanical analyses, range from 4 < Me < 23 kg mol-1 depending on the polymer composition (PCHC:PCPC). These polymers show superior mechanical performance to PCHC; specifically the lead material (PCHC0.28-grad-PCPC0.72) shows 25% greater tensile strength and 160% higher tensile toughness. These new plastics are recycled, using cycles of reprocessing by compression molding (150 °C, 1.2 ton m-2, 60 min), four times without any loss in mechanical properties. They are also efficiently chemically recycled to selectively yield the two epoxide monomers, CHO and CPO, as well as carbon dioxide, with high activity (TOF = 270-1653 h-1, 140 °C, 120 min). The isolated recycled monomers are repolymerized to form thermoplastic showing the same material properties. The findings highlight the benefits of the terpolymer strategy to deliver thermoplastics combining the beneficial low entanglement molar mass, high glass transition temperatures, and tensile strengths; PCHC properties are significantly improved by incorporating small quantities (23 mol %) of cyclopentene carbonate linkages. The general strategy of designing terpolymers to include chain segments of low entanglement molar mass may help to toughen other brittle and renewably sourced plastics.

High Molar Mass Polycarbonates as Closed-Loop Recyclable Thermoplastics

Using carbon dioxide (CO2) to make recyclable thermoplastics could reduce greenhouse gas emissions associated with polymer manufacturing. CO2/cyclic epoxide ring-opening copolymerization (ROCOP) allows for >30 wt % of the polycarbonate to derive from CO2; so far, the field has largely focused on oligocarbonates. In contrast, efficient catalysts for high molar mass polycarbonates are underinvestigated, and the resulting thermoplastic structure-property relationships, processing, and recycling need to be elucidated. This work describes a new organometallic Mg(II)Co(II) catalyst that combines high productivity, low loading tolerance, and the highest polymerization control to yield polycarbonates with number average molecular weight (Mn) values from 4 to 130 kg mol-1, with narrow, monomodal distributions. It is used in the ROCOP of CO2 with bicyclic epoxides to produce a series of samples, each with Mn > 100 kg mol-1, of poly(cyclohexene carbonate) (PCHC), poly(vinyl-cyclohexene carbonate) (PvCHC), poly(ethyl-cyclohexene carbonate) (PeCHC, by hydrogenation of PvCHC), and poly(cyclopentene carbonate) (PCPC). All these materials are amorphous thermoplastics, with high glass transition temperatures (85 < Tg < 126 °C, by differential scanning calorimetry) and high thermal stability (Td > 260 °C). The cyclic ring substituents mediate the materials' chain entanglements, viscosity, and glass transition temperatures. Specifically, PCPC was found to have 10× lower entanglement molecular weight (Me)n and 100× lower zero-shear viscosity compared to those of PCHC, showing potential as a future thermoplastic. All these high molecular weight polymers are fully recyclable, either by reprocessing or by using the Mg(II)Co(II) catalyst for highly selective depolymerizations to epoxides and CO2. PCPC shows the fastest depolymerization rates, achieving an activity of 2500 h-1 and >99% selectivity for cyclopentene oxide and CO2.

Depolymerization within a Circular Plastics System

The societal importance of plastics contrasts with the carelessness with which they are disposed. Their superlative properties lead to economic and environmental efficiency, but the linearity of plastics puts the climate, human health, and global ecosystems at risk. Recycling is fundamental to transitioning this linear model into a more sustainable, circular economy. Among recycling technologies, chemical depolymerization offers a route to virgin quality recycled plastics, especially when valorizing complex waste streams poorly served by mechanical methods. However, chemical depolymerization exists in a complex and interlinked system of end-of-life fates, with the complementarity of each approach key to environmental, economic, and societal sustainability. This review explores the recent progress made into the depolymerization of five commercial polymers: poly(ethylene terephthalate), polycarbonates, polyamides, aliphatic polyesters, and polyurethanes. Attention is paid not only to the catalytic technologies used to enhance depolymerization efficiencies but also to the interrelationship with other recycling technologies and to the systemic constraints imposed by a global economy. Novel polymers, designed for chemical depolymerization, are also concisely reviewed in terms of their underlying chemistry and potential for integration with current plastic systems.

Evaluating Heterodinuclear Mg(II)M(II) (M = Mn, Fe, Ni, Cu, and Zn) Catalysts for the Chemical Recycling of Poly(cyclohexene carbonate)

Polymer chemical recycling to monomers (CRM) is important to help achieve a circular plastic economy, but the "rules" governing catalyst design for such processes remain unclear. Here, carbon dioxide-derived polycarbonates undergo CRM to produce epoxides and carbon dioxide. A series of dinuclear catalysts, Mg(II)M(II) where M(II) = Mg, Mn, Fe, Co, Ni, Cu, and Zn, are compared for poly(cyclohexene carbonate) depolymerizations. The recycling is conducted in the solid state, at 140 °C monitored using thermal gravimetric analyses, or performed at larger-scale using laboratory glassware. The most active catalysts are, in order of decreasing rate, Mg(II)Co(II), Mg(II)Ni(II), and Mg(II)Zn(II), with the highest activity reaching 8100 h-1 and with >99% selectivity for cyclohexene oxide. Both the activity and selectivity values are the highest yet reported in this field, and the catalysts operate at low loadings and moderate temperatures (from 1:300 to 1:5000, 140 °C). For the best heterodinuclear catalysts, the depolymerization kinetics and activation barriers are determined. The rates in both reverse depolymerization and forward CHO/CO2 polymerization catalysis show broadly similar trends, but the processes feature different intermediates; forward polymerization depends upon a metal-carbonate intermediate, while reverse depolymerization depends upon a metal-alkoxide intermediate. These dinuclear catalysts are attractive for the chemical recycling of carbon dioxide-derived plastics and should be prioritized for recycling of other oxygenated polymers and copolymers, including polyesters and polyethers. This work provides insights into the factors controlling depolymerization catalysis and steers future recycling catalyst design toward exploitation of lightweight and abundant s-block metals, such as Mg(II).

Toughening CO2 -Derived Copolymer Elastomers Through Ionomer Networking

Utilizing carbon dioxide (CO2 ) to make polycarbonates through the ring-opening copolymerization (ROCOP) of CO2 and epoxides valorizes and recycles CO2 and reduces pollution in polymer manufacturing. Recent developments in catalysis provide access to polycarbonates with well-defined structures and allow for copolymerization with biomass-derived monomers; however, the resulting material properties are underinvestigated. Here, new types of CO2 -derived thermoplastic elastomers (TPEs) are described together with a generally applicable method to augment tensile mechanical strength and Young's modulus without requiring material re-design. These TPEs combine high glass transition temperature (Tg ) amorphous blocks comprising CO2 -derived poly(carbonates) (A-block), with low Tg poly(ε-decalactone), from castor oil, (B-block) in ABA structures. The poly(carbonate) blocks are selectively functionalized with metal-carboxylates where the metals are Na(I), Mg(II), Ca(II), Zn(II) and Al(III). The colorless polymers, featuring <1 wt% metal, show tunable thermal (Tg ), and mechanical (elongation at break, elasticity, creep-resistance) properties. The best elastomers show >50-fold higher Young's modulus and 21-times greater tensile strength, without compromise to elastic recovery, compared with the starting block polymers. They have wide operating temperatures (-20 to 200 °C), high creep-resistance and yet remain recyclable. In the future, these materials may substitute high-volume petrochemical elastomers and be utilized in high-growth fields like medicine, robotics, and electronics.

A circular polyester platform based on simple gem-disubstituted valerolactones

Geminal disubstitution of cyclic monomers is an effective strategy to enhance the chemical recyclability of their polymers, but it is utilized for that purpose alone and often at the expense of performance properties. Here we present synergistic use of gem-α,α-disubstitution of available at-scale, bio-based δ-valerolactones to yield gem-dialkyl-substituted valerolactones ([Formula: see text]), which generate polymers that solve not only the poor chemical recyclability but also the low melting temperature and mechanical performance of the parent poly(δ-valerolactone); the gem-disubstituted polyesters ([Formula: see text]) therefore not only exhibit complete chemical recyclability but also thermal, mechanical and transport properties that rival or exceed those of polyethylene. Through a fundamental structure-property study that reveals intriguing impacts of the alkyl chain length on materials performance of [Formula: see text], this work establishes a simple circular, high-performance polyester platform based on [Formula: see text] and highlights the importance of synergistic utilization of gem-disubstitution for enhancing both chemical recyclability and materials performance of sustainable polyesters.

Advances in the Synthesis of Chemically Recyclable Polymers

The development of modern society is closely related to polymer materials. However, the accumulation of polymer materials and their evolution in the environment causes not only serious environmental problems, but also waste of resources. Although physical processing can be used to reuse polymers, the properties of the resulting polymers are significantly degraded. Chemically recyclable polymers, a type of polymer that degrades into monomers, can be an effective solution to the degradation of polymer properties caused by physical recycling of polymers. The ideal chemical recycling of polymers, i. e., quantitative conversion of the polymer to monomers at low energy consumption and repolymerization of the formed monomers into polymers with comparable properties to the original, is an attractive research goal. In recent years, significant progress has been made in the design of recyclable polymers, enabling the regulation of the "polymerization-depolymerization" equilibrium and closed-loop recycling under mild conditions. This review will focus on the following aspects of closed-loop recycling of poly(sulfur) esters, polycarbonates, polyacetals, polyolefins, and poly(disulfide) polymer, illustrate the challenges in this area, and provide an outlook on future directions.

Solid-State Chemical Recycling of Polycarbonates to Epoxides and Carbon Dioxide Using a Heterodinuclear Mg(II)Co(II) Catalyst

Polymer chemical recycling to monomers (CRM) could help improve polymer sustainability, but its implementation requires much better understanding of depolymerization catalysis, ensuring high rates and selectivity. Here, a heterodinuclear [Mg(II)Co(II)] catalyst is applied for CRM of aliphatic polycarbonates, including poly(cyclohexene carbonate) (PCHC), to epoxides and carbon dioxide using solid-state conditions, in contrast with many other CRM strategies that rely on high dilution. The depolymerizations are performed in the solid state giving very high activity and selectivity (PCHC, TOF = 25700 h^–1, CHO selectivity >99 %, 0.02 mol %, 140 °C). Reactions may also be performed in air without impacting on the rate or selectivity of epoxide formation. The depolymerization can be performed on a 2 g scale to isolate the epoxides in up to 95 % yield with >99 % selectivity. In addition, the catalyst can be re-used four times without compromising its productivity or selectivity.

Efficient and Selective Chemical Recycling of CO2 -Based Alicyclic Polycarbonates via Catalytic Pyrolysis

Chemical recycling of polymers to their constituent monomers is the foremost challenge in building a sustainable circular plastics economy. Here, we report a strategy for highly efficient depolymerization of various CO₂ -based alicyclic polycarbonates to epoxide monomers in solvent-free conditions by a simple Cr^III -Salen complex mediated catalytic pyrolysis process. The chemical recycling of the widely studied poly(cyclohexene carbonate) exhibits excellent reactivity (TOF up to 3000 h^-1 , 0.1 mol % catalyst loading) and high epoxide monomer selectivity (>99 %). Mechanistic investigation reveals that the process proceeds in a sequential fashion via a trans-carbonate intermediate.

Modulating Polymerization Thermodynamics of Thiolactones Through Substituent and Heteroatom Incorporation

A central challenge in the development of next-generation sustainable materials is to design polymers that can easily revert back to their monomeric starting material through chemical recycling to monomer (CRM). An emerging monomer class that displays efficient CRM are thiolactones, which exhibit rapid rates of polymerization and depolymerization. This report details the polymerization thermodynamics for a series of thiolactone monomers through systematic changes to substitution patterns and sulfur heteroatom incorporation. Additionally, computational studies highlight the importance of conformation in modulating the enthalpy of polymerization, leading to monomers that display high conversions to polymer at near-ambient temperatures, while maintaining low ceiling temperatures (T_c). Specifically, the combination of a highly negative enthalpy (-19.3 kJ/mol) and entropy (-58.4 J/(mol·K)) of polymerization allows for a monomer whose equilibrium polymerization conversion is very sensitive to temperature.

Chemical Recycling of Poly(Cyclohexene Carbonate) Using a Di-MgII Catalyst

Chemical recycling of polymers to true monomers is pivotal for a circular plastics economy. Here, the first catalyzed chemical recycling of the widely investigated carbon dioxide derived polymer, poly(cyclohexene carbonate), to cyclohexene oxide and carbon dioxide is reported. The reaction requires dinuclear catalysis, with the di-Mg^II catalyst showing both high monomer selectivity (>98 %) and activity (TOF=150 h^-1 , 0.33 mol %, 120 °C). The depolymerization occurs via a chain-end catalyzed depolymerization mechanism and DFT calculations indicate the high selectivity arises from Mg-alkoxide catalyzed epoxide extrusion being kinetically favorable compared to cyclic carbonate formation.

Highly Reactive Cyclic Carbonates with a Fused Ring toward Functionalizable and Recyclable Polycarbonates

Monomer design plays an important role in the development of polymers with desired thermal properties and chemical recyclability. Here we prepared a class of seven-membered ring carbonates containing trans-cyclohexyl fused rings. These monomers showed excellent activity for ring-opening polymerization (ROP) with turnover frequency (TOF) up to 6 × 10⁵ h^-1 and catalyst loading down to 50 ppm, which yielded high-molecular-weight polycarbonates (M_n up to 673 kg/mol) with great thermostability (T_d > 300 °C). Ultimately, the resulting polycarbonates can completely depolymerize into their corresponding cyclic dimers that can repolymerize to synthesize the starting polymers in moderate yields, demonstrating a potential route to achieve chemical recycling. Postfunctionalization of the unsaturated polycarbonate was conducted through cross-linking reaction and "click" reaction under UV irradiation.

Facile synthesis of eight-membered cyclic(ester-amide)s and their organocatalytic ring-opening polymerizations

Facile modular synthesis of eight-membered cyclic(ester-amide)s based on phthalic anhydride and β-amino alcohols and organocatalitic ROP of the monomers to afford degradable semi-aromatic poly(ester-amides)s with tunable thermal properties.

Synthesis and characterization of novel poly(sulfone siloxane)s with good solubility and recyclability based on siloxane units

A controllable circulation between poly(sulfone siloxane)s (PSS) and sulfone-containing cyclosiloxane monomers (SCS) was acheived in the presence of KHSO4.