Publications

Chemical recycling of plastics into monomers offers a promising strategy for addressing global plastic pollution. However, most chemical recycling methods are limited to specific polymers. Pyrolysis offers generality for universal polymer recycling; however, overheating leads to energy inefficiencies and degradation products, which limits its practicality. Herein, we report a photothermal depolymerization approach that is general to numerous polymers with high selectivity. Distinct from previous approaches, we identify a porous carbon material derived from proteins as a high-surface-area photothermal agent that does not require direct incorporation into polymer resin. Using low carbon loadings, we successfully depolymerize a wide range of polymers, including polystyrene (PS), poly(methyl methacrylate) (PMMA), polyvinyl acetate (PVAc), polyvinyl alcohol (PVA), trans-polyisoprene, polypropylene carbonate (PPC), poly(L-lactide) (PLLA), polyethylene terephthalate (PET), and polycarbonate (BPA-PC). Additionally, the porous carbon material is reusable at least five times by sequentially adding polystyrene films to the post-reacted porous carbon materials. Excitingly, this method is broadly applicable to post-consumer plastics without preprocessing and is readily scalable, demonstrating the versatility of this closed-loop chemical recycling approach.

Chemical recycling of plastics into monomers is a promising strategy to achieve a circular economy. However, selective depolymerization methods for mixed plastics are still underdeveloped. Herein, we report a selective and sequential depolymerization strategy for mixed plastics, including poly(L-lactide) (PLLA), polystyrene (PS), and poly(ethylene terephthalate) (PET), using photothermal conversion. We were able to selectively depolymerize PLLA into L-lactide in the presence of PS and PET. Then, PS was selectively depolymerized to styrene, followed by the depolymerization of PET into its monomer. Our protocol was carried out in one pot without any additional purification of the unreacted plastics at each stage. This method was successfully applied to mixtures of post-consumer waste plastic.

Phenol is a commodity chemical frequently used in the manufacturing of resins, pharmaceuticals, and polymers. Yet, its industrial production from cumene requires harsh oxidative conditions followed by the Hock rearrangement. Polystyrene (PS) is a widely manufactured commodity polymer; however, it is among the least recycled plastics. Intriguingly, it is composed of contiguous cumene repeats that could serve as a phenol source, enhancing its value as a waste material. In this work, we leverage the similarity between cumene and PS to drive phenol production via a mild photooxidative HAT mechanism. Using Eosin Y under blue light irradiation and atmospheric pressure of oxygen, commercial PS resin was converted into hydroperoxyl PS. Subsequent acid-catalyzed Hock rearrangement afforded up to 21% yield of phenol. The photooxidation system was successfully applied to 8 post-consumer PS plastics, achieving phenol yields up to 23%, comparable to those of the current industrial Hock process. Our method introduces a mild catalytic strategy for converting post-consumer PS into phenol, offering a potential pathway for PS waste valorization.

Polyamides (PAs) exhibit excellent chemical stability and mechanical resistance, yet these same characteristics lead to their widespread accumulation in the environment as pollution. In this work, we developed an inclusive and operationally simple photothermal strategy to recycle PAs, overcoming the high energy barriers necessary to break down these materials. PAs can be depolymerized using photothermally mediated ring-closing depolymerization and acidic hydrolysis to afford cyclic and linear monomers using carbon black as a photothermal agent (PTA) under visible light irradiation. We showed that polyamide 6 is efficiently depolymerized to ε-caprolactam with 74% yield in 10 min. Similarly, in 1 h, the photothermal acidic hydrolysis of polyamide 6,6 afforded hexamethylene diamine and adipic acid with 97 and 96% yields, respectively. This method was further applied to a variety of aliphatic and aromatic PAs and mixed PA waste. Both photothermally promoted processes effectively depolymerize pigment-containing postconsumer waste by leveraging existing black pigments as PTAs. Here, photothermal conversion provided a general and rapid route for PA depolymerization under visible light irradiation, enabling high monomer yields with inexpensive reagents and a general tolerance to additives, demonstrating this approach’s potential for a circular plastic economy.

Widespread plastic pollution highlights the urgent need for materials with sustainable end-of-life management. Vitrimers, cross-linked polymers containing dynamic covalent bonds, combine the durability of thermosets with their recyclability. Here, we report a one-step photocopolymerization using multifunctional isocyanides as readily accessible cross-linkers that directly introduce vinylogous urethane-like linkages into polyacrylate networks, structures difficult to obtain via amine–β-ketoester condensation. The resulting materials show good reprocessability, maintaining a comparable mechanical performance after three processing cycles. Broadband dielectric spectroscopy (BDS) reveals that the temperature dependence of the bond-exchange relaxation times evolves from a kink-like response in fresh samples to Arrhenius behavior after annealing, visualizing topological rearrangement and defect healing. A scaling relationship between bond-exchange relaxation and electrical conductivity establishes that the former is the underlying mechanism for charge transport in vitrimers. Furthermore, dipolar intermediates generated during bond exchange increase the dielectric permittivity, providing new insight into designing sustainable dielectric materials.

Nucleophilic aromatic substitution (SNAr) is a valuable and widely used method for forming aromatic C–N bonds, which are of immense interest in both industry and academia. However, there remains a longstanding challenge in applying this approach to less activated electrophiles, due to negative charge buildup in the transition state, resulting in high activation energy barriers. Inspired by our previous work on the Newman Kwart Rearrangement (NKR), a high barrier (35-43 kcal/mol) C–S bond forming reaction, we sought to leverage photothermal conversion for challenging C–N bond forming SNAr reactions. In this work, we demonstrate that incorporation of an inexpensive photothermal agent, carbon black, and irradiation with red light affords several lightly activated, intermolecular substitution reactions. Attempts to apply this approach to electron neutral and electron rich aryl halides resulted in unproductive reactivity, leading us to examine the reaction barriers. Computations revealed elevated barriers which were within the range previously achieved during our photothermally mediated NKR. Electron neutral, intramolecular analogs were synthesized and found to undergo productive reactivity in short time frames (3-20 minutes), indicating that the inhomogeneous nature of the photothermal heating was a challenge in terms of colocalizing two reactants sufficiently close to the nanoparticle surface. This concept was further leveraged into a one pot, two step SNAr, where an initial intermolecular reaction occurs at a lower barrier, which then primes the substrate for a second, intramolecular substitution. This approach allowed for the formation of a diverse scope of fused heterocycles.

Multistep synthetic methods requiring harsh conditions and poor atom economy are central to the generation of aryne precursors. The Kobayashi method remains the standard for aryne generation, and newer methods have relied on similar strategies for activated precursors for mild aryne generation. Here, we show the generation of arynes from commercially available aryl or heteroaryl halides via photothermal conversion in air using red light. With photothermal conversion, a high-energy aryne intermediate is formed via inhomogeneous thermal gradients. Nitrogen nucleophiles then readily trap the aryne intermediate to forge C–N bonds, an essential bond in pharmaceutical and agrochemical synthesis, simplifying traditional methods that use aryl halides as aryne precursors. A wide range of readily available heteroaryl and aryl halides undergo amination in less than 20 min under red light irradiation of carbon black. The product regioselectivity matches the expected distribution from aryne trapping, further supporting an aryne mechanism. Additionally, we show aryne trapping via a [4 + 2] cycloaddition, with a sulfur nucleophile, and a one-pot double amination with 1,2-dibromobenzene. This photothermal method using commercially available aryl/heteroaryl halides enables on-demand access to arynes without inert conditions, solvent, and additional synthetic steps, overcoming operational barriers of existing approaches.

Design and synthesis of degradable polymers is a burgeoning area of research to combat plastic waste buildup. Here, a cobalt-catalysed atom transfer radical polymerisation (ATRP) turned on by photothermal conversion is used to synthesize photodegradable non-alternating acrylate-isocyanide copolymers. A simple one-step protocol enables control over copolymerisation with good chain-end fidelity under mild conditions. Although fundamentally a ground state reaction, we demonstrate temporal control via a unique localized heating strategy. Lastly, we demonstrate the advantage of this polymerisation compared to more traditional ATRP protocols.

Selective C–H activation via hydrogen atom transfer (HAT)
has gained significant interest recently. One industrial application for
HAT can be found in the tertiary benzylic C–H bond of cumene for producing phenol, a versatile chemical feedstock for various industries.
However, the overall phenol yield remains low using the existing Hock
process due to the poor selectivity towards the key reaction intermediate, cumene hydroperoxide (CHP). Here, we demonstrate an efficient
strategy for phenol production through photooxidation of cumene to
CHP using iron bromide under blue light irradiation, where bromine
radicals (generated in situ via ligand-to-metal charge transfer) are used
as hydrogen atom abstractors. Mechanistic studies revealed the cumene-to-CHP conversion occurred via a tertiary C–H bond abstraction
by a radical mechanism, and the acetic acid and water additives increased CHP selectivity through stabilizing peroxides. The cumene-phenol process achieved up to 88% yield of CHP in 1 hour and could be used
with a wide range of substrates. We thus developed a selective, mild,
and efficient phenol production method using iron bromide under photooxidative conditions.

Developing novel yet recyclable plastics is necessary as society advances and increases reliance on materials. Using new classes of monomers enables the synthesis of unprecedented polymers for new applications. Here, we report copolymerization with bicyclo[1.1.0]butane monomers to make polymers with strained backbone units and analyze the thermal property changes based on the % incorporation of strained monomers. We discover that even low incorporation (5%–20%) of methyl bicyclobutane-1-carboxylate (MBC) to poly(methyl methacrylate) (PMMA) increases degradation temperature while decreasing glass transition temperature. Efficient depolymerization is achieved for copolymers with 20% or fewer MBC units under photothermal conversion. Other comonomers (2-methoxyethyl methacrylate and oligo(ethylene glycol) monomethyl ethyl methacrylate) are also copolymerized with MBC, and we assess the changes in their thermal properties, including lower critical solution temperatures (LCSTs). Our work expands the understanding of how the strained MBC monomer plays a role in copolymerization, thermal characteristics, and depolymerization of copolymers.

Princeton Chem discovers that common plastic pigment promotes depolymerization – EurekAlert!

Photothermal Plastic Recycling | How Dr. Erin Stache and Hanning Jiang Use Carbon Black to Efficiently Depolymerise Polystyrene – ENGINEERINGNESS

Photoinduced ligand-to-metal charge transfer (LMCT) is a powerful technique for the formation of reactive radical species via homolytic cleavage of the metal–ligand bond. Here, we present that the excited state LMCT of a cobyrinate complex can be accessed by tuning its axial coordination with thiolates as ligands. We demonstrate the photoreduction of cobalt via the excited state Co–S bond homolytic cleavage, as guided by the DFT calculations, which signify the relevance of thiolate axial ligands facilitating the LMCT reactivity. By exploiting this excited state LMCT of cobyrinate, we developed a catalyst-controlled activator regeneration mechanism to catalyze an efficient atom transfer radical polymerization (ATRP) under low-energy light irradiation. Tuning the coordination sphere of cobyrinate provides further control over the electronic properties of the complex while also accessing photothermal conversion in mediating ATRP catalysis.

Using sunlight to recycle black plastics – ACS Newsroom

Chemists discover common plastic pigment promotes depolymerization – PHYS.org
New process recycles black plastics using sunlight – C&EN

Photothermal conversion can promote plastic depolymerization (chemical recycling to a monomer) through light-to-heat conversion. The highly localized temperature gradient near the photothermal agent surface allows selective heating with spatial control not observed with bulk pyrolysis. However, identifying and incorporating practical photothermal agents into plastics for end-of-life depolymerization have not been realized. Interestingly, plastics containing carbon black as a pigment present an ideal opportunity for photothermal conversion recycling. Herein, we use visible light to depolymerize polystyrene plastics into styrene monomers by using the dye in commercial black plastics. A model system is evaluated by synthesizing polystyrene–carbon black composites and depolymerizing under white LED light irradiation, producing styrene monomer in up to 60% yield. Excitingly, unmodified postconsumer black polystyrene samples are successfully depolymerized to a styrene monomer without adding catalysts or solvents. Using focused solar irradiation, yields up to 80% are observed in just 5 min. Furthermore, combining multiple types of polystyrene plastics with a small percentage of black polystyrene plastic enables full depolymerization of the mixture. This simple method leverages existing plastic additives to actualize a closed-loop economy of all-colored plastics.

Nonalternating poly(isocyanide-co-acrylate) copolymers enable access to novel polymer microstructures with versatile chemistry and allow for designed mechanical properties, an important facet of combating the plastic waste crisis. While previous syntheses used cobalt-mediated radical polymerizations, the cobalt complex exhibits side reactivity, complicating control over polymerization. This work describes the application of an orthogonal, photocontrolled RAFT polymerization to a range of isocyanides with excellent temporal control. Under solvent-free conditions, adding monomers with a standard chain transfer agent (CTA) results in the synthesis of copolymers that are then transformed into several novel microstructures. Interestingly, we discover that the isocyanide units can undergo cyclization of the backbone, resulting in an unexampled polyamide-like copolymer containing the pyrrolidone ring. Moreover, we generate poly(acrylic acid)-like copolymers with small amounts of ketone linkages through hydrolysis, enabling faster degradation rates for this class of polymer.

Photothermal conversion is a growing research area that promotes thermal transformations with visible light irradiation. However, few examples of dual photothermal conversion and catalysis limit the power of this phenomenon. Here, we take inspiration from nature’s ability to use porphyrinic compounds for nonradiative relaxation to convert light into heat to facilitate thermal polymerization catalysis. We identify the photothermal conversion catalytic activity of a vitamin B12 derivative, heptamethyl ester cobyrinate (HME-Cob), to perform atom transfer radical polymerization (ATRP) under irradiation. Rapid polymerization are obtained under photothermal activation while maintaining good control over polymerization with the aid of a photoinitiator to enable light-induced catalyst regeneration. The catalyst exhibits exquisite temporal control in photocontrolled thermal polymerization. Ultimately, the activation of this complex is accessed across a broad range of wavelengths, including near-IR light, with excellent temporal control. This work showcases the potential of developing photothermal conversion catalysts.

Poly(vinyl ethers) (PVEs) have many applications, such as adhesives, lubricants, and anticorrosive agents, thanks to their elastic, nonirritating, and chemically inert properties. The recycling of PVEs remains largely underexplored, and current methods lack generality towards other polymer classes. Thus, the chemical upcycling of PVE into small molecule feedstocks would provide an alternative approach to combat these current issues. Here, we report a visible light-mediated method of upcycling poly(isobutyl vinyl ether) (PIBVE) into small molecules via photooxidative degradation using chlorine or bromine radicals. PIBVE can be degraded to low molecular weight oligomers within 2 h, producing good yields of alcohols, aldehydes, and carboxylic acids. Mechanistic studies suggest that hydrogen atom transfer (HAT) from the backbone or the side chain leads to small molecule generation via oxidative cleavages. Additionally, this protocol was applied to a copolymer of poly(methyl acrylate-co-isobutyl vinyl ether) to demonstrate the preference for the degradation of polymers bearing more electron-rich C–H bonds through a judicious choice of abstraction agent. Ultimately, we show that photooxidative degradation enables the selective chemical upcycling of PVEs as a method of plastic waste valorization.

Leveraging electrochemistry to degrade robust polymeric materials has the potential to impact society’s growing issue of plastic waste. Herein, we develop an electrocatalytic oxidative degradation of polyethers and poly(vinyl ethers) via electrochemically mediated hydrogen atom transfer (HAT) followed by oxidative polymer degradation promoted by molecular oxygen. We investigated the selectivity and efficiency of this method, finding our conditions to be highly selective for polymers with hydridic, electron-rich C−H bonds. We leveraged this reactivity to degrade polyethers and poly(vinyl ethers) in the presence of polymethacrylates and polyacrylates with complete selectivity. Furthermore, this method made polyacrylates degradable by incorporation of ether units into the polymer backbone. We quantified degradation products, identifying up to 36 mol % of defined oxidation products, including acetic acid, formic acid, and acetaldehyde, and we extended this method to degrade a polyether-based polyurethane in a green solvent. This work demonstrates a facile, electrochemically-driven route to degrade polymers containing ether functionalities.

Installing ketones into a polymer backbone is a known method for introducing photodegradability into polymers; however, most current methods are limited to ethylene–carbon monoxide copolymerization. Here we use isocyanides in place of carbon monoxide in a copolymerization strategy to access degradable nonalternating poly(ketones) that either maintain or enhance the thermal properties. A cobalt-mediated radical polymerization of acrylates and isocyanides synthesizes nonalternating poly(acrylate-co-isocyanide) copolymers with tunable incorporation using monomer feed ratios. The kinetic product of the polymerization is a dynamic β-imine ester that tautomerizes to the β-enamine ester. Hydrolysis of this copolymer affords a third copolymer microstructure─the elusive nonalternating poly(ketone)─from a single copolymerization strategy. Analysis of the copolymer properties demonstrates tunable thermal properties with the degree of incorporation. Finally, we show that poly(acrylate-co-isocyanide) and poly(acrylate-co-ketone) are photodegradable with 390 nm light, enabling chain cleavage.

Advances in controlled radical polymerizations by cobalt complexes have primarily taken advantage of the reactivity of cobalt as a persistent radical to reversibly deactivate propagating chains by forming a carbon–cobalt bond. However, cobalt-mediated radical polymerizations require stoichiometric ratios of a cobalt complex, deterring its utility in synthesizing well-defined polymers. Here, we developed a strategy to use cobalt as a catalyst to control radical polymerizations via halogen atom transfer with alkyl halide initiators. Using a modified, hydrophobic analogue of vitamin B12 (heptamethyl ester cobyrinate) as a cobalt precatalyst, we controlled the polymerization of acrylate monomers. The polymerization efficiency of the cobalt catalyst was significantly improved by additional bromide anions, which enhanced the deactivation of propagating radicals yielding polymers with dispersity values <1.2 using catalyst concentrations as low as 5 mol %. We anticipate that the development of cobalt catalysis in atom transfer radical polymerization will enable new opportunities in designing catalytic systems for the controlled synthesis of polymers.

The Newman Kwart Rearrangement (NKR) offers an efficient and high-yielding method for producing substituted thiophenols from phenols. While an industrially important protocol, it suffers from high activation energy barriers (35-43 kcal/mol), requiring the use of extreme temperatures (>200 °C) and specialty equipment. This report details a highly efficient and straightforward method for facilitating the NKR using photothermal conversion. This underused, unique reactivity pathway arises from the irradiation of nanomaterials that relax via a non-radiative decay pathway to generate intense thermal gradients. We show carbon black (CB) can be an inexpensive and abundant photothermal agent under visible light irradiation to achieve a facile NKR under mild conditions. The scope includes a wide array of stereo- and electronically diverse substrates with increasing difficulty of rearrangement, including BHT and BINOL as effective substrates. Furthermore, we demonstrate the unique application for temporal control in a thermal reaction and tunability of thermal gradients by modulating light intensity. Ultimately, photothermal conversion enables high-temperature reactions with simple, visible light irradiation.