Iron-catalyzed ring-closing depolymerization of poly(tetrahydrofuran)

Electromagnetically Heating and Oscillating Liquid Metal for Catalyzing Polyester Depolymerization

Depolymerization and recycling of polyesters have shown great significance to economy, ecology, carbon neutrality and human health. Efficient catalysts for thermolysis depolymerization have long been pursued to achieve rapid depolymerization, high selectivity, and low energy consumption. In this study, it is found that liquid metal (LM) can serve as the efficient self-heater, mechanic disturber and catalyst for thermolysis depolymerization of polyesters under alternating electromagnetic fields. When dissolving different metals (e.g., Sn, Zn, Al, and Mg) into gallium, LMs may provide dynamic interactions between the catalyst and reactants, spontaneous metal enriching, and oxidation within the LM surface layer. Without any conventional heaters and mechanic shakers, polycaprolactone is catalytically depolymerized into ɛ-caprolactone at the rate of ≈700 mg h-1 mL-1 with the selectivity of 95.5%. The high surface tension and high mobility of LM also enable continuous depolymerization at an appropriate feeding speed of polyesters (including polyethylene terephthalate, polyhydroxybutyrate and polylactic acid). Thus, this study may offer an unprecedented "all-in-one" platform of liquid metal for continuous thermolysis depolymerization of polyesters, while without any requirement of external heater, mixer, and catalysts.

Using Redox-Switchable Polymerization Catalysis to Synthesize a Chemically Recyclable Thermoplastic Elastomer

In an effort to synthesize chemically recyclable thermoplastic elastomers, a redox-switchable catalytic system was developed to synthesize triblock copolymers containing stiff poly(lactic acid) (PLA) end blocks and a flexible poly(tetrahydrofuran-co-cyclohexene oxide) (poly(THF-co-CHO) copolymer as the mid-block. The orthogonal reactivity induced by changing the oxidation state of the iron-based catalyst enabled the synthesis of the triblock copolymers in a single reaction flask from a mixture of monomers. The triblock copolymers demonstrated improved flexibility compared to poly(l-lactic acid) (PLLA) and thermomechanical properties that resemble thermoplastic elastomers, including a rubbery plateau in the range of -60 to 40 °C. The triblock copolymers containing a higher percentage of THF versus CHO were more flexible, and a blend of triblock copolymers containing PLLA and poly(d-lactic acid) (PDLA) end-blocks resulted in a stereocomplex that further increased polymer flexibility. Besides the low cost of lactide and THF, the sustainability of this new class of triblock copolymers was also supported by their depolymerization, which was achieved by exposing the copolymers sequentially to FeCl3 and ZnCl2 /PEG under reactive distillation conditions.

A Simple, Selective, and General Catalyst for Ring Closing Depolymerization of Polyesters and Polycarbonates for Chemical Recycling

The ability to efficiently and selectively process mixed polymer waste is important to address the growing plastic waste problem. Herein, we report that the combination of ZnCl2 and an additive amount of poly(ethylene glycol) under vacuum can readily and selectively depolymerize polyesters and polycarbonates with high ceiling temperatures (Tc >200 °C) back to their constitute monomers. Mechanistic experiments implicate a random chain scission mechanism and a catalyst structure containing one equivalent of ZnCl2 per ethylene glycol repeat unit in the poly(ethylene glycol). In addition to being general for a wide variety of polyesters and polycarbonates, the catalyst system could selectively depolymerize a polyester in the presence of other commodity plastics, demonstrating how reactive distillation using the ZnCl2 /PEG600 catalyst system can be used to separate mixed plastic waste.

Ring-Opening Polymerization for the Goal of Chemically Recyclable Polymers

A crucial modern dilemma relates to the ecological crisis created by excess plastic waste production. An emerging technology for reducing plastic waste is the production of "chemically recyclable" polymers. These polymers can be efficiently synthesized through ring-opening polymerization (ROP/ROMP) and later recycled to pristine monomer by ring-closing depolymerization, in an efficient circular-type system. This Perspective aims to explore the chemistry involved in the preparation of these monomer/polymer systems, while also providing an overview of the challenges involved, including future directions.

Silicon Catalyzed C-O Bond Ring Closing Metathesis of Polyethers

The Lewis superacid bis(perchlorocatecholato)silane catalyzes C-O bond metathesis of alkyl ethers with an efficiency outperforming all earlier reported systems. Chemoselective ring contractions of macrocyclic crown ethers enable substrate-specific transformations, and an unprecedented ring-closing metathesis of polyethylene glycols allows polymer-selective degradation. Quantum chemical computations scrutinize a high Lewis acidity paired with a simultaneous low propensity for polydentate substrate binding as critical for successful catalysis. Based on these mechanistic insights, a second-generation class of silicon Lewis superacid with enhanced efficacy is identified and demonstrated.

Valorisation of plastic waste via metal-catalysed depolymerisation

Metal-catalysed depolymerisation of plastics to reusable building blocks, including monomers, oligomers or added-value chemicals, is an attractive tool for the recycling and valorisation of these materials. The present manuscript shortly reviews the most significant contributions that appeared in the field within the period January 2010–January 2020 describing selective depolymerisation methods of plastics. Achievements are broken down according to the plastic material, namely polyolefins, polyesters, polycarbonates and polyamides. The focus is on recent advancements targeting sustainable and environmentally friendly processes. Biocatalytic or unselective processes, acid–base treatments as well as the production of fuels are not discussed, nor are the methods for the further upgrade of the depolymerisation products.

Organic synthesis with the most abundant transition metal–iron: from rust to multitasking catalysts

The promising aspects of iron in synthetic chemistry are being explored for three-four decades as a green and eco-friendly alternative to late transition metals. This present review unveils these rich iron-chemistry towards different transformations.

Synthesis of cyclic ethers by cyclodehydration of 1,n-diols using heteropoly acids as catalysts

Heteropoly acids were used as catalysts for cyclodehydration of various 1,n-diols. Cyclodehydration of butane-1,4-diol, pentane-1,5-diol and hexane-1,6-diol catalysed by H3PW12O40 gave tetrahydrofuran, tetrahydropyran and oxepane, respectively. Cyclodehydration of diethylene glycol, triethylene glycol, diethylene glycol monomethyl ether and polyethylene glycol 200 catalysed by H3PW12O40 gave 1,4-dioxane. In particular, cyclodehydration of hexane-1,6-diol gave an excellent yield of oxepane (80%). The selectivity exhibited by the H3PW12O40 catalyst was even better than that exhibited by other reported catalyst systems for similar cyclodehydration reactions.

Synthesis of tetraline derivatives through depolymerization of polyethers with aromatic compounds using a heterogeneous titanium-exchanged montmorillonite catalyst

Synthesis of tetraline derivatives through depolymerization of poly(tetramethylene glycol) with several aromatic compounds is newly developed using a titanium-exchanged montmorillonite catalyst.

Low-temperature depolymerization of polysiloxanes with iron catalysis

The easy accessibility and high adjustability of polymers mainly accounts for the great impact of such materials on modern society. Besides this great success, an important matter is the accumulation of large amounts of end-of-life polymers, which are mainly deposited in landfills or converted by thermal recycling or down-cycling to low-quality materials. In contrast to that, the depolymerization of end-of-life polymers to monomers, which can be applied as feedstock in polymerization chemistry for high-quality polymers, is only carried out for a small fraction of waste. Polysiloxanes are extensively used in a diverse array of technological applications. Based on intrinsic properties of polymers, depolymerization is challenging and only a few high-temperature or less environment-friendly processes have been reported. In this regard, we have set up a capable low-temperature protocol for the depolymerization of poly(dimethylsiloxane) in the presence of catalytic amounts of simple iron salts in combination with different depolymerization reagents. The application of benzoyl fluoride, benzoyl chloride/potassium fluoride, or benzoic anhydride/potassium fluoride as depolymerization reagents affords difluorodimethylsilane or 1,3-difluoro-1,1,3,3-tetramethyldisilxanes as products, which are interesting building blocks for the synthesis of new polymers and allow an overall recycling of polysiloxanes.

Zinc-catalyzed depolymerization of end-of-life polysiloxanes

Polymers occupy an important role in our current society. Besides their great success, an issue is the accumulation of huge amounts of end-of-life polymers. Currently, the waste management is based primarily on landfills, thermal recycling, and downcycling. Notably, only a small portion of end-of-life materials is recycled by depolymerization, which refers to the creation of synthetic precursors that can be polymerized to new polymers to close the cycle. Widely used polymers in modern times are silicones (polysiloxanes), the intrinsic properties of which make their depolymerization demanding; only a few high-temperature or less environmentally friendly processes have been reported. In this regard, we have established an efficient low-temperature protocol for the depolymerization of silicones with benzoyl fluoride in the presence of cheap zinc salts as precatalysts to yield defined products. Notably, the products can be useful synthetic precursors for the preparation of new polymers, so that an overall recycling process is feasible.

The first zinc-catalyzed oxidation of sulfides to sulfones using H2O2 as green oxidant

The first example of the zinc-catalyzed oxidation of sulfides to sulfones has been developed. By using a catalytic amount of zinc salt as the catalyst, DBU as the ligand, various sulfones were produced in good yields with hydrogen peroxide as the terminal green oxidant.