Dual Organocatalysts Based on Ionic Mixtures of Acids and Bases: A Step Toward High Temperature Polymerizations

Thermally Reversible Organocatalyst for the Accelerated Reprocessing of Dynamic Networks with Creep Resistance

The industrial implementation of covalent adaptable networks hinges on the delicate task of achieving rapid bond exchange activation at specific temperatures while ensuring a sufficiently slow exchange at working temperatures to avoid irreversible deformation. In this pursuit, latent catalysts offer a potential solution, allowing for spatiotemporal control of dynamic exchange in vitrimer networks. However, the irreversible nature of their activation has led to undesired creep deformation after multiple cycles of reprocessing. In this work, we demonstrate that a tetraphenylborate tetramethyl guanidinium salt (TPB:TMG) undergoes a reversible thermal dissociation, releasing free TMG. This thermally reversible organocatalyst can be readily introduced as an additive in industrially relevant materials such as disulfide-containing polyurethane networks (PU) that undergo disulfide exchange in the presence of a base catalyst. In contrast with a free-base-catalyzed process, we demonstrate the dual benefit of adding the thermally reversible TPB:TMG in preventing creep at lower temperatures and also enabling reprocessability of disulfide-containing PU networks at elevated temperatures. The remarkable reversibility of this thermally activated catalyst allows for multiple reprocessing cycles while effectively maintaining a creep-free state at service temperature.

Selective deconstruction of mixed plastics by a tailored organocatalyst

Plastic represents an essential material in our society; however, a major imbalance between their high production and end-of-life management is leading to unrecovered energy, economic hardship, and a high carbon footprint. The adoption of plastic recycling has been limited, mainly due to the difficulty of recycling mixed plastics. Here, we report a versatile organocatalyst for selective glycolysis of diverse consumer plastics and their mixed waste streams into valuable chemicals. The developed organocatalyst selectively deconstructs condensation polymers at a specific temperature, and additives or other polymers such as polyolefin or cellulose can be readily separated from the mixed plastics, providing a chemical recycling path for many existing mixed plastics today. The Life Cycle Assessment indicates that the production of various condensation polymers from the deconstructed monomers will result in a significant reduction in greenhouse gas emissions and energy input, opening a new paradigm of plastic circularity toward a net-zero carbon society.

Elucidation of the Alternating Copolymerization Mechanism of Epoxides or Aziridines with Cyclic Anhydrides in the Presence of Halide Salts

Organic halide salts in combination with metal or organic compound are the most common and essential catalysts in ring-opening copolymerizations (ROCOP). However, the role of organic halide salts was neglected. Here, we have uncovered the complex behavior of organic halides in ROCOP of epoxides or aziridine with cyclic anhydride. Coordination of the chain-ends to cations, electron-withdrawing effect, leaving ability of halide atoms, chain-end basicity/nucleophilicity, and terminal steric hindrance cause three types of side reactions: single-site transesterification, substitution, and elimination. Understanding the complex functions of organic halide salts in ROCOP led us to develop highly active and selective aminocyclopropenium chlorides as catalysts/initiators. Adjustable H-bonding interactions of aminocyclopropenium with propagating anions and epoxides create chain-end coordination process that generate highly reactive carboxylate and highly selective alkoxide chain-ends.

Depolymerization of Polyesters by Transesterification with Ethanol Using (Cyclopentadienyl)titanium Trichlorides

Exclusive chemical conversions of polyesters [poly(ethylene adipate) (PEA), poly(butylene adipate) (PBA), poly(ethylene terephthalate) (PET), poly(butylene terephthalate) (PBT)] to the corresponding monomers (diethyl adipate, diethyl terephthalate, ethylene glycol, 1,4-butane diol) by transesterification with ethanol using Cp’TiCl3 (Cp’ = Cp, Cp*) catalyst have been demonstrated. The present acid-base-free depolymerizations by Cp’TiCl3 exhibited completed conversions (>99%) of PET, PBT to afford diethyl terephthalate and ethylene glycol or 1,4-butane diol exclusively (selectivity >99%) without formation of any other by-products in the NMR spectra (150–170 °C, Ti 1.0, or 2.0 mol%). The resultant reaction mixture after the depolymerization of PBA with ethanol via the CpTiCl3 catalyst (1.0 mol%, 150 °C, 3 h), consisting of diethyl adipate and 1,4-butane diol, was heated at 150 °C in vacuo for 24 h to afford high molecular weight recycled PBA with unimodal molecular weight distribution (Mn = 11,800, Mw/Mn = 1.6), strongly demonstrating a possibility of one-pot (acid-base-free) closed-loop chemical recycling.

Spotting Trends in Organocatalyzed and Other Organomediated (De)polymerizations and Polymer Functionalizations

Organocatalysis has evolved into an effective complement to metal‐ or enzyme‐based catalysis in polymerization, polymer functionalization, and depolymerization. The ease of removal and greater sustainability of organocatalysts relative to transition‐metal‐based ones has spurred development in specialty applications, e.g., medical devices, drug delivery, optoelectronics. Despite this, the use of organocatalysis and other organomediated reactions in polymer chemistry is still rapidly developing, and we envisage their rapidly growing application in nascent areas such as controlled radical polymerization, additive manufacturing, and chemical recycling in the coming years. In this Review, we describe ten trending areas where we anticipate paradigm shifts resulting from novel organocatalysts and other transition‐metal‐free conditions. We highlight opportunities and challenges and detail how new discoveries could lead to previously inaccessible functional materials and a potentially circular plastics economy.

Methanolysis of Poly(lactic Acid) Using Catalyst Mixtures and the Kinetics of Methyl Lactate Production

Polylactic acid (PLA) is a leading bioplastic of which the market share is predicted to increase in the future; its growing production capacity means its end-of-life treatment is becoming increasingly important. One beneficial disposal route for PLA is its chemical recycling via alcoholysis. The alcoholysis of PLA leads to the generation of value-added products alkyl lactates; this route also has potential for a circular economy. In this work, PLA was chemically recycled via methanolysis to generate methyl lactate (MeLa). Four commercially available catalysts were investigated: zinc acetate dihydrate (Zn(OAc)₂), magnesium acetate tetrahydrate (Mg(OAc)₂), 4-(dimethylamino)pyridine (DMAP), and triazabicyclodecene (TBD). Dual catalyst experiments displayed an increase in reactivity when Zn(OAc)₂ was paired with TBD or DMAP, or when Mg(OAc)₂ was paired with TBD. Zn(OAc)₂ coupled with TBD displayed the greatest reactivity. Out of the single catalyst reactions, Zn(OAc)₂ exhibited the highest activity: a higher mol% was found to increase reaction rate but plateaued at 4 mol%, and a higher equivalent of methanol was found to increase the reaction rate, but plateaued at 17 equivalents. PLA methanolysis was modelled as a two-step reversible reaction; the activation energies were estimated at: Ea₁ = 25.23 kJ∙mol⁻¹, Ea₂ = 34.16 kJ∙mol⁻¹ and Ea_-2 = 47.93 kJ∙mol⁻¹.

Direct Synthesis of Coumarin Derivatives from Alkynoic Esters via Dual-organocatalysis

An efficient synthetic method for coumarin derivatives was developed using a dual-organocatalytic reaction. A combination of p-toluenesulfonic acid monohydrate and piperidine was found to efficiently catalyze the cyclization between salicylaldehydes and alkynoic esters to give various coumarin derivatives in good yield and high selectivity. Mechanistic and kinetic data suggested that the conjugate addition between piperidine and alkynoic esters played a crucial role in the reaction mechanism.

Controlling polymer stereochemistry in ring-opening polymerization: a decade of advances shaping the future of biodegradable polyesters

This review highlights recent developments in the field of biodegradable polymeric materials intended to replace non-degradable conventional plastics, focusing on studies from the last ten years involving the stereoselective ring-opening polymerization of cyclic esters. This encompasses exciting advances in both catalyst design and monomer scope. Notably, the last decade has seen the emergence of metal-free stereocontrolled ROP for instance, as well as the synthesis and stereocontrolled polymerization of new types of chiral monomers. This study will emphasize recent stereoselective polymerization catalysts and chiral monomers and will focus on stereocontrol quantification, the mechanisms of stereocontrol and their differentiation if reported and studied for a specific catalyst system.

Stereoretention in the Bulk ROP of l-Lactide Guided by a Thermally Stable Organocatalyst

Polylactide (PLA) has emerged as one of the most promising bio-based alternatives to petroleum-based plastics, mainly because it can be produced from the fermentation of naturally occurring sugars and because it can be industrially compostable. In spite of these benefits, the industrial ring-opening polymerization (ROP) of l-lactide (L-LA) still requires the use of highly active and thermally stable metal-based catalysts, which have raised some environmental concerns. While the excellent balance between activity and functional group compatibility of organic acid catalysts makes them some of the most suitable catalysts for the metal-free ROP of L-LA, the majority of these acids are highly volatile and subject to decomposition at high temperature, which limits their use under industrially relevant conditions. In this work we exploit the use of a nonstoichiometric acid–base organocatalyst to promote the solvent-free and metal-free ROP of L-LA at elevated temperatures in the absence of epimerization and transesterification. To do so, a stable acidic complex was prepared by mixing 4-(dimethylamino)pyridine (DMAP) with 2 equiv of methanesulfonic acid (MSA). Both experimental and computational results indicate that DMAP:MSA (1:2) not only is highly thermally stable but also promotes the retention of stereoregularity during the polymerization of L-LA, leading to PLLA with a molar mass of up to 40 kg mol^–1 and a chiral purity in excess of 98%. This result provides a new feature to exploit in organocatalyzed polymerization and in the design of new catalysts to facilitate the path to market.

Advances, Challenges, and Opportunities of Poly(γ-butyrolactone)-Based Recyclable Polymers

The discovery and prosperous growth of synthetic polymers have presented both significant advantages and daunting challenges in the last century. To address the issues of environmental pollution and fossil consumption, recyclable, degradable, and/or biobased polymers have been given much attention in the polymer science community. This viewpoint focuses on the emerging fully chemical recyclable poly(γ-butyrolactone)-based polymers. The breakthrough from nonpolymerizable to efficient polymerization is highlighted by the benefits of the development of a series of catalysis for ring-opening polymerization of γ-butyrolactone. Subsequently, the design of γ-butyrolactone derivatives and synthesis of more recyclable polymers are summarized together with the discussions about the structure and property relationship. Finally, the remaining challenges and promising opportunities are suggested in order to provide insights into the further direction for sustainable polymers.

100th Anniversary of Macromolecular Science Viewpoint: Toward Catalytic Chemical Recycling of Waste (and Future) Plastics

The current global materials economy has long been inefficient due to unproductive reuse and recycling efforts. Within the wider materials portfolio, plastics have been revolutionary to many industries but they have been treated as disposable commodities leading to their increasing accumulation in the environment as waste. The field of chemistry has had significant bearing in ushering in the current plastics industry and will undoubtedly have a hand in transforming it to become more sustainable. Existing approaches include the development of synthetic biodegradable plastics and turning to renewable raw materials in order to produce plastics similar to our current petrol-based materials or to create new polymers. Additionally, chemists are confronting the environmental crisis by developing alternative recycling methods to deal with the legacy of plastic waste. Important emergent technologies, such as catalytic chemical recycling or upcycling, have the potential to alleviate numerous issues related to our current and future refuse and, in doing so, help pivot our materials economy from linearity to circularity.

Homoconjugated Acids as Low Cyclosiloxane-Producing Silanol Polycondensation Catalysts

Polycondensation of α,ω-disilanols is a foundational technology for silicones producers. Commercially, this process is carried out with strong Brønsted acids and bases, which generates cyclosiloxane byproducts. Homoconjugated acids (a 2:1 complex of acid:base or a 1:1 complex of acid:salt), a seldom used class of silanol polycondensation catalysts, were evaluated for their ability to polymerize α,ω-disilanols while forming low levels of cyclosiloxane byproducts. Homoconjugated acid catalysts were highly active for silanol polycondensation, even when made from relatively mild acids such as acetic acid. Both the acid and base (or cation) component of the homoconjugated species was important for activity and avoiding cyclosiloxane byproduct formation. Stronger acids and bases were found to positively affect reactivity, and the pK a of the acid was found to correlate with cyclosiloxane byproduct formation. The individual components of the homoconjugated species (the acid and base) were ineffective as catalysts by themselves, and compositions with fewer than 2 mol of acid to 1 mol of base were much less reactive. Homoconjugated trifluoroacetic acid tetramethylguanidinium and tetrabutylphosphonium complexes were found to be privileged catalysts, able to give high-molecular-weight siloxanes (M n > 60 kDa) while generating less than 100 ppm of octamethylcyclotetrasiloxane byproduct. Finally, a mechanism has been proposed where silanols are electrophilically and nucleophilically activated by the homoconjugated species, leading to silanol polycondensation.

Valorization of Plastic Wastes for the Synthesis of Imidazolium-Based Self-Supported Elastomeric Ionenes

Imidazolium-based ionenes are known to be high-performance materials for a great variety of applications. The preparation of these polymers requires the use of bis-imidazole starting monomers, which are commonly prepared by using toxic chloride reagents. In this study, bis-imidazole monomers are synthesized by organocatalytic chemical recycling of discarded plastics through chemical depolymerization. By using poly(ethylene terephthalate) and bisphenol A polycarbonate as starting materials, different monomers containing amide or urea functionalities are prepared to produce high-molecular-weight ionic polymers. These novel ionenes show excellent elastomeric and self-healing behavior, serving as a promising means to expand the exploration of plastic wastes as a source of new materials.

Synthesis of Functionalized Cyclic Carbonates through Commodity Polymer Upcycling

Functionalized cyclic carbonates are attractive monomers for the synthesis of innovative polycarbonates or polyurethanes for various applications. Even though their synthesis has been intensively investigated, doing so in a sustainable and efficient manner remains a challenge. Herein, we propose an organocatalytic procedure based on the depolymerization of a commodity polymer, bisphenol A based polycarbonate (BPA-PC). Different carbonate-containing heterocycles are obtained in good to excellent yields employing BPA-PC as a sustainable and inexpensive source of carbonate, including functionalized six-membered cyclic carbonates.

Aminolytic upcycling of poly(ethylene terephthalate) wastes using a thermally-stable organocatalyst

We report the potential of thermally stable acid-base mixtures for the upcycling of PET in the presence of amines.

Tunable hydantoin and base binary organocatalysts in ring-opening polymerizations

A (thio)hydantoin (HHyd) was deprotonated by a Brønsted base (B) to afford iminolate Hyd1 or Hyd3 that activated polymer chain-end (P), the conjugate acid (B–H+) activated monomer (M).

Poly(hydroxy acids) derived from the self-condensation of hydroxy acids: from polymerization to end-of-life options

Poly(hydroxy acids) derived from the self-condensation of hydroxy acid are biodegradable and can be fully recycled in a Circular Economy approach.

Dual-catalytic depolymerization of polyethylene terephthalate (PET)

Limiting our plastic waste and finding greener, more sustainable solutions for disposal is a current environmental priority.