Metal-Catalyzed Switching Degradation of Vinyl Polymers via Introduction of an "In-Chain" Carbon-Halogen Bond as the Trigger

Selective Electrochemical Modification and Degradation of Polymers

We demonstrate that electrochemical-induced decarboxylation enables reliable post-polymerization modification and degradation of polymers. Polymers containing N-(acryloxy)phthalimides were subjected to electrochemical decarboxylation under mild conditions, which led to the formation of transient alkyl radicals. By installing these redox-active units, we systematically modified the pendent groups and chain ends of polyacrylates. This approach enabled the production of poly(ethylene-co-methyl acrylate) and poly(propylene-co-methyl acrylate) copolymers, which are difficult to synthesize by direct polymerization. Spectroscopic and chromatographic techniques reveal these transformations are near-quantitative on several polymer systems. Electrochemical decarboxylation also enables the degradation of all-methacrylate poly(N-(methacryloxy)phthalimide-co-methyl methacrylate) copolymers with a degradation efficiency of >95 %. Chain cleavage is achieved through the decarboxylation of the N-hydroxyphthalimide ester and subsequent β-scission of the backbone radical. Electrochemistry is thus shown to be a powerful tool in selective polymer transformations and controlled macromolecular degradation.

Photocatalytic Upcycling and Depolymerization of Vinyl Polymers

Photocatalytic upcycling and depolymerization of vinyl polymers have emerged as promising strategies to combat plastic pollution and promote a circular economy. This mini review critically summarizes current developments in the upcycling and degradation of vinyl polymers including polystyrene and poly(meth)acrylates. Of these material classes, polymethacrylates possess the unique possibility to undergo a photocatalytic depolymerization back to monomer under thermodynamically favourable conditions, thus presenting significant advantages over traditional thermal strategies. Our perspective on current formidable challenges and potential future directions are also discussed.

Mechanically triggered on-demand degradation of polymers synthesized by radical polymerizations

Polymers that degrade on demand have the potential to facilitate chemical recycling, reduce environmental pollution and are useful in implant immolation, drug delivery or as adhesives that debond on demand. However, polymers made by radical polymerization, which feature all carbon-bond backbones and constitute the most important class of polymers, have proven difficult to render degradable. Here we report cyclobutene-based monomers that can be co-polymerized with conventional monomers and impart the resulting polymers with mechanically triggered degradability. The cyclobutene residues act as mechanophores and can undergo a mechanically triggered ring-opening reaction, which causes a rearrangement that renders the polymer chains cleavable by hydrolysis under basic conditions. These cyclobutene-based monomers are broadly applicable in free radical and controlled radical polymerizations, introduce functional groups into the backbone of polymers and allow the mechanically gated degradation of high-molecular-weight materials or cross-linked polymer networks into low-molecular-weight species.

Mechanism-Guided Discovery of Cleavable Comonomers for Backbone Deconstructable Poly(methyl methacrylate)

The development of cleavable comonomers (CCs) with suitable copolymerization reactivity paves the way for the introduction of backbone deconstructability into polymers. Recent advancements in thionolactone-based CCs, exemplified by dibenzo[c,e]-oxepine-5(7H)-thione (DOT), have opened promising avenues for the selective deconstruction of multiple classes of vinyl polymers, including polyacrylates, polyacrylamides, and polystyrenics. To date, however, no thionolactone CC has been shown to copolymerize with methacrylates to an appreciable extent to enable polymer deconstruction. Here, we overcome this challenge through the design of a new class of benzyl-functionalized thionolactones (bDOTs). Guided by detailed mechanistic analyses, we find that the introduction of radical-stabilizing substituents to bDOTs enables markedly increased and tunable copolymerization reactivity with methyl methacrylate (MMA). Through iterative optimizations of the molecular structure, a specific bDOT, F-p-CF3PhDOT, is discovered to copolymerize efficiently with MMA. High molar mass deconstructable PMMA-based copolymers (dPMMA, Mn > 120 kDa) with low percentages of F-p-CF3PhDOT (1.8 and 3.8 mol%) are prepared using industrially relevant bulk free radical copolymerization conditions. The thermomechanical properties of dPMMA are similar to PMMA; however, the former is shown to degrade into low molar mass fragments (<6.5 kDa) under mild aminolysis conditions. This work presents the first example of a radical ring-opening CC capable of nearly random copolymerization with MMA without the possibility of cross-linking and provides a workflow for the mechanism-guided design of deconstructable copolymers in the future.

Chemical recycling of bromine-terminated polymers synthesized by ATRP

Chemical recycling of polymers is one of the biggest challenges in materials science. Recently, remarkable achievements have been made by utilizing polymers prepared by controlled radical polymerization to trigger low-temperature depolymerization. However, in the case of atom transfer radical polymerization (ATRP), depolymerization has nearly exclusively focused on chlorine-terminated polymers, even though the overwhelming majority of polymeric materials synthesized with this method possess a bromine end-group. Herein, we report an efficient depolymerization strategy for bromine-terminated polymethacrylates which employs an inexpensive and environmentally friendly iron catalyst (FeBr2/L). The effect of various solvents and the concentration of metal salt and ligand on the depolymerization are judiciously explored and optimized, allowing for a depolymerization efficiency of up to 86% to be achieved in just 3 minutes. Notably, the versatility of this depolymerization is exemplified by its compatibility with chlorinated and non-chlorinated solvents, and both Fe(ii) and Fe(iii) salts. This work significantly expands the scope of ATRP materials compatible with depolymerization and creates many future opportunities in applications where the depolymerization of bromine-terminated polymers is desired.

Bulk Depolymerization of Methacrylate Polymers via Pendent Group Activation

In this study, we present an efficient approach for the depolymerization of poly(methyl methacrylate) (PMMA) copolymers synthesized via conventional radical polymerization. By incorporating low mol % phthalimide ester-containing monomers during the polymerization process, colorless and transparent polymers closely resembling unfunctionalized PMMA are obtained, which can achieve >95% reversion to methyl methacrylate (MMA). Notably, our catalyst-free bulk depolymerization method exhibits exceptional efficiency, even for high-molecular-weight polymers, including ultrahigh-molecular-weight (106-107 g/mol) PMMA, where near-quantitative depolymerization is achieved. Moreover, this approach yields polymer byproducts of significantly lower molecular weight, distinguishing it from bulk depolymerization methods initiated from chain ends. Furthermore, we extend our investigation to polymethacrylate networks, demonstrating high extents of depolymerization. This innovative depolymerization strategy offers promising opportunities for the development of sustainable polymethacrylate materials, holding great potential for various applications in polymer science.

Implementing a Doping Approach for Poly(methyl methacrylate) Recycling in a Circular Economy

To mitigate pollution by plastic waste, it is paramount to develop polymers with efficient recyclability while retaining desirable physical properties. A recyclable poly(methyl methacrylate) (PMMA) is synthesized by incorporating a minimal amount of an α-methylstyrene (AMS) analogue into the polymer structure. This P(MMA-co-AMS) copolymer preserves the essential mechanical strength and optical clarity of PMMA, vital for its wide-ranging applications in various commercial and high-tech industries. Doping with AMS significantly enhances the thermal, catalyst-free depolymerization efficiency of PMMA, facilitating the recovery of methyl methacrylate (MMA) with high yield and purity at temperatures ranging from 150 to 210 °C, nearly 250 K lower than current industrial standards. Furthermore, the low recovery temperature permits the isolation of pure MMA from a mixture of assorted common plastics.

The thermodynamics and kinetics of depolymerization: what makes vinyl monomer regeneration feasible?

Depolymerization is potentially a highly advantageous method of recycling plastic waste which could move the world closer towards a truly circular polymer economy. However, depolymerization remains challenging for many polymers with all-carbon backbones. Fundamental understanding and consideration of both the kinetics and thermodynamics are essential in order to develop effective new depolymerization systems that could overcome this problem, as the feasibility of monomer generation can be drastically altered by tuning the reaction conditions. This perspective explores the underlying thermodynamics and kinetics governing radical depolymerization of addition polymers by revisiting pioneering work started in the mid-20th century and demonstrates its connection to exciting recent advances which report depolymerization reaching near-quantitative monomer regeneration at much lower temperatures than seen previously. Recent catalytic approaches to monomer regeneration are also explored, highlighting that this nascent chemistry could potentially revolutionize depolymerization-based polymer recycling in the future.

1,1'- Thiocarbonyldiimidazole Radical Copolymerization for the Preparation of Degradable Vinyl Polymers

1,1'-Thiocarbonyldiimidazole (TCDI) readily undergoes radical copolymerization with tert-butyl acrylate (tBA), N,N-dimethylacrylamide, and styrene. 1H NMR monitoring of the comonomer reactivity revealed a notable compatibility between TCDI and comonomers, resulting in similar consumption rates when TCDI was introduced at a 10% feed ratio. Furthermore, trithiocarbonate-mediated RAFT copolymerization of TCDI with tBA gave polymers that exhibited a linear increase of molar mass (Mnth = 2-10 kg mol-1) with conversion with relatively low dispersities (1.2-1.4). Importantly, this process enabled a successful chain extension of the produced P(TCDI-co-tBA) copolymer with styrene to form a diblock copolymer. The copolymers generated through this method contain TCDI-derived diimidazolyl thioether moieties, as established through 1H NMR spectroscopy. Additionally, degradation experiments using isopropylamine, benzoyl peroxide, sodium methoxide, and bleach have provided further confirmation of the presence of degradable TCDI moieties in the vinyl copolymer backbone.

Umpolung Isomerization in Radical Copolymerization of Benzyl Vinyl Ether with Pentafluorophenylacrylate Leading to Degradable AAB Periodic Copolymers

This study revealed that benzyl vinyl ether (BnVE) shows a peculiar isomerization propagation in its radical copolymerization with an electron-deficient acrylate carrying a pentafluorophenyl group (PFA). The co-monomer pair inherently exhibits the cross-over propagation feature due to the large difference in the electron density. However, the radical species of PFA was found to undergo a backward isomerization to the penultimate BnVE pendant giving a benzyl radical species prior to propagation with BnVE. The isomerization brings a drastic change in the character of the growing radical species from electrophilic to nucleophilic, and thus the isomerized benzyl radial species propagates with PFA. Consequently, the two monomers were consumed in the order AAB (A: PFA; B: BnVE) and the unique periodic consumption was confirmed by the pseudo-reactivity ratios calculated by the penultimate model: r11 =0.174 and r21 =6600 for PFA (M1 ) with BnVE (M2 ). The pentafluorophenyl ester groups of the resulting copolymers are transformed into ester and amide groups by post-polymerization alcoholysis and aminolysis modifications. The unique isomerization in the AAB sequence allowed the periodic introduction of a benzyl ether structure in the backbone leading to efficient degradation under acid conditions.

Chemically Recyclable Vinyl Polymers by Free Radical Polymerization of Cyclic Styrene Derivatives

To achieve a sustainable society supported by resource circulation, vinyl monomers that can radically polymerize and be recovered from vinyl polymers (VPs) are desirable. However, the chemical recycling of VPs remains challenging because of the difficulty in quantitative and selective main-chain scission or depolymerization. In this study, VPs of cyclic styrene derivatives, such as 3-methylene phthalide, were investigated to be chemically recyclable. The ring-opening of the pendant groups by saponification enhanced the steric hindrance of the pendants, which resulted in main-chain scission and depolymerization to the monomer precursors. Highly efficient chemical recycling was achieved by suspending the polymer in aqueous KOH. These results facilitate resource circulation toward achieving a sustainable society.

Degradation of Polyacrylates by One-Pot Sequential Dehydrodecarboxylation and Ozonolysis

We establish a synthetically convenient method to degrade polyacrylate homopolymers. Carboxylic acids are installed along the polymer backbone by partial hydrolysis of the ester side chains, and then, in a one-pot sequential procedure, the carboxylic acids are converted into alkenes and oxidatively cleaved. This process enables the robustness and properties of polyacrylates to be maintained during their usable lifetime. The ability to tune the degree of degradation was demonstrated by varying the carboxylic acid content of the polymers. This method is compatible with a wide range of polymers prepared from vinyl monomers through copolymerization of acrylic acid with different monomers including acrylates, acrylamides, and styrenics.

Rapid and Selective Photo-degradation of Polymers: Design of an Alternating Copolymer with an o-Nitrobenzyl Ether Pendant

The development of polymers with on-demand degradability is required to alleviate the current global issues on polymer-waste pollution. Therefore, we designed a vinyl ether monomer with an o-nitrobenzyl (oNBn) group as a photo-deprotectable pendant (oNBnVE) and synthesized an alternating copolymer with an oNBn-capped acetal backbone via cationic copolymerization with p-tolualdehyde (pMeBzA). The resultant alternating copolymer could be rapidly degraded into lower-molecular-weight compounds upon simple exposure to UV irradiation without any reactants or catalysts, while it was sufficiently stable toward heat and ambient light. This degradation proceeds via cleavage of the hemiacetal structure generated upon photo-deprotection of the oNBn pendant. The oNBn-peculiar degradability allowed the exclusive photo-degradation of the oNBnVE/pMeBzA segments in a diblock copolymer composed of oNBnVE/pMeBzA and benzyl vinyl ether (BnVE)/pMeBzA segments.

Synthesis and Degradation of Vinyl Polymers with Evenly Distributed Thioacetal Bonds in Main Chains: Cationic DT Copolymerization of Vinyl Ethers and Cyclic Thioacetals

We report a novel method to synthesize degradable poly(vinyl ether)s with cleavable thioacetal bonds periodically arranged in the main chains using controlled cationic copolymerization of vinyl ethers with a 7-membered cyclic thioacetal (7-CTA) via degenerative chain transfer (DT) to the internal thioacetal bonds. The thioacetal bonds, which are introduced into the main chain by cationic ring-opening copolymerization of 7-CTA with vinyl ethers, serve as in-chain dormant species to allow homogeneous propagation of vinyl ethers for all internal segments to afford copolymers with controlled overall and segmental molecular weights. The obtained polymers can be degraded into low- and controlled-molecular-weight polymers with narrow molecular weight distributions via hydrolysis. Various vinyl ethers with hydrophobic, hydrophilic, and functional pendants are available. Finally, one-pot synthesis of multiblock copolymers and their degradation into diblock copolymers are also achieved.

Photoassisted Radical Depolymerization

Controlled radical polymerization techniques enable the synthesis of polymers with predetermined molecular weights, narrow molecular weight distributions, and controlled architectures. Moreover, these polymerization approaches have been routinely shown to result in retained end-group functionality that can be reactivated to continue polymerization. However, reactivation of these end groups under conditions that instead promote depropagation is a viable route to initiate depolymerization and potentially enable closed-loop recycling from polymer to monomer. In this report, we investigate light as a trigger for thermal depolymerization of polymers prepared by reversible-addition-fragmentation chain-transfer (RAFT) polymerization. We study the role of irradiation wavelength by targeting the n → π* and π → π* electronic transitions of the thiocarbonylthio end-groups of RAFT-generated polymers to enhance depolymerization via terminal bond homolysis. Specifically, we explore depolymerization of polymers with trithiocarbonate, dithiocarbamate, and p-substituted dithiobenzoate end groups with the purpose of increasing depolymerization efficiency with light. As the wavelength decreases from the visible range to the UV range, the rate of depolymerization is dramatically increased. This method of photoassisted depolymerization allows up to 87% depolymerization efficiency within 1 h, results that may further the advancement of recyclable materials and life-cycle circularity.

Backbone Degradation of Polymethacrylates via Metal-Free Ambient-Temperature Photoinduced Single-Electron Transfer

Polymeric materials comprised of all-carbon backbones are ubiquitous to modern society due to their low cost, impressive robustness, and unparalleled physical properties. It is well-known that these materials often persist long beyond their intended usage lifetime, resulting in environmental accumulation of plastic waste. A substantial barrier to the breakdown of these polymers is the relative chemical inertness of carbon-carbon bonds within their backbone. Herein, we describe a photocatalytic strategy for cleaving carbon-based polymer backbones. Inclusion of a low mole percent of a redox-active comonomer allows for a dramatic reduction in polymer molecular weight upon exposure to light. The N-(acyloxy)phthalimide comonomer, upon reception of an electron from a single-electron transfer (SET) donor, undergoes decarboxylation to yield a backbone-centered radical. Depending on the nature of this backbone radical, as well as the substitution on neighboring monomer repeat units, a β-scission pathway is thermodynamically favored, resulting in backbone cleavage. In this way, polymers with an all-carbon backbone may be degraded at ambient temperature under metal-free conditions.