Radical Approach to Thioester-Containing Polymers

General and Mild Method for the Synthesis of Polythioesters from Lactone Feedstocks

Polythioesters are attracting increasing interest in applications requiring degradability or recyclability. However, few general methods exist for the synthesis of these polymers. This report presents a fast and versatile method for synthesizing polythioesters from readily available lactone feedstocks. The two-step process begins with the thionation of lactones to thionolactones, followed by the ring-opening polymerization of the thionolactones to polythioesters. Unlike previous methods that rely on harsh reagents to accomplish this transformation, we demonstrate that the mild tetrabutylammonium thioacetate is a competent initiator for polymerization. This method exhibits broad applicability, as demonstrated by the successful polymerizations of an unstrained 17-membered macrocycle and an N-substituted cyclic thionocarbamate. Furthermore, the generality of this scheme enables the synthesis of polythioesters with highly tunable properties, as demonstrated here by the synthesis of a set of polymers with glass transition temperatures spanning 180 °C. Finally, the polythioesters are efficiently depolymerized into the corresponding thiolactones.

Degradable N-Vinyl Copolymers through Radical Ring-Opening Polymerization of Cyclic Thionocarbamates

A thiocarbonyl radical ring-opening polymerization approach was implemented with cyclic thionocarbamates to generate degradable copolymers with N-vinyl monomers. The rigid structures of cyclic N-substituted thionocarbamates have been revealed by X-ray crystallography and NMR spectroscopy. The corresponding copolymers show incorporation of the thiocarbamates within the carbon backbone of polyvinylpyrrolidone influenced by acyl substituents through radical ring-opening copolymerization. The phenyl-substituted cyclic thionocarbamate copolymerized with N-vinyl carbazole and N-vinyl caprolactam, while little to no incorporation occurred with tBu acrylate and styrene, respectively. Further, these copolymers can undergo hydrolytic degradation under mild conditions. A new family of cyclic thionocarbamates capable of radical ring-opening copolymerization with N-vinyl monomers has been established.

Additive-Controlled Divergent Synthesis of Fluorenone-4-carboxylic Acids and Diphenic Anhydrides via Rhodium-Catalyzed Oxidative Dimeric Cyclization of Aromatic Acids

A rhodium-catalyzed one-pot access to valuable polycyclic frameworks of fluorenone-4-carboxylic acids and diphenic anhydrides via the oxidative dimeric cyclization of aromatic acids has been developed. This transformation proceeded via carboxyl-assisted 2-fold C-H activation followed by intramolecular Friedel-Crafts or dehydration reactions. The silver salt additive plays a vital role in the chemoselectivity of the products. Diphenic anhydride 3l exhibits a maximum fluorescence quantum yield of up to 59%.

Mechanochemical Backbone Editing for Controlled Degradation of Vinyl Polymers

The chemically inert nature of fully saturated hydrocarbon backbones endows vinyl polymers with desirable durability, but it also leads to their significant environmental persistence. Enhancing the sustainability of these materials requires a pivotal yet challenging shift: transforming the inert backbone into one that is degradable. Here, we present a versatile platform for mechanochemically editing the fully saturated backbone of vinyl polymers towards degradable polymer chains by integrating cyclobutene-fused succinimide (CBS) units along backbone through photo-iniferter reversible addition-fragmentation chain-transfer (RAFT) copolymerization. Significantly, the evenly insertion of CBS units does not compromise thermal or chemical stability but rather offers a means to adjust the properties of polymethylacrylate (PMA). Meanwhile, reactive acyclic imide units can be selectively introduced to the backbone through mechanochemical activation (pulse ultrasonication or ball-milling grinding) when required. Subsequent hydrolysis of the acyclic imide groups enables efficient degradation, yielding telechelic oligomers. This approach holds promise for inspiring the design and modification of more environmentally friendly vinyl polymers through backbone editing.

Main chain selective polymer degradation: controlled by the wavelength and assembly

The advent of reversible deactivation radical polymerization (RDRP) revolutionized polymer chemistry and paved the way for accessing synthetic polymers with controlled sequences based on vinylic monomers. An inherent limitation of vinylic polymers stems from their all-carbon backbone, which limits both function and degradability. Herein, we report a synthetic strategy utilizing radical ring-opening polymerization (rROP) of complementary photoreactive cyclic monomers in combination with RDRP to embed photoresponsive functionality into desired blocks of polyvinyl polymers. Exploiting different absorbances of photoreactive cyclic monomers, it becomes possible to degrade blocks selectively by irradiation with either UVB or UVA light. Translating such primary structures of polymer sequences into higher order assemblies, the hydrophobicity of the photodegradable monomers allowed for the formation of micelles in water. Upon exposure to light, the nondegradable blocks detached yielding a significant reduction in the micelle hydrodynamic diameter. As a result of the self-assembled micellar environment, telechelic oligomers with photoreactive termini (e.g., coumarin or styrylpyrene) resulting from the photodegradation of polymers in water underwent intermolecular photocycloaddition to photopolymerize, which usually only occurs efficiently at longer wavelengths and a much higher concentration of photoresponsive groups. The reported main chain polymer degradation is thus controlled by the irradiation wavelength and the assembly of the polymers.

Plenty of Space in the Backbone: Radical Ring-Opening Polymerization

Radical polymerization is the most widely applied technique in both industry and fundamental science. However, its major drawback is that it typically yields polymers with non-functional, non-degradable all-carbon backbones-a limitation that radical ring-opening polymerization (rROP) allows to overcome. The last decade has seen a surge in rROP, primarily focused on creating degradable polymers. This pursuit has resulted in the creation of the first readily degradable materials through radical polymerization. Recent years have witnessed innovations in new monomers that address previous design limitations, such as ring strain and reactivity ratios. Furthermore, advances in integrating rROP with reversible deactivation radical polymerization (RDRP) have facilitated the incorporation of complex, customizable chemical payloads into the main polymer chain. This short review discusses the latest developments in monomer design with a focused analysis of their limitations in a broader historical context. Recently evolving strategies for compatibility of rROP monomers with RDRP are discussed, which are key to precision polymer synthesis. The latest chemistry surveyed expands the horizon beyond mere hydrolytic degradation. Now is the time to explore the chemical potential residing in the previously inaccessible polymer backbone.

Recent Advances in Environmentally Friendly Dual-crosslinking Polymer Networks

Environmentally friendly crosslinked polymer networks feature degradable covalent or non-covalent bonds, with many of them manifesting dynamic characteristics. These attributes enable convenient degradation, facile reprocessibility, and self-healing capabilities. However, the inherent instability of these crosslinking bonds often compromises the mechanical properties of polymer networks, limiting their practical applications. In this context, environmentally friendly dual-crosslinking polymer networks (denoted EF-DCPNs) have emerged as promising alternatives to address this challenge. These materials effectively balance the need for high mechanical properties with the ability to degrade, recycle, and/or self-heal. Despite their promising potential, investigations into EF-DCPNs remain in their nascent stages, and several gaps and limitations persist. This Review provides a comprehensive overview of the synthesis, properties, and applications of recent progress in EF-DCPNs. Firstly, synthetic routes to a rich variety of EF-DCPNs possessing two distinct types of dynamic bonds (i.e., imine, disulfide, ester, hydrogen bond, coordination bond, and other bonds) are introduced. Subsequently, complex structure- and dynamic nature-dependent mechanical, thermal, and electrical properties of EF-DCPNs are discussed, followed by their exemplary applications in electronics and biotechnology. Finally, future research directions in this rapidly evolving field are outlined.

New Chemistries for Degradable Pressure-Sensitive Adhesive Networks

With the increasing use of pressure-sensitive adhesives (PSAs) in various industries, there is a need for greater sustainability, particularly in developing polymer materials from renewable resources, as well as the reuse and recycling of materials to reduce environmental impact, reduce waste, or extend their life. Here, we outlined the required properties of PSAs which are governed by the molecular parameters (molecular weights, dispersities, molecular weight between entanglement, molecular weight between cross-links and gel content) of polymer materials which subsequently define the physical properties (storage and loss moduli, glass transition temperature) that are required for good performance in peel, tack and shear tests. The sustainable approach discussed here is the development of degradable polymer materials featuring selectively degradable linkages in the backbone. This provides a viable alternative for the design of PSAs that could overcome the 'stickies' problem and make the recycling of glass and cardboard more efficient.

Defining Reactivity-Deconstructability Relationships for Copolymerizations Involving Cleavable Comonomer Additives

The incorporation of cleavable comonomers as additives into polymers can imbue traditional polymers with controlled deconstructability and expanded end-of-life options. The efficiency with which cleavable comonomer additives (CCAs) can enable deconstruction is sensitive to their local distribution within a copolymer backbone, which is dictated by their copolymerization behavior. While qualitative heuristics exist that describe deconstructability, comprehensive quantitative connections between CCA loadings, reactivity ratios, polymerization mechanisms, and deconstruction reactions on the deconstruction efficiency of copolymers containing CCAs have not been established. Here, we broadly define these relationships using stochastic simulations characterizing various polymerization mechanisms (e.g., coltrolled/living, free-radical, and reversible ring-opening polymerizations), reactivity ratio pairs (spanning 2 orders of magnitude between 0.01 and 100), CCA loadings (2.5% to 20%), and deconstruction reactions (e.g., comonomer sequence-dependent deconstruction behavior). We show general agreement between simulated and experimentally observed deconstruction fragment sizes from the literature, demonstrating the predictive power of the methods used herein. These results will guide the development of more efficient CCAs and inform the formulation of deconstructable materials.

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.

Polymeric Nanoparticles for Drug Delivery

The recent emergence of nanomedicine has revolutionized the therapeutic landscape and necessitated the creation of more sophisticated drug delivery systems. Polymeric nanoparticles sit at the forefront of numerous promising drug delivery designs, due to their unmatched control over physiochemical properties such as size, shape, architecture, charge, and surface functionality. Furthermore, polymeric nanoparticles have the ability to navigate various biological barriers to precisely target specific sites within the body, encapsulate a diverse range of therapeutic cargo and efficiently release this cargo in response to internal and external stimuli. However, despite these remarkable advantages, the presence of polymeric nanoparticles in wider clinical application is minimal. This review will provide a comprehensive understanding of polymeric nanoparticles as drug delivery vehicles. The biological barriers affecting drug delivery will be outlined first, followed by a comprehensive description of the various nanoparticle designs and preparation methods, beginning with the polymers on which they are based. The review will meticulously explore the current performance of polymeric nanoparticles against a myriad of diseases including cancer, viral and bacterial infections, before finally evaluating the advantages and crucial challenges that will determine their wider clinical potential in the decades to come.

Ring-Opening Terpolymerisation of Elemental Sulfur Waste with Propylene Oxide and Carbon Disulfide via Lithium Catalysis

Elemental sulfur, a waste product of the oil refinement process, represents a promising raw material for the synthesis of degradable polymers. We show that simple lithium alkoxides facilitate the polymerisation of elemental sulfur S8 with industrially relevant propylene oxide (PO) and CS2 (a base chemical sourced from waste S8 itself) to give poly(monothiocarbonate-alt-Sx) in which x can be controlled by the amount of supplied sulfur. The in situ generation of thiolate intermediates obtained by a rearrangement, which follows CS2 and PO incorporation, allows to combine S8 and epoxides into one polymer sequence that would otherwise not be possible. Mechanistic investigations reveal that alkyl oligosulfide intermediates from S8 ring opening and sulfur chain length equilibration represent the better nucleophiles for inserting the next PO if compared to the trithiocarbonates obtained from the competing CS2 addition, which causes the sequence selectivity. The polymers can be crosslinked in situ with multifunctional thiols to yield reprocessable and degradable networks. Our report demonstrates how mechanistic understanding allows to combine intrinsically incompatible building blocks for sulfur waste utilisation.

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.

Light-driven polymer recycling to monomers and small molecules

Only a small proportion of global plastic waste is recycled, of which most is mechanically recycled into lower quality materials. The alternative, chemical recycling, enables renewed production of pristine materials, but generally comes at a high energy cost, particularly for processes like pyrolysis. This review focuses on light-driven approaches for chemically recycling and upcycling plastic waste, with emphasis on reduced energy consumption and selective transformations not achievable with heat-driven methods. We focus on challenging to recycle backbone structures composed of mainly C‒C bonds, which lack functional groups i.e., esters or amides, that facilitate chemical recycling e.g., by solvolysis. We discuss the use of light, either in conjunction with heat to drive depolymerization to monomers or via photocatalysis to transform polymers into valuable small molecules. The structural prerequisites for these approaches are outlined, highlighting their advantages as well as limitations. We conclude with an outlook, addressing key challenges, opportunities, and provide guidelines for future photocatalyst (PC) development.

Degradable Block Copolymer Nanoparticles Synthesized by Polymerization-Induced Self-Assembly

Polymerization-induced self-assembly (PISA) combines polymerization and in situ self-assembly of block copolymers in one system and has become a widely used method to prepare block copolymer nanoparticles at high concentrations. The persistence of polymers in the environment poses a huge threat to the ecosystem and represents a significant waste of resources. There is an urgent need to develop novel chemical approaches to synthesize degradable polymers. To meet with this demand, it is crucial to install degradability into PISA nanoparticles. Most recently, degradable PISA nanoparticles have been synthesized by introducing degradation mechanisms into either shell-forming or core-forming blocks. This Minireview summarizes the development in degradable block copolymer nanoparticles synthesized by PISA, including shell-degradable, core-degradable, and all-degradable nanoparticles. Future development will benefit from expansion of polymerization techniques with new degradation mechanisms and adaptation of high-throughput approaches for both PISA syntheses and degradation studies.

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.

pH-Controlled Reversible Folding of Copolymers via Formation of β-sheet Secondary Structures

Protein functions are enabled by their perfectly arranged 3D structure, which is the result of a hierarchical intramolecular folding process. Sequence-defined polypeptide chains form locally ordered secondary structures (i.e., α-helix and β-sheet) through hydrogen bonding between the backbone amides, shaping the overall tertiary structure. To generate similarly complex macromolecular architectures based on synthetic materials, a plethora of strategies have been developed to induce and control the folding of synthetic polymers. However, the degree of complexity of the structure-driving ensemble of interactions demonstrated by natural polymers is unreached, as synthesizing long sequence-defined polymers with functional backbones remains a challenge. Herein, we report the synthesis of hybrid peptide-N,N-Dimethylacrylamide copolymers via radical Ring-Opening Polymerization (rROP) of peptide containing macrocycles. The resulting synthetic polymers contain sequence-defined regions of β-sheet encoding amino acid sequences. Exploiting the pH responsiveness of the embedded sequences, protonation or deprotonation in water induces self-assembly of the peptide strands at an intramacromolecular level, driving polymer chain folding via formation of β-sheet secondary structures. We demonstrate that the folding behavior is sequence dependent and reversible.

Quantitative, Regiospecific, and Stereoselective Radical Ring-Opening Polymerization of Monosaccharide Cyclic Ketene Acetals

Cyclic ketene acetals (CKAs) are among the most well-studied monomers for radical ring-opening polymerization (rROP). However, ring-retaining side reactions and low reactivities in homopolymerization and copolymerization remain significant challenges for the existing CKAs. Here, we report that a class of monosaccharide CKAs can be facilely prepared from a short and scalable synthetic route and can undergo quantitative, regiospecific, and stereoselective rROP. NMR analyses and degradation experiments revealed a reaction mechanism involving a propagating radical at the C2 position of pyranose with different monosaccharides exhibiting distinct stereoselectivity in the radical addition of the monomer. Furthermore, the addition of maleimide was found to improve the incorporation efficiency of monosaccharide CKA in the copolymerization with vinyl monomers and produced unique degradable terpolymers with carbohydrate motifs in the polymer backbone.

Stereoselective Ring-opening Polymerization of S-Carboxyanhydrides Using Salen Aluminum Catalysts: A Route to High-Isotactic Functionalized Polythioesters

Polythioesters are important sustainable polymers with broad applications. The ring-opening polymerization (ROP) of S-Carboxyanhydrides (SCAs) can afford polythioesters with functional groups that are typically difficult to prepare by ROP of thiolactones. Typical methods involving organocatalysts, like dimethylaminopyridine (DMAP) and triethylamine (Et3 N), have been plagued by uncontrolled polymerization, including epimerization for most SCAs resulting in the loss of isotacticity. Here, we report the use of salen aluminum catalysts for the selective ROP of various SCAs without epimerization, affording functionalized polythioester with high molecular weight up to 37.6 kDa and the highest Pm value up to 0.99. Notably, the ROP of TlaSCA (SCA prepared from thiolactic acid) generates the first example of a isotactic crystalline poly(thiolactic acid), which exhibited a distinct Tm value of 152.6 °C. Effective ligand tailoring governs the binding affinity between the sulfide chain-end and the metal center, thereby maintaining the activity of organometallic catalysts and reducing the occurrence of epimerization reactions.

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.

Selective Electrocatalytic Degradation of Ether-Containing Polymers

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.

Synthesis of Poly(propylene oxide)-Poly(N,N'-dimethylacrylamide) Diblock Copolymer Nanoparticles via Reverse Sequence Polymerization-Induced Self-Assembly in Aqueous Solution

Sterically-stabilized diblock copolymer nanoparticles comprising poly(propylene oxide) (PPO) cores are prepared via reverse sequence polymerization-induced self-assembly (PISA) in aqueous solution. N,N'-Dimethylacrylamide (DMAC) acts as a cosolvent for the weakly hydrophobic trithiocarbonate-capped PPO precursor. Reversible addition-fragmentation chain transfer (RAFT) polymerization of DMAC is initially conducted at 80% w/w solids with deoxygenated water. At 30-60% DMAC conversion, the reaction mixture is diluted to 5-25% w/w solids. The PPO chains become less solvated as the DMAC monomer is consumed, which drives in situ self-assembly to form aqueous dispersions of PPO-core nanoparticles of 120-190 nm diameter at 20 °C. Such RAFT polymerizations are well-controlled (Mw/Mn ≤ 1.31), and more than 99% DMAC conversion is achieved. The resulting nanoparticles exhibit thermoresponsive character: dynamic light scattering and transmission electron microscopy studies indicate the formation of more compact spherical nanoparticles of approximately 33 nm diameter on heating to 70 °C. Furthermore, 15-25% w/w aqueous dispersions of such nanoparticles formed micellar gels that undergo thermoreversible (de)gelation on cooling to 5 °C.

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.

Development of an Efficient Thionolactone for Radical Ring-Opening Polymerization by a Combined Theoretical/Experimental Approach

The environmental impact of plastic waste has been a real problem for the past decades. The incorporation of cleavable bonds in the polymer backbone is a solution to making a commodity polymer degradable. When radical polymerization is used, this approach is made possible by radical ring-opening polymerization (rROP) of a cyclic monomer that allows for the introduction of a weak bond into the polymer backbone. Among the various cyclic monomers that could be used in rROP, thionolactones are promising structures due to the efficiency of the C═S bond to act as a radical acceptor. Nevertheless, only a few structures were reported to be efficient. In this work, we used DFT calculations to gain a better understanding of the radical reactivity of thionolactones, and in particular, we focused on the transfer rate constant ktr value and its ratio with the propagation rate constant kp of the vinyl monomer. The closer to 1, the better is the statistical incorporation of the two comonomers into the backbone. These theoretical results were in good agreement with all of the experimental data reported in the literature. We thus used this approach to understand the key parameters to tune the reactivity of thionolactone to prepare random copolymers. We identified and prepared the 7-phenyloxepane-2-thione (POT) thionolactone that led to statistical copolymers with styrene and acrylate derivatives that were efficiently degraded under accelerated conditions (KOH in THF/MeOH, TBD in THF, or mCPBA in THF), confirming the theoretical approach. The compatibility with RAFT polymerization as well as the homopolymerization behavior of POT was established. This theoretical approach paves the way for the in-silico design of new efficient thionolactones for rROP.

Controlled-Radical Polymerization of α-Lipoic Acid: A General Route to Degradable Vinyl Copolymers

Here, we present the synthesis and characterization of statistical and block copolymers containing α-lipoic acid (LA) using reversible addition-fragmentation chain-transfer (RAFT) polymerization. LA, a readily available nutritional supplement, undergoes efficient radical ring-opening copolymerization with vinyl monomers in a controlled manner with predictable molecular weights and low molar-mass dispersities. Because lipoic acid diads present in the resulting copolymers include disulfide bonds, these materials efficiently and rapidly degrade when exposed to mild reducing agents such as tris(2-carboxyethyl)phosphine (Mn = 56 → 3.6 kg mol-1). This scalable and versatile polymerization method affords a facile way to synthesize degradable polymers with controlled architectures, molecular weights, and molar-mass dispersities from α-lipoic acid, a commercially available and renewable monomer.

Precise construction of weather-sensitive poly(ester-alt-thioesters) from phthalic thioanhydride and oxetane

We report the selective ring opening copolymerisation (ROCOP) of oxetane and phthalic thioanhydride by a heterobimetallic Cr(III)K catalyst precisely yielding semi-crystalline alternating poly(ester-alt-thioesters) which show improved degradability due to the thioester links in the polymer backbone.

Efficient Synthesis of Hydrolytically Degradable Block Copolymer Nanoparticles via Reverse Sequence Polymerization-Induced Self-Assembly in Aqueous Media

Hydrolytically degradable block copolymer nanoparticles are prepared via reverse sequence polymerization-induced self-assembly (PISA) in aqueous media. This efficient protocol involves the reversible addition-fragmentation chain transfer (RAFT) polymerization of N,N'-dimethylacrylamide (DMAC) using a monofunctional or bifunctional trithiocarbonate-capped poly(ϵ-caprolactone) (PCL) precursor. DMAC monomer is employed as a co-solvent to solubilize the hydrophobic PCL chains. At an intermediate DMAC conversion of 20-60 %, the reaction mixture is diluted with water to 10-25 % w/w solids. The growing amphiphilic block copolymer chains undergo nucleation to form sterically-stabilized PCL-core nanoparticles with PDMAC coronas. 1 H NMR studies confirm more than 99 % DMAC conversion while gel permeation chromatography (GPC) studies indicate well-controlled RAFT polymerizations (Mw /Mn ≤1.30). Transmission electron microscopy (TEM) and dynamic light scattering (DLS) indicate spheres of 20-120 nm diameter. As expected, hydrolytic degradation occurs within days at 37 °C in either acidic or alkaline solution. Degradation is also observed in phosphate-buffered saline (PBS) (pH 7.4) at 37 °C. However, no degradation is detected over a three-month period when these nanoparticles are stored at 20 °C in deionized water (pH 6.7). Finally, PDMAC30 -PCL16 -PDMAC30 nanoparticles are briefly evaluated as a dispersant for an agrochemical formulation based on a broad-spectrum fungicide (azoxystrobin).

Thiol-triggered deconstruction of bifunctional silyl ether terpolymers via an SNAr-triggered cascade

While Si-containing polymers can often be deconstructed using chemical triggers such as fluoride, acids, and bases, they are resistant to cleavage by mild reagents such as biological nucleophiles, thus limiting their end-of-life options and potential environmental degradability. Here, using ring-opening metathesis polymerization, we synthesize terpolymers of (1) a "functional" monomer (e.g., a polyethylene glycol macromonomer or dicyclopentadiene); (2) a monomer containing an electrophilic pentafluorophenyl (PFP) substituent; and (3) a cleavable monomer based on a bifunctional silyl ether . Exposing these polymers to thiols under basic conditions triggers a cascade of nucleophilic aromatic substitution (SNAr) at the PFP groups, which liberates fluoride ions, followed by cleavage of the backbone Si-O bonds, inducing polymer backbone deconstruction. This method is shown to be effective for deconstruction of polyethylene glycol (PEG) based graft terpolymers in organic or aqueous conditions as well as polydicyclopentadiene (pDCPD) thermosets, significantly expanding upon the versatility of bifunctional silyl ether based functional polymers.

Degradable, Ultraviolet-Crosslinked Pressure-Sensitive Adhesives Made from Thioester-Functional Acrylate Copolymers

Pressure-sensitive adhesives (PSAs) are made from soft, irreversibly lightly crosslinked polymers. Even after removal from surfaces, they retain insoluble networks which pose problems during the recycling of glass and cardboard. Herein, degradable PSAs are presented that provide the required performance in use but have networks that can be degraded after use. A series of copolymers was prepared through radical copolymerization of n-butyl acrylate, 4-acryloyloxy benzophenone (ABP) photo-crosslinker, and dibenzo[c,e]oxepin-5(7H)-thione (DOT) to provide degradable backbone thioesters. The optimum tack and peel strengths were found for molar contents of 0.05 mol% ABP and 0.25 mol% DOT. Degradation of the backbone thioesters through aminolysis or thiolysis led to the full dissolution of the networks, loss of adhesive properties of films (decreases in the measured tack and peel strengths), and the quick detachment of model labels from a substrate. Inclusion of DOT into PSAs offers a viable route toward degradable and recyclable packaging labels.

Radical ring-opening polymerization of sustainably-derived thionoisochromanone

We present the synthesis, characterization and radical ring-opening polymerization (rROP) capabilities of thionoisochromanone (TIC), a fungi-derivable thionolactone. TIC is the first reported six-membered thionolactone to readily homopolymerize under free radical conditions without the presence of a dormant comonomer or repeated initiation. Even more, the resulting polymer is fully degradable under mild, basic conditions. Computations providing molecular-level insights into the mechanistic and energetic details of polymerization identified a unique S,S,O-orthoester intermediate that leads to a sustained chain-end. This sustained chain-end allowed for the synthesis of a block copolymer of TIC and styrene under entirely free radical conditions without explicit radical control methods such as reversible addition-fragmentation chain transfer polymerization (RAFT). We also report the statistical copolymerization of ring-retained TIC and styrene, confirmed by elemental analysis and energy-dispersive X-ray spectroscopy (EDX). Computations into the energetic details of copolymerization indicate kinetic drivers for ring-retaining behavior. This work provides the first example of a sustainable feedstock for rROP and provides the field with the first six-membered monomer susceptible to rROP, expanding the monomer scope to aid our fundamental understanding of thionolactone rROP behavior.

Building Tunable Degradation into High-Performance Poly(acrylate) Pressure-Sensitive Adhesives

Pressure-sensitive adhesives (PSAs) based on poly(acrylate) chemistry are common in a wide variety of applications, but the absence of backbone degradability causes issues with recycling and sustainability. Here, we report a strategy to create degradable poly(acrylate) PSAs using simple, scalable, and functional 1,2-dithiolanes as drop-in replacements for traditional acrylate comonomers. Our key building block is α-lipoic acid, a natural, biocompatible, and commercially available antioxidant found in various consumer supplements. α-Lipoic acid and its derivative ethyl lipoate efficiently copolymerize with n-butyl acrylate under conventional free-radical conditions leading to high-molecular-weight copolymers (M_n > 100 kg mol^-1) containing a tunable concentration of degradable disulfide bonds along the backbone. The thermal and viscoelastic properties of these materials are practically indistinguishable from nondegradable poly(acrylate) analogues, but a significant reduction in molecular weight is realized upon exposure to reducing agents such as tris (2-carboxyethyl) phosphine (e.g., M_n = 198 kg mol^-1 → 2.6 kg mol^-1). By virtue of the thiol chain ends produced after disulfide cleavage, degraded oligomers can be further cycled between high and low molecular weights through oxidative repolymerization and reductive degradation. Transforming otherwise persistent poly(acrylates) into recyclable materials using simple and versatile chemistry could play a pivotal role in improving the sustainability of contemporary adhesives.

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.

Polymer Colloids: Current Challenges, Emerging Applications, and New Developments

Polymer colloids are complex materials that have the potential to be used in a vast array of applications. One of the main reasons for their continued growth in commercial use is the water-based emulsion polymerization process through which they are generally synthesized. This technique is not only highly efficient from an industrial point of view but also extremely versatile and permits the large-scale production of colloidal particles with controllable properties. In this perspective, we seek to highlight the central challenges in the synthesis and use of polymer colloids, with respect to both existing and emerging applications. We first address the challenges in the current production and application of polymer colloids, with a particular focus on the transition toward sustainable feedstocks and reduced environmental impact in their primary commercial applications. Later, we highlight the features that allow novel polymer colloids to be designed and applied in emerging application areas. Finally, we present recent approaches that have used the unique colloidal nature in unconventional processing techniques.

Embedding Peptides into Synthetic Polymers: Radical Ring-Opening Copolymerization of Cyclic Peptides

Biopolymers such as proteins and nucleic acids are the key building blocks of life. Synthetic polymers have nevertheless revolutionized our everyday life through their robust synthetic accessibility. Combining the unmatched functionality of biopolymers with the robustness of tailorable synthetic polymers holds the promise to create materials that can be designed ad hoc for a wide array of applications. Radical polymerization is the most widely applied polymerization technique in both fundamental science and industrial polymer production. While this polymerization technique is robust and well controlled, it generally yields unfunctional all-carbon backbones. Combinations of natural polymers such as peptides, with synthetic polymers, are thus limited to tethering peptides onto the side chains or chain ends of the latter. This synthetic limitation is a critical restraint, considering that the function of biopolymers is programmed into the sequence of their main chain (i.e., primary structure). Here, we report the radical copolymerization of peptides and synthetic comonomers yielding synthetic polymers with defined peptide sequences embedded into their main chain. Key was the development of a solid-phase peptide synthesis (SPPS) approach to generate synthetic access to peptide conjugates containing allylic sulfides. Following cyclization, the obtained peptide monomers can be readily copolymerized with N,N-dimethylacrylamide (DMA)─controlled by reversible addition-fragmentation chain transfer (RAFT). Importantly, the developed synthetic strategy is compatible with all 20 standard amino acids and uses exclusively standard SPPS chemicals or chemicals accessible in one-step synthesis─prerequisite for widespread and universal application.

Easy Access to Diverse Multiblock Copolymers with On-Demand Blocks via Thioester-Relayed In-Chain Cascade Copolymerization

Multiblock copolymers are envisioned as promising materials with enhanced properties and functionality compared with their diblock/triblock counterparts. However, the current approaches can construct multiblock copolymers with a limited number of blocks but tedious procedures. Here, we report a thioester-relayed in-chain cascade copolymerization strategy for the easy preparation of multiblock copolymers with on-demand blocks, in which thioester groups with on-demand numbers are built in the polymer backbone by controlled/living polymerizations. These thioester groups further serve as the in-chain initiating centers to trigger the acyl group transfer ring-opening polymerization of episulfides independently and concurrently to extend the polymer backbone into multiblock structures. The compositions, number of blocks, and block degree of polymerization can be easily regulated. This strategy can offer easy access to a library of multiblock copolymers with ≈100 blocks in only 2 to 4 steps.

Divergent Acyl and Decarbonylative Liebeskind-Srogl Cross-Coupling of Thioesters by Cu-Cofactor and Pd-NHC (NHC = N-Heterocyclic Carbene) Catalysis

Transition-metal-catalyzed cross-coupling reactions of thioesters by selective acyl C(O)-S cleavage have emerged as a powerful platform for the preparation of complex molecules. Herein, we report divergent Liebeskind-Srogl cross-coupling of thioesters by Pd-NHC (NHC = N-heterocyclic carbene) catalysis. The reaction provides straightforward access to functionalized ketones by highly selective C(acyl)-S cleavage under mild conditions. Most crucially, the conditions enable direct functionalization of a range of complex pharmaceuticals decorated with a palette of sensitive functional groups, providing attractive products for medicinal chemistry programs. Furthermore, decarbonylative Liebeskind-Srogl cross-coupling by C(acyl)-S/C(aryl)-C(O) cleavage is reported. Cu metal cofactor directs the reaction pathway to acyl or decarbonylative pathway. This reactivity is applicable to complex pharmaceuticals. The reaction represents the mildest decarbonylative Suzuki cross-coupling discovered to date. The Cu-directed divergent acyl and decarbonylative cross-coupling of thioesters opens up chemical space in complex molecule synthesis.

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.

Metal-free synthesis of unsymmetric bis(thioesters)

The first metal-free protocol for efficient synthesis of unsymmetric bis(thioesters) via functionalization of dithiols with two different α,β-unsaturated aldehydes is presented. The methodology described leads to a novel class of dithiol-based building blocks.

Degradable polyisoprene by radical ring-opening polymerization and application to polymer prodrug nanoparticles

Radical ring-opening copolymerization of isoprene and dibenzo[c,e]oxepane-5-thione via free-radical and controlled radical polymerizations led to degradable polyisoprene under basic, oxidative and physiological conditions with application to prodrug nanoparticles.

Addition-Fragmentation Ring-Opening Polymerization of Bio-Based Thiocarbonyl l-Lactide for Dual Degradable Vinyl Copolymers

This study is designed to synthesize novel degradable polymers by radical addition-fragmentation ring-opening copolymerization of bio-based thiocarbonyl compounds with various vinyl monomers. Thiocarbonyl l-lactide is capable of radical copolymerization with acrylates and styrene via radical addition to the carbon-sulfur double bonds followed by ring-opening as well as controlled copolymerization in conjunction with the reversible addition-fragmentation chain transfer (RAFT) process. The obtained polymers possess ring-opened thioester and ring-retained thioacetal functionalities in the backbone, both of which could be cleaved under appropriate conditions with different chemical stimuli.

Polythioesters Prepared by Ring-Opening Polymerization of Cyclic Thioesters and Related Monomers

Polyhydroxyalkanoates (PHAs) are biodegradable and biocompatible polyesters with a wide range of applications; in particular, they currently stand as promising alternatives to conventional polyolefin-based "plastics". The introduction of sulfur atoms within the PHAs backbone can endow the resulting polythioesters (PTEs) with differentiated, sometimes enhanced thermal, optical and mechanical properties, thereby widening their versatility and use. Hence, PTEs have been gaining increasing attention over the past half-decade. This review highlights recent advances towards the synthesis of well-defined PTEs by ring-opening polymerization (ROP) of cyclic thioesters - namely thiolactones - as well as of S-carboxyanhydrides and thionolactones; it also covers the ring-opening copolymerization (ROCOP) of cyclic thioanhydrides or thiolactones with epoxides or episulfides. Most of the ROP reactions described are of anionic type, mediated by inorganic, organic or organometallic initiators/catalysts, along with a few enzymatic reactions as well. Emphasis is placed on the reactivity of the thio monomers, in relation to their ring-size ranging from 4- to 5-, 6- and 7-membered cycles, the nature of the catalyst/initiating systems implemented and their efficiency in terms of activity and control over the PTE molar mass, dispersity, topology, and microstructure.