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.

Water-Soluble Reversible Photo-Cross-Linking Polymer Dielectrics

Effective recycling of mixed materials requires the separation of the different components without the need for toxic solvents. One approach involves utilizing a water-soluble coating with reversible photo-cross-linkers, making it robust until end of life where it can then be dissolved in water after de-cross-linking. Here, a novel coumarin methacrylate monomer and its nitroxide-mediated copolymerization to create poly((methacrylic acid)-co-(styrene sulfonate)-co-(coumarin methacrylate)) for water-soluble thin films are reported. Under exposure to light, the coumarin functional groups produce reversible [2+2] cycloadditions which cross-link the resulting polymer films, making them no longer water soluble. Characterization of reversible cross-linking behavior is reported through changes in contact angle and in situ rheological characterization. The resulting polymers are successfully integrated into metal-insulator-metal capacitors, demonstrating the potential use for water-soluble reversible photo-cross-linkable dielectric materials for organic electronics.

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.

A Regioselective [3 + 2] Cycloaddition of Alkynols and Ketones To Access Diverse 1,3-Dioxolane Scaffolds

This study presents a stepwise exoselective [3 + 2] cycloaddition reaction of alkynols with ketones, leading to the synthesis of 4-methylene-1,3-dioxolane derivatives. Remarkably, without any Thorpe-Ingold induced effect, the cyclization reaction was demonstrated with complete regio- and chemoselectivity, which was solely promoted by cesium carbonate. A wide range of unactivated ketones are viable under these mild reaction conditions, and both primary and tertiary alkynols are compatible with these cyclization reactions. We have prepared a diverse array of highly dense exomethylene 1,3-dioxolane rings demonstrating a remarkable tolerance for various functional groups.

Radical Ring Opening Polymerization of Cyclic Ketene Acetals Derived From d-Glucal

A cyclic ketene acetal (CKA) derived from d-glucal was synthesized, and its polymerization using free radicals has been investigated. NMR analysis of the resulting polymers revealed the formation of polyacetal-polyester copolymers, with up to 78% of ester linkages formed by radical ring-opening polymerization (rROP). Conversely, the polymerization of the monomer-saturated analogue only produced acetal linkages, demonstrating that the alkene functionality within the d-glucal pyranose ring is essential to promote ring-opening and ester formation, likely via the stabilization of an allyl radical. The thermal properties of the polymers were linked to the ratio of the ester and acetal linkages. Copolymerization with methyl methacrylate (MMA) afforded statistically PMMA-rich copolymers (66-98%) with linkages prone to hydrolytic degradation and decreased glass-transition temperatures. The retention of the pseudoglucal alkene function offers opportunities to functionalize further these bioderived (co)polymers.

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).

Current Standing on Radical Ring-Opening Polymerizations of Cyclic Ketene Acetals as Homopolymers and Copolymers with one another

Radical Ring-opening polymerization (RROP) of cyclic ketene acetals (CKAs) emerges to be a valuable polymerization technique. In attracting more attention, RROP has seen a new spike in publications, which the authors will put into perspective. This review will hence address the progress made on the number of available CKAs and the synthetic strategies to get them. In grouping, the available monomers into distinct categories, the enormous variety of available CKAs will be highlighted. Polymerizations of CKAs without vinylenes have the potential to yield fully biodegradable polymers, which is why this kind of polymerization is the focus of this review. Detailing the current understanding of the mechanism, the various side reactions will be noted and also their effect on the overall properties of the final polymers. Current attempts to control the ring-retaining and branching reactions will be discussed as well. In addition to the polymerization itself, the available materials will be discussed as well as homopolymers, copolymers of CKAs, and block-copolymers with pure CKA-blocks have significantly widened the range of possible applications of materials from RROP. Altogether this review highlights the progress in the entire field of RROP just of CKAs to give a holistic overview of the field.

Increasing the Hydrophilicity of Cyclic Ketene Acetals Improves the Hydrolytic Degradation of Vinyl Copolymers and the Interaction of Glycopolymer Nanoparticles with Lectins

Radical ring-opening polymerization (rROP) of cyclic ketene acetals (CKAs) with traditional vinyl monomers allows the synthesis of degradable vinyl copolymers. However, since the most commonly used CKAs are hydrophobic, most degradable vinyl copolymers reported so far degrade very slowly by hydrolysis under physiological conditions (phosphate-buffered saline, pH 7.4, 37 °C), which can be detrimental for biomedical applications. Herein, to design advanced vinyl copolymers by rROP with high CKA content and enhanced degradation profiles, we reported the copolymerization of 2-methylene-1,3,6-trioxocane (MTC) as a CKA with vinyl ether (VE) or maleimide (MI) derivatives. By performing a point-by-point comparison between the MTC/VE and MTC/MI copolymerization systems, and their counterparts based on 2-methylene-1,3-dioxepane (MDO) and 5,6-benzo-2-methylene-1,3-dioxepane (BMDO), we showed negligible impact on the macromolecular characteristics and similar reactivity ratios, suggesting successful substitution of MDO and BMDO by MTC. Interestingly, owing to the hydrophilicity of MTC, the obtained copolymers exhibited a faster hydrolytic degradation under both accelerated and physiological conditions. We then prepared MTC-based glycopolymers, which were formulated into surfactant-free nanoparticles, exhibiting excellent colloidal stability up to 4 months and complete degradation under enzymatic conditions. Importantly, MTC-based glyconanoparticles also showed a similar cytocompatibility toward two healthy cell lines and a much stronger lectin affinity than MDO-based glyconanoparticles.

Degradable Glycopolyester-like Nanoparticles by Radical Ring-Opening Polymerization

A small library of degradable polyester-like glycopolymers was successfully prepared by the combination of radical ring-opening copolymerization of 2-methylene-1,3-dioxepane as a cyclic ketene acetal (CKA) with vinyl ether (VE) derivatives and a Pd-catalyzed thioglycoconjugation. The resulting thioglycopolymers were formulated into self-stabilized thioglyconanoparticles, which were stable up to 4 months and were enzymatically degraded. Nanoparticles and their degradation products exhibited a good cytocompatibility on two healthy cell lines. Interactions between thioglyconanoparticles and lectins were investigated and highlighted the presence of both specific carbohydrate/lectin interactions and nonspecific hydrophobic interactions. Fluorescent thioglyconanoparticles were also prepared either by encapsulation of Nile red or by the functionalization of the polymer backbone with rhodamine B. Such nanoparticles were used to prove the cell internalization of the thioglyconanoparticles by lung adenocarcinoma (A549) cells, which underlined the great potential of P(CKA-co-VE) copolymers for biomedical applications.

Vinyl copolymers with faster hydrolytic degradation than aliphatic polyesters and tunable upper critical solution temperatures

Vinyl polymers are the focus of intensive research due to their ease of synthesis and the possibility of making well-defined, functional materials. However, their non-degradability leads to environmental problems and limits their use in biomedical applications, allowing aliphatic polyesters to still be considered as the gold standards. Radical ring-opening polymerization of cyclic ketene acetals is considered the most promising approach to impart degradability to vinyl polymers. However, these materials still exhibit poor hydrolytic degradation and thus cannot yet compete with traditional polyesters. Here we show that a simple copolymerization system based on acrylamide and cyclic ketene acetals leads to well-defined and cytocompatible copolymers with faster hydrolytic degradation than that of polylactide and poly(lactide-co-glycolide). Moreover, by changing the nature of the cyclic ketene acetal, the copolymers can be either water-soluble or can exhibit tunable upper critical solution temperatures relevant for mild hyperthermia-triggered drug release. Amphiphilic diblock copolymers deriving from this system can also be formulated into degradable, thermosensitive nanoparticles by an all-water nanoprecipitation process.

Degradable Vinyl Random Copolymers via Photocontrolled Radical Ring-Opening Cascade Copolymerization

Degradable vinyl polymers by radical ring-opening polymerization are promising solutions to the challenges caused by non-degradable vinyl plastics. However, achieving even distributions of labile functional groups in the backbone of degradable vinyl polymers remains challenging. Herein, we report a photocatalytic approach to degradable vinyl random copolymers via radical ring-opening cascade copolymerization (rROCCP). The rROCCP of macrocyclic allylic sulfones and acrylates or acrylamides mediated by visible light at ambient temperature achieved near-unity comonomer reactivity ratios over the entire range of the feed compositions. Experimental and computational evidence revealed an unusual reversible inhibition of chain propagation by in situ generated sulfur dioxide (SO2), which was successfully overcome by reducing the solubility of SO2. This study provides a powerful approach to degradable vinyl random copolymers with comparable material properties to non-degradable vinyl polymers.

Synthesis of degradable and chemically recyclable polymers using 4,4-disubstituted five-membered cyclic ketene hemiacetal ester (CKHE) monomers

Novel degradable and chemically recyclable polymers were synthesized using five-membered cyclic ketene hemiacetal ester (CKHE) monomers. The studied monomers were 4,4-dimethyl-2-methylene-1,3-dioxolan-5-one (DMDL) and 5-methyl-2-methylene-5-phenyl-1,3-dioxolan-4-one (PhDL). The two monomers were synthesized in high yields (80-90%), which is an attractive feature. DMDL afforded its homopolymer with a relatively high molecular weight (M n >100 000, where M n is the number-average molecular weight). DMDL and PhDL were copolymerized with various families of vinyl monomers, i.e., methacrylates, acrylates, styrene, acrylonitrile, vinyl pyrrolidinone, and acrylamide, and various functional methacrylates and acrylate. Such a wide scope of the accessible polymers is highly useful for material design. The obtained homopolymers and random copolymers of DMDL degraded in basic conditions (in the presence of a hydroxide or an amine) at relatively mild temperatures (room temperature to 65 °C). The degradation of the DMDL homopolymer generated 2-hydroxyisobutyric acid (HIBA). The generated HIBA was recovered and used as an ingredient to re-synthesize DMDL monomer, and this monomer was further used to re-synthesize the DMDL polymer, demonstrating the chemical recycling of the DMDL polymer. Such degradability and chemical recyclability of the DMDL polymer may contribute to the circular materials economy.

ε-Thionocaprolactone: an accessible monomer for preparation of degradable poly(vinyl esters) by radical ring-opening polymerization

Readily accessible ε-thionocaprolactone can be copolymerized with vinyl esters under radical polymerization conditions to produce copolymers containing degradable thioester and thioacetal linkages.

Towards nanoparticles with site-specific degradability by ring-opening copolymerization induced self-assembly in organic medium

Radical ring-opening copolymerization-induced self-assembly (rROPISA) was successfully applied to the synthesis of core-, surface- or surface plus core-degradable nanoparticles in heptane, leading to site-specific degradability by rROPISA.

100th Anniversary of Macromolecular Science Viewpoint: Degradable Polymers from Radical Ring-Opening Polymerization: Latest Advances, New Directions, and Ongoing Challenges

Radical ring-opening polymerization (rROP) allows facile incorporation of labile groups (e.g., ester) into the main chain of vinyl polymers to obtain (bio)degradable materials. rROP has focused a lot of attention especially since the advent of reversible deactivation radical polymerization (RDRP) techniques and is still incredibly moving forward, as attested by the numerous achievements in terms of monomer synthesis, macromolecular engineering, and potential biomedical applications of the resulting degradable polymers. In the present Viewpoint, we will cover the latest progress made in rROP in the last ∼5 years, such as its recent directions, its remaining limitations, and the ongoing challenges. More specifically, this will be achieved through the three different classes of monomers that recently caught most of the attention: cyclic ketene acetals (CKA), thionolactones, and macrocyclic monomers.

Controlled Synthesis of Poly(pentafluorostyrene-ran-methyl methacrylate) Copolymers by Nitroxide Mediated Polymerization and Their Use as Dielectric Layers in Organic Thin-film Transistors

A library of statistically random pentafluorostyrene (PFS) and methyl methacrylate (MMA) copolymers with narrow molecular weight distributions was produced, using nitroxide mediated polymerization (NMP) to study the effect of polymer composition on the performance of bottom-gate top-contact organic thin-film transistors, when utilized as the dielectric medium. Contact angle measurements confirmed the ability to tune the surface properties of copolymer thin films through variation of its PFS/MMA composition, while impedance spectroscopy determined the effect of this variation on dielectric properties. Bottom-gate, top-contact copper phthalocyanine (CuPc) based organic thin-film transistors were fabricated using the random copolymers as a dielectric layer. We found that increasing the PFS content led to increased field-effect mobility, until a point after which the CuPc no longer adhered to the polymer dielectric.

DFT-calculation-assisted prediction of the copolymerization between cyclic ketene acetals and traditional vinyl monomers

The ring-opening polymerization of cyclic ketene acetals (CKAs) and vinyl monomers is an elegant method to produce degradable copolymers. Owing to DFT calculations, we are now able to better understand the reactivity of CKAs & common vinyl monomers.

Reversible-deactivation radical polymerization of cyclic ketene acetals

This review discusses the history of reversible-deactivation radical ring-opening polymerization of cyclic ketene acetals, focusing on the preparation of degradable complex polymeric architectures.

Degradable Copolymer Nanoparticles from Radical Ring-Opening Copolymerization between Cyclic Ketene Acetals and Vinyl Ethers

2-Methylene-1,3-dioxepane (MDO) and different vinyl ether (VE) monomers were successfully copolymerized by free-radical radical ring-opening copolymerization (rROP) to yield P(MDO- co-VE) copolymers with M_n = 7 000-13 000 g·mol^-1 and high molar fractions of MDO ( F_MDO = 0.7-0.9). By using VE derivatives of different aqueous solubilities or by grafting PEG chains onto the copolymers by "click" chemistry via azide-containing VE units, hydrophobic, amphiphilic and water-soluble copolymers were obtained. The different copolymers were then formulated into nanoparticles by nanoprecipitation using Pluronics for hydrophobic copolymers, without surfactant for amphiphilic copolymers, or blended with PMDO for water-soluble copolymers. Most of the copolymers led to nanoparticles with average diameters in the 130-250 nm with narrow particle size distributions and satisfying colloidal stability for a period of at least 1-2 weeks and up to 6 months. The copolymers were successfully degraded under accelerated, hydrolytic or enzymatic conditions. Hydrophobic copolymers led to degradation kinetics in PBS similar to that of PCL and complete degradation (-95% in M_n decrease) was observed in the presence of enzymes (lipases). Preliminary cytotoxicity assays were performed on endothelial cells (HUVEC) and macrophages (J774.A1) and revealed high cell viabilities at 0.1 mg·mL^-1.

Degradable polymer prodrugs with adjustable activity from drug-initiated radical ring-opening copolymerization

Degradable polymer prodrugs based on gemcitabine (Gem) as an anticancer drug were synthesized by 'drug-initiated' nitroxide-mediated radical ring-opening copolymerization (NMrROP) of methacrylic esters and 2-methylene-4-phenyl-1,3-dioxolane (MPDL). Different structural parameters were varied to determine the best biological performances: the nature of the monomer [i.e., oligo(ethylene glycol) methacrylate (OEGMA) or methyl methacrylate (MMA)], the nature of the Gem-polymer linker (i.e., amide or amide and diglycolate) and the MPDL content in the copolymer. Depending on the nature of the methacrylate monomer, two small libraries of water-soluble copolymer prodrugs and nanoparticles were obtained (M n ∼10 000 g mol-1, Đ = 1.1-1.5), which exhibited tunable hydrolytic degradation under accelerated conditions governed by the MPDL content. Drug-release profiles in human serum and in vitro anticancer activity on different cell lines enabled preliminary structure-activity relationships to be established. The cytotoxicity was independently governed by: (i) the MPDL content - the lower the MPDL content, the greater the cytotoxicity; (ii) the nature of the linker - the presence of a labile diglycolate linker enabled a greater Gem release compared to a simple amide bond and (iii) the hydrophilicity of the methacrylate monomer-OEGMA enabled a greater anticancer activity to be obtained compared to MMA-based polymer prodrugs. Remarkably, the optimal structural parameters enabled reaching the cytotoxic activity of the parent (free) drug.

Metal Free Reversible-Deactivation Radical Polymerizations: Advances, Challenges, and Opportunities

A considerable amount of the worldwide industrial production of synthetic polymers is currently based on radical polymerization methods. The steadily increasing demand on high performance plastics and tailored polymers which serve specialized applications is driven by the development of new techniques to enable control of polymerization reactions on a molecular level. Contrary to conventional radical polymerization, reversible-deactivation radical polymerization (RDRP) techniques provide the possibility to prepare polymers with well-defined structures and functionalities. The review provides a comprehensive summary over the development of the three most important RDRP methods, which are nitroxide mediated radical polymerization, atom transfer radical polymerization and reversible addition fragmentation chain transfer polymerization. The focus thereby is set on the newest developments in transition metal free systems, which allow using these techniques for biological or biomedical applications. After each section selected examples from materials synthesis and application to biomedical materials are summarized.

Radical Ring-Opening Copolymerization of Cyclic Ketene Acetals and Maleimides Affords Homogeneous Incorporation of Degradable Units

Radical copolymerization of donor-acceptor (D-A) monomer pairs has served as a versatile platform for the development of alternating copolymers. However, due to the use of conventional radical polymerization, the resulting copolymers have generally been limited to nondegradable vinyl polymers. By combining radical D-A copolymerization with radical ring-opening polymerization (rROP), we have synthesized an alternating copolymer with a high incorporation of degradable backbone units. Copolymerization of N-ethyl maleimide (NEtMI) with the cyclic ketene acetal (CKA) 2-methylene-4-phenyl-1,3-dioxolane (MPDL) was demonstrated to proceed in an alternating fashion, and controlled polymerization was achieved using reversible addition-fragmentation chain transfer (RAFT) polymerization. Spontaneous copolymerization, in the absence of an exogenous initiating source, occurred when the mixture of monomers was heated, presumably due to the large electron disparity between the comonomers. Chain-extension with styrene afforded well-defined P(MPDL-alt-NEtMI)-b-polystyrene copolymers, and degradation of the homopolymers and block copolymers showed complete breakdown of the alternating copolymer.