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

Multi-Generation Recycling of Thermosets Enabled by Fragment Reactivation

Thermosets are used in numerous industrial applications due to their excellent stabilities and mechanical properties; however, their covalently cross-linked structures limit chemical circularity. Cleavable comonomers (CCs) offer a practical strategy to impart new end-of-life opportunities, such as deconstructability or remoldability, to thermosets without altering critical properties, cost, or manufacturing workflows. Nevertheless, CC-enabled recycling of thermosets has so far been limited to one cycle with a 25% recycled content. Here, we introduce a "fragment reactivation" strategy, wherein the oligomeric fragments obtained from CC-enabled thermoset deconstruction are activated with functional groups that improve fragment solubility and reactivity for subsequent rounds of recycling. Using polydicyclopentadiene (pDCPD), an industrial hydrocarbon thermoset material, containing low loadings of a siloxane-based CC, we first demonstrate two rounds of chemical recycling by incorporating 40 wt % norbornene silyl ether-reactivated fragments derived from the prior generation's deconstruction. Then, we show that the two-step sequence of deconstruction and reactivation can be unified into a single-step process, referred to as "deconstructive reactivation." Using this approach, we demonstrate three rounds of chemical recycling with 40-45 wt % fragments incorporated per cycle while maintaining key material properties and deconstructability. These three generations of recycling effectively extend the lifespan of deconstructable pDCPD thermosets by ∼2.6 times. Combined with CCs, fragment reactivation presents a promising and potentially generalizable strategy to improve the chemical recycling efficiency of thermosets.

Monomer Design Enables Mechanistic Mapping of Anionic Ring-Opening Polymerization of Aromatic Thionolactones

Degradable chalcogenide polyesters, e.g., polythioesters (PTEs), typically exhibit improved thermal, mechanical, and optical properties. Anionic ring-opening polymerization (ROP) of thionolactones, an intrinsically promising yet underexplored approach to accessing PTEs, however, is still limited by: intolerance of metal catalysts, inadequate control over chain growth, and the absence of aromatic system. Monomer design-boosted mechanistic studies may address the above challenges. Here, we present a new and highly reactive thionolactone synthesized from 1,1'-binaphthyl-2,2'-diol (BINOL). Our investigations into polymerization kinetics and thermodynamics have underscored the importance of rapid initiation, eventually leading to the discovery of tetrabutylammonium 2-naphthyl-thiocarboxylate as a distinctive initiator that enables genuinely controlled and living polymerization of thionolactones. Ultimately, the atropisomerism inherent in BINOL has resulted in the creation of axially chiral PTE materials with tailored molecular weights, enantiomeric compositions, and topologies.

Polysulfones Prepared by Radical Ring-Opening Polymerization of Cyclic Sulfolane Derivatives: Density Functional Theory Calculations, Synthesis, Structure, and Polymer Reactions

Recently, the radical ring-opening polymerization of cyclic monomers has become one of the most important topics because it can impart degradability to vinyl polymers by introducing functional groups and heteroatoms into the main chain through copolymerization with vinyl monomers. In this study, we investigated the possibility of the ring-opening polymerization of the cyclic sulfone compounds, including sulfolane and sulfolene derivatives. First, the reactions of 2,5-dimethyl-3-sulfolene (DMS) were predicted using density functional theory (DFT) calculations, and the reaction product was actually examined after heating in the presence of a radical initiator. Next, we synthesized and polymerized 2-vinylsulfolane (2VS), which has been reported to undergo radical ring-opening polymerization in the literature and tried to modify the resulting polymers containing the unsaturated groups in the main chain via post-polymerization reaction by ene-thiol additions. We also examined the decomposition behavior of the polymer of 2VS before and after hydrogenation. Furthermore, we discussed the reactivity of 3-exomethylenesulfolane (3EMS), which is expected to exhibit radical ring-opening polymerization similar to 2VS, as well as the structure and reactivity of the corresponding polymer.

Radical Ring-Opening Polymerization: Unlocking the Potential of Vinyl Polymers for Drug Delivery, Tissue Engineering, and More

Synthetic vinyl polymers have long been recognized for their potential to be utilized in drug delivery, tissue engineering, and other biomedical applications. The synthetic control that chemists have over their structure and properties is unmatched, allowing vinyl polymer-based materials to be precisely engineered for a range of therapeutic applications. Yet, their lack of biodegradability compromises the biocompatibility of vinyl polymers and has held back their translation into clinically used treatments for disease thus far. In recent years, radical ring-opening polymerization (rROP) has emerged as a promising strategy to render synthetic vinyl polymers biodegradable and bioresorbable. While rROP has long been touted as a strategy for preparing biodegradable vinyl polymers for biomedical applications, the translation of rROP into clinically approved treatments for disease has not yet been realized. This review highlights the opportunities for leveraging rROP to render vinyl polymers biodegradable and unlock their potential for use in biomedical applications.

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.

Scalable Synthesis of Degradable Copolymers Containing α-Lipoic Acid via Miniemulsion Polymerization

A robust method is described to synthesize degradable copolymers under aqueous miniemulsion conditions using α-lipoic acid as a cheap and scalable building block. Simple formulations of α-lipoic acid (up to 10 mol %), n-butyl acrylate, a surfactant, and a costabilizer generate stable micelles in water with particle sizes <200 nm. The ready availability of these starting materials facilitated performing polymerization reactions at large scales (4 L), yielding 600 g of poly(n-butyl acrylate-stat-α-lipoic acid) latexes that degrade under reducing conditions (250 kg mol-1 → 20 kg mol-1). Substitution of α-lipoic acid with ethyl lipoate further improves the solubility of dithiolane derivatives in n-butyl acrylate, resulting in copolymers that degrade to even lower molecular weights after polymerization and reduction. In summary, this convenient and scalable strategy provides access to large quantities of degradable copolymers and particles using cheap and commercially available starting materials.

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.

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.

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.