Programming Photodegradability into Vinylic Polymers via Radical Ring-Opening Polymerization

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.

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.

Photochemical [2 + 2] Cycloaddition Enables the Synthesis of Highly Thermally Stable and Acid/Base-Resistant Polyesters from a Nonpolymerizable α,β-Conjugated Valerolactone

We report a simple strategy to transform a nonpolymerizable six-membered α,β-conjugated lactone, 5,6-dihydro-2H-pyran-2-one (DPO), into polymerizable bicyclic lactones via photochemical [2 + 2] cycloaddition. Two bicyclic lactones, M1 and M2, were obtained by the photochemical [2 + 2] cycloaddition of tetramethylethylene and DPO. Ring-opening polymerization (ROP) of M1 and M2 catalyzed by diphenyl phosphate (DPP), La[N(SiMe3)2]3, and 1-tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris (dimethylamino) phosphoranylide-namino]-2λ5, 4λ5-catenadi(phosphazene) (tBu-P4) were conducted. M1 is highly polymerizable, either DPP or La[N(SiMe3)2]3 could catalyze its living ROP under mild conditions, affording the well-defined PM1 with a predictable molar mass and low dispersity. M2 could only be polymerized with tBu-P4 as the catalyst, also generating the same polymer PM1. PM1 has high thermal stability, with a Td,5% being up to 376 °C. Ring-opening copolymerization (ROcP) of M1 and δ-valerolactone (δ-VL) catalyzed by La[N(SiMe3)2]3 afforded a series of random copolymers with enhanced thermal stabilities. Both PM1 and the copolymer containing 10 mol % M1 exhibited excellent resistance to acidic and basic hydrolysis. Our results demonstrate that direct photochemical [2 + 2] cycloaddition of α,β-conjugated valerolactone is not only a strategy to tune its polymerizability, but also allows for the synthesis of highly thermally stable aliphatic polyesters, inaccessible by other methods.

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.

Reversible Thiyl Radical Addition-Fragmentation Chain Transfer Polymerization

Developing reversible-deactivation radical polymerization (RDRP) methods that could directly control the thiyl radical propagation is highly desirable yet remains challenging in modern polymer chemistry. Here, we reported the first reversible thiyl radical addition-fragmentation chain transfer (SRAFT) polymerization strategy, which utilizes allyl sulfides as chain transfer agents for reversibly deactivating the propagating thiyl radicals, thus allowing us to directly control a challenging thiyl radical chain polymerization to afford polymers with well-defined architectures. A linear dependence of molecular weight on conversion, high chain-end fidelity, and efficient chain extension proved good controllability of the polymerization. In addition, density functional theory calculations provided insight into the reversible deactivation ability of allyl sulfides. The SRAFT strategy developed in this work represents a promising platform for discovering new controlled polymerizations based on thiyl radical chemistry.

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.

A Strategy for Controlling the Polymerizations of Thiyl Radical Propagation by RAFT Agents

The ability to extend the polymerizations of thiyl radical propagation to be regulated by existing controlled methods would be highly desirable, yet remained very challenging to achieve because the thiyl radicals still cannot be reversibly controlled by these methods. In this article, we reported a novel strategy that could enable the radical ring-opening polymerization of macrocyclic allylic sulfides, wherein propagating specie is thiyl radical, to be controlled by reversible addition-fragmentation chain transfer (RAFT) agents. The key to the success of this strategy is the propagating thiyl radical can undergo desulfurization with isocyanide and generate a stabilized alkyl radical for reversible control. Systematic optimization of the reaction conditions allowed good control over the polymerization, leading to the formation of polymers with well-defined architectures, exemplified by the radical block copolymerization of macrocyclic allylic sulfides and vinyl monomers and the incorporation of sequence-defined segments into the polymer backbone. This work represents a significant step toward directly enabling the polymerizations of heteroatom-centered radical propagation to be regulated by existing reversible-deactivation radical polymerization techniques.