Degradable Polystyrene via the Cleavable Comonomer Approach

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.

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.

Marine litter and microplastic pollution in mangrove sediments in the Sea of Oman

Marine litter and microplastic pollution in mangroves pose significant threats. This study of litter in Omani mangroves revealed an average density of 0.83 to 21.92 items/m2. Quriyat lagoon emerged as the most contaminated area, hosting 133 items/m2, while Qurum Natural Reserve lagoon showed the least contamination at 10 items/m2. Plastics constituted 73-96 % of the litter, with microplastic levels in sediment ranging from 6 to 256 pieces/kg. Al-Sawadi's lagoon had the highest microplastic abundance (27.52 ± 5.32 pieces/kg), in contrast to Al Qurum's Marine Protected Area with the lowest (0.60 ± 1.12 pieces/kg). Microplastics, categorized as fragments, pellets, and fibers, were distributed across zones 40.05 % landward, 30.97 % seaward, and 28.98 % in the middle mangrove zones. Primary plastic polymers identified were Polyethylene (PE) at 40 % and High-Density Polyethylene (HDPE) at 28 %, along with others found in specific areas. Our findings provide essential baseline data for future monitoring efforts and management strategies in Oman and other countries.

Chitinous Bioplastic Enabled by Noncovalent Assembly

Natural polymeric-based bioplastics usually lack good mechanical or processing performance. It is still challenging to achieve simultaneous improvement for these two usual trade-off features. Here, we demonstrate a full noncovalent mediated self-assembly design for simultaneously improving the chitinous bioplastic processing and mechanical properties via plane hot-pressing. Tannic acid (TA) is chosen as the noncovalent mediator to (i) increase the noncovalent cross-link intensity for obtaining the tough noncovalent network and (ii) afford the dynamic noncovalent cross-links to enable the mobility of chitin molecular chains for benefiting chitinous bioplastic nanostructure rearrangement during the shaping procedure. The multiple noncovalent mediated network (chitin-TA and chitin-chitin cross-links) and the pressure-induced orientation nanofibers structure endow the chitinous bioplastics with robust mechanical properties. The relatively weak chitin-TA noncovalent interactions serve as water mediation switches to enhance the molecular mobility for endowing the chitin/TA bioplastic with hydroplastic processing properties, rendering them readily programmable into versatile 2D/3D shapes. Moreover, the fully natural resourced chitinous bioplastic exhibits superior weld, solvent resistance, and biodegradability, enabling the potential for diverse applications. The full physical cross-linking mechanism highlights an effective design concept for balancing the trade-off of the mechanical properties and processability for the polymeric materials.

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.

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).

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.

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.

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.