Photocatalytic Upcycling and Depolymerization of Vinyl Polymers.

Photocatalytic upcycling and depolymerization of vinyl polymers have emerged as promising strategies to combat plastic pollution and promote a circular economy. This mini review critically summarizes current developments in the upcycling and degradation of vinyl polymers including polystyrene and poly(meth)acrylates. Of these material classes, polymethacrylates possess the unique possibility to undergo a photocatalytic depolymerization back to monomer under thermodynamically favourable conditions, thus presenting significant advantages over traditional thermal strategies. Our perspective on current formidable challenges and potential future directions are also discussed.

Chemical Recycling of Polymethacrylates Synthesized by RAFT Polymerization.

Reversing controlled radical polymerization and regenerating the monomer has been a long-standing challenge for fundamental research and practical applications. Herein, we report a highly efficient depolymerization method for various polymethacrylates synthesized by reversible addition-fragmentation chain-transfer (RAFT) polymerization. The depolymerization process, which does not require any catalyst, exhibits near-quantitative conversions of up to 92%. The key aspect of our approach is the utilization of the high end-group fidelity of RAFT polymers to generate chain-end radicals at 120 °C. These radicals trigger a rapid unzipping of the polymethacrylates. The depolymerization product can be utilized to either reconstruct the linear polymer or create an entirely new insoluble gel that can also be subjected to depolymerization. Our depolymerization strategy offers a promising route towards the development of sustainable and efficient recycling methods for complex polymer materials.

Temporal Regulation of PET-RAFT Controlled Radical Depolymerization.

A photocatalytic RAFT-controlled radical depolymerization method is introduced for precisely conferring temporal control under visible light irradiation. By regulating the deactivation of the depropagating chains and suppressing thermal initiation, an excellent temporal control was enabled, exemplified by several consecutive "on" and "off" cycles. Minimal, if any, depolymerization could be observed during the dark periods while the polymer chain-ends could be efficiently re-activated and continue to depropagate upon re-exposure to light. Notably, favoring deactivation resulted in the gradual unzipping of polymer chains and a stepwise decrease in molecular weight over time. This synthetic approach constitutes a simple methodology to modulate temporal control during the chemical recycling of RAFT-synthesized polymers while offering invaluable mechanistic insights.

Fate of the RAFT End-Group in the Thermal Depolymerization of Polymethacrylates.

Thermal RAFT depolymerization has recently emerged as a promising methodology for the chemical recycling of polymers. However, while much attention has been given to the regeneration of monomers, the fate of the RAFT end-group after depolymerization has been unexplored. Herein, we identify the dominant small molecules derived from the RAFT end-group of polymethacrylates. The major product was found to be a unimer (DP = 1) RAFT agent, which is not only challenging to synthesize using conventional single-unit monomer insertion strategies, but also a highly active RAFT agent for methyl methacrylate, exhibiting faster consumption and yielding polymers with lower dispersities compared to the original, commercially available 2-cyano-2-propyl dithiobenzoate. Solvent-derived molecules were also identified predominantly at the beginning of the depolymerization, thus suggesting a significant mechanistic contribution from the solvent. Notably, the formation of both the unimer and the solvent-derived products remained consistent regardless of the RAFT agent, monomer, or solvent employed.

Reversed Controlled Polymerization (RCP): Depolymerization from Well-Defined Polymers to Monomers.

Controlled polymerization methods are well-established synthetic protocols for the design and preparation of polymeric materials with a high degree of precision over molar mass and architecture. Exciting recent work has shown that the high end-group fidelity and/or functionality inherent in these techniques can enable new routes to depolymerization under relatively mild conditions. Converting polymers back to pure monomers by depolymerization is a potential solution to the environmental and ecological concerns associated with the ultimate fate of polymers. This perspective focuses on the emerging field of depolymerization from polymers synthesized by controlled polymerizations including radical, ionic, and metathesis polymerizations. We provide a critical review of current literature categorized according to polymerization technique and explore numerous concepts and ideas which could be implemented to further enhance depolymerization including lower temperature systems, catalytic depolymerization, increasing polymer scope, and controlled depolymerization.

Light-accelerated depolymerization catalyzed by Eosin Y.

Retrieving the starting monomers from polymers synthesized by reversible deactivation radical polymerization has recently emerged as an efficient way to increase the recyclability of such materials and potentially enable their industrial implementation. To date, most methods have primarily focused on utilizing high temperatures (typically from 120 °C to 180 °C) to trigger an efficient depolymerization reaction. In this work, we show that, in the presence of Eosin Y under light irradiation, a much faster depolymerization of polymers made by reversible addition-fragmentation chain-transfer (RAFT) polymerization can be triggered even at a lower temperature (i.e. 100 °C). For instance, green light, in conjunction with ppm amounts of Eosin Y, resulted in the accelerated depolymerization of poly(methyl methacrylate) from 16% (thermal depolymerization at 100 °C) to 37% within 1 hour, and finally 80% depolymerization after 8 hours, as confirmed by both 1H-NMR and SEC analyses. The enhanced depolymerization rate was attributed to the activation of a macroCTA by Eosin Y, thus resulting in a faster macroradical generation. Notably, this method was found to be compatible with different wavelengths (e.g. blue, red and white light irradiation), solvents, and RAFT agents, thus highlighting the potential of light to significantly improve current depolymerization approaches.

Investigating the Effect of End-Group, Molecular Weight, and Solvents on the Catalyst-Free Depolymerization of RAFT Polymers: Possibility to Reverse the Polymerization of Heat-Sensitive Polymers.

Reversing reversible deactivation radical polymerization (RDRP) to regenerate the original monomer is an attractive prospect for both fundamental research and industry. However, current depolymerization strategies are often applied to highly heat-tolerant polymers with a specific end-group and can only be performed in a specific solvent. Herein, we depolymerize a variety of poly(methyl methacrylate) materials made by reversible addition-fragmentation chain-transfer (RAFT) polymerization and terminated by various end groups (dithiobenzoate, trithiocarbonate, and pyrazole carbodithioate). The effect of the nature of the solvent on the depolymerization conversion was also investigated, and key solvents such as dioxane, xylene, toluene, and dimethylformamide were shown to facilitate efficient depolymerization reactions. Notably, our approach could selectively regenerate pure heat-sensitive monomers (e.g., tert-butyl methacrylate and glycidyl methacrylate) in the absence of previously reported side reactions. This work pushes the boundaries of reversing RAFT polymerization and considerably expands the chemical toolbox for recovering starting materials under relatively mild conditions.

Photoinduced Iron-Catalyzed ATRP of Renewable Monomers in Low-Toxicity Solvents: A Greener Approach.

Producing polymers from renewable resources via more sustainable approaches has become increasingly important. Herein we present the polymerization of monomers obtained from biobased renewable resources, employing an environmentally friendly photoinduced iron-catalyzed atom transfer radical polymerization (ATRP) in low-toxicity solvents. We demonstrate that renewable monomers can be successfully polymerized into sustainable polymers with controlled molecular weights and narrow molar mass distributions (D as low as 1.17). This is in contrast to reversible addition-fragmentation chain-transfer (RAFT) polymerization, arguably the most commonly employed method to polymerize biobased monomers, which led to poorer molecular weight control and higher dispersities for these specific monomers (Ds ∼ 1.4). The versatility of our approach was further highlighted by the temporal control demonstrated through intermittent "on/off" cycles, controlled polymerizations of a variety of monomers and chain lengths, oxygen-tolerance, and high end-group fidelity exemplified by the synthesis of block copolymers. This work highlights photoinduced iron-catalyzed ATRP as a powerful tool for the synthesis of renewable polymers.

The Impact of Postoperative Radiotherapy on Dietary Function of Head and Neck Cancer Patients after Pharyngoesophageal Reconstruction with Free Jejunal Flap.

This study investigated the impact of postoperative radiotherapy (PORT) on dietary function in patients who underwent pharyngoesophageal defect reconstruction using a free jejunal flap. A retrospective chart review of 36 patients who underwent circumferential pharyngoesophageal defect reconstruction using a free jejunal flap was performed. The European Organization for the Research and Treatment of Cancer head and neck cancer module questionnaire was used. Five items related to dietary function were selected and analyzed for changes in scores before and after PORT. Both the PORT and non-PORT groups showed improved dietary function before surgery, and no significant changes were noted at 3 and 12 months postoperatively. Repeated measures ANOVA showed that PORT had no significant impact on dietary function. In univariate analysis, no variable was a significant predictor of the score at 12 months, postoperatively. Previous radiotherapy and neck dissection had a close statistically significant relation. The multivariate analysis showed that neoadjuvant chemotherapy, previous radiotherapy, and neck dissection were significant predictors of the score at 12 months, postoperatively. PORT did not show a significant effect on the 12 months postoperative score. Free jejunal flap is an effective pharyngoesophageal defect reconstruction method that does not cause any dietary function disruption after PORT.

Reversing RAFT Polymerization: Near-Quantitative Monomer Generation Via a Catalyst-Free Depolymerization Approach.

The ability to reverse controlled radical polymerization and regenerate the monomer would be highly beneficial for both fundamental research and applications, yet this has remained very challenging to achieve. Herein, we report a near-quantitative (up to 92%) and catalyst-free depolymerization of various linear, bulky, cross-linked, and functional polymethacrylates made by reversible addition-fragmentation chain-transfer (RAFT) polymerization. Key to our approach is to exploit the high end-group fidelity of RAFT polymers to generate chain-end radicals at 120 °C. These radicals trigger a rapid unzipping of both conventional (e.g., poly(methyl methacrylate)) and bulky (e.g., poly(oligo(ethylene glycol) methyl ether methacrylate)) polymers. Importantly, the depolymerization product can be utilized to either reconstruct the linear polymer or create an entirely new insoluble gel that can also be subjected to depolymerization. This work expands the potential of polymers made by controlled radical polymerization, pushes the boundaries of depolymerization, offers intriguing mechanistic aspects, and enables new applications.

Controlling dispersity in aqueous atom transfer radical polymerization: rapid and quantitative synthesis of one-pot block copolymers.

The dispersity (D) of a polymer is a key parameter in material design, and variations in D can have a strong influence on fundamental polymer properties. Despite its importance, current polymerization strategies to control D operate exclusively in organic media and are limited by slow polymerization rates, moderate conversions, significant loss of initiator efficiency and lack of dispersity control in block copolymers. Here, we demonstrate a rapid and quantitative method to tailor D of both homo and block copolymers in aqueous atom transfer radical polymerization. By using excess ligand to regulate the dissociation of bromide ions from the copper deactivator complexes, a wide range of monomodal molecular weight distributions (1.08 < D < 1.60) can be obtained within 10 min while achieving very high monomer conversions (∼99%). Despite the high conversions and the broad molecular weight distributions, very high end-group fidelity is maintained as exemplified by the ability to synthesize in situ diblock copolymers with absolute control over the dispersity of either block (e.g. low D → high D, high D → high D, high D → low D). The potential of our approach is further highlighted by the synthesis of complex pentablock and decablock copolymers without any need for purification between the iterative block formation steps. Other benefits of our methodology include the possibility to control D without affecting the M n, the interesting mechanistic concept that sheds light onto aqueous polymerizations and the capability to operate in the presence of air.

High-Fidelity, Sub-5 nm Patterns from High-χ Block Copolymer Films with Vapor-Deposited Ultrathin, Cross-Linked Surface-Modification Layers.

Despite their capability, sub-10 nm periodic nano-patterns formed by strongly segregating block copolymer (BCP) thin films cannot be easily oriented perpendicular to the substrate due to the huge surface energy differences of the constituent blocks. To produce perpendicular nano-patterns, the interfacial energies of both the substrate and free interfaces should be controlled precisely to induce non-preferential wetting. Unfortunately, high-performance surface modification layers are challenging to design, and different kinds of surface modification methods must be devised respectively for each neutral layer and top coat. Furthermore, conventional approaches, largely based on spin-coating processes, are highly prone to defect formation and may readily cause dewetting at sub-10 nm thickness. To date, these obstacles have hampered the development of high-fidelity, sub-5 nm BCP patterns. Herein, an all-vapor phase deposition approach initiated chemical vapor deposition is demonstrated to form 9-nm-thick, uniform neutral bottom layer and top coat with exquisite control of composition and thickness. These layers are employed in BCP films to produce perpendicular cylinders with a diameter of ≈4 nm that propagate throughout a BCP thickness of up to ≈60 nm, corresponding to five natural domain spacings of the BCP. Such a robust approach will serve as an advancement for the reliable generation of sub-10 nm nano-patterns.

Thermal Approaches to Perpendicular Block Copolymer Microdomains in Thin Films: A Review and Appraisal.

Block copolymer thin films are highly versatile and accessible materials capable of producing nanofeatures in the size regime of a few to hundreds of nanometers by a simple spin-coating-and-anneal process. Unfortunately, this simple protocol usually leads to parallel microdomains, which limits the applicability of such nanofeatures. A great deal of effort has been put into achieving perpendicular microdomains, but those that incorporate thermal annealing are arguably the most practical and reproducible in the lab and industry. This review discusses the recent ongoing efforts on various thermal approaches to achieving perpendicular microdomains in order to provide the readers with a toolbox to work with.

Effect of isoflavone-enriched whole soy milk powder supplementation on bone metabolism in ovariectomized mice.

BACKGROUND/OBJECTIVE: There is intense interest in soy isoflavone as a hormone replacement therapy for the prevention of postmenopausal osteoporosis. A new kind of isoflavone-enriched whole soy milk powder (I-WSM) containing more isoflavones than conventional whole soy milk powder was recently developed. The aim of this study was to investigate the effects of I-WSM on bone metabolism in ovariectomized mice. MATERIALS/METHODS: Sixty female ICR mice individually underwent ovariectomy (OVX) or a sham operation, and were randomized into six groups of 10 animals each as follows: Sham, OVX, OVX with 2% I-WSM diet, OVX with 10% I-WSM diet, OVX with 20% I-WSM diet, and OVX with 20% WSM diet. After an 8-week treatment period, bone mineral density (BMD), calcium, alkaline phosphatase (ALP), tartrate-resistant acid phosphatase (TRAP) 5b, osteocalcin (OC), procollagen 1 N-terminal propeptide (P1NP), and osteoprotegenin (OPG) were analyzed. RESULTS: BMD was significantly lower in the OVX group compared to the Sham group but was significantly higher in OVX + 10% I-WSM and OVX + 20% I-WSM groups compared to the OVX group (P < 0.05). Serum calcium concentration significantly increased in the OVX + 10% and 20% I-WSM groups. Serum ALP levels were significantly lower in the OVX + 10% and 20% I-WSM groups compared to the other experimental groups (P < 0.05). OC was significantly reduced in the OVX group compared to the Sham group (P < 0.05), but a dose-dependent increase was observed in the OVX groups supplemented with I-WSM. P1NP and OPG levels were significantly reduced, while TRAP 5b level was significantly elevated in the OVX group compared with the Sham group, which was not affected by I-WSM (P < 0.05). CONCLUSIONS: This study suggests that I-WSM supplementation in OVX mice has the effect of preventing BMD reduction and promoting bone formation. Therefore, I-WSM can be used as an effective alternative to postmenopausal osteoporosis prevention.

Effect of isoflavone-enriched whole soy milk powder supplementation on bone metabolism in ovariectomized mice

BACKGROUND/OBJECTIVE: There is intense interest in soy isoflavone as a hormone replacement therapy for the prevention of postmenopausal osteoporosis. A new kind of isoflavone-enriched whole soy milk powder (I-WSM) containing more isoflavones than conventional whole soy milk powder was recently developed. The aim of this study was to investigate the effects of I-WSM on bone metabolism in ovariectomized mice. MATERIALS/METHODS: Sixty female ICR mice individually underwent ovariectomy (OVX) or a sham operation, and were randomized into six groups of 10 animals each as follows: Sham, OVX, OVX with 2% I-WSM diet, OVX with 10% I-WSM diet, OVX with 20% I-WSM diet, and OVX with 20% WSM diet. After an 8-week treatment period, bone mineral density (BMD), calcium, alkaline phosphatase (ALP), tartrate-resistant acid phosphatase (TRAP) 5b, osteocalcin (OC), procollagen 1 N-terminal propeptide (P1NP), and osteoprotegenin (OPG) were analyzed. RESULTS: BMD was significantly lower in the OVX group compared to the Sham group but was significantly higher in OVX + 10% I-WSM and OVX + 20% I-WSM groups compared to the OVX group (P < 0.05). Serum calcium concentration significantly increased in the OVX + 10% and 20% I-WSM groups. Serum ALP levels were significantly lower in the OVX + 10% and 20% I-WSM groups compared to the other experimental groups (P < 0.05). OC was significantly reduced in the OVX group compared to the Sham group (P < 0.05), but a dose-dependent increase was observed in the OVX groups supplemented with I-WSM. P1NP and OPG levels were significantly reduced, while TRAP 5b level was significantly elevated in the OVX group compared with the Sham group, which was not affected by I-WSM (P < 0.05). CONCLUSIONS: This study suggests that I-WSM supplementation in OVX mice has the effect of preventing BMD reduction and promoting bone formation. Therefore, I-WSM can be used as an effective alternative to postmenopausal osteoporosis prevention.

Three-Dimensional Multilayered Nanostructures from Crosslinkable Block Copolymers.

Block copolymer (BCP) lithography has generally been synonymous to one- or two-dimensional single layered lithographic templates as a means to fabricate simple nanoscaled structures. Recently, the rapidly increasing demand for complex nanostructures and the corresponding evolution in BCP lithography have led to three-dimensional (3D) BCP nanostructures, which can be fabricated in various ways such as directed self-assembly or stacking of cross-linked BCP patterns. This review covers the recent advances in the 3D multilayered structures from cross-linkable BCPs, which provide an easy and robust means for integrating various BCP structures into one scaffold. In this case, wetting-optimized adjustment of BCP microdomains at the layer interface plays a critical role in the formation of well-defined 3D multilayer nanostructures.