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 bromine-terminated polymers synthesized by ATRP.

Chemical recycling of polymers is one of the biggest challenges in materials science. Recently, remarkable achievements have been made by utilizing polymers prepared by controlled radical polymerization to trigger low-temperature depolymerization. However, in the case of atom transfer radical polymerization (ATRP), depolymerization has nearly exclusively focused on chlorine-terminated polymers, even though the overwhelming majority of polymeric materials synthesized with this method possess a bromine end-group. Herein, we report an efficient depolymerization strategy for bromine-terminated polymethacrylates which employs an inexpensive and environmentally friendly iron catalyst (FeBr2/L). The effect of various solvents and the concentration of metal salt and ligand on the depolymerization are judiciously explored and optimized, allowing for a depolymerization efficiency of up to 86% to be achieved in just 3 minutes. Notably, the versatility of this depolymerization is exemplified by its compatibility with chlorinated and non-chlorinated solvents, and both Fe(ii) and Fe(iii) salts. This work significantly expands the scope of ATRP materials compatible with depolymerization and creates many future opportunities in applications where the depolymerization of bromine-terminated polymers is desired.

Photocatalytic ATRP Depolymerization: Temporal Control at Low ppm of Catalyst Concentration.

A photocatalytic ATRP depolymerization is introduced that significantly suppresses the reaction temperature from 170 to 100 °C while enabling temporal regulation. In the presence of low-toxicity iron-based catalysts and under visible light irradiation, near-quantitative monomer recovery could be achieved (up to 90%), albeit with minimal temporal control. By employing ppm concentrations of either FeCl2 or FeCl3, the depolymerization during the dark periods could be completely eliminated, thus enabling temporal control and the possibility to modulate the rate by simply turning the light "on" and "off". Notably, our approach allowed preservation of the end-group fidelity throughout the reaction, could be carried out at high polymer loadings (up to 2M), and was compatible with various polymers and light sources. This methodology provides a facile, environmentally friendly, and temporally regulated route to chemically recycle ATRP-synthesized polymers, thus opening the door for further opportunities.

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.

Oxygen-Enhanced Atom Transfer Radical Polymerization through the Formation of a Copper Superoxido Complex.

In controlled radical polymerization, oxygen is typically regarded as an undesirable component resulting in terminated polymer chains, deactivated catalysts, and subsequent cessation of the polymerization. Here, we report an unusual atom transfer radical polymerization whereby oxygen favors the polymerization by triggering the in situ transformation of CuBr/L to reactive superoxido species at room temperature. Through a superoxido ARGET-ATRP mechanism, an order of magnitude faster polymerization rate and a rapid and complete initiator consumption can be achieved as opposed to when unoxidized CuBr/L was instead employed. Very high end-group fidelity has been demonstrated by mass-spectrometry and one-pot synthesis of block and multiblock copolymers while pushing the reactions to reach near-quantitative conversions in all steps. A high molecular weight polymer could also be targeted (DPn = 6400) without compromising the control over the molar mass distributions (D < 1.20), even at an extremely low copper concentration (4.5 ppm). The versatility of the technique was demonstrated by the polymerization of various monomers in a controlled fashion. Notably, the efficiency of our methodology is unaffected by the purity of the starting CuBr, and even a brown highly-oxidized 15-year-old CuBr reagent enabled a rapid and controlled polymerization with a final dispersity of 1.07, thus not only reducing associated costs but also omitting the need for rigorous catalyst purification prior to polymerization.

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.

Transformer-Induced Metamorphosis of Polymeric Nanoparticle Shape at Room Temperature.

Controlled polymerizations have enabled the production of nanostructured materials with different shapes, each exhibiting distinct properties. Despite the importance of shape, current morphological transformation strategies are limited in polymer scope, alter the chemical structure, require high temperatures, and are fairly tedious. Herein we present a rapid and versatile morphological transformation strategy that operates at room temperature and does not impair the chemical structure of the constituent polymers. By simply adding a molecular transformer to an aqueous dispersion of polymeric nanoparticles, a rapid evolution to the next higher-order morphology was observed, yielding a range of morphologies from a single starting material. Significantly, this approach can be applied to nanoparticles produced by disparate block copolymers obtained by various synthetic techniques including emulsion polymerization, polymerization-induced self-assembly and traditional solution self-assembly.

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.

Shape-Controlled Nanoparticles from a Low-Energy Nanoemulsion.

Nanoemulsion technology enables the production of uniform nanoparticles for a wide range of applications. However, existing nanoemulsion strategies are limited to the production of spherical nanoparticles. Here, we describe a low-energy nanoemulsion method to produce nanoparticles with various morphologies. By selecting a macro-RAFT agent (poly(di(ethylene glycol) ethyl ether methacrylate-co-N-(2-hydroxypropyl) methacrylamide) (P(DEGMA-co-HPMA))) that dramatically lowers the interfacial tension between monomer droplets and water, we can easily produce nanoemulsions at room temperature by manual shaking for a few seconds. With the addition of a common ionic surfactant (SDS), these nanoscale droplets are robustly stabilized at both the formation and elevated temperatures. Upon polymerization, we produce well-defined block copolymers forming nanoparticles with a wide range of controlled morphologies, including spheres, worm balls, worms, and vesicles. Our nanoemulsion polymerization is robust and well-controlled even without stirring or external deoxygenation. This method significantly expands the toolbox and availability of nanoemulsions and their tailor-made polymeric nanomaterials.

Tuning Dispersity by Photoinduced Atom Transfer Radical Polymerisation: Monomodal Distributions with ppm Copper Concentration.

Dispersity significantly affects the properties of polymers. However, current methods for controlling the polymer dispersity are limited to bimodal molecular weight distributions, adulterated polymer chains, or low end-group fidelity and rely on feeding reagents, flow-based, or multicomponent systems. To overcome these limitations, we report a simple batch system whereby photoinduced atom transfer radical polymerisation is exploited as a convenient and versatile technique to control dispersity of both homopolymers and block copolymers. By varying the concentration of the copper complex, a wide range of monomodal molecular weight distributions can be obtained (D=1.05-1.75). In all cases, high end-group fidelity was confirmed by MALDI-ToF-MS and exemplified by efficient block copolymer formation (monomodal, D=1.1-1.5). Importantly, our approach utilises ppm levels of copper (as low as 4 ppm), can be tolerant to oxygen and exhibits perfect temporal control, representing a major step forward in tuning polymer dispersity for various applications.

Effect of Polymerization Components on Oxygen-Tolerant Photo-ATRP.

Photo-ATRP has recently emerged as a powerful technique that allows for oxygen-tolerant polymerizations and the preparation of polymers with low dispersity and high end-group fidelity. However, the effect of various photo-ATRP components on oxygen consumption and polymerization remains elusive. Herein, we employ an in situ oxygen probe and UV-vis spectroscopy to elucidate the effects of ligand, initiator, monomer, and solvent on oxygen consumption. We found that the choice of photo-ATRP components significantly impacts the rate at which the oxygen is consumed and can subsequently affect both the polymerization time and the dispersity of the resulting polymer. Importantly, we discovered that using the inexpensive ligand TREN results in the fastest oxygen consumption and shortest polymerization time, even though no appreciable reduction of CuBr2 is observed. This work provides insight into oxygen consumption in photo-ATRP and serves as a guideline to the judicious selection of photo-ATRP components for the preparation of well-defined polymers.

Multiphoton 3D Printing of Biopolymer-Based Hydrogels.

Multiphoton lithography, based on multiphoton polymerization, is a powerful technique for the fabrication of complex three-dimensional (3D) structures. Herein, we report on the photostructuring of novel biopolymer-based hybrid hydrogels, comprising gelatin methacrylamide and a water-soluble chitosan derivative, via multiphoton polymerization. The nontoxic, Food and Drug Administration-approved, biocompatible photosensitizer eosin Y was exploited as the sole photoinitiator, without the coinitiators and/or comonomer that are commonly used, allowing for further expansion of the available wavelengths up to 800 nm. Importantly, the obtained hybrid material exhibits excellent biocompatibility, evidenced by the increased proliferation of dental pulp stem cells, compared with the individual components and the polystyrene control, after 7 days in culture. Additionally, the 3D hybrid scaffolds promote the matrix mineralization, following their functionalization with bone morphogenetic protein 2. These tailor-made synthetic, biocompatible materials pave the way for further opportunities in 3D scaffold fabrication, including in situ and in vivo biofabrication.

Tailoring polymer dispersity and shape of molecular weight distributions: methods and applications.

The width and shape of molecular weight distributions can significantly affect the properties of polymeric materials and thus are key parameters to control. This mini-review aims to critically summarise recent approaches developed to tailor molecular weight distributions and highlights the strengths and limitations of each technique. Special emphasis will also be given to applications where tuning the molecular weight distribution has been used as a strategy to not only enhance polymer properties but also to increase the fundamental understanding behind complex mechanisms and phenomena.