Unleashing the Power of PET-RAFT Polymerization: Journey from Porphyrin-Based Photocatalysts to Combinatorial Technologies and Advanced Bioapplications

The emergence of photoinduced energy/electron transfer-reversible addition-fragmentation chain transfer polymerization (PET-RAFT) not only revolutionized the field of photopolymerization but also accelerated the development of porphyrin-based photocatalysts and their analogues. The continual expansion of the monomer family compatible with PET-RAFT polymerization enhances the range of light radiation that can be harnessed, providing increased flexibility in polymerization processes. Furthermore, the versatility of PET-RAFT polymerization extends beyond its inherent capabilities, enabling its integration with various technologies in diverse fields. This integration holds considerable promise for the advancement of biomaterials with satisfactory bioapplications. As researchers delve deeper into the possibilities afforded by PET-RAFT polymerization, the collaborative efforts of individuals from diverse disciplines will prove invaluable in unleashing its full potential. This Review presents a concise introduction to the fundamental principles of PET-RAFT, outlines the progress in photocatalyst development, highlights its primary applications, and offers insights for future advancements in this technique, paving the way for exciting innovations and applications.

Near-Infrared Light and Acid/Base Dual-Regulated Polymerization Utilizing Imidazole-Anion-Fused Perylene Diimides as Photocatalysts

This work presents the first example of acid/base-responsive and near-infrared (NIR)-absorbing photocatalysts based on imidazole-anion-fused perylene diimide chromophores. The photocatalysts were in situ generated by deprotonation of imidazole-fused perylene diimide under an alkaline environment. NIR (λ = 730 nm, 128 mW/cm2) photoinduced atom transfer radical polymerization (ATRP) was implemented, exhibiting high efficiency and excellent livingness under ppm level of photocatalysts (15 ppm relative to monomer) and Cu(II) complex (10 ppm relative to monomer) concentrations. The method showed capabilities to polymerize behind opaque barriers (i.e., paper and pig skin) and under aerobic condition. Notably, this work demonstrated a dual temporal control of polymerization by adding weak base/acid and switching NIR light on/off. The polymerization can even be halted by bubbling CO2 and was then fully recovered by adding triethylamine. The NIR photoATRP of acrylamide monomers in aqueous solution was also performed, which can be regulated by the change of pH.

Tris-benzo[cd]indole Cyanine Enables the NIR-photosensitized Radical and Thiol-ene Polymerizations at 940 nm

A near-infrared-absorbing heptamethine (HM+ ) incorporating three bulky benzo[cd]indole heterocycles was designed to efficiently prevent self-aggregation of the dye, which results in a strong enhancement of its photoinitiating reactivity as compared to a parent bis-benzo[cd]indole heptamethine (HMCl+ ) used as a reference system. In this context, we highlight an efficient free-radical NIR-polymerization up to a 100 % acrylates C=C bonds conversion even under air conditions. Such an important initiating performance was obtained by incorporating our NIR-sensitizer into a three-component system leading to its self-regeneration. This original photoredox cycle was thoroughly investigated through the identification of each intermediary species using EPR spectroscopy.

Switchable DNA Photocatalysts for Radical Polymerization Controlled by Chemical Stimuli

Polymerization catalysts that activate in response to specific chemical triggers offer spatial and temporal control over polymer synthesis, facilitating the development of responsive materials and custom polymer coatings. However, existing catalysts switch their activity through mechanisms that are not generalizable to chemically diverse stimuli. To approach the level of control exhibited in biological polymer synthesis, switchable polymerization catalysts need to be configurable for activation in response to diverse chemical stimuli. Here, we combine synthetic photocatalysts with conformation-switching DNA aptamers to create polymerization catalysts that respond to diverse chemical stimuli. We use the secondary structure of DNA to bring a photocatalyst and quencher dye into proximity, turning off photocatalysis. The DNA structure can be precisely designed to change conformation in response to a molecular trigger, moving the photocatalyst far from the quencher and activating photocatalysis. We show these photocatalysts can initiate free-radical polymerization to form bulk hydrogels in response to complementary DNA, a metal ion (Zn2+), or small molecules (glucose and hydrocortisone). We demonstrate the biocompatibility of these switchable photocatalysts by triggering their activation on the surface of yeast cells. Finally, we perform reversible-deactivation radical polymerization through photoinduced electron/energy transfer reversible addition-fragmentation chain-transfer in a dual-stimulus manner, in which catalytic activity is regulated reversibly by photoirradiation and the conformational state of the DNA catalyst. These results demonstrate that DNA conformational changes triggered by chemically diverse stimuli can regulate the activity of radical polymerization photocatalysts. This platform offers new capabilities in spatially and temporally controlled polymer synthesis, with potential applications in diagnostics, sensing, and environmentally responsive materials.

Biomimetic controlled radical photopolymerization in a two-dimensional organized environment under visible light

Fast and well-controlled photoinduced atom transfer radical polymerization (photoATRP) in the organized medium of a bilayer activated by visible light under environmentally friendly mild aqueous conditions leads to polymers with predetermined molecular weight and low dispersity. The decisive parameter for photoATRP of monomers in the organized medium was their mobility and orientation with respect to the bilayer and the photoredox catalyst localized in the interstitial layer.

Azacalixphyrins as an innovative alternative for the free-radical photopolymerization under visible and NIR irradiation without the need of co-initiators

Azacalixphyrins are unique aromatic macrocycles featuring strong absorption from the visible to the near-infrared (NIR) spectral ranges. This work demonstrates through EPR spin-trapping experiments that the N-alkyl tetrasubstituted azacalixphyrin (ACP) can lead to the formation of carbon-centered radicals initiating for the free-radical photopolymerization (FRP) of bio-based acrylate monomer upon the irradiation of several light emitting diodes, which emissions range from 455 to 660 nm. Compared to other previously reported systems, the tremendous advantage of the ACP photoinitiating system is its ability to promote photopolymerization on its own, avoiding the introduction of co-initiators. A new potential application of this promising photoinitiator is highlighted through the fabrication of well-defined microstructures under NIR laser diode irradiation at λ = 800 nm.

One-step synthesis of poly(methacrylate)-b-polyester via “one organocatalyst, two polymerizations”

In a “one organocatalyst, two polymerizations” system, triarylsulfonium hexafluorophosphate salt could spontaneously catalyze photo-ATRP and ROP. A well-defined PTMC-b-PMMA block copolymer was successfully synthesized in one-step.

Selective Monofunctionalization Enabled by Reaction-History-Dependent Communication in Catalytic Rotaxanes

Selective monofunctionalization of substrates with distant, yet equally reactive functional groups is difficult to achieve, as it requires the second functional group to selectively modulate its reactivity once the first functional group has reacted. We now show that mechanically interlocked catalytic rings can effectively regulate the reactivity of stoppering groups in rotaxanes over a distance of about 2 nm. Our mechanism of communication is enabled by a unique interlocked design, which effectively removes the catalytic rings from the substrates by fast dethreading as soon as the first reaction has taken place. Our method not only led to a rare example of selective monofunctionalization, but also to a "molecular if function". Overall, the study presents a way to get distant functional groups to communicate with each other in a reaction-history-dependent manner by creating linkers that can ultimately perform logical operations at the molecular level.

Magnetic Janus nanocomposites with iridium(iii) complexes for heterogeneous catalysis of logic controlled RAFT polymerization using multiplexed external switching

Photoinduced electron/energy transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization has emerged as a versatile and highly-efficient method for the polymerization of more activated monomers including N,N-dimethylacrylamide and methyl acrylate, and less activated monomers including N-vinylpyrrolidone and vinyl acetate, whilst imposing composition, sequence and spatiotemporal regulation. Although significant progress has been achieved in terms of ability to regulate PET-RAFT polymerization through the implementation of myriad environmental cues, it is still a great challenge to introduce multiple external switches within a single catalyst to accomplish logic toggling of controlled radical polymerization (CRP). Herein, we report the synthesis and characterization of Fe₃O₄@aSiO₂@PNMIr Janus nanocomposites coupled with immobilized heteroleptic iridium(iii) complexes for heterogeneous catalysis of PET-RAFT polymerization. With this catalytic nanoarchitecture, we demonstrate multi-stimuli switching of CRPs using three different external physical manipulations: light "ON"/"OFF", magnet "OUT"/"IN" and temperature "LOW"/"HIGH". In addition, these magnetic Janus nanocomposites endowed radical polymerization with various attractive characteristics such as compatibility of myriad monomer formulations including "more activated" and "less activated" monomers, unique oxygen tolerance and ppm-level catalyst dosage. Logic-controlled polymerization with Fe₃O₄@aSiO₂@PNMIr nanocomposites provides a straightforward, robust and user-friendly strategy for realizing multiplexed external switching of polymer propagation using a single nanocatalyst without the involvement of exogenous reagents.

Chemiluminescence-Induced Free Radical-Promoted Cationic Polymerization

Chemiluminescence (CL) has recently been featured as a new external light source for various photoinduced reactions with attractive features such as eliminating continuous energy supply and advanced light source setups. In the present study, the free-radical-promoted cationic polymerization of cyclohexene oxide, n-butyl vinyl ether, and N-vinyl carbazole under CL irradiation is described. The method is based on the visible-light-induced generation of electron donor radicals from bis-(4-methoxybenzoyl)diethyl germane (BAG), bis(2,4,6-trimethylbenzoyl) phenyl phosphinate, and camphorquinone by CL illumination followed by electron transfer to diphenyl iodonium hexafluorophosphate (Ph₂ I⁺ PF₆ ^- ) to form corresponding cations capable of initiating cationic polymerization. The applicability of the process to network formation is also demonstrated by using a bifunctional monomer, tri(ethylene glycol) divinyl ether.

Role of External Field in Polymerization: Mechanism and Kinetics

The past decades have witnessed an increasing interest in developing advanced polymerization techniques subjected to external fields. Various physical modulations, such as temperature, light, electricity, magnetic field, ultrasound, and microwave irradiation, are noninvasive means, having superb but distinct abilities to regulate polymerizations in terms of process intensification and spatial and temporal controls. Gas as an emerging regulator plays a distinctive role in controlling polymerization and resembles a physical regulator in some cases. This review provides a systematic overview of seven types of external-field-regulated polymerizations, ranging from chain-growth to step-growth polymerization. A detailed account of the relevant mechanism and kinetics is provided to better understand the role of each external field in polymerization. In addition, given the crucial role of modeling and simulation in mechanisms and kinetics investigation, an overview of model construction and typical numerical methods used in this field as well as highlights of the interaction between experiment and simulation toward kinetics in the existing systems are given. At the end, limitations and future perspectives for this field are critically discussed. This state-of-the-art research progress not only provides the fundamental principles underlying external-field-regulated polymerizations but also stimulates new development of advanced polymerization methods.

Diphenyl functional porphyrins and their metal complexes as visible-light photoinitiators for free-radical, cationic and thiol–ene polymerizations

Diphenyl functional porphyrin derivatives as new visible-light photoinitiators for efficient free-radical, cationic and thiol–ene polymerizations.

Robust hollow nanocomposites with ruthenium-bipyridine complexes for heterogeneous catalysis of logic-controlled RAFT polymerization

Photoinduced electron/energy transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization has become a powerful and eco-friendly toolkit to create well-defined macromolecular buildups while exhibiting composition, sequence and spatiotemporal control. Although PET-RAFT polymerization is generally much more convenient than living ionic polymerization, it is still a great challenge to regulate the polymerization upon multiple external stimuli and to simplify the procedures of post-polymerization purification. In this contribution, hHPGE-PFPPNRu nanocomposites were engineered as catalyst supports to firmly accommodate ruthenium-bipyridine complexes for heterogeneous catalysis of PET-RAFT polymerization. The manipulation of reaction temperature modulated the performance of the nanocatalysts, with a pronounced acceleration of the polymerization kinetics being identified at a temperature above the lower critical solution temperature (LCST) of poly(N-isopropylacrylamide) (PNIPAM) brushes compared to that below it. Consequently, the control of RAFT polymerization can be achieved upon the dual-stimuli of light and heat. Moreover, these nanocatalysts conferred radical polymerizations with myriad attractive features such as the adaptability of diverse monomer formulations and reaction media, exquisite control over the molecular variables, oxygen tolerance, and catalyst doses in the ppm range. Owing to the robust mechanical nature of nanocomposites, the separation and reuse of the nanocatalysts were readily realized by rapid centrifugation, and they showed inappreciable catalyst leakage along with consistent catalytic performance even after multiple polymerization runs.

100th Anniversary of Macromolecular Science Viewpoint: Photochemical Reaction Orthogonality in Modern Macromolecular Science

The ability to perform multiple chemical reactions independently (orthogonally) in a single reaction vessel can allow simplified reaction protocols for intricate chemical syntheses. Light is an especially advantageous external stimuli to enact such orthogonal chemical reactions due to its independence with other stimuli, instantaneous spatiotemporal control, and material penetrability. The potential to combine orthogonal chemistry and polymerization is also very appealing, as these systems may open the door for polymeric materials to find applications in emerging and high-tech fields, including biotechnology, microelectronics, sensors, energy, and others. We highlight the use of light in orthogonal polymerization protocols, particularly for living and controlled polymerization, and explore potential future directions and challenges for this technology.

Seeing the Light: Advancing Materials Chemistry through Photopolymerization

The application of photochemistry to polymer and material science has led to the development of complex yet efficient systems for polymerization, polymer post-functionalization, and advanced materials production. Using light to activate chemical reaction pathways in these systems not only leads to exquisite control over reaction dynamics, but also allows complex synthetic protocols to be easily achieved. Compared to polymerization systems mediated by thermal, chemical, or electrochemical means, photoinduced polymerization systems can potentially offer more versatile methods for macromolecular synthesis. We highlight the utility of light as an energy source for mediating photopolymerization, and present some promising examples of systems which are advancing materials production through their exploitation of photochemistry.

The influence of mixing on chain extension by photo-controlled/living radical polymerization under continuous-flow conditions

Continuous-flow chemistry holds powerful potential for polymer synthesis, and has attracted increasing attention in recent years.

Photochemistry for Well-Defined Polymers in Aqueous Media: From Fundamentals to Polymer Nanoparticles to Bioconjugates

This review article highlights recent developments in the field of photochemistry and photochemical reversible deactivation radical polymerization applied to aqueous polymerizations. Photochemistry is a topic of significant interest in the fields of organic, polymer, and materials chemistry because it allows challenging reactions to be performed under mild conditions. Aqueous polymerization is of significant interest because water is an environmentally benign solvent, and the use of water enables complex polymer self-assembly and bioconjugation processes to occur. This review focuses on powerful new developments in photochemical aqueous polymerization reactions and their applications to the synthesis of well-defined polymer nano-objects and bioconjugates. It is anticipated that these aqueous photopolymerizations will enable the next generation of self-assembled structures and biohybrid materials to be developed under mild and environmentally friendly conditions.

A sustainable photocontrolled ATRP strategy: facile separation and recycling of a visible-light-mediated catalyst fac-[Ir(ppy)3]

A visible-light-mediated catalyst (fac-[Ir(ppy)₃]) in situ separation and recycling ATRP system for PEG-based water-soluble monomers was constructed.

Mechanistic Insight into the Photocontrolled Cationic Polymerization of Vinyl Ethers

The mechanism of the recently reported photocontrolled cationic polymerization of vinyl ethers was investigated using a variety of catalysts and chain-transfer agents (CTAs) as well as diverse spectroscopic and electrochemical analytical techniques. Our study revealed a complex activation step characterized by one-electron oxidation of the CTA. This oxidation is followed by mesolytic cleavage of the resulting radical cation species, which leads to the generation of a reactive cation-this species initiates the polymerization of the vinyl ether monomer-and a dithiocarbamate radical that is likely in equilibrium with the corresponding thiuram disulfide dimer. Reversible addition-fragmentation type degenerative chain transfer contributes to the narrow dispersities and control over chain growth observed under these conditions. Finally, the deactivation step is contingent upon the oxidation of the reduced photocatalyst by the dithiocarbamate radical concomitant with the production of a dithiocarbamate anion that caps the polymer chain end. The fine-tuning of the electronic properties and redox potentials of the photocatalyst in both the excited and the ground states is necessary to obtain a photocontrolled system rather than simply a photoinitiated system. The elucidation of the elementary steps of this process will aid the design of new catalytic systems and their real-world applications.

Visible light-induced PET-RAFT polymerization of methacrylates with novel organic photocatalysts

PET-RAFT polymerization was successfully carried out under irradiation of various LED lights at room temperature by using two novel organic agents 4-methoxybenzaldehyde and 2,4,6-tri-(p-methoxyphenyl) pyrylium tetrafluoroborate for the first time.