Natural RAFT Polymerization: Recyclable-Catalyst-Aided, Opened-to-Air, and Sunlight-Photolyzed RAFT Polymerizations

Recent Advances in Monocomponent Visible Light Photoinitiating Systems Based on Sulfonium Salts

During the last decades, multicomponent photoinitiating systems have been the focus of intense research efforts, especially for the design of visible light photoinitiating systems. Although highly reactive three-component and even four-component photoinitiating systems have been designed, the complexity to elaborate such mixtures has incited researchers to design monocomponent Type II photoinitiators. Using this approach, the photosensitizer and the radical/cation generator can be combined within a unique molecule, greatly simplifying the elaboration of the photocurable resins. In this field, sulfonium salts are remarkable photoinitiators but these structures lack absorption in the visible range. Over the years, various structural modifications have been carried out in order to redshift their absorptions in the visible region. In this work, an overview of the different sulfonium salts activable under visible light and reported to date is proposed.

Tyrosine residues initiated photopolymerization in living organisms

Towards intracellular engineering of living organisms, the development of new biocompatible polymerization system applicable for an intrinsically non-natural macromolecules synthesis for modulating living organism function/behavior is a key step. Herein, we find that the tyrosine residues in the cofactor-free proteins can be employed to mediate controlled radical polymerization under 405 nm light. A proton-coupled electron transfer (PCET) mechanism between the excited-state TyrOH* residue in proteins and the monomer or the chain transfer agent is confirmed. By using Tyr-containing proteins, a wide range of well-defined polymers are successfully generated. Especially, the developed photopolymerization system shows good biocompatibility, which can achieve in-situ extracellular polymerization from the surface of yeast cells for agglutination/anti-agglutination functional manipulation or intracellular polymerization inside yeast cells, respectively. Besides providing a universal aqueous photopolymerization system, this study should contribute a new way to generate various non-natural polymers in vitro or in vivo to engineer living organism functions and behaviours.

Recent Advances on Furan-Based Visible Light Photoinitiators of Polymerization

Photopolymerization is an active research field enabling to polymerize in greener conditions than that performed with traditional thermal polymerization. At present, a great deal of effort is devoted to developing visible light photoinitiating systems. Indeed, the traditional UV photoinitiating systems are currently the focus of numerous safety concerns so alternatives to UV light are being actively researched. However, visible light photons are less energetic than UV photons so the reactivity of the photoinitiating systems should be improved to address this issue. In this field, furane constitutes an interesting candidate for the design of photocatalysts of polymerization due to its low cost and its easy chemical modification. In this review, an overview concerning the design of furane-based photoinitiators is provided. Comparisons with reference systems are also established to demonstrate evidence of the interest of these photoinitiators in innovative structures.

Excitation Dependence in Photoiniferter Polymerization

We report on a fundamental feature of photoiniferter polymerizations mediated with trithiocarbonates and xanthates. The polymerizations were found to be highly dependent on the activated electronic excitation of the iniferter. Enhanced rates of polymerization and greater control over molecular weights were observed for trithiocarbonate- and xanthate-mediated photoiniferter polymerizations when the n → π* transition of the iniferter was targeted compared to the polymerizations activating the π → π* transition. The disparities in rates of polymerization were attributed to the increased rate of C-S photolysis which was confirmed using model trapping studies. This study provides valuable insight into the role of electronic excitations in photoiniferter polymerization and provides guidance when selecting irradiation conditions for applications where light sensitivity is important.

A Perspective on the History and Current Opportunities of Aqueous RAFT Polymerization

Reversible addition-fragmentation chain transfer (RAFT) polymerization has proven itself as a powerful polymerization technique affording facile control of molecular weight, molecular weight distribution, architecture, and chain end groups - while maintaining a high level of tolerance for solvent and monomer functional groups. RAFT is highly suited to water as a polymerization solvent, with aqueous RAFT now utilized for applications such as controlled synthesis of ultra-high molecular weight polymers, polymerization induced self-assembly, and biocompatible polymerizations, among others. Water as a solvent represents a non-toxic, cheap, and environmentally friendly alternative to organic solvents traditionally utilized for polymerizations. This, coupled with the benefits of RAFT polymerization, makes for a powerful combination in polymer science. This perspective provides a historical account of the initial developments of aqueous RAFT polymerization at the University of Southern Mississippi from the McCormick Research Group, details practical considerations for conducting aqueous RAFT polymerizations, and highlights some of the recent advances aqueous RAFT polymerization can provide. Finally, some of the future opportunities that this versatile polymerization technique in an aqueous environment can offer are discussed, and it is anticipated that the aqueous RAFT polymerization field will continue to realize these, and other exciting opportunities into the future.

A dual initiator approach for oxygen tolerant RAFT polymerization

Reversible-deactivation radical polymerizations are privileged approaches for the synthesis of functional and hybrid materials. A bottleneck for conducting these processes is the need to maintain oxygen free conditions. Herein we report a broadly applicable approach to "polymerize through" oxygen using the synergistic combination of two radical initiators having different rates of homolysis. The in situ monitoring of the concentrations of oxygen and monomer simultaneously provided insight into the function of the two initiators and enabled the identification of conditions to effectively remove dissolved oxygen and control polymerization under open-to-air conditions. By understanding how the surface area to volume ratio of reaction vessels influence open-to-air polymerizations, well-defined polymers were produced using acrylate, styrenic, and methacrylate monomers, which each represent an expansion of scope for the "polymerizing through" oxygen approach. Demonstration of this method in tubular reactors using continuous flow chemistry provided a more complete structure-reactivity understanding of how reaction headspace influences PTO RAFT polymerizations.

Air-tolerant Reversible Complexation Mediated Polymerization (RCMP) Using Aldehyde as Oxygen Removera

An air-tolerant reversible complexation mediated polymerization (RCMP) technique, which can be carried out without prior deoxygenation, was developed. The system contains a monomer, an alkyl iodide initiating dormant species, air (oxygen), an aldehyde, N-hydroxyphthalimide (NHPI), and a base. Oxygen is consumed via the NHPI-catalyzed conversion of the aldehyde (RCHO) to a carboxylic acid (RCOOH). The generated RCOOH is further converted to a carboxylate anion (RCOO^- ) by the base. The RCOO^- generated in situ works as an RCMP catalyst; the polymerization proceeds with the monomer, alkyl iodide dormant species, and RCOO^- catalyst. Thus, the system is not only air-tolerant but also does not require additional RCMP catalysts, which is a notable feature of this system. (NHPI is used as an oxidation catalyst for converting RCHO to RCOOH.) This technique is amenable to methyl methacrylate, butyl methacrylate, benzyl methacrylate, 2-hydroxyethyl methacrylate, and styrene, yielding polymers with relatively low-dispersity (M_w /M_n = 1.20-1.49), where M_w and M_n are the weight- and number-average molecular weights, respectively. This article is protected by copyright. All rights reserved.

Organocatalytic PET-RAFT polymerization with a low ppm of organic photocatalyst under visible light

The development of light-mediated controlled radical polymerization has benefited from the discovery of novel photocatalysts, which could allow precise light control over the polymerization process and the production of well-defined polymers.

Aqueous broadband photopolymerization on microreactor arrays: from high throughput polymerization to fabricating artificial cells

Microreactor arrays combining ZnO and polyaniline are fabricated onto the bottom of multi-well plates to catalyze broadband sunlight-driven open-to-air polymerization in aqueous media.

Reusable polymer brush-based photocatalysts for PET-RAFT polymerization

Fluorescein polymer-brush functionalized glass beads synthesize polymers via photoelectron reversible addition fragmentation chain transfer (PET-RAFT) polymerization. These shelf stable heterogeneous catalysts can be recycled after simple filtration.

Photo-Iniferter RAFT Polymerization

Light-mediated polymerization techniques offer distinct advantages over polymerization reactions fueled by thermal energy, such as high spatial and temporal control as well as the possibility to work under mild reaction conditions. Reversible addition-fragmentation chain-transfer (RAFT) polymerization is a highly versatile radical polymerization method that can be utilized to control a variety of monomers and produce a vast number of complex macromolecular structures. The use of light to drive a RAFT-polymerization is possible via multiple routes. Besides the use of photo-initiators, or photo-catalysts, the direct activation of the chain transfer agent controlling the RAFT process in a photo-iniferter (PI) process is an elegant way to initiate and control polymerization reactions. Within this review, PI-RAFT polymerization and its advantages over the conventional RAFT process are discussed in detail.

Machine-Learning-Guided Discovery of 19F MRI Agents Enabled by Automated Copolymer Synthesis

Modern polymer science suffers from the curse of multidimensionality. The large chemical space imposed by including combinations of monomers into a statistical copolymer overwhelms polymer synthesis and characterization technology and limits the ability to systematically study structure-property relationships. To tackle this challenge in the context of 19F magnetic resonance imaging (MRI) agents, we pursued a computer-guided materials discovery approach that combines synergistic innovations in automated flow synthesis and machine learning (ML) method development. A software-controlled, continuous polymer synthesis platform was developed to enable iterative experimental-computational cycles that resulted in the synthesis of 397 unique copolymer compositions within a six-variable compositional space. The nonintuitive design criteria identified by ML, which were accomplished by exploring <0.9% of the overall compositional space, lead to the identification of >10 copolymer compositions that outperformed state-of-the-art materials.

Automation and data-driven design of polymer therapeutics

Polymers are uniquely suited for drug delivery and biomaterial applications due to tunable structural parameters such as length, composition, architecture, and valency. To facilitate designs, researchers may explore combinatorial libraries in a high throughput fashion to correlate structure to function. However, traditional polymerization reactions including controlled living radical polymerization (CLRP) and ring-opening polymerization (ROP) require inert reaction conditions and extensive expertise to implement. With the advent of air-tolerance and automation, several polymerization techniques are now compatible with well plates and can be carried out at the benchtop, making high throughput synthesis and high throughput screening (HTS) possible. To avoid HTS pitfalls often described as "fishing expeditions," it is crucial to employ intelligent and big data approaches to maximize experimental efficiency. This is where the disruptive technologies of machine learning (ML) and artificial intelligence (AI) will likely play a role. In fact, ML and AI are already impacting small molecule drug discovery and showing signs of emerging in drug delivery. In this review, we present state-of-the-art research in drug delivery, gene delivery, antimicrobial polymers, and bioactive polymers alongside data-driven developments in drug design and organic synthesis. From this insight, important lessons are revealed for the polymer therapeutics community including the value of a closed loop design-build-test-learn workflow. This is an exciting time as researchers will gain the ability to fully explore the polymer structural landscape and establish quantitative structure-property relationships (QSPRs) with biological significance.

New Light in Polymer Science: Photoinduced Reversible Addition-Fragmentation Chain Transfer Polymerization (PET-RAFT) as Innovative Strategy for the Synthesis of Advanced Materials

Photochemistry has attracted great interest in the last decades in the field of polymer and material science for the synthesis of innovative materials. The merging of photochemistry and reversible-deactivation radical polymerizations (RDRP) provides good reaction control and can simplify elaborate reaction protocols. These advantages open the doors to multidisciplinary fields going from composite materials to bio-applications. Photoinduced Electron/Energy Transfer Reversible Addition-Fragmentation Chain-Transfer (PET-RAFT) polymerization, proposed for the first time in 2014, presents significant advantages compared to other photochemical techniques in terms of applicability, cost, and sustainability. This review has the aim of providing to the readers the basic knowledge of PET-RAFT polymerization and explores the new possibilities that this innovative technique offers in terms of industrial applications, new materials production, and green conditions.

Multipath oxygen-mediated PET-RAFT polymerization by a conjugated organic polymer photocatalyst under red LED irradiation

TCPP-DMTA-COP has been synthesized and serves as a heterogeneous photocatalyst in a multipath aerobic-mediated reductive quenching pathway (O-RQP) for a PET-RAFT polymerization process.

The in situ “grafting from” approach for the synthesis of polymer brushes on upconversion nanoparticles via NIR-mediated RAFT polymerization

Through NIR-mediated RAFT polymerization, surface growth of polymer brushes on UCNPs was realized based on an efficient in situ ligand exchange.

Heterogeneous photocatalytic reversible deactivation radical polymerization

Photocatalytic reversible deactivation radical polymerization (RDRP) permits the use of sustainable solar light for spatiotemporal regulation of radical polymerization under mild conditions.

Universal Antifogging and Antimicrobial Thin Coating Based on Dopamine-Containing Glycopolymers

A novel strategy for preparing universal antifogging and antimicrobial coating is reported by the means of one-step coating and Ag nanoparticle (AgNP) formation in situ. A series of hydrophilic glycopolymers including poly(N-3,4-dihydroxybenzenethyl methacrylamide-co-2-deoxy-2-(methacrylamido)glucopyranose) (P1s) and poly(N-3,4-dihydroxybenzenethyl methacrylamide-co-methacrylic acid-co-2-deoxy-2-(methacrylamido)glucopyranose) (P2s) were synthesized by sunlight-induced reverse addition-fragmentation chain transfer (RAFT) polymerization. With the ability to strongly immobilize onto organic and inorganic surfaces (i.e., glass slide, silicon wafer, and polycarbonate) via catechol groups, P1s are very convenient to form superhydrophilic and transparent thin coatings, which result in a unique antifogging property. Additionally, the antimicrobial property is realized by in situ AgNPs forming P2 coatings, facilitated by the presence of carboxyl groups and catechol groups in the polymer chain, rendering it superior antimicrobial activity against both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus microorganisms. This antifogging and antimicrobial thin coating shows strong prospects in medical and optical devices, with the extra benefits of avoiding potential pathogen infection in vitro or while in storage.

Polymer Nanofibers Exhibiting Remarkable Activity in Driving the Living Polymerization under Visible Light and Reusability

Visible light-driving syntheses have emerged as a powerful tool for organic synthesis and for the preparation of macromolecules under mild and environmentally benign conditions. However, precious but nonreusable photosensitizers or photocatalysts are often required to activate the reaction, limiting its practicality. Here, it is reported that poly(1,4-diphenylbutadiyne) (PDPB) nanofibers exhibit remarkable activity in driving the living free radical polymerization under visible light. Moreover, PDPB nanofibers are very stable under irradiation of visible light and can be reused without appreciable loss of activity even after repeated cycling. The nanofiber will be a promising photocatalyst with excellent reusability and stability for the reactions driven by visible light.

Light and magnetism dual-gated photoinduced electron transfer-reversible addition–fragmentation chain transfer (PET-RAFT) polymerization

PET-RAFT polymerization can be reversibly ceased in the absence of light or under an external magnetic field with a novel raspberry-like γ-Fe₂O₃@CD nanocatalyst.

Recoverable and recyclable nickel–cobalt magnetic alloy nanoparticle catalyzed reversible deactivation radical polymerization of methyl methacrylate at 25 °C

Recyclable Ni–Co alloy catalyzed synthesis of well-defined poly(methyl methacrylate) (PMMA, up to 129 500 g mol⁻¹) with narrow-dispersity (Đ = 1.30) via a reversible deactivation radical polymerization technique is reported.

Polymerization techniques in polymerization-induced self-assembly (PISA)

The development of controlled/“living” polymerization greatly stimulated the prosperity of the fabrication and application of block copolymer nano-objects.

Photoregulated reversible addition–fragmentation chain transfer (RAFT) polymerization

Different strategies on photoregulated RAFT polymerization are developed. This minireview summarizes recent advances in photoregulated RAFT polymerization and its applications.

Enzyme-initiated reversible addition-fragmentation chain transfer (RAFT) polymerization: Precision polymer synthesis via enzymatic catalysis

Enzyme-initiated reversible addition-fragmentation chain transfer (RAFT) polymerization provides a sustainable strategy for efficient production of well-defined polymers under mild conditions. Horseradish peroxidase (HRP), a heme-containing metalloenzyme, catalyzes oxidation of acetylacetone (ACAC) by hydrogen peroxide (H₂O₂) to generate ACAC radicals, initiating polymerization of vinyl monomers. This HRP/H₂O₂/ACAC ternary initiating system is applied to RAFT polymerization of different types of vinyl monomers. Furthermore, to overcome the inherent limitation of necessity for oxygen-free conditions, another enzyme, glucose oxidase (GOx) or pyranose 2-oxidase (P2Ox), with excellent deoxygenation capability, is introduced to consume oxygen by catalyzing oxidation of glucose to generate H₂O₂. The generated H₂O₂ is directly supplied to HRP catalysis for radical generation. Both GOx-HRP and P2Ox-HRP cascade catalysis afford RAFT polymerization with oxygen tolerance. In this chapter, we mainly focus on detailed synthetic protocols of RAFT polymerizations initiated by HRP/H₂O₂/ACAC ternary initiating system and P2Ox-HRP cascade catalysis. The general characterization and analytical methods used in these enzyme-initiated RAFT polymerizations are also included.

High Throughput Screening of Glycopolymers: Balance between Cytotoxicity and Antibacterial Property

To search for synthetic agents with low cytotoxicity and good antibacterial activity is essential for antimicrobial applications. Here we report a high throughput technique that carried out in multiwell plates via recyclable-catalyst-aided, opened-to-air, and sunlight-photolyzed RAFT (ROS-RAFT) polymerization. By using this method, three key monomers (MAG the sugar unit, DMAPMA the positively charged monomer, and DEMAA the hydrophobic monomer) can be polymerized in a controlled manner to afford glycopolymers. This simple high throughput technology is used to synthesize glycopolymers with variable compositions. The bacterial adhesion/killing ability and cytotoxicity of synthesized polymers have been evaluated, and glycopolymers with certain composition can achieve a balance of low cytotoxic and good antibacterial activity.

Visible-Light-Driven MADIX Polymerisation via a Reusable, Low-Cost, and Non-Toxic Bismuth Oxide Photocatalyst

The continuous amalgamation of photocatalysis into existing reversible deactivation radical polymerisation (RDRP) processes has initiated a rapidly propagating area of polymer research in recent years. We introduce bismuth oxide (Bi₂ O₃ ) as a heterogeneous photocatalyst for polymerisations, operating at room temperature with visible light. We demonstrate formidable control over degenerative chain-transfer polymerisations, such as macromolecular design by interchange of xanthate (MADIX) and reversible addition-fragmentation chain-transfer (RAFT) polymerisation. We achieved narrow molecular weight distributions and attribute the excellent temporal control of a photo-induced electron transfer (PET) process. This methodology was employed to synthesise diblock copolymers combining differently activated monomers. The Bi₂ O₃ catalyst system has the additional benefits of low toxicity, reusability, low-cost, and ease of removal from the reaction mixture.

Visible light induced aqueous RAFT polymerization using a supramolecular perylene diimide/cucurbit[7]uril complex

A water-soluble perylene diimide (PDI), in the presence of triethanolamine (TEOA), is used as a metal-free photocatalyst for aqueous reversible addition–fragmentation chain transfer (RAFT) polymerization under green light.

Photo-controlled one-pot strategy for the synthesis of asymmetric three-arm star polymers

A one-pot tandem strategy for the synthesis of asymmetric star polymers was developed. Two wavelengths of light were used to provide spatial and temporal control for the reactions. A series of stars were synthesised using this procedure giving well-defined structures.

A cocktail of vitamins for aqueous RAFT polymerization in an open-to-air microtiter plate

We report a highly biocompatible photoinitiation strategy based on Vitamin B₂ and Vitamin C. This two-component photoinitiator enables RAFT polymerization to be conducted in high throughput in an open-to-air microtiter plate.

Up in the air: oxygen tolerance in controlled/living radical polymerisation

The requirement for deoxygenation in controlled/living radical polymerisation (CLRP) places significant limitations on its widespread implementation by necessitating the use of large reaction volumes, sealed reaction vessels as well as requiring access to specialised equipment such as a glove box and/or inert gas source. As a result, in recent years there has been intense interest in developing strategies for overcoming the effects of oxygen inhibition in CLRP and therefore remove the necessity for deoxygenation. In this review, we highlight several strategies for achieving oxygen tolerant CLRP including: "polymerising through" oxygen, enzyme mediated deoxygenation and the continuous regeneration of a redox-active catalyst. In order to provide further clarity to the field, we also establish some basic parameters for evaluating the degree of "oxygen tolerance" that can be achieved using a given oxygen scrubbing strategy. Finally, we propose some applications that could most benefit from the implementation of oxygen tolerant CLRP and provide a perspective on the future direction of this field.

Oxygen Tolerant and Room Temperature RAFT through Alkylborane Initiation

A reversible addition-fragmentation chain transfer (RAFT) process was developed capable of being performed at room temperature and in the presence of oxygen by initiating polymerization through an alkylborane-amine complex. This air-stable alkylborane-amine complex was chemically deblocked with carboxylic acid or isocyanate functionalities to liberate a reactive trialkylborane that consumes oxygen and generates radicals to mediate RAFT. Alkylborane-initiated RAFT (AI-RAFT) was demonstrated to allow the synthesis of a wide range of polymer molecular weights with narrow distributions. Rapid polymerization was also possible within minutes under an ambient environment without any prior deoxygenation. Optimal conditions were investigated revealing that carboxylic acids are required in larger excess to alkylborane versus isocyanates and that deblocker functionality can have an impact on polymerization kinetics, achievable molecular weight, and dispersity. Living chain-ends were confirmed by synthesizing block copolymers using AI-RAFT-derived macro-chain transfer agents. In this preliminary study, a chemically induced RAFT process is introduced without requirement of any thermal, photochemical, electrical, or mechanical stimulus capable of polymerizing acrylamide, acrylate, and methacrylate monomers in limited amounts of oxygen at room temperature.