Near-Infrared Light-Induced Reversible Deactivation Radical Polymerization: Expanding Frontiers in Photopolymerization

Photoinduced reversible deactivation radical polymerization (photo-RDRP) or photoinduced controlled/living radical polymerization has emerged as a versatile and powerful technique for preparing functional and advanced polymer materials under mild conditions by harnessing light energy. While UV and visible light (λ = 400-700 nm) are extensively employed in photo-RDRP, the utilization of near-infrared (NIR) wavelengths (λ = 700-2500 nm) beyond the visible region remains relatively unexplored. NIR light possesses unique properties, including enhanced light penetration, reduced light scattering, and low biomolecule absorption, thereby providing opportunities for applying photo-RDRP in the fields of manufacturing and medicine. This comprehensive review categorizes all known NIR light-induced RDRP (NIR-RDRP) systems into four mechanism-based types: mediation by upconversion nanoparticles, mediation by photocatalysts, photothermal conversion, and two-photon absorption. The distinct photoinitiation pathways associated with each mechanism are discussed. Furthermore, this review highlights the diverse applications of NIR-RDRP reported to date, including 3D printing, polymer brush fabrication, drug delivery, nanoparticle synthesis, and hydrogel formation. By presenting these applications, the review underscores the exceptional capabilities of NIR-RDRP and offers guidance for developing high-performance and versatile photopolymerization systems. Exploiting the unique properties of NIR light unlocks new opportunities for synthesizing functional and advanced polymer materials.

Sonochemistry-assisted photocontrolled atom transfer radical polymerization enabled by manganese carbonyl

Sonochemistry-assisted photocontrolled atom transfer radical polymerization (SAP-ATRP) is developed to circumvent the problem caused by the low penetration depth of light.

Dithiocarbamate-mediated controlled copolymerization of ethylene with cyclic ketene acetals towards polyethylene-based degradable copolymers

In this article, degradable polyethylene (PE)-based copolymers containing ester units in the backbone were prepared through the hybrid copolymerization of ethylene and cyclic ketene acetals (CKAs) mediated by dithiocarbamate successfully.

Near-Infrared, Light-Induced Cationic and Radical RAFT Polymerization Catalyzed by Iron Complex

A near-infrared (NIR) light induced controlled cationic polymerization is presented here. The halide abstraction reaction between the cyclopentadienyl iron dicarbonyl dimer (Fe₂(Cp)₂(CO)₄) and an organic halide is utilized to generate initial radicals or cations under mild conditions, which can be further combined with both radical and cationic reversible addition-fragmentation chain transfer (RAFT) polymerization. Well-defined poly(vinyl ether)s and polyacrylates are prepared successfully under NIR light by this method. The excellent penetration ability of NIR light through thick barriers has been verified by polymerization in the presence of an A4 paper. In addition, iron-based radical polymerization has been used for three-dimensional (3D) printing to fabricate materials with different thicknesses.

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.

Manganese carbonyl induced cationic reversible addition–fragmentation chain transfer (C-RAFT) polymerization under visible light

Vinyl ethers were polymerized by C-RAFT polymerization on the basis of halide abstraction reaction of manganese carbonyl and organic halide.

Grafting of Polymer Brushes from Xanthate-Functionalized Silica Particles

Monodisperse silica particles (SiPs) were surface-modified with a newly designed silane coupling agent comprising a triethoxysilane and an alkyl halide, namely, 6-(triethoxysilyl)hexyl 2-bromopropionate, which was further treated with potassium O-ethyl dithiocarbonate (PEX) to immobilize xanthate molecules on the particle surfaces. Surface-initiated macromolecular design via interchange of xanthates (MADIX) polymerization of vinyl acetate (VAc) was conducted with the xanthate-functionalized SiPs. The polymerization was well controlled and produced SiPs coated with poly(vinyl acetate) (PVAc) with a well-defined target molar mass and a graft density of about 0.2 chains nm^-2 . Dynamic light scattering and TEM measurements revealed that the hybrid particles were highly dispersible in good solvents without any aggregation. The PVAc brushes were hydrolyzed with hydrochloric acid to produce poly(vinyl alcohol) brushes on the SiP surfaces. In addition, the number of xanthate molecules introduced on the SiP surfaces could be successfully controlled by adjusting the concentration of PEX. Thus, the SiPs have two functionalities: xanthates able to act as a MADIX chain-transfer agent and alkyl bromide initiation sites for atom transfer radical polymerization (ATRP). By using these unique bifunctional particles, mixed polymer brushes were constructed on the SiPs by MADIX of VAc followed by ATRP of styrene or methyl methacrylate.

Visible light induced controlled cationic polymerization by in situ generated catalyst from manganese carbonyl

Controlled cationic polymerization of various vinyl ethers is achieved under light radiation, utilizing commercially available reagents, under mild conditions and the molecular weight distributions can be modulated by simply regulating the irradiation time.

Manganese phosphinocarbodithioate for RAFT polymerisation with sunlight-induced chain end post-treatment

A new manganese complex of the formula Cp(CO)₂MnP(Ph)₂C(S)SCH(CH₃)Ph is an efficient RAFT agent for the preparation of SH-terminated polymers by simple visible light photocleavage of the organometallic end-group.

Photoinduced controlled radical polymerization of methyl acrylate and vinyl acetate by xanthate

A block copolymer of PMA-b-PVAc was successfully synthesized using photo-induced RAFT polymerization with a xanthate.

RAFT polymerization to form stimuli-responsive polymers

Stimuli-responsive polymers respond to a variety of external stimuli, which include optical, electrical, thermal, mechanical, redox, pH, chemical, environmental and biological signals. This paper is concerned with the process of forming such polymers by RAFT polymerization.

Polymerization-induced self-assembly of PVAc-b-PVDF block copolymers via RAFT dispersion polymerization of vinylidene fluoride in dimethyl carbonate

This article reports the results of the study of the RAFT dispersion polymerization of VDF in dimethyl carbonate using PVAc macroCTAs. The morphology of the resulting crystalline polymer aggregates is likely governed by crystallization of the PVDF.

Beyond Traditional RAFT: Alternative Activation of Thiocarbonylthio Compounds for Controlled Polymerization

Recent developments in polymerization reactions utilizing thiocarbonylthio compounds have highlighted the surprising versatility of these unique molecules. The increasing popularity of reversible addition-fragmentation chain transfer (RAFT) radical polymerization as a means of producing well-defined, 'controlled' synthetic polymers is largely due to its simplicity of implementation and the availability of a wide range of compatible reagents. However, novel modes of thiocarbonylthio activation can expand the technique beyond the traditional system (i.e., employing a free radical initiator) pushing the applicability and use of thiocarbonylthio compounds even further than previously assumed. The primary advances seen in recent years are a revival in the direct photoactivation of thiocarbonylthio compounds, their activation via photoredox catalysis, and their use in cationic polymerizations. These synthetic approaches and their implications for the synthesis of controlled polymers represent a significant advance in polymer science, with potentially unforeseen benefits and possibilities for further developments still ahead. This Research News aims to highlight key works in this area while also clarifying the differences and similarities of each system.

Visible Light-Mediated Polymerization-Induced Self-Assembly in the Absence of External Catalyst or Initiator

We report the use of visible light to mediate a RAFT dispersion polymerization in the absence of external catalyst or initiator to yield nanoparticles of different morphologies according to a polymerization-induced self-assembly (PISA) mechanism. A POEGMA macro-chain transfer agent (macro-CTA) derived from a 4-cyano-4-((dodecylsulfanylthiocarbonyl)sulfanyl)pentanoic acid (CDTPA) RAFT agent can be activated under blue (460 nm, 0.7 mW/cm²) or green (530 nm, 0.7 mW/cm²) light and act simultaneously as a radical initiator, chain transfer agent, and particle stabilizer under ethanolic dispersion conditions. In particular, the formation of worm-like micelles was readily monitored by the increase of reaction viscosity during the polymerization; this method was shown to be particularly robust to different reaction parameters such as macro-CTAs of varying molecular weight. Interestingly, at high monomer conversion, different morphologies were formed depending on the wavelength of light employed, which may be due to differing degrees of polymerization control. Finally, the in situ encapsulation of the model hydrophobic drug, Nile Red, was demonstrated, suggesting applications of this facile process for the synthesis of nanoparticles for drug delivery applications.

A novel solid state photocatalyst for living radical polymerization under UV irradiation

This study presents the development of a novel solid state photocatalyst for the photoinduced controlled radical polymerization of methacrylates under mild UV irradiation (λmax ≈ 365 nm) in the absence of conventional photoinitiators, metal-catalysts or dye sensitizers. The photocatalyst design was based on our previous finding that organic amines can act in a synergistic photochemical reaction with thiocarbonylthio compounds to afford well controlled polymethacrylates under UV irradiation. Therefore, in the current contribution an amine-rich polymer was covalently grafted onto a solid substrate, thus creating a heterogeneous catalyst that would allow for facile removal, recovery and recyclability when employed for such photopolymerization reactions. Importantly, the polymethacrylates synthesized using the solid state photocatalyst (ssPC) show similarly excellent chemical and structural integrity as those catalysed by free amines. Moreover, the ssPC could be readily recovered and re-used, with multiple cycles of polymerization showing minimal effect on the integrity of the catalyst. Finally, the ssPC was employed in various photo-"click" reactions, permitting high yielding conjugations under photochemical control.

Photomediated Controlled Radical Polymerization and Block Copolymerization of Vinylidene Fluoride

This review summarizes recent research on novel photochemical methods for the initiation and control of the polymerization of main chain fluorinated monomers as exemplified by vinylidene fluoride (VDF) and for the synthesis of their block copolymers. Such reactions can be carried out at ambient temperature in glass tubes using visible light. Novel, original protocols include the use of hypervalent iodide carboxylates alone or in conjunction with molecular iodine, as well as the use of photoactive transition metal carbonyls in the presence of alkyl, fluoroalkyl, and perfluoroalkyl halides. An in-depth study of the reaction parameters highlights the use of dimethyl carbonate as a preferred polymerization solvent and outlines the structure-property relationship for hypervalent iodide carboxylates and halide initiators in both the free radical and iodine degenerative transfer controlled radical polymerization (IDT-CRP) of VDF. Finally, the rational selection of metal carbonyls that are successful not only as IDT mediators but, more importantly, in the quantitative activation of both PVDF-CH2-CF2-I and PVDF-CF2-CH2-I chain ends toward the synthesis of well-defined PVDF block copolymers is presented.

RAFT synthesis of well-defined PVDF-b-PVAc block copolymers

This article reports that PVDF-b-PVAc diblock copolymers can be synthesized by RAFT polymerization from PVDF macroCTAs and rationalizes this discovery using DFT calculations.

A logic gate for external regulation of photopolymerization

The use of photocatalysts for visible light mediated reversible deactivation radical polymerization (RDRP) provides an efficient route for the synthesis of well-defined polymers with spatial, temporal and sequence control.

Stereo-, Temporal and Chemical Control through Photoactivation of Living Radical Polymerization: Synthesis of Block and Gradient Copolymers

Nature has developed efficient polymerization processes, which allow the synthesis of complex macromolecules with a perfect control of tacticity as well as molecular weight, in response to a specific stimulus. In this contribution, we report the synthesis of various stereopolymers by combining a photoactivated living polymerization, named photoinduced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) with Lewis acid mediators. We initially investigated the tolerance of two different photoredox catalysts, i.e., Ir(ppy)3 and Ru(bpy)3, in the presence of a Lewis acid, i.e., Y(OTf)3 and Yb(OTf)3, to mediate the polymerization of N,N-dimethyl acrylamide (DMAA). An excellent control of tacticity as well as molecular weight and dispersity was observed when Ir(ppy)3 and Y(OTf)3 were employed in a methanol/toluene mixture, while no polymerization or poor control was observed with Ru(bpy)3. In comparison to a thermal system, a lower amount of Y(OTf)3 was required to achieve good control over the tacticity. Taking advantage of the temporal control inherent in our system, we were able to design complex macromolecular architectures, such as atactic block-isotactic and isotactic-block-atactic polymers in a one-pot polymerization approach. Furthermore, we discovered that we could modulate the degree of tacticity through a chemical stimulus, by varying [DMSO]0/[Y(OTf)3]0 ratio from 0 to 30 during the polymerization. The stereochemical control afforded by the addition of a low amount of DMSO in conjunction with the inherent temporal control enabled the synthesis of stereogradient polymer consisting of five different stereoblocks in one-pot polymerization.

Dimanganese decacarbonyl/2-cyanoprop-2-yl-1-dithionaphthalate: toward sunlight induced RAFT polymerization of MMA

Methyl methacrylate was polymerized in the presence of dimanganese decacarbonyl [Mn₂(CO)₁₀]/2-cyanoprop-2-yl-1-dithionaphthalate (CPDN) via a photo-induced controlled radical polymerization under visible (green LED with λ_max of 565 nm) or sunlight irradiation at a moderate temperature.