Photoinduced Radical Polymerization Research Articles

Metal-Free Photoinduced Decarboxylative Radical Polymerization Using Carboxylic Acids as Benign Radical Initiators: Introduction of Complex Molecules into Polymer Chain Ends

Metal-free photoinduced decarboxylative radical polymerization of aliphatic carboxylic acids with a variety of monomers was found to proceed smoothly to give the corresponding polymers under mild conditions. Complex carboxylic acids such as those of sugars, steroids, and peptides can function as benign radical initiators via decarboxylation and can be incorporated at the polymer chain ends. This synthetic methodology represents a facile introduction of molecules and functionalities to polymers by using commercially available carboxylic acids.

Photoinduced Fe-mediated atom transfer radical polymerization in aqueous media

Photoinduced atom transfer radical polymerization with an Fe catalyst was successfully performed in aqueous media for the first time.

Conventional Type II photoinitiators as activators for photoinduced metal-free atom transfer radical polymerization

A novel methodology for photoinduced metal-free Atom Transfer Radical Polymerization (ATRP) by using conventional Type II photoinitiators such as benzophenone, thioxanthone, isopropyl thioxanthone and camphorquinone as sensitizers is presented.

Investigation of photoinduced polymerization of doxycycline-imprinted hydrogels: effect of template on initiator reactivity, conversion, and reaction rate

Photopolymerization kinetics of doxycycline hyclate (DOX)-imprinted hydrogels were monitored by real-time Fourier transform infrared spectroscopy and differential photocalorimetry. 2-Hydroxyethyl methacrylate-based hydrogels were synthesized by using ethylene glycol dimethacrylate as a cross-linker, acrylic acid (AA) as a functional monomer, and 2,2-dimethyl-2-hydroxy acetophenone as a photoinitiator. For imprinting DOX in hydrogels, the molar ratio of template to functional monomer (DOX:AA) was chosen as 1:8 and 1:16. The polymerization was achieved at two different initiator concentrations. The conversion and reaction rate were calculated as a function of the molar ratio of template, and the results were compared to those of nonimprinted ones. In order to reveal the effect of DOX on the photoinduced radical polymerization, thermal polymerization was also performed for imprinted and nonimprinted hydrogels by using 2,2$' $-azobis(2,4-dimethyl-pentanenitrile) as a thermal initiator. All results showed that there is a significant effect of DOX concentration on the conversion and reaction rate of the photopolymerization reaction. The conversion and reaction rate decreased during photopolymerization when the template concentration increased in the monomer mixture.

A di‐tert‐butyl acrylate monomer for controlled radical photopolymerization

ABSTRACTA new di‐tert‐butyl acrylate (diTBA) monomer for controlled radical polymerization is reported. This monomer complements the classical use of tert‐butyl acrylate (TBA) for synthesis of poly(acrylic acid) by increasing the density of carboxylic acids per repeat unit, while also increasing the flexibility of the carboxylic acid side‐chains. The monomer is well behaved under Cu(II)‐mediated photoinduced controlled radical polymerization and delivers polymers with excellent chain‐end fidelity at high monomer conversions. Importantly, this new diTBA monomer readily copolymerizes with TBA to further the potential for applications in areas such as dispersing agents and adsorbents. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 801–807

Metal-Free Atom Transfer Radical Polymerization of Methyl Methacrylate with ppm Level of Organic Photocatalyst.

It is well known that the recently developed photoinduced metal-free atom transfer radical polymerization (ATRP) has been considered as a promising methodology to completely eliminate transition metal residue in polymers. However, a serious problem needs to be improved, namely, large amount of organic photocatalysts should be used to keep the controllability over molecular weights and molecular weight distributions. In this work, a novel photocatalyst 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) with strong excited state reduction potential is successfully used to mediate a metal-free ATRP of methyl methacrylate just with parts per million (ppm) level usage under irradiation of blue light emitting diode at room temperature, using ethyl α-bromophenyl-acetate as a typical initiator with high initiator efficiency. The polymerization kinetic study, multiple controlled "on-off" light switching cycle regulation, and chain extension experiment confirm the "living"/controlled features of this promising photoinduced metal-free ATRP system with good molecular weight control in the presence of ppm level photocatalyst 4CzIPN.

Grafting-from cellulose nanocrystals via photoinduced Cu-mediated reversible-deactivation radical polymerization

In this work we describe the grafting of cellulose nanocrystals (CNCs) by surface-initiated photoinduced Cu-mediated reversible-deactivation radical polymerization (RDRP). Initially, CNCs obtained through sulfuric acid hydrolysis were functionalized with a tertiary bromo-ester moiety as an initiating group for the subsequent RDRP of methyl acrylate, targeting three different degrees of polymerization for the polymer grafts: 50, 300 and 600. The polymerizations proceeded in DMSO in the presence of CuBr2 and Me6TREN as the catalytic system utilizing a UV source (λmax≈360nm). The technique proved highly versatile for the modification of CNCs with poly(methyl acrylate), where considerably high grafting was achieved in short reaction times (90min), with simple purification steps. CNC morphology was maintained and polymer grafts were evident through FT-IR spectroscopy, thermal analysis, contact angle measurements, X-ray photoelectron microscopy and x-ray diffraction.

Improved Protein Conjugation with Uniform, Macroporous Poly(acrylamide-co-acrylic acid) Hydrogel Microspheres via EDC/NHS Chemistry.

We demonstrate a robust and tunable micromolding method to fabricate chemically functional poly(acrylamide-co-acrylic acid) (p(AAm-co-AA)) hydrogel microspheres with uniform dimensions and controlled porous network structures for rapid biomacromolecular conjugation. Specifically, p(AAm-co-AA) microspheres with abundant carboxylate functional groups are fabricated via surface-tension-induced droplet formation in patterned poly(dimethylsiloxane) molds and photoinduced radical polymerization. To demonstrate the chemical functionality, we enlisted rapid EDC/NHS (1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide (EDC) and N-hydroxysuccinimide (NHS)) chemistry for fluorescent labeling of the microspheres with small-molecule dye fluorescein glycine amide. Epifluorescence imaging results illustrate the uniform incorporation of carboxylate groups within the microspheres and rapid conjugation kinetics. Furthermore, protein conjugation results using red fluorescent protein R-phycoerythrin demonstrate the highly porous nature of the microspheres as well as the utility of the microspheres and the EDC/NHS scheme for facile biomacromolecular conjugation. Combined, these results illustrate the significant potential for our fabrication-conjugation strategy in the development of biofunctionalized polymeric hydrogel microparticles toward rapid biosensing, bioprocess monitoring, and biodiagnostics.

Photoinduced Metal-Free Atom Transfer Radical Polymerization of Biomass-Based Monomers

Well-defined polymers derived from biomass feedstock (e.g., soybean oil, rosin acid, and furfural) were successfully prepared by metal-free atom transfer radical polymerization (ATRP). In the presence of photoredox catalysts and UV irradiation, three biomass-based methacrylate monomers were efficiently polymerized with good control over molecular weight and dispersity. NMR and MALDI-TOF MS confirmed high fidelity of chain end groups originated from initiators. Furthermore, block copolymers from these monomers were also achieved through chain extension by metal-free ATRP.

Aqueous Copper(II) Photoinduced Polymerization of Acrylates: Low Copper Concentration and the Importance of Sodium Halide Salts.

Photoinduced metal-mediated radical polymerization is a rapidly developing technique which allows for the synthesis of macromolecules with defined molecular weight and narrow molecular weight distributions, although typically exhibiting significant limitations in aqueous media. Herein we demonstrate that the presence of alkali metal halide salts, in conjunction with low copper concentration and UV irradiation, allows for the controlled polymerization of water-soluble acrylates in aqueous media, yielding narrow molecular weight distributions and high conversions. Despite the aqueous environment which typically compromises polymer end group fidelity, chain extensions have also been successfully performed and different degrees of polymerization were targeted. Importantly, no conversion was observed in the absence of UV light and the polymerization could be switched "on" and "off" upon demand, as demonstrated by intermittent light and dark periods and thus allowing access to spatiotemporal control.

Mechanism of Photoinduced Metal-Free Atom Transfer Radical Polymerization: Experimental and Computational Studies.

Photoinduced metal-free atom transfer radical polymerization (ATRP) of methyl methacrylate was investigated using several phenothiazine derivatives and other related compounds as photoredox catalysts. The experiments show that all selected catalysts can be involved in the activation step, but not all of them participated efficiently in the deactivation step. The redox properties and the stability of radical cations derived from the catalysts were evaluated by cyclic voltammetry. Laser flash photolysis (LFP) was used to determine the lifetime and activity of photoexcited catalysts. Kinetic analysis of the activation reaction according to dissociative electron-transfer (DET) theory suggests that the activation occurs only with an excited state of catalyst. Density functional theory (DFT) calculations revealed the structures and stabilities of the radical cation intermediates as well as the reaction energy profiles of deactivation pathways with different photoredox catalysts. Both experiments and calculations suggest that the activation process undergoes a DET mechanism, while an associative electron transfer involving a termolecular encounter (the exact reverse of DET pathway) is favored in the deactivation process. This detailed study provides a deeper understanding of the chemical processes of metal-free ATRP that can aid the design of better catalytic systems. Additionally, this work elucidates several important common pathways involved in synthetically useful organic reactions catalyzed by photoredox catalysts.

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.

The active role of excited states of phenothiazines in photoinduced metal free atom transfer radical polymerization: singlet or triplet excited states?

The active role of phenothiazine excited states in photoinduced metal-free atom transfer radical polymerization (ATRP) was investigated by using laser flash photolysis, fluorescence, phosphorescence and electron spin resonance spectroscopy.

A degradable copolymer of 2-methylene-1,3-dioxepane and vinyl acetate by photo-induced cobalt-mediated radical polymerization

A degradable copolymer of 2-methylene-1,3-dioxepane (MDO) and vinyl acetate (VAc) was synthesized through photo-induced cobalt-mediated radical polymerization under UV irradiation (450 μW cm−2 at 365 nm) at 30 °C by using (2,4,6-trimethylbenzoyl)diphenylphosphine oxide as an initiator and Co(acac)2 as a regulating agent. Living polymerization behavior was proved via pseudo linear kinetics, the molecular weight of the copolymers increasing with conversion, narrow molecular weight distribution of copolymers and successful chain extension. The structure of the copolymer was characterized by NMR and MALDI-TOF MS. The reactivity ratios for MDO and VAc were determined to be rMDO = 0.14 and rVAc = 1.89. It was possible to vary the content of MDO in a wide range of copolymers by changing the feed ratio of VAc and MDO. The glass transition temperature increased gradually with increasing the amount of VAc. Such copolymers were easily degraded no matter in acidic or alkaline solution.

All Solid-State Electrolytes for Lithium Microbatteries: Fabrication of an Ionic Liquid-Based Membrane

Through the permanent miniaturization of portable autonomous systems, the development of reliable, integrated micro power sources is necessary. Currently, Li-ion batteries are limited by their size, which make them unsuitable to new technologies on the millimeter scale or less. In this context, lithium-based microbatteries raise great interest at both academic and industrial levels. These systems are less than 10µm thick and a few square centimeters wide with storage capacities going from 50 to 200µAh/cm². Their convenient size make them an effective solution for applications such as self-powered microelectronics (connected smart-cards, ultra-thin watches, autonomous sensors, etc…), medical devices (pacemaker, hearing aid, etc…) and energy harvesting devices (solar cells). However, microbatteries are built by successive thin films deposition, mainly by PVD processes, which involve expensive technologies and time-costly processes. In order to reduce manufacturing costs and also enable conformal deposition on 3D substrates to increase energy storage capabilities, our laboratory is currently developing milder deposition routes for microbatteries films. For instance, standard electrolyte, LiPON, is a glassy thin film deposited at a rate of 0.3µm/hour. This project focuses on the study of new solid electrolytes for microbatteries through an innovative wet process. Our strategy consists in confining a mixture of an ionic liquid and a lithium salt in an organic membrane in-situ polymerized. Ionic liquids (IL) are interesting electrolyte solvents because of their high ionic conductivity, paired with their excellent chemical and electrochemical stability vs Li+/Li. Moreover, IL have important features in terms of security and process flow, due to their low flammability and thermal stability up to 350°C. A wide range of anion-cation couples can tailor the material properties. Thus, we studied the characteristics of several ionic liquid/lithium salts binary solutions. Three different cations families, 1-Ethyl-3-Methylimidazolium (EMI), N-alkykl-N-methylpyrrolidinium (Pyr1A +) and 1-Alkyl-1-Methylpiperidinium (PIP1A +), were combined to two anions, bis(trifluoromethane-sulfonyl)imide (TFSI-) and bis(fluorosulfonyl)imide (FSI-). Electrochemical stability window, ionic conductivity and ionic transport were investigated as well as temperature stability. The chosen electrolytic solution was then confined in an organic matrix by photo-induced (UV light) radical polymerization of a di-methacrylate precursor. The reticulation was tracked by differential scanning calorimetry associated with a photo-calorimetric accessory (PCA-DSC) and by infrared spectroscopy (FTIR). Additional interesting results were obtained by following the conductivity evolution during the polymerization. For this purpose, we designed a completely original planar interdigitated cell to measure reliable electrochemical impedance during UV-curing. The influence of curing parameters such as precursor concentration and light intensity on the final gel conversion and structure were also investigated. After optimal polymerization, thermally stable (up to 300°C), transparent and flexible gelled electrolytes called “ion-gels” were obtained – Figure 1. Synthesized ion-gels displayed ionic conductivity around 10-4 S.cm-1 at 25°C, significantly better than the standard electrolyte (LiPON) conductivity of 10-6 S.cm-1. Our electrolytes also had broad stability window vs Li+/Li up to 5V. Indeed, electrochemical behavior of ionic liquids is almost completely preserved after their confinement in the matrix. A Li/ion-gel/LiCoO2 coin cell was assembled and characterized by impedance spectroscopy (EIS) and galvanostatic cycling (GCPL) – Figure 2. Since the electrolyte precursor solution is liquid before UV-curing, a thin film with controlled thickness (50-200µm) can be easily deposited on the positive electrode. The battery has successfully cycled at different rates (C/10, C/5, C/2, C = 30µA/cm²) between 3 and 4.2V at 25°C. Its total capacity was equivalent to 80% of the “reference” capacity obtained with a common carbonate-based liquid electrolyte (EC-PC-DMC + 1M LiPF6). The capacity loss has been assigned to the polarization, which increases with the cycling rate, caused by pristine rough interfaces and consequently higher electrolyte resistivity. However, first results showed that the interfaces and the whole electrolyte film are stable for hundreds of cycles. A functional Li-microbattery Li/ion-gel film/LiCoO2 prototype was also produced. The lithium layer was deposited on the ion-gel by evaporation deposition without degradation of the electrolyte during the process. Figure 1

Surface Grafting via Photo-Induced Copper-Mediated Radical Polymerization at Extremely Low Catalyst Concentrations.

Surface-initiated photo-induced copper-mediated radical polymerization is employed to graft a wide range of polyacrylate brushes from silicon substrates at extremely low catalyst concentrations. This is the first time that the controlled nature of the reported process is demonstrated via block copolymer formation and re-initiation experiments. In addition to unmatched copper catalyst concentrations in the range of few ppb, film thicknesses up to almost 1 μm are achieved within only 1 h.

Photoinduced Metal-Free Atom Transfer Radical Polymerization of Acrylonitrile.

Photoinduced metal-free atom transfer radical polymerization has been successfully extended to the synthesis of polyacrylonitrile (PAN) with predictable molecular weights and low dispersities. This was achieved using phenothiazine derivatives as photoredox catalysts, which activate dormant alkyl bromides to reversibly form propagating radicals. Both 1H NMR spectroscopy and chain-end extension polymerization show highly preserved Br chain-end functionality in the synthesized PAN.

Tertiary amine catalyzed photo-induced controlled radical polymerization of methacrylates

A novel tertiary amine catalyst and trithiocarbonate synergistic photo-induced controlled radical polymerization of methacrylates has been realized under mild UV irradiation, yielding polymethacrylates with low molecular weight distributions and excellent end-group fidelity.

Ligand switch in photoinduced copper-mediated polymerization: synthesis of methacrylate–acrylate block copolymers

The use of photo-induced copper-mediated radical polymerization (photoCMP) to synthesize mixed acrylate/methacrylate (methyl acrylate, MA and methyl methacrylate, MMA) block copolymers is investigated.

Cellulose grafting by photoinduced controlled radical polymerisation

Cellulose surfaces, in the form of filter paper, have been grafted utilizing UV-induced surface-initiated controlled radical polymerization of acrylates.