Free Radical Photopolymerization Research Articles

Precision synthesis of diblock copolymers via free radical photopolymerization.

Diblock copolymers of phenyl methacrylate (PMA) and butyl methacrylate (BMA) were synthesized using dual-wavelength photopolymerization with a bifunctional photoinitiator 2-(4-(2-hydroxy-2-methylpropanoyl)phenoxy)ethyl(E)-3-(1-methyl-1H-pyrrol-2-yl)acrylate (PAA-2959).

Charge Transfer Complexes, Based on 2,4-Diethyl-9-oxo-10-(4-heptyloxyphenyl)-9H-thioxanthenium Hexafluorophosphate and Thiazole Derivatives, as Photoinitiators of Holographic Free Radical Photopolymerization

Charge Transfer Complexes, Based on 2,4-Diethyl-9-oxo-10-(4-heptyloxyphenyl)-9H-thioxanthenium Hexafluorophosphate and Thiazole Derivatives, as Photoinitiators of Holographic Free Radical Photopolymerization

Robust antifouling Poly(ionic liquid)s membrane with cyclodextrin skeleton for separation of oil/water emulsions

Membrane fouling remains a persistent and arduous challenge in the industrial treatment of different surfactants-stabilized oily wastewater. Constructing a robust antifouling and sustainable hydration coating on the membrane surface to efficiently repel oils or surfactants is a critical requirement. Herein, cyclodextrin-based poly(ionic liquid)s (PILs) coatings are cross-linked via the co-deposition on poly(vinylidene fluoride) (PVDF) of quaternized heptkis(6-deoxy-6-vinylimidazolium)-β-cyclodextrin (CD-Vi) derivatives as skeleton and acrylic acid (AA) monomers, and following one-step free radical photopolymerization. Combining with the strong hydration of the CD-based PILs coating, the resultant membrane exhibits fabulous superhydrophilicity and underwater superoleophobicity, oil repulsion, and self-cleaning capability. It demonstrates robust mechanical durability and chemical stability in harsh environments without losing its superhydrophilicity. Besides, the PIL-CD/AA coating can separate various surfactant-free and different surfactants (cationic CTAB, anionic SDS, and nonionic TWEEN 80) stabilized oil / water emulsions. During the long-term cycle operations, it also realizes stable recyclability with a series of permeation fluxes as high as removal efficiencies (>99.6 %) after 20 cycles for surfactant-free cyclohexane-in-water emulsion and 10 cycles for CTAB-stabilized isooctane-in-water emulsion.

Macromolecular coumarin sulfonium salt with side chain effect constructed by copolymerization strategy for free radical, cationic, and hybrid photopolymerizations

Macromolecular coumarin sulfonium salt with side chain effect constructed by copolymerization strategy for free radical, cationic, and hybrid photopolymerizations

Capsanthin-iodonium salt system for free radical photopolymerization under LED irradiation

The current photoinitiator (PI) is effective under mercury lamps but exhibits limited selectivity towards different radiation sources, which hinders its compatibility with eco-friendly LED lighting and raises concerns about toxicity and deep coloration. Capsanthin (Cap), with its strong photo-absorption in the 400–500 nm range, utilizes its conjugated structure to align the PI's absorption wavelength with the LED's emission spectrum. This study introduced Cap as a natural photosensitizer (PS) for free radical photopolymerization under LED irradiation. The Cap-based initiation system demonstrated promising photoinitiating capabilities under a 405 nm LED, achieving over 50 % double bond conversion in polymerized monomers. Furthermore, the system's photobleaching properties were exceptional, and its cytotoxicity was significantly lower than that of traditional PIs, such as benzoin methyl ether. The Cap's photochemical behavior under LED irradiation was explored, investigating its photoinitiating capacity and mechanism through fluorescence, photolysis, and electron paramagnetic resonance techniques. This research is vital for developing bio-based photoinitiating systems for photopolymerization in LED light environments.

Advancing photopolymerization and 3D printing: High-Performance NiII complexes bearing N2O2 Schiff-base ligands as photocatalysts

New high-performance photocatalysts based on metal structures are being developed for polymerization. However, using low-cost metals while ensuring effectiveness is challenging. In this study, two NiII complexes NiL1 and NiL2 were synthesized using N2O2 Schiff-base ligands bearing π-extended rings (naphthaldehyde (H2L1) and phenylsalicylaldehyde (H2L2) groups), respectively. These complexes were characterized by FTIR, UV–Vis and 1H NMR spectroscopy, elemental analysis, cyclic voltammetry, and MALDI-TOF mass spectrometry. NiL1 and NiL2 were evaluated in photoinitiating system as new photocatalysts for the Free Radical Photopolymerization of Ethoxylated (3) Trimethylolpropane Triacrylate (TMPETA) for violet and green light. Three-component systems were employed in the presence of Di-tert-butyl-diphenyl iodonium hexafluorophosphate (Iod) and ethyl dimethylaminobenzoate (EDB). The absorption, luminescence, and redox properties of nickel complexes and ligands were explored and compared for a better understanding of their structure/reactivity relationship. The designed nickel complexes showed good photoinitiation abilities. NiL2 exhibited the highest monomer conversion, and it was investigated in the on–off process under green light irradiation, demonstrating the efficiency of the system in reactivating the polymerization process. An oxidative pathway mechanism was proposed based on free energy, steady state photolysis and electron spin resonance spin trapping experiments. The best system was successfully applied in cationic photopolymerization and 3D printing.

Aryl structural effect on the photoinitiation abilities of aryl glycine derivatives for polymerization upon exposure to blue light

AbstractThe design and development of photoinitiating systems applicable to visible light delivered from light‐emitting diodes (LEDs) have attracted increasing attention owing to the wide application of photopolymerization. In this study, four aryl glycine derivatives are designed and synthesized, and their applicability as visible light‐sensitive photoinitiators is thoroughly investigated. Specifically, the photoinitiation mechanism of these aryl glycine derivatives, when combined with iodonium salt, is investigated using steady‐state photolysis, fluorescence, and electron paramagnetic resonance spin trapping techniques. It is revealed that radicals can be generated from aryl glycine derivatives/iodonium salt combinations upon exposure to blue LEDs (410 and 445 nm) to induce free radical photopolymerization (FRP) of (meth)acrylates. Additionally, besides FRP, a photobase generator based on one of the investigated aryl glycine derivatives is synthesized and demonstrates the capability to initiate epoxy‐thiol polymerization under light irradiation. The remarkable photolatent characteristics demonstrate the significant potential in broadening the application of aryl glycine derivatives in controlled photopolymerization processes.

New monomers or co-monomers based on the alkoxyfuranone Scaffold: Toward new alternatives to Petroleum-Based structures

Over the past decade, the circular economy has become a key component of sustainability due to environmental issues and more stringent environmental regulations. In this connection, the valuation of bio-based monomers possessing innovative structures remains an important challenge. Furthermore, several goals must be achieved: 1) the homopolymerization of such bio-based monomers by free radical photopolymerization which requires small energy inputs with limited emission of volatile organic compounds (COV) and 2) their copolymerization with Elium® thermoplastic resins to increase biogenic carbon in these novel low-viscosity methacrylic resins developed by Arkema. The challenge will be, in this context, to maximize the biomass carbon content while maintaining or even surpassing the properties exhibited by their petrochemical counterpart. In this work, a series of alkoxyfuranones derived from furfural have been synthesized. The four alkoxyfuranones (AF1, AF2, AF3 and AF4) varying by the functionality of the side chains (but-3-en-1-yl, acrylate, isopropyl and ethyl) attached via acetalization of the 5-hydroxy-2(5H)-furanone were studied as potential monomers or co-monomers for free radical polymerization. The synthesis of AF1 and AF2 has never been reported in the literature. The homopolymerization of two of them (AF1 and AF2) was successfully achieved, but only one led to a high-glass transition temperature (Tg) bio-based polymer. On the other hand, copolymerization with Elium® thermoplastic resins was obtained with different ratios of AF2 as a co-monomer. As a result, the expected gain value, such as faster photopolymerization was successfully obtained. Nevertheless, the copolymerization doesn’t lead to obtaining high-Tg polymers.

Triphenylamine based oxime ester photoinitiator for LED-induced free radical photopolymerization and 3D printing

With the development of light-emitting diode (LED), it is increasingly important to design and prepare photoinitiators that can be used for LED excitation. We designed and synthesized four new LED oxime ester photoinitiators (TOXEs) based on the triphenylamine group. Among them, the maximum absorption wavelengths of TOXE-3 and TOXE-4 are red-shifted to 402 nm and 397 nm, and have high molar absorption coefficients (ε) of 24315 M-1cm−1 and 29915 M-1cm−1, respectively, which are higher than that of ethenone, 1-[9-ethyl-6-(2-methylbenzoyl)-9H-carbazol-3-yl] ethanone 1-(O-acetyloxime) (OXE02, 342 nm, 21836 M-1cm−1), indicating that it can be used in LED polymerization. Investigating the photopolymerization properties of the TOXEs/α, ω-diacryloyl poly(ethylene glycol) (PEGDA) system, under LED@365 nm irradiation, the double bond conversion rate (DC) of TOXE-1 and TOXE-2 were 83 % and 92 %, which were higher than OXE02 (80 %). Under LED@405 nm irradiation, the DC of TOXE-3 and TOXE-4 are 80 % and 78 % respectively, which is higher than that of OXE02 (52 %). To improve the photoinitiation ability of TOXEs, a two-component photoinitiation system using bis(4-tert-butylphenyl) iodonium (Iod) as an additive was tested. The DC of TOXEs has been improved under different light sources. Under LED@480 nm irradiation, the final DC of TOXE-4/Iod/PEGDA is 83 %. In addition, TOXE-3 and TOXE-4 can be applied to the laser cladding deposition (LCD) 3D printers. At the same time, we conduct research and verification on the polymerization mechanism of TOXEs through steady-state photolysis and electron spin resonance spin trapping experiments (ESR-ST). In addition, cytotoxicity assay and thermal stability testing showed that TOXEs exhibited excellent biocompatibility and thermal stability.

Typ‐I‐Photoinitiator auf der Basis nachhaltiger Kohlenstoffpunkte

AbstractNachhaltige Kohlenstoffpunkte, die an der Oberfläche Oximestergruppen enthalten, welche einer homolytischen Bindungsspaltung folgen, zeigen ein Potential als Photoinitiatoren für die traditionelle freie radikalische Photopolymerisation. Die Kohlenstoffpunkte wurden nach einem solvothermalen Verfahren aus nachhaltigem Furfural, welches über Lignocellulose erhältlich ist, hergestellt. Aldehydgruppen der Oberfläche reagierten mit Hydroxylamin zu dem entsprechendem Oxim, während die Reaktion mit Benzoylchlorid zu einem bio‐basierten Typ I Photoinitiator (CD‐PI) führte, welcher nachhaltige Kohlenstoffpunkte enthält. Die Fähigkeit zur Photoinitiierung wurde unter UV‐Bestrahlung bei 365 nm mit dem traditionellen Photoinitiator (PI) Ethyl (2,4,6‐trimethyl benzoyl) phenyl phosphinat (TPO‐L) mittels Echtteit FTIR Spektroskopie verglichen. Die auf einem Gemisch von Urethandimethacrylat (UDMA) und Tripropylenglykoldiacrylat (TPGDA) basierte Photopolymerzusammensetzung führte unter Verwendung von CD‐PI oder TPO‐L zu einem ähnlichen Endumsatz von etwa 70 %. Dennoch erschien das vernetzte Material im Fall der Zusammensetzungen, die mit CD‐PI verarbeitet wurden, homogen, während jene, die mit TPO‐L hergestellt wurden, heterogen waren, was durch die beiden Glasübergangstemperaturen gezeigt wurde. Außerdem war die Migrationsgeschwindigkeit von CD‐PI in den gehärteten Proben geringer im Vergleich zu jenen Proben für die TPO‐L als PI eingesetzt wurde.

Type I Photoinitiator Based on Sustainable Carbon Dots.

Sustainable carbon dots comprising surficial oxime ester groups following homolytic bond cleavage exhibit potential as photoinitiators for traditional free radical photopolymerization. Carbon dots were made following a solvothermal procedure from sustainable furfural available from lignocellulose. Surficial aldehyde moieties reacted with hydroxylamine to the respective oxime while reaction with benzoyl chloride resulted in a biobased Type I photoinitiator comprising sustainable carbon dot (CD-PI). Photoinitiating ability was compared with the traditional photoinitiator (PI) ethyl (2,4,6-trimethyl benzoyl) phenyl phosphinate (TPO-L) by real-time FTIR with UV exposure at 365 nm. Photopolymer composition based on a mixture of urethane dimethacrylate (UDMA) and tripropylene glycol diacrylate (TPGDA) resulted in a similar final conversion of about 70 % using either CD-PI or TPO-L. Nevertheless, it appeared homogeneous in the case of compositions processed with CD-PI, while those made with TPO-L were heterogeneous as shown by two glass transition temperatures. Moreover, the migration rate of CD-PI in the cured samples was lower in comparison with those samples using TPO-L as PI.

The role of the Terminal Benzene Derivatives in Triphenylamine‐Based Oxime Esters for Free Radical Photopolymerization

AbstractIn this study, we investigate the effect of terminal benzene derivatives in triphenylamine‐based oxime esters on their photoreactivity. Five novel oxime esters (Miko series) that containing different benzene derivatives as terminal groups, were synthesized. The electronic properties of the substituents on the benzene ring range from electron‐donating to electron‐withdrawing. Specifically, a methoxy benzene for Miko‐MOB, tert‐butyl benezene for Miko‐t‐Bu, chloro benzene for Miko‐CB, trifluoromethyl benzene for Miko‐TFM, and nitro benzene for Miko‐NB. Additionally, a compound with benzene only as the terminal substituent (TP‐1) was selected for comparision. All the new compounds exhibited higher molar extinction coefficients than TP‐1. Additionally, the steady‐state photolysis and electron spin resonance (ESR) properties would decrease from the electron‐donating substitutes to electron‐withdrawing substitutes. Finally, these oxime esters are utilized in Type I photoinitiating systems for the free radical polymerization of trimethylolpropane triacrylate (TMPTA) under UV or LED@405 nm light irradiation conditions. Among all formulations, Miko‐MOB demonstrated the highest double bond conversion efficiency under both UV and LED@405 nm light irradiation. Thus, terminal benzene derivatives in oxime esters play an important role in tuning the optical and photochemical reaction performance, offering significant value for the design strategy of such photoinitiators.

Novel D-π-A hemicyanine dye as photoinitiators for insitu hydrogel formation and DLP printing.

The field of biofabrication imposes stringent requirements on the polymerization activity and biosafety of photopolymeric hydrogel systems. In this investigation, we designed and synthesized four hemicyanine dyes with a D-π-A structure specifically tailored for biofabrication purposes. These novel dyes, incorporating carbazole (CZ), triphenylamine (TPA), anthracene (AN), and benzodithiophene (BDT) as electron donors, along with heterocyclic salt (IN) as electron acceptors, were prepared using a straightforward synthesis method. The absorption maxima of ANIN, CZIN, and TPAIN exceeded 500 nm, rendering them suitable co-initiators for the free radical photopolymerization of acrylates under green-red light exposure facilitated by light-emitting diodes (LEDs) and the co-initiator iodonium salt (ION). Notably, CZIN and TPAIN, due to their robust dye absorption and efficient electron transfer to ION, functioned as high-performance photosensitizers. Meanwhile, BDTIN, with its strong and broad absorption range (400-600 nm), enhanced the accuracy of visible light photopolymerization. These dyes exhibit characteristics such as facile synthesis, heightened photo stability, and non-toxicity and also demonstrate the ability to discern the alkalinity of a solution to some extent. Furthermore, we explored the application of these hemicyanine dyes in 3D printing, showing potential to enhance printing resolution in DLP 3D printing (digital light process 3D printing).

Coumarin based glyoxylate photoinitiators for free radical and cationic Photopolymerizations with UV-Visible LED irradiation

High-performance photoinitiators (PIs) play a crucial role in ultraviolet–visible (UV–Vis) photopolymerization. In this study, three coumarin-3-oxoacetic acid methyl esters (COAEs) which demonstrate significant UV–Vis absorption within the 300–500 nm range were synthesized and studied. Real-time fourier transform infrared (RT-FTIR) and photo-differential scanning calorimetry (photo-DSC) experiments demonstrated that these PIs can function as Type I and Type II PIs for initiating free radical polymerization (FRP) of acrylate monomers. Additionally, they can serve as photosensitizers for iodonium salts and sulfonium salts, enabling cationic photopolymerization (CP) of epoxy and oxetane monomers. Remarkably, even at low concentrations of 0.01 wt%, they can effectively initiate the deep curing of Tri-(propylene glycol) diacrylate (TPGDA) to a depth of approximately 9 cm after 5 min of irradiation with a 415 nm or 450 nm LED light source. Furthermore, the COAEs exhibit satisfactory photobleaching properties, making them suitable for colorless applications in photopolymerization. The photochemical and photophysical properties of the COAEs were thoroughly investigated using absorption spectroscopy, electron spin resonance, cyclic voltammetry experiments, and photoinduced decarboxylation experiments to elucidate the photoinitiation mechanism. Cytotoxicity experiments revealed that COAE-N exhibited good cytocompatibility. Therefore, the exceptional polymerization properties of COAEs offer promising prospects for FRP and CP applications. Additionally, their efficient photobleaching characteristics make them highly suitable for deep photopolymerization applications. Moreover, the low cytotoxicity of COAEs suggests significant potential for biomedical applications such as dental restoration.

Photobleachable coumarin-thioester derivatives with low cytotoxicity as visible-light photoinitiators for free radical and cationic photopolymerization

The development of photoinitiators is suffering from deep color, low solubility, and biotoxicity caused by the introduction of large-conjugated structures, also the complex synthetic procedures constrain the application of photoinitiators. To overcome this issue, nine kinds of coumarin thioester photoinitiators (CTEs) have been delicately designed and synthesized through a simple, metal-free two-step reaction. CTEs with excellent light absorption properties in the visible light range can be used not only as a type I photoinitiator but also as a type II photoinitiator, and sensitizer in the cationic photoinitiating system. CTEs effectively initiate the photopolymerization of tripropylene glycol diacrylate (TPGDA) under the irradiation of 405 nm LED, and the double bond conversions are similar to or better than those of commercial photoinitiators. Also, the cationic photopolymerization of bisphenol A diglycidyl ether (BADGE) is successfully initiated by CTEs/IOD systems. Notably, the epoxy group conversion initiated by 6O-CTE/IOD reaches 72.1 %, surpassing that of the commercial photoinitiating system. The photoinitiating mechanism of CTEs is proposed and supported by theoretical calculations. Furthermore, the bond cleavage abilities of CS bonds endow CTEs with photobleaching properties. The colorless cured film initiated by 6O-CTE was obtained under only 15 s irradiation, indicating their application prospects of CTEs in colorless photopolymerization. More importantly, the cytotoxicity of CTEs is much lower than that of commercial photoinitiator TPO. This work provides new insight into the design of biocompatible photoinitiators with a photobleaching property for LED photopolymerization.

Novel High-Performance Glyoxylate Derivative-Based Photoinitiators for Free Radical Photopolymerization and 3D Printing with Visible LED.

Investigations concerning the glyoxylate moiety as a photocleavable functional group for visible light photoinitiators, particularly in the initiation of free radical photopolymerization remain limited. This study introduces nine innovative carbazole-based ethyl glyoxylate derivatives (CEGs), which are synthesized and found to exhibit excellent photoinitiation abilities as monocomponent photoinitiating systems. Notably, these structures demonstrate robust absorption in the near-UV/visible range, surpassing the commercial photoinitiators. Moreover, the newly developed glyoxylate derivatives show higher acrylate function conversions compared to a benchmark photoinitiator (MBF) in free radical photopolymerization. Elucidation of the photoinitiation mechanism of CEGs is achieved through a comprehensive analysis involving the decarboxylation reaction and electron spin resonance spin trapping. Furthermore, their practical utility is confirmed during direct laser writing and 3D printing processes, enabling the successful fabrication of 3D printed objects. This study introduces pioneering concepts and effective strategies in the molecular design of novel photoinitiators, showcasing their potential for highly advantageous applications in 3D printing.

Designing unimolecular photoinitiator by installing NHPI esters along the TX backbone for acrylate photopolymerization and their applications in coatings and 3D printing

Developing efficient and long wavelength sensitive unimolecular photoinitiators (PIs) is still facing a great challenge. In this work, a series of thioxanthone-based N-hydroxyphthalimide esters (TX-NHPIEs) were synthesized by installing NHPIEs along the TX backbone and characterized. The investigated TX-NHPIEs have a 60 nm redshift and demonstrate sterling initiating efficiency for free radical photopolymerization (FRP) under LED@450 nm light irradiation compared with the commercialized isopropylthioxanthone (ITX). Real-time 1Hnuclear magnetic resonance (1H NMR), electron spin resonance (ESR), decarboxylation and gas chromatograph-mass spectrometer (GC–MS) experiments and density functional theory (DFT) reveal that TX-NHPIEs can generate one alkyl radical and one N-centered iminyl radical, which can initiate FRP directly and indirectly, respectively. In other words, TX-NHPIEs absorb one photon and can generate two active radicals, which break through the limitations of common PIs. TX-NHPIE-Cpe demonstrates the highest initiating efficiency, and its application in coatings and 3D printing was also studied, indicating TX-NHPIEs have broad potential applications in photopolymerization processes.

Naphthoquinone-imidazolyl derivatives-based oxime esters as photoinitiators for blue LED-induced free radical photopolymerization

In this study, we report the successful design and synthesis of nine novel oxime esters based on the naphthoquinone-imidazolyl scaffold and serving as photoinitiators to trigger photopolymerization processes under visible light. In the presence of acrylate monomers and upon irradiation at 405 nm with a LED, these photoinitiators showed remarkable photoinitiating abilities. Newly developed photoinitiators exhibited a multifunctional photoinitiating ability since these structures could act both as Type I photoinitiators or in multicomponent systems, with an improvement of the monomer conversions in the second case. This performance enhancement was notably evidenced in two-component systems comprising the new compounds in combination with either an iodonium salt or an amine. This enhancement of the monomer conversion can be assigned to favorable interactions between the dyes and the different additives, enabling to improve the free radical generation. The underlying chemical mechanisms that govern the radical chemistry during the different photopolymerization processes were explored through a series of systematic experiments. To end, usefulness of these photoinitiators was confirmed by the successful production of 3D objects using the Direct Laser Write method. This suggests promising applications of these photoinitiators in 3D printing technologies.

Photoinitiators from bio-sourced naphthoquinone – the application of naphthoquinone-based vitamins K1 and K3 in free radical photopolymerization

Two common bio-sourced naphthoquinone derivatives, vitamin K1 (phylloquinone) and vitamin K3 (menadione), were employed in free radical photopolymerization under LED light exposure at 405 nm.

Bisbenzothieno[b]-fused BODIPYs in panchromatic photoinitiating for the free radical and cationic photopolymerization and application in 3D printing

Abstract: Polymerization reactions induced by lights occupy an increasing share in the area of polymer synthesis; however, the effective photopolymerization under visible light emitting diodes (LEDs) rather than UV still...