Photoinitiation Ability Research Articles

Photoinitiation Mechanism and Ability of Thioxanthone-Based Versatile Visible Photoinitiators

Photoinitiation Mechanism and Ability of Thioxanthone-Based Versatile Visible Photoinitiators

Visible-Light-Sensitive Triazine-Coated Silica Nanoparticles: A Dual Role Approach to Polymer Nanocomposite Materials with Enhanced Properties.

Nanocomposite materials are of great interest because of their superior properties. Besides the traditional synthesis methods that require high temperatures or toxic solvents, photopolymerization technology provides a simple, low-cost, and environmentally friendly route in preparing nanocomposites. In this research, the preparation of blue-light-sensitive triazine derivative-coated silica nanoparticles is presented. The resulting triazine-coated silica nanoparticles can play a dual role, i.e., acting as both photoinitiators to trigger photopolymerization reactions under the irradiation of LED@410 nm and fillers to endow the produced photopolymer nanocomposite materials with enhanced properties. Specifically, the triazine-coated silica nanoparticles can successfully induce free radical polymerization of trimethylolpropane triacrylate efficiently under the irradiation of LED@410 nm and demonstrate comparable photoinitiation ability to the triazine derivative-based photoinitiator. The effects of different loading amounts of triazine-coated silica nanoparticles toward the photopolymerization kinetics are also evaluated. By coating with the triazine derivative, the nanoparticles show good dispersion in the polymer matrix and significantly reduce the shrinkage of the samples during the photopolymerization. Moreover, the photocured nanocomposites exhibit enhanced migration stability and mechanical properties when an optimal amount of triazine-coated silica nanoparticles is added in the formulation.

Significantly improve the photoinitiation ability of hydroxyalkyl-derived polymerizable α-hydroxyalkylacetophenone photoinitiators by blocking hyperconjugation

Five ester derivatives from hydroxyalkyl of 2-hydroxy-2-methyl-1-phenyl propanone, one of the most successfully applied α-hydroxyalkyl acetophenone photoinitiators, are synthesized. From the spectroscopy results and theory calculation, it is proved that the much lower photocleavage rates of the directly linked unsaturated ester derivatives are due to that the strong electron withdrawing effect of carbonyl increase the bond strength between the benzoyl and quaternary carbon and the effective energy transfer from benzoyl to double bond because of the existence of p-σ-π hyperconjugation, which consequently decreased their initiating ability. Excitingly, simple introduction of two-methylene spacer between the carbonyl and alkoxyl can effectively block the hyperconjugation and significantly increase the photocleavage rate up to the same level of their mother commercial photoinitiator, which can open a gate for design of new high efficiency and polymerizable type I photoinitiators.

Enone dyes as visible photoinitiator in radical polymerization: The influence of peripheral N-alkylated (hetero)aromatic amine group

In this work, three enone dyes (C3s) bearing N-alkylated pyrrole (C3PY), indole (C3ID) and carbazole (C3CZ) group were synthesized and characterized to explore the effect of peripheral group on their photoinitiation ability. C3s display good light absorption in the region of 300–500 nm. Under 405/460 nm LED irradiation, the polymerization kinetics of a diacrylate monomer in the presence of one-component C3s or bimolecular C3s/triethanolamine (TEOA) were investigated by Real-time FT-IR technology. Polymerization profiles show that C3s could initiate the polymerization and TEOA significantly promote the initiation efficiency of C3CZ, while this acceleration effect is relatively small for C3PY and C3ID. To decipher these phenomenon, steady state photolysis, cyclic voltammetry and theoretical calculation were jointly performed. Accordingly, it is proposed that C3PY and C3ID tend to photolyze through intramolecular hydrogen abstraction. This work show that the peripheral group strongly affect the initiation ability of enone photoinitiator.

Substituted Stilbene-based D-π-A and A-π-A type oxime esters as photoinitiators for LED photopolymerization

Three novel oxime esters based on the π-conjugated trans-stilbene group were designed and synthesized as photoinitiators (PIs) for light-emitting diode (LED) photopolymerization. Three substituents (thioether, sulfoxide, and sulfone) with different electron pushing and withdrawing abilities were introduced into the chromophore to constitute D-π-A (OXE-S) and A-π-A (OXE-SO, OXE-SO2) type structures of PIs to rationalize their relevant effects on free radical photoinitiation ability. Structure-activity relationship, electronic properties, and photodissociation mechanism were investigated using steady-state photolysis, density functional theory (DFT) calculations, and real-time 1H NMR. The compounds show good absorption under exposure to LED irradiation wavelength (i.e., 365–405 nm). The polarity of solvent influences the photoreactivity of this series of oxime esters. The photolysis rates and free radical generation quantum yields for the three PIs increase from polar to nonpolar solvent. For D-π-A type OXE-S, the photolysis rate increases 70 times from acetonitrile to toluene. The three PIs can efficiently initiate the polymerization of acrylate monomer under exposure to LED irradiation at 365–405 nm. The excellent absorption and efficient photoinitiation properties of the PIs demonstrate their great potential in photocuring applications.

Novel phenylamine‐based oxime ester photoinitiators for LED‐induced free radical, cationic, and hybrid polymerization

AbstractFour new photoinitiators (PI's) containing various phenylamine‐based groups as chromophores and oxime ester (OXE) as initiating groups are proposed for the free radical polymerization, the cationic polymerization, and interpenetrated polymer networks upon visible light exposure using a light emitting diodes at 405 nm. These compounds showed outstanding polymerization photoinitiation ability, that is, high‐polymerization rates and good final reactive function conversions were obtained. The chemical mechanisms by which these OXE's generate reactive species were investigated through different techniques including real‐time Fourier transform infrared spectroscopy, UV–visible spectroscopy, fluorescence (time‐resolved or steady state) as well as cyclic voltammetry. Markedly, the proposed structures can be used as thermal initiators indicating all of them can be polymerized under suitable reaction temperature. In addition, due to their high efficiency, these optimized PI's were found to be useful @405 nm for both direct laser write and 3D printing applications.

Terthiophene Derivative-Based Photoinitiating Systems for Free Radical and Cationic Polymerization under Blue LEDs

Terthiophene derivatives were prepared and utilized as photoinitiators of photopolymerization reactions upon exposure to LEDs (410 and 445 nm). The photochemical mechanisms of the terthiophene-based photoinitiating systems were investigated and confirmed by steady-state photolysis, fluorescence quenching, and electron paramagnetic resonance spin-trapping techniques. The photoinitiation ability of terthiophene derivatives in combination with the iodonium salt and optionally with a tertiary amine in the free radical polymerization of multifunctional acrylates and cationic polymerization of epoxides under mild light irradiation was investigated. Exceptionally, a sodium salt form of a terthiophene derivative presented exceeding water solubility and admirable efficiency in rapid hydrogel formation from the 2-hydroxy ethyl acrylate and water blend. The storage stability of the effective photoinitiating systems in the formulations was also studied under moderate conditions.

Penta‐Hydroxy Flavones‐Based Photoinitiating Systems for Free Radical, Cationic, and Thiol‐Ene Polymerization upon Exposure to Mild Blue LEDs

AbstractNatural flavone‐derivatives (i.e., morin and quercetin) are investigated as visible light‐sensitive photoinitiators. The photochemical mechanisms of the radical generation from flavone‐derivative‐based photoinitiating systems (PISs) upon exposure to mild blue light‐emitting diodes are studied. The impact of substituent site of functional groups on the photoinitiation ability is determined. The quercetin‐based PISs exhibit more reactivity than the morin‐based systems on free radical photopolymerization. Moreover, the investigated flavone‐based PISs present their stabilities at room temperature, indicating that resin containing these systems barely require special storage requirements. Thiol‐ene polymerization initiated by the flavone‐derivatives‐based PISs is also investigated.

Synthesis and photoinitiating ability of substituted 4,5-di-tert-alkyl-o-benzoquinones in radical polymerization

Synthesis and photoinitiating ability of substituted 4,5-di-tert-alkyl-o-benzoquinones in radical polymerization

Newly Synthesized Chromophore‐linked Iodonium Salts as Photoinitiators of Free Radical Photopolymerization

AbstractIodonium salts are the topic of interest in the field of photopolymerization as they act as effective photoinitiators and generators of photoacid. Yet, the current commercially available iodonium salts have low absorption above 300 nm whereby limiting the use of the long‐wavelength of light irradiations to trigger polymerization. Iodonium salts are thus generally used in couples with photosensitizers (e.g., dyes) to extend the usage under irradiation of red‐shifted light wavelength. In this research, the synthesis of chromophore‐linked (anthraquinone‐based and flavone‐based chromophores) iodonium salts is present. They can initiate free radical polymerization with 320–480 nm irradiation and show much better photoinitiation ability as compared to commercial iodonium salts, especially when they act as mono‐component photoinitiators. Steady‐state photolysis and electron spin resonance spin trapping experiments are also carried out to evaluate the relevant photochemical mechanism.

Type I Photoinitiator-Functionalized Block Copolymer Nanoparticles Prepared by RAFT-Mediated Polymerization-Induced Self-Assembly.

Type I photoinitiators have been widely used in UV-vis curing technology for the fabrication of functional polymer materials such as coatings, inks, and adhesives. To overcome the drawbacks of using small molecular type I photoinitiators and expand the potential applications of UV-vis curing technology, attaching type I photoinitiators onto the surface of polymer colloids is an attractive strategy. Here we report a robust strategy for the efficient preparation of type I photoinitiator-functionalized block copolymer nanoparticles with various morphologies via aqueous reversible addition-fragmentation chain transfer (RAFT)-mediated polymerization-induced self-assembly (PISA), in which the photoinitiating ability of the type I photoinitiator end group provides a landscape for further functionalization. These block copolymer nanoparticles could also be used as heterogeneous photoinitiators to generate hydrogels with nanoparticles embedded inside. Significantly, the properties and functionalities of these hydrogels could be further controlled by using different block copolymer nanoparticles. This study provides a robust strategy toward the preparation of type I photoinitiator-functionalized block copolymer nanoparticles with the capacity to be modified with varying functionalities.

Substituent effects on the photoinitiation ability of coumarin-based oxime-ester photoinitiators for free radical photopolymerization

In this study, a series of coumarin chromophore-based oxime-esters were designed and synthesized as visible light photoinitiators (PIs).

Design of ketone derivatives as highly efficient photoinitiators for free radical and cationic photopolymerizations and application in 3D printing of composites

AbstractHerein, thirteen ketone derivatives composed of different cyclohexanone cores and peripheral moieties are designed, among which 10 ketones have never been synthesized before. These ketones are proposed as high‐performance photoinitiators for both free radical polymerizations and cationic polymerizations under soft conditions (visible LED@405 nm irradiation at room temperature). In combination with an amine and an iodonium salt (Iod), these ketones could be used in three−component photoinitiating systems to initiate the free radical polymerization of acrylates with distinct final conversions, among which the ketone−1/amine/Iod combination proved to be the most efficient one. Besides, the ketone−1/Iod two−component system also showed a remarkable photoinitiation ability for the cationic polymerization of epoxides. The photochemical sensitivity of ketone−1 in the presence of an amine and an iodonium salt was systematically investigated by steady state photolysis and excited state fluorescence quenching characterizations, respectively. Interestingly, macroscopic 3D patterns with excellent spatial resolution could be generated using the ketone−1/amine/Iod photoinitiating system for the free radical polymerization of acrylates. This high performance is also found useful to overcome the light penetration issue for the access to filled samples (silica) and the preparation of composites.

In-silico based development of photoinitiators for 3D printing and composites: Search on the coumarin scaffold

In this work, the in-silico rational design of new photoinitiators by molecular modeling for specific wavelength (here 405 nm) and specific applications (3D printing, composites) is reported. A large number of (keto)coumarin derivatives were investigated by molecular modeling and their synthesis and more detailed photochemical investigations are based on obtaining structures having both excellent predicted light absorption properties @405 nm and high excited state energy levels (to ensure high photochemical reactivity). More particularly, four new families of coumarins were designed (4 of the 19 proposed coumarins were never synthesized (N2,M6,T1,T6)): the first family is based on Nitrocoumarins (N1-N6), the second one on Methoxybenzene-based coumarins and Ethoxycoumarins (M1-M6), the third one on Thiophene-based coumarins (T1-T6) and the last family studied concerns Alkyne-based coumarin (A1). The purpose of this work concerns the study of the photoinitiating ability of these compounds in different monomers for different polymerization processes (free radical, cationic) using FTIR technique. The different compounds reported in this work are very efficient to initiate the free radical polymerization of (meth)acrylates but also the cationic polymerization of epoxides upon mild irradiation conditions using a Light Emitting Diode (LED) at 405 nm as visible light source. Nitrocoumarins were identified as the best candidates for photoinitiation among the different families of coumarins investigated in this work. More precisely, nitrocoumarins are characterized by very good polymerization profiles, great final reactive function conversions (FC) and also high rates of polymerization (Rp). The electrochemical and photochemical properties of the different compounds were also studied to get a deeper insight into the photochemical mechanisms supporting the initiation process. A full picture of the involved photochemical mechanisms is provided. Thanks to the astounding polymerization initiating ability of these coumarins, their use in 3D printing applications can be worthwhile. Remarkably, using these compounds, the preparation of photocomposites was possible even in difficult light penetration conditions resulting from the presence of fibers inside the resins.

Substituent Effects on Photoinitiation Ability of Monoaminoanthraquinone-Based Photoinitiating Systems for Free Radical Photopolymerization under LEDs.

Three monoamino-substituted anthraquinone derivatives (AAQs), that is, 1-aminoanthraquinone (AAQ), 1-(methylamino)anthraquinone (MAAQ), and 1-(benzamido)anthraquinone (BAAQ), incorporated with various additives [e.g., triethanolamine (TEAOH) and phenacyl bromide (PhC(═O)CH2 Br)] are investigated for their roles as photoinitiating systems of free radical photopolymerization of (meth)acrylate monomers upon the exposure to UV to green LEDs. The AAQs-based photoinitiating systems, AAQ/TEAOH/PhC(═O)CH2 Br and BAAQ/TEAOH/PhC(═O)CH2 Br photoinitiators exhibit the highest efficiency for the free radical photopolymerization of DPGDA under the irradiation of blue LED and UV LED, respectively, which is consistent with the extent of overlap between their absorption spectra and the emission spectra of the LEDs. AAQ/TEAOH/PhC(═O)CH2 Br photoinitiator can also initiate the free radical photopolymerization of different (meth)acrylate monomers, with an efficiency dependent on the chemical structures of these monomers.

Naphthoquinone derivatives: Naturally derived molecules as blue-light-sensitive photoinitiators of photopolymerization

Two naphthoquinone derivatives, i.e. 5-hydroxy-1,4-naphthoquinone (5HNQ) and 2-hydroxy-1,4-naphthoquinone (2HNQ), combined with different additives such as diphenyliodonium hexafluorophosphate (Iod), N-vinylcarbazole (NVK), phenacyl bromide (R-Br), N-phenyl glycine (NPG) and ethyl 4-dimethylamino benzoate (EDB), are investigated as photoinitiating systems (PISs) for free radical polymerization of trimethylolpropane triacrylate (TMPTA) and cationic polymerization of triethyleneglycol divinyl ether (DVE-3). The photopolymerization of the thiol-ene system trimethylolpropane tris(3-mercaptopropionate) (Trithiol)/DVE-3 with 5HNQ or 2HNQ based PISs are also investigated. 5HNQ shows maximum absorption in visible light region (blue light) which thus has the potential to perform under the irradiation of blue LED light. For the free radical polymerization of TMPTA, 5HNQ/Iod/NPG shows the highest photoinitiating ability among all the naphthoquinone derivative-based PISs, while 2HNQ-based systems are inefficient. The addition of Trithiol to TMPTA can significantly increase the double bond conversion of TMPTA during the photopolymerization. For the cationic photopolymerization of DVE-3, 2HNQ/Iod shows the highest photoinitiating ability among all the naphthoquinone derivative-based photoinitiating systems. 5HNQ-based systems also demonstrate good photoinitiation ability for DVE-3, but with slower polymerization rate compared to 2HNQ-based systems. In thiol-ene photopolymerization, 5HNQ PISs shows better photoinitiation ability than 2HNQ PIs systems. The results illustrate that the position of hydroxyl group in 1,4-naphthoquinone molecules plays an important role on photoinitiation ability of naphthoquinone derivatives. Steady state photolysis and electron spin resonance trapping experiments are also carried out to evaluate the relevant photochemical mechanism.

1,2‐Diketones as photoinitiators of both cationic and free‐radical photopolymerization under UV (392 nm) or Blue (455 nm) LEDs

ABSTRACTThe photoinitiation abilities of three 1,2‐diketones [i.e., acenaphthenequinone (ANPQ), aceanthrenequinone (AATQ), and 9,10‐phenanthrenequinone (PANQ)]‐based photoinitiating systems [PISs, with additives such as iodonium salt, N‐vinylcarbazole (NVK), tertiary amine, and phenacyl bromide (R‐Br)] for cationic photopolymerization and free‐radical photopolymerization under the irradiation of ultraviolet (UV; 392 nm) or blue (455 nm) light‐emitting diode (LED) bulb are investigated. All 1,2‐diketones studied exhibit ground state absorption that match with the emission spectra of UV (392 nm) or blue LED (455 nm) better than that of the well‐known blue‐light‐sensitive photoinitiator camphorquinone (CQ). In particular, AATQ/iodonium salt/NVK can show high photoinitiating ability (with epoxide conversion yield >70%) under the UV light irradiation due to the effect of NVK. In addition, 1,2‐diketone/iodonium salt (and optional NVK) systems are capable of initiating free‐radical photopolymerization of methacrylates, with conversions of 50–58%. Furthermore, some 1,2‐diketone/tertiary amine (and optional R‐Br) combinations are found to demonstrate high efficiency to initiate free‐radical photopolymerization, and 71% of methacrylate conversion can be achieved with PANQ/tertiary amine/R‐Br PIS. Some 1,2‐ketone‐based PISs can even exhibit higher efficiency than the CQ‐based systems. The photochemical mechanism of the radical generation from the 1,2‐diketone‐based PISs is investigated and found to be consistent with the related photopolymerization efficiency. © 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020, 58, 792–802

Design of Iodonium Salts for UV or Near-UV LEDs for Photoacid Generator and Polymerization Purposes.

Iodonium salts are well established photoacid generators, cationic photoinitiators, as well as additives commonly used in photoredox catalytic cycles. However, as a strong limitation, iodonium salts are characterized by low light absorption properties for λ > 300 nm so that these latter cannot be activated with cheap, safe, and eco-friendly near UV or even visible light emitting diodes (LEDs). To overcome this drawback, the covalent linkage of an iodonium salt to a chromophore absorbing at longer wavelength is actively researched. With aim at red-shifting the absorption spectrum of the iodonium salt, the synthesis of new compounds combining within a unique chemical structure both the chromophore (here the naphthalimide scaffold) and the iodonium salt is presented. By mean of this strategy, a polymerization could be initiated at 365 nm with the modified iodonium salts whereas no polymerization could be induced with the benchmark iodonium salt i.e., Speedcure 938 at this specific wavelength. To examine the effect of the counter-anion on the photoinitiating ability of these different salts, five different counter-anions were used. Comparison between the different anions revealed the bis(trifluoromethane)sulfonimide salt to exhibit the best photoinitiating ability in both the free radical polymerization of acrylates and the cationic polymerization of epoxides. To support the experimental results, molecular orbital calculations have been carried out. By theoretical calculations, the initiating species resulting from the photocleavage of the iodonium salts could be determined. The cleavage selectivity and the photochemical reactivity of the new iodoniums are also discussed.

Yellow Triazine as an Efficient Photoinitiator for Polymerization and 3D Printing under LEDs

AbstractThe search for new efficient photoinitiators for polymerization under the illumination of visible light is under extensive investigation due to their wide application in 3D printing. 2‐(4‐Methoxystyryl)‐4,6‐bis(trichloromethyl)‐1,3,5‐triazine (yellow triazine, R‐Cl) alone or the R‐Cl/additive systems have been indicated as promising photoinitiating systems in the free radical polymerization at 405 nm in our previous study. Here, further investigation of R‐Cl in the polymerization of a series of difunctional (meth)acrylate monomers is described. The effect of monomers on the polymerization performance is elucidated. The interplay of functional group, double bond concentration, and fluidity determines the polymerization rate and final conversions. Remarkably, R‐Cl shows better photoinitiating ability than well‐known commercial photoinitiator bisacylphosphine oxide in the polymerization of bis‐GMA/TEGDMA blend and EB605 under LED at 400 nm. The optimum concentrations of R‐Cl in the photopolymerization of TetEGDA and DPGDA monomers are revealed, the results of which are used to guide the 3D printing of DPGDA.

Photoinitiation Mechanism and Ability of Monoamino-Substituted Anthraquinone Derivatives as Cationic Photoinitiators of Polymerization under LEDs.

The design and development of photoinitiating systems applicable to UV or even visible light delivered from light-emitting diodes (LEDs) has been attracting increasing attention due to their great potential applications in various fields. Compared to the strategy of synthesizing novel compounds, the exploration of existing chemicals with interesting photochemical/photophysical properties for their usage as photoinitiators is more appealing and easily commercialized. Nevertheless, a number of compounds such as monoamino-substituted anthraquinone derivatives, which are intensively investigated for their photophysical and photochemical properties, have seldom been studied for their roles as photoinitiators under LED irradiation. Herein, three monoamino-substituted anthraquinone derivatives, that is, 1-aminoanthraquinone, 1-(methylamino)anthraquinone and 1-(benzamido)anthraquinone, are studied for their potential as photoinitiators. The photoinitiation mechanism of these monoamino-substituted anthraquinone derivatives, when combined with iodonium salt, is first clarified using computational quantum chemistry, fluorescence, steady-state photolysis, and electron spin resonance spin-trapping techniques. Then, their photoinitiation ability for the cationic photopolymerization of epoxide and divinyl ether monomers is also investigated.