Photoinitiating systems for cationic photopolymerization: Ongoing push toward long wavelengths and low light intensities

Recently, long-wavelength photoinitiating systems for cationic photopolymerization (LWCPIs) have attracted growing interest due to the wide applications of visible light emission sources. The approaches used to expand the wavelength range of the cationic photoinitiating (CPI) system range from designing new onium salts with long-wavelength absorbing chromophores to photosensizing conventional onium salts with various visible light photosenzitizers or free radical sources. In this review, the initiating mechanisms and characteristics of each approach are introduced, and the latest developments are highlighted. Based on analysis and comparison of recent progress, forthcoming developments are proposed as well.     

A novel ferrocenium salt as visible light photoinitiator for cationic and radical photopolymerization

A novel hybrid photoinitiator for visible light photopolymerization, (η⁶-3-benzoyl-4-chlorodiphenylamine) (η⁵-cyclopentadienyl) iron hexafluorophosphate (Fc-NBP), was synthesized and studied. Its absorption in the UV and visible light regions showed much stronger activity than those of either the commercialized cationic photoinitiator I-261 or the conventional free radical photoinitiator benzophenone, especially above wavelengths of 350 nm. When exposed to visible light, the photoinitiator under study initiates both cationic polymerization and radical polymerization. The photoinitiator's abilities in the photopolymerization of acrylates and epoxides were evaluated by near infrared (NIR) spectroscopy. The results from NIR clearly indicate that Fc-NBP exhibited high efficiency in photopolymerizing acrylate monomers. In the same lamp, however, benzophenone showed no photoinitiating ability. The photopolymerization rate of the diglycidyl ether of the bisphenol-A epoxy (DGEBA) oligomer was found to be slower than that of acrylates when using Fc-NBP as the photoinitiator. This study shows that the polymerization of epoxide DGEBA can be speeded up by adding a photosensitizer benzoyl peroxide (BPO), but BPO and tertiary amines do not affect the free radical photopolymerization of tripropylene glycol diacrylate (TPGDA). We conclude by providing a possible photoinitiation mechanism.     

Macromolecular design and application using Mn2(CO)10‐based visible light photoinitiating systems

The environmentally friendly Mn₂(CO)₁₀‐based visible light photoinitiating system is a powerful method for the preparation of linear and crosslinked polymeric structures. From a practical point of view, the most important feature of this initiating system is its optical characteristics in the visible range with high quantum yield and good solubility properties. This photoinitiating system is applicable for a variety of monomers that can be polymerized via either radical or cationic mechanisms. Various complex macromolecular structures such as telechelic polymers, block and graft copolymers, polymer networks and surface modifications can be simply prepared by using halogenated precursors. This photoinitiating system is also combined with controlled radical polymerization techniques providing a mild and efficient method for polymer synthesis. © 2016 Society of Chemical Industry     

Efficient visible photoinitiator with high‐spectrum stability in an acid medium for free‐radical and free‐radical‐promoted cationic photopolymerization based on erythrosine B derivatives

Three dye‐linked photoinitiators with excellent spectral stability in an acid medium were synthesized by the covalent bonding of a coinitiator (tertiary amine) and a benzoyl group or two coinitiators to the phenolic and carboxyl group position of erythrosine B. The combination of the iodonium salt and free tertiary amino used to initiate the free‐radical/cationic visible photopolymerization was investigated to acquire the relationship between the structure and performance. The photoinitiating ability of the derivative with the phenolic position bearing a coinitiator was poorer than of the derivative with the carboxyl group bearing one because of the strong back electron transfer of the former. For the derivative with a linked coinitiator on the carboxyl position, the proximity effect between the sensitizer and the coinitiator moiety resulted in an excellent photoinitiating ability for radical/cationic polymerization; this suggested its potential for application. Although radical/cationic photopolymerization could be initiated by the derivative/coinitiator/iodonium salt, the different component ratios between them had different effects on these two polymerizations; this provided useful information for the design of effective photoinitiators for different polymerizations. On the basis of the fluorescence quenching and photopolymerization results, a corresponding synergistic mechanism was proposed. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43035.     

Spatial and temporal evolution of the photoinitiation rate for thick polymer systems illuminated by polychromatic light: selection of efficient photoinitiators for LED or mercury lamps

BACKGROUND: Four common free radical photoinitiators were evaluated for use in thick photopolymerizations illuminated with a medium‐pressure 200 W mercury–xenon arc lamp and a high‐intensity 400 nm light‐emitting diode (LED) lamp. For each photoinitiator/lamp combination, the spatial and temporal evolution of the photoinitiation rate profile was analyzed by solving the set of differential equations that govern the light intensity gradient and initiator concentration gradient for polychromatic illumination. RESULTS: The simulation results revealed that two of the four photoinitiators evaluated were ineffective for photoinitiating thick polymer systems. The photoinitiator bis(2,4,6‐trimethylbenzoyl)‐phenylphosphine oxide, in combination with the 400 nm LED lamp, was shown to be the most efficient photoinitiator/light source combination for photoinitiation of thick systems. CONCLUSION: The results show that some photoinitiators commonly used for photopolymerization of thin coatings are ineffective for curing thick systems. LED light sources provide advantages over traditional mercury lamps, and may have tremendous potential in the effective photoinitiation of thick polymer systems. Copyright © 2008 Society of Chemical Industry     

Radical photopolymerization reactions under air upon lamp and diode laser exposure: The input of the organosilane radical chemistry

A new concept based on the conversion of the peroxyl radicals generated under air (ROO) with the concomitant formation of new initiating radicals to overcome the oxygen inhibition in particular free radical photopolymerization reactions (low intensity, low viscosity, relatively low film thickness) is proposed. This is achieved using organosilanes and the specific input of this new radical chemistry to various Type I or Type II photoinitiating systems (PI). Through many selected examples corresponding to a broad choice of experimental conditions—lamps (UV or visible lights) and laser diodes, the outstanding performance of a lot of organosilanes containing photoinitiating systems is disclosed.     

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

Natural 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.     

Photopolymerization of wood coatings under visible lights

The possibility of using visible lights and sun to cure acrylate based coatings onto wood and the performance attained, compared to an artificial UV light exposure, have been studied. The kinetics of the polymerization, carried out on glass plates and wood substrates, as well as two physical properties (hardness and final coloration) were determined as a function of the photoinitiating systems PIS. Several PIS which allow a satisfactorily compromise between reactivity and yellowing are selected.     

Kinetics studies of hybrid structure formation by controlled photopolymerization

Real-time FT-NIR spectroscopy was used to monitor the individual monomer photopolymerization kinetics within the hybrid methacrylate/vinyl ether system composed of 2-phenoxyethyl methacrylate and tri(ethylene glycol) methyl vinyl ether. Photopolymerization processing conditions, such as light intensity, photoinitiator type (both free radical and cationic) and initiator ratios and concentrations, that provide preferential direction of polymer formation based on individual monomer photopolymerization kinetics and overall conversion have been evaluated. Single source UV-light irradiation was employed to produce either single or dual-stage hybrid polymerization, validating the potential of one-step, one-pot methodology for initiating stage-curable polymerizations.     

Oxygen mediated and wavelength tunable cationic photopolymerization reactions under air and low intensity: A new concept

A new concept which allows efficient cationic photopolymerization reactions under air, at any excitation wavelength in the UV–visible range and under low light intensities (conventional Xe lamp and/or laser diodes) is presented. It is based on the use of a versatile free radical promoted cationic process for the photoinitiation step which only requires a radical source, a silane and an usual iodonium salt and takes advantage of the presence of oxygen. The tunable character of the absorption is connected with the choice of the radical source. The oxygen enhancement is due to the particular role played by the silane. In every case, the silylium cation is the most efficient structure for the cationic ring opening reaction. Examples of the performance achieved (e.g. a conversion close to 100%) with such systems over the 390–580 nm range using various irradiation devices is shown and discussed.     

新型杂化光聚合体系

光固化方式的复合化是对光固化体系进行改性的重要方法。通过将自由基和阳离子以及阳离子和阳离子配合组成新的光固化体系,可以得到优良的固化体系。本论文以多种丙烯酸酯,乙烯基醚,环氧树脂,以及实验室合成的杂化单体等做为基础材料进行混合杂化光聚合体系,杂化单体光聚合体系的光固化动力学比较,并对羟基化合物对各体系的影响进行分析。实验结果表明,杂化单体在转化率,转化速度,对羟基化合物不敏感性等多方面都比混合杂化体系有明显优势。     

Synthesis and photopolymerization of 4‐(1‐propenyl)oxybutyl acrylate

Hybrid monomer, 4‐(1‐propenyl)oxybutyl acrylate, with cationic and free radical polymerizable group was synthesized. Real‐time Fourier transform infrared spectroscopy (FTIR) was used to monitor the photopolymerization kinetics of the monomer. Photopolymerization processing conditions, such as light intensity, photoinitiator concentrations have been evaluated. It was found that hybrid monomer showed higher efficiency of photopolymerization in comparison with the blend system. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Asymmetric cyanine dyes as fluorescence probes and visible‐light photoinitiators of free‐radical polymerization processes

This article introduces asymmetric cyanine dyes employed as visible‐light photoinitiators of vinyl monomer polymerization and as fluorescence probes monitoring the progress of polymerization. A degree of polymer cure from the measurement of the changes in the probe emission intensity and position shifts during the thermally initiated polymerization of monoacrylate was obtained. A distinct increase in the intensity of the probe fluorescence was observed during polymerization when the degree of monomer conversion was gradually increasing. This effect was accompanied by a blue‐shift of the probe emission maxima. The second part of this work is focused on the possibility of an application of the tested dyes, in combination with borate anions, as photoinitiating systems. The kinetics of polymerization of trimethylolpropane triacrylate, with cyanine borates as photoinitiators, was studied by a microcalorimetric method. Asymmetric cyanine borates were found to be effective photoinitiators, and both the initiator and coinitiator concentration as well as the light intensity strongly affected the progress of photopolymerization, leading, for example, to an increase in the polymerization rate. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 207–217, 2006     

自引发纯丙烯酸酯的合成及光聚合性质研究

合成了一种全新的自引发纯丙烯酸酯,利用实时红外光谱技术研究了自引发感光高分子的光聚合过程,比较了不同引发剂浓度下的聚合速率和转化率;结果表明,引入到高分子侧链的自由基引发剂仍具有较高的引发活性,双键基团也具有很好的光聚合反应活性。     

Synthesis and photopolymerization of dicarboxylic acid dimethacrylates and their application as dental monomers

New six dicarboxylic acid dimethacrylates (DMAs) were synthesized and studied as photocurable dental monomers. The photopolymerization behavior of the monomers was investigated by FTIR spectroscopy using camphorquinone (CQ) and 2‐(dimethylamino)ethyl MA (DMAEMA) as a photoinitiating system. Relatively high conversions (>55%) resulted from photopolymerization of the monomers by visible light. The volume shrinkage of the monomers during photopolymerization was significantly influenced by their molecular weight and degree of conversion. Preliminary tests of the composites formulated with a dicarboxylic acid DMA, a diluent, CQ, DMAEMA, and a glass filler were carried out including a three‐point bending test and fluoride release. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1802–1808, 2000     

Specific cationic photoinitiators for near UV and visible LEDs: Iodonium versus ferrocenium structures

Two iodonium salts based on a coumarin chromophore are investigated for polymerization upon light emitting diode irradiations (LEDs). They work as one‐component photoinitiators. They initiate the cationic polymerization of epoxides (under air) and vinylethers (laminate) upon exposure to violet LEDs (385 and 405 nm). Excellent polymerization profiles are recorded. Their efficiency is quite similar to that of a ferrocenium salt. Interpenetrating polymer networks can also be obtained through a concomitant cationic/radical photopolymerization of an epoxy/acrylate blend monomer. The light absorption properties of these new salts as well as the involved photochemical mechanisms are investigated for the first time through electron spin resonance, laser flash photolysis, steady state photolysis experiments. Molecular orbital calculations are also used to shed some light on the initiation mechanisms. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42759.     

Preparation and characterization of hybrid nanocomposite coatings by photopolymerization and sol-gel process

Organic-inorganic nanocomposite hybrid coatings were prepared through a dual-cure process involving cationic photopolymerization of epoxy groups and subsequent condensation of alkoxysilane groups. The kinetics of the photopolymerization and condensation reactions were investigated; suitable conditions for obtaining a complete conversion of both reactive groups were found. The obtained films are transparent to visible light. The T_g values of the hybrids increase by increasing the TEOS content in the photocurable formulation. Also, a significant increase in surface hardness could be obtained for the hybrid systems. AFM analysis gave evidence of a strong interaction between the organic and inorganic phase with the formation of silica domains in the nanoscale range.     

Diketopyrrolopyrrole dyes: Structure/reactivity/efficiency relationship in photoinitiating systems upon visible lights

Several diketopyrrolopyrrole derivative based dyes (DPP), combined with an iodonium salt or an amine (and optionally an additive), are studied as photoinitiating systems for the cationic polymerization CP of epoxides or the free radical polymerization FRP of acrylates under different irradiation sources i.e. a very soft halogen lamp as well as laser diodes at 473 nm (blue light) and 532 nm (green light). The diketopyrrolopyrrole-furan derivative (FuDPP) is very efficient in CP. The structure/reactivity/efficiency relationships in this series of DPP derivatives are investigated. A comparison with a well known reference for visible light photoinitiating system (i.e. camphorquinone based photoinitiating system) is also provided showing the better efficiency of the new proposed structures. The photochemical mechanisms are studied by steady state photolysis, fluorescence, cyclic voltammetry, laser flash photolysis and electron spin resonance spin-trapping techniques.     

Novel visible photoinitiators systems for free-radical/cationic hybrid photopolymerization

Free radical/cationic hybrid photopolymerization of acrylates and epoxides was induced using an initiator system comprised of the dye derivative 5,12-dihydroquinoxalino[2,3-b]quinoxaline or 5,12-dihydroquinoxalino[2,3-b]pyridopyrazine as the sensitizers and hexafluoroantimonate triarylsulfonium salt as the initiators. The curing experiments were carried out in the presence of air and the consumption of each monomer upon VIS-radiation was monitored in situ by real-time infrared spectroscopy. Hardness (H), elastic modulus (E) and the H/E ratio of the coatings obtained by visible-initiated acrylate/epoxide hybrid photopolymerization were determined using nanoindentation. DSC measurements show that the initiator system presented here may produce an interpenetrating polymer network. The pencil hardness of the obtained coatings indicates that the dye/hexafluoroantimonate triarylsulfonium salts systems studied here may have practical applications as visible-light hybrid initiators.     

Novel N‐ethyl‐2‐styrylquinolinium iodides as sensitizers in the photoinitiated free‐radical polymerization of trimethylolopropane triacrylate. II

Results of kinetic studies of two‐component photoinitiator systems used in the visible‐light photoinduced polymerization of 2‐ethyl‐2‐(hydroxymethyl)‐1,3‐propanediol triacrylate are presented. Nine different styrylquinolinum dyes coupled with n‐butyltriphenylborate as a coinitiator have been used as photoinitiating systems. Reactive radicals that initiate the polymerization are formed by the well‐known mechanism of photoinduced electron transfer between dye cations acting as electron acceptors and borate anions acting as electron donors. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010