Photoinitiators with Functional Groups 9: New Derivatives of Covalently Linked Benzophenone-amine Based Photoinitiators

As it had been shown that some covalently bound benzophenone (BP)—phenylglycine derivatives act as remarkable monomolecular Type II photoinitiators for radical polymerization, structural variations were investigated. The influence of the linking position between BP and the coinitiator and the coinitiator structure itself were considered. Furthermore, water soluble monomolecular BP-glycine based PIs were developed. While in UV-spectroscopy similar absorption behavior were found for all compounds, photo-DSC experiments revealed that derivative 2 has the most effective constitution.     

Photoinitiators with functional groups. IX. Hydrophilic bisacylphosphine oxides for acidic aqueous formulations

Because of the poor photoreactivity of camphorquinone–amine‐based photoinitiator systems in dental acidic aqueous primer formulations, we were interested to evaluate a commercially available bisacylphosphine oxide as photoinitiator. Because of the promising results on photoreactivity and storage stability, new oligo(ethylene glycol)‐substituted bisacylphosphine oxides were synthesized and investigated. Beside good solubility and significantly increased reactivity in aqueous acidic formulations, sufficient storage stability was found. Additionally, the influence of the type of substitution on the photopolymerization behavior in aqueous, hydrophilic, and hydrophobic resins is discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1686–1700, 2006     

Photoinitiators with double and triple bonds

In continuation of our research on new chromophores for photoinitiators (PIs) we investigated a series of double and triple bond containing PIs based on benzophenone (BP), thioxanthone, benzil (BZ), benzildimethylketal (BDK), and hydroxyalkylphenone. UV‐Vis spectroscopy revealed an absorption shifted up to 50 nm to the visible range of the spectrum. With steady state photolysis experiments, the rates of decomposition (R_d) were determined. Compared to the commercially available reference PIs the R_d of the new PIs were nearly identical. By quenching reactions with 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO), α‐cleavage was identified for Type I initiators. Generally, all new compounds are suitable as PI especially for longer wavelengths. Nevertheless it has to be noted that the PI concentration has to be selected carefully to achieve optimum reactivity. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 289–301, 2008     

大分子二苯甲酮光引发剂的合成及性能研究

合成了两种大分子光引发剂PPI 1和PPI 2,紫外吸收行为均与4-BP相似,摩尔消光系数比4-BP高,且光解速率快。在不加助引发剂EDAB条件下引发单体HDDA聚合效率良好,对比PPI 1和PPI 2,随着吸光基团官能度（分子量）增大,聚合速率增加、最终转化率增高。将各自诱导单体PEGDA聚合的固化膜,在二氯甲烷溶液中浸泡5天后,PPI 1的萃取量是4-BP的44.5%,PPI 2的萃取量是4-BP的40%,说明PPI 1和PPI 2都是引发效率良好的低迁移光引发剂。     

Fluorescent Brighteners as Visible LED‐Light Sensitive Photoinitiators for Free Radical Photopolymerizations

The photochemical and electrochemical investigations of commercially available, safe, and cheap fluorescent brighteners, namely, triazinylstilbene (commercial name: fluorescent brightener 28) and 2,5‐bis(5‐tert‐butyl‐benzoxazol‐2‐yl)thiophene, as well as their original use as photoinitiators of polymerization upon light emitting diode (LED) irradiation are reported. Remarkably, their excellent near‐UV–visible absorption properties combined with outstanding fluorescent properties allow them to act as high‐performance photoinitiators when used in combination with diaryliodonium salt. These two‐component photoinitiating systems can be employed for free radical polymerizations of acrylate. In addition, this brightener‐initiated photopolymerization is able to overcome oxygen inhibition even upon irradiation with low LED light intensity. The underlying photochemical mechanisms are investigated by electron‐spin resonance‐spin trapping, fluorescence, cyclic voltammetry, and steady‐state photolysis techniques.

     

New Developments in Photoinitiators

New developments of photoinitiators are the base for the further extension of the radiation curing technology. Three different approaches are presented: The optimized combination of a bisacylphosphine oxide photoinitiator with a suitable stabilizer package allows the UV curing of clear coatings with an excellent performance in outdoor use. The design of new oxime ester derivatives allowed the introduction of a new tailor-made photoinitiator for color filter resists applications that considerably improves the color quality of the color filter. A novel photoinitiator class capable of releasing strong amine bases opens new opportunities for expanding radiation curing into resins types that crosslink by base-catalyzed processes.     

Phenacyl onium salt photoinitiators: synthesis, photolysis, and applications

Onium salts, namely sulfonium, phosphonium, ammonium, and pyridinium salts containing phenacyl group are photoinitiators appropriate for the polymerization of monomers such as oxiranes and vinyl ethers, which are not polymerizable by a free-radical mechanism. The initiation is accomplished by direct or indirect (sensitized) photolysis of the salts. Depending on the type of the salt, the direct photoinitiation of cationic polymerization involves reversible or irreversible processes. The photolysis of phenacylsulfonium compounds proceeds by a reversible process, while the other types undergo irreversible photolysis leading to complete fragmentation of the photoinitiator. An additionally useful tool, namely photosensitized generation of initiating species enlarges the versatility of these salts as photoinitiators. Photoinitiated free-radical and zwitterionic polymerizations by using phenacyl-type salts are also addressed. Keto-enol tautomerization of phenacyl pyridinium salts is discussed. Moreover, an interesting application concerning in situ synthesis of clay-poly(methyl methacrylate) nanocomposites with the aid of the phenacyl anilinium salt-based photopolymerization technique is noted.     

Photoinitiators with functional groups. VII. Covalently bonded camphorquinone—amines

Camphorquinone (CQ), a widely used photoinitiator (PI) in dental applications, was covalently bonded to aromatic amines to enhance the rate of electron and proton transfer effect due to the close vicinity of the diketone and the amine group. 10‐bromocamphorquinone and 10‐bromomethylcamphorquinone were selected as suitable precursors for esterification with the carboxyl group containing aromatic amines based on 4‐dimethylaminobenzoic acid. Properties of the new photoinitiating systems were investigated by UV spectroscopy and differential scanning photocalorimetry in lauryl acrylate. Compared to physical mixtures, in all cases similar or even better performance was obtained. Surprisingly, 10‐acetyl derivatives 7 – 9 and 18 especially, were found to be highly reactive. Compared to CQ/ethyl 4‐dimethylaminobenzoate, the rate of photopolymerization was increased by a factor of up to 2. Intramolecular reaction was confirmed by photo‐differential scanning calorimetry experiments with varying PI concentrations. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4948–4963, 2004     

Photoinitiators with Functional Groups. Part IV. Water-Soluble Photoinitiators Containing Quaternary Ammonium Groups

Abstract

The syntheses and properties of a series of new water-soluble hydroxyalkylphenone-based photoinitiators (PIs) for UV curing of water-borne systems is described. By reaction of tosyl- (2), chloro- (3), and dimethylamino- (4) derivatives of the commercially available PI 2-hydroxy-1-[4-(2-hydroxy-ethoxy)phenyl]-2-methylpropan-1-one (1, Darocur 2959) with tertiary amines and halogenides, respectively, PIs containing quaternary ammonium groups exhibiting high water solubility were obtained. Preliminary photocalorimetric tests of the activity demonstrated excellent efficiencies for the PIs in a commercial water-based acrylate formulation, exceeding the photoactivity of the reference PI (1) substantially.     

Photoinitiators with Functional Groups. Part I. Polymer Photoinitiators

A series of new polymerizable photoinitiators (PIs) based on hydroxyalkylphenones has been synthesized by Grignard reaction of 4-vinylphenylmagnesiumchloride with suitable carbonyl compounds. Radical homopolymerization and copolymerization of the PIS with various vinyl monomers yielded polymer PIS compatible with either hydrophilic or hydrophobic resin systems. The PI monomers and polymers are characterized by excellent photoinitiating activity comparable with the most efficient known PIS and exhibit high migration stability.     

Free-radical polymerization upon near-infrared light irradiation,merging photochemical and photothermal initiating methods

Polymerization of (meth)acrylate resins upon near-infrared (NIR) light remains a huge challenge. In this study, a new photoinduced method of polymerization of methacrylic monomers is presented, originally merging a photochemical and a photothermal pathway. A four-component system is proposed comprising an NIR dye combined with an iodonium salt, a phosphine, and a thermal initiator. A selection of dyes is suggested regarding electron transfer properties and/or light-to-heat conversion abilities. Several thermal initiators are studied: an alkoxyamine (BlocBuilder MA), an azo derivative, and a peroxide. For the first time, an NIR absorbing dye is used in photopolymerization using both its capacities of light-to-heat conversion and its ability to initiate an electron transfer reaction. Three wavelengths of irradiation will be presented here: 785, 940, and 1064 nm. These long wavelengths are challenging because the energy of photons is extremely low but these wavelengths offer significant advantages in term of light penetration (e.g., for the access to composites through photopolymerization processes). The different systems presented here exhibit high and rapid conversions of methacrylate functions. The underlying chemical mechanism will be fully depicted by real-time Fourier transform infrared spectroscopy and thermal imaging measurements. © 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 300–308     

Naphthodioxinone‐1,3‐benzodioxole as photochemically masked one‐component type II photoinitiator for free radical polymerization

A 1,3‐benzodioxole derivative of naphthodioxinone, namely 2‐(benzo[d][1,3]dioxol‐5‐yl)‐9‐hydroxy‐2‐phenyl‐4H‐naphtho[2,3‐d][1,3]dioxin‐4‐one was synthesized and characterized. Its capability to act as caged one‐component Type II photoinitiator for free radical polymerization was examined. Upon irradiation, this photoinitiator releases 5‐benzoyl‐1,3‐benzodioxole possessing both benzophenone and 1,3‐dioxole groups in the structure as light absorbing and hydrogen donating sites, respectively. Subsequent photoexcitation of the benzophenone chromophore followed by hydrogen abstraction generates radicals capable of initiating free radical polymerization of appropriate monomers. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Photopolymers (I): photoinitiating role of monochloroacetic acid in the synthesis of poly(methyl methacrylate)

Photopolymerization of methyl methacrylate in bulk and in solution at 40 °C using monochloroacetic acid –dimethyl aniline (MCAA–DMA) combination as photoinitiator was studied kinetically. The apparent activation energy was found to be 4.39 kcal/mol (18.37 kJ/mol) while the kinetic parameter k_p²/k_t was 1.27 × 10⁻² 1/mol/sec. The kinetic data indicated that polymerization followed a radical mechanism. The initiator order was found to be 0.25, indicating significant deviation from the square root dependence for normal free radical kinetics. The non‐ideality in the kinetics can be explained on the basis of significant initiator‐dependent termination through primary radicals or degradative initiator transfer. The observed monomer order was significantly less than unity (i.e. nonideal behavior) for use of carbon tetrachloride, chloroform, methylethyl ketone and acetic acid as diluents, but it was unity (i.e. ideal behavior) for use of benzene as the diluent. Solvents other than benzene contributed to enhancement of rate of polymerization by influencing the radical generation step. End‐group analysis indicated the incorporation of DMA and MCAA moieties as end‐groups in the polymers. Copyright © 1999 John Wiley & Sons, Ltd.     

Phenacyl Ethyl Carbazolium as a Long Wavelength Photoinitiator for Free Radical Polymerization

A new phenacyl‐type photoinitiator based on ethyl carbazole as a long wavelength photo­initiator is developed for free radical polymerization. Phenacyl ethyl carbazolium hexafluoroantimonate (PECH) photoinitiator is synthesized in a two‐step, one‐pot manner by quaternizing ethyl carbazole with phenacyl bromide and subsequent ion exchange reaction with potassium hexafluoroantimonate. Under irradiation, PECH tends to undergo homolytic bond cleavage bringing about initiating free radicals. However, as evidenced by cyclic voltammetry and real‐time photobleaching studies, formation of initiating cationic species is highly unlikely as the photochemically formed charged carbazole units tend to couple.

     

Visible light‐emitting diode‐sensitive thioxanthone derivatives used in versatile photoinitiating systems for photopolymerizations

Novel thioxanthone (TX) derivatives are used as versatile photoinitiators upon visible light‐emitting diode (LED; e.g., 405, 425, and 450 nm) exposure. The mechanisms for the photochemical generation of reactive species (i.e., cations and free radicals) produced from photoinitiating systems based on the photoinitiator and an iodonium salt, tris(trimethylsilyl)silane, or an amine, were studied by UV–vis spectroscopy, fluorescence, cyclic voltammetry, steady‐state photolysis, and electron spin resonance spin‐trapping techniques. The reactive species are particularly efficient for cationic, free radical, hybrid, and thiol‐ene photopolymerizations upon LED exposure. The optimized photoinitiating systems exhibit higher efficiency than those of reference systems (i.e., isopropyl TX‐based photoinitiating systems), especially in the visible range. According to their beneficial features, these photoinitiating systems have considerable potential in photocuring applications. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 4037–4045     

Thioxanthone–anthracene‐9‐carboxylic acid as radical photoinitiator in the presence of atmospheric air

Thioxanthone–anthracene‐9‐carboxylic acid (TX‐ANCA) namely 14‐oxo‐14H‐naphthol [2,3‐b]thioxanten‐12‐carboxylic acid, is synthesized and characterized as part of our continuing interest for syntheses of polyaromatic initiators. Photoinitiator, TX‐ANCA have good absorption properties in the UV and visible region of the electromagnetic spectrum (ɛ₃₇₀: 9080 M⁻¹cm⁻¹, ɛ₄₃₀: 6151 M⁻¹ cm⁻¹). The fluorescence quantum yield is calculated as 0.1 which is slightly higher than of the parent thioxanthone compound (φ_f: 0.07). The phosphorescence lifetime is found to be 39 ms. The possible initiating mechanism of TX‐ANCA is based on photoexcitation of TX‐ANCA and quenching of triplet excited states of TX‐ANCA by molecular oxygen generates singlet oxygen. Singlet oxygen reacts with the anthracene moiety of TX‐ANCA possibly forms an endoperoxide. The endoperoxides undergoes photochemical or thermal decomposition to form radicals which are able to initiate free radical polymerization. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1878–1883     

Photoinitiators with functional groups. VI. Chemically bound sensitizers

Several benzophenone‐ and thioxanthone‐based photosensitizers (PSs) were covalently bonded to hydroxyalkylphenone‐ and aminoalkylphenone‐based photoinitiators (PIs) to enhance the rate of the excitation‐transfer effect due to the close vicinity of the PS to the PI. The properties of these new systems were investigated with UV spectroscopy and photo‐differential scanning calorimetry. Broadband irradiation experiments and selective excitation of the PS were carried out for the physical mixtures and covalently bonded PI/PS combinations to investigate the effect of excitation transfer. Selective excitation of the PS chromophore revealed that the energy transfer was significantly increased in covalently bonded initiators in comparison with the physical mixtures. This effect was most pronounced for the hydroxyalkylphenones that were sensitized by suitable benzophenone derivatives, especially at low PI concentrations. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2285–2301, 2004     

Monitoring photopolymerization reactions with optical pyrometry

This article describes the development of optical pyrometry (OP) as a new analytical technique for the continuous monitoring of the progress of both free‐radical and cationic photopolymerizations. The method is rapid, reproducible, and very easy to implement. A temperature profile of a photopolymerization can be obtained. Preliminary studies have shown that the temperatures of some polymerizing monomers can easily reach temperatures in excess of 250 °C. The effects of the mass and reactivity of the monomer, light intensity, structures, and concentrations of the photoinitiators and monomers as well as the presence or absence of oxygen on various free‐radical and cationic photopolymerizations were examined with this method. Coupling of real‐time infrared spectroscopy with OP provides a convenient method for simultaneously monitoring both the chemical conversion and the temperature of a photopolymerization. This combined technique affords new insights into the effects of temperature‐induced autoacceleration on the course of photopolymerizations. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 579–596, 2003     

Benzoin type photoinitiator for free radical polymerization

Benzoin, a popular photoinitiator for free radical polymerization of vinyl monomers, was improved by introduction of two methyl thioether substituents. This new benzoin derivative showed an about 50 times higher light absorption in the near‐UV spectral region and performed better than the unsubstituted benzoin in polymerization experiments in bulk solutions or films of acrylate monomers when low initiator concentrations are used. Laser flash photolysis, low temperature luminescence experiments and photoproduct studies by mass spectrometry suggest that a slow α‐cleavage mechanism (k_α = 2.2 × 10⁵ s^?1) from the electronic triplet state with a quantum yield of 0.1 is the primary photoreaction to generate the initiating free radicals. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013