JPH0834808A - Photopolymerization initiator - Google Patents

Abstract

PURPOSE:To obtain a photopolymerization initiator which can initiate polymerization even in a lowly light-transmiting system such as a system containing a pigment or a thick film system by mixing a cationic color having an absorption in the near-IR region with a boron catalyst and an electron-accepting substance. CONSTITUTION:This initiator essentially consists of a cationic color represented by the formula: D<+>.A<-> (wherein D<+> is a cation having an absorption in the near-IR region; and A<-> is an anion), a boron catalyst represented by the formula (wherein R1 to R4 are each an (substituted) alkyl, an (substituted) aryl, a (substituted) allyl, an (substituted) aralkyl, an (substituted) alkenyl, a (substituted) alkynyl, (substituted) silyl, a heterocyclic ring or a halogeno; and Z<+> is cation). It is particularly desirable that A<-> is a 4-coordinated boron anion. The electron- accepting compound is desirably one having a reduction potential as measured by using an Li/Li<+> electrode as the reference electrode more desirably one having a difference of at most 10.0 eV between the potential in the maximum occupied orbit and that in the minimum vacant orbit.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a photopolymerization initiator having an improved polymerization rate. More specifically, even if the composition contains a pigment and a dye having a high hiding rate, or even a composition having a large thickness or poor light transmittance, it can be polymerized and cured at a practical speed by near-infrared light. The present invention relates to a photopolymerization initiator capable of polymerization.

[0002]

2. Description of the Related Art Conventionally, photopolymerization is used for curing, drying and printing coatings.
It is used for various applications such as resin convex printing, print substrate making, resist or photomask, black and white or color transfer coloring sheet or making coloring sheet,
In particular, recently, from the viewpoints of global environment problems, energy saving, labor saving in response to rising labor costs, etc., attention has been paid to the features of photopolymerization, such as the fact that polymerization is possible at room temperature, quick drying, and the possibility of solvent-free use. , Is being developed. For example, curing with ultraviolet light is a method in which a polymerizable monomer is rapidly cured by cation polymerization or radical polymerization by irradiating with ultraviolet light of 200 to 400 nm, but a thick film is coated, and a pigment with a high hiding rate is contained. The use of such a colored coating film is restricted because it has low ultraviolet light transmittance.

Further, even in a transparent system containing no pigment, coloring dye, etc., ultraviolet rays do not reach the inside of the curing system sufficiently in a system containing a composition having a thickened curing system or a composition having poor transparency, and It had a problem of insufficient curing. Further, the ultraviolet light source is not necessarily inexpensive, such as a high-pressure mercury lamp, and it cannot be said to be a sufficiently satisfactory method in terms of safety such as ozone generation and the possibility of skin cancer. As a method for solving these problems, a polymerization system using a dye that absorbs visible light or near infrared light having a wavelength longer than ultraviolet light has been proposed (for example, JP-A-5-194619), and a pigment addition system. Although photo-curing of thick film system and the like has been achieved, it was not always a satisfactory technique in terms of polymerization rate and use of a relatively expensive light-absorbing dye.

[0004]

DISCLOSURE OF THE INVENTION The present invention is a near-infrared light which can be polymerized and cured even in a polymerization system having low ultraviolet light transmittance such as a pigment addition system or a thick film system, which has been difficult in conventional ultraviolet light curing. An object of the present invention is to provide a polymerization initiator having an improved polymerization rate in a photopolymerization initiation system and capable of reducing the amount of addition of a light absorbing dye which is an essential component of the initiator.

[0005]

The inventors of the present invention have conducted extensive studies to solve the above problems, and as a result, in addition to a cationic dye having an absorption in the near infrared light region and a boron-based catalyst, an electron-accepting substance. The inventors have found that the above problems can be solved by using the compound as a photopolymerization initiator by adding, and have completed the present invention. That is, according to the present invention, the cationic dye represented by the general formula (1) is represented by the general formula (1);

D ⁺ · A ⁻ (wherein D ⁺ is a cation having absorption in the near infrared wavelength region, A ⁻ represents various anions) and the general formula (2)
A boron-based catalyst represented by the general formula (2);

[0007]

Embedded image

(In the formula, R ₁ , R ₂ , R ₃ and R ₄ are each independently an alkyl group, an aryl group, an allyl group, an aralkyl group, an alkenyl group, an alkynyl group, a silyl group, a heterocyclic group and a halogen atom. , A substituted alkyl group, a substituted aryl group, a substituted allyl group, a substituted aralkyl group, a substituted alkenyl group, a substituted alkynyl group or a substituted silyl group, Z ⁺
Represents a cation.) Further, by combining an electron-accepting compound to form a photopolymerization initiator, a photopolymerization initiator capable of polymerizing a polymerizable unsaturated compound at a practical rate by near-infrared light irradiation can be obtained. .

The polymerization initiator in the present invention is used for coating various photocurable materials such as printing inks, resists, adhesives, furniture, electronic materials, building materials, carpentry, metals, plastics, automobile bodies, interiors. It can also be used for painting bumpers, coating inorganic materials, and the like. The combined use of the cation dye of the general formula (1) and the boron-based catalyst of the general formula (2) constituting the present invention causes decomposition of the cation dye by near-infrared light, and the color of the cation dye is decolorized. As a result, radicals are generated, and when a polymerizable unsaturated compound coexists, polymerization is initiated. The decolorization reaction of the cationic dye is an irreversible reaction, and the color of the cationic dye does not impair the hue of the polymer. When an electron-accepting compound is added as a measure for improving the polymerization rate while taking advantage of the characteristics of this photopolymerization initiator system, the polymerization rate is significantly improved, and the photopolymerization reaction is completed within a practical polymerization time.

The function of the electron-accepting compound is estimated as follows. That is, it is generally considered that electron transfer occurs from a boron-based catalyst to a cationic dye excited by light energy, and a radical generated from the boron-based catalyst functions as a polymerization initiator (for example, Japanese Patent Laid-Open No. 62-62).
No. 143044), in which the electron-accepting compound receives an electron from a radical derived from a cation dye (hereinafter referred to as a dye radical), which is a by-product, and returns the dye radical to the original cation dye to regenerate a cation. It is considered that the cycle in which the dye undergoes the photoreaction again is established, the number of generated radicals is increased, and the polymerization rate is improved.

An example in which an electron-accepting radical generator is used as a photopolymerization initiator in combination with a light-absorbing sensitizer is conventionally known (see, for example, “Surface”, 1989, No. 7, page 551). Up to now, no example has been known in which an electron-accepting compound was used to regenerate a cationic dye from a dye radical to improve photopolymerization efficiency. This has made it possible to increase the polymerization rate if the amount of the light absorbing dye is the same, and to achieve the conventional polymerization rate by adding a small amount of the dye.

The cation dye of the general formula (1) of the present invention may be any dye having absorption in the near infrared wavelength region, and specifically, it may have absorption in the wavelength region of 740 nm or more. Among them, a cation having an absorption in the wavelength region of 780 nm is particularly preferable in consideration of the light transmittance and the light stability of the initiator. Near-infrared light has a long wavelength and is excellent in light transmittance as compared with ultraviolet light which is generally used in the past, and therefore, a composition of various pigments and the like having high light hiding property which was difficult with conventional ultraviolet light, Good photopolymerization can be performed even for thick compositions and the like.

The cation (D ⁺ ) of the present invention is not particularly limited as long as it has absorption in the near infrared region, but preferred examples include methine, polymethine, indoline,
Examples thereof include cyanine, xanthene, oxazine, thiazine, diarylmethane, triarylmethane, and cations of pyrylium-based cation dyes. Typical examples of such cation dyes include cations as shown in Table 1.

[0014]

[Table 1]

[0015]

[Table 1]

The counter anion A ⁻ is an arbitrary anion, but a tetracoordinated boron anion represented by the following general formula (3) is particularly preferable. General formula (3);

[0017]

Embedded image

(Wherein R ₅ , R ₆ , R ₇ and R ₈ are each independently an alkyl group, an aryl group, an allyl group, an aralkyl group, an alkenyl group, an alkynyl group, a silyl group, a heterocyclic group or a halogen atom. , A substituted alkyl group, a substituted aryl group, a substituted allyl group, a substituted aralkyl group, a substituted alkenyl group, a substituted alkynyl group or a substituted silyl group)

As specific examples, n-butyltriphenylboron ion, n-octyltriphenylboron ion, triphenylsilyltriphenylboron ion, n
-Butyltrianisylboron ion, di-n-dodecyldiphenylboron ion, tetraphenylboron ion,
Triphenylnaphthylboron ion, tetrabutylboron ion, tri-n-butyl (dimethylphenylsilyl)
Examples thereof include boron ions, and more specifically, the anions and the like described in the patent specification filed by the present inventors earlier (Japanese Patent Application No. 4-175767).

The boron-based catalyst according to the present invention is represented by the general formula (2), and specific examples of the anionic four-coordinated boron ion include n-butyltriphenylboron ion and n-octyl. Triphenylboron ion, triphenylsilyltriphenylboron ion, n
-Butyltrianisylboron ion, di-n-dodecyldiphenylboron ion, tetraphenylboron ion,
Triphenylnaphthylboron ion, tetrabutylboron ion, tri-n-butyl (dimethylphenylsilyl)
Examples include boron ions. Further, the cation (Z ⁺ ) described in the formula is arbitrary, but the general formula (4) is the general formula (4);

[0021]

[Chemical 4]

(Wherein R ₉ , R ₁₀ , R ₁₁ and R ₁₂ are each independently a hydrogen atom, an alkyl group, an aryl group, an allyl group, an aralkyl group, an alkenyl group, an alkynyl group, a heterocyclic group or a substituted alkyl group. Group, substituted aryl group, substituted allyl group, substituted aralkyl group, substituted alkenyl group or substituted alkynyl group)

A quaternary ammonium cation represented by
Quaternary pyridinium cations, quaternary quinolinium cations, phosphonium cations, sulfonium cations, metal cations such as sodium, potassium, lithium, magnesium, calcium, cations having a charge on an oxygen atom, charge on a carbon atom And a cation of a metal compound such as arsenic, cobalt, palladium, titanium and tin. Specific examples of the boron-based catalyst of the present invention have been previously filed (Patent application 4-1
No. 75767) There are detailed descriptions in patent specifications and the like. These cationic dyes and boron-based catalysts may be used alone or in admixture of two or more.

Generally, there are various definitions of electron-accepting substances. For example, in the “Electron Acceptor” section of the Chemical Dictionary (published by Kyoritsu Publishing Co. in 1936), “Atoms and molecules that easily accept electrons from the other party” are mentioned. , Or an ion. ”, Examples thereof include halogen atoms such as iodine, various quinones, trinitrobenzene, tin tetrachloride, aluminum chloride, boron trifluoride, and the like. In the same place, as the classifications of Lapworth and Robinson, protons, proton sources such as acids, cations such as H ₃ O ⁺ and diazonium salts, metals with valence nuclei lacking octagon, esters, and halogenated compounds. Although it is described as an alkyl, a quaternary ammonium compound, an oxidant such as ozone and peroxide, a nitrogen compound such as nitro and a nitroso compound, and a sulfur compound such as sulfur trioxide, the electron acceptor of the present invention is defined as above. Shows the electron acceptor.

Further, when considering electron acceptance electrochemically,
A compound having a higher reduction potential is more likely to receive an electron from another compound. The reduction potential is cyclic voltammetry,
It can be easily measured by a method such as polarography (for example, "electrochemical measurement method", published by Gihodo Publishing Co., Ltd. in 1984). The electron accepting substance of the present invention is preferably a compound having the above reduction potential of 2.0 V or more when the Li / Li ⁺ electrode is used as a reference electrode.

Further, in the molecular orbital calculation, it is considered that a compound having a smaller difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is more likely to receive an electron from another compound. The electron acceptor of the present invention is preferably a compound having a difference between HOMO and LUMO of 10.0 eV or less as calculated by MOPAC ver6. Compounds with low reduction potential or H
A compound having a large difference between OMO and LUMO is not preferable because it has insufficient electron accepting ability.

Specific examples of the electron accepting compound include triazine compounds and aromatic onium salt compounds. Preferred examples of the triazine compound include:
A trihalomethyl-s-triazine compound is mentioned.
Specific examples include 2-phenyl-4,6-bis (tribromomethyl) -s-triazine and 2,4,6-tris (trichloromethyl) -1,3,5-triazine. For a more detailed example, see Bull. Chem. Soc. Jp
n. Vol. 42, page 2924 (1969), JP-A-53
The triazine compounds described in JP-A-133428, JP-A-60-198868, JP-A-62-161802 and the like can be mentioned.

Specific examples of the aromatic onium salt compound of the present invention include aromatic iodonium salts such as diphenyliodonium salt (any counter anion), aromatic sulfonium salts such as triphenylsulfonium salt,
An azinium salt etc. are mentioned. Further detailed description is given in, for example, Japanese Patent Publication No. 52-14277. Particularly preferred is the triphenyliodonium salt of trifluoromethanesulfonic acid.

The photopolymerization initiator of the present invention is generally used as a photopolymerizable composition in combination with a polymerizable unsaturated compound. The polymerizable unsaturated compound used at that time includes a polymerizable oligomer and a polymerizable monomer, and as the polymerizable oligomer, a urethane acrylate resin, a polyester acrylate resin, an unsaturated polyester resin, an epoxy acrylate resin, a silicone acrylate resin, a melamine acrylate resin. And (meth) acrylic functional polyorganosilsesquioxane disclosed in JP-A-4-28722. These oligomers have various compounds produced by various manufacturers and are easily available. The above-mentioned polymerizable oligomer is the main component that determines the physical properties of the polymerization product, and is selected according to the physical properties required for various applications such as hardness, strength, durability, adhesion, etc., and is mixed in an arbitrary amount.

As the polymerizable monomers, esterified products of (meth) acrylic acid and monohydric alcohol, vinylbenzenes such as styrene, vinyltoluene and chlorostyrene, vinyl ethers such as vinyl isobutyl ether and 2-ethylhexyl vinyl ether, (Meth) acrylonitrile, (meth) acrylamide, (meth) acrylic compounds such as methylenebisacrylamide, vinyl acetates such as vinyl acetate, vinyl propionate, vinyl benzoate, allyl alcohol, allyl acetate, diallyl phthalate, etc. Examples thereof include monomers containing an allyl group. Further, as the monomer, a hydroxyl group-containing monomer such as 2-hydroxyethyl (meth) acrylate or 2-hydroxypropyl (meth) acrylate;

Addition products of polyisocyanates such as butyl isocyanate and phenyl isocyanate with the above hydroxyl group-containing monomers; addition products of phosphoric acid with the above hydroxyl group containing monomers; N-vinylpyrrolidone, N-vinylcarbazole, N-vinylacetamide, vinyl. Nitrogen-containing unsaturated monomers such as pyridines can also be used. Further, as the monomer, diethylene glycol diacrylate,
Di-, tri- or tetravinyl compounds such as 1,6-hexanediol dimethacrylate, trimethylolpropane triacrylate, pentaerythritol trimethacrylate, and pentaerythritol tetraacrylate; ) A product obtained by reacting acrylic acid; a product obtained by reacting (meth) acrylic acid with the above-mentioned adduct of polyhydric alcohol and propylene oxide; and an adduct of the above-mentioned polyhydric alcohol with ε-caprolactone ( A product obtained by reacting (meth) acrylic acid; a phosphorus-containing polymerizable unsaturated monomer and the like are included. More detailed specific examples are described in the patent specification filed previously (Japanese Patent Application No. 4-175767).

The polymerization reaction of the composition containing the polymerization initiator and the polymerizable unsaturated compound of the present invention is achieved by irradiation with light having a spectrum in the wavelength region of 740 nm or more, depending on the absorption wavelength of the light absorbing dye. It For example halogen lamps,
Examples include xenon lamps and sunlight. As mentioned above, these light sources are generally cheaper than mercury lamps, metal halide lamps, etc. used in the conventional UV photopolymerization, and are free from ozone generation, skin cancer, etc., and easy to take safety measures. It also has the feature of being.
In the photopolymerization reaction, it is preferable to perform light irradiation under a low oxygen concentration in order to reduce polymerization inhibition by oxygen. For example, it is possible to accelerate the polymerization by reducing the oxygen concentration by blowing the above inert gas onto the surface of the cured product in an atmosphere of an inert gas such as nitrogen, argon, carbon dioxide or the like.

The cationic dye represented by the general formula (1) and the boron-based catalyst represented by the general formula (2) in the present invention can be used in an arbitrary ratio, and generally 10: 1 to 1:50.
Although it is used in a (molar ratio) ratio, it is preferable to use a larger amount of the boron-based catalyst than the cationic dye for the purpose of the present invention in which a more expensive cationic dye is recycled in the system. That is, the range of 1: 1 to 1:50 (molar ratio) is particularly preferable. Further, the electron-accepting compound of the present invention exerts a corresponding function even if added in a very small amount, but it is preferably used in the range of 10: 1 to 1:20 (molar ratio) with respect to the cationic dye. However, since the addition of the electron-accepting compound of the present invention more than necessary may adversely affect the physical properties of the polymerizable composition in terms of cost, the optimum value of each electron-accepting compound should be determined and used. Is desirable.

The object of the present invention can be achieved by using the polymerization initiator of the present invention generally in an amount of 0.01% by weight or more based on the entire polymerizable composition. If it is less than that, the polymerization may not be performed sufficiently. It is preferably in the range of 0.05 to 10% by weight. It is not preferable from the economical point of view to use too much. It is also possible to use two or more cationic dyes, a boron-based catalyst, and an electron-accepting compound in combination.

Further, optional additives and fillers can be added to the polymerizable composition containing the initiator of the present invention. Examples of the additive here include a thickener, a leveling agent, a thixotropic agent, a dispersant and the like which are generally used as an additive for paints, and the filler includes
Examples thereof include organic substances, inorganic substances, or their composites and mixtures. Further, as described above, since the polymerization initiator of the present invention uses a cationic dye having absorption in a long wavelength region, a black pigment such as carbon black having low light transmittance,
Even if it contains a metallic pigment such as aluminum or titanium, photopolymerization can be caused, and various pigments thereof can be added.

As described above, the photopolymerization initiator of the present invention is suitable as a polymerization initiator for photocurable materials such as paints, inks and adhesives. These photo-curable materials can be used as solvent-free materials and are expected to contribute to the improvement of the global environment. Conventional reaction curable compositions required a large amount of solvent to dissolve the polymer component before curing, but photo-curable materials can use a reactive compound as a diluent, so essentially solvent-free is possible. Therefore, it becomes possible to eliminate the solvent or to drastically reduce the amount of the solvent.

Of course, there is no problem even if it is used in a form diluted with a conventional solvent, but the amount of solvent used is as small as possible from the viewpoints of resource saving from the viewpoint of global environment, prevention of environmental damage, etc. and cost. Is desirable. As the solvent that can be used at that time, as long as it is a compound that does not impair the chemical stability of the cationic dye and the sensitizer, a general solvent that has been conventionally used for paints, for example, an aromatic solvent such as toluene or xylene. Hydrocarbons, aliphatic hydrocarbons such as heptane, hexane and pentane, ketones such as acetone, methyl ethyl ketone and methyl isobutyl ketone, ethers such as diethylene glycol dimethyl ether and triethylene glycol dimethyl ether, esters such as ethyl acetate, butyl acetate and amyl acetate. And monoethers of ethylene glycol such as methyl cellosolve and ethyl cellosolve can be used. These solvents can be used alone or in combination of two or more. Further, it is also possible to use water as a solvent or a dispersion medium.

[0038]

EXAMPLES The present invention will be described below with reference to examples. (Example 1) Pentaerythritol triacrylate-
Hexamethylene diisocyanate-urethane prepolymer 70 g, nonaethylene glycol diacrylate 30
g, and 20 g of acetone are thoroughly mixed, and then 0.1 g of a near-infrared light absorbing cation dye (No. 3, Table 1; anion is n-butyltriphenylboron anion), a boron-based catalyst (tetrabutylammonium n- Butyltriphenylboron) 0.5 g, 2,4,6-tris (trichloromethyl) -1,3,5-triazine (reduction potential 3.1 V described in the text, HOMO-LUM by the calculation method described in the text)
Sample 1 was prepared by dissolving 0.5 g of O (difference of 8.902 eV).

Example 2 In place of 2,4,6-tris (trichloromethyl) -1,3,5-triazine, 2-phenyl-4,6-bis (trichloromethyl) -1,3.
5-triazine (difference in HOMO-LUMO is 8.32)
Sample 2 was prepared in the same manner as in Example 1 except that 0.5 g of 0 eV) was used.

(Example 3) 2,4,6-tris (trichloromethyl) -1,3,5-triazine instead of 2-
(P-Methoxyphenyl) -4,6-bis (trichloromethyl) -1,3,5-triazine (the same 7.800e
Sample 3 was prepared in the same manner as in Example 1 except that 0.5 g of V) was used.

(Example 4) 0.5 g of bis (4-t-butylphenyl) iodonium salt of hexafluorophosphate (6.236 eV) was used instead of the triazine compound.
Sample 4 was prepared in the same manner as in Example 1 except that it was used.

(Example 5) Instead of the triazine compound,
Sample 5 was prepared in the same manner as in Example 1 except that 0.5 g of diphenyliodonium salt of trifluoromethanesulfonic acid (3.956 eV in the same) was used.

(Example 6) Instead of the triazine compound,
Sample 6 was prepared in the same manner as in Example 1 except that 0.5 g of triphenylsulfonium salt of trifluoromethanesulfonic acid (6.540 eV in the same) was used.

(Example 7) Instead of the triazine compound,
3,3 ′, 4,4′-tetrakis (t-butyldioxycarbonyl) benzophenone (the above-mentioned reduction potential of 2.7 V,
The HOMO-LUMO difference of 9.423 eV) is 0.5
Sample 8 was prepared in the same manner as in Example 1 except that g was used.

Comparative Example 1 Comparative sample 1 was prepared in the same manner as in Example 1 except that the triazine compound which was a polymerization accelerator was not added.

Example 8 Sample 8 was prepared in the same manner as in Example 1 except that 0.1 g of the near-infrared light absorbing cation dye was changed from No. 3 in Table 1 to No. 1.

Comparative Example 2 Comparative sample 2 was prepared in the same manner as in Example 8 except that the triazine compound which was the polymerization accelerator was not added.

[Sample Polymerization Test and Polymerization Evaluation] The prepared samples 1 to 8 and comparative samples 1 and 2 were coated on an aluminum substrate with an applicator to a thickness of 100 μm.
Under a nitrogen atmosphere, a halogen lamp with an output of 1500 W having a spectral distribution in the wavelength region of 740 nm or more was irradiated for 5 minutes. The obtained coating film was peeled off, and the residual double bond was quantified by FT-IR. Table 2 shows the results. The less residual double bonds, the more the polymerization is progressing.

(Comparative Example 3) Six substrates were prepared by coating the sample of Comparative Example 1 by the same method as in the above-mentioned evaluation of polymerizability, and light irradiation was carried out by the same method. One substrate was taken out every 5 minutes after irradiation and the residual double bond was measured. It took 20 minutes until the reaction was completed (until the residual double bond remained unchanged), and the residual double bond at that time was 33%.

(Example 9) The amount of near infrared light absorbing dye was adjusted to 0.
Sample 9 was prepared in the same manner as in Example 1 except that the amount was changed from 1 g to 0.02 g, and six coated substrates were prepared in the same manner as in Comparative Example 3. As in Comparative Example 3, the reaction rate was measured every 5 minutes of light irradiation. As a result, the time required until the reaction was completed was 20 minutes, and the residual double bond at that time was 32%. It was revealed that the reaction can be terminated at the same curing speed as in the past even if the amount of expensive dye is reduced.

[0051]

[Table 2]

[0051]

INDUSTRIAL APPLICABILITY According to the present invention, it is useful and safe for the polymerization of a polymerizable composition having a high concealment ratio, which is capable of improving the polymerization rate or reducing the amount of the light-absorbing dye as an initiator, as compared with the prior art. A photopolymerization initiator having high property was provided.

Claims (4)

[Claims] 1. A photopolymerization initiator comprising a cationic dye represented by the general formula (1), a boron catalyst represented by the general formula (2), and an electron-accepting compound as essential components. General formula (1); D ⁺ · A ⁻ (wherein, D ⁺ is a cation having absorption in the near infrared wavelength region, and A ⁻ represents various anions) General formula (2); 1] (In the formula, R ₁ , R ₂ , R ₃ and R ₄ are each independently an alkyl group, an aryl group, an allyl group, an aralkyl group, an alkenyl group, an alkynyl group, a silyl group, a heterocyclic group, a halogen atom or a substituted alkyl group. Group, a substituted aryl group, a substituted allyl group, a substituted aralkyl group, a substituted alkenyl group, a substituted alkynyl group or a substituted silyl group, and Z ⁺ represents a cation) 2. The photopolymerization initiator according to claim 1, wherein the electron-accepting compound is a compound having a reduction potential of 2.0 V or more when a Li / Li + electrode is used as a reference electrode. 3. The highest occupied molecular orbit (H
The difference between OMO) and the lowest unoccupied orbit (LUMO) is 10.0 eV
The photopolymerization initiator according to claim 1, which is the following compound. 4. The electron-accepting compound is a triazine compound,
The photopolymerization initiator according to claim 1, which is one kind or a mixture of two or more kinds selected from the aromatic onium salt compounds.