JPH08314139A - Photopolymerizable composition - Google Patents

Abstract

PURPOSE: To ensure satisfactory sensitivity to visible light by using a photopolymn. initiation system contg. a specified sensitizer and an activator which can generate active radicals when irradiated with light under coexistence with the sensitizer. CONSTITUTION: This photopolymerizable compsn. contains a photopolymn. initiation system contg. a sensitizer represented by the formula and an activator which can generate active radicals when irradiated with light under coexistence with the sensitizer. In the formula, each of R<1> -R<4> is H or <=10C alkyl, R<1> and R<2> may form alkylene by bonding to each other, each of R<5> and R<6> is H, <=10C alkyl, alkoxy or acyloxy, each of Y and Z is cyano, phenyl which may have a substituent, a heterocyclic residue, carboxyl, carboalkoxy, carbamoyl or acyl and (m) is 0 or 1. The pref. activator is, e.g., a biimidazole compd., a titanocene compd. or an org. peroxide.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a photopolymerizable composition. In particular, the present invention relates to a photopolymerizable composition having high sensitivity to light rays in the visible region.

[0002]

2. Description of the Related Art Conventionally, many image forming methods utilizing a photopolymerization system are known. For example, a photopolymerizable composition comprising a compound containing an addition-polymerizable ethylenic double bond, a photopolymerization initiator, and an organic polymer binder optionally used is prepared, and the photopolymerizable composition is supported. A photosensitive material having a layer of the photopolymerizable composition applied on the body is prepared, and the desired image is imagewise exposed to polymerize and cure the exposed portion, and the unexposed portion is dissolved and removed to form a cured relief image. And a layer of a photopolymerizable composition is provided between two supports in which at least one of the above-mentioned light-sensitive materials is transparent, and the transparent support side is imagewise exposed to cause a change in adhesive strength due to light. After that, there is a method of forming an image by peeling the support, and an image forming method utilizing the change of toner adherability of the photopolymerizable composition layer due to light. Conventionally, benzoin, benzoin alkyl ether, benzyl ketal, benzophenone, anthraquinone, benzyl, or Michler's ketone has been used as the photopolymerization initiator of the photopolymerizable composition applied to these methods.

However, these photopolymerization initiators are
Compared with the ability to initiate photopolymerization for light in the ultraviolet region below 00 nm, it is significantly lower than that for light in the visible light region above 400 nm, thus limiting the range of application of photopolymerizable compositions containing them.

In recent years, with the development of image forming technology, there has been a strong demand for photopolymers having a high sensitivity to light rays in the visible region. For example, it is a light-sensitive material suitable for non-contact type projection exposure plate making, laser plate making by visible light laser, and the like. Among these techniques, the plate making method using an 488 nm oscillation beam of an argon ion laser is considered to be one of the most promising techniques in the future.

Several proposals have hitherto been made regarding a photopolymerization composition containing a photopolymerization initiation system which is sensitive to light rays in the visible light region. For example, a system of hexaarylbiimidazole and (p-dialkylaminobenzylidene) ketone (JP-A-47-2528, JP-A-54-155).
292, JP-A-56-166154), a system of ketocoumarin and an activator (JP-A-52-112681,
JP-A-58-15503, JP-A-60-88005), a system of a substituted triazine and a sensitizer (JP-A-54-54).
151024, JP-A-58-29803, JP-A-5-
8-40302), biimidazole, styrene derivative, thiol system (JP-A-59-56403), organic peroxide and dye system (JP-A-59-1402).
03, JP-A-59-189340)
A system of a titanocene compound and a sensitizer (JP-A-63-2211).
No. 10, JP-A-4-26154, JP-A-4-2197.
No. 56). In addition, a system using an aminobenzylidene sensitizer has been proposed. For example, JP-A-61-270747 and JP-A-62-180359.
And JP-A-60-92302 disclose an initiation system containing a specific benzylidene sensitizer having a dialkylamino group. These are certainly effective to some extent, but due to recent market demands, further improvement in sensitivity has been desired.

The present invention meets that need, that is, the object of the present invention is to provide a photopolymerizable composition having sufficient sensitivity to visible light. The present inventors have found that the above-mentioned object is achieved by the following specific composition.

That is, the present invention provides a photopolymerizable composition containing a compound having at least one ethylenically unsaturated double bond capable of addition polymerization and a photopolymerization initiation system, wherein the photopolymerization initiation system is (a). Sensitizer represented by the following general formula [I]

[0008]

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(Wherein R ¹ , R ² , R ³ and R ⁴ are
Each independently represents a hydrogen atom or an alkyl group having 10 or less carbon atoms, but R ¹ and R ² may be connected to each other to form an alkylene group. R ⁵ and R ⁶ independently represent a hydrogen atom, an alkyl group having 10 or less carbon atoms, an alkoxy group or an acyloxy group. Y and Z each independently represent a cyano group, a phenyl group which may have a substituent, a heterocyclic residue, a carboxyl group, a carboalkoxy group, a carbamoyl group or an acyl group, and m represents 0 or 1. And (b) in the coexistence with the sensitizer (a), a photopolymerizable composition characterized by containing an activator capable of generating an active radical upon irradiation with light. Hereinafter, the present invention will be described in detail.

The compound having at least one addition-polymerizable ethylenically unsaturated double bond contained as the first essential component in the photopolymerizable composition of the present invention (hereinafter abbreviated as "ethylenic compound") is When the photopolymerizable composition is irradiated with actinic rays, it is a compound having an ethylenically unsaturated double bond that undergoes addition polymerization by the action of the photopolymerization initiation system that is the second essential component and cures. For example, it is a monomer having the above double bond or a polymer having an ethylenically unsaturated double bond in the side chain or main chain. Incidentally, the meaning of the monomer in the present invention is a concept corresponding to a so-called polymer substance, and therefore includes not only a narrowly defined monomer but also a dimer, a trimer and an oligomer. .

Examples of the monomer having an ethylenically unsaturated bond include unsaturated carboxylic acid, ester of aliphatic polyhydroxy compound and unsaturated carboxylic acid; ester of aromatic polyhydroxy compound and unsaturated carboxylic acid; Examples thereof include esters obtained by an esterification reaction of a saturated carboxylic acid with a polyvalent carboxylic acid and the above-mentioned aliphatic polyhydroxy compound, polyvalent hydroxy compound such as aromatic polyhydroxy compound, and the like.

The ester of the aliphatic polyhydroxy compound and the unsaturated carboxylic acid is not limited, but specific examples thereof include ethylene glycol diacrylate, triethylene glycol diacrylate, trimethylolpropane triacrylate, trimethylolethane triacrylate. Acrylic esters such as pentaerythritol diacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, dipentaerythritol tetraacrylate, dipentaerythritol pentaacrylate, dipentaerythritol hexaacrylate, glycerol acrylate, etc. Methacrylic acid ester replaced, Itaconic acid ester similarly replaced with itaconate Crotonate is maleic acid ester, and was replaced with crotonic acid ester or maleate was replaced.

Examples of the ester of an aromatic polyhydroxy compound and an unsaturated carboxylic acid include hydroquinone diacrylate, hydroquinone dimethacrylate, resorcin diacrylate, resorcin dimethacrylate and pyrogallol triacrylate.

The ester obtained by the esterification reaction of an unsaturated carboxylic acid with a polyvalent carboxylic acid and a polyvalent hydroxy compound is not necessarily a single substance, but typical examples include acrylic acid, phthalic acid and Examples thereof include condensates of ethylene glycol, condensates of acrylic acid, maleic acid and diethylene glycol, condensates of methacrylic acid, terephthalic acid and pentaerythritol, condensates of acrylic acid, adipic acid, butanediol and glycerin.

In addition to the above ester (meth) acrylates, there are so-called urethane (meth) acrylates and epoxy (meth) acrylates. The former can be prepared by an addition reaction between a polyvalent isocyanate and hydroxyacrylic esters, and the latter can be prepared by an addition reaction between a polyvalent epoxy compound and hydroxyacrylic esters. Other examples of the ethylenic compound used in the present invention include acrylamides such as ethylenebisacrylamide; allyl esters such as diallyl phthalate; and vinyl group-containing compounds such as divinylphthalate.

The above-mentioned polymer having an ethylenically unsaturated bond in the main chain is, for example, a polyester obtained by a polycondensation reaction of an unsaturated divalent carboxylic acid and a dihydroxy compound, or an unsaturated divalent carboxylic acid and a diamine. There are polyamides and the like obtained by polycondensation reaction. The polymer having an ethylenically unsaturated bond in the side chain is a divalent carboxylic acid having an unsaturated bond in the side chain, for example itaconic acid, propylidene succinic acid,
There is a condensation polymer of ethylidene malonic acid and the like and a dihydroxy or diamine compound. Further, a polymer having a functional group having a reactive activity such as a hydroxy group or a methyl halide group in its side chain, for example, polyvinyl alcohol, poly (2-hydroxyethyl methacrylate), polyepichlorohydrin, etc. and acrylic acid, methacrylic acid, crotonic acid, etc. Polymers obtained by the macromolecular reaction with the unsaturated carboxylic acid can also be suitably used.

Among the above-mentioned ethylenic compounds, acrylic acid ester or methacrylic acid ester monomers can be particularly preferably used. Next, the photopolymerization initiation system, which is the second essential component of the photopolymerization composition of the present invention, will be described.
The photopolymerization initiation system of the present invention is composed of a combination of two kinds of components, and the first component (a) thereof is a specific benzylidene sensitizer represented by the general formula [I]. Conventionally,
Benzylidene-based sensitizers having a lower dialkylamino substituent on the benzene ring are known. On the contrary, the present invention provides a photopolymerization initiation system containing a specific benzylidene sensitizer having a tetrahydroquinoline structure or a julolidine structure. It has been found that a sensitizer having such a structure brings about a great improvement in sensitivity characteristics.

In the formula [I], R ^{1 to} R ⁴ and R ^{5 to} R
^The alkyl group represented by ⁶ has 10 or less carbon atoms,
It is preferably 6 or less. In particular, it is preferable that R ¹ and R ² are connected to each other to form a propylene group, and R ^{3 to} R ^{6 are}
Is preferably a hydrogen atom. Examples of the substituent of the phenyl group represented by Y and Z include groups such as halogen, alkyl groups and alkoxy groups, which do not impair the sensitizing effect. Further, the heterocyclic residue represented by Y and Z is benzothiazole or the like, and the carboalkoxy group is 2 to 10 carbon atoms in total.
Examples of the carboalkoxy group include an acetyl group, a benzoyl group, a phthaloyl group, and the like. still,
When Y and Z are phthaloyl groups, Y and Z communicate with each other to form a ring. m represents 0 or 1, but is 0 from the viewpoint of synthesis.
Is preferred. Specific examples of the suitable sensitizer (a) include those having the following structures.

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[0020]

[Chemical 4]

[0021]

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[0022]

[Chemical 6]

[0023]

[Chemical 7]

These compounds can be easily prepared, for example, by a base-catalyzed dehydration condensation reaction between a corresponding aldehyde and an active methylene compound. Next, the activator of the second component (b) of the photopolymerization system of the present invention will be described. Any of these may be used as long as it has a property of generating active radicals when irradiated with light in the presence of the sensitizer (a). Preferred activators include:
Examples thereof include a biimidazole compound, a titanocene compound, a halogenated hydrocarbon derivative, an organic peroxide, and a diaryliodonium salt.

Specific examples of suitable biimidazole include:
For example, 2,2'-bis (o-chlorophenyl) -4,
4 ', 5,5'-Tetraphenylbiimidazole (hereinafter abbreviated as B-1), 2,2'-bis (o-chlorophenyl) -4,4', 5,5'-tetra (p-carbethoxy) Phenyl) biimidazole (hereinafter abbreviated as B-2), 2,2'-bis (o-chlorophenyl) -4,
4 ', 5,5'-tetra (p-bromophenyl) biimidazole, 2,2'-bis (o-chlorophenyl)-
4,4 ', 5,5'-tetra (o, p-dichlorophenyl) biimidazole (hereinafter abbreviated as B-3), 2,2'
-Bis (o-bromophenyl) -4,4 ', 5,5'-
Tetraphenylbiimidazole, 2,2'-bis (o,
p-dichlorophenyl) -4,4 ', 5,5'-tetraphenylbiimidazole, 2,2'-bis (o-chlorophenyl) -4,4', 5,5'-tetra (m-methoxyphenyl) bi Examples include imidazole and the like. Suitable titanocene compounds include, for example, dicyclopentadienyl-Ti-di-chloride, di-cyclopentadienyl-Ti-bis-phenyl, di-cyclopentadienyl-
Ti-bis-2,3,4,5,6-pentafluorophen-1-yl, dicyclopentadienyl-Ti-bis-
2,3,5,6-tetrafluorophen-1-yl, di-cyclopentadienyl-Ti-bis-2,4,6-trifluorophen-1-yl, di-cyclopentadienyl-Ti- Bis-2,6-di-fluorophen-1-yl, di-cyclopentadienyl-Ti-bis-2,4-
Di-fluorophen-1-yl, di-methylcyclopentadienyl-Ti-bis-2,3,4,5,6-pentafluorophen-1-yl, di-methylcyclopentadienyl-Ti-bis -2,3,5,6-tetrafluorophen-1-yl, di-methylcyclopentadienyl-
Ti-bis-2,6-difluorophenyl-1-yl,
Examples thereof include di-cyclopentadienyl-Ti-bis-2,6-difluoro-3- (pyr-1-yl) -phen-1-yl (hereinafter abbreviated as B-4).

Specific examples of the halogenated hydrocarbon derivative include 2,4,6-tris (trichloromethyl) -s-triazine, 2,4,6-tris (tribromomethyl) -s-triazine, and the like. 2-Methyl-4,6-di (trichloromethyl) -s-triazine, 2- (p-methoxyphenyl) -4,6-di (trichloromethyl)-
s-triazine, 2,2,2-trichloromethylacetophenone, tribromomethylphenyl sulfone, 2-trichloromethyl-5- (p-methoxystyryl) -1,
3,4-oxadiazole and the like can be mentioned.

Examples of the diaryliodonium salt include diphenyliodonium hexafluorophosphate, diphenyliodonium tosylate, diphenyliodonium fluoroborate, diphenyliodonium hexafluoroarsenate, diphenyliodonium chloride, ditolyliodonium hexafluorophosphate, phenyl (p-anisyl). Examples thereof include iodonium hexafluorophosphate and bis (m-nitrophenyl) iodonium hexafluorophosphate.

Other than the above, the activator in the present invention is known, for example, ketoxime esters such as 3-benzoyloxyiminobutan-2-one, 3,3 ',
4,4'-Tetra (t-butylperoxycarbonyl)
Organic peroxides such as benzophenone, N-phenylglycine, dimedone and the like can also be used. The activator used in the photopolymerizable composition of the present invention can be preferably used alone or in combination of two or more kinds.

Among the above-mentioned activators, more preferable ones are a biimidazole compound and a titanocene compound. Further, among the biimidazole compounds, particularly preferable are those in which the phenyl group at the 4-position or 5-position has a halogen atom such as chlorine atom or bromine atom or a carboalkoxy group. The total carbon number of carboalkoxy is preferably about 2-8.

The photopolymerization initiation system component of the present invention may contain a heterocyclic thiol compound as the component (c) in addition to the components (a) and (b). Generally, the inclusion of this improves the stability of the composition. Examples of suitable heterocyclic thiol compounds include, for example, 2-
Mercaptobenzothiazole (hereinafter abbreviated as C-1), 2
-Mercaptobenzoxazole, 2-tolucaptobenzimidazole, etc. are mentioned.

The amount of each component of the photoinitiating system of the photopolymerizable composition of the present invention to be used is preferably 0.1 to 20 parts by weight of the component (a), preferably 0.
2 to 10 parts by weight, component (b) to 0.1 to 60 parts by weight, preferably 0.5 to 30 parts by weight and component (c) to 50 parts by weight or less, preferably 30 parts by weight or less.
In addition to the above-mentioned photoinitiator component, other property improving agent,
For example, P-dialkylaminobenzoic acid ester, Michler's ketone, etc. may be further contained.

In the photopolymerizable composition of the present invention, in addition to the above-mentioned constituents, an organic polymer substance may be further added as a binder for the purpose of modifying the composition and improving the physical properties after photocuring. it can. The binder may be appropriately selected depending on the improvement purpose such as compatibility, film forming property, developability, and adhesiveness. Specifically, for example, to improve the water-based developability, acrylic acid copolymer, methacrylic acid copolymer, itaconic acid copolymer, partially esterified maleic acid copolymer, acidic cellulose modified product having a carboxyl group in the side chain, Examples include polyethylene oxide and polyvinylpyrrolidone. To improve film strength and adhesion, a polyether of epichlorohydrin and bisphenol A; soluble nylon; alkyl polymethacrylate such as polymethylmethacrylate or alkyl polyacrylate; alkyl methacrylate and acrylonitrile, acrylic acid, methacrylic acid , Copolymers with vinyl chloride, vinylidene chloride, styrene, etc .; copolymers with acrylonitrile and vinyl chloride, vinylidene chloride; vinylidene chloride, chlorinated polyolefins, copolymers with vinyl chloride and vinyl acetate; polyvinyl acetate; Acrylonitrile / styrene copolymer; Acrylonitrile / butadiene / styrene copolymer; Polyvinyl alkyl ether; Polyvinyl alkyl ketone; Polystyrene; Polyamide; Polyurethane; Polyethylene terephthalate isophthalate Mention may be made of acetyl cellulose and polyvinyl butyral. These binders can be added and mixed in a weight ratio of generally 500% or less, preferably 200% or less with respect to the ethylenic compound.

The photopolymerizable composition of the present invention may further contain a thermal polymerization inhibitor, a coloring agent, a plasticizer, a surface protective agent, a smoothing agent, if necessary.
Coating aids and other additives can be added. Examples of the thermal polymerization inhibitor include hydroquinone, p-methoxyphenol, pyrogallol, catechol, 2,6-di-
There are t-butyl-p-cresol, β-naphthol, and the like. Examples of the colorant include phthalocyanine pigments, azo pigments, carbon black, pigments such as titanium oxide, ethyl violet, crystal violet, azo dyes, anthraquinone dyes. , There are cyanine dyes. When the binder is used, the thermal polymerization inhibitor and the colorant are added in an amount of 0.01 to 5% of the thermal polymerization inhibitor and 0.1% of the colorant with respect to the total weight of the ethylenic compound and the binder. ~ 4
0% is preferable.

Examples of the plasticizer include dioctyl phthalate, didodecyl phthalate, triethylene glycol dicaprylate, dimethyl glycol phthalate, tricresyl phosphate, dioctyl adipate, dibutyl sebacate, triacetyl glycerin, and the like. When used, 30% or less is generally added to the total weight of the ethylenic compound and the binder.

When using the photopolymerizable composition of the present invention,
The light-sensitive material can be prepared by forming the light-sensitive material without a solvent or dissolving it in a suitable solvent to form a solution, which is coated on a support and dried. Examples of the solvent include methyl ethyl ketone, cyclohexanone, butyl acetate, amyl acetate, methyl lactate, toluene, xylene, monochlorobenzene, carbon tetrachloride, trichloroethylene, trichloroethane, dimethylformamide, methyl cellosolve, ethyl cellosolve, tetrahydrofuran, pentoxone, propylene glycol methyl ether acetate. Etc., and one kind or two or more kinds can be used in combination.

The support applied when preparing a light-sensitive material using the photopolymerizable composition of the present invention may be any of those usually used. For example, a sheet of metal such as aluminum, magnesium, copper, zinc, chromium, nickel, iron or an alloy containing them as a main component; high-quality paper, art paper,
Papers such as release paper; inorganic sheets such as glass and ceramics; polyethylene terephthalate, polyethylene, polymethylmethacrylate, vinyl chloride, vinyl chloride-vinylidene chloride copolymer, polystyrene, 6-nylon, cellulose triacetate, cellulose acetate butyrate, etc. Polymer sheets, etc.

Further, the photopolymerizable composition of the present invention is further provided with a known technique for preventing adverse effects such as sensitivity deterioration and storage stability deterioration due to oxygen, for example, providing a peelable transparent cover sheet on the photosensitive layer. Wax-like substance with low oxygen permeability,
It is also possible to provide a coating layer made of a water-soluble polymer or the like.

The exposure light source applicable to the composition of the present invention is not particularly limited, but carbon arc, high pressure mercury lamp, xenon lamp, metal halide lamp, fluorescent lamp, tungsten lamp, helium cadmium laser, argon ion laser, YAG laser, etc. 400
A general-purpose light source containing visible light of nm or more can be used more suitably.

[0039]

EXAMPLES The present invention will be described in more detail below with reference to Reference Examples, Examples and Comparative Examples, but the present invention is not limited to these Examples unless it exceeds the gist. Reference Example Preparation of Sensitizer A-3 4 g of julolidine-9-carbaldehyde and cyanoacetic acid ethyl ester 2.8 synthesized by a conventional method from julolidine using dimethylformamide and phosphorus oxychloride.
was added to 15 cc of ethanol, and 0.4 g of piperidine was further added dropwise. The reaction was continued for 2 hours at a bath temperature of 80 ° C. After completion of the reaction, the reaction mixture was cooled to room temperature and allowed to stand to obtain the desired product A-3 [1- (julolidin-9-yl) -2-cyano-2-carbethoxyethylene]. Reddish brown crystals, yield 5.3g.

Examples 1-20, Comparative Examples 1-5 Methyl methacrylate / methacrylic acid copolymer (weight average molecular weight 45,000, polymerization ratio 85/15) 1.8 g and trimethylolpropane triacrylate 2.2 g Was dissolved in 36 g of methyl ethyl ketone. A predetermined amount of each component shown in Table 1 was added to this to obtain a sample photosensitive solution.
This is applied to a grained and anodized aluminum sheet using a whirler to a dry film thickness of 2 μm, and a polyvinyl alcohol aqueous solution is further applied to the surface thereof.
A photosensitive material sample was prepared by coating so that the dry film thickness was 3 μm.

A xenon lamp; UI-501C manufactured by USHIO INC. Was used as a sample of this photosensitive material, and a spectral sensitivity measuring device manufactured by Narumi Co., Ltd. was used for 10 seconds so that the horizontal axis represents wavelength and the vertical axis logarithmically weakened the light intensity. Irradiated. The exposed sample was developed with an aqueous solution containing 6% by weight of butyl cellosolve and 1% by weight of sodium silicate, and the minimum amount of exposure required for photocured image formation was calculated from the height of the cured image corresponding to each wavelength obtained. Then, the sensitivity of each wavelength of the photosensitive composition was used. Here, all the sensitivities are shown by energy values corresponding to a light beam of 488 nm. The results are shown in Table 1. Table-1
The parts described in are the values expressed in parts by weight based on 100 parts by weight of the total weight of the binder polymer and the acrylate monomer. Also, the abbreviation R- used in the table
1 to R-4 represent the following compounds.

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[0043]

[Table 1]

[0044]

INDUSTRIAL APPLICABILITY The photopolymerizable composition of the present invention has extremely high sensitivity to visible light, and therefore, the composition is useful in a wide range of fields of application, such as printing plates such as planographic printing plates, intaglio printing plates and letterpress printing plates. It is industrially very useful because it can be used as a photoresist for production, printed wiring and IC production, a dry film, image formation such as relief image and image duplication, photocurable ink, paint, adhesive and the like.

Claims (4)

[Claims] 1. A photopolymerizable composition comprising a compound having at least one ethylenically unsaturated double bond capable of addition polymerization and a photopolymerization initiation system, wherein the photopolymerization initiation system is (a) I] a sensitizer represented by (In the formula, R ¹ , R ² , R ³ and R ⁴ independently represent a hydrogen atom or an alkyl group having 10 or less carbon atoms, but R ¹ and R ² are connected to each other to form an alkylene group. R ⁵ and R ⁶ independently represent a hydrogen atom, an alkyl group having 10 or less carbon atoms, an alkoxy group or an acyloxy group, and Y and Z may independently have a cyano group or a substituent. Represents a phenyl group, a heterocyclic residue, a carboxyl group, a carboalkoxy group, a carbamoyl group or an acyl group, and m represents 0 or 1.) and (b) in the presence of the sensitizer (a) and upon irradiation with light. A photopolymerizable composition comprising an activator capable of generating an active radical. 2. The photopolymerizable composition according to claim 1, wherein the activator contains a biimidazole compound or a titanocene compound. 3. R ¹ and R ^{2 in the} general formula [I] are connected to each other to represent a propylene group, and R ³ , R ⁴ and R ⁵
And R ⁶ is a hydrogen atom, The photopolymerizable composition according to claim 1. 4. The photopolymerizable composition according to claim 1, wherein the photopolymerization initiation system further contains a heterocyclic thiol compound.