JPH0513305B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a novel photosensitive elastomer composition for flexographic printing plates, more specifically, a photosensitive elastomer that provides a flexographic printing plate with high productivity and excellent thermal stability, ink acceptance transfer characteristics, ozone resistance, and resolution. The present invention relates to a composition. In general, rubber plates for flexographic printing are manufactured by producing an original plate by corroding a metal plate, and then using this original plate as a matrix and pouring rubber into it and solidifying it under pressure. However, this method involves many steps, which requires a lot of money and time, and the resulting rubber plate itself has low precision, so it has the disadvantage of requiring finishing such as back-shaving before use. It was hot. Recently, along with the development of photosensitive resins, their use in printing technology has increased, and many methods have been proposed for directly producing flexographic printing plates using photosensitive elastomer compositions. has already been put into practical use. For example, a printing plate for flexographic printing is produced using a photosensitive composition containing as main components an elastomeric polymer such as chloroprene rubber or polyurethane rubber, an ethylenically unsaturated compound having a terminal ethylenic double bond, and a photopolymerization initiator. (U.S. Pat. No. 2,948,611, U.S. Pat. No. 3,024,180), in which an ABA type thermoplastic block copolymer and an addition-polymerizable acrylate or methacrylate are used to impart insufficient elasticity to a photosensitive resin plate. , a method using a photosensitive composition containing a photopolymerization initiator and a liquid rubber having a molecular weight of 750 to 3000 (British Patent No. 1395822), unvulcanized polybutadiene rubber, chloroprene rubber, and acrylonitrile-butadiene as elastomeric polymers At least one liquid prepolymer selected from rubber, and further selected from polybutadiene, butadiene-styrene copolymer, butadiene-acrylonitrile copolymer, or derivatives thereof, having a molecular weight of 1000 to 5000. A method using a photosensitive composition in which one type is used in combination (Japanese Patent Application Laid-Open No. 51-106501
Publication No. 2), etc. have been proposed. In addition, at least two thermoplastic non-elastomeric polymer blocks with a glass transition temperature of 25° C. or higher are used to overcome the problems of lack of elasticity, thermal stability and cold flow associated with the use of photosensitive compositions. A solvent-soluble thermoplastic elastomer block copolymer having a structure in which elastomeric blocks are connected by elastomeric blocks having a glass transition temperature of 10°C or less is used as an elastomeric polymer (Japanese Unexamined Patent Application Publication No. 1989-1992).
It is also known to improve the physical properties of printing plates using esters of fumaric acid and maleic acid (Japanese Patent Application Laid-open No. 127004/1983). The development of technology for manufacturing flexographic printing plates using such photosensitive elastomer compositions has significantly improved the shortcomings of conventional flexographic printing plate manufacturing methods, such as shortening the manufacturing process and omitting finishing processing. Although it has become possible to print even finer designs than conventional rubber plates, there are still many problems that need to be solved before it can be put into practical use, and it cannot be said that it is necessarily satisfactory. The photosensitive elastomer compositions for flexo printing plates conventionally used have poor resolution, and when exposed to normal image exposure, forming fine dots of 100 lines/inch at 3% results in smudges such as white lines, resulting in white spots in printing. It is not possible to obtain quality reproductions of nude images. For this reason, it is necessary to perform masking exposure by first covering the part of the negative film that corresponds to the blank image area with light-shielding tape to prevent light from passing through, and then exposing it again using the light-shielding tape. This was time-consuming and had poor productivity. Furthermore, if there is even a partial error in this masking operation, that part will become dull on the plate and cannot be reproduced in printed matter, resulting in a defective plate and having to be remade. There was a problem. In addition, when printing using the obtained plate,
There is a poor balance in the ink acceptance and transfer characteristics between bright areas and dark areas or solid areas such as uppercase letters in a photo, and if you try to print solid areas uniformly and darkly, small letters may become unclear or the photo may become dark. , Also, if you try to make small letters clearer or brighten a photo area, it may cause problems such as uneven solid areas, so even when printing in the same color, the photo area and the solid area are printed on separate plates and printed separately. I had to. Furthermore, when printing on film, corona discharge treatment is often performed prior to the printing process, but the ozone generated during this process deteriorates the plate surface and causes cracks, so printing with a small number of copies is difficult. The drawback was that it could only be implemented. The present invention overcomes various drawbacks of photosensitive elastomer compositions used in conventional flexographic printing plates, and achieves high productivity, thermal stability,
As a result of intensive research to develop a photosensitive elastomer composition that can provide a flexographic printing plate with excellent ink acceptance transfer characteristics, ozone resistance, and resolution,
It has been discovered that the object can be achieved by a composition containing a combination of a specific thermoplastic elastomer block copolymer, a specific polybutadiene, and a maleic acid derivative together with a photopolymerization initiator, and based on this knowledge, the present invention has been developed. I was able to accomplish this. That is, the present invention has the following features: (a) General formula (A-B) _o ...(1) (A in the formula is a thermoplastic non-elastomeric polymer block having a glass transition temperature of 25°C or higher;
B is a polybutadiene block, n is an integer of 2 to 10); (b) the molar ratio of vinyl structure/cis and trans structure is 1 to 2.3; At least one type of terminal hydrogen type polybutadiene having an average molecular weight of 500 to 5000, (c) General formula R ₁ OOCCH=CHCOOR ₂ or

[Formula] …(2) (R ₁ and R ₂ in the formula each have 2 to 30 carbon atoms
at least one kind of ethylenically unsaturated compound represented by ( _d ) A photosensitive elastomer composition for flexographic printing plates containing a photopolymerization initiator as an essential component is provided. By the way, conventional photosensitive elastomer compositions using a thermoplastic elastomeric block copolymer and liquid polybutadiene in combination include:
For example, (1) A-B-A type thermoplastic block copolymer, (2) addition-polymerizable methacrylate or acrylate, (3) photoinitiator, and (4) molecular weight 750-3000.
A composition comprising a liquid rubber has been proposed (UK Patent No. 1395822). In this proposal, liquid polybutadiene is mentioned as an example of the liquid rubber, and the liquid rubber is added for the purpose of lowering the hardness. This composition differs from the present invention in the structure of the thermoplastic elastomeric block copolymer and the type of ethylenically unsaturated compound, and furthermore, nothing is mentioned about the microstructure of the liquid rubber. Due to such compositional differences, the composition undergoes a photocuring reaction differently from the composition of the present invention, and the compatibility before and after curing is poor, making it impossible to obtain sufficiently satisfactory resolution. Also, the one described in Japanese Patent Publication No. 53-37764 is also known,
This is also the same as above. Furthermore, JP-A-47-37521 lists polybutadiene as an example of a polymer that can be added as an anti-ozonant to a photosensitive elastomer composition based on a thermoplastic elastomer block copolymer. There is. However, the microstructure and molecular weight of this polybutadiene are not described, and if ordinary polybutadiene is used, it has poor compatibility and further deteriorates resolution, so it cannot be used in practice. Examples of A in the thermoplastic elastomeric block copolymer used as component (a) of the composition of the present invention, that is, a thermoplastic non-elastomeric polymer block having a glass transition temperature of 25°C or higher, include:
Mention may be made of polystyrene and poly(α-methylstyrene). Therefore, examples of component (a) are (polystyrene-polybutadiene) _2-10 and poly[α-methylstyrene)-polybutadiene]
In particular, polystyrene- _{polybutadiene} -polystyrene-polybutadiene and poly(α-methylstyrene)-polybutadiene-poly(α-methylstyrene) can provide good rubber elasticity and are easily available. )-polybutadiene and the like are preferred. Such a thermoplastic elastomeric block copolymer has rubber elasticity even in an unvulcanized state due to physical crosslinking of the thermoplastic non-elastomeric polymer blocks sandwiching the elastomeric polymer block, and has rubber elasticity even in an unvulcanized state. It has the characteristic of not causing flow. However, if the molecular weight of the thermoplastic non-elastomeric polymer block is too small, the effect of physical crosslinking will not appear, and if it is too large, the rubber elasticity will be impaired. Furthermore, if the molecular weight of the polybutadiene block is too small, it will not be able to exhibit rubber elasticity, and if it is too large, the effect of physical crosslinking will not be fully exhibited. From this point of view, GPC using polystyrene as the standard
The number average molecular weight obtained by measurement is 2000 for thermoplastic non-elastomeric polymer blocks.
~100,000, and preferably 25,000 to 1,000,000 for polybutadiene blocks. This thermoplastic elastomeric block copolymer can be produced, for example, by the method described in U.S. Pat. No. 3,265,765, in which sec-butyllithium is added to a benzene solution of styrene and subjected to living polymerization to generate polystyrene anion radicals. ,
Next, butadiene is added to this reaction mixture to perform polymerization to form a polystyrene-polybutadiene anion radical, and the operation of adding styrene is repeated in order to obtain the desired (polystyrene-polybutadiene) _o anion radical. It can be produced by adding a hydroxyl compound such as water, alcohol, or phenol at the end to deactivate anion radicals, which are active species of living polymerization. The A portion in the thermoplastic elastomeric block copolymer thus obtained must have a glass transition temperature of 25°C or higher. If this material has a glass transition temperature of less than 25° C., it has the disadvantage that cold flow is likely to occur. The (A-B) _o (n≧2) type thermoplastic elastomeric block copolymer used in the present invention is A-
Unlike the B-A type, since B blocks are protruded at the edges, compatibility with other components in the photosensitive elastomer composition is improved, contributing to improved resolution. Next, the hydrogen-terminated polybutadiene used as component (b) of the composition of the present invention has a ratio of 1,2-bond moieties to 1,4-bond moieties present therein, that is, vinyl structure/cis and trans structure. The molar ratio of 1 to 2.3 and the number average molecular weight of 500 to 5000 are required. The molar ratio of vinyl structure/cis and trans structure is
If it is larger than 2.3, the resolution of the photosensitive composition obtained will be reduced, and if it is smaller than 1.0, the sensitivity will be reduced. On the other hand, if the number average molecular weight is greater than 5,000, resolution will be impaired, and if it is less than 500, sufficient rubber elasticity will not be obtained. If a functional group such as a hydroxyl group or a carboxyl group is present at the end, the compatibility with component (a) will be poor, resulting in turbidity and a decrease in resolution, so it is recommended to use a product with hydrogen at the end. is necessary. Such terminal hydrogen type polybutadiene has a number average molecular weight of 500 to 3000 from the viewpoint of easy availability and a wide range of compatibility with other components.
Preferably. Next, used as component (c) of the composition of the present invention,
The esters of maleic acid or fumaric acid represented by the general formula (2) include, for example, diethyl fumarate, dibutyl fumarate, dioctyl fumarate, distearyl fumarate,
Fumaric acid butyloctyl ester, fumaric acid diphenyl ester, fumaric acid dibenzyl ester,
Examples include dibutyl maleate, dioctyl maleate, bis(3-phenylpropyl) fumarate, and the like, but are not limited to these. These may be used alone or in combination of two or more. If methacrylic acid or acrylic acid esters having an ethylene group at the end are used as the ethylenically unsaturated compound, not only will the resulting photosensitive composition have poor thermal stability, but it will also be susceptible to addition polymerization due to its own addition polymerizability. This is not preferable because a homopolymer is formed and the compatibility with other components deteriorates during curing, resulting in a decrease in resolution. In addition, as the N-substituted maleimide compound represented by the general formula (3), hydrocarbon groups having 1 to 22 carbon atoms, such as linear, branched or cyclic alkyl groups, phenyl groups, naphthyl groups, lower alkyl groups, etc. Mention may be made of N-substituted maleimides with phenyl groups or naphthyl groups which are nuclearly substituted with groups or halogen atoms. In addition, maleimide N-substituted with an aralkyl group such as a lower alkyl group substituted with a phenyl group, or an alkylene group having 1 to 12 carbon atoms, a phenylene group, a biphenylene group, a 1,6-hexamethylenebiphenylene group Examples include maleimide compounds in which two maleimide groups are bonded at the N position to a divalent hydrocarbon residue such as , methylenebiphenylene group, or 2,2-diphenylenepropane group. Specific examples include N-methylmaleimide, N-ethylmaleimide, N-propylmaleimide, N-butylmaleimide, N-bentylmaleimide, N-n-hexylmaleimide, N-
Cyclohexylmaleimide, N-n-octylmaleimide, N-2-ethylhethysylmaleimide,
N-n-decylmaleimide, N-laurylmaleimide, N-stearylmaleimide, N-behenylmaleimide, N-phenylmaleimide, N-chlorophenylmaleimide, N-tolylmaleimide,
N-xylylmaleimide, N-benzylmaleimide, N-(6-phenylhexyl)maleimide,
Methylene bismaleimide, ethylene bismaleimide, trimethylene bismaleimide, hexamethylene bismaleimide, decamethylene bismaleimide, dodecamethylene bismaleimide, phenylene bismaleimide, 2,2-diphenylenepropane bismaleimide, diphenylenemethane bismaleimide Examples include, but are not limited to, the following. Among these N-substituted maleimide compounds, N-alkylmaleimide, N-phenylmaleimide, and N-benzylmaleimide are preferred in terms of availability and price. In addition, N-alkylmaleimide whose alkyl group has a small number of carbon atoms has a pungent odor and is not preferred in terms of workability. Therefore, it is desirable that the alkyl group has 6 or more carbon atoms. Furthermore, if the number of carbon atoms in the alkyl group is too large, it becomes difficult to obtain, so it is desirable that the number of carbon atoms is 22 or less. Note that unsubstituted maleimide has a strong irritating odor and is poorly compatible with photosensitive elastomer compositions, so it cannot be used practically. In addition, substances with aryl groups such as N-phenylmaleimide have a large UV absorption and reduce the amount of light reaching the center, so when making plates with a thickness of 3 mm or more, , is disadvantageous in plate making compared to the case where N-alkylmaleimide is added. This maleimide compound is used in an amount of 10% by weight or less based on the entire photosensitive composition. If the amount is more than this, it is not preferable because the N-substituted maleimide compound itself absorbs ultraviolet light, which may be disadvantageous in making thick plates. By the way, in order to improve the resolution width, it is necessary to have good transparency of the photosensitive composition before and after curing, and to improve the reactivity of the photosensitive composition, when the photosensitive composition is imagewise exposed. It is required that the composition not be cured by scattered light generated within the composition or by weak light reflected from the opposite side. Regarding transparency before and after curing, it is necessary that the components that make up the photosensitive elastomer composition have good compatibility, and that oligomer or polymer components that would inhibit compatibility are not generated during the photocuring reaction process. It is. From the latter point of view regarding transparency and the fact that it does not cure with weak light, ethylenically unsaturated compounds having an ethylene group (CH ₂ ═C) at the end that undergoes addition polymerization themselves are unsuitable, and the composition It is necessary to use ethylenically unsaturated compounds, such as those used in the present invention, of a type that will undergo a crosslinking reaction with the main polymer in the product. Photopolymerization initiators useful in the photosensitive elastomer composition of the present invention include benzoin and its derivatives, such as benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin isobutyl ether, α-methylolbenzoin, α-methoxybenzoin, α
Examples include -methoxybenzoin methyl ether, α-methylolbenzoin methyl ether, benzoin phenyl ether, α-ethoxybenzoin ethyl ether, α-tert-butylbenzoin, benzophenone, and 2-ethylanthraquinone. The blending ratio of each component in the composition of the present invention is (a)
Per 100 parts by weight of component, the amount of component (b) is selected from 5 to 100 parts by weight, and the amount of component (c) is selected from 2 to 40 parts by weight. If the amount of component (b) is less than this, the resulting printing plate will lack elasticity, and if the amount is too large, the hardness will be low and the uneven boundary portion of the plate surface will be easily crushed during use. It is inevitable that the clarity of fine patterns will deteriorate. On the other hand, if the amount of component (c) is too small, sufficient photosensitivity cannot be obtained, while if it is too large, rubber elasticity will be impaired. Further, the amount of component (d), that is, the photopolymerization initiator, added to the composition of the present invention is selected to be at least 0.01% by weight, and usually in the range of 0.1 to 3% by weight, based on the total amount of the composition. In addition, various auxiliary additive components commonly used in ordinary photosensitive resin compositions, such as thermal polymerization inhibitors, ultraviolet absorbers, antihalation agents, etc., may be added to the composition of the present invention as desired. can. Examples of the thermal polymerization inhibitor include 2,6-di-tert-butyl-p-cresol, p-methoxyphenol, and pentaerythritol tetrakis [3
-(3',5'-di-tert-butyl-4'-hydroxy)phenylpropionate], hydroquinone, tert-butylcatechol, tert-butylhydroxyanisole, 4,4'-butylidenebis(3-methyl- 6-
(tertiary butyl) phenol and the like. These may be used alone or in combination of two or more. The appropriate amount of the thermal polymerization inhibitor to be added is usually in the range of 0.001 to 2% by weight. Since the composition of the present invention exhibits tackiness depending on its composition, in order to improve contact with a transparent image carrier overlaid thereon and to enable reuse of the transparent image carrier, A laminate layer of a thin film of polyethylene, polypropylene, polyester, polystyrene, etc. can be provided on the surface. The film is peeled off after exposure through the overlying transparent image carrier. For similar purposes, the film may be replaced by a thin flexible layer that is soluble in solvents. In this case, when the unexposed area is eluted after exposure through the transparent image carrier, that layer is also removed by dissolution or the like at the same time. The composition of the present invention can be prepared by any method. For example, chloroform, carbon tetrachloride, 1,
1,1-trichloroethane, tetrachlorethylene, trichlorethylene, methyl ethyl ketone,
This photosensitive elastomer composition can be dissolved in a suitable solvent such as methyl isobutyl ketone, toluene, tetrahydrofuran, etc., mixed, cast into a mold, and evaporated to form a plate as it is. If the plate is heated and pressed, a layer with high precision can be obtained. Further, the compositions can be mixed in a kneader or roll mill without using a solvent, and then formed into a sheet of a desired thickness by extrusion, injection, pressing, calendering, or the like. The protective film and support film can be brought into close contact with the photosensitive layer by roll lamination after forming the sheet. After lamination, a highly accurate photosensitive layer can be obtained by hot pressing. When a thin flexible layer, such as a layer of crystalline 1,2-polybutadiene, soluble polyamide, partially saponified polyvinyl acetate, etc., is to be provided on the surface of the photosensitive layer, the layer is dissolved in a suitable solvent and the solution is prepared. The photosensitive layer may be coated directly on the surface of the photosensitive layer, or it may be coated on a film of polyester, polypropylene, etc., and then the entire film may be laminated onto the photosensitive layer and transferred. In the photosensitive composition of the present invention, cellulose esters are particularly suitable as the thin flexible layer. Examples of active light sources used to insolubilize the composition of the present invention include low-pressure mercury lamps, high-pressure mercury lamps,
Examples include ultraviolet fluorescent lamps, carbon arc lamps, xenon lamps, zirconium lamps, and sunlight. After irradiating the photosensitive elastomer composition of the present invention with light through a transparent image carrier to form an image,
The solvent used to elute the unexposed area must be one that dissolves the unexposed area well and has little effect on the exposed image area, for example, 1, 1, 1. - Trichloroethane, tetrachlorethylene, trichlorethylene, tetrachloroethane, toluene, etc., and mixtures thereof with 60% by weight or less of alcohol, such as ethanol, isopropanol, n-butanol, etc. The unexposed areas are eluted by spraying from a nozzle or by brushing with a brush. Next, the present invention will be explained in detail with reference to Examples. Example 1 Polystyrene-polybutadiene-polystyrene-polybutadiene block copolymer obtained by the method described in U.S. Pat. No. 3,265,765 (styrene unit content 40% by weight, number average molecular weight by GPC measurement using polystyrene as standard) 200000) 6Kg,
Liquid polybutadiene (polystyrene is standard)
Number average molecular weight 2000 by GPC measurement, molar ratio of vinyl structure/cis and trans structure by IR analysis
2.03) 3Kg, di-n-octyl fumarate 500g,
250 g of N-laurylmaleimide, 200 g of 2,2-dimethoxyphenylacetophenone, and 20 g of 2,6-di-t-butyl-p-cresol were kneaded in a kneader (composition E1). The photosensitive composition thus obtained was placed between two films: a 100μ thick polyester film and a 100μ thick polyester film coated with a 3μ thick cellulose acetate butyrate film. A sheet with a thickness of 3 mm was made by using scissors and hot pressing in contact with the photosensitive composition. The side of this sheet without the cellulose acetate butyrate film is exposed to ultraviolet fluorescent lamps.
Exposure to light with an intensity of 2 mW/cm ² for 5 minutes, then peel off the polyester film on the cellulose acetate butyrate membrane side, stick a negative film with an image on it, and expose it to 2 mW/cm ² of ultraviolet fluorescent lamp through this film. exposed for 20 minutes at a light intensity of .
Next, the unexposed areas were washed out with a brush using a mixture of 1,1,1-trichloroethane/isopropanol (3/1), dried at 60°C for 30 minutes, cooled to room temperature, and used for image exposure. Post-exposure was performed using the same light source as in In this edition, 100
A highlighted area of 3% line/inch was formed, and the depth of the 500μ wide white line measured using a depth microscope was as large as 150μ. Example 2 and Comparative Example 1 Polystyrene-polybutadiene-polystyrene-polybutadiene block copolymer obtained by the method described in U.S. Pat. No. 3,265,765 (styrene unit content 35% by weight, GPC measurement using polystyrene as standard) A photosensitive elastomer composition (E2-4, C1
A sheet of ~5) was obtained. The polyester film on the cellulose acetate butyrate film side of these sheets was peeled off, a negative film bearing an image was adhered thereto, and the film was exposed to light of an intensity of 2 mW/cm ² from an ultraviolet fluorescent lamp through this film. Next, in the same manner as in Example 1, the uncured portion was washed out, dried,
Post-exposure was performed to obtain a printing plate. A plate-making operation was performed while changing the exposure time, and the formation state of the highlight portion and the depth of the 500 μ wide blank line on the obtained plate were measured. The formulation contents of each photosensitive elastomer composition are determined first.
The plate making results are shown in Table 2. Table 2 shows that when the vinyl group content of the liquid rubber used is too low, the formation of highlighted areas is extremely poor, and when it is too high, white lines become thick and the depth becomes small. Furthermore, it has been shown that when dimethacrylate having a terminal ethylenic group is used as an ethylenically unsaturated compound, white lines become thick.

【table】

【table】

[Table] Comparative Example 2 Polystyrene-polybutadiene-polystyrene in the photosensitive elastomer composition of Example 1
Polybutadiene block copolymer is polystyrene-polybutadiene-polystyrene block copolymer (styrene unit content 40% by weight, number average molecular weight by GPC measurement using polystyrene as standard).
100000), or polystyrene-polyisoprene-
Photosensitive elastomer composition sheets replaced with polystyrene block copolymer (styrene unit content 14% by weight, number average molecular weight 100000 by GPC measurement using polystyrene as standard) (respectively C6,
C7) was obtained in the same manner as in Example 1. Next, plate making was performed in the same manner as in Example 1 to obtain a printing plate. Observation results of the highlight formation state of the obtained plate,
Table 3 shows the results of measuring the depth of a 500 μ wide white blank line.
Comparing these results with Example 1, it was found that the resolving power changes depending on the type of thermoplastic elastomeric block copolymer, and that the combination of polystyrene-polybutadiene-polystyrene-polybutadiene block copolymer has excellent resolving power. I understand.

[Table] Example 3 The printing plate obtained in Example 1 was wrapped around a cylinder with a diameter of 15 cm and left at 40°C and an ozone concentration of 50 ppm. When the condition of the plate surface was observed, cracks appeared. It took 18 hours, and even at that point the cracks were still very small. Comparative Example 3 Polystyrene-polyisoprene-polystyrene block copolymer (styrene unit content 14% by weight,
Number average molecular weight by GPC measurement using polystyrene as standard: 200000) 4 kg, 1,6-hexanediol dimethacrylate 400 g, 2,2-dimethoxy-2-phenylacetophenone 88 g, 2,6-di-t-butyl- A photosensitive elastomer composition sheet was prepared from 0.9 g of p-cresol in the same manner as in Example 1, and a printing plate was prepared in the same manner as in Example 1. When this printing plate was examined for deterioration due to ozone in the same manner as in Example 3, large cracks appeared on the surface in just 2 hours.

Claims (1)

[Claims] 1 (a) General formula (A-B) _o (A in the formula is a thermoplastic non-elastomeric polymer block having a glass transition temperature of 25°C or higher,
B is a polybutadiene block, n is an integer of 2 to 10); (b) the molar ratio of vinyl structure/cis and trans structure is 1 to 2.3; At least one type of terminal hydrogen type polybutadiene with an average molecular weight of 500 to 5000, (c) General formula R ₁ OOCCH=CHCOOR ₂ or [Formula] (R ₁ and R ₂ in the formula each have 2 to 30 carbon atoms.
and ( _d ) A photosensitive elastomer composition for flexographic printing plates containing a photopolymerization initiator as an essential component.