Thermosensitive colour-forming polymer

Abstract: Provided is a polymer represented by the following formula (6): wherein A represents a repeating unit obtained by polymerizing at least one vinyl monomer having a function of generating an acid by the action of an acid and further breeding an acid by the action of an acid generated from the vinyl monomer itself; B represents a repeating unit obtained by polymerizing at least one vinyl monomer having a partial structure of causing an absorption region of from 360 to 900 nm by the action of an acid; C represents a repeating unit obtained by polymerizing at least one vinyl monomer copolymerizable with A and B; and x, y and z each represents % by weight and satisfy the conditions of 1≦x≦99, 1≦y≦99, 0≦z≦98 and x+y+z=100.

Description: The present invention relates to a copolymer of a monomer having a partial structure capable of generating an acid by the heating or action of an acid and a monomer having a partial structure capable of causing variation in the absorption by the contact with an acid. BACKGROUND OF THE INVENTION The thermosensitive recording material expresses the image area and the non-image area as a temperature difference distribution and many systems therefor have been proposed, such as transfer by the melting or sublimation of a coloring agent, color formation reaction between two components due to heat-melting or rupture of a capsule, or change in the optical properties due to phase transition. These kinds of thermosensitive recording mediums are advantageous in that a recording image can be obtained by a dry and simple system and the maintenance is not necessary, thus, are widely used as an output material of various printers, word processors, facsimiles and the like. Furthermore, to keep up with the progress of laser recording devices in recent years, application of the thermosensitive recording medium to optical disks or photomechanical materials is being studied. The photomechanical material heretofore used is a silver halide light-sensitive material requiring a wet processing. However, because of demands for simplification of the processing steps and problems of environmental pollution by the processing solutions, development of a dry process is required and several technical proposals using a thermosensitive recording system have been recently made. In view of the resolution, recording of an image using a laser is preferred and for example, a system using a high output laser, called dye ablation, has been developed. Recording materials therefor are disclosed in JP-A-7-164755 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-7-149063 and JP-A-7-149065, and image forming apparatuses are disclosed in JP-A-8-48053 and JP-A-8-72400. According to this system, a recording material comprising a support having coated thereon a dye composition comprising an image dye, a substance having an absorption in the laser wavelength region (infrared ray absorptive substance) and a binder is irradiated with a laser beam from the dye layer side to record an image. The energy given by the laser beam brings about abrupt local changes in the image forming layer at the spot of the laser beam thrusting the material and thereby expels the substance from the layer. According to the above-described patent publications, this is not a completely physical change (for example, melting, evaporation or sublimation) but a kind of chemical change (for example, bond destruction), and the image dye is not partially removed but completely removed. The dye ablation system has, however, problems that a high output laser is indispensable so as to increase the dye removal efficiency at the site exposed to a laser beam and a dust collector for collecting dyes removed is necessary to be installed in combination. As a system which can dispense with a dust collector, an ablation transfer image recording method using a laser heat source is disclosed in U.S. Patent 5,171,650. This system uses a dye donor sheet containing a dynamic release layer formed by overcoating an ablative carrier topcoat, where an image is transferred to a separate receiving sheet which is placed adjacently and aligned. Therefore, the sheet left useless after the recording of an image raises a problem of waste materials. Furthermore, also in this case, a high output laser is indispensable so as to increase the transfer efficiency. Thus, conventional thermosensitive recording systems using ablation by a laser cannot dispense with a high output laser and are bound to the problem of dusts or waste materials. On the other hand, a method expanded from a system called "dry silver" is described in JP-A-6-194781 as a heat-recording system involving no ablation using a laser. According to this system, recording is performed by a laser on a recording material containing a silver source capable of thermal reduction, a reducing agent for silver ion and a light-heat conversion dye. This system is, however, deficient in the practical performance in view of storability of the non-image area and heat sensitivity. Another example of the thermosensitive recording system using a laser is a compound capable of changing the absorption due to thermal decomposition of carbamate described in U.S. Patents 4,602,263 and 4,826,976. U.S. Patent 5,243,052 describes a compound of forming a yellow color due to thermal decomposition of the t-butoxycarbonyl group introduced into the hydroxyl group. These systems use an irreversible monomolecular reaction and are suited for the image recording within a short time using a laser, however, the sensitivity is not sufficiently high and improvement for higher sensitivity is needed. Higher sensitivity may be achieved by allowing an acid catalyst present together but in turn a problem arises in the storage stability. With respect to the method for forming a UV mask image (360 nm to 420 nm; corresponding to the exposure light source for PS plate) used for the photomechanical material, heat mode systems using a laser have not been proposed in practice. U.S. Patents 5,286,612, 5,395,736 and 5,441,850, W094/10606 and W094/10607 describe a recording material comprising a compound which generates an extra strong acid of pKa<0 by the action of an ultraviolet ray, a secondary acid generating agent which generates a secondary acid in the presence of an extra strong acid, and a compound which changes in the color upon contacting with the secondary acid. This system is effective for achieving high sensitivity of a recording material. However, generation of the secondary acid from a secondary acid generating agent described in those patent publications is catalyzed only by an extra strong acid generated from an extra strong acid precursor and although the patent publications state that about 20 molecules of the secondary acid precursor are catalyzed to decompose per one molecule of the extra strong acid, the secondary acid produced has no catalytic capability. Accordingly, increase in the sensitivity by the acid breeding in geometrical progression cannot be expected. As a compound which breeds an acid by the action of an acid, K. Ichimura, Chem. Lett. , 551 (1995) and JP-A-8-248561 describe a compound capable of producing an organic acid due to decomposition by an acid catalyst. In these publications, an acid reactive polymer composition comprising a combination of a photoacid generating agent and a substance of causing structural change by the action of an acid is described. However, the structural change caused by the action of an acid generated from a photoacid generating agent is to cause solubilization or insolubilization in a polar solvent or an aqueous alkali solution and the end use thereof is a so-called photoresist. These publications are completely silent on the recording material using change in the absorption. The present inventors have made extensive investigations on the compounds developed by Ichimura et al and have found that the compounds reported by Ichimura et al mostly have a function as a thermal acid generating agent and can be used for the image formation because of their capability to act as an acid generating agent even if a photo or thermal acid generating agent is not newly added. SUMMARY OF THE INVENTION GB-A-2 203 438 discloses a photosensitive and heat sensitive polymer having conjugated polyenes and sulphonic acid groups and an absorbence ratio of a strong band in the region of 1200 cm -1 to 1300 cm -1 assignable to sulfonic acid groups adjacent to conjugated polyenes of at least 3 sequence lengths to a strong band near 1050 cm -1 assignable to sulfonic acid groups in the IR spectrum of at least 0.6. It is an object of the present invention to provide a novel polymer capable of coloring (or decolorizing) only by an acid in a catalytic amount or by the action of heat. This object of the present invention can be attained by the following inventions 1) and 2). 1) A copolymer represented by the formula (6) wherein: A represents a repeating unit obtainable by polymerising at least one vinyl monomer of formula (7) capable of generating an acid by the action of an acid and further breeding an acid by the action of an acid generated from the vinyl monomer itself; B represents a repeating unit obtainable by polymerising at least one vinyl monomer having a partial structure capable of causing a change in the absorption region of from 360-900 nm by the action of an acid; C represents a repeating unit obtainable by polymerising at least one vinyl monomer copolymerisable with A and B; and x, y and z each represents % by weight and satisfy the conditions of 1≦x≦99, 1≦y≦99, 0≦z≦98 and x+y+z=100; wherein R 21 represents an electron-withdrawing group having a Hammett's σp value greater than 0; R 22 represents an alkyl group or an aryl group; R 23 represents a secondary or tertiary alkyl group having a hydrogen atom at the β-position; W 2 represents a residue of an acid represented by W 2 OH; and any one of the substituents R 21 , R 22 , R 23 and W 2 represents a polymerisable vinyl group. 2) The polymer as described in 1), wherein A has a function of generating an acid by the action of heat. Detailed Description of the Invention The present invention is described in detail below. In formula (6), A represents a repeating unit obtained by polymerizing at least one vinyl monomer having a function of generating an acid by the action of an acid and further breeding an acid by the action of an acid generated from the monomer itself or a function of generating an acid by heat. Examples of the compound having such a function include compounds described in Chemistry Letter, p. 551 (1995) and JP-A-8-248561 and cyclohexyl sulfonate. The polymer having a thermal acid generating function of the present invention is advantageous in that when it is applied to an image recording medium in a laser heat mode system, another acid generating agent is not necessary to be added. The embodiment of A having such a function includes the structure represented by formula (7). In formula (7), R 21 represents an electron-withdrawing group having a Hammet's σp value greater than 0 (having preferably from 0 to 60 carbon atoms, for example, an acyl group such as acetyl and benzoyl, a sulfonyl group such as methanesulfonyl and benzenesulfonyl, a heterocyclic residue such as benzothiazolyl and benzoxazolyl, and a trifluoromethyl group), R 2 represents an alkyl group (inclusive of an alkyl group having a substituent, preferably having from 1 to 60 carbon atoms, e.g., methyl, ethyl, iso-propyl, t-butyl, trifluoromethyl, ethoxymethyl) or an aryl group (inclusive of an aryl group having a substituent, preferably having from 6 to 60 carbon atoms, e.g., phenyl, naphthyl, 4-chlorophenyl, 2-methoxyphenyl, 4-nitrophenyl, 3-methanesulfonylphenyl), R 3 represents a secondary or tertiary alkyl group having a hydrogen atom at the β-position (inclusive of an alkyl group having a substituent, preferably having from 3 to 60 carbon atoms, e.g., t-butyl, cyclohexyl, tetrahydropyranyl, tetrahydrofuranyl, 4,5-dihydro-2-methylfuran-5-yl, 2-cyclohexenyl), W 2 represents a residue of an acid represented by W 2 OH (W 2 OH preferably having a pKa of 3 or less, e.g., p-toluenesulfonic acid, benzenesulfonic acid, benzenesulfonic acid, methanesulfonic acid, polyvinylbenzenesulfonic acid, p-nitrobenzoic acid), and at least one of R 21 , R 22 , R 23 and W 2 contains a polymerizable vinyl group (for example, groups shown below) at the site capable of substitution. (1) (2) (3) (4) (5) (6) Specific examples of the vinyl monomer for forming A of formula (6) are set forth below, however, the present invention is by no means limited thereto. These compounds can be synthesized in the same manner as in the method described in JP-A-8-248561. In formula (6), B represents a repeating unit obtained by the polymerization of at least one vinyl monomer having a partial structure capable of causing change in the absorption region of from 360 to 900 nm by the action of an acid. The partial structure capable of causing change in this absorption region by the action of an acid is described above. The monomer for forming a repeating unit represented by B in formula (6) by the polymerization is obtained by introducing a polymerizable vinyl group (examples thereof include those described above for A) into the site capable of substitution in the above-described partial structure of causing change in the absorption by the action of an acid. Examples thereof are set forth below, however, the present invention is by no means limited thereto. C in formula (6) represents a repeating unit obtained by polymerizing at least one vinyl monomer capable of forming a copolymer together with A and B, and the storage stability, coloring activity and the like can be controlled by adjusting the polarity, glass transition temperature or the like. Preferred examples thereof include acrylic ester, methacrylic ester, acrylamide, styrene and vinyl ether. Specific examples of the monomer for forming C are set forth below, however, the present invention is by no means limited thereto. In formula (6), x, y and z represent % by weight of respective compositions, x, y and z satisfy the conditions of 1≦x≦99, 1≦y≦99, 0≦z≦98 and x+y+z=100. x and y are preferably in the relation of 0.01y≦x≦10y, more preferably 0.1y≦x≦5y. z preferably satisfies the condition of 0≦z≦50. The polymer represented by formula (6) preferably has a molecular weight of from 1,000 to 1,000,000, more preferably from 2,000 to 100,000. The polymer may have any form of a random copolymer, an alternate copolymer and a block copolymer, however, a random copolymer easy to synthesize is commonly used. The monomers for forming A, B and C and the preferred combinations of x, y and z in the polymer represented by formula (6) are shown below, however, the present invention is by no means limited thereto. A B C x y z P-(1) A-(1) B-(1) - 67 33 - P-(2) A-(1) B-(1) - 75 25 - P-(3) A-(1) B-(1) C-(17) 50 25 25 P-(4) A-(1) B-(1) C-(18) 50 25 25 P-(5) A-(1) B-(1) C-(1) 50 25 25 P-(6) A-(1) B-(1) C-(11) 40 20 40 P-(7) A-(1) B-(5) - 67 33 - P-(8) A-(1) B-(6) - 67 33 - P-(9) A-(1) B-(13) - 67 33 - P-(10) A-(1) B-(19) - 67 33 - P-(11) A-(1) B-(21) - 67 33 - P-(12) A-(1) B-(25) - 67 33 - P-(13) A-(1) B-(29) - 67 33 - P-(14) A-(1) B-(41) - 67 33 - P-(15) A-(1) B-(41) - 75 25 - P-(16) A-(1) B-(41) - 80 20 - P-(17) A-(1) B-(43) - 75 25 - P-(20) A-(2) B-(1) - 67 33 - P-(21) A-(6) B-(1) - 67 33 - P-(22) A-(8) B-(1) - 67 33 - P-(23) A-(13) B-(1) - 67 33 - P-(24) A-(28) B-(5) C-(10) 50 10 40 P-(25) A-(30) B-(41) C-(14) 60 20 20 P-(26) A-(41) B-(1) - 67 33 - P-(27) A-(42) B-(41) - 67 33 - P-(28) A-(43) B-(26) - 67 33 - P-(29) A-(45) B-(1) - 67 33 - P-(30) A-(47) B-(1) - 67 33 - P-(31) A-(1) B-(1) - 50 50 - P-(32) A-(1) B-(1) - 20 80 - P-(33) A-(1) B-(1)/B-(28)/B-(42) (=1/1/1) C-(1) 40 40 10 P-(34) A-(1) B-(1)/B-(28) (=1/1) - 50 50 - The polymer of the present invention can be synthesized by, for example, solution polymerization, precipitation polymerization, suspension polymerization, bulk polymerization or emulsion polymerization. The polymerization may be initiated by a method of using a radical initiator or a method of irradiating light or radiant. These polymerizations methods and polymerization initiating methods are described, for example, in T. Tsuruta, Kobunshi Gosei Ho'ho, kaitei-ban (Polymer Synthesis Method, revised edition) , Nikkan Kogyo Shinbun Sha (1971) and T. O'tsu and M. Kinoshita, Kobunshi Gosei no Jikken-Ho (Experimental Method of Polymer Synthesis) , pp. 124-154, Kagaku Dojin (1972). Of those polymerization methods, solution polymerization using a radical initiator is particularly preferred. Examples of the solvent for use in the solution polymerization include various organic solvents such as ethyl acetate, methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, acetone, dioxane, N,N-dimethylformamide, N,N-dimethylacetamide, benzene, toluene, acetonitrile, methylene chloride, chloroform and dichloroethane. These solvents may be used individually or in combination of two or more thereof or may be used as a mixed solvent with water. The polymerization temperature is necessary to be selected in relation with the molecular weight of the polymer produced or the kind of initiator. The polymerization temperature may be from 0 to 100°C but it is usually from 30 to 100°C. In the present invention, the polymerization is preferably performed at from 30 to 60°C because the monomer for forming the site of A in formula (1) may decompose at a high temperature. Preferred examples of the radical initiator used for the polymerization include aza-based initiators such as 2,2'-azobisisobutyronitile,2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(2-amidinopropane)dihydrochloride and 4,4'-azobis(4-cyanopentanoic acid) and peroxide-based initiators such as benzoyl peroxide, t-butyl hydroperoxide and potassium persulfate (which may be used as a redox initiator by combining, for example, with sodium hydrogensulfite). In the present invention, an initiator of giving a half-value period of 10 hours at a temperature of 70°C or less (for example, 2,2'-azobis(2,4-dimethylvaleronitrile),2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), dimethyl 2,2'-azobis(2-methylpropionate) and 2,2'-azobis[2-(3,4,5,6-tetrahydropropane)-dihydrochloride]) is particularly preferred. The amount of the polymerization initiator used may be controlled according to the polymerizability of the monomer or the molecular weight of the polymer intended but it is preferably from 0.01 to 5.0 mol% based on the monomers. In synthesizing the polymer of the present invention, the polymerization may be performed by mixing monomers for forming A, B and C, placing the mixture in a reaction vessel and then adding an initiator or by passing through a step of adding dropwise those monomers to a polymerization solvent. An image recording medium can be manufactured by coating a polymer represented by formula (6) on a support. In the case where the polymer of the present invention has also a function of generating an acid by heat, there occurs change in the absorption region only by the action of heat and accordingly, the thermosensitive recording material as such used exhibits high sensitivity and good image sharpness. Furthermore, the image recording medium which can be manufactured using the polymer of the present invention is advantageous in that the film thickness can be reduced, a fairly sharp image can be obtained and ablation is difficult to take place. A slight amount of base may be added for the purpose of increasing the storage stability of the image recording medium, a compound capable of generating an acid by the action of light or heat may be separately added so as to increase the sensitivity, or if desired, additives such as a pigment, an antistatic agent and a sticking inhibitor may be added. Furthermore, an overcoat layer may be provided in order to protect the image forming layer or a backcoat layer may be provided on the back surface of the support. In addition, various known techniques for the thermosensitive material may be used, for example, a single or plurality of undercoat layers comprising a pigment or resin may be provided between the image recording layer and the support. In the case of adding a base, an organic base is preferred and preferred examples thereof include guanidine derivatives (e.g., 1,3-diphenylguanidine, 1,3-dimethylguanidine, 1,3-dibutylguanidine, 1-benzylguanidine, 1,1,3,3-tetramethylguanidine), aniline derivatives (e.g., aniline, p-t-butylaniline, N,N'-dimethylaniline, N,N'-dibutylaniline, triphenylamine), alkylamine derivatives (e.g., tributylamine, octylamine, laurylamine, benzylamine, dibenzylamine) and heterocyclic compounds (e.g., N,N'-dimethylaminopyridine, 1,8-diazabicyclo[5.4.0]-undecene, triphenylimidazole, rutidine, 2-picolin). The base is preferably added in an amount of from 1 to 50 mol%, more preferably from 5 to 20 mol%, based on the composition represented by A of formula (1). The polymer represented by formula (6) may contain a compound capable of generating an acid by the action of light or heat and specific examples thereof are as described above. The compound is preferably added in an amount of from 1 to 50 mol%, more preferably from 1 to 20 mol%, based on the site of A in the polymer. In the case of adding a pigment, examples thereof include diatomaceous earth, talc, kaolin, calcined kaolin, titanium oxide, silicon oxide, magnesium carbonate, calcium carbonate, aluminum hydroxide and urea-formalin resin. Examples of other additives include an ultraviolet absorbent such as benzophenone-based ultraviolet absorbent and benzotriazole-based ultraviolet absorbent, a head abrasion or sticking inhibitor comprising a higher fatty acid metal salt such as zinc stearate and calcium stearate, and waxes such as paraffin, paraffin oxide, polyethylene, polyethylene oxide and caster wax, and these may be added if desired. Examples of the support for use in the image recording medium which can be manufactured using the polymer of the present invention include papers such as wood free paper, baryta paper, coat paper, cast coat paper and synthetic paper, polymer film such as polyethylene, polypropylene, polyethylene terephthalate, polyethylene-2,6-naphtylene dicarboxylate, polyarylene, polyimide, polycarbonate and triacetyl cellulose, glass, metal foil and non-woven fabric. In the case when the image recording medium which can be manufactured using the polymer of the present invention is used for forming a transmitting image, for example, as OHP film or photomechanical film, a transparent support is used. For the photomechanical film, a support having a small coefficient of thermal expansion and good dimensional stability and having no absorption in the photosensitive region of the PS plate is selected. In the formation of an image using the image recording medium which can be manufactured using the polymer of the present invention, heating may be performed by a method of contacting the medium with a heated block or plate, a method of contacting the medium with a heat roller or heat drum, a method of irradiating a halogen lamp or an infrared or far infrared lamp heater, a method of imagewise heating the medium by a thermal head of a thermosensitive printer or a method of irradiating a laser ray. When the image recording medium which can be manufactured using the polymer of the present invention is used for the material required to have high resolution, such as photomechanical material, a scan exposure system using a laser ray is preferred. In order to form an image with a smaller heat energy, the thermosensitive recording material which can be manufactured using the polymer of the present invention may be previously heated at an appropriate temperature. After the imagewise heating according to the above-described method, the entire surface is heated at a temperature of from 60 to 150°C (preferably from 60 to 120°C) so that the picture image can be amplified. More specifically, in the polymer of the present invention, the site of A first decomposes by the heating to generate an acid and further, by the action of the acid generated, change in the absorption of B is caused. This reaction proceeds within a very short time upon irradiation of a laser ray but the picture image can be amplified by the after-treatment. In the case of forming an image by the irradiation of a laser ray, a dye of absorbing light at the wavelength of the laser ray is necessary to be present so as to convert the laser ray to the heat energy. Examples of the laser light source include an excimer laser, an argon laser, a helium-neon laser, a semiconductor laser, a glass (YAG) laser, a carbon dioxide gas laser and a dye laser. Of these, a helium-neon laser, a semiconductor laser and a glass laser are useful in the present invention. In particular, a semiconductor laser is more useful because the device is compact and inexpensive. The semiconductor usually has an oscillation wavelength of from 670 to 830 nm and a dye having absorption in this near infrared region is used. Examples of the dye having near infrared absorption include a cyanine dye, a squarylium dye, a merocyanine dye, an oxonol dye and a phthalocyanine dye. Specific examples thereof include the materials described in U.S. Patents 4,973,572, 4,948,777, 4,950,640, 4,950,639, 4,948,776, 4,948,778, 4,942,141, 4,952,552, 5,036,040 and 4,912,083. In the case when a compound capable of generating an acid by light is used, a laser is selected according to the absorption characteristics of the photoacid generating agent as means for forming a latent image. For the purpose of spectral sensitization, various dyes may be added but in this case, the laser used must be selected taking account of the absorption wavelength of the dye. The present invention is described in greater detail below by referring to the Examples, however, it should be understood that the present invention is not limited thereto. SYNTHESIS EXAMPLE 1 Synthesis of P-(1) : 1) Synthesis of A-(1) 18 g of t-butyl 2-methyl-2-(2-hydroxymethyl)acetoacetate synthesized according to the method described in JP-A-8-248561, 19.8 g of triethylamine and 2 g of 4-dimethylaminopyridine were dissolved in 90 ml of methylene chloride. To the resulting solution, 18 g of p-vinylbenzenesulfonyl chloride (synthesized by allowing thionyl chloride to act on sodium p-vinylbenzenesulfonate) was added, and the solution was stirred at room temperature for 4 hours. To the reaction solution 100 ml of water was added, and then the organic layer was extracted and washed twice with 100 ml of water. The organic layer was dried over magnesium sulfate and after added thereto 2 mg of hydroquinone monomethyl ether, concentrated under reduced pressure. The oily product obtained was purified by a silica gel column chromatography (eluent: n-hexane/ethyl acetate =3/1) to obtain 17.7 g (yield 54.1%) of a colorless transparent oil. The structure obtained was identified by the mass spectrometry, elemental analysis and 1 H-NMR. Mass spectrometry: M + =367 Elemental analysis: Calculated C: 58.68%, H: 6.57%, S: 8.70% Found C: 58.69%, H: 6.59%, S: 8.71% 1 H-NMR (CDCl 3 ) δ (ppm): 1.40 (s, 3H), 1.42 (s, 9H), 2.15 (s, 3H), 4.30 (Abq, 2H), 5.50 (d, 1H), 5.93 (d, 1H), 6.78 (dd, 1H), 7.58 (d, 2H), 7.85 (d, 2H) 2) Synthesis of B-(1) 50 g of 4-(2-methacryloyloxyethyl)-2-(2H-benzotriazol-2-yl)phenol (produced by Otsuka Kagaku KK) was dissolved in 300 ml of THF and thereto, 17.4 g of potassium t-butoxy was added under ice cooling. To the resulting solution, 50.4 g of di-t-butyl dicarbonate was added, and the mixture was stirred for 1.5 hours under ice cooling. To the solution obtained, 300 ml of water was added and further 500 ml of ethyl acetate was added to extract the organic layer. The organic layer was washed twice with water, dried over magnesium sulfate, and after adding thereto 10 mg of hydroquinone monomethyl ether, concentrated under reduced pressure. The concentrate obtained was recrystallized from acetonitrile to obtain 54 g (yield: 82.3%) of white crystals. The structure was identified by the mass spectrometry, elemental analysis and 1 H-NMR. Mass spectrometry: M + =422 Elemental analysis for C 23 H 25 N 3 O 5 : Calculated C: 65.23%, H: 5.95%, N: 9.92% Found C: 65.31%, H: 5.97%, N: 9.90% Melting point: 125-127°C 1 H-NMR (CDCl 3 ) δ (ppm): 1.51 (s, 9H), 1.95 (s, 3H), 3.11 (t, 2H), 4.45 (t, 2H), 5.55 (s, 1H), 6.10 (s, 1H), 7.27-7.45 (m, 4H), 7.90 (d, 1H), 7.92 (d, 1H), 8.10 (s, 1H) 3) Synthesis of P-(1) 20 g of A-(1) and 11.5 g of B-(1) were dissolved in 50 ml of benzene and thereto 202.4 mg of 2,2'-azobis(2,4-dimethylvaleronitrile), followed by stirring under heating at 60°C for 10 hours. The reaction solution was added to 500 ml of MeOH and the white polymer precipitated was isolated. As a result, 14.5 g (yield: 46.03%) of a polymer (P-(1)) having a molecular weight of 5,200 and a molecular weight distribution (Mw/Mn) of 3.7 was obtained. This polymer was identified to be a copolymer of A-(1) and B-(1) by 1 H-NMR and x:y was verified to be 67:33 by the elemental analysis. The thermal decomposition temperature was from 145 to 148°C. SYNTHESIS EXAMPLE 2 Synthesis of P-(14) : 2 g of A-(1) and 1.52 g of B-(41) were dissolved in 5 ml of benzene and thereto 20.2 mg of 2,2'-azobis(2,4-dimethylvaleronitrile) was added, followed by stirring at 60°C for 10 hours. The reaction solution was added to 30 ml of methanol and then white amorphous was obtained. The supernatant was removed by decantation and the polymer was dissolved in a slight amount of methylene chloride and added to methanol to cause reprecipitation. The supernatant was removed and the residue was dried to obtain 220 mg (yield: 6.25%) of a polymer (P-(14)) having a molecular weight of 3,800 and a molecular weight distribution (Mw/Mn) of 3.2 was obtained. This polymer was identified to be a copolymer of A-(1) and B-(41) by 1 H-NMR and x:y was verified to be 67:33 by the elemental analysis. The melting point was from 120 to 123°C, the thermal decomposition temperature was from 145 to 148°C and formation of a dark green color was confirmed at the same time with the decomposition. EXAMPLE 4 The following compounds were dissolved in chloroform and the solutions obtained each was coated on a 100 µm-thick polyethylene terephthalate film and dried to prepare a transparent thermosensitive recording sheet. Sample 1': P-(1) benzotriazole moiety: 2 mmol/m 2 IR dye 113 mg/m 2 Reference Sample 1': p-Toluenesulfonic acid 4 mmol/m 2 Comparative compound benzotriazole: 2 mmol/m 2 IR dye 113 mg/m 2 Reference Sample 2': Comparative compound benzotriazole moiety: 2 mmol/m 2 IR dye 113 mg/m 2 Comparative Compound : Laser Exposure Condition for Image Recording: Wavelengths of 8 lines of Spectra Diode Labs No. SDL-2430 (wavelength range: 800 to 830 nm) were combined and the output was set at 400 mW to prepare a laser for image writing. Each of the samples prepared above was exposed using this laser by setting the beam system at 160 µm, the laser scanning speed at 0.5 m/s (scan center part), the sample feeding speed at 15 mm/s and the scanning pitch at 8 lines/mm so as to obtain an image of 22 mm × 9 mm. At this time, the laser energy density on the sample was 5 mJ/mm 2 . After Sample 1' and Reference Samples 1' and 2' were scan exposed under the above-described laser exposure conditions, the color density at 360 nm in the exposed area was examined. Thereafter, each sample after the laser exposure was heat developed under the heating conditions of 120°C and 60 and then, the color density in the exposed area and unexposed area was determined. The results obtained are shown in Table 4 (the color density shown is a value corrected by subtracting the absorption density (about 0.2) of the raw film). Color Density Laser Exposed Area (5 mJ/mm 2 ) Laser Unexposed Area No Heating Heating (120°C, 60 s) Heating (120°C, 60 s) Sample 1' 0.8 2.3 0 Reference Sample 1' 1.2 2.3 2.3 Reference Sample 2' 0 0 0 It is seen from Table 4 that Reference Sample 2' was not colored at all by the laser exposure or the heat development after the exposure, whereas Sample 1' of the present invention exhibited coloring property equal to or superior to the coloring property of Reference Sample 1', only the by laser exposure. Furthermore, it is seen that in the Sample 1' of the present invention, the color density in the exposure area increased by the heat development after the laser exposure but since the unexposed area was not colored at all, a high contrast image could be obtained. EXAMPLE 5 After Sample 1' and Reference Sample 1' were stored at 60°C and 70% RH for 3 days, the color density at 360 nm was examined. Thereafter, the samples each was heat developed under the heating conditions of 120°C and 60 s and then the color density was examined. Furthermore, the color density when the samples after storage each was laser exposed under the above-described exposure conditions and the color density when each sample was heat developed (120°C, 60 s) after the laser exposure were examined. The results obtained are shown in Table 5. 60°C and 70% RH for 3 days No Laser Exposure Laser Exposure (5 mJ/mm 2 ) No Heating Heating (120°C, 60 s) No Heating Heating (120°C, 60 s) Sample 1' 0 0 1.8 2.3 Reference Sample 1' 2.1 2.3 2.3 2.3 It is seen from Table 5 that Reference Sample 1' was colored under the above-described storage conditions, whereas Sample 1' of the present invention was completely free of coloring after the storage and even after the heat development only was performed, was not colored at all, thus, revealed to have high storage stability. Furthermore, it is seen that Sample 1' of the present invention exhibited excellent coloring property at the laser exposure after the storage and at the heat development after the exposure, the same as the results shown in Table 4. EXAMPLE 6 The following compounds were dissolved in chloroform and the solutions obtained each was coated on a 100 µm-thick polyethylene terephthalate film and dried to prepare a transparent thermosensitive recording sheet. Sample 1': P-(1) benzotriazole moiety: 2 mmol/m 2 IR Dye (2) 40 mg/m 2 Sample 2': P-(2) benzotriazole moiety: 2 mmol/m 2 IR Dye (2) 40 mg/m 2 Sample 3': P-(1) benzotriazole moiety: 4 mmol/m 2 IR Dye (2) 50 mg/m 2 Laser Exposure Condition for Image Recording : The energy density was varied as shown in Table 4 by changing the laser scan speed or the laser output in the laser exposure conditions described in Example 4. Evaluation of Image Formation Efficiency : Samples 1' to 3' of the present invention was heat developed under the heating conditions of 120°C and 60 s after the laser exposure. Then, the maximum color density at the laser scan center part (image area) was determined and by comparing the value obtained with the theoretical value in the case of 100% coloring, the image formation efficiency (coloring efficiency) was calculated. The results obtained are shown in Table 6. Color Density Laser Exposure (3.5 mJ/m 2 ) Laser Exposure + Heat Development (3.5 mJ/mm 2 + 120°C/60 s) Sample 1' 1.4 2.3 Sample 2' 1.3 2.3 Sample 3' 2.5 4.6 It is apparent from Table 6 that the compounds of the present invention were sufficiently colored even without adding a thermal acid generating agent and the polymer itself had a function as a thermal acid generating agent. EXAMPLE 7 The compounds shown below were dissolved in chloroform and the solutions obtained each was coated on a 100 µm-thick polyethylene terephthalate film and dried to obtain a transparent thermosensitive recording sheet. Sample 2': P-(2) benzotriazole moiety: 3 mmol/m 2 IR dye 113 mg/m 2 Sample 3': P-(3) benzotriazole moiety: 2 mmol/m 2 IR dye 113 mg/m 2 Sample 4': P-(7) benzotriazole moiety: 2 mmol/m 2 IR dye 113 mg/m 2 Sample 5': P-(9) benzotriazole moiety: 2 mmol/m 2 IR dye 113 mg/m 2 Sample 6': P-(13) leuco dye moiety: 2 mmol/m 2 IR dye 113 mg/m 2 Sample 7': P-(14) leuco dye moiety: 2 mmol/m 2 IR dye 113 mg/m 2 Sample 8': P-(16) 2 mmol/m 2 IR dye 113 mg/m 2 Sample 9': P-(21) 2 mmol/m 2 IR dye 113 mg/m 2 Sample 10': P-(22) benzotriazole moiety: 2 mmol/m 2 IR dye 113 mg/m 2 Sample 11': P-(1) benzotriazole moiety: 2 mmol/m 2 Thermal acid generating agent 0.1 mmol/m 2 IR dye 113 mg/m 2 Sample 12': P-(29) benzotriazole moiety: 2 mmol/m 2 Thermal acid generating agent 0.1 mmol/m 2 IR dye 113 mg/m 2 Sample 13': P-(47) benzotriazole moiety: 2 mmol/m 2 Thermal acid generating agent 0.1 mmol/m 2 IR dye 113 mg/m 2 Reference Sample 3': Nitrocellulose (viscosity: 1,000 sec, produced by Dicel Kogyo KK) 0.85 g/m 2 Reference Dye 1 0.35 g/m 2 Reference Dye 2 0.55 g/m 2 IR Dye 1 113 mg/m 2 Laser Exposure Condition for Image Recording: The energy density was varied as shown in Table 7 by changing the laser scan speed or the laser output in the laser exposure conditions described in Example 4. Comparison of Image Formation Efficiency : Samples 1' to 3' of the present invention was heat developed under the heating conditions of 120°C and 60 s after the laser exposure. Then, the maximum color density at the laser scan center part (image area) was determined and by comparing the value obtained with the theoretical value in the case of 100% coloring, the image formation efficiency (coloring efficiency) was calculated. With respect to Reference Example 3', the image formation efficiency (decolorization efficiency) was calculated from the comparison between the density of the laser scan center part (image area) and the density of the non-image area. The results obtained are shown in Table 7. Image Formation Efficiency (Laser Exposure + Heat Development) Laser Energy Density (mJ/mm 2) 5 3 Sample 1' (Invention) 99% 92% Sample 2' (Invention) 100% 99% Sample 3' (Invention) 93% 82% Sample 4' (Invention) 98% 96% Sample 5' (Invention) 97% 90% Sample 6' (Invention) 89% 58% Sample 7' (Invention) 98% 67% Sample 8' (Invention) 99% 92% Sample 9' (Invention) 79% 63% Sample 10' (Invention) 97% 88% Sample 11' (Invention) 97% 96% Sample 12' (Invention) 60% 30% Sample 13' (Invention) 43% 25% Reference Sample 3' 35% 8% It is seen from Table 7 that the samples of the present invention could form efficiently an image with a low energy of 5 mJ/mm 2 and were superior to Reference Sample 3' using ablation by a laser ray.

Claims: A copolymer represented by the formula (6) wherein: A represents a repeating unit obtainable by polymerising at least one vinyl monomer of formula (7) capable of generating an acid by the action of an acid and further breeding an acid by the action of an acid generated from the vinyl monomer itself; B represents a repeating unit obtainable by polymerising at least one vinyl monomer having a partial structure capable of causing a change in the absorption region of from 360-900 nm by the action of an acid; C represents a repeating unit obtainable by polymerising at least one vinyl monomer copolymerisable with A and B; and x, y and z each represents % by weight and satisfy the conditions of 1≦x≦99, 1≦y≦99, 0≦z≦98 and x+y+z=100; wherein R 21 represents an electron-withdrawing group having a Hammett's σp value greater than 0; R 22 represents an alkyl group or an aryl group; R 23 represents a secondary or tertiary alkyl group having a hydrogen atom at the β-position; W 2 represents a residue of an acid represented by W 2 OH; and any one of the substituents R 21 , R 22 , R 23 and W 2 represents a polymerisable vinyl group. A copolymer according to Claim 1, wherein A has a function of generating an acid by the action of heat.

Inventors: OBAYASHI TATSUHIKO, OHKAWA ATSUHIRO, YAMANOUCHI JUNICHI, OBAYASHI, TATSUHIKO, OHKAWA, ATSUHIRO, YAMANOUCHI, JUNICHI, Obayashi, Tatsuhiko, Fuji Photo Film Co., Ltd., OHKAWA, ATSUHIRO, FUJI PHOTO FILM CO., LTD., YAMANOUCHI, JUNICHI, FUJI PHOTO FILM CO., LTD.
Applicants: FUJI PHOTO FILM CO LTD, FUJI PHOTO FILM CO., LTD.
Application year: 1998
Main classification: C08F28
IPC codes: B41M5, C08F246