Salicylic acid copolymers and their metal salts, production process thereof, color-developing agents comprising metal salts of the copolymers and color-developing sheets employing the agents

Abstract: Disclosed herein are salicylic acid copolymers having a weight average molecular weight of 500 10,000 and obtained (a) by condensing salicylic acid (i) with a benzyl alcohol (ii) and/or benzyl ether (iii) or a benzyl halide (iv) in the presence of an acid catalyst or (b) by reacting a co-condensation resin of (i) and (ii) and/or (iii) with (iv) or (c) by reacting a co-condensation resin of (i) and (ii) and/or (iii) or (iv) with a styrene derivative in the presence of an acid catalyst; multivalent metal salts of the salicylic acid copolymers; color-developing agents comprising the multivalent metal salts; and color-developing sheets suited for pressure-sensitive copying paper and comprising the color-developing agents. Their production processes are also disclosed.

Description: BACKGROUND OF THE INVENTION 1) Field of the Invention: This invention relates to novel salicylic acid resins and metal salts thereof, color-developing agents containing the metal salts as effective components and suited for pressure-sensitive copying paper, as well as color-developing sheets making use of the color-developing agents and suited for use in pressure-sensitive copying paper. 2) Description of the Prior Art: Pressure-sensitive copying paper sheets are also called carbonless copying paper sheets. They produce a color by mechanical or impactive pressure, for example, by writing strokes or typewriter impression, thereby allowing to make a plurality of copies at the same time. Among such pressure-sensitive copying paper sheets, there are those called "transfer type copying paper sheets", those called "self-contained copying paper sheets", etc. Their color-producing mechanisms are each based on a color-producing reaction between an electron-donating colorless dyestuff precursor and an electron-attracting color-developing agent. Taking a pressure-sensitive copying paper sheet of the transfer type by way of example, it will be described with reference to FIGURE 1 which is a schematic crosssectional view showing the structure of the exemplary pressure-sensitive copying paper sheet. The back sides of a CB-sheet 1 and CF/CB-sheet 2 are coated with microcapsules 4 which have diameters of several micrometers to somewhat greater than 10 micrometers and have been obtained by dissolving a colorless pressure-sensitive dyestuff precursor in a non-volatile oil and then encapsulating the resultant solution with high-molecular films such as gelatin films. On the other hand, the front sides of the CF/CB-sheets 2 and a CF-sheet 3 are coated with a coating formulation containing a color-developing agent 5 which has such properties that upon contact with the pressure-sensitive dyestuff precursor, the color-developing agent 5 undergoes a reaction with the dyestuff precursor, thereby causing the dyestuff precursor to produce its color. In order to make copies, they are stacked in the order of the CB-sheet, (CF/CB-sheet), (CF/CB-sheet) and CF-sheet with the sides coated with the dyestuff precursor maintained in contiguous relation with the sides coated with the color-developing agent. The CF/CB-sheets are optional. When a pressure is applied locally by a ballpoint pen 6 or a typewriter, the capsules 4 are ruptured there. As a result, the solution containing the pressure-sensitive dyestuff precursor is transferred to the color-developing agent 5 so that one or more copied records are obtained. As electron-attracting color-developing agents, there have been proposed (1) inorganic solid acids such as acid clay and attapulgite, as disclosed in U.S. Patent No. 2,712,507; (2) substituted phenols and diphenols, as disclosed in Japanese Patent Publication No. 9309/1965; (3) p-substituted phenol-formaldehyde polymers, as disclosed in Japanese Patent Publication No. 20144/1967; (4) metal salts of aromatic carboxylic acids, as disclosed in Japanese Patent Publication Nos. 10856/1974 and 1327/1977; etc. Some of them have already been employed actually. As performance conditions which a color-developing sheet is supposed to satisfy, may be mentioned little yellowing during storage and upon exposure to radiant rays such as sunlight and good fastness of produced color marks so that they do not disappear or fade easily by radiant rays, water or a plasticizer, to say nothing of excellent color-developing ability not only right after its fabrication but also after its storage over a long period of time. Color-developing agents, which have been proposed to date, and sheets coated with such conventional color-developing agents have both advantages and disadvantages in performance. For example, inorganic solid acids are inexpensive but adsorb gas and moisture in the air during storage. They hence result in yellowing of paper surfaces and reduced color-producing performance. Substituted phenols have insufficient color-producing ability and produced color marks have low color densities. Para-phenylphenol-novolak resins which are usually employed as p-substituted phenol-formaldehyde polymers have excellent color-producing ability, but their coated paper sheets undergo yellowing and produced color marks are faded significantly upon exposure to sunlight or during storage (especially, by nitrogen oxides in the air). In addition, metal salts of aromatic carboxylic acids are good in yellowing resistance but their color-producing ability at low temperatures, resistance to water or plasticizers and light fastness cannot still be considered sufficient. SUMMARY OF THE INVENTION A first object of this invention is to provide anovel copolymer which has excellent properties in flexibility, oxidation resistance, formability and moldability, etc. and can form multivalent metal salts excellent as color-developing agents for pressure-sensitive copying paper. A second object of this invention is to provide an excellent color-developing agent for pressure-sensitive copying paper, which is a metal-modified product composed of a multivalent metal salt of the above copolymer or a molten mixture containing a multivalent metal salt of the above copolymer. A third object of this invention is to provide an excellent pressure-sensitive copying paper which makes use of the above color-developing agent. The present invention therefore provides a copolymer comprising 5 - 20 mole % of structural units (I), 10 - 85 mole % of structural units (II), and 4 - 85 mole % of structural units (III) and having a weight average molecular weight of 500 - 10,000, each of said structural units (I) being coupled via the α-carbon of one of said structural units (II) with the benzene ring of one of said structural units (II), one or more of said structural units (II) being coupled via the α-carbon or α-carbons thereof to the benzene ring or rings of other structural units (II), one or more of said structural units (III) being coupled via the α-carbon or α-carbons thereof with the benzene ring or rings of one or more of the structural units (II) or ring or rings of one or more of structural units (III), and said structural units (I), (II), and (III) being represented respectively by the following formulae (I), (II), and (III): wherein R₁ and R₂ mean independently a hydrogen atom or a C₁₋₁₂ alkyl, aralkyl, aryl, or cycloalkyl group, R₃ and R₆ denote independently a hydrogen atom or a C₁₋₄ alkyl group, R₄ means a hydrogen atom or a methyl group, R₅ means a hydrogen and R₇ stands for -CH₂-R₈ (R₈: hydrogen atom or methyl group); obtainable by reacting (vi) a styrene derivative, which is represented by the following general formula (VII): wherein R₄, R₆, and R₈ have the same meaning as defined above, with a co-condensation resin, which has in turn been obtained by condensing in the presence of at least one acid catalyst (i) salicylic acid with (ii) a benzyl alcohol, (iii) a benzyl ether and/or (iii') a dibenzyl ether, both represented by the following general formula (IV): wherein R₁, R₂, and R₃ have the same meaning as defined above, and R₉ stands for a hydrogen atom, a C₁₋₄ alkyl group, or or (iv) a benzyl halide represented by the following general formula (V): wherein R₁, R₂, and R₃ have the same meaning as defined above, and X denotes a halogen atom. Further embodyments of the invention result from the subclaims. The copolymer and their multivalent metal salt thereof, both of which pertain to the present invention, are a novel high molecular compound and salt which have not been produced to date. Accordingly, the color-developing agent of this invention is also a novel color-developing agent. The color-developing sheet making use of the novel color-developing agent of this invention has either comparable or superior color-producing properties compared with color-developing sheets making use of an inorganic solid acid or p-phenylphenol novolak resin. Compared with color-developing sheets making use of a metal salt of an aromatic carboxylic acid, the color-developing sheet making use of the novel color-developing agent of this invention has better color-producing properties at low temperatures. Further, color marks produced by the color-developing sheet making use of the novel color-developing agent of this invention have high resistance that they are not easily faded by water, plasticizers or light. The present invention has a still further merit that a color-developing sheet, which has improved yellowing tendency upon exposure to sunlight, especially, significantly improved yellowing resistance to oxygen oxides in the air and is extremely advantageous in handling and storage, can be provided at a low cost. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 illustrates the structure of a pressure-sensitive copying paper sheet. DETAILED DESCRIPTION OF THE INVENTION AND PREFERED EMBODIMENTS The salicylic acid resins (copolymers) of this invention are novel resins which have not been produced to date. Namely, the resins of this invention are salicylic acid resins obtained by condensing in the presence of an acid catalyst a benzyl halide or styrene derivative with a co-condensation product which has in turn been obtained by condensing salicylic acid and a benzyl alcohol or benzyl ether or a mixture thereof, or a benzyl halide in the presence of an acid catalyst, containing 5 - 20 mole % of structural units represented by the structural formula (I), 10 - 85 mole % of structural units represented by the structural formula (II) and 4 - 85 mole % of structural units represented by the structural formula (III) and having a weight average molecular weight of 500 - 10,000, each structural unit (I) being coupled via the α-carbon of one of said structural units (II) with the benzene ring of said one of said structural units (II), one or more of said structural units (II) being coupled via the α-carbon or α-carbons thereof to the benzene ring or rings of another or other structural units (II), one or more of said structural units (III) being coupled via the α-carbon or α-carbons thereof with the benzene ring or rings of one or more of the structural units (II) and ring or rings of structural units (III). A detailed description will hereinafter be made of production processes of salicylic acid resins which can be obtained by two reaction stages. First-stage reaction The first-stage reaction comprises condensing, in the presence of an acid catalyst, salicylic acid and a benzyl alcohol and/or benzyl ether represented by the following general formula (IV): wherein R₁ and R₂ may be the same or different and mean independently a hydrogen atom or a C₁₋₁₂ alkyl, aralkyl, aryl or cycloalkyl group, R₃ denotes a hydrogen atom or a C₁₋₄ alkyl group, and R₉ stands for a hydrogen atom, a C₁₋₄ alkyl group or or a benzyl halide represented by the following general formula (V): wherein R₁, R₂ and R₃ have the same meaning as in the formula (IV) and X denotes a halogen atom. (A)-1. Condensation between salicylic acid and the benzyl alcohol: A description will be made firstly of condensation of a benzyl alcohol of the formula (IV) in which R₉ is a hydrogen atom. It has conventionally been practised routinely to obtain alkyl-substituted salicylic acids by reacting various alcohols with salicylic acid and alkylating the latter. Several processes have been known. Included in such conventional processes are, for example, to react isobutanol with salicylic acid to obtain tert-butylsalicylic acid ["Jikken Kagaku Koza (Handbook of Experimental Chemistry)", 18 , 30 (1958), The Maruzen Publishing Co., Ltd.] and to react 2 moles of phenyl ethanol with 1 mole of salicylic acid to obtain 5-[α-methyl-4'-(α-methylbenzyl)-benzyll-salicylic acid (Japanese Patent Laid-Open Nos. 100493/1986 and 96449/1987). These conventional processes are all related to the production of substituted salicylic acid compounds. It has not been known to produce a co-condensation resin of salicylic acid by using an alcohol as in the present invention. In the process of this invention, a benzyl alcohol is caused to exist in an excess amount so as to obtain a novel co-condensation resin between the benzyl alcohol and salicylic acid. Namely, the present inventors have found that when a benzyl alcohol is caused to exist in an amount of 2 moles or more per mole of salicylic acid and reacted with salicylic acid in a molten state at a reaction temperature of 80°C or higher by an acid catalyst, one or more benzyl groups can be introduced to one or both positions ortho to and/or the position para to the hydroxyl group of salicylic acid and moreover, excess benzyl alcohol undergoes successive condensation with the benzyl groups introduced in salicylic acid while undergoing concurrent self-condensation, thereby to form a resinous substance. When a salicylic acid composition obtained as such a resin is modified by a metal and is then used as a color-developing agent, it exhibits superb miscibility with an oil in microcapsules on a CB-sheet and the velocity of color production at low temperatures is improved significantly compared with conventional products. Owing to the lack of any water solubility, the waterproofness of produced color marks is extremely good. In the process for the production of such a useful salicylic acid resin and its metal-modified product, various starting benzyl alcohols are represented by the general formula (V) in which R₉ stands for H. Although a hydrogen atom or a lower alkyl group having 1 - 4 carbon atoms is bonded to the α-position of the benzyl group, a hydrogen atom or methyl group is preferred. The nucleus of the benzyl group is either unsubstituted or substituted at o-, m- and/or p-positions with at least one C₁₋₁₂ alkyl, aralkyl, aryl or cycloalkyl group. As various benzyl alcohols useful in the practice of this invention to afford co-condensation resins, may hence be mentioned benzyl alcohol, o-methylbenzyl alcohol, m-methylbenzyl alcohol, p-methylbenzyl alcohol, o-ethylbenzyl alcohol, m-ethylbenzyl alcohol, p-ethylbenzyl alcohol, o-isopropylbenzyl alcohol, p-n-propylbenzyl alcohol, p-tert-butylbenzyl alcohol, p-nonylbenzyl alcohol, o-phenylbenzyl alcohol, p-cyclohexylbenzyl alcohol, p-(benzyl)benzyl alcohol, p-(α-methylbenzyl)benzyl alcohol, p-(α,α-dimethyl)benzyl alcohol, 2,3-dimethylbenzyl alcohol, 2,4-dimethylbenzyl alcohol, 2,5-dimethylbenzyl alcohol, 2,6-dimethylbenzyl alcohol, 3,4-dimethylbenzyl alcohol, 3,5-dimethylbenzyl alcohol, 2,4-diethylbenzyl alcohol, 3-methyl-4-ethylbenzyl alcohol, 3-(α-methylbenzyl)-4-methylbenzyl alcohol, 2,5-(dimethylbenzyl)benzyl alcohol, α-methylbenzyl alcohol, α-ethylbenzyl alcohol, α-n-propylbenzyl alcohol, α-isopropylbenzyl alcohol, α-n-butylbenzyl alcohol, o-methyl-α-methylbenzyl alcohol, p-methyl-α-methylbenzyl alcohol, m-methyl-α-methylbenzyl alcohol, p-methyl-α-ethylbenzyl alcohol, p-methyl-α-isopropylbenzyl alcohol, p-methyl-α-n-propylbenzyl alcohol, p-methyl-α-n-butylbenzyl alcohol, p-ethyl-α-methylbenzyl alcohol, m-ethyl-α-ethylbenzyl alcohol, o-ethyl-α-methylbenzyl alcohol, p-isopropyl-α-methylbenzyl alcohol, p-phenyl-α-methylbenzyl alcohol, p(α-methylbenzyl)-α-methylbenzyl alcohol, p-(α,α-dimethylbenzyl)-α-methylbenzyl alcohol, 2,3-dimethyl-α-methylbenzyl alcohol, 2,4-dimethyl-α-methylbenzyl alcohol, 2,5-dimethyl-α-methylbenzyl alcohol, 3,4-dimethyl-α-methylbenzyl alcohol, 2-methyl-5-tert-butyl-α-methylbenzylalcohol, etc. It should however be borne in mind that the process of the present invention is not necessarily limited to the use of the above specific benzyl alcohols. Preferable benzyl alcohols include benzyl alcohol, p-methylbenzyl alcohol, α-methylbenzyl alcohol, p-methyl-α-methylbenzyl alcohol, and the like. In the production of the co-condensation resin in accordance with the present invention, the benzyl alcohol may be used in an amount of 2 - 30 moles, preferably, 2 - 10 moles per mole of salicylic acid. So long as the benzyl alcohol is used within the above amount range, the weight average molecular weight of the resulting resin falls within a range of 500 - 10,000, preferably, 500 - 5,000 and the content of salicylic acid in the resultant resin composition ranges from 3 wt.% to 70 wt.%, preferably, from 5 wt.% to 50 wt.%. The reaction temperature is required to be at least 80°C. The reaction velocity becomes extremely slow at any temperatures lower than 80°C. In order to shorten the reaction time as much as possible, a temperature range of 100 - 240°C is desired. The reaction time may range from 1 to 20 hours. As the acid catalyst, inorganic and organic acids, especially, mineral acids, for example, hydrochloric acid, phosphoric acid and sulfuric acid, formic acid, Friedel-Crafts catalysts such as zinc chloride, stannic chloride and ferric chloride, and organic sulfonic acids such as methanesulfonic acid and p-toluenesulfonic acid may be used either singly or in combination. The catalyst may be used in an amount of about 0.01 - 20 wt.%, preferably, 0.5 - 5 wt.% based on the total weight of salicylic acid and the benzyl alcohol. As a usual procedure for producing the resin useful in the practice of this invention, salicylic acid, the benzyl alcohol and the catalyst are added in their respective predetermined amounts at the same time and are then heated to conduct a reaction at a predetermined temperature or the reaction is allowed to proceed while adding dropwise a portion of the benzyl alcohol in the course of the reaction. Water, which is formed as the reaction proceeds, is taken out of the reaction system by means of nitrogen gas. After completion of the reaction, the contents are taken out and then cooled to obtain the intended product. Where the benzyl alcohol is used at a smaller molar ratio with a view toward obtaining a resin having a composition of a relatively low molecular weight, unreacted salicylic acid remains. As a method for the elimination of the remaining salicylic acid, it is practised, for example, to wash the resin with warm water either as is or after dissolving the resin in an organic solvent such as methyl isobutyl ketone or cyclohexanone. (A)-2. Condensation between salicylic acid and benzyl ethers: The benzyl ethers are other than those of the formula (IV) in which R₉ is a hydrogen atom. A benzyl ether can introduce a benzyl group into certain aromatic compounds. In the case of benzyl propyl ether by way of example, it has been known to introduce its benzyl group into benzene, naphthalene, phenol and the like ["Jikken Kagaku Koza (Handbook of Experimental Chemistry)", 18 , 84 (1958), The Maruzen Publishing Co., Ltd.]. These reactions make use of an extremely strong acid catalyst such as boron fluoride and moreover, compounds relatively prone to nucleophilic substitution have been chosen. It has however not been known to react salicylic acid with a benzyl ether as in the present invention. This reaction have not yet been contemplated of, since difficulties have been expected to lie in reacting a benzyl ether with a compound, which is rather resistant to nucleophilic substitution due to inclusion of a carboxylic acid like salicylic acid, to introduce a benzyl or substituted benzyl group therein. Upon production of the resin of this invention, an alcohol is formed through a dealcoholytic reaction. It is easily foreseeable that salicylic acid, one of the reactants, would yield a mixture of a salicylic ester and its resin through a reaction with the resulting alcohol in the presence of an acid catalyst and difficulties would be encountered in obtaining the intended product. The present inventors have surprisingly found that when various benzyl ethers are each reacted with salicylic acid at a reaction temperature of 80°C or higher in the presence of an acid catalyst by overcoming such difficult circumstances, the various benzyl groups can be introduced to positions ortho and/or para to the hydroxyl group of the salicylic acid without inducing any substantial side reactions such as corresponding esterifications. It has also been found that when a benzyl ether is used in an excess amount, the excess portion of the benzyl ether undergoes condensation with the benzyl groups introduced in salicylic acid while undergoing its self-condensation reaction concurrently, thereby to form a resinous substance. The condensation with the above-described benzyl ethers will next be divided into four cases for more detailed description. (a) Benzyl alkyl ethers: The benzyl alkyl ethers are represented by the formula (IV), in which R₃ means a hydrogen atom and R₉ denotes a C₁₋₄ alkyl group [p-methylbenzyl alkyl ethers will however be described in a subsequent item (b)]. So long as R₉ contains 4 or fewer carbon atoms and the reaction is performed at a temperature to be described subsequently, the reaction proceeds fast and no esterification occurs, thereby facilitating the provision of making a good resin. Where R₉ contains 4 carbon atoms, namely, R₉ is a butyl group, tert-butyl group tends to result in a slow reaction velocity. As illustrative examples of the benzyl alkyl ether useful in the practice of this invention to afford a co-condensation resin, may therefore be mentioned benzyl methyl ether, benzyl ethyl ether, benzyl isopropyl ether, benzyl n-butyl ether, o-methylbenzyl methyl ether, o-methylbenzyl ethyl ether, o-methylbenzyl isobutyl ether, m-methylbenzyl methyl ether, m-methylbenzyl ethyl ether, m-methylbenzyl isopropyl ether, o-ethylbenzyl methyl ether, o-ethylbenzyl ethyl ether, o-ethylbenzyl isopropyl ether, p-ethylbenzyl methyl ether, p-ethylbenzyl ethyl ether, p-ethylbenzyl isopropyl ether, p-ethylbenzyl n-butyl ether, o-isopropylbenzyl ethyl ether, p-isopropylbenzyl ethyl ether, p-n-propylbenzyl ethyl ether, p-tert-butylbenzyl ethyl ether, p-tert-butylbenzyl isopropyl ether, p-tert-octylbenzyl ethyl ether, p-nonylbenzyl methyl ether, p-dodecylbenzyl methyl ether, p-phenylbenzyl methyl ether, o-phenylbenzyl isopropyl ether, p-cyclohexylbenzyl ethyl ether, p-(α-methylbenzyl)-benzyl methyl ether, p-(α-methylbenzyl)benzyl ethyl ether, p-(α,α-dimethylbenzyl)benzyl methyl ether, 2,3-dimethylbenzyl methyl ether, 2,4-dimethylbenzyl methyl ether, 2,5-dimethylbenzyl methyl ether, 2,6-dimethylbenzyl methyl ether, 3,4-dimethylbenzyl methyl ether, 3,5-dimethylbenzyl methyl ether, 2,4-diethylbenzyl methyl ether, 3,4-dimethylbenzyl ethyl ether, 3,5-dimethylbenzyl ethyl ether, 2,4-diethylbenzyl methyl ether, 2,3-dimethylbenzyl isopropyl ether, 2,4-dimethylbenzyl n-propyl ether, 3,4-dimethylbenzyl n-butyl ether, 2,4-dimethylbenzyl sec-butyl ether, 3,5-dimethylbenzyl n-amyl ether, 2,4-diethylbenzyl methyl ether, 3,5-diethylbenzyl isopropyl ether, 2,3-diethylbenzyl n-butyl ether, 2,4-diisopropylbenzyl methyl ether, 3,5-diisopropylbenzyl ethyl ether, 3-methyl-4-ethylbenzyl methyl ether, 3-methyl-5-tertbutylbenzyl methyl ether, 2-methyl-4-(α-methylbenzyl)-benzyl methyl ether, and so on. It should however be borne in mine that the present invention is not necessarily limited to the use of such exemplary benzyl alkyl ethers. The amount of the benzyl alkyl ether to be used in the production of the co-condensation resin and the weight average molecular weight and salicylic acid content of the resulting co-condensation resin are similar to those mentioned above with respect to the benzyl alcohol in (A) - 1. Similar reaction conditions, post-treatment and the like are also employed. (b) p-Methylbenzyl alkyl ethers: As exemplary p-methylbenzyl alkyl ethers, may preferably be mentioned p-methylbenzyl methyl ether, p-methylbenzyl ethyl ether, p-methylbenzyl n-propyl ether, p-methylbenzyl isopropyl ether, p-methylbenzyl n-butyl ether, p-methylbenzyl sec-butyl ether, p-methylbenzyl isobutyl ether, etc. Reaction conditions for the production of a co-condensation resin from a p-methylbenzyl alkyl ether, for example, the amount of the p-methylbenzyl alkyl ether to be used, and the weight average molecular weight and salicylic acid content of the resulting co-condensation resin are similar to those mentioned above with respect to the benzyl alkyl ethers (a). (c) α-Alkylbenzyl alkyl ethers: These α-alkylbenzyl alkyl ethers are represented by the general formula (IV) in which R₃ and R₉ are independently a C₁₋₄ alkyl group. So long as the carbon number of the O-bonded alkyl group is 4 or smaller in various α-alkylbenzyl alkyl ether and the reaction is performed at a temperature to be described subsequently, the reaction proceeds at a high velocity and no esterification takes place, so that a good resin is obtained. Where the O-bonded alkyl group contains 4 carbon atoms, namely, the O-bonded alkyl group is a butyl group, tert-butyl group tends to result in a slow reaction velocity. On the other hand, the alkyl group bonded to the α-position of the benzyl group is a C₁₋₄ alkyl group, preferably, a methyl or ethyl group from the industrial standpoint. Illustrative examples of the α-alkylbenzyl alkyl ether useful in the practice of this invention to afford a co-condensation resin may therefore include α-methylbenzyl methyl ether, α-methylbenzyl ethyl ether, α-methylbenzyl isopropyl ether, α-methylbenzyl n-propyl ether, α-methylbenzyl n-butyl ether, α-ethylbenzyl methyl ether, α-ethylbenzyl ethyl ether, α-ethylbenzyl isopropyl ether, α-ethylbenzyl n-butyl ether, α-n-propylbenzyl methyl ether, α-isopropylbenzyl ethyl ether, α-n-butylbenzyl isopropyl ether, o-methyl-α-methylbenzyl methyl ether, p-methyl-α-methylbenzyl methyl ether, m-methyl-α-methylbenzyl methyl ether, p-methyl-α-methylbenzyl ethyl ether, p-methyl-α-methylbenzyl isopropyl ether, p-methyl-α-methylbenzyl n-propyl ether, p-methyl-α-methylbenzyl n-butyl ether, p-methyl-α-ethylbenzyl methyl ether, p-methyl-α-ethylbenzyl ethyl ether, p-methyl-α-ethylbenzyl isopropyl ether, p-methyl-α-ethylbenzyl n-butyl ether, o-methyl-α-ethylbenzyl methyl ether, o-methyl-α-ethylbenzyl isopropyl ether, m-methyl-α-ethylbenzyl methyl ether, p-ethyl-α-methylbenzyl methyl ether, o-ethyl-α-ethylbenzyl ethyl ether, m-ethyl-α-n-propylbenzyl methyl ether, p-isopropyl-α-methylbenzyl methyl ether, o-isopropyl-α-methylbenzyl methyl ether, p-secbutyl-α-methylbenzyl methyl ether, p-tert-butyl-α-methylbenzyl methyl ether, p-nonyl-α-methylbenzyl methyl ether, p-dodecyl-α-methylbenzyl methyl ether, p-phenyl-α-methylbenzyl methyl ether, o-phenylbenzyl-α-ethylbenzyl methyl ether, p-cyclohexyl-α-methylbenzyl methyl ether, p-(α-methylbenzyl)-α-methylbenzyl methyl ether, p-(α,α-dimethylbenzyl)-α-methylbenzyl methyl ether, 2,3-dimethyl-α-methylbenzyl methyl ether, 2,4-dimethyl-α-methylbenzyl methyl ether, 2,5-dimethyl-α-methylbenzyl methyl ether, 2,6-dimethyl-α-methylbenzyl methyl ether, 3,4-dimethyl-α-methylbenzyl methyl ether, 3,5-dimethyl-α-methylbenzyl methyl ether, 2,3-dimethyl-α-methylbenzyl ethyl ether, 2,4-dimethyl-α-methylbenzyl isopropyl ether, 2,5-dimethyl-α-methylbenzyl n-propyl ether, 2,6-dimethyl-α-methylbenzyl n-butyl ether, 3,4-dimethyl-α-methylbenzyl sec-butyl ether, 2,3-dimethyl-α-ethylbenzyl methyl ether, 2,4-dimethyl-α-ethylbenzyl methyl ether, 2,5-dimethyl-α-ethylbenzyl ethyl ether, 2,6-dimethyl-α-ethylbenzyl n-propyl ether, 3,4-dimethyl-α-ethylbenzyl isopropyl ether, 3,5-dimethyl-α-ethylbenzyl n-butyl ether, 2,4-dimethyl-α-propylbenzyl methyl ether, 2,4-diethyl-α-methylbenzyl methyl ether, 2,6-diethyl-α-methylbenzyl methyl ether, 3,4-diethylbenzyl-α-ethylbenzyl methyl ether, 3,5-diethylbenzyl-α-propylbenzyl isopropyl ether, 2,4-diisopropyl-α-methylbenzyl methyl ether, 2,4-diisopropyl-α-ethylbenzyl methyl ether, 2,6-diisopropyl-α-propylbenzyl methyl ether, 3,5-diisopropyl-α-butylbenzyl methyl ether, 2-methyl-4-ethyl-α-methylbenzyl methyl ether, 2-methyl-4-tert-butyl-α-methylbenzyl methyl ether, 2-methyl-4-(α-methylbenzyl)-α-methylbenzyl methyl ether, 2-methyl-4-(α,α-dimethylbenzyl)-α-methylbenzyl methyl ether, etc. It should however be borne in mind that the present invention is not necessarily limited to the use of the above specific α-alkylbenzyl alkyl ethers. Reaction conditions for the production of a co-condensation resin from an α-alkylbenzyl alkyl ether, for example, the amount of the α-alkylbenzyl alkyl ether to be used, and the weight average molecular weight and salicylic acid content of the resulting co-condensation resin are similar to those mentioned above with respect to the benzyl alkyl ethers (a). (d) Dibenzyl ethers: These dibenzyl ethers are represented by the formula (IV) in which R₉ stands for The substitutent bonded to the α-position of the benzyl group is a hydrogen atom or C₁₋₄ lower alkyl group. Of these, a hydrogen atom or a methyl group is preferred. As various dibenzyl ethers capable of affording co-condensation resins according to this invention, may hence be mentioned dibenzyl ether, di(o-methylbenzyl) ether, di(m-methylbenzyl) ether, di(p-methylbenzyl) ether, di(o-ethylbenzyl) ether, di(m-ethylbenzyl) ether, di(p-ethylbenzyl) ether, di(o-isopropylbenzyl) ether, di(p-n-propylbenzyl) ether, di(p-tert-butylbenzyl) ether, di(p-nonylbenzyl) ether, di(o-phenylbenzyl) ether, di(p-cyclohexylbenzyl) ether, di[(p-benzyl)benzyl] ether, di[p-(α-methylbenzyl)benzyl] ether, di[p-(α,α-dimethyl)benzyl] ether, di(2,3-dimethylbenzyl) ether, di(2,4-dimethylbenzyl) ether, di(2,5-dimethylbenzyl) ether, di(2,6-dimethylbenzyl) ether, di(3,4-dimethylbenzyl) ether, di(3,5-dimethylbenzyl) ether, di(2,4-diethylbenzyl) ether, di(3-methyl-4-ethylbenzyl) ether, di[3-(α-methylbenzyl)-4-methylbenzyl] ether, di[2,5-(dimethylbenzyl)benzyl] ether, di(α-methylbenzyl) ether, di(α-ethylbenzyl) ether, di(α-n-propylbenzyl) ether, di(α-isopropylbenzyl) ether, di(α-n-butylbenzyl) ether, di(o-methyl-α-methylbenzyl) ether, di(p-methyl-α-methylbenzyl ether, di(m-methyl-α-methylbenzyl) ether, di(p-methyl-α-ethylbenzyl) ether, di(p-methyl-α-isopropylbenzyl) ether, di(p-methyl-α-n-propylbenzyl) ether, di(p-methyl-α-n-butylbenzyl) ether, di(p-ethyl-α-methylbenzyl) ether, di(m-ethyl-α-ethylbenzyl) ether, di(o-ethyl-α-methylbenzyl) ether, di(p-isopropyl-α-methylbenzyl) ether, di(p-phenyl-α-methylbenzyl) ether, di[p-(α-methylbenzyl)-α-methylbenzyl] ether, dil(p-(α,α-dimethylbenzyl)-α-methylbenzyl] ether, di(2,3-dimethyl-α-methylbenzyl) ether, di(2,4-dimethyl-α-methylbenzyl) ether, di(2,5-dimethyl-α-methylbenzyl) ether, di(3,4-dimethyl-α-methylbenzyl) ether, di(2-methyl-5-tertbutyl-α-methylbenzyl) ether, etc. It should however be borne in mind that the present invention is not necessarily limited to the use of the above specific dibenzyl ethers. Among these exemplary dibenzyl ethers, dibenzyl ether, di(p-methylbenzyl) ether, di(α-methylbenzyl) ether, di(p-methyl-α-methylbenzyl) ether and the like are preferred. Upon production of a co-condensation resin, the dibenzyl ether may be used in an amount of 1.0 - 15 moles, preferably, 1.0 - 5 moles per mole of salicylic acid. Reaction conditions for the production of a co-condensation resin from a dibenzyl ether, for example, the amount of the dibenzyl ether to be used, and the weight average molecular weight and salicylic acid content of the resulting co-condensation resin are similar to those mentioned above with respect to the benzyl alkyl ethers (a). Where the dibenzyl ether is used at a smaller molar ratio with a view toward obtaining a resin having a composition of a relatively low molecular weight, unreacted salicylic acid remains. As a method for the elimination of the remaining salicylic acid, it is practised, for example, to wash the resin with warm water either as is or after dissolving the resin in an organic solvent such as benzene, toluene, monochlorobenzene, methyl isobutyl ketone or cyclohexanone. (A)-3: Condensation between salicylic acid and benzyl halides: Specifically, this reaction is to obtain a co-condensation resin by causing a benzyl halide represented by the general formula (V) to react with salicylic acid in the presence of a Friedel-Crafts catalyst which has been described collectively as an acid catalyst. It has been found that when a benzyl halide is caused to exist in an amount of 2.0 moles or more per mole of salicylic acid and they are reacted at a reaction temperature of 20°C or higher by a Friedel-Crafts catalyst, the benzyl group or groups can be introduced to one or both positions ortho to and/or the position para to the hydroxyl group of salicylic acid and moreover, excess benzyl halide undergoes successive condensation with the benzyl groups introduced in salicylic acid while undergoing concurrent self-condensation, thereby to form a resinous substance. In the starting benzyl halides, the nuclei of the benzyl groups are either unsubstituted or substituted with at least one C₁₋₁₂ alkyl, aralkyl, aryl or cycloalkyl group at an o- or m- position or the p-position and a hydrogen atom or C₁₋₄ lower alkyl group is contained at the α-position. As halogens, chlorine, bromine and fluorine may be mentioned with chlorine being preferred. Illustrative examples of the benzyl halide may therefore include benzyl chloride, benzyl bromide, o-methylbenzyl chloride, m-methylbenzyl chloride, p-methylbenzyl chloride, o-ethylbenzyl chloride, m-ethylbenzyl chloride, p-ethylbenzyl chloride, o-isopropylbenzyl chloride, p-n-propylbenzyl chloride, p-tert-butylbenzyl bromide, p-nonylbenzyl chloride, o-phenylbenzyl fluoride, p-cyclohexylbenzyl chloride, p-(benzyl)benzyl chloride, p-(α-methylbenzyl)benzyl chloride, p-(α,α-dimethyl)benzyl chloride, 2,3-dimethylbenzyl fluoride, 2,4-dimethylbenzyl chloride, 2,5-dimethylbenzyl chloride, 2,6-dimethylbenzyl chloride, 3,4-dimethylbenzyl chloride, 3,5-dimethylbenzyl chloride, 2,4-diethylbenzyl chloride, 3-methyl-4-ethylbenzyl chloride, 3-(α-methylbenzyl)-4-methylbenzyl chloride, 2,5-(dimethylbenzyl)benzyl chloride, α-methylbenzyl chloride, α-ethylbenzyl bromide, α-isopropylbenzyl chloride, α-n-butylbenzyl chloride, o-methyl-α-methylbenzyl chloride, p-methyl-α-methylbenzyl chloride, m-methyl-α-methylbenzyl chloride, p-ethyl-α-methylbenzyl chloride, o-ethyl-α-methylbenzyl chloride, p-isopropyl-α-methylbenzyl chloride, p-phenyl-α-methylbenzyl chloride, p-(α-methylbenzyl)-α-methylbenzyl chloride, p-(α,α-dimethylbenzyl)-α-methylbenzyl chloride, 2,3-dimethyl-α-methylbenzyl chloride, 2,4-dimethyl-α-methylbenzyl chloride, 2,5-dimethyl-α-methylbenzyl chloride, 3,4-dimethyl-α-methylbenzyl chloride, 2-methyl-5-tert-butyl-α-methylbenzyl chloride, etc. It should however be borne in mind that the present invention is not necessarily limited to the use of the above specific dibenzyl halides. Among these exemplary benzyl halides, benzyl chloride, p-methylbenzyl chloride, α-methylbenzyl chloride, p-methyl-α-methylbenzyl chloride and the like are preferred. In the production of co-condensation resins of this invention, the various benzyl halides may be used in an amount of 2 - 30 moles, preferably, 2 - 10 moles per mole of salicylic acid. If the benzyl halide is used in any mounts smaller than the lower limit, a metal-modified product of the resulting resin is somewhat insufficient in its miscibility with a non-volatile oil in microcapsules on a CB-sheet and in its water insolubility when the metal-modified product is used as a color-developing agent for pressure-sensitive copying paper. On the other hand, an amount greater than the upper limit result in a reduction to the relative proportion of salicylic acid, so that the density of a color to be produced will not reach a desired level. The weight average molecular weight of a resin produced by using a benzyl halide within the above range falls within a range of 500 - 10,000, preferably, 500 - 5,000 As exemplary catalysts useful in the above reaction, may be mentioned Lewis acid catalysts such as ferric chloride, zinc chloride, aluminum chloride, stannic chloride, titanium tetrachloride and boron trifluoride; perfluoroalkanesulfonic acids known as super strong acids, for example, trifluoromethanesulfonic acid; and "Nafion H" (trade name; product of E.I. du Pont de Nemours & Co., Inc.) as a perfluoroalkanesulfonic acid resin. Among these, zinc chloride is particularly preferred. The catalyst may be used in an amount of 0.05 - 200 mole % based on salicylic acid, with 1 - 100 mole % being preferred from the economical viewpoint. In the process of this invention, the metal species of the catalyst employed in the reaction reacts with the salicylic acid resin when the catalyst is neutralized with a base in the reaction mixture subsequent to the reaction, whereby the metal species of the catalyst is obtained as the metal-modified product of the salicylic acid. This is also an advantageous feature of the process of this invention. This is economical because the catalyst is effectively used and its processing and/or treatment as a waste material is unnecessary. In the above reaction, a solvent may also be used. Illustrative examples of such a solvent may include those inert to the reaction, for example, ethyl ether, ethylene glycol dimethyl ether, methylene chloride, 1,2-dichloroethane, acetic acid, propionic acid, carbon disulfide, nitromethane and the like. These solvents may be used desirably in an amount of 30 or less times by volume/weight as much as the total weight of the starting materials from the economical viewpoint. Upon practice of this reaction, the reaction temperature may be in a range of 20 - 180°C, preferably, 50 - 160°C. The reaction time may be 1 - 30 hours. As a procedure for producing a resin by the above reaction, it is usual to charge all the starting materials at once and then to heat them in situ so as to react them at a predetermined temperature or as an alternative, to charge salicylic acid and the catalyst and then to conduct the reaction at a predetermined temperature while adding dropwise the other starting material, i.e., benzyl halide. Here, the time of the dropwise addition may preferably account for at least 50% of the overall reaction time and may usually range from 1 to 20 hours. When a solvent is used and the solvent is insoluble in water, water may be added to the reaction mixture after the reaction. After washing the reaction mixture with the water and allowing the resultant mixture to separate into two layers, the solvent may be distilled off to obtain the resin. As an alternative, the resin may be extracted out with a dilute aqueous alkali solution for use in the next step. Where the solvent is soluble in water, the reaction mixture may be poured into water and the resin may then be obtained as a precipitate. Second-stage reaction The second-stage reaction comprises reacting the styrene derivative represented by the general formula (VII) with the co-condensation resin, which has been obtained in the first-stage reaction, in the presence of an acid catalyst. formula (VII): wherein R₄ and R₈ mean independently a hydrogen atom or a methyl group and R₆ denotes a hydrogen atom or a C₁₋₄ alkyl group In this reaction, the styrene derivative acts at the α-position thereof on one or more aromatic substituents of the salicylic acid resin obtained in the first-stage reaction, thereby introducing the corresponding number of benzyl groups in the resin. In other words, the styrene derivative reacts at the α-position thereof with one or more benzene rings other than the skeletal benzene ring of salicylic acid. In addition, an excess portion of the styrene derivative also reacts with aromatic substituents of the salicylic acid resin and also with benzene rings of the styrene derivative bonded to such aromatic substituents, whereby the molecular weight increases. As illustrative examples of the styrene derivative useful in the second-stage reaction, may be mentioned styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, o-ethylstyrene, p-ethylstyrene, o-isopropylstyrene, m-isopropylstyrene, p-isopropyl-styrene, p-tert-butylstyrene, α-methylstyrene, β-methylstyrene and so on. It should however be borne in mind that the present invention is not necessarily limited to the use of the above exemplified specific styrene derivatives. It is economical styrene that is preferable from the industrial standpoint. The styrene derivative may be used in an amount of 0.2 - 20 moles, preferably, 0.5 - 10 moles per mole of salicylic acid which is contained in the resin obtained in the first-stage reaction. If the styrene derivative is used in an amount smaller than the lower limit specified in the present invention, a color-developing agent containing a metal-modified product of the resulting resin has somewhat insufficient miscibility with a non-volatile oil in microcapsules on a CB-sheet of the above-mentioned copying paper and water insolubility. On the other hand, an amount greater than the upper limit results in a decrease to the relative proportion of salicylic acid and the density of a color to be produced will not reach a desired level. The weight average molecular weight of a resin produced by using the styrene derivative within the above-specified range falls within a range of 500 - 10,000, prefeably, 500 - 5,000. In this second-stage reaction, an acid catalyst is used. Illustrative examples of the acid catalyst may include mineral acids such as hydrochloric acid, sulfuric acid and phosphoric acid, Friedel-Crafts catalysts such as ferric chloride, zinc chloride, aluminum chloride, stannic chloride, titanium tetrachloride and boron trifluoride, methanesulfonic acid, triluoromethanesulfonic acid, etc. It is inexpensive sulfuric acid that is particularly preferable among these exemplary strong acid catalysts. The catalyst may be used in an amount of 0.05 - 200 wt.% based on the total weight of the salicylic acid resin obtained in the first-stage reaction and the styrene derivative, with 1 - 100 wt.% being preferable from the economical standpoint. Further, a solvent may also be used in the second-stage reaction. Illustrative examples of the solvent may include those inert to the reaction, for example, aliphatic hydrocarbons such as n-hexane, n-heptane, n-pentane and cyclohexane; ethers such as ethyl ether and ethylene glycol dimethyl ether; esters such as ethyl acetate and butyl acetate; halogenated hydrocarbon solvents such as methylene chloride, 1,2-dichloroethane, 1,1,2-trichloroethane, carbon tetrachloride, chloroform and monochlorobenzene; organic acids such as acetic acid and propionic acid; ketones such as acetone and methyl ethyl ketone; and in addition, carbon disulfide, nitromethane, acetonitrile, tetrahydrofuran and the like. In view of economy, it is desirable to use such a solvent in an amount not more than 30 times by volume/weight the total weight of the starting materials. The reaction temperature upon practice of the second-stage reaction may range from -20°C to 180°C, preferably, from 0°C to 120°C. The reaction time may be in a range of 1 - 30 hours. As a procedure for producing the resin by the second-stage reaction, it is usual to add a catalyst to an organic solvent solution of the salicylic acid resin obtained in the first-stage reaction and then to react the further starting material, i.e., the styrene derivative with the salicylic acid resin at a predetermined temperature while adding the styrene derivative dropwise. Here, the time of the dropwise addition may preferably account for at least 50% of the whole reaction time and may usually range from 1 to 20 hours. When the solvent used is insoluble in water, water may be added to the reaction mixture after the reaction. After washing the reaction mixture with the water and allowing the resultant mixture to separate into two layers, the solvent may be distilled off to obtain the resin. As an alternative, the resin may be extracted out with a dilute aqueous alkali solution for use in the next step. Where the solvent is soluble in water, the reaction mixture may be poured into water and the resin may then be obtained as a precipitate. Further condensation is effected with styrene derivative subsequent to the condensation between salicylic acid and benzyl alcohol or benzyl ether, with a view toward improving the miscibility between the non-volatile oil in microcapsules coated on the back sides of the CB-sheet 1 and CB/CF-sheet 2 and a color-developing agent coated on the front sides of the CB/CF-sheet 2 and the CF-sheet 3 composed of a multivalent metal-modified product of such a salicylic acid resin upon application of pressures. The color-developing agent composed of a multivalent metal-modified product of the copolymer formed of structural units represented by the formulae (I), (II) and (III) respectively reacts promptly with a colorless pressure-sensitive dyestuff precursor dissolved in the non-volatile oil upon application of pressures, thereby bringing about a merit that vivid pressure-sensitive marks are formed instantaneously. A description will next be made of multivalent metal-modified products of the above-described salicylic acid resins. As described above, the term "multivalent metal-modified product of a salicylic acid resin" as used herein means either a multivalent metal salt of the salicylic acid resin or a molten mixture containing the multivalent-metal salt. Several known processes may be applied to produce a multivalent metal salt of the salicylic acid resin described above. For example, it may be produced by reacting an alkali metal salt of the resin and a water-soluble multivalent metal in water or a solvent in which the alkali metal salt and water-soluble multivalent metal can be dissolved. Namely, the multivalent metal salt can be formed by reacting the hydroxide, the carbonate or an alkoxide of an alkali metal with the resin to obtain the alkali metal salt of the resin, an aqueous solution thereof, an alcohol solution thereof or a water-alcohol mixed solution thereof and then reacting a water-soluble multivalent metal salt further. More specifically, the copolymer is dispersed in an aqueous solution, alcohol solution or water-alcohol solution containing the hydroxide, the carbonate or an alkoxide of an alkali metal in an amount at least equivalent to carboxyl groups in the copolymer, whereby the copolymer is dissolved at 0 - 100°C. A water-soluble multivalent metal salt is then added as is or as an aqueous solution, alcohol solution or water-alcohol solution, and reacted with the resulting solution at 0 -100°C, so that the metal salt of the copolymer is obtained as a precipitate. It is desirable to react the water-soluble multivalent metal salt in an amount of about 0.5 - 1 equivalent relative to the carboxyl groups in the copolymer. In order to obtain a molten mixture containing a multivalent metal salt of a salicylic acid resin, the resin is mixed with an organic carboxylic acid such as formic acid, acetic acid, propionic acid, valeric acid, caproic acid, stearic acid or benzoic acid, heating and reacting them in a molten state and then cooling the resultant mixture. In some instances, a basic substance, for example, ammonium carbonate, ammonium bicarbonate, ammonium acetate or ammonium benzoate may be added further, followed by heating and reaction in a molten state. It may also be obtained by using the resin and the carbonate, oxide or hydroxide of a multivalent metal, heating them together with a basic substance, e.g., the ammonium salt of an organic carboxylic acid such as ammonium formate, ammonium acetate, ammonium caproate, ammonium stearate or ammonium benzoate, reacting the resultant mixture in a molten state, and then cooling the reaction mixture. When the metal-modified product of the resin is produced by heating and melting the resin and multivalent metal salt, the melting is conducted usually at 100 - 180°C, and the reaction time is about 1 - several hours although it varies depending on the composition of the resin, the melting temperature and the kind and amount of the multivalent metal salt used. Further, the multivalent metal salt may desirably be used as an organic carboxylic acid salt thereof or the carbonate, oxide or hydroxide thereof in an amount such that the multivalent metal is contained in an amount of 1 wt.% - about 20 wt.% based on the total weight of the resin. Although no specific limitation is imposed on the amount of the basic substance to be used, it may generally be used in an amount of 1 - 15 wt.% based on the total weight of the resin. When the basic substance is used, it is more preferable to used it after mixing same with the multivalent metal salt in advance. The softening points (as measured by the ring and ball softening point measuring method prescribed in JIS K-2548) of metal-modified resins produced by the heating and melting method fall within a range of 50 - 120°C. Illustrative examples of the metal of the metal-modified salicylic acid co-condensation resin of this invention may embrace metals other than alkali metals such as lithium, sodium and potassium. As preferable multivalent metals, may be mentioned calcium, magnesium, aluminum, copper, zinc, tin, barium, cobalt, nickel and the like. Among these, zinc is particularly effective. Each of these multivalent metals forms a multivalent metal salt between carboxyl groups within the same molecule of the salicylic acid resin or between carboxylic groups in different molecules of the salicylic acid resin. The color-developing agent of this invention may be used in combination with one or more of known color-developing agents, namely, inorganic solid acids such as activated clay, organic polymers such as phenol-formaldehyde resin and metal salts of aromatic carboxylic acids without any problems or inconvenience. The color-developing agent of this invention may also be used in combination with at least one of the oxide, hydroxide and carbonate of a multivalent metal selected from the group consisting of zinc, magnesium, aluminum, lead, titanium, calcium, cobalt, nickel, manganese and barium. As a method for the fabrication of the color-developing sheet of this invention suitable for use in pressure-sensitive copying paper, any one of the following methods can be employed: (1) to apply a water-base coating formulation, which makes use of an aqueous suspension of the metal-modified product, to a base material such as paper web; (2) to incorporate the metal-modified product in a base paper web when the base paper web is produced; and (3) to coat a base material with a solution or suspension of the metal-modified product in an organic solvent. Upon preparation of a coating formulation, kaolin clay or its analogous clay, calcium carbonate, starch, synthetic or natural latex, etc. are added to obtain the coating formulation with suitable viscosity and coating applicability. The proportion of the color-developing agent in the coating formulation may preferably be 10 - 70% of the whole solids. If the proportion of the color-developing agent is smaller than 10%, it is impossible to exhibit sufficient color-producing ability. Any proportions greater than 70% result in color-developing sheets having poor paper surface characteristics. The coating formulation may be applied at a rate of 0.5 g/m² or more, preferably, 1 - 10 g/m² in terms of dry weight. In the color-developing sheet of this invention which is suitable for pressure-sensitive copying paper, it is possible to use the color-developing agent and coating formulation in smaller amounts. In addition, the concentration, viscosity, etc. of the coating formulation can be varied over relatively wide ranges. Both on-machine and off-machine coating operations are hence feasible. A great merit has hence be brought about not only from the viewpoint of performance but also from the standpoint of fabrication steps for pressure-sensitive copying paper. By the present invention, there have been provided the novel salicylic acid resins and their multivalent metal salts, color-developing agents composed of multivalent metal-modified resins of the salicylic acid resins and suited for pressure-sensitive copying paper as well as color-developing sheets containing the color-developing agents and suited for pressure-sensitive copying paper. The color-developing sheets of this invention are substantially free from yellowing by light and gases in the air such as nitrogen oxides. Produced color marks are stable to light, plasticizers and the like and are not substantially lowered in color density. They have good waterproofness. The present invention is therefore possible to broaden the utility of pressure-sensitive copying paper to such fields that conventional color-developing sheets are not suited owing to the requirement for long-term storage stability. Accordingly, this invention has extremely great practical significance. The present invention will hereinafter be described in further detail by the following Examples. The following methods were relied upon for the determination of performance of color-developing sheets for pressure-sensitive copying paper. 1. Color-producing speed and produced color density [(conducted in an air-conditioned room of 20°C and 65% R.H. except for those indicated specifically as low-temperature color production (5°C, 60% R.H.)]: Using (1) a commercial blue-color producing CB-sheet containing Crystal Violet Lactone (CVL) as a principal pressure-sensitive dyestuff precursor ("NW-40T", trade name; product of Jujo Paper Co., Ltd.) or (2) a commercial black-color producing CB-sheet containing 3-diethylamino-6-methyl-7-phenyl-aminofluoran (ODB) as a principal pressure-sensitive dyestuff precursor ("KW-40T", trade name; product of Jujo Paper Co., Ltd.) in the case of blue color production or black color production, a sample color-developing sheet (CF-sheet) coated with a water-base coating formulation was stacked with their coated sides maintained in contiguous relation. The thus-stacked pressure-sensitive copying paper was typed by an electronic typewriter to produce a color. The reflectance of the sample color-developing sheet was measured by a "Σ-80 Color Difference Meter" (trade name; manufactured by Tokyo Denshoku Kogyo K.K.). The same color difference meter was also used in the subsequent measuring tests.) twice, namely, 1 minute and 30 seconds after the typing and 24 hours after the typing. The results are expressed in terms of Y value. Y value is a value obtained in accordance with the double-field display method established by CIE (Commision Internationale de l'Eclairage). It is expressed by the following equation (the same definition also applies to Y values in the subsequent measuring tests): where P(λ): distribution of standard light source. y (λ): color matching function in the double field. τ(λ): spectral characteristics of each sample. Regarding P(λ) and y(λ), reference may be had to JIS Z8722. 2. Light fastness of produced color marks: Each sample color-developing sheet, which had produced a color in the manner described above in the testing method 1, was exposed for 2 hours (and for 4 hours) to light on a carbon arc fade meter (manufactured by Suga Testing Machine Co., Ltd.). After the exposure, its reflectance was measured by the "Σ-80 Color Difference Meter". The results are expressed in terms of Y-value. The smaller the Y value and the smaller its difference from the Y value before the test, the less the fading by the light and the more preferable. 3. Plasticizer resistance: DOP microcapsule coated paper sheets were prepared by forming microcapsules, which contained dioctyl phthalate (DOP) as a core substance, had an average capsule size of 5.0 µm, and were equipped with melamine-formaldehyde resin capsule walls, adding a small amount of a starch-type binder, applying the thus-prepared coating formulation by an air-knife coater on a high-quality paper web to achieve a dry coat weight of 5 g/m² and then drying the thus-coated paper web. One of the DOP microcapsule coated paper sheets and the color-developing sheet with color marks produced above in the testing method 1 were brought into contiguous relation with their coated sides facing each other. They were thereafter caused to pass under a linear pressure of 100 Kg/cm through a super calender roll, so that DOP was allowed to penetrate uniformly into the colored surface. One hour after the test, the reflectance of the color-developing sheet was measured by the "Σ-80 Color Difference Meter". The results are expressed in terms of Y value. The smaller the Y value and the smaller its difference from the Y value before the test, the better the plasticizer resistance of the produced color marks. 4. Waterproofness of produced color marks: Each sample color-developing sheet, which had been colored by the testing method 1, was dipped for 2 hours in water. Density changes of the produced color marks were observed visually. 5. Yellowing property of color-developing sheets: (5-1) Yellowing by NO x : Following JIS L-1055 (Testing Method for NO x Gas Fastness of Dyed Materials and Dyes), each sample color-developing sheet was stored for 1 hour in a closed vessel of an atmosphere of NO x occurred by the reaction of NaNO₂ (sodium sulfite) and H₃PO₄ (phosphoric acid). The degree of its yellowing was investigated. Upon an elapsed time of 1 hour after completion of the storage, the reflectance of the color-developing sheet was measured by the "Σ-80 Color Difference Meter". The measurement results are expressed in terms of WB value. The greater the WB value and the smaller its difference from the WB value before the test, the smaller the yellowing property in an NO x atmosphere. (5-2) Yellowing by light: Each sample color-developing sheet was exposed for 4 hours to light on the carbon arc fade meter (manufactured by Suga Testing Machine Co., Ltd.). After the test, the reflectance of the sample color-developing sheet was measured by the "Σ-80 Color Difference Meter". The measurement results are expressed in terms of WB value. The greater the WB value and the smaller its difference from the WB value before the test, the smaller the yellowing property upon exposure to light. Reference Example 1: (a) Synthesis of salicylic acid resin: Charged in a glass-made reactor were 27.6 g (0.2 mole) of salicylic acid, 24.4 g (0.2 mole) of α-methylbenzyl alcohol, and 3.0 g of p-toluenesulfonic acid as a catalyst. Under a nitrogen gas stream, the reactants were subjected to condensation for 3 hours at a reaction temperature of 150 - 160°C, followed by a dropwise addition of 48.8 g (0.4 mole) of α-methyl benzyl alcohol at the same temperature over 5 hours. The contents were then heated to 170 -180°C, at which they were aged for 3 hours. The contents were then immediately poured into an enameled shallow pan at the same temperature. When left over, the resinous reaction product became solid to obtain 86 g of a transparent resin of a light yellowish color. The weight average molecular weight of the thus-obtained resin was 750. Its softening point was measured by the ring and ball softening point measuring apparatus in accordance with JIS K-2548. It was found to be 54°C. As a result of an analysis by high-performance chromatography, the content of unreacted salicylic acid was found to be 5 wt.% or less. A tetrahydrofuran solution of the resin was titrated with a 1/10 N aqueous solution of sodium carbonate to determine the content of salicylic acid moieties in the resin. As a result of subtraction of the unreacted portion, the content of the salicylic acid moieties was found to be 23.2 wt.%. (b) Synthesis of a multivalent metal-modified product of the salicylic acid resin: Twenty-five grams of the above resin were charged in a flask and then heated to melt same at 150 - 160°C. A mixture of 6.8 g of zinc benzoate and 4 g of ammonium bicarbonate, which had been prepared in advance, was then added gradually to the molten resin over 30 minutes. Thereafter, the contents were stirred for 1 hour at 155- 165°C to complete their reaction. After the completion of the reaction, the molten resin was taken out of the flask, cooled and then ground, thereby obtaining 27 g of powder of a zinc benzoate modified product of the salicylic acid resin. The softening point of the zinc-modified product was measured by a ring and ball softening point measuring apparatus in accordance with the method prescribed in JIS K-2548. It was found to be 78°C. (c) Use of the zinc-modified product as a color-developing agent and its performance: Using the zinc-modified product of the salicylic acid resin as a color-developing agent, it was dispersed in accordance with the following composition by a sand grinding mill so as to prepare a suspension. Parts by weight Color-developing agent 6 10% Aq. PVA solution (PVA : "Kuraray #117", product of Kuraray Co., Ltd.) 3 Water 22.5 Then, a coating formulation of the following composition was prepared by using the above suspension of the color-developing agent. Parts by weight Suspension 10 Light calcium carbonate 10 Starch 0.8 Synthetic rubber latex 0.8 Water 32.5 The coating formulation was applied on a high-quality paper web to give a dry coat weight of 5.0 - 5.5 g/m² and was then dried, thereby obtaining color-developing sheets. Performance of the color-developing sheets for pressure-sensitive copying paper was determined in accordance with the measurement methods described above. Comparative Example 1: Charged in a glass-made reactor were 170 g of p-phenylphenol, 22.5 g of 80% para-formaldehyde, 2.0 g of p-toluenesulfonic acid and 200 g of benzene. The contents were heated with stirring and while distilling out the resulting water as an azeotropic mixture with benzene from the reaction system, they were reacted at 70 - 80°C for 2 hours. After the reaction, 320 g of a 10% aqueous solution of sodium hydroxide was added and benzene was distilled out by steam distillation. The reaction mixture was then cooled, followed by dropwise addition of dilute sulfuric acid. The precipitated p-phenylphenol-formaldehyde polymer was collected by filtration, washed with water and then dried, thereby obtaining 176 g of white powder. Using the p-phenylphenol-formaldehyde polymer, color-developing sheets were obtained in the same manner as in Reference Example 1. Comparative Example 2: Charged in a reactor equipped with a thermometer, reflux condenser, dropping funnel and stirrer were 9.4 g of phenol and 0.2 g of sulfuric acid. Then, 23.6 g of α-methylstyrene was added dropwise under stirring at 50°C from a dropping funnel. After completion of the dropwise addition, the reaction mixture was aged for 5 hours and then poured into a dilute aqueous solution of sodium carbonate. The resultant mixture was separated to obtain an oil layer. The oil layer was then distilled in vacuo. At a vacuum level of 3 - 4 mmHg, 22 g of a fraction having a boiling point of at least 220°C was obtained. Thereafter, 7.5 g of a 40% aqueous solution of caustic soda was added to the fraction, followed by dehydration under reflux of xylene. After the dehydration, the xylene mixture was transferred to an autoclave and carbon dioxide was then charged to 30 kg/cm² at 160°C. The contents were then reacted for 5 hours at the same temperature. After the reaction, the autoclave was cooled, the remaining gas was purged, the reaction mixture was extracted with hot water and the extract was neutralized with dilute sulfuric acid. As a result, crystals were caused to precipitate. Those crystals were then collected by filtration, followed by recrystallization from an aqueous acetic acid solution to obtain 8 g of 3,5-di(α,α-dimethylbenzyl)salicylic acid. A zinc salt was produced from the salicylic acid compound in the same manner as in Reference Example 1. Using the zinc 3,5-di(α,α-dimethylbenzyl)salicylate thus produced, color-developing sheets were obtained in the same manner as in Reference Example 1. Example 1 : Charged in a glass-made reactor were 27.6 g (0.2 mole) of salicylic acid, 44.8 g (0.4 mole) of benzyl methyl ether, and 0.76 g of p-toluene sulfonic acid and 0.76 g of anhydrous zinc chloride as catalysts. They were condensed for 3 hours at 125 -135°C under a nitrogen gas stream. The reaction temperature was thereafter raised to 145°C, at which the reaction was allowed to proceed further for 2 hours. The internal temperature was then cooled to 70°C, followed by an addition of 150 mℓ of 1,2-dichloroethane. The resultant mixture was cooled to room temperature. Thereafter, 7.5 g of 96% sulfuric acid was charged, followed by a dropwise addition of 83.2 g (0.8 mole) of styrene with vigorous stirring at 20 - 30°C over 5 hours. After the dropwise addition, the reaction mixture was aged for 5 hours at the same temperature to complete the reaction. To the reaction mixture, 60 g of water was added under stirring and the resultant mixture was allowed to stand and to separate into layers. The weight average molecular weight of the resultant condensation resin was 1380. The lower layer, namely, the solvent layer was charged in another glass-made reactor, to which 20 g of 28% aqueous ammonia and 8.1 g (0.1 mole) of zinc oxide were added. The resultant mixture was stirred for 1 hour at room temperature. The reaction mixture was then heated and reacted at 60 - 70°C for 1 hour. The reaction mixture was thereafter heated to distil out the solvent. After raising the internal temperature to 150°C, the reaction product was degasified for 30 minutes in a vacuum of 20 mmHg and was then taken out of the reactor, thereby obtaining 156 g of a transparent zinc-modified salicylic acid resin of a reddish brown color (stoichiometric yield). The softening point of the zinc-modified resin was measured by the ring and ball softening point measuring apparatus in accordance with the method prescribed in JIS K-2548. It was found to be 85°C. (c) Use of the zinc-modified resin as a color-developing agent: Using the zinc-modified salicylic acid resin as a color-developing agent, color-developing sheets were obtained in the same manner as in the procedure (c) of Reference Example 1. Example 2 : Charged in a reactor were 27.6 g (0.2 mole) of salicylic acid, 40.8 g (0.3 mole) of p-methyl-α-methylbenzyl alcohol, 100 mℓ of monochlorobenzene, and 0.7 g of anhydrous zinc chloride as a catalyst. They were reacted for 5 hours under reflux of the solvent. Water, which was distilled out in the course of the reaction, was removed by a water separator. After the reaction, 300 mℓ of warm water was added, the resultant mixture was stirred at 90°C or higher for 20 minutes, and the upper water layer was removed. That warm water washing and separation procedure was repeated two more times to remove unreacted salicylic acid. The monochlorobenzene solution was cooled to 5°C, followed by an addition of 10 g of concentrated sulfuric acid. To the resultant mixture, 31.2 g (0.3 mole) of styrene was added dropwise at 5 - 10°C over 7 hours. After the reaction, the reaction mixture was aged for 3 hours at the same temperature. The weight average molecular weight of the thus-formed resin was 1150. The resin was added with 1500 mℓ of water, followed by a dropwise addition of 36 g (0.4 mole) of a 45% aqueous solution of caustic soda. The reaction mixture was then heated to distil out the solvent azeotropically, whereby an aqueous solution was obtained in a somewhat clouded state. The solution was cooled to 40°C, to which was added dropwise an aqueous solution formed in advance by dissolving 29 g (0.1 mole) of zinc sulfate heptahydrate in 200 mℓ of water. A white precipitate was formed. The precipitate was collected by filtration, washed with water and then dried in vacuo, thereby obtaining 92 g of the zinc salt of the salicylic acid resin. As a result of an elemental analysis, the content of zinc was found to be 6.78%. Using the zinc-modified salicylic acid resin, color-developing sheets were obtained in the same manner as in Reference Example 1. Examples 3 to 16 Various multivalent metal-modified salicylic acid resins were produced in the same manner as in Example 1 except that the kind of the benzyl alcohol or benzyl alkyl ether, its molar ratio to salicylic acid, the kind and amount of the styrene, the kind and amount of the catalyst used and reaction conditions were changed as shown in Table 18. The softening points of the resultant metal-modified products are also shown in Table 18. Using those multivalent metal modified salicylic acid resins, color-developing sheets were produced in the same manner as in Reference Example 1 . Examples 17 & 18 Suspensions of the color-developing agents obtained respectively in Examples 85 and 90 were formulated separately into coating formulations of the composition employed in Examples 13 and 14, and color-developing sheets were then obtained correspondingly. The color-developing sheets obtained in Examples 1 to 18 were subjected along with the color-developing sheets of Comparative Examples 1 and 2 to a performance test. Results are shown in Table 2. Example 19 (a) Synthesis of salicylic acid resin and its multivalent metal-modified product: Charged in a glass-made reactor were 27.6 g (0.2 mole) of salicylic acid, 39.7 g (0.2 mole) of dibenzyl ether, and 0.7 g of p-toluenesulfonic acid and 0.7 g of anhydrous zinc chloride as catalysts. The reactants were condensed for 3 hours at 125 - 135°C under a nitrogen gas stream. Thereafter, the reaction temperature was raised to 145°C and the reaction was allowed to proceed further for 2 hours. The internal temperature was cooled to 70°C and 150 mℓ of 1,2-dichloroethane was charged. The reaction mixture was allowed to cool down to room temperature. Thereafter, 7.5 g of 96% sulfuric acid was charged, followed by a dropwise addition of 83.2 g (0.8 mole) of styrene with vigorous stirring at 20 - 30°C over 5 hours. After the dropwise addition, the reaction mixture was aged for 5 hours at the same temperature to complete the reaction. To the reaction mixture, 60 g of water was added under stirring and the resultant mixture was allowed to stand and to separate into layers. The weight average molecular weight of the resultant condensation resin was 1100. The lower layer, namely, the solvent layer was charged in another glass-made reactor, to which 10 g of 28% aqueous ammonia and 8.1 g (0.1 mole) of zinc oxide were added. The resultant mixture was stirred for 1 hour at room temperature. The reaction mixture was then heated and reacted at 60 - 70°C for 1 hour. The reaction mixture was thereafter heated to distil out the solvent. After raising the internal temperature to 150°C, the reaction product was degasified for 30 minutes in a vacuum of 20 mmHg and was then taken out of the reactor, thereby obtaining 155 g of a transparent zinc-modified salicylic acid resin of a reddish brown color (stoichiometric yield). The softening point of the zinc-modified resin was measured by the ring and ball softening point measuring apparatus in accordance with the method prescribed in JIS K-2548. It was found to be 78°C. (c) Use of the zinc-modified resin as a color-developing agent: Using the zinc-modified salicylic acid resin as a color-developing agent, color-developing sheets were obtained in the same manner as in the procedure (c) of Reference Example 1. Example 20 Charged in a reactor were 27.6 g (0.2 mole) of salicylic acid, 38.1 g (0.15 mole) of di(p-methyl-α-methylbenzyl) ether, 100 mℓ of monochlorobenzene, and 0.7 g of anhydrous zinc chloride as a catalyst. They were reacted for 5 hours under reflux of the solvent. Water, which was distilled out in the course of the reaction, was removed by a water separator. After the reaction, 300 mℓ of warm water was added, the resultant mixture was stirred at 90°C or higher for 20 minutes, and the upper water layer was removed. That warm water washing and separation procedure was repeated two more times to remove unreacted salicylic acid. The monochlorobenzene solution was cooled to 5°C, followed by an addition of 10 g of concentrated sulfuric acid. To the resultant mixture, 31.2 g (0.3 mole) of styrene was added dropwise at 5 - 10°C over 7 hours. After the reaction, the reaction mixture was aged for 3 hours at the same temperature. The weight average molecular weight of the thus-formed resin was 850. The resin was added with 1500 mℓ of water, followed by a dropwise addition of 36 g (0.4 mole) of a 45% aqueous solution of caustic soda. The reaction mixture was then heated to distil out the solvent azeotropically, whereby an aqueous solution was obtained in a somewhat clouded state. The solution was cooled to 40°C, to which was added dropwise an aqueous solution formed in advance by dissolving 29 g (0.1 mole) of zinc sulfate heptahydrate in 200 mℓ of water. A white precipitate was formed. The precipitate was collected by filtration, washed with water and then dried in vacuo, thereby obtaining 90 g of the zinc salt of the salicylic acid resin. As a result of an elemental analysis, the content of zinc was found to be 6.50%. Using the zinc-modified salicylic acid resin, color-developing sheets were obtained in the same manner as in Example 19 Examples 21 - 24 Various multivalent metal-modified salicylic acid resins were produced in the same manner as in Example 19 except that the kind of the dibenzyl ether, its molar ratio to salicylic acid, the kind and amount of the styrene, the kind and amount of the catalyst used and reaction conditions were changed as shown in Table 3 . The softening points of the resultant metal-modified products are also shown in Table 3 . Using those multivalent metal modified salicylic acid resins, color-developing sheets were produced in the same manner as in Reference Example 1. Examples 25 & 26 Suspensions of the color-developing agents obtained respectively in Examples 19 and 24 were formulated separately into coating formulations of the composition employed in Examples 13 and 14, and color-developing sheets were then obtained correspondingly. The color-developing sheets obtained in Examples 19 - 26 were subjected along with the color-developing sheets of Comparative Examples 1 and 2 to a performance test. Results are shown in Table 4 .

Claims: A copolymer comprising 5 - 20 mole % of structural units (I), 10 - 85 mole % of structural units (II), and 4 - 85 mole % of structural units (III) and having a weight average molecular weight of 500 - 10,000, each of said structural units (I) being coupled via the α-carbon of one of said structural units (II) with the benzene ring of one of said structural units (II), one or more of said structural units (II) being coupled via the α-carbon or α-carbons thereof to the benzene ring or rings of other structural units (II), one or more of said structural units (III) being coupled via the α-carbon or α-carbons thereof with the benzene ring or rings of one or more of the structural units (II) or ring or rings of one or more of structural units (III), and said structural units (I), (II), and (III) being represented respectively by the following formulae (I), (II), and (III): wherein R₁ and R₂ mean independently a hydrogen atom or a C₁₋₁₂ alkyl, aralkyl, aryl, or cycloalkyl group, R₃ and R₆ denote independently a hydrogen atom or a C₁₋₄ alkyl group, R₄ means a hydrogen atom or a methyl group, R₅ means a hydrogen and R₇ stands for -CH₂-R₈ (R₈: hydrogen atom or methyl group); obtainable by reacting (vi) a styrene derivative, which is represented by the following general formula (VII): wherein R₄, R₆, and R₈ have the same meaning as defined above, with a co-condensation resin, which has in turn been obtained by condensing in the presence of at least one acid catalyst (i) salicylic acid with (ii) a benzyl alcohol, (iii) a benzyl ether and/or (iii') a dibenzyl ether, both represented by the following general formula (IV): wherein R₁, R₂, and R₃ have the same meaning as defined above, and R₉ stands for a hydrogen atom, a C₁₋₄ alkyl group, or or (iv) a benzyl halide represented by the following general formula (V): wherein R₁, R₂, and R₃ have the same meaning as defined above, and X denotes a halogen atom. The copolymer as claimed in Claim 1, wherein R₄ in each structural formular (III) is a hydrogen atom. A process for the production of the copolymer according to Claim 1, which comprises reacting (vi) a styrene derivative, which is represented by the following general formula (VII): wherein R₄ and R₈ mean independently a hydrogen atom or a methyl group and R₆ denotes a hydrogen atom or a C₁₋₄ alkyl group, with a co-condensation resin, which has in turn been obtained by reacting in the presence of at least one acid catalyst (i) salicylic acid and (ii) a benzyl alcohol and/or (iii) a benzyl ether, both, represented by the following general formula (IV): wherein R₁ and R₂ may be the same or different and mean independently a hydrogen atom or a C₁₋₁₂ alkyl, aralkyl, aryl or cycloalkyl group, R₃ denotes a hydrogen atom or a C₁₋₄ alkyl group, and R₉ stands for a hydrogen atom, a C₁₋₄ alkyl group or in the presence of at least one acid catalyst, at 0 - 120°C and a styrene derivative (vi)/salicylic acid (i) molar ratio of 0.2 - 20. A multivalent metal salt of the copolymer according to Claim 1 or 2. The multivalent metal salt as claimed in Claim 4 , wherein said at least one multivalent metal atom is a metal atom selected from the group consisting of magnesium, aluminum, copper, zinc, tin, barium, cobalt, and nickel atoms. The multivalent metal salt as claimed in Claim 4, wherein said at least one multivalent metal atom is a zinc atom. A process for the preparation of multivalent metal salt, which is one of the following processes (a), (b), or (c) (a) (i) Dispersing and dissolving at 0 - 100°C the copolymer according to Claims 1 -2 in an aqueous solution, alcohol solution, or water-alcohol solution which contains the hydroxide or carbonate or an alkoxide of an alkali metal in an amount equivalent to or more than the carboxyl groups in the copolymer, thereby obtaining a substantial solution of an alkali salt of the copolymer, and (ii) adding and reacting at 0 - 100°C a water-soluble multivalent metal salt in an amount of 0.5 - 1 gram equivalent based on the carboxyl groups in the copolymer either as is or in the form of an aqueous solution, alcohol solution, or water-alcohol solution to the substantial solution of the alkali salt of the copolymer, thereby obtaining the metal salt of the copolymer as a precipitate; (b) (i) heating, melting, and reacting at 100 - 180°C the copolymer according to Claims 1 - 2 and a multivalent metal salt of an organic carboxylic acid in such an amount that the multivalent metal salt contains the multivalent metal in an amount of 1 - 20 wt.% of the weight of the copolymer, optionally in the co-presence of an inorganic basic salt, and (ii) cooling the reaction mixture to obtain a molten mixture containing the metal salt of the copolymer; or (c) (i) heating, melting, and reacting the copolymer according to Claims 1 - 2, the carbonate, oxide, or hydroxide of a multivalent metal in such an amount that the carbonate, oxide, or hydroxide contains the multivalent metal in an amount of 1 - 20 wt.% of the weight of the copolymer and an organic basic salt in an amount equivalent to 1 - 15 wt.% of the weight of the copolymer, and (ii) cooling the reaction mixture to obtain a molten mixture containing the metal salt of the copolymer. The preparation process as claimed in Claim 7, wherein the multivalent metal is at least one metal selected from the group consisting of magnesium, aluminum, copper, zinc, tin, barium, cobalt, and nickel. The preparation process as claimed in Claim 7, wherein the multivalent metal is zinc. A color-developing agent for pressure-sensitive copying paper, which contains as an effective component the multivalent metal salt of the copolymer according to Claims 4 - 6. A color-developing sheet for pressure-sensitive copying paper, comprising the color-developing agent according to Claim 7 distributed evenly on at least one side of the color-developing sheet. A process for the fabrication of a color-developing paper sheet for pressure-sensitive copying paper, which comprises any one of the following steps (a) - (c): (a) coating a water-base coating formulation, which contains an aqueous suspension of the color-developing agent according to Claim 10, on a base paper web, (b) incorporating the color-developing agent according to Claim 10 in a base paper web upon production of the base paper web, and (c) coating a base paper web with a solution or suspension of the color-developing agent according to Claim 10 in an organic solvent.

Inventors: ASANO MAKOTO, TANABE YOSHIMITSU, YAMAGUCHI AKIHIRO, YAMAGUCHI KEIZABURO, ASANO, MAKOTO, TANABE, YOSHIMITSU, YAMAGUCHI, AKIHIRO, YAMAGUCHI, KEIZABURO
Applicants: MITSUI TOATSU CHEMICALS, MITSUI TOATSU CHEMICALS, INC.
Application year: 1987
Main classification: C08G61
IPC codes: C08G61, B41M5