RAW MATERIAL FOR THE PRODUCTION OF HEAT-RESISTANT RESIN, HEAT-RESISTANT RESIN AND PROCESS FOR THE PRODUCTION OF THE RESIN

Abstract: A raw material used for producing heat-resistant resins of the present invention contains N -cyclohexyl maleimide containing 0.001 to 1 weight percent of cyclohexylamino succinic anhydride, as a principal component. Heat-resistant resins and the process for producing the heat-resistant resins of the present invention are achieved by polymerizing the raw material and at least one kind of monomer which is copolymerizable with the raw material. Therefore, the heat-resistant resins produced from the raw material have excellent transparency, i.e., a low coloring factor, even when higher heat resistance is imparted to the heat-resistant resins by increasing the amount of the raw material containing N -cyclohexyl maleimide as a principal component. Thus, the heat-resistant resins are suitably used in fields requiring heat resistance as well as a low coloring factor.

Description: TECHNICAL FIELD The present invention relates to heat-resistant resins which are produced from a raw material and have excellent heat resistance, fine appearance and a low coloring factor, and a process for producing the heat-resistant resins. BACKGROUND ART Methacrylic resins made from methyl methacrylate as a principal component have excellent weatherproof and optical properties, and relatively balanced mechanical, thermal and fabrication properties. The methacrylic resins are used in various fields, such as automobile parts, electrical equipment parts, face plates, signs, covers for lights, ornaments, and variety goods, because of their properties mentioned above. However, the methacrylic resins are insufficient in terms of heat resistance, and therefore their use in the fields requiring shape stability at high temperatures is limited. Thus, there is a strong demand for an improvement of the heat resistance of the methacrylic resins. Many methods have been proposed for improving the heat resistance of the methacrylic resins. Examples of the proposed methods include the copolymerization of methyl methacrylate and N -aryl maleimide (Japanese Publication for Examined Patent Application, No. 9753/1968), and blending the copolymer of methyl methacrylate, α-methyl styrene and maleic anhydride with a methyl methacrylate copolymer (Japanese Publication for Unexamined Patent Application, No. 122536/1984). Moreover, as known methods for producing transparent methacrylic resins of improved heat resistance and reduced coloring factor, there are methods including copolymerizing methyl methacrylate and N -cyclohexylmaleimide within a specific range (Japanese Publication for Unexamined Patent Application, No. 156115/1987 and 177009/1987), and a method including copolymerizing methyl methacrylate and N-cyclohexyl maleimide and then reducing raw material monomers remaining in the resultant copolymer (Japanese Publication for Unexamined Patent Application, No. 112612/1987). By copolymerizing a methacrylic ester such as methyl methacrylate and N -substituted maleimide such as N -cyclohexylmaleimide, it is possible to achieve some improvements in respect of the heat resistance, transparency, and coloring factor, thereby providing a transparent heat-resistant resin for optical uses. However, in such a heat-resistant resin, the improvement of the heat resistance depends on the amount of N -substituted maleimide. Therefore, in order to obtain a heat-resistant resin of improved heat-resistance, it is necessary to increase the N -substituted maleimide unit in the heat-resistant resin. However, the increase of the N -substituted maleimide unit causes a problem that the resultant heat-resistant resin tends to color (yellow). In the case of N -substituted maleimide-based heat-resistant resins produced from N -substituted maleimide as one of the raw materials, the coloration of resin products is generally known. However, some uses of the resin products strongly require a reduction in the coloration of the resin products. Thus, there has been a demand for the development of N -substituted maleimide-based heat-resistant resins of reduced coloration (referred to as "low coloring factor" in the present invention). In the production of molded articles by molding the heat-resistant resins obtained by the above-mentioned methods, the molded articles sometimes had lowered surface smoothness, and showed deterioration in their optical properties including transparency, and appearance. The present inventor studied to find the cause of the lowering of the smoothness, and discovered that non-polymerizable impurities derived from the raw material in producing the N-substituted maleimide were the causes of such a lowering of the smoothness. Since most of such impurities have their melting points at temperatures lower than 200 °C, when a heat-resistant resin containing the impurities is molded, the impurities volatilize due to heat in molding, and adhere to the mold surfaces. As a result, the smoothness of the mold surfaces is impaired. The impairing of the smoothness of the mold surfaces deteriorates the smoothness of the surface of the molded article obtained by the molding, and worsens the appearance and transparency of the molded article. It was also discovered by the present inventor that such impurities adhere to an air pressure-reducing line and adhere as gum to an outlet of the heat-resistant resin in the process of producing a heat-resistant resin by copolymerizing the N -substituted maleimide and a monomer components containing, for example, methacrylic ester as a principal component, for example, in the solvent removing step of removing a solvent, etc. under conditions of heat and reduced pressure by an extruder, a flash tank, etc. after solution polymerization. Such adhesion of impurities may prevent stable production of heat-resistant resins. Examples of the impurities are non-radical polymerizable compounds having a functional group, such as carboxyl group, acid anhydride group, primary amino group and secondary amino group. According to Japanese Publication for Unexamined Patent Applications, No. 221974/1993, 201972/1993, and 135931/1994, when the resultant N -substituted maleimide is stored, these impurities cause a change in the color of the N -substituted maleimide with time. These publications also disclose the inclusion of another purifying step like water treatment in the production steps as a method for reducing the impurities. However, the conventional method disclosed in the above-mentioned publications can not easily eliminate disadvantages of the resultant heat-resistant resin and the production thereof which are caused by the impurities. The reason for such disadvantages is that this method additionally requires a special purifying step like water treatment in order to reduce the impurities, or has difficulty in reducing the impurities depending on the type of N -substituted maleimide. In the case where an increase in the heat-resistance of the resultant heat-resistant resin is required, it is necessary to increase the amount of N -substituted maleimide units. However, if a monomer mixture containing a large amount of N -substituted maleimide is polymerized, the heat-resistant resin tends to color during polymerization and heating in the molding process. Consequently, it is difficult to obtain heat-resistant resins of low coloring factors. Therefore, the use of antioxidant such as a phosphorus-based antioxidant and a phenol-based antioxidant was proposed as a method for reducing the coloring factor of a heat-resistant resin containing a large amount of N -substituted maleimide units in heating. For example, Japanese Publication for Unexamined Patent Application, No. 256551/1989 discloses a method for producing a styrene-methyl methacrylate-based resin composition by mixing a thioether-based organic sulfur stabilizer and a phosphite compound with a styrene-methyl methacrylate-based resin obtained by polymerizing monomer components containing methyl methacrylate, styrene and a maleimide compound, and further mixing a bisphenol-based antioxidant with the styrene-methyl methacrylate-based resin, if desired, for the purpose of improving the thermal stability. On the other hand, Japanese Publication for Unexamined Patent Application, No. 304045/1988 discloses the addition of an organic phosphorus-based compound to a methacrylate-based polymer obtained by copolymerizing methyl methacrylate and a maleimide compound as a method for producing a methacrylate-based resin composition of improved heat decomposition resistance. Additionally, this publication discloses the addition of an organic sulfur-based compound to the methacrylate-based polymer, and the addition of an organic sulfur-based compound as a chain transfer agent during polymerization based on the above-mentioned method. Japanese Publication for Unexamined Patent Application, No. 116331/1994 discloses a method for producing a transparent heat-resistance resin by polymerizing a composition formed by adding a phosphorus-based compound to a monomer mixture containing N -substituted maleimide for the purpose of reducing yellowing in the heating step. Thus, the above-mentioned publications describe the use of antioxidants, such as phosphorous-based compound and phenol-based compound, to achieve improvements in respect of the thermal stability and coloring factor. However, in the methods disclosed in Japanese Publication for Unexamined Patent Application, No. 256551/1989 and 304045/1988 above, since the antioxidant is not added during polymerization, i.e., the antioxidant is all added after the completion of the polymerization, the effect on a coloring source produced during the polymerization may not be fully exhibited. Furthermore, in the method disclosed in Japanese Publication for Unexamined Patent Application, No. 116331/1994 above, since the antioxidant is all added to the monomer mixture and then cast polymerization is performed, the polymerization of the monomer mixture is interfered by the antioxidant, and the conversion rate of the monomers to polymer may be lowered. Namely, the methods disclosed in the above-mentioned publications suffer from the problems that the efficiency of producing heat-resistant resins is low, and the heat-resistant resins tend to color more easily. In the method disclosed in Japanese Publication for Unexamined Patent Application, Nc. 304045/1988, even when the organic sulfur-based compound as the chain transfer agent is added during polymerization, a sufficient coloration reducing effect is not produced in the polymerization. In addition, for example, the following publications propose a method for reducing the coloration of heat-resistant resins containing N-substituted maleimide units during polymerization and molding. Japanese Publication for Unexamined Patent Application, No. 252211/1986 proposes reducing the residual maleimide by washing polymer beads obtained by suspension polymerization with an alcohol having 1 to 4 carbons. Japanese Publication for Unexamined Patent Application, No. 304013/1988 proposes polymerization using a specific initiator. Japanese Publication for Unexamined Patent Application, No. 310853/1993 proposes performing cast polymerization by adding a benzyl alcohol or substituted benzyl alcohol to the monomer mixture. However, these conventional methods suffer from the problems that the number of production steps is increased, and the coloration reducing effect is not sufficient. The method disclosed in Japanese Publication for Unexamined Patent Application, No. 310853/1993 is cast polymerization, and alcohol is not removed. In this method, if a large quantity of alcohol is used, a lowering of the heat resistance and the expansion of the heat-resistant resin in heating tend to occur due to the alcohol remaining in the resin. Thus, there is a possibility of impairing the appearance. Consequently, in this method, the amount of alcohol to be added is limited to a small amount not greater than 1 percent by weight of the total amount of monomers, for example, 0.5 percent by weight. As a result, a sufficient coloration reducing effect is not exhibited. As a method for reducing the coloration of heat-resistant resins containing N -substituted maleimide units during polymerization or molding, Japanese Publication for Unexamined Patent Application No. 234063/1987 discloses heat stabilization of N -cyclohexyl maleimide by adding maleamic acid, etc. to the N -cyclohexyl maleimide. In this method, suspension copolymerization of N -cyclohexyl maleimide and vinyl chloride is performed to produce a copolymer of a reduced coloring factor. However, in this conventional method, since the N -cyclohexyl maleimide to which the maleamic acid is added is copolymerized, a radical polymerization (polymerization reaction) of radical polymerizable maleamic acid and monomers of N -cyclohexyl maleimide, etc. proceeds. This may impair the transparency and heat-resistance of the resultant heat-resistant resin. It is an object of the present invention to provide a heat-resistant resin produced from the above-mentioned raw material, more particularly an N -substituted maleimide-based heat-resistant resin having excellent heat resistance, fine appearance including transparency, and a lower coloring factor, and to provide a process for producing such a heat-resistant resin. DISCLOSURE OF THE INVENTION The present inventor studied the coloration of N -cyclohexyl maleimide-based heat-resistant resins, and discovered that cyclohexylamino succinic anhydride which is present as an impurity in N -cyclohexyl maleimide as a raw material is a causative agent of coloration. The present invention was completed based on this discovery. More specifically, the raw material used for producing heat-resistant resins of the present invention contains N -cyclohexyl maleimide as a principal component, and the content of cyclohexylamino succinic anhydride is in the range from 0.001 to 1 weight percent (based on the N -cyclohexyl maleimide). The heat-resistant resin of the present invention is obtained by copolymerizing the above-mentioned raw material with at least one kind of monomer copolymerizable with the raw material. The yellowness (YI) of the heat-resistant resin is not higher than 2, or the yellowness (YIsol.) thereof in a solution is not higher than 3, preferably not higher than 2. The following description will discuss the present invention in detail. The raw material used for producing the heat-resistant resin of the present invention contains N -cyclohexyl maleimide as a principal component, and the content of cyclohexylamino succinic anhydride is adjusted in the range from 0.001 to 1 weight percent (based on the N -cyclohexyl maleimide), more preferably in the range from 0.01 to 0.5 weight percent. The heat-resistant resin of the present invention is obtained by copolymerizing the above-mentioned raw material with at least one kind of monomer copolymerizable with the raw material. The yellowness (YI) of the heat-resistant resin is not higher than 2, or the yellowness (YIsol.) thereof in a solution is not higher than 3, preferably not higher than 2. The N -cyclohexyl maleimide is a known substance, and the production method thereof is also well known. More specifically, N -cyclohexyl maleimide is easily obtained by the ring closure of N -cyclohexylmaleamic acid obtained by reacting, for example, maleic anhydride with cyclohexyl amine. However, it has not been known that the N-cyclohexyl maleimide thus obtained contains cyclohexylamino succinic anhydride as an impurity. Moreover, it was first discovered by the present inventor that the cyclohexylamino succinic anhydride is a causative agent of coloration of the N -cyclohexyl maleimide-based heat-resistant resin. The N -cyclohexylamino succinic anhydride is represented by general formula (1) below, and would be produced as a byproduct in producing N -cyclohexyl maleimide. When the cyclohexylamino succinic anhydride content exceeds 1 weight percent, the resultant N -cyclohexyl maleimide-based heat-resistant resin colors (see Comparative Examples 1 and 2). On the other hand, even when the cyclohexylamino succinic anhydride content is reduced to a value less than 0.001 weight percent, the reduction in coloration corresponding to the reduction in the cyclohexylamino succinic anhydride content is not observed. Considering the cost of distillation for removing the cyclohexylamino succinic anhydride, it is desirable to set the minimum content of the cyclohexylamino succinic anhydride to 0.001 weight percent. A raw material containing cyclohexylamino succinic anhydride in an amount ranging from 0.001 to 1 weight percent is easily obtained by distilling a raw material containing crude N-cyclohexyl maleimide as a principal component after the completion of reaction and recrystallizing the raw material using a solvent like acetone, or repeating these processes. In the present invention, the cyclohexylamino succinic content was measured as follows. Cyclohexylamino succinic anhydride content The cyclohexylamino succinic anhydride content was measured using a high-speed chromatography (LC-10A) of Shimadzu Corporation according to the method explained in the Example section given later. In the present invention, the term "heat-resistant resin" refers to a resin which is produced from the above-mentioned raw material, has a glass transition temperature of not lower than 120 °C, and yellowness (YI) of not higher than 2 or yellowness in solution (YIsol.) of not higher than 3, preferably not higher than 2, more preferably not higher than 1 when measured by the methods described below. Measurement of glass transition temperature The glass transition temperature of the resultant copolymer was calculated from the midpoint of a DSC (differential scanning calorimetry) curve measured under atmosphere of nitrogen gas from room temperature to 220 °C at a heating rate of 10 °C per minute using α-alumina as a reference by a differential scanning calorimeter (DSC-8230 of Rigaku Denki K.K.) according to JIS(Japanese Industrial Standards)-K-7121. Yellownesses (YI and YIsol.) The yellowness (YI) of a 3.0-mm thick polymer plate formed by the heat-resistant resin of the present invention by the cast polymerization described in Example 1 given below, and of a 3.0-mm thick copolymer molded article produced from the heat-resistant resin by the method described in Example 3 below were measured using a color difference meter (Σ80 of Nippon Denshoku Kogyo) according to JIS-K-7103. 15 weight percent of chloroform solution of a copolymer pellet produced from the heat-resistant resin of the present invention by the method described in Example 3 below was prepared. The chloroform solution was placed on a 10-mm thick glass cell, and its tristimulus values were measured using transmitted light according to JIS-K-7103. The yellowness (YIsol.) in solution was measured based on the tristimulus values. Examples of monomers which are copolymerizable with the raw material containing N -cyclohexyl maleimide as a principal component used for producing heat-resistant resins of a low coloring factor of the present invention include methacrylic esters, such as methyl methacrylate; acrylic esters such as methyl acrylate; and aromatic vinyl compounds such as styrene and α-methyl styrene. The methyl methacrylate, methyl acrylate, styrene, α-methyl styrene are particularly suitable because they have excellent reactivity and give resins of high heat resistance. Preferred methacrylic esters are those having either an alkyl group, cyclohexyl group or benzyl group containing 1 to 18 carbons. Examples of such methacrylic esters include methyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, amyl methacrylate, isoamyl methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate, decyl methacrylate, lauryl methacrylate, cyclohexyl methacrylate, benzyl methacrylate, 2-phenoxyethyl methacrylate, 3-phenylpropyl methacrylate, and 2-hydroxyethyl methacrylate. Among these compounds, the methyl methacrylate is particularly preferred. It is possible to use only one kind or a combination of more than one kind of such methacrylic esters. One of the heat-resistant resins with a low coloring factor of the present invention is a bicomponents-copolymer of the above-mentioned raw material and copolymerizable monomer. Typical examples of such a bicomponents-copolymer include the copolymer of the raw material and methyl methacrylate, the copolymer of the raw material and cyclohexyl methacrylate, and the copolymer of the raw material and isobornyl methacrylate. The polymerization of such bicomponents-copolymers is not particularly limited in its method, and can be performed according to a conventional known method. For example, when copolymerizing the raw material and methyl methacrylate, a radical polymerization of 5 to 50 weight parts of the raw material and 95 to 50 weight parts of methyl methacrylate (100 weight parts in total) is performed using a radical polymerization initiator to be described later by the conventional procedure. As another heat-resistant resin with a low coloring factor of the present invention, a tricomponents-copolymer or polycomponents-copolymer obtained by copolymerizing the raw material, methacrylic ester (particularly, methyl methacrylate), and at least one kind of monomer selected from the above-mentioned copolymerizable monomers. Typical examples thereof include the copolymer of the raw material, methyl methacrylate and α-methyl styrene, the copolymer of the raw material, methyl methacrylate and styrene, and the copolymer of the raw material, methyl methacrylate and methyl acrylate. Like the polymerization of bicomponents-copolymer, the polymerization of such a tricomponents- or polycomponents-copolymer is not particularly limited in its method, and can be performed by a conventional method. For example, in the case of the copolymer of the raw material, methyl methacrylate and styrene, a radical polymerization of 5 to 50 weight parts of the raw material, 95 to 50 weight parts of methyl methacrylate, styrene in an amount of not higher than 30 weight parts based on 100 weight parts in total of the raw material and methyl methacrylate is performed using radical polymerization initiator to be described later according to the conventional procedure. Another process for producing a heat-resistant resin of the present invention is characterized by copolymerizing a maleimide-based monomer (maleimide-based polymerizable monomer) whose yellowness in solution is not higher than 1.0 and a polymerizable monomer which is copolymerizable with the maleimide-based monomer. Examples of such a maleimide-based monomer include N -cyclohexyl maleimide, N -phenyl maleimide, N -methyl maleimide, N -ethyl maleimide, N -isopropyl maleimide, N -t-butyl maleimide, N -tribromophenyl maleimide, N -lauryl maleimide, and N -benzyl maleimide. Considering the industrial values, such as the transparency, low coloring factor and heat resistance of the resultant heat-resistant resin, the maleimide-based monomer is preferably N -phenyl maleimide or N -cyclohexyl maleimide, and more preferably N -cyclohexyl maleimide. If N -tribromophenyl maleimide is used as the maleimide-based monomer, it is possible to impart fire retardancy to the resultant heat-resistant resin as well as transparency and heat resistance. It is possible to use only one kind, or a combination of more than one kind of the above-exemplified maleimide-based monomers. As the polymerizable monomer which is copolymerizable with the above-mentioned maleimide-based monomer, it is desirable to use the one containing methacrylic ester like methyl methacrylate as a principal component because it can impart the transparency. In this method, it is possible to improve the heat resistance of the heat-resistant resins by using a large amount of maleimide-based monomer. Moreover, even when such a large amount of maleimide-based monomer is used, if a maleimide-based monomer whose yellowness in solution is not higher than 1.0 is used, it is possible to reduce the coloration of the resultant heat-resistant resin, thereby achieving stable production of heat-resistant resins having excellent transparency and a low coloring factor, whose yellowness in solution is not higher than 4. It is possible to use only a maleimide-based monomer, or a mixture of a maleimide-based monomer and a polymerizable monomer which is copolymerizable with the maleimide-based polymerizable monomer (hereinafter referred to as the "monomer mixture") as the above-mentioned monomer components containing a maleimide-based monomer. The above-mentioned compounds can be used as the above-mentioned maleimide-based monomer. Considering the industrial values, such as the transparency, low coloring factor and heat resistance, of the resultant heat-resistant resins, N -phenyl maleimide and N -cyclohexyl maleimide are preferred, and N -cyclohexyl maleimide is more preferred. In addition, if N -tribromophenyl maleimide is used, it is possible to impart flame retardancy to the resultant heat-resistant resins as well as the transparency and heat resistance. It is possible to use only one kind or a mixture of more than one kind of the above-exemplified maleimide-based monomers. As the copolymerizable polymerizable monomer (hereinafter referred to as the "copolymerizable monomer"), it is desirable to use a compound having an unsaturated bonding copolymerizable with the maleimide-based monomer. Examples of such a compound are the above-mentioned methacrylic ester, and other monomers which are copolymerizable with both the maleimide-based monomer the methacrylic ester. It is possible to use one kind or more than one kind of the above-mentioned other monomers. As the copolymerizable monomer, it is preferred to use methacrylic ester, and it is more preferred to use the methacrylic ester together with an aromatic vinyl or acrylic ester in order to obtain highly transparent heat-resistant resins. Further, the polymerizable monomer preferably contains methacrylic ester as a principal component. The ratio of the maleimide-based monomer to the monomer components (maleimide-based monomer or monomer mixture) is 10 to 60 weight percent, and more preferably 15 to 50 weight percent. If the ratio of the maleimide-based monomer is less than 10 weight percent, there is a possibility of lowering the heat resistance of the heat-resistant resins. On the other hand, if the ratio of the maleimide-based monomer is more than 60 weight percent, since the ratio of the copolymerizable monomer as other monomer components inevitably becomes less than 40 weight percent, there is a possibility of lowering of the physical properties, such as the transparency, of the resultant heat-resistant resin. As a method of polymerizing the above-mentioned monomer components, it is possible to use polymerization methods using a solvent, such as solution polymerization, bulk polymerization, and suspension-bulk polymerization. Among these polymerization methods, solution polymerization is particularly suitable because it allows easy control of the composition and molecular weight of the heat-resistant resin. Next, the following description will discuss in detail the monomer mixture used for the production of the heat-resistant resins. As the above-mentioned polymer mixture, it is desirable to use a mixture containing 10 to 60 weight percent, more preferably 15 to 50 weight percent of maleimide-based monomer, and 90 to 40 weight percent, more preferably 85 to 50 weight percent of methacrylic ester, and, if necessary, 0 to 20 weight percent of other monomer copolymerizable with the maleimide-based monomer and the methacrylic ester. Such a loading allows stable production of highly transparent heat-resistant resins while maintaining excellent optical properties and moldability. In this specification, the expression "weight percent" means that the monomer components are loaded so that the total thereof always becomes 100 weight percent. For instance, when 15 weight percent of the maleimide-based monomer is loaded, the loading of methacrylic ester is readily set at 85 weight percent. The ratio of the methacrylic ester to the monomer mixture is preferably in the range from 40 to 90 weight percent. If the ratio of the maleimide-based monomer is less than 40 weight percent, there is a possibility that the excellent properties of methacrylic ester, such as the transparency, deteriorate in the resultant heat-resistant resins. On the other hand, if the ratio of the methacrylic ester is more than 90 weight percent, since the ratio of the maleimide-based monomer inevitably becomes less than 10 weight percent, there is a possibility that the resultant heat-resistant resins show lowered heat resistance. The above-mentioned monomer mixture may contain other monomers, if necessary. It is desirable to use compounds copolymerizable with the maleimide-based monomer and the methacrylic ester as the other monomers. Examples of the other monomers include: aromatic vinyls; unsaturated nitriles; acrylic esters; olefins; dienes; vinyl ethers; vinyl esters; vinyl fluorides; allyl esters or methacrylic esters of saturated aliphatic monocarboxylic acids such as allyl propionate; poly(meth)acrylates; polyallylates; glycidyl compounds; and unsaturated carboxylic acids. Among these exemplified compounds, aromatic vinyls are particularly preferred. Examples of the aromatic vinyls include styrene, α-methylstyrene, paramethylstyrene, isopropenyl styrene, vinyl toluene, and chlorostyrene. Among these aromatic vinyls, styrene is particularly preferred. Examples of the unsaturated nitriles include acrylonitrile, methacrylonitrile, ethacrylonitrile, and phenyl acrylonitrile. As the above-mentioned acrylic esters, an acrylic ester having at least one group selected from the group consisting of alkyl, cyclohexyl and benzyl groups having 1 to 18 carbons. Examples of the acrylic esters include methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, butyl acrylate, isobutyl acrylate, t -butyl acrylate, amyl acrylate, isoamyl acrylate, octyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, lauryl acrylate, cyclohexyl acrylate, benzyl acrylate, and 2-hydroxylethyl acrylate. Examples of the above-mentioned olefins include ethylene, propylene, isobutylene, and diisobutylene. Examples of the dienes are buthadiene, and isoprene. Examples of the vinyl ethers are methyl vinyl ether, and butyl vinyl ether. Examples of the vinyl esters are vinyl acetate, and vinyl propionate. One example of the vinyl fluoride is vinylidene fluoride. Examples of the above-mentioned poly(meth)acrylates include ethylene glycol di(meth)acrylate, diethylene glycol (meth)acrylate, divinyl benzene, diallyl phthalate, trimethylolpropane tri(meth)acrylate, hexanediol di(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, di(meth)acrylate of bisphenol A-ethylene oxide or propylene oxide addition product, di(meth)acrylate of halogenated bisphenol A-ethylene oxide or propylene oxide addition product, and di or tri(meth)acrylate of isocyanurate-ethylene oxide or propylene oxide addition product. One example of the above-mentioned polyallylates is triallyl isocyanurate. Examples of the above-mentioned glycidyl compounds are glycidyl (meth)acrylate, and allyl glycidyl ether. Examples of the above-mentioned unsaturated carboxylic acids include acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, and their semi-esterified compounds or anhydrides. It is possible to use only one kind or a mixture of more than one kind of the exemplified compounds as the other monomers. The ratio of the other monomers to the monomer mixture is preferably in the range from 0 to 20 weight percent. The ratio of the aromatic vinyls to the monomer mixture is preferably less than 15 weight percent. If the ratio of the other monomers to the monomer mixture is more than 20 weight percent, or if the ratio of the aromatic vinyls to the monomer mixture is not less than 15 weight percent, there is a possibility of lowering the transparency, heat resistance and weather resistance of the resultant heat-resistant resins. The above-mentioned invention will be explained in detail below. A process for producing heat-resistant resins of the present invention includes polymerizing the above-mentioned monomer mixture in the presence of a non-radical polymerizable acid anhydride and/or non-radical polymerizable carboxylic acid. Alternatively, a process of producing a heat-resistant resin of the present invention includes adding a non-radical polymerizable acid- anhydride and/or non-radical polymerizable carboxylic acid after polymerizing the above-mentioned monomer mixture. As the copolymerizable monomer, it is preferred to use methacrylic ester in order to obtain a highly transparent heat-resistant resin. It is more preferred to use aromatic vinyls and acrylic esters together. The principal component of the copolymerizable monomer is desirably methacrylic ester. As a method of polymerizing the monomer mixture, solution polymerization, bulk polymerization, suspension polymerization, emulsion polymerization, etc. can be used. The solution polymerization is particularly suitable because it can easily control the composition and molecular weight of the heat-resistant resin. On the other hand, in a method in which the removal of solvent is carried out by performing solution polymerization or bulk polymerization, it is possible to further enhance the effect of reducing the coloration of the heat-resistant resin. When performing polymerization using a solvent, it is desirable to use the above-mentioned common organic solvents. The amount of solvent is not particularly limited. Moreover, when initiating the polymerization of the monomer mixture, a polymerization initiator is added. Examples of the polymerization initiator include organic peroxides, such as benzoyl peroxide, lauroyl peroxide, methyl ethyl ketone peroxide, t -butyl peroxy octoate, t -butyl peroxy benzoate, t -butyl peroxy-2-ethyl hexanoate, t -amyl peroxy-2-ethyl hexanoate, cumene hydroperoxide, cyclohexanone peroxide, dicumyl peroxide, bis(4- t -butyl cyclohexyl) peroxy dicarbonate, and t -butyl peroxy isopropyl carbonate; and azo compounds, such as 2,2'-azobisisobutyronitrile, and 2-phenyl azo-2,4-dimethyl-4-methoxy valero nitrile. It is possible to use only one kind, or a mixture of more than one kind of these polymerization initiators. The amount of the polymerization initiator added to the monomer mixture is not particularly limited. For the control of the molecular weight of copolymer, it is possible to use a chain transfer agent, for example, alkyl mercaptans such as butyl mercaptan and octyl mercaptan, and α-styrene dimer. In the production process of the present invention, it is possible to add various types of additives, for example, known additives like ultraviolet light absorbers and stabilizers, during or after the polymerization. The heat-resistant resins produced by the production process of the present invention can be used as they are, or blended with other synthetic resins, for example, polymethylmethacrylate resin, styrene-methyl methacrylate resin, ABS resin, AES resin, MBS resin, polystyrene, and PPE resin, using an extruder, etc. Since the heat-resistant resins of the present invention have excellent heat resistance and transparency, and a low coloring factor, they are suitable for use as material of parts which require not only heat resistance and transparency but also beautiful appearance, such as a base of optical material like optical disks, automobile parts, covers for lights, and electrical equipment parts. BRIEF EXPLANATION OF THE DRAWING Fig. 1 is a graph showing a change with time in the coloration of N -cyclohexyl maleimide used in a process for producing heat-resistant resins of the present invention, measured by an accelerated heating test. THE BEST MODE FOR CARRYING OUT THE INVENTION The following description will explain the present invention in detail by presenting some examples. The terms "part" and "percent" in the following description means "part by weight" and "percent by weight", respectively. A raw material used for producing a heat-resistant resin of the present invention contains N -cyclohexyl maleimide unit, as a principal component, containing 0.001 to 1 percent of cyclohexylamino succinic anhydride (based on the N -cyclohexyl maleimide). The identification and determination of the structure of the cyclohexylamino succinic anhydride were performed as follows. Identification A compound in the N-cyclohexyl maleimide was fractionated using high-speed liquid chromatography (LC-10A of Shimadzu Corporation). The fractionated liquid was dried under vacuum at 30 °C. The resultant solids content was recrystallized with a mixed solvent containing chloroform and hexane in a ratio of 1 to 1, and then dried. Subsequently, the solids content was measured with an infrared spectrophotometer (Japan Biorat SPC-3200), a super-conductive Fourier transform nuclear magnetic resonance spectrometer (Varian VXR-300S), a gas chromatograph mass spectrometer (JMC-AX505WA of JEOL Ltd.), and a melting point measuring apparatus (Buch 501K). The spectral values and analytic values are as follows. IR(KBr) : ν(cm -1 ) = 3400s, 2930s, 1695 vs, 1390s, 1195s, 1120s, 735m, 590m 1 H-NMR(300MHz, DMSO-d 6 ) : δ(ppm) = 6.05(1H, J = 6.0Hz, d), 4.40(1H, m), 3.80(1H, m), 2.95(1H, J = 18, J = 7.2, dd), 2.40(1H, J = 18, J = 5, dd), 2.00(2H, J = 12.7, g), 1.80(2H, J = 15, d), 1.6(3H, m), 1.20(3H, m) 13 C-NMR(300MHz, DMSO-d 6 ) : δ(ppm) = 178.13(1C), 174.8(1C), 65.8(1C), 50.4(1C), 37.6(1C), 28.6(1C), 28.3(1C), 25.4(2C), 24.9(1C) GC-MS = 197* Melting point = 155.5 to 156.5 °C. Quantification After preparing a working curve of cyclohexylamino succinic anhydride using the high-speed liquid chromatography (LC-10A of Shimadzu Corporation), the content of the cyclohexylamino succinic anhydride in the N-cyclohexylmaleimide was measured under the following conditions. Column: Shimadzu Zorbax-ODS (4.6 mmΦ × 25 cm) Mobile Phase: Mixed Solution of aqueous solution containing 0.005 mol potassium dihydrogen phosphate/liter and methanol in a ratio of 1:0.818 by weight Column Temperature: 45 °C Flow Rate: 0.8 mℓ/min. Detector: UV220nm (6.55A, Hitachi, Ltd). Preparation of Sample: 0.4 g of sample was precisely measured in the order of milligrams, dissolved in the mobile phase in a 50 mℓ measuring flask to 50 mℓ, and then 20 µℓ of the sample solution was injected into the column for analysis. [Reference Example 1] Maleic anhydride and an amount of ortho-xylene which was five times by weight per unit weight of the maleic anhydride were placed in a flask provided with a thermometer, a cooling pipe having a water separator, a dropping funnel and an agitator, and dissolved at 56 °C so as to prepare an ortho-xylene solution in which maleic anhydride was evenly dissolved. Thereafter, a solution containing cyclohexylamine in an amount which is the same as the maleic anhydride by a molar ratio and ortho-xylene in an amount which is the same as the cyclohexylamine by weight was all dropped into the maleic anhydride solution in 0.5 hours while agitating the solution at 56 °C. As a result, a slurry of ortho-xylene of N-cyclohexyl maleic amido acid was synthesized. Next, ortho-phosphoric acid as an acid catalyst in an amount as same as the cyclohexyl amine by weight, and 200 ppm of copper dibutyldithiocarbamate as a polymerization inhibitor to the N-cyclohexyl maleic amido acid were added to the slurry. The mixture was heated and maintained at 140 °C while agitating the mixture, and reacted for 7 hours while distilling off water which was produced by the reaction and ortho-xylene from the reaction system. After the completion of the reaction, a layer of the acid catalyst separated as a lower layer from the reaction solution at 140 °C was removed. Subsequently, the reaction solution was heated to 60 °C, and an amount of water which was the same as the organic layer by weight was added and washed for 30 minutes while agitating the mixture. Then, the organic layer was separated from the water layer. Thereafter, ortho-xylene was removed from the organic layer under reduced pressure of 10 mmHg. Further, by performing simple distillation under reduced pressure of 5 mmHg at a bulk temperature in the range from 130 to 150 °C, a raw material containing white N-cyclohexylmaleimide as a principal component was obtained. The content of cyclohexylamino succinic anhydride in the raw material thus obtained was measured to 2.5 percent. In the following Examples of the present invention and Comparative Examples, the content of cyclohexylamino succinic anhydride in the raw material containing N -cyclohexyl maleimide as a principal component was adjusted by changing the degree of purification performed by distillation and recrystallization. [Example 1] A monomer mixture was prepared by mixing 20 parts of a raw material formed by N-cyclohexylmaleimide containing 0.9 percent of cyclohexylamino succinic anhydride and 80 parts of methyl methacrylate, and deaerated under reduced pressure of 200 mmHg for 10 minutes. 0.4 parts of lauryl peroxide as a polymerization initiator was dissolved in the monomer mixture, and molding is performed by cast polymerization as follows. Cast Polymerization A solution was prepared by blowing a nitrogen gas into the monomer mixture and sufficiently saturating the monomer mixture. The solution was gently poured into a frame formed by fixing glass plates (240 × 180 × 5 mm) with a soft vinyl tube (7 × 4 mm) so that the space between the glass plates was 4 mm, and heated in a water bath at 55 °C for 20 hours. Then, a transparent polymer plate (1) with a thickness of 3.0 mm was obtained. The yellowness (YI) of the polymer plate (1) obtained by the above-mentioned cast polymerization was measured with a color difference meter (Σ80 of Nippon Denshoku Kogyo) according to JIS-K-7103. It was found that the yellowness (YI) of the polymer plate (1) was 0.8. Moreover, the glass transition temperature of the polymer plate (1) was measured with a thermal analyzer (DSC-8230/TAS-100 of Rigaku Denki K.K.) according to JIS K-7121. It was found that the glass transition temperature of the polymer plate (1) was 137 °C. [Example 2] A monomer mixture was prepared by mixing 20 parts of a raw material formed by N -cyclohexylmaleimide containing 0.01 percent of cyclohexylamino succinic anhydride and 80 parts of methyl methacrylate, and deaerated under reduced pressure of 200 mmHg for 10 minutes. 0.4 parts of lauryl peroxide as a polymerization initiator was dissolved in the monomer mixture. A transparent polymer plate (2) with a thickness of 3.0 mm was obtained by performing cast polymerization under the same conditions as in Example 1. The yellowness (YI) and glass transition temperature of the polymer plate (2) were 0.8 and 137 °C, respectively. It was found from the results of Examples 1 and 2 that a N -cyclohexyl maleimide-methyl methacrylate copolymer produced from a raw material formed by N -cyclohexyl maleimide containing cyclohexylamino succinic anhydride in an amount of not higher than 1 percent is transparent, has a low coloring factor as given by yellowness (YI) of not higher than 1, and excellent heat resistance as indicated by a glass transition temperature of not lower than 120 °C. [Comparative Example 1] A monomer mixture was prepared by mixing 20 parts of a comparative raw material formed by N -cyclohexylmaleimide containing 1.8 percent of cyclohexylamino succinic anhydride and 80 parts of methyl methacrylate, and deaerated under reduced pressure of 200 mmHg for 10 minutes. 0.4 parts of lauryl peroxide as a polymerization initiator was dissolved in the monomer mixture. A transparent comparative polymer plate (1) with a thickness of 3.0 mm was obtained by performing cast polymerization under the same conditions as in Example 1. The yellowness (YI) and glass transition temperature of the comparative polymer plate (1) were measured in the same manner as in Example 1. The yellowness (YI) and glass transition temperature were 2.4 and 138 °C, respectively. [Comparative Example 2] A monomer mixture was prepared by mixing 20 parts of a comparative raw material formed by N -cyclohexylmaleimide containing 2.5 percent of cyclohexylamino succinic anhydride and 80 parts of methyl methacrylate, and deaerated under reduced pressure of 200 mmHg for 10 minutes. 0.4 parts of lauryl peroxide as a polymerization initiator was dissolved in the monomer mixture. A transparent comparative polymer plate (2) with a thickness of 3.0 mm was obtained by performing cast polymerization under the same conditions as in Example 1. The yellowness (YI) and glass transition temperature of the comparative polymer plate (2) were measured in the same manner as in Example 1. The yellowness (YI) and glass transition temperature were 4.0 and 137 °C, respectively. It was found from the results of Comparative Examples 1 and 2 that an N -cyclohexyl maleimide-methyl methacrylate copolymer produced from the raw material formed by N -cyclohexyl maleimide containing cyclohexylamino succinic anhydride in an amount higher than 1 percent is transparent and has excellent heat resistance, but is highly colored (yellowed). [Comparative Example 3] 0.4 parts of lauryl peroxide as a polymerization initiator was dissolved in 100 parts of methyl methacrylate which was deaerated under reduced pressure of 200 mmHg for 10 minutes. A transparent comparative polymer plate (3) with a thickness of 3.0 mm was obtained by performing cast polymerization under the same conditions as in Example 1. The yellowness (YI) and glass transition temperature of the comparative polymer plate (3) were measured in the same manner as in Example 1. The yellowness (YI) and glass transition temperature were 0.8 and 114 °C, respectively. It was found from the results of Comparative Example 3 that the comparative polymer plate (3) as a methyl methacrylate polymer obtained by solely polymerizing methyl methacrylate without using N -cyclohexyl maleimide is transparent and has a low coloring factor. However, the comparative polymer plate (3) had lower heat resistance because its glass transition temperature was 114 °C. [Example 3] 15.75 parts of methyl methacrylate, 6.25 parts of a raw material formed by N -cyclohexyl maleimide containing 0.01 percent of cyclohexylamino succinic anhydride, 25 parts of toluene, and 0.0025 parts of pentaerythrityltetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] as a stabilizer were placed in a 20-liter stainless polymerizing tank having an agitator, and three dropping vessels, namely first, second and third dropping vessels. On the other hand, a first mixed solution containing 15.75 parts of methyl methacrylate, 6 parts of styrene and 10 parts of toluene was placed in the first dropping vessel, and the dissolved oxygen was removed from the first mixed solution by performing bubbling with nitrogen gas in advance. Moreover, a second mixed solution containing 6.25 parts of the raw material formed by N -cyclohexyl maleimide containing 0.01 percent of cyclohexylamino succinic anhydride, and 10 parts of toluene was placed in the second dropping vessel, and the dissolved oxygen was removed from the second mixed solution by performing bubbling with nitrogen gas in advance. Furthermore, a third mixed solution containing 0.108 parts of tert -butyl peroxy isopropyl carbonate and 5 parts of toluene was placed in the third dropping vessel, and the dissolved oxygen was removed from the third mixed solution by performing bubbling with nitrogen gas in advance. Bubbling of the solution in the polymerizing tank was performed for 10 minutes using nitrogen gas while agitating the solution at 300 rpm so as to remove the dissolved oxygen from the solution by nitrogen gas substitution. Thereafter, heating of the solution was initiated under the atmosphere of nitrogen gas. When the solution reached 110 °C, 0.02 parts of tert-butyl peroxy isopropyl carbonate as a polymerization initiator was added to the solution. Subsequently, the first, second and third mixed solutions were dropped into the solution in the polymerizing tank from the first, second and third dropping vessels, respectively, over 3.5 hours, and a polymerization was performed under refluxing at a polymerization temperature of 110 °C for 7 hours. Thereafter, 0.0475 parts of the above-mentioned stabilizer was further added to the solution in the polymerizing tank. The polymeric liquid containing the copolymer in the polymerizing tank was supplied to a 30-mm twin screw extruder with a vent port and cylinder temperature being controlled at 240 °C, and the solvent in the polymeric liquid was removed under vacuum through the vent port. The strand from the vent port was made into a pellet to produce a copolymer pellet (3) as a heat-resistant resin of the present invention. The yellowness (YIsol.) of the copolymer pellet (3) in a solution was 0.8. Subsequently, the copolymer pellet (3) was molded using an injection molding machine which was controlled so as to have a cylinder temperature in the range from 250 to 260 °C, a mold temperature of 100 °C, and an injection pressure of 950 kg/cm 2 . As a result, a transparent molded article with a thickness of 3 mm was obtained. The yellowness (YI) and glass transition temperature of the molded article were measured in the same manner as in Example 1. The yellowness (YI) and glass transition temperature were 1.6 and 136 °C, respectively. It was found from the results of Example 3 that the N -cyclohexyl maleimide-methyl methacrylate copolymer produced from the raw material formed by N -cyclohexyl maleimide containing cyclohexylamino succinic anhydride in an amount of not higher than 1 percent has a low coloring factor and excellent heat resistance because the yellowness (YI) of the molded article was 1.6 and the glass transition temperature was not lower than 120 °C. [Comparative Example 4] A comparative copolymer pellet (4) was obtained in the same manner as in Example 3 except that a comparative raw material formed by N -cyclohexyl maleimide containing 1.8 percent of cyclohexylamino succinic anhydride was used instead of the raw material formed by N -cyclohexyl maleimide containing 0.01 percent of cyclohexylamino succinic anhydride used in Example 3. The yellowness (YIsol.) of the comparative copolymer pellet (4) in a solution was 4.1. Subsequently, the comparative copolymer pellet (4) was molded in the same manner as in Example 3 so as to produce a transparent molded article with a thickness of 3 mm. The yellowness (YI) and glass transition temperature of the molded article were measured in the same manner as in Example 1. The yellowness (YI) and glass transition temperature were 6.4 and 136 °C, respectively. It was found from the results of Comparative Example 4 that the N -cyclohexyl maleimide-methyl methacrylate copolymer produced from the comparative raw material formed by N -cyclohexyl maleimide containing cyclohexylamino succinic anhydride in an amount higher than 1 percent is transparent and has excellent heat resistance, but is highly colored (yellowed). It is clear from Examples 1 to 3 and Comparative Examples 1 to 4 that since the raw materials used for producing heat-resistant resins of the present invention contains N -cyclohexyl maleimide as a principal component, and 0.001 to 1 weight percent of cyclohexylamino succinic anhydride (based on the N -cyclohexyl maleimide), the heat-resistant resins of the present invention produced by copolymerizing the raw materials and at least one monomer copolymerizable with the raw materials contains N -cyclohexyl maleimide unit as a heat-resistance imparting component. Therefore, the resultant N -cyclohexyl maleimide-based resins have excellent heat resistance as shown by a glass transition temperate of not lower than 120 °C. Moreover, since the heat-resistant resins are produced using the raw materials formed by N -cyclohexyl maleimide containing 0.001 to 1 weight percent of cyclohexylamino succinic anhydride, they have a low coloring factor, and yellowness (YI) of not higher than 2, or yellowness in a solution (YIsol.) of not higher than 3, preferably not higher than 2. In order to produce a heat-resistant resin with higher heat resistance, namely, with a glass transition temperature of not lower than 120 °C, even if the ratio of the N -cyclohexyl maleimide unit is increased, the resultant heat-resistant resin can have a reduced coloring factor and excellent transparency. Thus, by preparing a raw material containing N -cyclohexyl maleimide as a principal component and controlling the content of cyclohexylamino succinic anhydride within the range from 0.001 to 1 weight percent (based on N -cyclohexyl maleimide), it is possible to reduce the coloration of the heat-resistant resins produced from the raw material. Moreover, since the heat-resistant resins produced from the above-mentioned raw material has excellent transparency and heat resistance, and a lower coloring factor, it is suitably used in various fields, such as automobile parts, electrical equipment parts, face plates, signs, covers for lights, ornaments, and variety goods. [Example 4] 15.75 parts of methyl methacrylate, 6.25 parts of N -cyclohexyl maleimide (a), 25 parts of toluene, and 0.0025 parts of pentaerythrityl-tetrakis[3-(3,5-di- t -butyl-4-hydroxyphenyl)propionate] were placed in the polymerizing tank described in Example 1. On the other hand, a first mixed solution containing 15.75 parts of methyl methacrylate, 6 parts of styrene and 10 parts of toluene was placed in the first dropping vessel, and the dissolved oxygen was removed from the first mixed solution by performing bubbling with nitrogen gas in advance. Moreover, a second mixed solution containing 6.25 parts of the N -cyclohexyl maleimide (a) and 10 parts of toluene was placed in the second dropping vessel, and the dissolved oxygen was removed from the second mixed solution by performing bubbling with nitrogen gas in advance. Furthermore, a third mixed solution containing 0.108 parts of t -butyl peroxy isopropyl carbonate and 5 parts of toluene was placed in the third dropping vessel, and the dissolved oxygen was removed from the third mixed solution by performing bubbling with nitrogen gas in advance. Bubbling of the solution in the polymerizing tank using nitrogen gas was performed for 10 minutes while agitating the solution at 300 rpm so as to remove the dissolved oxygen from the solution by nitrogen gas substitution. Thereafter, heating of the solution was initiated under the atmosphere of nitrogen gas. When the solution reached 110 °C, 0.02 parts of t -butyl peroxy isopropyl carbonate as a polymerization initiator was added to the solution. Subsequently, the first, second and third mixed solutions were dropped into the solution in the polymerizing tank from the first, second and third dropping vessels, respectively, over 3.5 hours, and a polymerization was performed under refluxing at a polymerization temperature of 110 °C for 7 hours. Thereafter, 0.0475 parts of the above-mentioned stabilizer was added to the solution in the polymerizing tank. The polymeric liquid containing the copolymer in the polymerizing tank was supplied to a 30-mm twin screw extruder with a vent port and cylinder temperature being controlled at 240 °C, and the solvent in the polymeric liquid was removed under vacuum through the vent port. The strand of the polymer from the vent port was made into a pellet to produce a polymer (4) as a heat-resistant resin of the present invention. The N -cyclohexyl maleimide (a) used in Example 4 was used immediately after being purified by distillation. The YIsol. of the N -cyclohexyl maleimide (a) was 0.1 when measured by the above-mentioned measuring method. Moreover, the YIsol. of all of other raw materials, like the methyl methacrylate, etc. was not higher than 0.1. The physical properties of the polymer (4) were measured according to the above-mentioned measuring methods. The YIsol. and glass transition temperature of the polymer (4) were 3.1 and 136 °C, respectively. The results are shown in Table 1. YIsol. of raw material CHMI (-) YIsol. of polymer (-) Glass transition temperature (°C) Example 4 0.1 3.1 136 Example 5 0.2 3.2 135 Comparative Example 4 1.1 4.4 136 Comparative Example 5 1.5 5.9 136 CHMI is N -cyclohexyl maleimide [Example 5] N -cyclohexyl maleimide (b) having YIsol of 0.2 was prepared by the accelerated heating test as shown in Fig. 1. A polymer (5) of Example 5 was produced in the same manner as in Example 4 except that the N -cyclohexyl maleimide (b) was used instead of the N -cyclohexyl maleimide (a) with YIsol. of 0.1 of Example 4. The physical properties of the polymer (5) were measured according to the above-mentioned measuring methods. The YIsol. and glass transition temperature of the polymer (5) were 3.2 and 135 °C, respectively. The results are shown in Table 1. In the accelerated heating test, the N -cyclohexyl maleimide which was just produced was left at rest in an oven at 86 °C, and the YIsol. of the N -cyclohexyl maleimide was measured with the passage of days so as to observe the coloration of the N -cyclohexyl maleimide. Then, N -cyclohexyl maleimides having varying YIsol. were produced. Next, in order to explain the effect of the production process of the present invention, the following description will explain Comparative Examples. [Comparative Example 5] N -cyclohexyl maleimide (c) was prepared by leaving the N -cyclohexyl maleimide, which was just purified by distillation, under atmosphere of oxygen at 86 °C for 8 days. A comparative polymer (5) of Comparative Example 5 was prepared in the same manner as in Example 4 except that the N -cyclohexyl maleimide (c) was used. The YIsol. of the N -cyclohexyl maleimide (c) was 1.1 according to the measuring method. The physical properties of the comparative polymer (5) were measured according to the above-mentioned measuring methods. The YIsol. and glass transition temperature of the comparative polymer (5) were 4.4 and 136 °C, respectively. The results are shown in Table 1. [Comparative Example 6] N -cyclohexyl maleimide (d) was prepared by leaving the N -cyclohexyl maleimide, which was just purified by distillation, under natural light for 4 days. A comparative polymer (6) of Comparative Example 6 was prepared in the same manner as in Example 4 except that the N -cyclohexyl maleimide (d) was used. The YIsol. of the N -cyclohexyl maleimide (d) was 1.5 according to the measuring method. The physical properties of the comparative polymer (6) were measured according to the above-mentioned measuring methods. The YIsol. and glass transition temperature of the comparative polymer (6) were 5.9 and 136 °C, respectively. The results are shown in Table 1. It is clear from Examples 4 and 5 and Comparative Examples 5 and 6 that the process for producing heat-resistant resins of the present invention can stably produce a heat-resistant resin which contains a large amount of monomer units of N -cyclohexyl maleimide, for example, 20 to 30 percent, based on the sum of the whole monomer units required for producing the heat-resistant resin, and has excellent heat resistance as indicated by a glass transition temperature of not lower than 120 °C. Moreover, in this production process, even when the amount of N -cyclohexyl maleimide by monomer unit is increased, if N -cyclohexyl maleimide whose YIsol. is not higher than 1.0 is used, it is possible to cause the YIsol. of the polymer as the resultant heat-resistant resin not to be higher than 4, and stably produce heat-resistant resins having a reduced coloring factor and excellent transparency. Since such a heat-resistant resin is excellent in its transparency and heat resistance, it is suitable for use as parts which are often brought into high temperature states around 100 °C, like automobile parts and electrical equipment parts. INDUSTRIAL APPLICABILITY The raw material used for producing heat-resistant resins of the present invention can reduce the coloration of heat-resistant resins produced from the raw material. Since the heat-resistant resins of the present invention produced from the above-mentioned raw material have excellent transparency and heat resistance, and a low coloring factor, they are suitably used in various fields, such as automobile parts, electrical equipment parts, face plates, signs, covers for lights, ornaments, and variety goods. The process for producing heat-resistant resins of the present invention allows simple and stable production of heat-resistant resins having the above-mentioned excellent properties.

Claims: A heat-resistant resin produced by copolymerizing a raw material containing N-cyclohexyl maleimide as a principal component, and 0.001 to 1 weight percent of cyclohexyl amino succinic anhydride (based on the N-cyclohexyl maleimide) and at least one kind of monomer which is copolymerizable with the raw material. The heat-resistant resin according to claim 1, whose yellowness (YI) is not higher than 2, or yellowness (YIsol.) in a solution is not higher than 3. A process for producing heat-resistant resins, comprising the step of copolymerizing N-cyclohexyl maleimide whose yellowness in a solution is not higher than 1.0 and a polymerizable monomer which is copolymerizable with the N-cyclohexyl maleimide, in the presence of 0.001 to 1 weight percent of cyclohexyl amino succinic anhydride (based on the N-cyclohexyl maleimide). The process according to claim 3, wherein the polymerizable monomer which is copolymerizable with the N-cyclohexyl maleimide is methacrylic ester.

Inventors: FUJIOKA KAZUMI, KANTO TERUYUKI, KISHINO KAZUO, UEDA KENICHI, FUJIOKA, KAZUMI, KANTO, TERUYUKI, KISHINO, KAZUO, UEDA, KENICHI
Applicants: NIPPON CATALYTIC CHEM IND, NIPPON SHOKUBAI CO., LTD.
Application year: 1996
Main classification: C08F222
IPC codes: C08F220, C08F222