HIGHLY POLYMERIZABLE N-VINYLCARBOXYLIC ACID AMIDE AND PRODUCTION PROCESS THEREOF

Abstract: A highly polymerizable N-vinylcarboxylic acid amide having an N-1,3-butadienylcarboxylic acid amide content of 30 ppm or less, a process for producing the same, and a process for producing a homopolymer of N-vinylcarboxylic acid amide or a copolymer thereof with another copolymerizable monomer using the same. Also, a highly polymerizable N-vinylcarboxylic acid amide is produced by thermal cracking or catalytic cracking of N-(1-alkoxyethyl)carboxylic acid amide or ethylidenebiscarboxylic acid amide, wherein the N-vinylcarboxylic acid amide content of the N-(1-alkoxyethyl)-carboxylic acid amide or ethylenebiscarboxylic acid amide is 10 wt.% or less.

Description: HIGHLY POLYMERIZABLE N-VINYLCARBOXYLIC ACID AMIDE AND PRODUCTION PROCESS THEREOF FIELD OF THE INVENTION The present invention relates to a highly polymerizable N-vinylcarboxylic acid amide, a production process thereof and a process for producing a high molecular weight polymer of N-vinylcarboxylic acid amide using the same. More specifically, the present invention provides an N- vinylcarboxylic acid amide having improved polymerizability and a process for producing the same. The N-vinylcarboxylic acid amide is advantageously used in industry as a monomer for the production of an N-vinylcarboxylic acid amide polymer, which polymer is used as a coagulant, a liquid absorbent or a thickener. The present invention also provides a process for producing the polymer. Furthermore, the present invention provides a high-quality N-vinylcarboxylic acid amide that is useful as a raw material for industrial chemicals or medical products in various fields. BACKGROUND OF THE INVENTION A large number of methods for producing an N- vinylcarboxylic acid amide have hitherto been proposed. For example, a method is known where an N-{ 1-alkoxyethyl )- carboxylic acid amide as an intermediate is produced from a carboxylic acid amide, acetaldehyde and an alcohol, and an N- vinylcarboxylic acid amide is synthesized by cracking or catalytic cracking of the product. Another useful method for synthesizing an N-vinylcarboxylic acid amide includes synthesizing an ethylidene biscarboxylic acid amide from acetaldehyde and a carboxylic acid amide. This product is then cracked into a carboxylic acid amide and an N-vinyl¬ carboxylic acid amide. In these methods, a purification step, such as distillation, extraction and recrystallization of the resulting N-vinylcarboxylic acid amide is provided. For example, JP-A-61-286069 (the term "JP-A" as used herein means an "unexamined published Japanese patent application" ) discloses an extraction separation using water and an aromatic hydrocarbon. This is because distillation fails to prevent mingling of formamide as an unreacted raw material into the N-vinylformamide. Furthermore, JP-A-63-132868 discloses cooling crystallization from a mixed organic solvent, JP-A-2- 188560 discloses extraction using an aqueous inorganic salt solution and an aromatic hydrocarbon, and U.S. Patent 4,401,516 discloses extractive distillation using a polyhydric alcohol. On the other hand, an N-vinylcarboxylic acid amide polymer is obtained by polymerizing an N-vinylcarboxylic acid amide alone or copolymerizing with another monomer. The polymer is used as a coagulant, a liquid absorbent or a thickener, and in any case the polymer must have a high molecular weight. However, it has been difficult to obtain an N-vinylcarboxylic acid amide which consistently exhibits good polymerizability in any of the uses described above. SUMMARY OF THE INVENTION An object of the present invention is to provide a highly polymerizable N-vinylcarboxylic acid amide having good and improved polymerizability. Another object of the present invention is to produce a high molecular weight N-vinylcarboxylic acid amide polymer. As a result of extensive investigations on the production process of an N-vinylcarboxylic acid amide having good polymerizability, the present inventors have found that a high molecular weight polymer can be produced using an N- vinylcarboxylic acid amide having an N-l, 3-butadienyl- carboxylic acid amide content of 30 ppm or less, preferably 10 ppm or less, more preferably 1 ppm or less, to thereby achieve the present invention. More specifically, the present invention provides a highly polymerizable N-vinylcarboxylic acid amide, wherein the N-vinylcarboxylic acid amide has an N-l , 3-butadienyl- carboxylic acid amide content of 30 ppm or less, a production process thereof, and a process for producing a high molecular weight polymer of an N-vinylcarboxylic acid amide using the same. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 shows the relationship between viscosity an d the N-l,3-butadienylacetamide content of crude N- vinylacetamide (NVA) as measured following the procedure o f Example 1. The viscosity is a measure of the degree of polymerizability. DETAILED DESCRIPTION OF THE INVENTION The present invention is described in detail below. The N-vinylcarboxylic acid amide for use in the present invention is represented by formula (I): CH 2 =CH-NR 1 -COR 2 ( I ) (wherein R 1 and R 2 each represents a hydrogen atom or an alkyl group having from 1 to 5 carbon atoms). Examples thereof include N-vinylformamide, N-methyl-N-vinylformamide, N- vinylacetamide, N-methyl-N-vinylacetamide, N-vinylpropion- a ide, N-methyl-N-vinylpropionamide, N-vinylbutylamide and N- vinylisobutylamide. Among these, N-vinylformamide and N- vinylacetamide are preferred and N-vinylacetamide is more preferred. The N-l,3-butadienylcarboxylic acid amide is represented by formula (II): CH 2 =CH-CH=CH-NR 1 -COR 2 ( II ) (wherein R ! and R2 each has the same meaning as defined above ) and includes cis and trans forms which are present as geometrical isomers thereof. In the present invention, a highly polymerizable N- vinylcarboxylic acid amide is obtained by setting the content of N-l,3-butadienylcarboxylic acid amide in the crude N - vinylcarboxylic acid amide to 30 ppm or less, but a desired high polymerizability may be obtained by setting the content preferably to 10 ppm or less, and more preferably to 1 ppm or lees. If the content exceeds the above-described range, high polymerizability is difficult to achieve. This is verified, for example, by the relationship (Fig " 1) between the content of N-l,3-butadienylacetamide in crude N-vinylacetamide and viscosity (degree of polymerizability) . There is no particular limitation on the production process of an N-vinylcarboxylic acid amide which is used in the process for producing a highly polymerizable N-vinyl¬ carboxylic acid amide of the present invention, as long as the crude N-vinylcarobxylic acid amide thus obtained has an N-l,3- butadienylcarboxylic acid amide content in excess of 30 ppm, and in some cases, in excess of 10 ppm or 1 ppm. However, in one preferred embodiment, the N-vinylcarboxylic acid amide is produced through dealcoholization of an N-(l- alkoxyethyl)carboxylic acid amide or dealcoholization of an N-(l-alkoxyethyl)carboxylic acid amide obtained as an intermediate from carboxylic acid amide, acetaldehyde and alcohol or from carboxylic acid amide and acetaldehyde dialkylacetal. In this case, the dealcoholization reaction is preferably conducted by thermal cracking or catalytic cracking. In another preferred embodiment, the N- vinylcarboxylic acid amide is produced through cracking of ethylidene biscarboxylic acid amide or cracking of an ethylidene biscarboxylic acid amide obtained as an intermediate from acetaldehyde and carboxylic acid amide. In this case, the ethylidene biscarboxylic acid is cracked into a carboxylic acid amide and an N-vinylcarboxylic acid amide. In the present invention, a highly polymerizable N- vinylcarboxylic amide having an N-l,3-butadienylcarboxylic acid amide content of 30 ppm or less may be produced by a purification processing method where an N-l,3-butadienyl- carboxylic acid amide is reduced or removed from the crude N- vinylcarboxylic acid amide, or by a purification processing method where an N-l,3-butadienylcarboxylic acid amide or a precursor thereof is reduced or removed from the production raw material or intermediate of N-vinylcarboxylic acid amide. A method of reducing or removing an N-l,3- butadienylcarboxylic acid amide from the crude N-vinyl¬ carboxylic acid amide is described below. The purification processing method for reducing or removing an N-l,3-butadienylcarboxylic acid amide from the crude N-vinylcarboxylic acid amide includes, for example, a physical purification processing method of processing the crude N-vinylcarboxylic acid amide or a solution thereof by rectification, a recrystallization method, pressure crystallization or treatment with an adsorbent such as activated carbon, and a purification processing method using chemical conversion of N-l,3-butadienylcarboxylic acid amide, such as a method of processing N-l,3-butadienylcarboxylic acid amide with p-benzoquinone using a Dielε-Alder reaction and a method of processing the 1,3-butadienyl group using selective hydrogenation. These methods may be used either individually or in combination. Other than the above-described methods, any method may be used if the N-l,3-butadienylcarboxylic acid amide is easily separated from the N-vinylcarboxylic acid amide or chemically converted. The method for removing or reducing the amount of N-l,3-butadienylcarboxylic acid amide from the crude N-vinyl- carobxylic acid amide is described in greater detail below. In the process of the present invention, there is no particular limitation on the distillation apparatus for use in separating by rectification, and a plate column or packed column having a theoretical plate number of from 1 to 50 may be used. However, a rectifying column is preferably used which causes little pressure loss and has excellent rectifiability, and examples thereof include a packed column using a regular filling. The distillation is preferably conducted at a temperature that is as low as possible because the N-vinylcarboxylic acid amide is readily deteriorated due to heat. Accordingly, the distillation is conducted under a reduced pressure of from 0.01 to 100 mmHg. The rectification may be conducted either continuously or diεcontinuously, however, a continuous operation is preferred in view of productivity and safety in operation. The reflux ratio is not particularly limited and can be set according to the content of N-l,3-butadienyl- carboxylic acid amide, the kind of N-vinylcarboxylic acid amide and the capabilities of the distillation column. However, the reflux ratio is generaly from 0.1 to 20, an d preferably from 0.5 to 10. In the process of the present invention, when the separation is made by a recrystallization method comprising cooling a crude N-vinylcarboxylic acid amide, the N-vinyl¬ carboxylic acid amide may be directly cooled, or a recrystallization solvent which does not react with the N - vinylcarboxylic acid amide and having an appropriate solubility may be used. Examples of the recrystallization solvent include an aromatic hydrocarbon such as benzene, toluene and xylene; an aliphatic hydrocarbon such as pentane, cyclopentane, hexane, cyclohexane and heptane; an alcohol suc h as methanol, ethanol, n-propyl alcohol, isopropyl alcohol, n- butanol, isobutanol, sec-butanol, tert-butanol and cyclo- hexanol; a halogenated hydrocarbon such as chloroform and chlorobenzene; a ketone such as acetone, methyl ethyl ketone and cyclohexanone; an ester such as methyl acetate, ethyl acetate, propyl acetate and butyl acetate; an ether such as diethyl ether; an amide such as N,N-dimethylformamide and N N- dimethylacetamide; and a dimethylsulfoxide. Toluene cyclohexane, methanol and isopropyl alcohol are preferred. These solvents may also be used in combination. The cooling temperature varies depending upon the kind and the amount of the N-vinylcarboxylic acid amide and the recrystallization solvent, but it is usually from -20 to 50°C, preferably from -10 to 40°C. The crystallization apparatus for use in the present invention is not particularly limited with respect to its structural form and may be either a continuous system or a batch system. Also, the crystallization method may either employ heat exchange with a cooling medium, or concentration and cooling of the solvent upon evaporation. The apparatus for separating crystals for use in the present invention is also not particularly limited. An apparatus using vacuum pressure or the application of pressure, or an apparatus using gravity or centrifugal force may be used. In the present invention, a solid-liquid separator which combines crystallization and separation in the same apparatus may also be used. Preferred examples of the apparatus include, in the case of not using a crystallization solvent, a pressure cryεtallizer, a falling film crystallizer (e.g., a fractional crystallization apparatus) and a tower type continuous crystallization purifier (e.g., a BM C or b ack mixing column cryεtallizer apparatus). in the case of filtering a high concentration slurry, an automatic Nutεche filter such aε a Roεemund filter is preferably used. In the process of the present invention, when the crude N-vinylcarboxylic acid amide is separated by treating with an adsorbent such as activated carbon, the adsorbent for use in the preεent invention is not particularly limited aε long as it εelectively adεorbs the N-l,3-butadienylcarboxylic acid amide in comparison with the N-vinylcarboxylic aci d amide. Examples of the adsorbent include activated carbon, clays, alumina, silica, zeolite and adsorptive resin, an d among these, activated carbon is preferred. In the adsorption operation of the present invention, the crude N-vinylcarboxylic acid amide may be placed into direct contact with the adsorbent. Alternatively, the crude solution may be diεεolved in a εolvent which does not react with the N-vinylcarboxylic acid amide and which has an appropriate solubility, and then placed into contact with the adsorbent. Examples of the solvent include water an d those described above with respect to the metho d of recrystallization, and when activated carbon is used as an adsorbent, water and methanol are particularly preferred. The ratio of the solvent to the crude N-vinylcarboxylic acid amide is not particularly limited, however, when water or methanol is used as the εolvent, it is preferably from 0 to 10 : 1 (by weight), more preferably from 0.1 to 3 : 1 (by weight). The adsorption temperature suitable for conducting the adsorption operation of the present invention varies depending upon the kind of adsorbent, but it is preferably from -20 to 100°C, more preferably from 0 to 80°C. If the adsorption temperature is lower than -20°C, diffusion into the pores of the adsorbent is considerably retarded to disadvantageouεly prolong the adsorption time. If the temperature is higher than 100°C, stability of the N- vinylcarboxylic acid amide is lowered and at the same time, the equilibrium adsorption quantity is greatly reduced. The adsorption method of the present invention is not particularly limited with respect to the structural form thereof, but may either be a continuous system or a batch system. In the present invention, the content of N-l,3- butadienylcarboxylic acid amide in the N-vinylcarboxylic acid amide may be reduced to 30 ppm or less by chemically processing the crude N-vinylcarboxylic acid amide solution. There is no particular limitation on the chemical reaction to be used aε long as it uses the difference in reactivity between the N-vinylcarboxylic acid amide and the N-l,3- butadienylcarboxylic acid amide. For example, the reaction with a diene is highly active but the reaction with a monoene is inactive. The method of procesεing the crude N-vinylcarboxylic acid amide solution uεing a Diels-Alder reaction, and the method of processing the solution uεing selective hydrogenation are described below. In the present invention, when the crude N- vinylcarboxylic acid amide εolution is processed using a Diels-Alder reaction, the procesεing is conducted by allowing a dienophilic compound (dienophile) in the Diels-Alder reaction to be present in the crude N-vinylcarboxylic acid amide εolution. The dienophilic compound of the present invention is not particularly limited aε long as it is a compound commonly used aε a dienophilic compound in the Diels- Alder reaction, more specifically, a compound which is selected from α,β-unsaturated compounds subεtituted by one or more electron attractive group(ε) and which does not react with the N-vinylcarboxylic acid amide. Examples of the dienophilic compound include an unsaturated carboxylic ester; such as acrylic ester maleic ester and fumaric ester, an unsaturated ketone such aε methyl vinyl ketone and p- benzoquinone; an unsaturated nitrile such as acrylonitrile; and an unsaturated imide such as maleic acid imide. Among these, p-benzoquinone is preferred. The amount of the dienophilic compound iε not particularly limited as long as it is equivalent to or more than the amount of the N-l,3- butadienylcarboxylic acid amide contained in the crude N - vinylcarboxylic acid amide. However, the molar ratio of the dienophilic compound to the N-l,3-butadienylcarboxylic aci d amide contained in the crude N-vinylcarboxylic acid amide solution iε generally from 1 to 100 equivalents, an d preferably from 1.2 to 10 equivalents. In the present invention, when the crude N- vinylcarboxylic acid amide solution iε processed by a Diels- Alder reaction, the crude N-vinylcarboxylic acid amide may be directly contacted with the - dienophilic compound. Alternatively, the crude solution may be dissolved in a εolvent which does not react with the N-vinylcarboxylic aci d amide and has an appropriate εolubility, and then εubjecte d to a Dielε-Alder reaction. Examples of the solvent include the solvents described above with respect to the adsorption method of the present invention. Furthermore, although a catalyst is not required when the crude N-vinylcarboxylic acid amide εolution is processed by a Diels-Alder reaction in the present invention, those known to have catalytic activity in the Diels-Alder reaction and which do not react with the N - vinylcarboxylic acid amide may be used. Examples of the catalyst include Lewis acids such as aluminum trichloride, boron trifluoride and lanthanide complex. The reaction temperature suitable for the Diels- Alder reaction of the present invention varies depending upon the kind of the dienophilic compounds used, but it is preferably from -20 to 100°C, and more preferably from 0 to 80°C. If the reaction temperature is lower than -20°C, the reaction rate iε too slow, whereas if the reaction temperature exceeds 100°C, stability of the N-vinylcarboxylic acid amide iε diεadvantageouεly reduced. The N-vinylcarboxylic acid amide εolution processed by a Diels-Alder reaction of the present invention contains a Diels-Alder adduct produced by the Diels-Alder reaction, but this compound has almost no harmful effect on the polymeriz¬ ation of the N-vinylcarboxylic acid amide. The Diels-Alder adduct produced in the present invention has a low vapor preεεure aε compared with the N-l,3-butadienylcarboxylic acid amide. Accordingly, if the N-vinylcarboxylic acid amide solution iε purified by diεtillation after processing by a Diels-Alder reaction of the present invention, the Diels-Alder adduct can be separated more easily than N-l,3- butadienylcarboxylic acid amide that is removed by distillation alone. In other words, the compound can be separated using simple distillation equipment. The processing of the crude N-vinylcarboxylic acid amide using a hydrogenation reaction of the present invention is conducted by contacting the crude N-vinylcarboxylic acid amide with hydrogen in the presence of a catalyst. The catalyst iε not particularly limited as long aε it has general activity in the selective hydrogenation reaction of olefins, however, the catalyst preferably has high diene hydrogenation selectively when a monoolefin and a diene are present together. Examples of the catalyst include a Pd series, C o series or Ni-Co-Cr series metal or a modified product of these metals provided on alumina, activated carbon or silica an d among these, Pd-alumina, Pd-Ag-alumina, Pd-Pb-alumina, Pd-Cr- alumina and Pd-alumina-based catalysts are preferred. When Pd iε uεed aε the metal component, the carried amount thereof iε preferably from 0.001 to 5 wt%, -more preferably from 0.01 to 1 wt%. If the carried amount is less than 0.001 wt%, the reaction rate is extremely slow, whereas if it exceeds 5 wt%, N-ethylacetamide reεulting from hydrogenation of the N- vinylcarboxylic acid amide diεadvantageouεly increases. In the present invention, when the N-vinylcarboxylic acid amide is proceεsed by a hydrogenation reaction, the N- vinylcarboxylic acid may be directly contacted with hydrogen, or may be diεεolved in a solvent, which does not react with the N-vinylcarboxylic acid amide and which has an appropriate solubility, and then subjected to a hydrogenation reaction. Examples of the solvent include the solvents described above with respect to the adsorption method of the present invention. Among these, an alcohol is preferred, and methanol and isopropyl alcohol are particularly preferred. The reaction temperature εuitable for the hydrogenation reaction of the present invention varieε depending upon the kind of catalyst used, but it is preferably from -20 to 100°C, more preferably from 0 to 80°C. If the reaction temperature iε lower than -20°C, the reaction rate iε extremely εlow, whereaε if it exceedε 100°C, the stability of the N-vinylcarboxylic acid amide iε reduced. The hydrogen partial pressure suitable for the selective hydrogenation reaction of the present invention is from 0.01 to 100 kg/cm 2 , preferably from 0.5 to 50 kg/cm 2 . If the hydrogen partial presεure is less than 0.01 kg/cm 2 , the reaction rate iε extremely slow, whereas if it exceeds 100 kg/cm 2 , the production of N-ethylacetamide resulting from hydrogenation of N-vinylcarboxylic acid amide increases and the equipment becomes more expensive. When the catalyst iε charged and fluidized, the proper conditionε of liquid εpace velocity vary depending upon the hydrogen partial preεεure, the reaction temperature and the content of N-l,3-butadinenylcarboxylic acid amide, but the liquid space velocity is preferably from 0.05 to 1000. If it exceeds 1000, the conversion of N-l,3-butadienylcarboxylic acid amide is insufficient, whereas if it is less than 0.05, the reaction efficiency is reduced. The reaction method for use in the present invention may either be conducted continuously or batchwise, and the reaction vesεel iε not particularly limited in itε εtructural form. A ny of gas-solid contact, gaε-liquid-εolid contact and solid-liquid contact may be used, however, a εolid-liquid reactor capable of uniform contact of the raw material with the catalyst under relatively mild conditions iε preferred. When the εolid-liquid reaction is conducted, hydrogen is previously disεolved in the crude N-vinylcarboxylic acid amide εolution εo aε to supply the hydrogen necessary for the reaction. A fter the hydrogenation reaction, the reaction solution contains a by-product accompanying the product produced by the hydrogenation reaction. Methods for purifying the reaction εolution include, for example, rectification, recryεtallization by cooling and pressure crystallization of the εolution, and these methodε may be uεed individually or in combination. Other than these methodε, any method may be used without particular restriction if the by-product and the N -vinylcar b oxylic acid amide can be easily separated. A s described above, the N-l,3-butadienylcarboxylic acid amide has a cis form and a tranε form and due to difference in the physical properties or reactivity of these, the degree of separation may vary in the present invention. I n that case, a cis-trans isomerization reaction of the N-l,3- butadienylcarboxylic acid amide may be conducted under appropriate reaction conditions in combination with the above- described operations such as separation. In any case, the N-vinylcarboxylic acid amide causes solvolysis or hydrolysis when an acid is present. Accordingly, it is preferred to place auxiliary facilities for use in the present invention, such as a production apparatus, a separation apparatuε, a raw material tank, a product container and a filtrate tank, under an atmosphere of inert gas (nitrogen) or dry air. Furthermore, in order to prevent a hydrolysis reaction of N-vinylcarboxylic acid amide, a slight amount of a drying agent such as magnesium sulfate may be added to the raw material. When a base is present, a dimerization reaction results. Accordingly, the pH of the N-vinylcarboxylic acid amide εolution is preferably adjusted prior to the distillation and adsorption operation to a pH of from 3 to 11, preferably from 4 to 10, more preferably from 5 to 6. When the crude N-vinylcarboxylic acid amide solution iε acidic, the pH is ad uεted by adding a baεic compound. Examples of the basic compound include sodium salts such as sodium carbonate, sodium hydrogencarbonate, sodium hydroxide, sodium {hydrogen ) - phosphate and sodium acetate, potassium saltε such aε potassium carbonate, potassium hydrogencarbonate, potassium hydroxide, potassium (hydrogen)phosphate and potassium acetate, and aromatic amines such as N-phenyl-α-naphthylamine, 4,4'-bis(α, -dimethylbenzyl)diphenylamine, N-phenyl-N'- ( 1,3- dimethylbutyl)-p-phenylenediamine, N-phenyl-N'-isopropy1-p- phenylenediamine, N-phenyl-N' -( l-methylheptyl ) -p- phenylenediamine, N-phenyl-N'-cyclohexyl-p-phenylenediamine, N ,N'-diphenyl-p-phenylenediamine, N,N'-di-β-naphthy1-p- phenylenediamine, N,N'-bis(1,4-dimethylpentyl)-p-phenylene- diaraine,N,N'-biε(l-ethyl-3-methylpentyl)-p-phenylenediamine, N,N'-biε(l-methylheptyl)-p-phenylenediamine and N-phenyl-N'- (p-tolueneεulfonyl)-p-phenylenediamine, with εodium hydroxide being particularly preferred. The addition amount of the basic compound iε preferably from 1 to 10,000 ppm, more preferably from 10 to 1,000 ppm. If the addition amount exceeds 10,000 ppm, in the case of an inorganic εalt, the entire amount is not dissolve d and no additional effect is expected from the excess amount. In case of an aromatic amine, it iε difficult to completely remove the aromatic amine at the purification εtep, and the polymerizability of N-vinylcarboxylic acid amide is reduced. If the addition amount iε less than 1 ppm, almost no stabilizing effect iε obtained. When the crude N-vinylcarboxylic acid amide solution is basic, the adjustment is conducted by adding an acidic compound. Examples of the acidic compound include acidic inorganic compounds such as hydrochloric acid, sulfuric aci d , nitric acid, phosphoric acid and a salt thereof, and acidic organic compounds including carboxylic acids such as acetic acid, phthalic acid and citric acid, carbolic acids such aε phenol, hydroquinone and catechol, and εalts thereof. Among the methodε for use in the present invention, a method for reducing or removing the N-l,3-butadienyl- carboxylic acid amide or a precursor thereof contained in the production raw material or intermediate of N-vinylcarboxylic acid amide iε described below. The precursor content iε expressed in terms of the content of N-l,3-butadienyl- carboxylic acid amide when all precursors are converted into N -l,3- b utadienylcarboxylic acid amide. In the process of the- present invention, the production raw material or intermediate of N-vinylcarboxylic acid amide includes N-(1-alkoxyethyl)carboxylic acid amide, dialkylacetal and ethylidenebiscarboxylic acid amide. Examples of the alkoxyl group in N-(1-alkoxyethyl)carboxylic acid amide and dialkylacetal include an aliphatic alkoxyl group such aε methoxy, ethoxy, n-propyl, iεopropoxy, n-butoxy and sec-butoxy. Examples of the carboxylic acid amide group in N -(1-alkoxyethyl)carboxylic acid amide and ethylidene¬ biscarboxylic acid amide include formamide, N-methylformamide, acetamide, N -methylacetamide, propionamide, butyramide and isobutyramide. Examples of the corresponding compound inclu d e: aε the N-(1-alkoxyethyl)carboxylic acid amide, N-(l- methoxyethyl ) acetamide, N-(1-methoxyethyl)formamide, N-(l- ethoxyethyl)acetamide, N-(l-ethoxyethyl)formamide, N-(l-iso- propoxyethyl)acetamide and N-(1-isopropoxyethyl)formamide; as the dialkylacetal, dimethylacetal, diethylacetal and diiso- propylacetal; aε the ethylidenebiscarboxylic acid amide, ethylidenebiεacetamide, ethylidenebiεformamide, ethylidenebiε- (N-methylformamide) and ethylidenebiεpropionamide. An N-(l-alkoxyethyl)carboxylic acid amide or an ethylidenebiεcarboxylic acid amide iε converted into an N- vinylcarboxylic acid amide by a known method εuch as thermal cracking or catalytic cracking. The reaction conditions are, for example, εuch that a gaε phaεe or a liquid phaεe is used. The reaction temperature iε from 60 to 600°C, the reaction time is from 0.3 second to 2 hours and the reaction pressure iε from 0.1 mmHg to atmospheric presεure. Examples of the catalyst used in case of catalytic cracking include an alkali metal εalt of carboxylic acid εuch aε potaεεium acetate and an oxide of an alkali metal or an alkaline earth metal, εuch aε magnesium oxide. The N-l,3-butadienylcarboxylic acid amide is produced upon elimination of two alcohol molecules induced by the thermal cracking or catalytic cracking of N-(l,3- dialkoxybutyl)carboxylic acid amide, or upon elimination of alcohol and carboxylic acid amide induced by the thermal cracking or catalytic cracking of 3-alkoxybutylidene- biscarboxylic acid amide. The N-(1,3-dialkoxybutyl)carboxylic acid amide and 3-alkoxybutylidenebiscarboxylic acid amide are produced by the reaction of 1,1,3-trialkoxybutane with carboxylic acid amide. Accordingly, in the present invention, examples of the precursor of N-l,3-butadienylcarboxylic acid amide include 1,1,3-trialkoxybutane, N-(l,3-dialkoxybutyl)carboxylic acid amide and 3-alkoxybutylidenebiscarboxylic acid amide, and examples of the alkoxyl group and the carboxylic acid amide group in these precursors include those described above for the precursor of N-vinylcarboxylic acid amide. Therefore, the 1,1,3-trialkoxybutane includes 1,1,3-trimethoxybutane, 1,1,3- triethσxybutane and 1,1,3-triisopropoxybutane; the N-(l,3- dialkoxybutyl)carboxylic acid amide includes N-(l,3- dimethoxybutyl)acetamide, N-(l,3-dimethoxybutyl)formamide, N-(1, 3-diethoxybutyl)acetamide, N-(1, 3-diethoxybutyl )- formamide, N-(l,3-diiεopropoxybutyl)acetamide and N-(l,3- diisopropoxybutyl)formamide; and the 3-alkoxybutylidene- biεcarboxylic acid amide includes 3-methoxybutylidene- bisacetamide, 3-methoxybutylidenebiεformamide, 3-ethoxy- butylidenebiεacetamide, 3-ethoxybutylidenebisformamide, 3- isopropoxybutylidenebisacetamide and 3-isopropoxy- butylidenebisformamide. The embodiment for reducing the content of N-l, 3- butadienylcarboxylic acid amide or a precursor thereof in the production raw material or intermediate of N-vinylcarboxylic acid amide to 30 ppm or less includes, for example, a rectification method, a recrystallization method by cooling the precursor solution of N-vinylcarboxylic acid amide, a pressure crystallization method of the precursor, a physical adsorption method by treating the precursor with an adεorbent such aε activated carbon and a method of chemically processing the precursor, and these methodε may be uεed individually or in combination. Other methodε may be uεed without particular limitation aε long as the N-l,3-butadienylcarboxylic acid amide or a precursor thereof can be easily separated from the production raw material or intermediate of the N- vinylcarboxylic acid amide. Furthermore, aε a reεult of extenεive investigations on the process for producing an N-vinylcarboxylic acid amide having good polymerizability, the present inventors have found that an N-vinylcarboxylic acid amide for use in the synthesis of a high molecular weight polymer is produced when, in producing an N-vinylcarboxylic acid amide by thermal cracking or catalytic cracking of N-( 1-alkoxyethyl )carboxylic acid amide or ethylidenebiscarboxylic acid amide, the N- vinylcarboxylic acid amide content of the N-(1-alkoxyethyl )- carboxylic acid amide or ethylidenebiscarboxylic acid amide is reduced to 10 wt% or less and the pH of a 33 wt% aqueous solution (hereinafter simply referred to as "pH" ) of the N-(l- alkoxyethyl)carboxylic acid amide or ethylenebiscarboxylic acid amide iε adjusted to from 5 to 10. More εpecifically, the present invention provides a procesε for producing an N-vinylcarboxylic acid amide having good polymerizability, wherein, when an N-vinylcarboxylic acid amide iε produced by thermal cracking or catalytic cracking of N-(l-alkoxyethyl)carboxylic acid amide or ethylidene ¬ biscarboxylic acid amide, the N-vinylcarboxylic acid amide content of the N-(l-alkoxyethyl)carboxylic acid amide or ethylidenebiεcarboxylic acid amide is reduced to 10 wt % or less, preferably 5 wt% or less, more preferably 3 wt% or less, the pH of the N-(l-alkoxyethyl)carboxylic acid amide or ethylidenebiεcarboxylic acid amide * is adjusted to from 5 to 10, preferably from 6 to 8, more preferably from 6.3 to 7.5, and still more preferably, the N-vinylcarboxylic acid amide content is reduced to 3 wt% or less and at the same time, the pH is adjusted to from 6.3 to 7.5. There are various reasons why the N-(l-alkoxy- ethyl)carboxylic acid amide or ethylidenebiscarboxylic acid amide generally contains more than 10 wt% of N-vinylcarboxylic acid amide. One reason is that when a reaction solution containing N-(l-alkoxyethyl)carboxylic acid amide or ethylidenebiεcarboxylic acid amide obtained by the a b ove- described synthesis method is purified and separated by a distillation operation or the like, a part of the N-(l- alkoxyethyl ) carboxylic acid or ethylidenebiεcarboxylic acid amide iε cracked into N-vinylcarboxylic acid amide. A nother reason iε that when an N-vinylcarboxylic acid amide iε synthesized by thermal cracking or catalytic cracking of N-(l-alkoxylethyl)carboxylic acid amide or ethylidenebiscarboxylic acid amide and the N-vinylcarboxylic acid iε separated, the unreacted N-(l-alkoxylethyl)carboxylic aci d amide or ethylidenebiεcarboxylic acid amide which containε N-vinylcarboxylic acid is recycled for cracking. A method for reducing the N-vinylcarboxylic acid amide content of N-(l-alkoxyethyl)carboxylic acid amide or ethylidenebiscarboxylic acid amide to 10 wt% or less iε deεcribed below. The method iε not particularly limited as long aε it iε a method for reducing the N-vinylcarboxylic acid amide content of N-(l-alkoxyethyl)carboxylic acid amide or ethylidenebiscarboxylic acid amide to 10 wt% or less . The method includes, for example, the proceεεing of N-(l- alkoxyethyl)carboxylic acid amide, ethylidenebiεcarboxylic acid amide or a solution of these by a rectification method, an azeotropic distillation method, a recrystallization method or a preεεure cryεtallization method, and these methods may be used individually or in combination. Other than these methods, any method may be used without any particular restriction if it is a method where N-(l-alkoxyethyl)- carboxylic acid amide or ethylidenebiscarboxylic acid amide and N-vinylcarboxylic acid amide can be easily separated. The embodiment for reducing the N-vinylcarboxylic acid amide content of N-(l-alkoxyethyl)carboxylic acid amide or ethylidenebiscarboxylic acid amide to 10 wt% or less is described in greater detail below. In the procesε of the present invention, the distillation apparatuε used for separating by a rectification method iε not particularly limited and a plate column or packed column having a theoretical plate number of from 1 to 50 may be used, but it iε preferable to use a rectification column which causeε little preεεure loss and has excellent rectification capabilities. An example of εuch a rectification column iε a packed column uεing a regular filling. N-(1-alkoxyethyl)carboxylic acid amide, ethylidene¬ biscarboxylic acid amide and N-vinylcarboxylic acid amide are readily deteriorated due to heat. Thuε, it iε preferable to effect distillation at aε low a temperature aε possible. Accordingly, the distillation iε conducted under a reduced presεure of from 0.01 to 100 mmHg. The present invention may be conducted either continuously or batchwise, however, in view of productivity and operation safety, a continuous operation is preferred. The reflux ratio is not particularly limited and can be set according to the content and kind of N-vinylcarboxylic acid amide and the capabilities of the distillation column. However, the reflux ratio is generally from 0.1 to 20, and preferably from 0.5 to 10. In the process of the present invention, when the separation is effected according to a recrystallization method by cooling the N-(l-alkoxyethyl)carboxylic acid amide or ethylidenebiscarboxylic acid amide solution, the recrystallization εolvent, apparatuε and process described above with respect to the recrystallization method for N- vinylcarboxylic acid amide can be used. A method for adjusting the pH of N-(1-alkoxyethyl)- carboxylic acid amide or ethylidenebiscarboxylic acid amide to from 5 to 10 is described below. The pH of N-(l-alkoxyethyl)carboxylic acid amide or ethyli d ene b iεcarboxylic acid amide becomes less than 5 due to the presence of a carboxylic acid εuch aε acetic acid therein. Although it iε not clearly known why the carboxylic acid iε mixed therein, it iε conεidered that this results from oxidation of acetaldehyde used in the synthesis of N-(l- alkoxyethyl ) carboxylic acid amide or ethylidenebiscarboxylic aci d amide, or the production of carboxylic acid amide by solvolysis. The method for adjusting the pH of N-(l- alkoxyethyl ) carboxylic acid amide or ethylidenebiscarboxylic acid amide to from 5 to 10, is not particularly limited. The method includes, for example, the procesεing of N-(1- alkoxyethyl)carboxylic acid amide, ethylidenebiscarboxylic acid amide or a εolution of these by a rectification metho d an azeotropic distillation method, a recrystallization method, a preεεure crystallization method, a phyεical method uεing treatment with an adεorbent or a chemical method uεing neutralization by an acid or a baεe, and these methodε may b e used either individually or in combination. Among these methods, the physical method including procesεing by a rectification method, an azeotropic distillation method, a recrystallization method or a pressure crystallization method may be made in the same manner as described above with respect to the removal of N - vinylcarboxylic acid amide. The method using an adsorbent includes a method where an anion exchange resin is uεed aε an adsorbent. Exampleε of the basic compound for use in neutralizing an acid aε a chemical method include a εodium salt εuch as sodium carbonate, sodium hydrogencarbonate, sodium hydroxide, sodium (hydrogen)phosphate and sodium acetate, and a potassium salt such as potasεium carbonate, potassium hydrogencarbonate, potassium hydroxide, potassium (hydrogen)phosphate and potassium acetate. When the N-(1-alkoxyethyl )carboxylic acid amide or ethylidenebiscarboxylic acid amide solution is basic, the pH thereof iε adjuεted by adding an acidic compound. Examples of the acidic compound include acidic inorganic compounds such aε hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid and a salt thereof, and acidic organic compounds including carboxylic acids εuch aε acetic acid, phthalic acid and citric acid, carbolic acids such as phenol, hydroquinone and catechol, and εaltε thereof. The conversion of N-(l-alkoxyethyl)carboxylic acid amide or ethylidenebiscarboxylic acid amide into N- vinylcarboxylic acid amide iε effected by the above-described thermal cracking or catalytic cracking. In the present invention, the N-(l- alkoxyethyl)carboxylic acid amide or ethylidenebiscarboxylic acid amide causes solvolyεis or hydrolysis when an acid is present. Accordingly, it is preferred to install auxiliary facilities for use in the present invention, such as a production apparatus, a separation apparatus, a raw material tank, a product container and a filtrate tank, in an atmosphere of inert gas (nitrogen) or dry air. Furthermore in order to prevent a hydrolysis reaction of N-(l- alkoxyethyl)carboxylic acid amide or ethylidenebiscarboxylic acid amide, a slight amount of a drying agent such as magnesium sulfate may be added to the raw material. In the present invention, when an N-vinylcarboxylic acid amide is produced by thermal cracking or catalytic cracking of N-(l-alkoxyethyl)carboxylic acid amide or ethylidenebiεcarboxylic acid amide, the N-vinylcarboxylic acid amide content of the N-( 1-alkoxyethyl)carboxylic acid amide or ethylidenebiscarboxylic acid amide iε reduced to 10 wt% or less, preferably 5 wt% or leεε, more preferably 3 wt% or eεε the pH of the N-(l-alkoxyethyl)carboxylic acid amide or ethylidenebiεcarboxylic acid amide iε adjuεted to from 5 to 10, preferably from 6 to 8, more preferably from 6.3 to 7.5, and εtill more preferably, the N-vinylcarboxylic acid amide content is reduced to 3 wt% or lesε and at the εame time the pH iε adjusted to from 6.3 to 7.5, whereby an N-vinyl ¬ carboxylic acid amide having the desired polymerizability is obtained. If the N-vinylcarboxylic acid amide content exceeds the above described range, it tends to be difficult to obtain a highly polymerizable N-vinylcarboxylic acid amide by thermal cracking or catalytic cracking of N-(1-alkoxyethyl)carboxylic acid amide or ethylidenebiscarboxylic acid amide. Also, if the pH exceeds the above-described range, it tends to be difficult to obtain a highly polymerizable N-vinylcarboxylic acid amide by thermal cracking or catalytic cracking of N-(l- alkoxyethyl)carboxylic acid amide or ethylidenebiscarboxylic acid amide. The present invention provides a process for producing a homopolymer or copolymer of N-vinylcarboxylic acid amide using the above-described N-vinylcarboxylic acid amide. By using a highly polymerizable N-vinylcarboxylic acid amide in which the crude N-vinylcarboxylic acid amide has an N-l,3-butadienylcarboxylic acid amide content of 30 ppm or lesε, a homopolymer of a high molecular weight N-vinyl¬ carboxylic acid amide or a copolymer thereof with other copolymerizable monomer can be obtained. As used herein, "highly polymerizable" means that the viscosity as measured following the procedure of Example 1 iε 30 cps or more, preferably 60 cps or more, and also means that the peak achieving time as measured following the procedure of Example 13 is 15 hours or less, preferably 10 hours or lesε. The term "high molecular weight" means a weight average molecular weight of 500,000 or more, preferably 1,000,000 or more. Specific examples of representative monomers copolymerizable with the N-vinylcarboxylic acid amide of the present invention include: an alkali metal salt such as a sodium salt or potassium salt of acrylic acid or methacrylic acid; an alkyl eεter of the alkali metal salt, εuch as methyl ester, ethyl ester, propyl eεter, butyl eεter, pentyl ester, hexyl ester, heptyl ester, octyl ester, nonyl ester, decyl ester, stearyl ester and palmityl ester; a hydroxy lower alkyl ester of the alkali metal salt, such as hydroxyethyl ester, hydroxypropyl ester and hydroxybutyl ester; a lower alkyl ester substituted by a lower alkylamino group of the alkali metal salt, such aε dimethylaminomethyl eεter, dimethylaminoethyl ester, dimethylaminopropyl ester, dimethylaminobutyl ester, diethylaminomethyl ester, diethylaminoethyl ester, diethyl- aminopropyl ester and diethylaminobutyl ester; a lower alkyl eεter halide (the halide being preferably chloride or bromide) substituted by a quaternary ammonium group of the alkali metal salt, εuch as triraethylammonioethyl ester halide, trimethyl- ammoniopropyl ester halide, triethylammonioethyl ester halide and triethylammoniopropyl ester halide; an amide of the alkali metal salt; an amide εubstituted by a lower alkylamino group of the alkali metal salt, εuch aε dimethylaminomethylamide, dimethylaminoethylamide, dimethylaminoproylamide, dimethyl- aminobutylamide, diethylaminomethylamide, diethylamino- ethylamide, diethylaminopropylamide and diethylamino- butylamide; a lower alkylamide εubεtituted by a quaternary ammonium group of the alkali metal εalt, such as trimethyl- amraonioethylamide halide, triethylammoniopropylamide halide, triethylammonioethylamide halide and triethylammoniopropyl¬ amide halide; a lower alkylamide substituted by a sulfonic acid or an alkali metal sulfonic acid of the alkali metal salt, such as sulfomethylamide, sulfoethyjamide, sulfopropylamide, sulfobutylamide, sodium sulfomethyiamide, sodium sulfoethylamide, potassium sulfopropylamide, potassium εulfobutylamide, potasεium εulfomethylamide, potaεsium εulfoethylamide, potaεεium εulfopropylamide and potassium εulfobutylamide; an acrylonitrile; a vinyl ether εuch aε methyl vinyl ether, ethyl vinyl ether, propyl vinyl ether and butyl vinyl ether; a vinyl ketone εuch aε methyl vinyl ketone and ethyl vinyl ketone; a lower vinyl carboxylate εuch aε vinyl acetate and vinyl propionate; a maleic anhydride; a maleic acid; εodium maleate; and potaεεium maleate. Among theεe preferred are (meth)acrylic acid, sodium (meth)acrylate, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, trimethyl- aminoethyl (meth)acrylate chloride, acrylamide, sodium εulfopropylacrylamide, sodium εulfobutylacrylamide, acrylo¬ nitrile, methyl vinyl ether, ethyl vinyl ether, methyl vinyl ketone, ethyl vinyl ketone, vinyl acetate, N-vinyl-2- pyrrolidone and maleic anhydride. As uεed herein, " (meth)acrylic acid" means acrylic acid or methacrylic acid. In the present invention, a crosslinking monomer or crosslinking agent which is a compound having two or more unsaturated groups in one molecule, may be used as a monomer copolymerizable with N-vinylcarboxylic acid amide. The polymerization procesε iε not necessarily limited and conventionally known processes may be used. Usually, a εolution polymerization proceεs, a reverse-phase suspension polymerization proceεs or a reverse-phaεe emulsion polymerization proceεε is preferably used. For example, in the solution polymerization proceεε monomer componentε and a croεεlinking agent are uniforml y dissolved in water, an organic solvent or a mixed solvent of these, the dissolved oxygen in the system iε removed by vacuum deaeration or displacement by an inactive gas such aε nitrogen or carbon dioxide gaε and then, a polymerization initiator iε added to the syεtem to start the reaction. The polymerization initiation temperature is usually approximately from -10 to 60°C and the reaction time is approximately from 1 to 10 hours. The N-l,3-butadienylcarboxylic acid amide of the present invention is verified and identified by mass spectra (an electron impact method and a chemical ionization method ) , an infrared light absorption spectrum and an ultraviolet light absorption spectrum. For example, N-l,3-butadienylacetamide iε identified as follows. Physical properties of N-l .3-butadienylacetamide: Maεε εpectrum (electron impact method): 111, 69, 54, 43 Mass spectrum (chemical ionization method): 112 Infrared light absorption spectrum (cm "1 ): 3099, 1732, 1654, 1471 Ultraviolet light absorption spectrum (nm): 237, 276 In the present invention, the quantitative analysis of N-l,3-butadienylcarboxylic acid amide is preferably made by high-speed liquid column chromatography, but the present invention not limited to thiε method. For example, the following operating conditions of a high-speed liquid column chromatography are preferred. Unlesε otherwise indicated, the quantitative analysis was conducted under theεe conditions in the Examples of the present invention. Column: SHODEX* SIL5B (Showa Denko K.K.) Eluate: n-Hexane/iεopropyl alcohol (9/1), 1 mL/min Detector: Ultraviolet detector, 254 nm The preεent invention will be described below in greater detail by referring to the following Examples and Comparative Examples, however, the present invention should not be construed aε being limited thereto. EXAMPLE 1 To a three-necked flask (200 ml) with a thermometer and a dry ice-ethanol trap, 5.9 g (0.1 mol) of acetamide, 40 g (0.67 mol) of iεopropyl alcohol, 2.16 g (15 mmol) of ethylidenebiεacetamide and 14.6 g (0.1 mol) of acetaldehyde diiεopropylacetal were added, and the mixture was stirred and dissolved at a temperature of from 45 to 48°C until it became uniform. A solution containing 0.43 g (0.1 wt% baεed on the charge ) of concentrated sulfuric acid dissolved in 2 g (33 mmol ) of isopropyl alcohol (the same in the following examples) was added and after stirring, 17.6 g (0.4 mol) of acetaldehyde was added thereto through a dropping funnel over a period of 3 minutes. After the dropping addition, the mixture was allowed to react at 50°C for 3 hours to neutralize the catalyst. The reaction mixture was then analyzed by gas chromatography to find that the acetamide converεion was 88%, the selectivity of N-(l-propoxyethyl)acetamide was 94% and the selectivity of ethylidenebisacetamide aε a by-product was 5.3%. From the resulting reaction solution, N-(l- propoxyethyl)acetamide was obtained by distillation under reduced pressure and thermally cracked into N-vinylacetamide and iεopropyl alcohol at 450°C and a reεidence time of 1 second. The cracked εolution was cooled to 20°C and the mother εolution was separated therefrom at 1800 kg/cm 2 and 20°C in a high-presεure cryεtallizer. As a result, N- vinylaceta ide in a purity of 99.9% and having an N-l, 3- butadienyl content of 1 ppm was obtained. In order to evaluate the polymerizability of the N-vinylacetamide, diεtillated water was added thereto to give a concentration of 17 wt% and after displacement by nitrogen, 500 ppm of V-50 (N,N' -azobis-(2-amidinopropane) dihydrochloride was added thereto and the container was dipped in a constant-temperature water bath of 45°C. After 10 minutes, the system was diluted by 9 wt% of a 1% aqueous solution of hydroquinone and the viscosity was measured at 30°C and a revolution number of 30 rpm using a BL-type (rotary) viεcometer and found to be 130 cpε . COMPARATIVE EXAMPLE 1 The thermally cracked εolution obtained in Example 1 waε εubjected to εi ple distillation at a pressure of 0.5 mmHg to provide N-vinylacetamide in a purity of 97.5% and having an N-l,3-butadienylacetamide content of 200 ppm. The polymerizability evaluation waε conducted in the εame manner aε in Example 1, and the viεcoεity was found to be 10 cps or leεε . EXAMPLE 2 Synthesis of acetal: Into the fifth stage from the top of a glass-made Oldershaw-type rectification column having 25 stages, methanol containing 0.5 wt% of sulfuric acid was introduced in an amount of 180 g per hour and into the fifteenth stage from the top, acetaldehyde waε introduced in an amount of 27 g per hour. At the bottom of the rectification column, a 500 ml- volu e flask having placed therein 100 g of water waε provided and heated at 100°C to extract the content of the flask in an amount of 29 g per hour. The solution extracted from the flask contained substantially no organic matter. From the top of column, 221 g/h of a dimethylacetal-methanol mixture waε extracted at a reflux ratio of 2. The distillate contained substantially no water and aldehyde. The acetaldehyde conversion was 100% and the yield of di ethylacetal was 100%. Separation of acetal: Into the first stage from the top of a glass-made Olderεhaw-type rectification column having 25 stages, n-hexane waε introduced in an amount of 56 g per hour and into the tenth εtage from the top, dimethylacetal containing 28 wt % of methanol waε introduced in an amount of 71 g per hour. The column was heated so that the temperature of the column top waε maintained at 50 β C at a reflux ratio of 6. At the lower part of the rectification column, a 500 ml-volume flask having placed therein 100 g of dimethylacetal waε provided and heated by dipping it in an oil bath at 110°C to thereby extract the content of the flask in an amount- of 47 g per hour. The εolution extracted from the flask was dimethylacetal substantially free of n-hexane and containing 0.3% of methanol. From the top of column, 80 g/h of a dimethylacetal- methanol-n-hexane mixture waε extracted. The distillate and the bottom product each contained substantially no water or acetaldehyde. Svnthesiε of N-f1-methoxyethyl^acetamide: The high-purity dimethylacetal obtained in the acetal separation process and the dimethylacetal containing methanol obtained in the methanol recovery process were mixed, and dry acetamide was dissolved therein to prepare a reaction raw material solution of acetamide/dimethylacetal/methanol at a molar ratio of 1/20/3. Thiε solution was introduced from the lower part of a reaction tube having an inner diameter of 40 mm charged with 60 ml of a strongly acidic ion exchange resin Amberlist 15, in an amount of 5 ml per hour. Into the jacket of the reaction tube, hot water at 55°C was circulated to control the reaction temperature to 55°C. The reaction εolution obtained from the exit at the upper portion of the reaction veεεel waε εubjected to quantitative analysis and aε a reεult, it was found that the reaction εolution had a molarcomposition comprisingapproximatelyacetamide/dimethyl- acetal/methanol/MEA of 0/19/4/0.9, the acetamide conversion was 98% and the yield of N-(1-methoxyethyl)acetamide (MEA ) waε 90%. Recovery of acetal: The reaction solution obtained in the synthesis step of N-(1-methoxyethy1)acetamide was fed to a thin film-type continuous flash evaporator with a jacket having a heating area of 0.04 m 2 under a reduced preεεure of 100 mmHg, in an amount of 600 g per hour. In the jacket, a heating medium waε circulated. A reεidue on evaporation εubεtantially comprising N-(1-methoxyethyl)acetamide was obtained in an amount o f 1 7 g per hour. A solution comprising dimethylacetal containing 7 wt% of methanol with the volatile components being condense d was obtained in an amount of 583 g per hour. Recovery of methanol: Into the tenth stage from the top of a glasε-made Olderεhaw-type rectification column having 25 stages the dimethylacetal fraction of distillate containing 7 wt % of methanol obtained in the acetal recovery εtep was intro d uced in an amount of 200 g per hour. The column was heate d so that the temperature of the column top was maintained at 58 C C at a reflux ratio of 6. At the lower potion of the rectification column, a 500 ml-volume flask was provided and heate d by dipping it in an oil bath at 110°C to extract the content of the flask in an amount of 185 g per hour. The solution extracted from the flask was dimethylacetal containing 5.6 wt % of methanol. From the column top, a dimethylacetal-methanol azeotropic mixture (methanol: 24 wt%) was extracted in an amount of 15 g per hour. Svnthesiε of N-vinylacetamide: The εolution εubεtantially comprising N - ( l- methoxyethyl)acetamide obtained in the acetal recovery step waε fed into a stainless steel reaction tube having an inner diameter of 20 mm and a total length of 6 m, which was under a reduced pressure of 100 mmHg and heated at 450°C, in an amount of 20 ml per minute. The mixture of N-vinylacetami d e and methanol produced by the thermal cracking reaction waε condensed in a condenser provided at the exit of the reaction tube and recovered. The conversion of N-(1-methoxye t hyl ) - acetamide waε 92%. Concentration of N-vinylacetamide: Into the tenth stage from the top of a glass-made Oldershaw-type rectification column having 10 stages the reaction solution obtained in the syntheεis εtep of N- vinylacetamide waε introduced in an amount of 200 g per hour The pressure was 200 mmHg and the column was heated so that the temperature of the column top was maintained at 40°C at a reflux ratio of 2. A 500 ml-volume flask was provided at the lower portion of the rectification column and heated by dipping in an oil bath at 80°C to extract the content of the flask in an amount of 155 g per hour. The solution extracted from the flask was a crude N-vinylacetamide solution containing 94 wt% of N-vinylacetamide. From the column top, methanol was extracted in an amount of 45 g per hour. The crude N-vinylacetamide contained 70 ppm of N-l 3- butadienylacetamide. Purification of N-vinylacetamide: The crude N-vinylacetamide solution obtained in the concentration step of N-vinylacetamide was introduced into the fifth stage from the top of a glass-made Oldershaw-type rectification column having 10 stages and rectification waε conducted under a reduced pressure of 0.15 mmHg and at a reflux ratio of 3. As a result, N-vinylacetamide in a purity of 98% and containing 4 ppm of N-l , 3-butadienylacetamide was obtained. The polymerizability of the resulting N- vinylacetamide waε evaluated in the εame manner aε in Example 1, and the viεcoεity was found to be 100 cps. EXAMPLE 3 The crude N-vinylacetamide containing 70 ppm of N- 1,3-butadienylacetamide obtained in the concentration step of N-vinylacetamide in Example 2 was adjusted to 50°C and the mother εolution waε separated therefrom at 1,800 kg/cm 2 and 50°C in a high-pressure cryεtallizer. As a result, N- vinylacetamide in a purity of 99.9% and having an N-l, 3- butadienylacetamide content of 1 ppm or less was obtained. The polymerizability of the resulting N-vinylacetamide waε evaluated in the same manner aε in Example 1 , and the viεcoεity was found to be 150 cpε . EXAMPLE 4 The methanol εolution of N-vinylacetamide containing 70 ppm of N-l,3-butadienylacetamide obtained in the synthesis step of N-vinylacetamide in Example 2 was introduced into a column charged with activated carbon at a LHSV of 2 Hr " ' and room temperature. The resulting solution contained 9 ppm of N-l, 3-butdienylacetamide. The resulting solution waε subjected to simple distillation under reduced pressure. The polymerizability of the distilled N-vinylacetamide was evaluated in the same manner as in Example 1 , and the viscosity was found to be 70 cps. EXAMPLE 5 To 50 parts by weight of N-vinylacetamide containing 200 ppm of N-l,3-butadienylacetamide obtained in Comparative Example 1, 50 parts by weight of toluene was added and diεεolved at 40°C in a nitrogen atmosphere. After cooling the mixture to 4°C, the precipitated cryεtalε were εeparated by filtration and dried under reduced preεεure. The resulting N-vinylacetamide contained 8 ppm of N-l, 3-butadienylacetamide. The polymerizability of this product was evaluated in the same manner aε in Example 1, and the viscosity was found to be 80 cps. EXAMPLE 6. To the methanol εolution of N-vinylacetamide containing 70 ppm of N-l,3-butadienylacetamide obtained in the εyntheεiε step of N-vinylacetamide in Example 2, p- benzoquinone was added to give a concentration of 97 ppm (1.5 equivalent to the N-l,3-butadienylacetamide) and stirred at room temperature for 1 hour. The reaction solution contained 9 ppm of N-l,3-butadienylacetamide. The solution was subjected to simple distillation under reduced pressure. The polymerizability of the distilled N-vinylacetamide was evaluated in the same manner as in Example 1, and the viscosity was found to be 70 cps. EXAMPLE 7 Five grams of solid catalyst comprising 0.5 wt% of palladium carried on an alumina carrier and 50 g of crude N- vinylacetamide containing 70 ppm of N-l, 3-butadienylacetamide 55 wt% of N-vinylacetamide, 15 wt% of N-( 1-methoxyethyl)- acetamide, 7 wt% of acetamide and 20 wt% of methanol were placed in a 200 ml-volume flask and reacted in a hydrogen atmosphere at room temperature for 30 minuteε while εtirring. After completion of the reaction, the catalyεt waε εeparated by filtration from the reaction solution and the filtrate waε analyzed. The N-l,3-butadienylacetamide waε analyzed by HPLC and other components were analyzed by gas chromatography. The reaction εolution had an N-l,3-butadienylacetamide content of 1 ppm or leεε and contained 0.4 wt% of N-ethylacetamide. After removing methanol by distillation under reduced presεure, N-vinylacetamide was crystallized in a high- pressure crystallizer under a pressure of 2,000 kg/cm 2 and the mother solution was εeparated at 40°C. The reεulting N- vinylacetamide had a purity of 99.3 wt% and an N-l,3- butadienylacetamide content of 1 ppm or lesε. The polymerizability was evaluated in the same manner as in Example 1, and the viscosity was found to be 160 cps. EXAMPLE 8 A solid catalyst (2.5 g) comprising 0.5 wt% of palladium carried on an alumina carrier was charged into a hydrogenation reactor in a nitrogen atmosphere. Under the nitrogen atmosphere, 95 kg of crude N-vinylacetamide containing 400 ppm of N-l, 3-butadienylacetamide, 55 wt% of N- vinylacetamide, 15 wt% of N-(1-methoxyethyl)acetamide and 20 wt% of methanol was circulated in the reaction vessel at a liquid space velocity (LHSV) of 60 Hr "1 for 6 hourε . The reaction εolution had an N-l,3-butadienylacetamide content of 1 ppm or leεε and contained 0.3 wt% of N-ethylacetamide. After removing methanol by diεtillation under reduced pressure from the reεulting reaction εolution, the mother solution was separated in a high-pressure crystallizer at 1,800 kg/cm 2 and 30°C. As a result, N-vinylacetamide in a purity of 99.5 wt% and having an N-l, 3-butadienylacetamide content of 1 ppm or less waε obtained. The polymerizability of the resulting N- vinylacetamide waε evaluated in the same manner as in Example 1, and the viscoεity waε found to be 170 cpε . EXAMPLE 9 A εolid catalyst (25 g) compriεing 0.5 wt% of palladium carried on an alumina carrier was charged in a hydrogenation reaction vesεel in a nitrogen atmoεphere and crude N-vinylacetamide containing 400 ppm of N-l, 3-butadienyl- acetamide, 55 wt% of N-vinylacetamide, 15 wt% of N- ( l- methoxyethyl)acetamide, 7 wt% of acetamide and 20 wt% of methanol was circulated therein. The reaction solution had an N-l, 3-butadienylacetamide content of 1 ppm or less and contained 0.5 wt% of N-ethylacetamide. After removing methanol from the resulting reaction solution, the mother εolution waε εeparated in a high-preεsure crystallizer at 1,800 kg/cm 2 and 30°C. As a result, N- vinylacetamide in a purity of 99.5 wt% and having an N-l, 3- butadienylacetamide content of 1 ppm or less was obtained. The polymerizability of the resulting N- vinylacetamide waβ evaluated in the εame manner as in Example 1, and the viscosity waε found to be 170 cpε. EXAMPLE 10 This experiment waε conducted in the same manner as in Example 7, except that a catalyst comprising 0.05 wt% of palladium and 0.3 wt% of silver carried on an alumina carrier was used in place of the catalyst comprising 0.5 wt% of palladium carried on an alumina carrier, and the reaction time waε changed to 1 hour. The reaction solution had an N-l,3- butadienylacetamide content of 1 ppm or less and contained 0.4 wt% of N-ethylacetamide. The polymerizability of the resulting N-vinylacetamide was evaluated in the same manner as in Example 1, and the viεcoεity was found to be 170 cps. EXAMPLE 11 The dimethylacetal containing 7 wt% of methanol obtained in the acetal recovery step in Example 2 was returned to the acetal separation step and methanol was separated from acetal. The resulting acetal contained 250 ppm of 1,1,3- trimethoxybutane . This acetal was diεtilled uεing a packed column having a theoretical plate number of 20 at a reflux ratio of 5 to obtain acetal containing 12 ppm of 1,1,3- trimethoxybutane. The resulting acetal was subjected to the processeε εubεequent to the εyntheεiε step of 1-methoxyethyl- acetamide as described in Example 3. However, in this example, simple distillation was conducted in place of rectification at the N-vinylacetalamide purification step. The resulting N-vinylacetamide contained 10 ppm of N-l, 3- butadienylacetamide. The polymerizability of the resulting product waε evaluated in the same manner as in Example 1 , and the viscosity waε found to be 70 cps. COMPARATIVE EXAMPLE 2 N-vinylacetamide was produced in the same manner as in Example 1 except for omitting the rectification of acetal. The resulting N-vinylacetamide contained 230 ppm of N-l, 3- butadienylacetamide. The polymerizability was evaluated in the εame manner as in Example 1, and the viscosity was found to be 10 cps or leεε. EXAMPLE 12 The residue on evaporation substantially comprising N-(l-methoxyethyl)acetamide obtained in the acetal recovery step in Example 2 was distilled using a packed column having a theoretical plate number of 20 at a reflux ratio of 6 to obtain N-( 1-methoxyethyl )acetamide containing 11 ppm of N- ( 1,3-dimethoxybutyl)acetamide. The resulting N-(l- methoxyethyl)acetamide waε εubjected to the synthesis step of N-vinylacetamide aε deεcribed in Example 3, except that εimple diεtillation waε conducted in place of rectification at the N-vinylacetamide purification εtep. The reεulting N- vinylacetamide contained 9 ppm of N-l, 3-butadienylacetamide. The polymerizability waε evaluated in the εame manner as in Example 1, and the viεcoεity waε found to be 80 cpε . COMPARATIVE EXAMPLE 3 N-vinylacetamide waε produced in the same manner aε in Example 9, except for omitting rectification of N-(l- methylethyl)acetamide. The resulting N-vinylacetamide contained 230 ppm of N-l,3-butadienylacetamide. The polymerizability waε evaluated in the εame manner aε in Example 1, and the viscosity was found to be 10 cps or lesε. EXAMPLE 13 To a glaεε-made reaction veεεel, 745 g of water, 250 g of N-vinylacetamide obtained in Example 2 and 0.409 g of N,N'~(diacetyl)-N,N'-(divinyl)-l,4-biε(aminomethyl)cyclohexane aε a croεεlinking agent were added and diεsolved. After removing the disεolved oxygen by a nitrogen gas, 0.075 g of 2,2'-azobis 2-( 2-imidazolin-2-yl )propane dihydrochloride disεolved in 5 ml of deaerated water was added, and the mixture was insulated from heat and allowed to stand. After 7 hours, the inner temperature of the reaction vessel reached 71°C due to polymerization heat and thereafter, the inner temperature gradually lowered due to heat loεε . The time period from the addition of a polymerization initiator to the time when the inner temperature of the reaction veεsel reaches a maximum iε called the "peak achieving time". In thiε example, the peak achieving time was 7 hours. COMPARATIVE EXAMPLE 4 The polymerization waε conducted in the εame manner aε in Example 13, except that N-vinylacetamide obtained in Comparative Example 1 waε used in place of N-vinylacetamide obtained in Example 2. An increase of inner temperature accompanying the polymerization was not observed even after 48 hours from the addition of the polymerization initiator. EXAMPLE 14 To a glass-made reaction vessel, 745 g of water, 225 g of N-vinylacetamide obtained in Example 2 and 27.6 g of sodium acrylate were added and dissolved. After removing the diεεolved oxygen by a nitrogen gaε, 0.075 g of 2,2'-azobis 2- (2-imidazolin-2-yl)propane dihydrochloride disεolved in 5 ml of deaerated water was added, and the mixture was isolated from heat and allowed to stand. The peak achieving time was 6 hours . COMPARATIVE EXAMPLE 5 The polymerization was conducted in the same manner as in Example 13, except that N-vinylacetamide was used in place of N-vinylacetamide obtained in Example 2. An increase of inner temperature accompanying the polymerization was not observed even after 48 hours from the addition of the polymerization initiator. EXAMPLE 15 A solution having a pH of 6.5 (determined with a 33 wt% aqueous εolution) and containing 98 wt% of N-(l- methoxyethyl)acetamide and 2 wt% of acetamide was fed in an amount of 35 g per minute into a stainless steel reaction tube having an inner diameter of 21 mm and a total length of 6 m. The reaction tube waε heated to 400°C, and the pressure was reduced to 100 mmHg. A mixture of N-vinylacetamide and methanol produced by the thermal cracking reaction waε condensed in a condenεer provided at the exit of the reaction tube and recovered. The N-vinylacetamide contained 28 ppm of N-l,3-butadienylacetamide, and the conversion of N-(l- methoxyethyl ) acetamide was 85%. The resulting reaction εolution waε introduced into the tenth εtage from the top of a glass-made Oldershaw-type rectification column having 10 stageε, in an amount of 200 g per hour. The column was heated so that the temperature of the column top was maintained at 40°C at a reduced pressure of 200 mH and a reflux ratio of 2. A 500 ml-volume flask waε provided at the lower portion of the rectification column and heated by dipping in an oil bath of 80°C to extract the content of the flask in an amount of 155 g per hours. The solution extracted from the flask was a crude N-vinylacetamide solution containing 94 wt% of N-vinylacetamide. From the top of column, methanol waε extracted in an amount of 45 g per hour. The reεulting crude N-vinylacetamide εolution was introduced into the fifth stage from the top of a glass-made Oldershaw-type rectification column having 10 stages and subjected to rectification in a reduced presεure of 0.15 mmHg and at a reflux ratio of 3. Aε a reεult, N-vinylacetamide in a purity of 98% and having an N-l,3-butadienylacetamide content of 0.9 ppm waε obtained. The polymerizability of the reεulting N-vinylacetamide waε evaluated in the εame manner aε in Example 1, and the viscosity -was found to be 100 cps. REFERENCE EXAMPLE 1 From the methanol solution of N-vinylacetamide containing 28 ppm of N-l,3-butadienylacetamide obtained in Example 15, the methanol was removed by distillation. After concentrating the N-vinylacetamide, the temperature waε adjusted to 50°C and the mother solution was separated in a high-preεεure cryεtallizer at 1,800 kg/cm 2 and 50°C. Aε a result, N-vinylacetamide in a purity of 99.9% and containing 2 ppm of N-l,3-butadienylacetamide was obtained. The polymerizability of the resulting N-vinylacetamide was evaluated in the same manner as in Example 1, and the viscosity was found to be 80 cpε. EXAMPLES 16 TO 19 AND COMPARATIVE EXAMPLES 6 TO 9 A reaction waε conducted in the same manner as in Example 15, except for using N-(1-methoxyethyl )acetamide containing N-vinylacetamide in various concentrations as a raw material. The reaction reεultε, the contentε of N-l,3- butadienylaceta ide after purification and the polymerizability of the N-vinylacetamide are εhown together in Table 1 below. TABLE 1 N-vinyl¬ 33Z Conversion N-Butadienyl- Polymeriz¬ acetamide PH (Z) acetamide ability (X) (ppm) (cpε) Example 16 3 6 85 2 80 Example 17 5 7 84 6 50 Example IB 7 8 86 10 30 Example 19 1 5 86 10 35 Comparative 1 A 87 50 8 Example 6 Comparative 3 87 400 5 Example 7 Comparative 11 8 86 40 5 Example 8 Comparative 13 8 86 50 5 Example 9 The crude N-vinylacetamide is obtained, for example, (1) by dealcoholization of an N-( 1-alkoxyethyl )carboxyl Lc acid amide, (2) by dealcoholization of an N-(l- alkoxyethyl)carboxylic acid amide obtained as an intermediate from carboxylic acid amide, acetaldehyde and alcohol or from carboxylic acid amide and acetaldehyde dialkylacetal, or (3 ) by cracking ethylidene biscarboxylic acid amide or by cracking an ethylidene biscarboxylic acid amide obtained aε an intermediate from acetaldehyde and carboxylic acid amide. The crude N-vinylacetamide obtained by these known production proceεεes does not have good polymerizability. However, when the N-l,3-butadienylcarboxylic acid amide content in the crude N-vinylcarboxylic acid amide iε reduced to 30 ppm or leεε, preferably 10 ppm or leεε, more preferably 1 ppm or leεs, and then the crude vinylcarboxylic acid amide is subjected to a purification proceεs, a highlypolymerizable N-vinylcarboxylic acid amide having improved polymerizability is obtained. Furthermore, the N-l,3-butadienylcarboxylic acid amide can be reduced to the above-described content range by subjecting a crude N-vinylcarboxylic acid amide or solution thereof to a purification proceεs such as a rectification method, a recrystallization method, a pressure crystallization method, a physical adsorption method using an activated carbon adsorbent, a Diels-Alder reaction method or a selective hydrogenation reaction method of a 1,3-butadienyl group. Furthermore, according to the present invention, in producing N-vinylcarboxylic acid amide by thermal cracking or catalytic cracking of N-(1-alkoxyethyl)carboxylic acid amide or ethylidenebiscarboxylic acid amide, a highly polymerizable N-vinylcarboxylic acid amide can be obtained. Furthermore, a polymer of a high molecular weight N-vinylcarboxylic aci d amide can be produced using the highly polymerizable monomer. While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Claims: CLAIMS 1. A highly polymerizable N-vinylcarboxylic acid amide having an N-l,3-butadienylcarboxylic acid amide content of 30 ppm or less. 2. A proceεs for producing a highly polymerizable N-vinylcarboxylic acid amide, which comprises reducing the content of N-l,3-butadienylcarboxylic acid amide in a crude N-vinylcarboxylic acid amide to 30 ppm or less . 3. The proceεε for producing a highly polymerizable N-vinylcarboxylic acid amide aε claimed in claim 2, wherein the N-vinylcarobxylic acid amide is produced by dealcoholization of an N-(l-alkoxyethyl)carboxylic acid amide or by dealcoholization of an N-(1-alkoxyethyl)carboxylic acid amide obtained aε an intermediate from carboxylic acid amide, acetaldehyde and alcohol or from carboxylic acid amide and acetaldehyde dialkylacetal. 4. The proceεε for producing a highly polymerizable N-vinylcarboxylic acid amide as claimed in claim 2, wherein the N-vinylcarobxylic acid amide is produced by cracking ethylidene biscarboxylic acid amide or by cracking an ethylidene biεcarboxylic acid amide obtained as an intermediate from acetaldehyde and carboxylic acid amide. 5. The procesε for producing a highly polymerizable N-vinylcarboxylic acid amide as claimed in claim 2, wherein said reducing step comprises purifying the crude N- vinylcarboxylic acid amide or a solution thereof using a rectification method, a recrystallization method, a presεure cryεtallization method, a phyεical adsorption method with an activated carbon adsorbent, a Diels-Alder reaction method or a method comprising selective hydrogenation of a 1,3- butadienyl group. 6. A proceεε for producing a homopolymer of N- vinylcarboxylic acid amide or a copolymer thereof with another copolymerizable monomer, compriεing polymerizing a highly polymerizable N-vinylcarboxylic acid amide having an N- l ,3- butadienylcarboxylic acid amide content of 30 ppm or leεε. 7. A proceεε for producing a highly polymerizable N-vinylcarboxylic acid amide aε claimed in claim 5, wherein said purifying compriseε conducting a catalytic hydrogenation of N-l,3-butadienylcarboxylic acid amide. 8. A process for producing a highly polymerizable N-vinylcarboxylic acid amide, comprising thermal cracking or catalytic cracking of N-(1-alkoxyethyl)carboxylic acid amide or ethylidenebiscarboxylic acid amide to produce an N - vinylcarboxylic acid amide, wherein the N-vinylcarboxylic acid amide content of the N-(l-alkoxyethyl)carboxylic acid amide or ethylenebiscarboxylic acid amide is 10 wt% or less. 9. A proceεs for producing a highly polymerizable N-vinylcarboxylic acid amide, comprising thermal cracking or catalytic cracking of N-(l-alkoxyethyl)carboxylic acid amide or ethylidenebiεcarboxylic acid amide, wherein a 33 wt% aqueouε εolution of the N-(1-alkoxyethyl)carboxylic acid amide or ethylenebiscarboxylic acid amide used to produce the N- vinylcarboxylic acid amide is adjusted to have a pH of from 5 to 10. 10. A proceεs for producing a highly polymerizable N-vinylcarboxylic acid amide, comprising thermal cracking or catalytic cracking of N-(1-alkoxyethyl)carboxylic acid amide or ethylidenebiεcarboxylic acid amide, wherein the N- vinylcarboxylic acid amide content of the N-(l- alkoxyethyl)carboxylic acid amide or ethylidenebiεcarboxylic acid amide iε 10 wt% or leεε, and a 33 wt% aqueous solution of the N-(l-alkoxyethyl)carboxylic acid amide or ethyli d enebiscarboxylic acid amide used to produce the N- vinylcarboxylic acid amide is adjusted to have a pH of from 5 to 10. 11. A homopolymer of N-vinylcarboxylic acid amide or a copolymer thereof with another copolymerizable monomer having a weight average molecular weight of 500,000 or more, prepared by polymerizing a highly polymerizable N - vinylcarboxylic acid amide having an N-l, 3- butadienylcarboxylic acid amide content of 30 ppm or l eεε . 12. The product of claim 11, having a weight average molecular weight of 1,000,000 or more.

Inventors: AIZAWA TOSHIYUKI, KUDO TETSUO, NAGAMATSU SHUN-ICHI, NAKAMURA HITOSHI, SHIMAMURA KENJI, WAKABAYASHI KUNITOSHI, AIZAWA, TOSHIYUKI, KUDO, TETSUO, NAGAMATSU, SHUN-ICHI, NAKAMURA, HITOSHI, SHIMAMURA, KENJI, WAKABAYASHI, KUNITOSHI
Applicants: SHOWA DENKO KK, SHOWA DENKO K.K.
Application year: 1995
Main classification: C07C233
IPC codes: C07C231, C07C233, C08F26