GB2044252A - Process for preparing polymethylene polyphenyl polycarbamates - Google Patents

Abstract

In the preparation of polymethylene polyphenyl polycarbamates of general formula <IMAGE> wherein R1 is C1-6 alkyl or C5-10 cycloalkyl, R2 is H, halogen, C1-6 alkyl or C1-6 alkoxy, n is 1-4 and m is 0-5 by reacting an N-phenyl carbamic acid ester of general formula <IMAGE> with formaldehyde or a formaldehyde-producing compound in the presence of water and an acid catalyst, the acid catalyst is used in the form of its aqueous solution having an acid catalyst concentration, at the start of the reaction, of at least 10% by weight and in an amount, at the start of the reaction, of from 0.01 to 100 moles per mole of the N-phenyl carbamic acid ester, and the reaction is carried out at a temperature of from 20 to 150 DEG C. After completion of the reaction, the aqueous acid catalyst solution containing small amounts of organic impurities may be recovered, subjected to adjustment of its acid catalyst concentration, and then reused directly for a subsequent reaction.

Description

SPECIFICATION Process for preparing polymethylene polyphenyl polycarbamates This invention relates to an improved process for preparing polymethylene polyphenyl poiycarbamates from N-phenyl carbamic acid esters and formaldehyde.

Polymethylene polyphenyl polycarbamates are substances that are useful in the manufacture of agricultural chemicals, drugs, polyamides, polyurethanes, and the like. In addition, polymethylene polyphenyl polycarbamates can be thermally decomposed to produce the corresponding polymethylene polyphenyl polyisocyanates.

Accordingly, it is desirable to develop new processes for preparing polymethylene polyphenyl polycarbamates with industrial advantages.

One well-known prior art process for preparing polymethylene polyphenyl polycarbamates comprises reacting a corresponding polymethylene polyphenyl polyisocyanate with alcohol. However, the preparation of the polymethylene polyphenyl polyisocyanate used as a starting material involves the use of highly toxic aniline and phosgene and, moreover, requires a complicated procedure.

Another well-known prior art process for preparing polymethylene polyphenyl polycarbamates comprises reacting a corresponding polymethylene polyphenyl polyamine with a chloroformic acid alkyl ester. However, the polymethylene polyphenyl polyamine and chloroformic acid alkyl ester used as starting materials both have such severe intoxicating and irritating properties that they are very difficult to handle, and the procedures for preparing them are complicated. For these reasons, this process cannot be regarded as useful in industrial applications.

There is still another well-known prior art process for preparing polymethylene polyphenyl polyca rbamates by reacting an N-phenyl carbamic acid ester with formaldehyde. For example, as is described in German Patent No. 1,042,891, an N-phenyl carbamic acid ester and formaldehyde may be heated in an aqueous solution of hydrochloric acid to obtain a condensation product which consists mainly of polymethylene polyphenyl polycarbamates.

While much is known about the reaction of an aromatic amine (e.g., aniline) with formaldehyde, the aforesaid German Patent is the only publication that deals with the reaction of an N-phenyl carbamic acid ester with formaldehyde. Accordingly, nothing is known about the reactivity of N-phenyl carbamic acid esters as well as the action of an aqueous acid solution used as catalyst and its activity after completion of the reaction. The commonly practiced reaction of aniline with formaldehyde involves a great difficulty in that the aqueous acid solution used as catalyst, which reacts with the resulting polyamine to form a salt, is neutralized and discharged as waste water instead of being recovered for recycling.

However, the process described in the aforesaid German Patent No. 1,042,891 exhibits such a low reaction rate that large amounts of unreacted starting materials remain even after the reaction has been carried out for a long period of time. Furthermore, the selectivity to the desired useful diphenylmethane - 4,4' - dicarbamic acid diester is so low that polymethylene polyphenyl polycarbamates, called polynuclear compounds, having three or more phenyl radicals as well as various by-products and intermediate products are formed in large amounts.

Thus, no process for preparing polymethylene polyphenyl polycarbamates with industrial advantages has been available so far.

It is an object of the present invention, at least in a preferred example, to provide a process for preparing polymethylene polyphenyl polycarbamates from N-phenyl carbamic acid esters and formaldehyde which process can achieve a higher reaction rate than has been attainable by prior art processes.

It is another object of the present invention, at least in a preferred example, to provide a process for preparing polymethylene polyphenyl polycarbamates from N-phenyl carbamic acid esters and formaldehyde which process can exhibit a high selectively to the corresponding diphenylmethane - 4,4' dicarbamic acid diester.

It is still another object of the present invention, at least in a preferred example, to provide a process for preparing polymethylene polyphenyl polycarbamates from N-phenyl carbamic acid esters and formaldehyde in which process, after completion of the reaction, the aqueous acid solution used as catalyst can be recycled to a subsequent reaction.

According to the present invention there is provided a process for preparing a polymethylene polyphenyl polycarbamate of the general formula

where R1 is an alkyl radical of from 1 to 6 carbon atoms or a cycloalkyl radical of from 5 to 10 carbon atoms, R2 is a hydrogen atom, a halogen atom, an alkyl radical of from 1 to 6 carbon atoms, or an alkoxy radical of from 1 to 6 carbon atoms, n is a positive integer of from 1 to 4, and m is zero or a positive integer of from 1 to 5, by reacting an N-phenyl carbamic acid ester of the general formula

wherein P1, R2, and n have the same meanings as described above for formula (I), with formaldehyde or a formaldehyde-producing compound in the presence of water and an acid catalyst, wherein the improvement comprises (a) using the acid catalyst in the form of its aqueous solution having an acid catalyst concentration, at the start of the reaction, of at least 10% by weight and in an amount, at the start of the reaction, of from 0.01 to 100 moles per mole of the N-phenyl carbamic acid ester and (b) carrying out the reaction at a temperature of from 20 to 1 500C.

After completion of the above-described reaction, the solid reaction product or the organic layer containing it is separated from the aqueous acid catalyst solution containing small amounts of organic impurities, and the desired polymethylene polyphenyl polycarbamate is isolated from the organic layer. On the other hand, the aqueous acid catalyst solution may be reused for a subsequent reaction of the N-phenyl carbamic acid ester with formaldehyde or a formaldehyde-producing compound without removing the organic impurities contained therein but after adjusting its acid catalyst concentration to such a level as to provide an aqueous acid catalyst solution having an acid catalyst concentration, at the start of the subsequent reaction, of at least 10% by weight.

Close examination of the reaction of an N-phenyl carbamic acid ester with formaldehyde has revealed that, if this reaction is carried out in the presence of water and an acid catalyst, a solid reaction product or an oily layer containing it spontaneously separates from the aqueous acid solution used as catalyst. Accordingly, the aqueous acid solution can be recovered with ease. The recovered aqueous acid solution contains, in addition to unreacted formaldehyde, a total of from 0.1 to 3% by weight of organic impurities which are considered to be unreacted carbamic acid ester, reaction product, by-products, and intermediate products. Most of the aqueous acid solution used as catalyst is recovered without mixing in the reaction product layer.However, if the recovered aqueous acid solution is reused directly for a subsequent reaction, the reaction rate becomes so low that it is difficult to repeat its reuse many times. Upon examination of the reason for this, it has been found that the reduction in the activity of the recovered aqueous acid solution is not attributable to the presence of organic impurities therein, but to the decrease in its acid concentration resulting from the formation of water during the reaction, the loss caused by aftertreatment, and the like. It has also been found that, when an aqueous acid solution is used as catalyst in this reaction, it should have an acid coilcentration of at least 10% by weight at the start of the reaction and higher acid concentrations give higher reaction rates.This is quite unexpected and surprising in view of the fact that, in the well-known reaction of an aromatic amino compound (particularly, aniline) with formaldehyde, the reaction rate becomes higher as the acid concentration is decreased (see, for example, Y. Ogata, et al.: J. Am. Chem. Soc.,37, 1715(1951)).

The N-phenyl carbamic acid ester used in the process of the present invention is a compound represented by the general formula (II). In this formula, R1 is an alkyl radical such as methyl, ethyl,n-propyl, isopropyl,n-butyl,sec-butyl, isobutyl,tert-butyl, any of the pentyl radicals derived from n-pentane and its isomers, any of the hexyl radicals derived from n-hexane and its isomers, etc.; or a cycloalkyl radical such as cyclopentyl, cyclohexyl, etc.; and R2 is a hydrogen atom; a halogen atom such as chlorine, bromine, fluorine, etc.; an alkyl radical such as methyl, ethyl,n-propyl, isopropyl, n-butyl, sec-butyi, isobutyl, tert-butyl, any of the pentyl radicals derived from n-pentane and its isomers, any of the hexyl radicals derived from n-hexane and its isomers, etc.; or an alkoxy radical composed of any one of the foregoing alkyl radicals and an oxygen atom.

More specifically, the useful N-phenyl carbamic acid esters include phenyl carbamic acid alkyl esters of the general formula (II) in which R1 is an alkyl radical as defined above and R2 is a hydrogen atom; halophenyl carbamic acid alkyl esters of the general formula (II) in which R1 is an alkyl radical as defined above and R2 is a halogen atom as defined above; alkylphenyl carbamic acid alkyl esters of the general formula (II) in which R1 and R2 are alkyl radicals as defined above; alkoxyphenyl carbamic acid alkyl esters of the general formula (II) in which R1 is an alkyl radical as defined above and R2 is an alkoxy radical as defined above; phenyl carbamic acid cyclopentyl orcyclohexyl ester of the general formula (II) in which R1 is a cyclopentyl or cyclohexyl radical and R2 is a hydrogen atom; halophenyl carbamic acid cyclopentyl orcyclohexyl esters of the general formula (II) in which P1 is a cyclopentyl or cyclohexyl radical and R2 is a halogen atom as defined above; alkylphenyl carbamic acid cyclopentyl or cyclohexyl esters of the general formula (II) in which R1 is a cyclopentyl or cyclohexyl radical and R2 is an alkyl radical as defined above; alkoxyphenyl carbamic acid cyclopentyl orcyclohexyl esters of the general formula (II) in which P1 is a cyclopentyl or cyclohexyl radical and R2 is an alkoxy radical as defined above; and the like.

The preferred N-phenyl carbamic acid esters are phenyl carbamic acid methyl ester, phenyl carbamic acid ethyl ester, phenyl carbamic acidn-propyl ester, phenyl carbamic acid isopropyl ester, phenyl carbamic acidn-butyl ester, phenyl carbamic acid secbutyl ester, phenyl carbamic acid isobutyl ester, phenyl carbamic acid tert-butyl ester, phenyl carbamic acid pentyl ester, phenyl carbamic acid hexyl ester, o-chlorophenyl carbamic acid methyl ester, o-chlorophenyl carbamic acid ethyl ester, o::chlorophenyl carbamic acid isopropyl ester, o-chlorophenyl carbamic acid isobutyl ester, o-methylphenyl carbamic acid methyl ester, o-methylphenyl carbamic acid ethyl ester, phenyl carbamic acid cyclohexyl ester, o-chlorophenyl carbamic acid cyclohexyl ester, o-methylphenyl carbamic acid cyclohexyl ester,m-methoxyphenyl carbamic acid methyl ester, phenyl carbamic acid cyclopentyl ester, and the like.

In the process of the present invention, the aforesaid N-phenyl carbamic acid ester is reacted with formaldehyde or a formaldehyde-producing compound. The formaldehyde-producing compound may be any compound that can produce formaldehyde under the reaction conditions of the present invention, and specific examples thereof include paraformaldehyde, methylal, and other formals. Usually, an aqueous solution of formaldehyde is used.

The acid used in the process of the present invention may be a mineral acid such as hydrochloric acid, sulfuric acid, phosphoric acid, boric acid, etc. or an organic acid such as formic acid, acetic acid, oxalic acid, toluenesulfonic acid, etc. The so-called super acids such as hydrobromic acid, perchloric acid, chlorosulfonic acid, trifluoromethanesulfonic acid, etc. may also be used effectively. Usually, hydrochloric acid or sulfuric acid is preferred. In order to recover and reuse it, the acid need be used in the form of its aqueous solution. However, the acid and water may be added to the reactor in any suitable way that results in the formation of an aqueous acid solution in the reactor. For example, the acid and water may be added separately to the reactor, or the acid and an aqueous formaldehyde solution may be added to the reactor.

Although the process of the present invention can be carried out in the absence of solvent, a suitable solvent may be used, for example, in order to faciiitate the handling of starting materials and/or reaction products having high melting points. In this case, the solvent must be inert to formaldehyde.

Specific examples of the suitable solvent include aliphatic hydrocarbons such as hexane, heptane, etc.; alicyclic hydrocarbons such as cyclopentane, cyclohexane, etc.; halogenated hydrocarbons such as chloroform, methylene chloride, carbon tetrachloride, dichloroethane, trichloroethane, tetrach loroethane, etc.; fatty acid alkyl esters such as ethyl acetate, etc.; and the like. Aromatic compounds such as benzene, toluene, etc. are generally unsuitable for use in the process of the present invention, because they tend to react with formaldehyde. However, they can be used under those conditions which do not allow them to react materially with formaldehyde.

In carrying out the process of the present invention, the formaldehyde (or formaldehyde-producing compound) is generally used in an amount of from 0.1 to 10 moles and preferably from 0.2 to 2 moles per mole of the N-phenyl carbamic acid ester. If the amount of formaldehyde used is too small, the remaining unreacted N-phenyl carbamic acid ester increases, while if it is too large, the formation of polymethylene polyphenyl polycarbamates (or polynuclear compounds) having three or more phenyl radicals results.

The acid should be used in the form of its aqueous solution having an acid concentration, atthe start of the reaction, of at least 10% by weight. If the acid concentration is lowerthan 10% by weight, the reaction becomes slow and the selectivitY to the corresponding diphenylmethane - 4,4' - dicarbamic acid ester is reduced. High concentrations in the vicinity of 100% by weight may be used effectively, but are practically unsuitable for use in industral applications. Generally, if the acid concentration is too high, side reactions such as hydrolysis are apt to take place and, therefore, the quality and yield of the desired product tend to be reduced. The preferred range is from 20 to 95% by weight.

The acid should also be used in an amount, at the start of the reaction, of from 0.01 to 100 moles per mole of the N-phenyl carbamic acid ester. If the amount of acid used is less than 0.01 mole, the reaction becomes slow and the selectivity to the desired product is reduced. Although it is possible to use more than 100 moles of the acid, such large amounts are not needed in ordinary cases. The preferred range is from 0.1 to 50 moles per mole of the N-phenyl carbamic acid ester and the most preferred range is from 0.2 to 10 moles per mole of the N-phenyl carbamic acid ester.

The amount of solvent used depends on the properties of the starting materials and the reaction product. However, the solvent is generally used in an amount of from 0.1 to 100 parts by weight and preferably from 0.2 to 50 parts by weight per part by weight of the N-phenyl carbamic acid ester.

The reaction is carried out at a temperature of from 20 to 1 50 C and preferably from 30 to 1 000C. If the reaction temperature is lower than 20"C, the reaction becomes very slow, while if it is higher than 1 500C, side reactions such as hydrolysis takes place so that the amount of by-products is increased and, therefore, the yield and purity of the desired product are reduced.

Generally speaking, the process of the present invention may be carried out by providing the N-phenyl carbamic acid ester as it is or in the form of its solution or suspension in a properly selected solvent, adding the formaldehyde or formaldehydeproducing compound and the aqueous acid solution thereto, and then stirring the resulting reaction mixture at a predetermined temperature. Alternatively, the process of the present invention may also be carried out by adding a formaldehyde solution drop by drop to a solution or suspension containing the N-phenyl carbamic acid ester and the aqueous acid solution.

Furthermore, the process of the present invention may be carried out in a continuous operation system in which a solution or suspension containing the starting materials, the solvent, and the aqueous acid solution in an appropriate proportion is continuously fed to a reactor and continuously withdrawn therefrom after a predetermined residence time.

The reaction time depends on the types or amounts of starting materials and acid used, the type of operation, reaction conditions, and the like.

In the case of batch operation, it may generally range from 1 to40 hours.

After completion of the reaction, the solid reaction product or the oily layer containing it is isolated from the aqueous acid solution by any suitable technique such as the use of a separating funnel, filtration, etc.

The recovered aqueous acid solution can be reused for a subsequent reaction after its acid concentration is adjusted to such a level as to provide an aqueous acid solution having an acid concentration, at the start of the subsequent reaction, of at least 10% by weight. More specifically, the recovered aqueous acid solution is diluted with an appropriate amount of water if its acid concentration is too high, or strengthened by concentration or other suitable techniques if its acid concentration is too low. It is of course that the recovered aqueous acid solution may be directly reused if its acid concentration falls within the above-defined range.However, owing to the formation of water during the reaction, the losses caused by migration to the reaction product layer and evaporation during the reaction, the loss caused by after-treatment, and the like, the acid concentration of the recovered aqueous acid solution usually differs from the original one and tends to depart from the above-defined range. Thus, adjust mentofthe acid concentration of the recovered aqueous acid solution is more or less required if it is desired to repeat its reuse many times. Especially when the recovered aqueous acid solution is used in a recycling manner, a constant reaction rate and hence consistent results can be obtained by adjusting the acid concentration of the aqueous acid solution recovered from each reaction to the same level as the original one.

In calculating the acid concentration at the start of the reaction, the water contained in the aqueous formaldehyde solution and the aqueous acid solution should be included. Although the recovered aqueous acid solution contains organic impurities such as unreacted formaldehyde, intermediate products, by-products, etc., part of the unreacted formaldehyde and intermediate products will effectively be consumed during its reuse. Accordingly, the recovered aqueous acid solution containing such organic impurities may be reused without purification, whereby little adverse influence is exerted on the reaction of the present invention.

After being isolated from the aqueous acid solution, the solid reaction product or the oily layer containing it may be either used directly for intended purposes or subjected to further treatment (for example washed with water and dried) to obtain an end product Where the end product still contains unreacted starting materials and other impurities, it may further be purified, if desired, by any suitable technique such as distillation, recrystallization, extraction, etc. to obtain a product of higher quality.

Thus, the process of the present invention can achieve a higher reaction rate than has been attainable by prior art processes. It can also exhibit a higher selectivity to the desired diphenylmethane 4, 4' - dicarbamic acid diesterthan has been attainable by prior art processes.

Moreover, according to the process of the present invention, a variety of methylene-bridged polyphenyl polycarbamates of the general formula (I) can be prepared depending on the N-phenyl carbamic acid ester used as a starting material. Under ordinary reaction conditions, the reaction product is a mixture comprising a larger amount of binuclear compounds of the general formula (I) in which m is equal to zero and a smaller amount of polynuclear compounds of the general formula (I) in which m is equal to 1 or more. According to the process of the present invention, the yield of the binuclear compounds becomes higher as the acid concentration is increased.

Furthermore, since the aqueous acid solution recovered from the reaction of the present invention is strongly acidic and contains considerable amount of organic substances, its direct discharge will cause severe environmental pollution. In order to treat the recovered aqueous acid solution appropriately prior to its discharge as waste water, very great expenses are required. However, the recovered aqueous acid solution may be recycled according to the process of the present invention. Thus, the process of the present invention has great industrial advantages in that the amounts of materials used can be saved as compared with prior art processes and in that a closed system can be established to eliminated the discharge of waste water and prevent the occurrence of environmental pollution.

The process of the present invention is further illustrated by the following examples. In each of these examples, the product was analyzed by liquid chromatography using naphthalene as an internal standard.

Example 1 Into a 100-mlflask fitted with a thermometer, a stirrer, and a dropping funnel were charged 20 g of phenyl carbamic acid ethyl ester, 37.5 g of 36% hydrochloric acid, and 20 g of water. After the flask was heated to 50"C in an oil bath while its contents were being stirred, 5.2 g of a 35% aqueous solution of formaldehyde was added thereto through the droppping funnel. The resulting reaction mixture was stirred at 98-100 C for 5 hours. The oily layer was isolated, washed with water, and then dried to obtain 19.1 g of a product.

The product was dissolved in tetrahydrofuran and analyzed by liquid chromatography. Consequently, the product was found to contain 47% by weight of diphenylmethane - 4,4' - dicarbamic acid diethyl ester, 21% by weight of trinuclear and higher polymethylene polyphenyl poly(ethyl carbamate)s, and 14% by weight of unreacted phenyl carbamic acid ethyl ester. This result meansthattho degree of conversion of phenyl carbamic acid ethyl ester used as a starting material was 86% and the yield of diphenylmethane -4, 4' - dicarbamic acid diethyl ester based on the amount of phenyl carbamic acid ethyl ester (i.e., the selectivity to diphenylmethane 4,4' - dicarbamic acid diethyl ester) was 51%.

Comparative Example 1 According to the process described in German Patent No. 1,042,891, a reaction mixture consisting of 20 g of phenyl carbamic acid ethyl ester, 10 g of 36% hydrochloric acid, 50 g of water, and 7.3 g of a 35% aqueous solution of formaldehyde was worked up in the same manner as in Example 1. That is, the reaction mixture was stirred at 98-1 000C for 5 hours.

As a result, the degree of conversion of phenyl car bamic acid ethyl ester was 29% and the selectivity to diphenylmethane - 4,4' - dicarbamic acid diethyl ester was 40%.

Examples 2 to 4 and Comparative Example 2 The procedure of Example 1 was repeated except that varying amounts of sulfuric acid were used in place of the hydrochloric acid and the reaction was carried out at 80"C for 5 hours. The results thus obtained are shown in Table 1.

Table 1 Charged Materials: Phenyl carbamic acid ethyl ester (0.12 mole) and formaldehyde (0.06 mole).

Reaction Temperature: 80 C.

Reaction Time: 5 hours.

Number of Amounts of Acid and Acid Concentration Conversion of Selectivity to Example or Water Used at the Start of Phenyl Carbamic diphenylmethane-4,4' Comparative Reaction Acid Ethyl Ester dicarbamic acid diethyl 98% Sulfuric Water Example Acid (g) (g) (% by weight) (%) (%) Example 2 35.4 33 50 80 63 Example 3 35.4 50 40 77 60 Example 4 23.6 33 71 54 Comparative 8.4 112 7 8 32 Example 2 Example 5 The procedure of Example 1 was repeated except that 18.1 g of phenyl carbamic acid methyl ester was used in place of the phenyl carbamic acid ethyl ester and the reaction was carried out at 80"C for 4 hours.

As a result, the degree of conversion of phenyl carbamic acid methyl ester was 88% and the selectivity to diphenylmethane - 4,4' - dicarbamic acid dimethyl ester was 68%.

Examples 6 to 8 and Comparative Example 3 The procedure of Example 5 was repeated except that varying amounts of sulfuric acid were used in place of the hydrochloric acid. The results thus obtained are shown in Table 2.

Table2 Charged Materials: Phenyl carbamic acid methyl ester (0.12 mole) and formaldehyde (0.06 mole).

Reaction Temperature: 80"C Reaction Time: 4 hours.

Number of Amounts of Acid and Acid concentration Conversion of Selectivity to Example or Water Used at the Start of Phenyl Carbamic diphenylmethane-4,4' Comparative 98% Sulfuric Water Example Acid (g) (g) (% by weight) (%) (%) Example 6 35.4 33 50 95 73 Example 7 35.4 50 40 88 68 Example 8 23.6 33 40 82 64 Comparative 8.4 112 7 12 40 Example 3 Example 9 Into a 100-ml flask fitted with a thermometer, a stirrer, and a dropping funnel were charged 20 g of phenyl carbamic acid ethyl ester, 37 g of 98% sulfuric acid, and 50 g of water. After the flask was heated to 805C in an oil bath while its contents were being stirred, 5.2 g of a 35% aqueous solution of formaldehyde was added thereto through the dropping funnel. The resulting reaction mixture was stirred at 805C for4 hours and then separated at 50"C or above to obtain 21.7 g of an oily layer containing reaction products and 88 g of a recovered aqueous acid solution.

The oily layer was dissolved in tetrahydrofuran and analyzed by liquid chromatography using naphthalene as an internal standard. Consequently, the oily layer was found to contain 47% by weight of binuclear compounds, 2% by weight of trinuclear compounds, 4% by weight of tetranucloar and higher polynuclear compounds, and 27% by weight of unreacted phenyl carbamic acid ethyl ester. This result means that the degree of conversion of the carbamic acid ester used as a starting material was 71%, the yield of binuclear compounds based on the amount of carbamic acid ester consumed was 60%, and the yield of trinuclear and higher polynuclear compounds based on the amount of carbamic acid ester consumed was approximately 8%.

The recovered aqueous acid solution had an acid concentration of 38% by weight and contained formaldehyde and very small amounts of unknown impurities in addition to the sulfuric acid.

Using the precedently recovered aqueous acid solution (without adjusting its acid concentration), 20 g of phenyl carbamic acid ethyl ester, and 5.2 g of a 35% aqueous solution of formaldehyde, the above-described procedure was repeated four times.

The results thus obtained are shown in Table 3.

Then, another series of repetitive runs was carried out in the same manner as described above. In this series, however, a small amount of concentrated sulfuric acid was added to the aqueous acid solution recovered from each run so that an acid concentration of 400/c by weight might be achieved at the start of the succeeding reaction. When the same procedure was repeated five times, substantially consistent results were obtained. That is, in all runs, the degree of conversion of the carbamic acid ester was kept at 70-72%, the yield of binuclear compounds at 58-60%, and the yield of trinuclear and higher polynuclear compounds at 8-10%.

Table3 Charged Materials for Each Run: Phenyl carbamic acid ethyl ester (20 g), a 35% aqueous solution of formaldehyde (5.2 g), and a recovered aqueous acid solution (85-88 g).

Reaction Temperature: 80 C.

Reaction Time: 4 hours.

Number Aqueous Acid Solution Used Acid Concen- Molar Ratio Results Repetitive Amount Concen- | Acid Content the Start of Ca Acid to Conversion Yield Based on Carbamic Acid Recovered Aqueous tration @@@ @@@@@ @@ @@@@@@@ of Carbamic Ester Consumption (%) Acid Solution @un (g)(% by g mole Reaction Acid Ester Acid Ester Binuclear Trinuclear and Higher Amount Concent weight) (% by weight) (%) Compounds Polynuclear Compounds (g) ration (% by weight) Original 37 96 36 0.36 40 3.0 71 60 ca. 8 89 38 1 88 38 33 0.34 36 2.8 74 53 ca. 11 90 37 2 89 37 33 0.34 36 | 2.8 71 54 ca. 10 87 36 3 86 36 31 0.32 35 2.7 65 53 c. 8 86 33 4 | 85 33 28 0.29 32 2.4 65 52 ca. 8 54 30 Example 10 An original run was carried out by following the procedure of Example 9 except that 18 g of phenyl carbamic acid methyl ester was used in place of the phenyl carbamic acid ethyl ester. After completion of the reaction, the reaction mixture was cooled to room temperature. The resulting precipitate was separated by filtration, washed with water, and then dried to obtain 21 g of a solid product. On the other hand, 76 g of an aqueous acid solution was recovered as the filtrate.

The solid product was dissolved in tetrahydrofuran and analyzed by liquid chromatography. Consequently, the product was found to contain 54% by weight of binuclear compounds, 5% by weight of trinuclear compounds, not more than 1% by weight of tetranuclear and higher polynuclear compounds, and 10% by weight of unreacted phenyl carbamic acid methyl ester. This result means that the degree of conversion of the carbamic acid ester used as a starting material was 88%, the yield of binuclear compounds based on the amount of carbamic acid ester consumed was 68%, and the yield of trinuclear and higher polynuclear compounds based on the amount of carbamic acid ester consumed was 6%.

The recovered aqueous acid solution had an acid concentration of 39% by weight and contained formaldehyde and very small amounts of unknown impurities in addition to the sulfuric acid.

Using the precedently recovered aqueous acid solution (without adjusting its acid concentration), phenyl carbamic acid methyl ester, and a 35% aqueous solution of formaldehyde, the above-described procedure was repeated three times. The results thus obtained are shown in Table 4.

Then, another series of repetitive runs was carried out in the same manner as described above. In this series, however, a small amount of concentrated sulfuric acid was added to the aqueous acid solution recovered from each run so that an acid concentration of 40% by weight might be achieved at the start of the succeeding reaction. When the same procedure was repeated five times, substantially consistent results were obtained. That is, in all runs, the deg ree of conversion of the carbamic acid ester was kept at 88-90%, the yield of binuclear compounds at 68-70%, and the yield of binuclear and higher polynuclear compounds at 3-5%.

Table 4 Charged Materials for Each Run: Phenyl carbamic acid methyl ester (18 g), a 35% aqueous solution of formaldehyde (5.2 g), and a recovered aqueous acid solution (52-75g).

Reaction Temperature: 805C.

Reaction Time: 4 hours.

Number of | Aqueous Acid Solution Used | Acid Concen- Ester Estis Results tration at of Acid to Repetitive Amount Concen- Acid Content Conversion Yield Based on Carbamic Acid Recovered Aqueous the Start of Carbamic of Carbamic of Carbamic Ester Consumption (%) Acid Solution Run (g) tration g mole Reaction Acid Ester (% by (by weight) (%) Binuclear Trinuclear and Higher Amount Concentration e w5fCht) - ~~~ Cyhy ~~~~~~~~~ Original | 37 | 96 | 36 | 0.36 | 40 | 3.0 88 68 75 39 29 0.30 37 2.5 78 68 2 | 64 | 37 | 24 | 0.24 | 36 | 2.0 | 89 | 74 3 52 35 18 0.18 32 1.5 79 69

Claims (17)

1. In a process for preparing a polymethylene DolvPhenyl polycarbamate of the general formula

where Rt is an alkylradical of from 1 to 6 carbon atoms or a cycloalkyl radical of from 5 to 10 carbon atoms, R2 is a hydrogen atom, a halogen atom, an alkyl radical of from 1 to 6 carbon atoms, or an alkoxy radical of from 1 to 6 carbon atoms, n is a positive integer of from 1 to 4, and m is zero or a positive integer of from 1 to 5, by reacting an N-phenyl carbamic acid ester of the general formula

where R1, R2, and n have the same meanings as described above, with formaldehyde or a formaldehyde-producing compound in the presence of water and an acid catalyst, the improvement which comprises (a) using the acid catalyst in the form of its aqueous solution having an acid catalyst concentration, at the start of the reaction, of at least 10% by weight in an amount, at the start of the reaction, of from 0.01 to 100 moles per mole of the N-phenyl carbamic acid ester and (b) carrying out the reaction at a temperature of from 20 to 1505C.

2. The process according to claim 1 wherein the acid catalyst is used in the form of its aqueous solution having an acid catalyst concentration, at the start of the reaction, of from 20 to 95% by weight.

3. The process according to claim 1 wherein the acid catalyst is used in an amount, at the start of the reaction, of from 0.1 to 50 moles per mole of the N-phenyl carbamic acid ester.

4. The process according to claim 1 wherein the reaction is carried out at a temperature of from 30 to 100"C.

5. The process according to claim 1 wherein the N-phenyl carbamic acid ester is phenyl carbamic acid methyl ester, phenyl carbamic acid ethyl ester, phenyl carbamic acid isopropyl ester, or phenyl carbamic acid isobutyl ester.

6. The process according to claim 1 wherein the acid catalyst is sulfuric acid, hydrochloric acid, phosphoric acid, boric acid, formic acid, acetic acid, oxalic acid, toluenesulfonic acid, hydrobromic acid, perchloric acid, chlorosulfonic acid, ortrifluoromethanesulfonic acid.

7. The process according to claim 6 wherein the acid catalyst is hydrochloric acid or sulfuric acid.

8. In a process for preparing a polymethylene polyphenyl polycarbamate of the general formula

where R, is an alkyl radical of from 1 to 6 carbon atoms or a cycloalkyl radical of from 5 to 10 carbon atoms, R2 is a hydrogen atom, a halogen atom, an alkyl radical of from 1 to 6 carbon atoms, or an alkoxy radical of from 1 to 6 carbon atoms, n is a positive integer of from 1 to 5, and m is zero or a positive integer of from 1 to 5, by reacting an N-phenyl carbamic acid ester ofthe general formula

where R1, R2, and n have the same meanings as described above, with formaldehyde or a formaldehyde-producing compound in the presence of water and an acid catalyst, the improvement which comprises (a) using the acid catalyst in the form of its aqueous solution having an acid catalyst concentration, at the start of the reaction, of at least 10% by weight and in an amount, at the start of the reaction, of from 0.01 to 100 moles per mole of the N-phenyl carbamic acid ester; (b) carrying out the reaction at a temperature of from 20 to 1 500C; (c) after completion of the reaction, separating the solid reaction product or the organic layer containing it from the aqueous acid catalyst solution containing small amounts of organic impurities; (d) isolating the desired polymethylene polyphenyl polycarbamate from the organic layer; and (e) reusing the aqueous acid catalyst solution for a subsequent reaction of the N-phenyl carbamic acid ester with formaldehyde or a formaldehyde-producing compound without removing the organic impurities contained therein but after adjusting its acid catalyst concentration to such a level as to provide an aqueous solution having an acid catalyst concentration, at the start of the subsequent reaction, of at least 10% by weight.

9. The process according to claim 8 wherein the acid catalyst is used in the form of its aqueous solution having an acid catalyst concentration, at the start of the reaction, of from 20 to 95% by weight.

10. The process according to claim 8 wherein the acid catalyst is used in an amount, at the start of the reaction, of from 0.1 to 50 moles per mole of the N-phenyl carbamic acid ester.

11. The process according to claim 8 wherein the reaction is carried out at a temperature of from 30 to 100"C.

12. The process according to claim 8 wherein the N-phenyl carbamic acid ester is phenyl carbamic acid methyl ester, phenyl, carbamic acid ethyl ester, phenyl carbamic acid isopropyl ester, or phenyl carbamic acid isobutyl ester.

13. The process according to claim 8 wherein the acid catalyst is sulfuric acid, hydrochloric acid, phosphoric acid, boric acid, formic acid, acetic acid, oxalic acid, toluenesulfonic acid, hydrobromic acid, perchloric acid, chlorosulfonic acid, ortrifluoromethanesulfonic acid.

14. The process according to claim 13 wherein the acid catalyst is hydrochloric acid or sulfuric acid.

15. A process according to claim 1, substantially as exemplified herein.

16. A process according to claim 8, substantially as exemplified herein.

17. A polymethylene polyphenyl polycarbamate prepared by a process according to any one of the preceding claims.