1. Field of the Invention
The invention pertains to the preparation of aliphatic, cycloaliphatic, arylaliphatic and aliphatic-cycloaliphatic N-substituted di- and polyurethanes by reacting primary aliphatic, cycloaliphatic, arylaliphatic or aliphatic-cycloaliphatic di- or polyamines with urea and alcohol.
2. Description of the Related Art
On an industrial scale, N-substituted urethanes are normally produces by the reaction of alcohols with isocyanates or by the reaction of amines with chlorocarbonates. The isocyanates and chlorocarbonates used in these reactions are obtained by phosgenation of the corresponding amines or alcohols. Houben-Weyl, Methods of Organic Chemistry, Vol. 8, pages 137, 120 and 101, (Georg Thieme Publishers, Stuttgart, 1952). These processes are very expensive and phosgene must be used with care because of its potential danger to man and the environment.
N-substituted urethane are used as both intermediates and end products. For instance, German Published Application No. 26 35 490 and U.S. Pat. No. 3,919,278 disclose the use of N-substituted urethanes for the manufacture of isocyanates. Because of their utility, many attempts have been made to develop better methods for preparing N-substituted urethanes. These methods and their shortcomings will be discussed in detail in the paragraphs which follow.
German Published Application No. 21 60 111 describes a process for the manufacture of N-substituted urethanes by reacting an organic carbonate with a primary or secondary amine in the presence of a Lewis acid. There are several problems with this process. The conversion rates are rather low, the reaction times are long, and N-alkylarylamines are always produced as by-products. Furthermore, the organic carbonate starting materials are themselves prepared from phosgene, and thus this process does not achieve the goal of a phoshene-free urethane preparation.
R. A. Franz et al, Journal of Organic Chemistry, Vol. 28, page 585 (1963) describe a process for making methyl-N-phenyl urethane from carbon monoxide, sulfur, aniline, and methanol. However, very low yields are produced by this method; the yield does not exceed 25 percent even when there is a long reaction period.
U.S. Pat. No. 2,409,712 discloses a method of preparing monoisocyanates by the pyrolysis of N-substituted urethanes, A process for making N-alkyl- and N-aryl-urethanes by the reaction of monoamines with urea and alcohol at temperatures of from 150.degree. C. to 350.degree. C. under increased pressure is disclosed. However, the disclosure describes only the manufacture of N-alkylmonourethanes and does not disclose the manufacture of N,N'-disubstutited diurethanes or poly-N-polysubstituted polyurethanes. The patent further discloses that the process is not suitable for all N-substituted urethanes. Furthermore, the yields disclosed are quite low and certainly unacceptable for commercial application.
U.S. Pat. No. 2,677,698 also describes a process for the manufacture of N-substituted monourethanes. In this process, urea is initially converted into the corresponding N,N'-disubstituted urea by reacting urea with a monoamine. The N,N'-disubstituted urea is then purified and reacted with an alcohol. The processes described are expensive and the yields are very low. Furthermore, the reaction is very slow, resulting in excessive reaction times. Attempts to improve the yield by improving the methods of preparing and purifying the N,N'-disubstituted ureas have not been successful.
A process similar to that mentioned in U.S. Pat. No. 2,409,712 is disclosed in U.S. Pat. No. 2,806,051. In this process, N-substituted urethanes are produced by reacting alkyl- or aryl-monoamines with urea and alcohol at a mole ratio of 1.0:1.2:2.0, preferably at temperatures of from 120.degree. to 175.degree. C., more preferably from 125.degree. C. to 160.degree. C. Even within the most preferably used temperature range, this process produces only small yields of N-substitued urethanes if the reaction time is limited to a period which is practical in an industrial setting.
The processes of U.S. Pat. Nos. 2,409,712 and 2,806,051 preferably take place below 160.degree. C. The reason for this preference for low temperatures is presumably the tendency for urea and substituted ureas to react to form biurets and other products at higher temperatures. For example, urea is known to condense to form biuret and cyanuric acid at temperatures of from 150.degree. to 175.degree. C. Erickson, in J. Am. Chem. Soc. 76, 3977-78, showed that alkyl amines react with urea at lower temperatures, i.e 160.degree. to 165.degree. C., to produce mono- and di-substituted ureas while at a higher temperature of 170.degree. to 200.degree. C., monosubstituted and 1,3-disubstituted biurets were formed. For these and other reasons, the use of higher temperatures in reactions involving urea, and especially urea and amines, has been avoided.
In view of the problems disclosed in U.S. Pat. Nos. 2,409,712 and 2,806,051 with respect to yields and reaction times, it is no wonder that future attempts to produce N-alkylurethanes have not involved the reaction between amines, urea, and alcohol. The inventors of U.S. Pat. No. 3,076,007, for example, in searching for a commercially viable, non-phosgene approach to N-substituted monourethanes describe the N-alkyl, N-alkoxylalkyl and N-alkoxyalkoxyalkyl monourethanes of U.S. Pat. No. 2,409,712 as requiring phosgene for their preparation due to the fact that the available non-phosgene methods produce poor yields with numerous side reactions.
It is thus surprising that aliphatic, cycloaliphatic, and aliphatic-cycloaliphatic N-substituted di- and polyurethanes can be produced in a single process with good yields by reacting a diamine with urea and alcohol at higher temperatures, preferably temperatures of from greater than 190.degree. C. to 300.degree. C., and most preferably from 195.degree. C. to 250.degree. C. Prior teachings indicate that diureas and polyureas are obtained from diamines and urea; for example, hexamethylenediurea is obtained from hexamethylenediamine and urea. The prior art also teaches that, although urea and alcohol may react to produce urethanes, they continue to react to form N,N'-disubstituted ureas in the presence of amines. See Houben-Weyl, Methods of Organic Chemistry, Vol. 8, pages 151 and 140, (Georg Thieme Publishers, Stuttgart, 1952). These side reactions decrease the yield of the desired product.
None of the references cited discloses the preparation of aliphatic, cycloaliphatic, arylaliphatic or aliphatic-cycloaliphatic N-substituted di- and polyurethanes by reacting diamines or polyamines with urea and alcohol. Neither do the references disclose the unexpectedly high yields obtainable at higher temperatures. The reaction temperatures utilized in U.S. Pat. No. 2,806,051, for example, are low and only monoamines are used in this process. If diamines are used under these process conditions, one obtains high yields of a polymeric precipitate with a polyurea structure similar to the polyureas which are formed from diamines and polyisocyanates.
The use of higher temperatures for the reaction between diamines, urea, and alcohol is neither taught nor suggested in the prior art. As a matter of fact, the prior art suggests that higher temperatures should be avoided, as the patent and non-patent literature is replete with examples wherein diamines and urea participate in numerous reactions at higher temperatures yielding substituted biurets or a variety of other byproducts; or participate in condensation reactions at higher temperatures to form polyurea thermoplastics.
For example, German Patent 896 412 indicates that high molecular weight, spinnable condensation products may be produced from the reaction of diamines with urea or other diamides of carbonic acid. This result is likely to occur if the amino groups of the diamines are separated by a chain of more than three atoms. Preparation of polyureas is taught in many other references also. In Great Britain Pat. No. 530,267, for example, urea reacts with aliphatic diamines in the presence of aromatic alcohols such as phenol and m-cresol at temperatures of from 100.degree. to 270.degree. C. High molecular weight polyureas are the product of this reaction. In U.S. Pat. No. 2,973,342, urea and diamines are reacted in the presence of water to form spinnable polyurea condensates at temperatures of from 130.degree. C. to 200.degree. C. U.S. Pat. No. 3,412,072 discloses the preparation of polyurea themoplastics by reacting diamines with urea in the presence of aliphatic alcohols such as ethanol and isopropanol at temperatures from 90.degree. to 300.degree. C.
In addition to the expected reaction of diamines with urea to form polyureas, any diurethanes formed may further react with unreacted diamine to form polyureas. For example, U.S. Pat. No. 2,181,663 and U.S. Pat. No. 2,568,885 disclose that high molecular polyureas with molecular weights of 8000 to 10,000 and greater, may be produced when diurethanes are condensed with diamines at temperatures of approximately 150.degree. C. to 300.degree. C. Moreover, as mono-, di-, and polyurethanes can be split thermally into isocyanates, alcohols, olefins, carbon dioxide, ureas, and carbodiimides, these products can further react to form numerous by-products such as biurets, allophanates, isocyanurates, and polycarbodiimides, among others. See The Journal of the American Chemical Society, Vol. 80, page 5495 (1958) and Vol. 48, page 1946 (1956).
In view of the problems disclosed in the prior art and the many possible side reactions, particularly polyurea formation, it was surprising that the process of the subject invention, which involves similar reaction conditions, would result in N-substituted di- and polyurethanes with excellent yields and in exceptional purity.