2,999,881 United States Patent Office Patented Sept. 12, 1961

2 tended period of time, particularly during the preparation 2,999,881 of amides under reduced pressures. ACRYLAM DE MONOMER POLYMERZATION In accordance with the present invention it has been INHIBITOR found that the polymerization of solutions of acrylamide George C. Gleckler, Springdale, and George L. Suther and c-alkyl acrylamides can be effectively inhibited for land, Stamford, Conn., assignors to American Cyal amid Company, New York, N.Y., a corporation of extended periods of time by producing therein as a poly Maine merization inhibitor a chelate of N-nitrosophenylhydrox No Drawing. Filed Mar. 2, 1959, Ser. No. 796,201 ylamine having the formula 8 Claims. (C. 260-561) CE-N-O - 10 This invention relates to acrylamides. More particul N=O> a Feh larly, it relates to acrylamide and cc-alkyl acrylamides and The of N-nitrosophenylhydroxylamine particularly to the stabilization of solutions of Such acryl (), a well-known compound, is employed in the amides against polymerization. formation of the inhibitors of this invention. This salt Acrylamide and c-alkyl acrylamides and the polymer 5 has found considerable use as a means for quantitatively ization and copolymerization products thereof are em. removing certain metals from solution by forming insol ployed in various fields. They are of particular im uble complexes with such metals. In view of this, it is portance in such fields as adhesives, dispersants, plasti a particularly surprising feature of the present invention cizers, surface coatings, thickening agents, as well as in that the ferrous iron complex of cupferron, which is the leather, paper, textile, and photographic arts. Because 20 practically insoluble in , is unusually soluble in aque of these many potential applications, methods of pre ous solutions of the amides contemplated herein. paring the monomers are of particular interest. Furthermore, the inhibitor within the purview of the The monomers are usually prepared by reacting the present invention has shown an unusually slow decay rate, corresponding with water and concentrated particularly at elevated temperatures. Accordingly, ex . The reaction mixture comprising the amide 25 cellent stabilization may be obtained for longer periods is then further treated to separate the free amide. of time than when employing, in like amount, other mate This may be conducted in various ways, one of which rials of excellent inhibiting properties. In addition, the comprises treating an of the amide Sul slower decay rate results in the formation of less color fate with an alkaline earth metal hydroxide or carbonate bodies than conventional polymerization inhibitors over to precipitate an insoluble sulfate. Another procedure 30 almost any given period of time. comprises treating a solution of the amide sulfate in an The inhibitor of the present invention may be incor organic solvent with or an alkaline metal hy porated in the monomer in different ways. The inhibitor, droxide or carbonate to form the corresponding sulfate for instance, may first be prepared by bringing together which is insoluble in the solvent. a ferrous iron salt, such as ferrous sulfate, and the like, An unusually successful procedure particularly adapted 35 and the ammonium salt of N-nitrosophenylhydroxyl for the neutralization of acrylamide sulfate comprises amine. The resulting solid metal complex is then dis treating an aqueous solution thereof with ammonia under solved in the liquid monomer contemplated herein, or a controlled conditions of temperature and hydrogen solution thereof, or the complex may be mixed with a concentration. Under these controlled conditions, the solid monomer. normally water-soluble ammonium sulfate precipitates 40 A particularly Successful method of incorporating the and may be separated. inhibitor into the liquid monomer or solution thereof is In each of these modes of neutralization, the residual to provide in the monomer stoichiometric amounts of the liquor is usually treated to recover a crystalline product. ammonium salt of N-nitrosophenylhydroxylamine and a Regardless of how the neutralization and crystalization ferrous iron salt so that the complex forms in situ. This are conducted, at some point in the process elevated tem 45 practice is advantageous in that the rate of of peratures are generally employed which aggravate the the complex in the monomer or solution thereof is thus problem of premature polymerization. This problem is rendered unusually high. still further aggravated in continuous neutralization sys The amount of inhibitor employed in accordance with tems because of the extended residence time within the this invention may be varied. The optimum amount em system of part of the original liquor due to the recycling 50 ployed in any particular situation may be readily deter of amide-bearing streams. mined by those skilled in the art. In general, however, Although the present invention is particularly con as little as 5 parts per million of complex by weight of cerned with inhibiting polymerization during the prepara monomer will provide stabilization to a certain degree. tion acrylamides, the problem exists in other situations. Continued improvement may be observed by increasing Thus, polymerization may also be a problem when storing 55 this amount to as high as 2000 parts per million and even a solution of an amide as well as when employing an higher, although it has been observed that these latter amide as an intermediate under conditions favorable for concentrations are usually unnecessary. For most situ polymerization. It is apparent, therefore, that there are ations, it has been observed that a concentration of about various situations in which stabilization of an amide solu 15-300 parts per million is adequate to obtain the desired tion is highly desirable. 60 stabilization. It would appear that this problem of premature poly By O-alkyl acrylamide, pursuant to the instant dis merization could be readily overcome by incorporating covery, is meant acrylamide substituted on the a-carbon in the amide-bearing solution any of known materials by an alkyl radical of not more than about 8 carbon which have been suggested as polymerization inhibitors. atoms. Many of these materials, however, have been found un 65 Satisfactory for one or more of various reasons. Some The invention is further illustrated by the following satisfactorily inhibit polymerization but discolor the solu example. Unless otherwise noted all parts are by weight. tion and/or are difficult to remove prior to use of the EXAMPLE Solution for polymerization. Still others have been found In a 50 percent by weight acrylamide solution (aque to be ineffective as polymerization inhibitors for any ex 70 Ous) is provided 100 parts per million, based on the 2,999,881 3 4. weight of the acrylamide, of the ferrous chelate of N nitrosophenylhydroxylamine as a polymerization inhibitor. nitrosophenylhydroxylamine 2. A method according to claim 1 in which the amount of inhibitor is at least 5 parts per million parts of solu tion. 3. A method according to claim 2 in which the amide Kd- N-O. 3 Feh is acrylamide and the amount of inhibitor is about The inhibitor is prepared in situ by first introducing fer 15-300 parts per million parts of solution. rous sulfate into the acrylamide solution and then a 4. A solution of an amide selected from the group corresponding stoichiometric quantity of the ammonium consisting of acrylamide and o-alkyl acrylamides sta Salt of N-nitrosophenylhydroxylamine, the ferrous sulfate O bilized against polymerization, said solution containing and N-nitrosophenylhydroxylamine being present in suffi the ferrous iron complex of N-nitrosophenylhydroxyl cient amount to provide 100 parts per million, based on amine as a polymerization inhibitor. the weight of the acrylamide, of the ferrous chelate. 5. A Solution according to claim 4 in which the amount Admixture of these materials is carried out at 50° C. of inhibitor is at least 5 parts per million of solution. and the resulting mixture maintained at this temperature 5 6. A solution according to claim 4 in which the amide and at a pressure of 50 millimeters mercury (absolute) is acrylamide and the amount of inhibitor is about for over 19 hours. 15-300 parts per million parts of solution. Likewise an identical standard solution (blank) of 7. A method of inhibiting the polymerization of a acrylamide containing no inhibitor is provided and main monomer selected from the group consisting of acryl tained at the same temperature and pressure for over 20 amide and O-alkyl acrylamides which comprises provid 19 hours. ing in an aqueous solution of said monomer stoichio At the end of 19 hours the viscosity in centipoises at metric equivalents of N-nitrosophenylhydroxylamine and 40 C. of the solution containing no inhibitor is over a ferrous iron salt. 30 times greater than the viscosity in centipoises at 40° 8. The method according to claim 7 wherein the fer C. of the Solution containing the ferrous chelate of 25 rous iron salt is first introduced into the aqueous solution N-nitrosophenylhydroxylamine. of said monomer and the stoichiometric equivalent of While the present invention has been described in de the ammonium salt of N-nitrosophenylhydroxylamine is tail as to specific embodiments thereof, it is not intended added thereto. that these details create undue limitations upon the scope of the discovery, except insofar as they appear in the 30 References Cited in the file of this patent appended claims. We claim: UNITED STATES PATENTS 1. A method of inhibiting the polymerization of a 2,720,545 Mallinckrodt et al. ------Oct. 11, 1955 Solution of an amide selected from the group consisting 2,758,131 Couvillon ------Aug. 7, 1956 of acrylamide and o-alkyl acrylamides which comprises 35 2,787,634 Coover et al. ------Apr. 2, 1957 incorporating therein the ferrous iron complex of N 2,840,611 Bikales et al. ------June 24, 1958