Salts of N-nitrosophenylhydroxylamine

Abstract: Disclosed are salts of N -nitrosophenylhydroxylamine and their use as polymerization inhibitors.

Description: This invention concerns salts of N -nitrosophenylhydroxylamine, solutions thereof and the use of such salts to inhibit polymerization of ethylenically unsaturated monomers. During the manufacture of ethylenically unsaturated monomers such as acrylic acid, concentration and purification is usually performed by thermal distillation techniques. During this process, losses of product can occur due to thermal polymerization of the monomers. Hence means for inhibiting this polymerization despite the presence of heat are required, and certain polymerization inhibitors are known for this purpose. The ammonium salt of N-nitrosophenylhydroxylamine (NPHA,or cupferron) is a known polymerization inhibitor, but it has several drawbacks including its limited solubility which makes it difficult to add to some process streams. Other derivatives of N -nitrosophenylhydroxylamine are also known (Gros, et al., U.S.-A-3,426,063 and U.S.-A-4,772,740). Solvents must be employed to prepare usable solutions and these solvents often cause problems during use. Cupferron has limited solubility in solvents such as water and lower alcohols. The following are limits of solubility at 25°C in the indicated solvents: water (12%), methanol (5.5%) and isopropanol (0.38%), while it is essentially insoluble in hydrophobic solvents - i.e. solvents which are immiscible with water. As a result of these limited solubilities, objectionably large amounts of such solvents are required in many applications where cupferron would otherwise be a desirable inhibitor. Solutions of cupferron also undergo degradation in the presence of air, as manifested by discoloration and formation of a black precipitate. Minimizing the extent and rate of degradation requires storing such solutions under an inert atmosphere such as nitrogen. US-A-4,772,740 discloses that the above-mentioned deficiencies of cupferron solutions are not overcome by the salts of N -nitrosophenylhydroxylamine disclosed in the above-cited US-A-3426063. On the contrary, these salts of NPHA are unsatisfactory for use in inhibiting polymer formation in acrylic acids and acrylate esters. In the presence of such monomers, the amine salts decompose to form organic aliphatic amines (e.g. ethylamine, etc.). The lower boiling amines (e.g. the C₁ to C₇ aliphatic amines) formed by decomposition of the corresponding amine salts may co-distill with the acrylic acid or acrylate ester monomers being purified by distillation, with resulting discolouration of polymers prepared from the monomers, such as, for example, poly(acrylic acid), poly(methyl methacrylate) and poly(ethyl acrylate). The higher boiling amines (e.g. the C₈-C₂₀ aliphatic amines) formed upon decomposition of the corresponding amine salts are so immiscible with water that such salts are not entirely satisfactory for addition to water-containing acrylic acid systems. Accordingly there is a need for a salt of NPHA which can be added to acrylic acid and acrylate ester systems and is effective for inhibiting undesired formation of polymer in such systems without the above drawbacks. Compounds have now been found which substantially fulfil the above-mentioned need. The risk of co-distillation of the amines formed upon decomposition of these corresponding amine salts of NPHA with the acrylic acid or acrylate ester monomers being purified by distillation is minimised. Accordingly one aspect the present invention provides a salt of N-nitrosophenylhydroxylamine (NPHA) and an amine, expressed in neutral form by the formula: wherein n is an integer of from one to the number of basic nitrogen atoms in the amine and preferably 1 to 3; R¹ is hydrogen, (C₁-C₆)alkyl, or (C₁-C₆)hydroxyalkyl; R² is hydrogen, (C₁-C₆)alkyl, or (C₁-C₁₀)aminoalkyl; R³ is (C₁-C₆)alkyl, (C₁-C₁₀)aminoalkyl or aminophenyl; or R² and R³ may be joined together with the nitrogen atom to which they are attached to form a substituted or unsubstituted morpholino ring ; with the proviso that when R¹ is hydroxylalkyl, R² and R³ are joined to form the morpholino ring, and when R¹ is alkyl, R² and R³ are alkyl. In a further aspect, this invention provides a method for inhibiting the formation of undesired polymer from monomers, especially acrylic acids, which comprises adding thereto the above-defined salts of NPHA in an amount effective to inhibit the formation of such polymer. The (stoichiometric or non-stoichiometric) salts of NPHA comprising a mono-, di- or higher substituted basic salt of the organic amine may be represented in the neutral form (for convenience) by the following formula: wherein n is an integer of from one to the number of basic nitrogen functions in the amine and preferably is 1 to 3; R¹ is: hydrogen; (C₁-C₆) alkyl, for example lower alkyl of from 1 to 5 carbon atoms such as methyl, ethyl, propyl, butyl, pentyl and the like; or, (C₁-C₆) hydroxyalkyl, for example, hydroxy lower alkyl of from 1 to 5 carbon atoms, such as, hydroxymethyl, hydroxyethyl, hydroxypropyl, hydroxybutyl, hydroxypentyl, and the like; R² is: hydrogen; (C₁-C₆)alkyl as defined above; or (C₁-C₁₀) aminoalkyl such as aminomethyl, aminoethyl, aminopropyl, aminobutyl, aminopentyl, aminohexyl, aminoheptyl, aminooctyl, aminononyl, aminodecyl and the like; R³ is: (C₁-C₆) alkyl as defined above, (C₁-C₁₀) aminoalkyl as defined above; or aminophenyl; or R² and R³ may be joined together with the nitrogen atom to which they are attached to form a substituted or unsubstituted morpholino ring, and where the substituent may be alkyl, for example, lower alkyl of from 1 to 5 carbon atoms, or hydroxyalkyl, for example, hydroxy lower alkyl, such as hydroxymethyl, hydroxyethyl, hydroxypropyl, hydroxybutyl, hydroxylpentyl and the like; with the proviso that when R¹ is hydroxyalkyl, R² and R³ are joined to form a morpholino ring, and when R¹ is alkyl, R² and R³ are alkyl. The NPHA may be bound to 1, 2 or 3 of the available nitrogen atoms to form mono-, di or higher substituted salts. It is not required that the salts be stoichiometric compounds, but only that the number of basic nitrogens is at least equal to the number of NPHA moieties present. The salts of NPHA of this invention may be monosubstituted, disubstituted, trisubstituted or even higher depending on the particular salt selected. Some examples of the salts include the following where A ⊖ is the symbol for are given below 1. Diethylene triamine Monosubstituted Disubstituted or Trisubstituted 2. Ethylene Diamine Monosubstituted A ⊖ N ⊕ H₃CH₂CH₂NH₂ Disubstituted A ⊖ N ⊕ H₃CH₂CH₂N ⊕ H₃A ⊖ 3. Meta-phenylene diamine Monosubstituted Disubstituted 4. 4-(2-hydroxyethyl)morpholine 5. Tributylamine 6. 1,4-Butylene Diamine Monosubstituted A ⊖ N ⊕ H₃CH₂CH₂CH₂CH₂NH₂ Disubstituted A ⊖ N ⊕ H₃CH₂CH₂CH₂CH₂N ⊕ H₃A ⊖ 7. 3,3'-Iminobispropylamine Monosubstituted Disubstituted Other amines which may be used include propylene diamine, dibutylene triamine, tetraethyl pentamine, tetrabutyl pentamine, tetraethylene diamine, tetrabutylene diamine, triethyl amine, pentaethylene hexamine, hexamethylene tetramine, 4-ethylmorpholine, 4-butyl-morpholine, o or p -aminoanilines. The salts of NPHA employed in this invention can be prepared by (i) reacting an equimolar ratio of the amine and cupferron (ie NPHA) in a solvent, and removing the solvent by subsequent evaporation to form a dry salt, or (ii) reacting the amine with cupferron in an excess of the amine as the solvent, preferably sparging the reaction system with nitrogen or other inert gases to aid in removing the ammonia which evolves in the course of the reaction. In general, where a solvent other than excess amine is desired in the synthesis of these salts of NPHA, polar organic solvents capable of dissolving amines may be used. Alcohols are preferred, and isopropanol is especially preferred for use as the reaction solvent. Preferably, cupferron is added to the reaction solvent and thereafter the amine is added to the rest of the solution with stirring. In general, cupferron is added in an approximately stoichiometric amount, i.e., in an amount such that a total of approximately one mole of cupferron is added per mole of amine. However, if excess amine is desired as the solvent, cupferron may be added in an amount less than stoichiometric. Preferably, equal parts by weight of the amine and cupferron are employed. The reaction may be carried out under any suitable conditions, including a temperature of, for example, about 50°C and atmospheric pressure. The time required to complete the reaction is dependent upon the amine employed, reaction temperature, and the relative amounts of reactants. The dry salt can be recovered from the alcohol solvent using well-known recovery methods. Recovery may be effected, for example, by cooling the reaction mixture to about 0°C to crystallize the salt of NPHA. The salts of NPHA can be stored in the atmosphere over long periods of time without degradation in the following ways: (i) as the dry salt of NPHA, (ii) as the salt of NPHA in excess amine or (iii) in solutions containing the salt of NPHA in a polar solvent such as water or an alcohol such as isopropanol and the like. No discolouration or formation of black solids or precipitate is observed over a period of months. Another aspect of this invention concerns a method for inhibiting formation of undesired polymer from an ethylenically unsaturated monomer selected from polymerizable ethylenically unsaturated acids, esters or mixtures thereof in a distillation unit which comprises adding an effective amount of the salt of NPHA to the unit. The salts of NPHA have been found to be effective inhibitors of undesired polymerization such as thermal polymerization and popcorn polymer formation in ethylenically unsaturated acids and esters, such as acrylic acids and esters thereof. They are effective for inhibiting undesired polymerization in both liquid and vapour phases. That is, inhibition can be effected not only in the liquid phases of such monomers, but also in the vapour spaces above the liquid. As used herein, the term "acrylic acids" includes substituted and unsubstituted acrylic acid, e.g. acrylic acid per se, methacrylic acid and the like. Monomeric esters of such acids include, for example, esters thereof with a lower alkanol having from 1 to about 8 carbon atoms, such as the methyl, ethyl, isopropyl, butyl and octyl (e.g. 2-ethylhexyl) alcohols. The various inhibited monomer solutions may be prepared by simply admixing at any suitable temperature (e.g. about 20°C to 25°C) one or more of the new salt compounds in the amine form with the monomer to be inhibited. It is understood that the salts of NPHA may, if desired, be introduced into, or otherwise admixed with, monomers or monomer containing systems to be inhibited. The salts may if necessary be added as a solution, for example, an aqueous solution or an alcohol solution. The salts of NPHA may be admixed with an all-monomer system or other monomer-containing system employing any effective amount of the NPHA salt. Expressed on the basis of the amount of NPHA in the salts of NPHA, effective amounts are generally in the range of from about 20 to about 1000 parts per million (ppm) of the monomer. It has been found that addition of such amounts to monomers or monomer-containing systems such as the acrylic acids and esters thereof effectively inhibits thermal polymerization of such monomers, while at the same time effectively inhibits formation of popcorn polymer. Advantageously, the inhibiting effects achieved are observed in both the liquid and the vapour phase. The % conversion of cupferron to the salts of the invention is extremely important. If the reaction is incomplete (ie residual amounts of cupferron remain in the reaction mix) then poorer inhibition is observed. Practice of the present invention is illustrated by the following examples, which are given by way of illustration and not by way of limitation. As indicated above, all amounts throughout this disclosure are by weight unless otherwise indicated. Preparation of The Salts of N -Nitrosophenylhydroxylamine The salts of N -nitrosophenylhydroxylamine can be prepared as both dry (Preparation I, below) and solution forms in excess amine (Preparation II, below). The two Preparations are similar but differ in the following ways: (i) 1:1 molar ratios of cupferron:amine are used for the preparation of the dry salt (however, if the amine has two amino functional groups, it is possible to prepare the 2 cupferron:1 amine molar ratio salt and so forth), (ii) the dry salt is prepared in the presence of isopropanol which is not required for the preparation of the solution form, and (iii) 50%:50% (w/w) ratios of cupferron:amine are used for the preparation of the solution form of the salts. (Since the molecular weight of cupferron is higher than that of any of the amines used in the following Examples, the amine is always present in excess). In all cases, with the exception of meta-phenylene diamine, the amines used were liquids and thus, the presence of a solvent was not necessary in the preparation of the solution forms of these salts. PREPARATION I Preparation of an Amine Salt of N -Nitrosophenylhydroxylamine Using 1:1 molar ratios of Cupferron and Amine Cupferron (5g, 0.0322 moles) and isopropanol (15 ml) are added to a 25 ml three neck round bottom flask. [Note: in the preparation of the meta-phenylene diamine salt of NPHA, MeOH was used as the solvent and not i-PrOH]. Stirring is begun and the system sparged with nitrogen (to remove ammonia) and the amine of choice (0.0322 moles) is added to the flask, via a pre-weighed syringe, through a septum on one of the necks on the flask. The syringe is then reweighed to determine the exact amount of amine used. Once the amine has been added, the flask is heated to 50°C and held at this temperature for about 2 hr. After heating for 2 hr., the oil bath is lowered and the flask allowed to cool under nitrogen. If necessary, the reaction mixture can be heated further until the desired conversion has been achieved (See Note b, Table I). When a satisfactory conversion has been achieved, the mixture is allowed to stand, undisturbed under nitrogen to allow the salt to crystallize. The solid is removed from the flask, vacuum filtered, washed with hexane and placed in a tared beaker to dry in a vacuum oven. The salt is then weighed and stored in a bottle until further use. PREPARATION II Preparation of an Amine Salt of N -Nitrosophenylhydroxylamine in Excess Amine (Solution Form) Using 50:50 (w/w) Cupferron and Amine Cupferron (∼5g) is added to a 25 ml. three neck round bottom flask. The stirring is begun and the system sparged with nitrogen (to remove ammonia), and the amine of choice (∼5g) is added to the flask, via a pre-weighed syringe, through the septum on one of the necks on the flask [Note: in the preparation of the solution of the meta-phenylene diamine (mPD) salt of NPHA in excess amine, it was necessary to use a solvent, MeOH, since mPD is a solid]. The syringe is then re-weighed to determine the exact amount of amine used. Once the amine is added, the flask is heated to 50°C and held at this temperature for about 2 hr. After heating for 2 hr., the oil bath is lowered and the flask allowed to cool under nitrogen. If necessary, the reaction mixture can be heated further until the desired conversion has been achieved. When a satisfactory conversion has been achieved, (See Note b, Table I) the mixture is removed from the flask, weighed and stored until further use. Example 7 - Evaluation The method of evaluation of the inhibitors involves monitoring the inhibiting performance of the amine salts of NPHA under distillation conditions. The distillation apparatus consists of a 1 litre five neck distillation flask surmounted by two 5-tray Oldershaw column sections. A standard magnetically controlled reflux distillation head containing a finger-type condensor and a thermometer joint is connected to the top of the Oldershaw column. Before the start of the experiment, an initial charge of acrylic acid (AA) (500g) containing 4-methoxyphenol (MEHQ; 200 ppm), phenothiazine (PTZ; 600 ppm) and hydroquinone (HQ; 600 ppm) is made to the flask. During the distillation, AA (inhibited with 200 ppm MEHQ) is fed to the flask at a rate of 90cc/hr. Dry air is also fed to the flask below the liquid level at a rate of 20cc/min. A reflux ratio of 4 (i.e. 20% collected as distillate and 80% refluxed) is maintained throughout the distillation experiment which lasts 4 hr. A bottoms bleed of 40cc/hr is removed manually, 20cc every 30 min. Liquid phase inhibitor solution (1.0 wt. % each of PTZ and HQ in acrylic acid) is fed to the condensor at the top of the column at a rate of 20cc/hr in each experiment. This is to ensure that (i) the distillate samples are adequately inhibited and that (ii) the experiment does not fail prematurely due to inadequate liquid phase inhibition on the trays in the column of the distillation apparatus. The salt of NPHA is delivered to the flask as a solution in AA (unless water is indicated) at 10cc/hr. The amount of inhibitor added is expressed as ppm active ingredient (as NPHA) based on liquid volume in the flask. During the 4 hour experiment, the distillation apparatus is monitored for any visual evidence of polymer in the distillation head. The method of presenting results involves counting specks of polymer in the distillation head, where no liquid splashing occurs and where condensation of vapor is more likely. The results for the salts of NPHA and also for some standards (including cupferron and the ethanolamine salt of NPHA) are given in Table II. Unless otherwise indicated, the numbers given in Table II represent the number of specks of polymer ≤ 1 mm in size. Example 8 - Study of Codistillation of Amines with Acrylic Acid The amines employed do not codistill with acrylic acid as shown by an HPLC study conducted on the distillate of an ethylene diamine salt which indicated 0 ppm of ethylene diamine. Example 9 - Percent Conversion of Cupferron vs. Activity The following shows the decrease in activity when there is a large amount of cupferron still remaining in the reaction mixture. Example 10 - Physical Properties The Table below shows viscosities of different concentrations of aqueous solutions of the ethylene diamine (EDA) salt of NPHA at temperatures of 30°C and 7°C. For comparison, water has a viscosity of 7.975 x 10⁻⁴Pas (0.7975 cp) and 1.428 x 10⁻³Pas (1.428 cp) at 30°C and 7°C, respectively. Ethylene Glycol has a viscosity of 1.99x 10⁻²Pas (1.99 cp) at 20°C. Viscosities of Different Concentrations of Aqueous Solutions of the Ethylene Diamine Salt of NPHA Salt of NPHA (wt. %) Other Compoments (wt.%) Viscosity, Pa s (cp) at 30°C Viscosity, Pa s (cp) at 7°C 40% EDA/NPHA 20% EDA 1.06 x 10⁻² (10.60) 3.861 x 10⁻² (38.61) 40% Water 19% EDA/NPHA 48% EDA 1.148 x 10⁻² (11.48) 4.657 x 10⁻² (46.57) 33% Water 20% EDA/NPHA 10% EDA 5.01 x 10⁻³ (5.01) 1.156 x 10⁻² (11.56) 70% Water 24% EDA/NPHA 13% EDA 2.9 x 10⁻³ (2.90) 8.13 x 10⁻³ (8.13) 63% Water The viscosities of the above solutions indicate that they could be easily handled under plant conditions at different temperatures. In addition to the inhibitor solution having a manageable viscosity, it is also important that such a solution does not freeze during cold weather, which could cause inhibition problems. The freezing point of a solution comprising 20% ethylene diamine salt of NPHA, 10% ethylene diamine and 70% water was measured to be between -12°C and -17°C. Aqueous solutions of the ethylene diamine salt have been monitored for a period of 3 months, and no black solids or decomposition products have been observed during this time period.

Claims: A salt of N -nitrosophenylhydroxylamine (NPHA) and an amine, expressed in neutral form by the formula: wherein n is an integer of from one to the number of basic nitrogens in the amine, and preferably 1 to 3; R¹ is hydrogen, (C₁-C₆)alkyl, or (C₁-C₆)hydroxyalkyl; R² is hydrogen, (C₁-C₆)alkyl, or (C₁-C₁₀)aminoalkyl R³ is (C₁-C₆)alkyl (C₁-C₁₀)aminoalkyl, or aminophenyl; or R² and R³ may be joined together with the nitrogen atom to which they are attached to form a substituted or unsubstituted morpholino ring; with the proviso that when R¹ is hydroxyalkyl, R² and R³ are joined to form the morpholino ring, and when R¹ is alkyl, R² and R³ are alkyl. Salt according to claim 1, wherein: R¹ is hydrogen, methyl, ethyl, propyl, butyl, pentyl, hydroxymethyl, hydroxyethyl, hydroxypropyl, hydroxybutyl, or hydroxypentyl; R² and R³ are each independently methyl, ethyl, propyl, butyl, pentyl, aminomethyl, aminoethyl, aminopropyl, aminobutyl, aminopentyl, aminohexyl, aminoheptyl, aminooctyl, aminononyl, or aminodecyl, R² may additionally be hydrogen, and R³ may additionally be aminophenyl; or when joined together with the nitrogen atom to which they are attached R² and R³ form an optionally substituted morpholino ring, where the substituent is (C₁-C₅)alkyl or (C₁-C₅)hydroxyalkyl. Salt according to claim 1 or 2 wherein R¹ is hydrogen or butyl, preferably hydrogen; R² is hydrogen, butyl or aminoethyl, preferably hydrogen or aminoethyl; and R³ is butyl or aminoethyl, preferably aminoethyl; with the proviso that when R¹ is butyl, R² and R³ are butyl. Salt according to claim 3 wherein R¹, R² and R³ are butyl. Method of inhibiting the formation of polymer from an ethylenically unsaturated monomer, preferably an unsaturated acid or ester, comprising adding to the monomer an effective amount, preferably from 20 to 1000 ppm based on said monomer, of a salt as defined in any preceding claim. Method according to claim 5 wherein the salt is added to the monomer prior to distillation of the monomer. Composition comprising an ethylenically unsaturated monomer and a polymerization-inhibiting amount of a salt as defined in any of claims 1 to 4. Process for producing a salt of NPHA as defined in any of claims 1 to 4, comprising either (i) reacting an equimolar ratio of amine of the formula N(R¹)(R²)(R³) and NPHA in a solvent, and removing the solvent by subsequent evaporation to form a dry salt, or (ii) reacting said amine with NPHA in an excess of the amine as the solvent, and preferably sparging the reaction system with nitrogen or other inert gases to aid in removing ammonia evolved in the course of the reaction. Use of a salt as defined in any of claims 1 to 4 as a polymerization inhibitor for ethylenically unsaturated acids or esters.

Inventors: BAUER WILLIAM JR, REED SAMUEL FRANKLIN JR, UPMACIS RITA KARINA, BAUER, WILLIAM JR, REED, SAMUEL FRANKLIN JR, UPMACIS, RITA KARINA
Applicants: ROHM & HAAS, ROHM AND HAAS COMPANY
Application year: 1992
Main classification: C07C243
IPC codes: C08F2, C07C57, C08K5, C07C239, C07C243, C07C241