Polymerization inhibitors for N-substituted aminoalkyl acrylic monomers
This application is a continuation-in-part of U.S. Ser. No. 477,126 filed June 6, 1974, now abandoned. 
    
    
     This invention relates to the use of an inhibitor system for preventing polymerization of N-substituted aminoalkyl acrylic monomers at ambient and above ambient temperatures, such as may occur in preparation, purification or storage of the acrylic monomers. 
     Acrylic ester monomers are well-known and their preparation has been amply disclosed in the literature and art. N-substituted aminoalkyl acrylic monomers are generally prepared by a transesterification reaction, typical examples of which are discussed in U.S. Pat. Nos. 2,744,884 and 3,714,234. Since the ester interchange of transesterification reaction is conducted in a temperature range of about 60° C. to about 150° C., polymerization of reaction product in the reaction zone due to heating above ambient temperatures is a persistent problem in monomer manufacture. Additionally, the almost universal method for purification of the crude acrylic ester monomers obtained by various processes involves the step of distillation. The problem encountered during this step which generally entails refluxing at elevated temperatures, is again the marked tendency for the monomer to polymerize. N-substituted aminoalkyl acrylic monomers tend to polymerize in the still pot as well as in the distillation columns. The heavy viscous polymer that forms in the still pot during distillation induces higher stress and strain on processing equipment, such as pumps, necessitating more frequent shutdowns. Likewise, polymerization on the plates of the distillation columns becomes so excessive as to force production shutdowns. These monomers are also subject to polymerization at ambient temperatures during prolonged storage in the absence of any inhibitors. 
     A variety of polymerization inhibitors have been developed and are known in the art. Among the most common are the phenol-based inhibitors such as the quinones, hydroquinones, nitrophenol as well as other organic and inorganic compounds. However, polymerization during distillation of N-substituted aminoalkyl acrylic monomers is still a problem due to the general ineffectiveness of the known inhibitors in suppressing the polymerization, with the resultant shutdowns discussed infra. 
     According to the present invention, an inhibitor is provided which is useful in inhibiting the polymerization of N-substituted aminoalkyl acrylic monomers. It is especially useful to prevent polymerization during the preparation of the monomers at elevated temperatures, as well as during distillation processes and during prolonged storage at ambient temperatures. N-substituted aminoalkyl acrylic monomers, the polymerization of which may be thus inhibited, include dimethylaminoethyl acrylate, diethylaminoethyl acrylate, dibutylaminoethyl acrylate, diethylaminopropyl acrylate, 2-dibutylaminopropyl acrylate, 3-dibutylaminopropyl acrylate, 2-morpholinoethyl acrylate, 2-[N-(2-hydroxyethyl)ethylamino]ethyl acrylate, t-butylaminoethyl methacrylate, dimethylaminoethyl methacrylate, 2-oxazolidinylethyl acrylate or methacrylate, and so forth. Such monomers may be generally represented by the formula: ##STR1## where R 1  is hydrogen or a methyl group, R 2  is a divalent alkylene radical of 1 to 5 carbon atoms, and R 3  is a hydrogen or an alkyl having 1 to 5 carbon atoms, R 4  is an alkyl having 1 to 5 carbon atoms, or R 3  and R 4  are taken together with the nitrogen atom to form a five or six membered saturated heterocyclic ring, optionally having at least one of the hetero atoms, N and O, in the ring. 
     The monomers are stabilized by contacting them with an amount of metallic salt, or complex in an amount effective to inhibit polymerization. The amount used may be as low as 0.1% by weight of the unsaturated monomer. As the salts and complexes are generally insoluble in the monomers no rigid upper limit on amount of inhibitor to be used can be spelled out, as even slurries can be used to inhibit polymerization. As a practical matter, about 4% by weight of the unsaturated monomer is a useful upper limit, as this amount is entirely sufficient to inhibit polymerization. The inhibitors of this invention are the salts of tin (II), copper (I), manganese (II), cobalt (II) as well as complexes and salts of manganese, sodium and zinc with ethylenebisdithiocarbamic acid. The useful salts include the halides, halogenates, sulfates, nitrates, chromates, phosphate as well as carboxylic acid salts such as acetates, citrates and so on. The useful zinc and maganese complexes and salts with ethylenebisdithiocarbamic acid include nabam, mancozeb, zineb and maneb. The stannous salts are preferred and the stannous halides and zineb are most preferred. Cuprous chloride is most preferred for use as an inhibitor during ambient temperature storage conditions. 
     As previously mentioned, the inhibitor of this invention may be employed to inhibit polymerization during the preparation of an N-substituted aminoalkyl acrylic monomers, such as by direct esterification or transesterification as well as during the purification of such acrylic monomers as by distillation. When the acrylic monomer is obtained in an aqueous medium, it may be extracted by means of an organic solvent from the aqueous solution and the solvent then stripped off in a distillation column. Alternatively, the acrylic monomer may be separated from the aqueous solution by distilling off the water. When inhibiting polymerization during a preparation step, as during a transesterification, the inhibitor is added directly to the reactants before or during the reaction. Likewise, the inhibitors can be contacted with the crude monomer in the still kettle of the distillation system prior to distillation. The inhibitors may also be added to the plates in the distillation column to prevent polymer formation therein. While the inhibitor of this invention does not always completely suppress the formation of the acrylic polymer, even if polymers do form, it is only after a considerably longer period of distillation than could be obtained without inhibitor. Additionally, purified monomers are effectively stabilized against polymerization during storage at ambient temperatures by the addition of the inhibitors of this invention. 
     Since the inhibitors are solids, and their vaporization points are higher than that of the monomers, the solid salts remain in the still pot with the advantage that the purified monomers collected overhead are free of inhibitor. 
     In the following examples, test are run showing the effect of these inhibitors on polymerization of t-butylaminoethyl methacrylate and 2-oxazolidinylethyl methacrylate under trial run conditions as well as conditions simulating actual full-scale production. 
    
    
     EXAMPLE I 
     Samples of crude t-butylaminoethyl methacrylate with different levels of stannous chloride are placed in 25 × 200 mm test tubes and heated in an oil bath over a range of temperatures. A control sample in run without stannous chloride. The results clearly demonstrate that even at low temperatures, crude t-butylaminoethyl methacrylate polymerizes without any inhibitor, while low levels of stannous chloride suppress polymerization even at higher temperatures. 
     
                       TABLE I                                                     
______________________________________                                    
Temper-           Control  + 0.5%                                         
ature             t-BAEMA  SnCl.sub.2                                     
                                  + 1.0% SnCl.sub.2                       
______________________________________                                    
       Duration   38.2 hrs.                                               
                           5 days 5 days                                  
 80° C                                                             
       Appearance                                                         
       of Sample  Solid    Fluid  Fluid                                   
       Duration   91 hrs.  91 hrs.                                        
                                  91 hrs.                                 
100° C                                                             
       Appearance                                                         
       of Sample  Solid    Slightly                                       
                                  Viscous                                 
                  (viscous viscous                                        
                  after                                                   
                  24 hrs.)                                                
       Duration   5 hrs.   48 hrs.                                        
                                  48 hrs.                                 
130° C                                                             
       Appearance                                                         
       of Sample  Solid    Viscous                                        
                                  Less vis-                               
                                  cous than                               
                                  with 0.5%                               
                                  SnCl.sub.2                              
       Duration   3 hrs.   24 hrs.                                        
                                  24 hrs.                                 
150° C                                                             
       Appearance                                                         
       of Sample  Solid    Very   Very vis-                               
                           viscous                                        
                                  cous                                    
______________________________________                                    
 t-BAEMA = tert-butylaminoethyl methacrylate                              
 
    
     EXAMPLE II 
     Samples of previously distilled oxazolidinylethyl methacrylate to which are added different levels of stannous chloride are placed in 25 × 200 mm test tubes and heated in an oil bath to a temperature of 75° C. A control sample is run without stannous chloride. The results are summarized in Table II. Again, untreated samples show rapid onset of polymerization, whereas treated samples remain clear after 24 hours. 
     
                       TABLE II                                                    
______________________________________                                    
Temper-           Uninhibited                                             
                             + 0.4%  + 1.2%                               
ature             OXEMA      SnCl.sub.2                                   
                                     SnCl.sub.2                           
______________________________________                                    
       Duration   4 hrs.     24 hrs. 24 hrs.                              
75° C                                                              
       Appearance                                                         
       of Sample  Viscous    no signs                                     
                                     no signs                             
                             of polymer                                   
                                     of polymer                           
       Duration   6 hrs.                                                  
75° C                                                              
       Appearance                                                         
       of Sample  Very thick                                              
                  and syrupy                                              
______________________________________                                    
 OXEMA = oxazolidinylethyl methacrylate                                   
 
    
     EXAMPLE III 
     350 gm samples of crude oxazolidinylethyl methacrylate (˜50% methyl methacrylate,˜50% oxazolidinylethyl methacrylate) are placed in a 1-liter flask fitted with a thermometer, stirrer and fractionation column (10 plate Oldershaw). A sample is run without stannous chloride. The samples are heated with reflux, and stirred, at 115° C. until polymer forms. The results are summarized in Table III. Again, the time at which polymer appears is delayed by the addition of inhibitor. 
     
                       TABLE III                                                   
______________________________________                                    
Inhibitor Added      Time For Polymer Formation                           
______________________________________                                    
None                  4.5 hrs.                                            
+ 0.2% SnCl.sub.2     5.5 hrs.                                            
+ 0.8% SnCl.sub.2    11.0 hrs.                                            
______________________________________                                    
 
    
     EXAMPLE IV 
     3,000 gm batches of crude t-butylaminoethyl methacrylate are distilled from a 5-liter flask equipped with a stirrer, thermometer, a 5-plate Oldershaw fractionation column and a distillation head filled with automatic reflux control (75% forward). The crude t-butylaminoethyl methacrylate is composed of approximately the following: 
     
         ______________________________________                                    
7.3%       methyl methacrylate                                            
2.1%       t-butylaminoethanol                                            
79.2%      t-butylaminoethyl methacrylate                                 
5.1%       high boiling fraction                                          
6.3%       solids                                                         
100.0%                                                                    
______________________________________                                    
 
    
     A theoretical distillation recovery would be about 2,660 gm of low boiling fraction and product. The experiment is run with a control distillation having no inhibitor and a distillation with inhibitor. The distillation is carried out until no more product comes overhead. The results are summarized in Table IV. The results show that stannous chloride can provide much higher yields in product recovery over inhibitor-free recoveries. 
     
                                           TABLE IV                                
__________________________________________________________________________
DISTILLATION OF CRUDE t-BAEMA                                             
Crude t-BAEMA             Crude t-BAEMA + 1% SnCl.sub.2                   
                   Total                     Total                        
Time Pot  Head                                                            
              Pressure                                                    
                   Amount Time Pot  Head                                  
                                        Pressure                          
                                             Amount                       
(Hrs)                                                                     
     (C°)                                                          
          (C°)                                                     
              (mm Hg)                                                     
                   Collected                                              
                          (Hrs)                                           
                               (C°)                                
                                    (C°)                           
                                        (mm Hg)                           
                                             Collected                    
__________________________________________________________________________
0     89  30  55   1st drop                                               
                          0     87  30  55   1st drop                     
2.5   93.5                                                                
          67  7    68  gm 2.0   97  84  8    123 gm                       
5.5   99  84  7    313 gm 6.5  100  88  8    663 gm                       
12.0 101  86  7    874 gm 14.0 103  90  8    1348                         
                                                 gm                       
18.5 101  86  7    1485                                                   
                       gm 21.5 104  89  8    2093                         
                                                 gm                       
25.0 102  87  7    1743                                                   
                       gm 27.5 129  84  8    2410                         
                                                 gm                       
31.5 109  86  7    2317                                                   
                       gm 29.0 138  82  8    2513                         
                                                 gm                       
__________________________________________________________________________
Yield = 87%               Yield = 94%                                     
 t-BAEMA = t-butylaminoethyl methacrylate                                 
 
    
     EXAMPLE V 
     Samples of previously distilled oxazolidinylethyl methacrylate, to which are added various inhibitors at the level of 1% per weight of monomer, are placed in 25 × 200 mm test tubes and heated in an oil bath to a temperature of 75° C. A control sample in run without any inhibitor. The results are summarized in Table V. Untreated samples show rapid onset of polymerization while some treated samples remain clear after 96 hours and others show polymer onset only after a long period of heating. 
     
                       TABLE V                                                     
______________________________________                                    
INHIBITOR  ONSET TIME (hrs)                                               
                           APPEARANCE                                     
______________________________________                                    
None        6-8            Polymer                                        
MnCl.sub.2 22.0            Polymer                                        
Co(NO.sub.3).sub.2                                                        
           27.0            Polymer                                        
CuCl        96.0.sup.1     No Polymer                                     
SnCl.sub.2  96.0.sup.1     No Polymer                                     
______________________________________                                    
 .sup.1 Sample heated for this length of time without showing polymer     
 onset.                                                                   
 
    
     EXAMPLE VI 
     Samples of previously distilled oxazolidinylethyl methacrylate, to which are added various inhibitors at the level of 1% per weight of monomer are heated in 25 × 200 mm test tubes to a temperature of 75° C. under nitrogen sparge. A control sample is run without any inhibitor. The results are summarized in Table VI. 
     
                       TABLE VI                                                    
______________________________________                                    
INHIBITOR  ONSET TIME (hrs)                                               
                           APPEARANCE                                     
______________________________________                                    
None       1 hour          Polymer                                        
ZnCl.sub.2 1 hour          Polymer                                        
Zineb      79.0            Polymer                                        
Mancozeb   96.0.sup.2      No Polymer                                     
Mancozeb.sup.1                                                            
           110.0           Polymer                                        
______________________________________                                    
 .sup.1 Different sample of mancozeb from previous sample.                
 .sup.2 Sample heated for this length of time without showing polymer     
 onset.                                                                   
 
    
     EXAMPLE VII 
     Samples of distilled oxazolidinylethyl methacrylate are stored at ambient temperature with and without inhibitor to determine the stabilizing effect of the inhibitors. The monomer is stored in capped bottles at a temperature of about 20° C. The results are summarized in Table VII. It is apparent from the results that while stannous chloride is extremely effective in preventing polymerization in above ambient temperature conditions, it is definitely unacceptable in ambient temperature monomer storage conditions. 
     
                       TABLE VII                                                   
______________________________________                                    
INHIBITOR  ONSET TIME (days)                                              
                           APPEARANCE                                     
______________________________________                                    
None       4               Solid                                          
1% SnCl.sub.2                                                             
           1               Solid                                          
1% CuCl    17.sup.1        Fluid                                          
                           (no polymer)                                   
______________________________________                                    
 .sup.1 Sample stored this long without showing polymer onset.