Preparation of polyvinylazides from polyvinylnitrate
GOVERNMENT RIGHTS 
     The invention described herein may be manufactured, used and licensed by the Government for Governmental purposes without the payment to me of any royalties thereon. 
    
    
     BACKGROUND OF THE INVENTION 
     Energetic compounds having a high nitrogen content are employed as ingredients of solid propellants for guns and rockets. Such compounds do not contribute to smoke and flash and their main combustion product is nitrogen, an extremely stable molecule showing little tendency to react even at the high temperatures generated in gun tubes. Propellant compounds, which yield combustion products rich in nitrogen, are further advantageous in that they cause less gun erosion than compounds giving the same flame temperature but of lower nitrogen content. 
     Compounds of high nitrogen content can be produced by introduction of the azide group into the molecule. Azide compounds are of interest, since they (1) provide good impetus, (2) yield low flame temperatures, (3) produce less flash, (4) accelerate the burn rate, and (5) can possess good thermal and chemical stability. 
    
    
     SUMMARY AND DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to the preparation of polyvinylazides, particularly polyvinylazidonitrates, i.e. vinyl polymers containing both azide (--N 3 ) and nitrate (--ONO 2 ) groups, which are useful as propellants for guns, rockets and the like. It is believed that polyvinylazidonitrates are novel although polymers containing only azide groups, e.g. glycerylazide polymer, and only nitrate groups, e.g. polyvinylnitrate, are well known as gun propellants. 
     The novel polyvinylazides can be prepared by reacting polyvinylnitrate with a metallic azide in a solvent for the polyvinylnitrate and the metal azide at a temperature and for a period of time sufficient to effect replacement of a nitrate group by an azide group. 
     The degree of substitution of the nitrate groups by azide groups can vary from about 5 to about 95%, and is determined by the stoichiometry, as well as by the time and temperature of reaction. Suitable metal azides are preferably alkali metal azides, e.g. sodium, potassium and lithium azide, and alkaline earth metal azides, such as calcium, magnesium and barium azides, although any stable metal azide can be employed in the process of the present invention. 
     The reaction is carried out in a solvent in which the polyvinylnitrate or the metal azide or both are soluble. Suitable solvents include dimethyl formamide (DMF), dimethylsulfoxide (DMSO), N-methylpyrrolidinone (NMP), acetone and 2-methoxyethanol. The preferred solvent is DMF, particularly with sodium azide, even though the sodium azide is only slightly soluble in DMF (about 0.5g/100 ml DMF at 155° C.), although a higher reaction rate is obtained with lithium azide apparently because of its much higher solubility in DMF. 
     The reaction is preferably carried out at a temperature not substantially exceeding 80° C. to avoid crosslinking of the polymeric azides and the production of crosslinked products, which are insoluble in solvents such as acetone. Both crosslinked and non-crosslinked products are suitable for use as energetic materials; however, the non-crosslinked products are preferred, since they are soluble in solvents such as acetone and can be employed to make solutions wih other materials such as nitrocellulose. Thus, in a series of six reactions each of seven hours duration carried out at temperatures ranging from 50° C. to 95° C. the products obtained at temperatures from 50° C. to 75° C. were all soluble in acetone, whereas at 85° C. 5% of the product was insoluble in acetone and at 95° C. over 50% of the product obtained was insoluble in acetone. When the reaction was carried out at 65° C. for a much longer period of 118 hours, the reaction product was still completely soluble in acetone. 
     Solvents also vary in their influence on the production of crosslinked reaction products. For example, considerable crosslinking resulted when polyvinylnitrate was reacted with sodium azide in DMSO at room temperature after 72 hours, whereas no crosslinking resulted when the reaction was carried out in DMF at 65° C. for 118 hours. Further, the method of isolation of the solid reaction product can influence crosslinking. For example, in experiments described and tabulated below, if the solid reaction product, after trituration with methanol, is dissolved in acetone shortly after preparation, e.g. within six hours, it remains in solution indefinitely. However, if the solid reaction product is held for a longer period, e.g. twelve hours, it becomes crosslinked and completely insoluble in acetone. 
     The following examples illustrate specific embodiments of the novel polyvinylazidonitrate propellants and method for producing same according to the process of the present invention. 
     EXAMPLE A 
     Polyvinyl nitrate (containing a minimum of 14.7% nitrogen, corresponding to 93.5% of the total possible nitrate groups) (1.0 g) was mixed with sodium azide (2.0 g) in dimethyl sulfoxide (25 ml.). The mixture was stirred at room temperature for 47 hours, and then poured into water. The resulting gummy solid was thoroughly triturated with water, then with methanol and dried, yielding 1.0 g. of product. An analysis of the product revealed a nitrogen content of 33.1%, and the infrared spectrum showed the present of both azide and nitrate groups. 
     EXAMPLE B 
     Polyvinylnitrate of the type employed in example 1 (1.0g) was heated with sodium azide (1.0 g) in dimethylformamide (25 ml) at 60° C. for 3 hours. The reaction mixture was worked up as described in example 1, yielding 1.0 g of a rubbery solid containing 33.9% nitrogen. 
     Table 1 sets forth the reaction conditions and results of experiments made using the same polyvinylnitrate starting material as in example 1. The reaction products and course of the reaction were identified by elemental analysis and infrared spectra. 
     
                                           TABLE 1                                 
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Example                                                                   
     Reagents  Solvent        Grms.                                       
                                   Analysis                               
No.  PVN  NaN.sub.3                                                       
               Name Ml.                                                   
                       °C.                                         
                          Hrs.                                            
                              Yld. C  H N   Total                         
                                                Remarks                   
__________________________________________________________________________
 1   1.0  1.0  DMSO 25 25 22  0.9  -- --                                  
                                        --  --                            
 2   &#34;    &#34;    DMF  &#34;  60 3   1.0  -- --                                  
                                        33.9                              
                                            --                            
 3   &#34;    &#34;    DMSO &#34;  25 46  &#34;    -- --                                  
                                        32.3                              
                                            --                            
 4   &#34;    &#34;    &#34;    &#34;  &#34;  97  1.6  -- --                                  
                                        --  --  Insolubles noted after 72 
                                                hrs.                      
 5   &#34;    0.5  &#34;    &#34;  &#34;  46  2.1  -- --                                  
                                        22.3                              
                                            --                            
 6   &#34;    2.0  &#34;    &#34;  &#34;  47  1.0  -- --                                  
                                        33.1                              
                                            --                            
 7   1.1  1.0(1)                                                          
               Acetone                                                    
                    &#34;  56 9.5 &#34;    -- --                                  
                                        --  --                            
 8   &#34;    1.0(1)                                                          
               N--CH.sub.3                                                
                    &#34;  25 46.5                                            
                              1.2  -- --                                  
                                        --  --                            
               Pyrr.                                                      
 9   &#34;    1.0  Acetone                                                    
                    &#34;  56 24  1.4  -- --                                  
                                        --  --                            
10   &#34;    &#34;    DMF  &#34;  60 3   1.0  -- --                                  
                                        --  --                            
11   &#34;    &#34;(1) N--CH.sub.3                                                
                    &#34;  25 96  0.9  -- --                                  
                                        --  --                            
               Pyrr.                                                      
12   1.0  &#34;    DMF  &#34;  50 5   &#34;    -- --                                  
                                        --  --                            
13   6.0  6.0  &#34;    150                                                   
                       &#34;  5   &#34;    -- --                                  
                                        --  --                            
14   1.1  1.0  &#34;    25 &#34;  7.5 0.9  36.9                                   
                                      4.5                                 
                                        39.8                              
                                            79.3                          
15   1.0  &#34;    (2)  &#34;  &#34;  &#34;   --   -- --                                  
                                        --  --                            
16   1.1  &#34;    DMF  &#34;  &#34;  16  0.9  -- --                                  
                                        --  --                            
17   &#34;    &#34;    &#34;    &#34;  &#34;  24  1.0  -- --                                  
                                        --  --                            
18   &#34;    2.0  &#34;    &#34;  &#34;  48  0.9  38.8                                   
                                      4.5                                 
                                        43.8                              
                                            87.1                          
19   &#34;    &#34;    &#34;    &#34;  &#34;  120 0.9  &#34;  &#34; &#34;   &#34;                             
20   4.8  6.0  &#34;    100                                                   
                       &#34;  48  3.2  38.8                                   
                                      4.8                                 
                                        43.9                              
                                            87.5                          
21   1.1  1.0  &#34;    25 65 9   0.9  -- --                                  
                                        --  --                            
22   &#34;    &#34;    &#34;    &#34;  &#34;  24  &#34;    -- --                                  
                                        --  --                            
23   &#34;    &#34;    &#34;    &#34;  &#34;  47  1.1  39.0                                   
                                      4.5                                 
                                        50.4                              
                                            93.9                          
24   &#34;    &#34;    &#34;    &#34;  &#34;  118 1.5  39.2                                   
                                      4.6                                 
                                        50.6                              
                                            94.4                          
25   5.3  5.0  &#34;    125                                                   
                       &#34;  47  5.4  39.9                                   
                                      4.5                                 
                                        48.8                              
                                            93.2                          
                                                (4)                       
26   5.5  5.0  &#34;    125                                                   
                       &#34;  47  5.6  -- --                                  
                                        --  --  (4)                       
27   5.5  5.0  &#34;    &#34;  &#34;  47  5.9(5)                                      
                                   -- --                                  
                                        --  --  (4)                       
28   1.1  1.0  &#34;    25 &#34;  24  0.9(5)                                      
                                   39.1                                   
                                      4.7                                 
                                        50.7                              
                                            94.5                          
29   &#34;    &#34;    &#34;    &#34;  &#34;  7   1.0(5)                                      
                                   38.4                                   
                                      4.7                                 
                                        49.6                              
                                            92.7                          
30   &#34;    &#34;    &#34;    &#34;  &#34;  15  1.0(5)                                      
                                   39.0                                   
                                      4.4                                 
                                        49.1                              
                                            92.5                          
31   &#34;    &#34;    &#34;    &#34;  75 7   0.7  -- --                                  
                                        --  --                            
32   &#34;    &#34;    &#34;    &#34;  85 &#34;   --   40.8                                   
                                      4.7                                 
                                        48.4                              
                                            93.9                          
33   &#34;    &#34;    &#34;    &#34;  95 &#34;   --   -- --                                  
                                        --  --                            
34   11.0 10.0 &#34;    200                                                   
                       65 24  7.3  -- --                                  
                                        --  --                            
35   1.1  1.0(1)                                                          
               DMF  25 65 24  --   -- --                                  
                                        --  --                            
36   11.0 10.0 &#34;    200                                                   
                       55 6   8.2  34.5                                   
                                      4.6                                 
                                        38.9                              
                                            78.0                          
37   5.5  5.0  &#34;    100                                                   
                       50 6   6.3(5)                                      
                                   34.5                                   
                                      4.2                                 
                                        37.0                              
                                            75.7                          
38   1.1  1.5(1)                                                          
               &#34;    25 80 164 0.7  41.5                                   
                                      4.5                                 
                                        48.1                              
                                            94.1                          
39   0.5  0.5  DMSO 20 25 162 --   -- --                                  
                                        --  --                            
40   1.1  2.0  DMF  25 50 161 0.7  39.2                                   
                                      4.4                                 
                                        50.7                              
                                            94.3                          
41   &#34;    0.8(1)                                                          
               &#34;    &#34;  55 3   --   -- --                                  
                                        35.8                              
                                            --                            
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 NOTES:                                                                   
 (1) Lithium azide used in place of sodium azide.                         
 (2) 2Methoxyethanol used as solvent.                                     
 (4) Analysis on combined experiments 25, 26, 27 given under Run 25.      
 (5) Undried product.                                                     
 
    
     Elemental analysis (see Table 1) shows that the reaction at fairly short periods of time is accompanied by olefin formation as well as replacement of a nitrate group by an azide group, as shown in Table 2. 
     
                       TABLE 2                                                     
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Example           Structure (1)                                           
                             Analysis                                     
No.    °C.                                                         
             Hours    X   Y    Z        C    H   N                        
______________________________________                                    
                                   Calc.                                  
                                        36.0 4.2 40.3                     
14     50    7        55  27   18  Fd.  36.9 4.5 39.8                     
                                   Calc.                                  
                                        35.0 4.2 39.3                     
36     55    7        54  31   15  Fd.  34.5 4.6 38.9                     
                                   Calc.                                  
                                        38.6 4.5 49.5                     
29     65    7        70  10   20  Fd.  38.4 4.7 49.6                     
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 (1) Mole percent based on the following structure:                       
 (CH.sub.2 CHN.sub.3) .sub.x (CH.sub.2 CHONO.sub.2) .sub.y                
 (CH═CH).sub.z                                                        
 
    
     Additional evidence of double bond formation was obtained by bromination of the products. Analysis of the brominated product showed a bromine uptake corresponding closely to the degree of unsaturation indicated by analysis of the unbrominated product. 
     Longer reaction periods yield products containing no nitrate groups, as shown by infrared analysis. However, the total content of C, H and N never exceeded approximateely 95%. The difference is accounted for by the uptake of atmospheric oxygen, as shown by a broad infrared peak at 3400 cm -  1 corresponding to the hydroxyl group. A maximum of about 51% nitrogen was attained, corresponding to about 83% of that theoretically present in pure polyvinylazide.