Solution polymerization of chlorophosphazene with sulfur modifier
This invention relates to the solution polymerization of cyclic polychlorophosphazenes represented by the general formula (NPCl 2 ) n  in which n is 15 or less, usually 3 or 4, to produce essentially linear polydichlorophosphazenes in which the degree of polymerization is selectively controlled within the range of 20 to 50,000. 
     The bulk polymerization of low molecular weight cyclic polydichlorophosphazenes is described in U.S. Pat. No. 3,370,020 issued Feb. 20, 1968 and elsewhere in the literature. 
     It is also known that the thermal polymerization of hexachlorocyclophosphazene and octachlorocyclophosphazene can be effected in the presence of various catalysts as reported in &#34;Phosphorus-Nitrogen Compounds&#34; published 1973 by Interscience Publishers, New York. 
     Previous attempts to polymerize cyclic dichlorophosphazenes in solution have resulted in excessive gelation and the production of unusable products. 
     The present invention resides in the use of aromatic hydrocarbon or cyclo-aliphatic hydrocarbon solvent, as the solvent medium in which the thermal polymerization of low molecular weight cyclic polydichlorophosphazenes is conducted. 
     Suitable hydrocarbon solvents include decalin, tetralin, napthalene, cyclohexane, toluene, dimethylbenzene, dihydroanthracene, tetramethylbenzene, cumene, cymen, mono-, di-, tri-, tetra-substituted aromatic compounds where the substituents are straight chain or branched chain hydrocarbons with up to 10 carbon atoms. 
     Other solvents which have been used are liquids having a dielectric constant of at least two, and which are solvents for both the cyclic dichlorophosphazene starting materials and for the sulfur, sulfur compounds or sulfur-donating compounds present as catalysts. 
     Further solvents which have been found suitable for such polymerizations include aromatic compounds carrying polar groups such as methoxy, N,N-disubstituted amino, halogens particularly mono-, di-, tri- or tetra-halogen substituents on the aromatic nucleus, and mixtures of said solvents or other hydrocarbon solvents of a similar nature. 
     Still other solvents which have been used include aromatic compounds including those which are monocyclic or polycyclic and which are either unsubstituted or substituted, especially with polar substituents such as halogen, N,N&#39;-dialkylamino, alkoxy, and other polar groups. Aromatic solvents with up to four polar substituents on the aromatic ring have been employed. These polar substituents can be on the ring or on a side chain. For example, the aromatic ring can have either electron withdrawing groups or electron supplying groups. Examples of such solvents are 2-chloro toluene; 2,3-dichlorotoluene; 2,3-dichloroethylbenzene; and 2,3,4-trichloroethylbenzene. 
     The invention also comprises the use of small but significant amounts of elemental sulfur or sulfur compounds or sulfur donating compounds such as tri- or tetra-sulfides of organic compounds or tri- or tetra-sulfides or inorganic compounds in inorganic sulfides and organic sulfides. Sulfur donor compounds which are suitable are compounds which release sulfur at the temperatures at which the solution polymerization is conducted. Such compounds are commonly used in the vulcanization of rubbers include tetramethylthiuram tetrasulfides, and other tetraalkyl thiuram tetrasulfides; the reaction products of alkylene chlorides and sodium polysulfides, amine disulfides such as 4,4&#39;-dithio dimorpholine and other compounds which release sulfur when heated to about 200° C. 
     It has been found that instead of controlling the temperature and other reaction conditions in order to obtain polymer of a desired molecular weight, it is now possible to obtain products with a desired molecular weight by choice of a suitable solvent as the reaction medium. 
     It has been further found that the presence of a small amount of sulfur either as elemental sulfur or as a sulfur compound in the solution in the polymerization mixtures appears to overcome a tendency of the solvents to react with cyclochlorophosphazenes at high temperatures forming an intractable gel. 
     It has also been found that the choice of solvent or mixture of solvents significantly affects the molecular weight of the resulting polymer and that as a consequence polymer of a desired molecular weight is produced by selecting an appropriate solvent and suitable reaction conditions. 
     A principal object of the present invention is to provide a new and improved process for the solution polymerization of cyclic dichlorophosphazenes which yields essentially linear polydichlorophosphazene polymers. 
     A further object of the invention is to provide a solution polymerization process which can be conducted at atmospheric pressure and at comparatively moderate temperatures and under a relatively inert and easily controlled atmosphere. 
     A further object of the invention is to polymerize cyclic dichlorophosphazene oligomers to produce linear polydichlorophosphazene polymers exhibiting a unique, very narrow range of molecular weights, heretofore unobtainable in previously practiced bulk polymerizations. 
     These and other objects are achieved by polymerizing solutions comprising cyclic dichlorophosphazene oligomers dissolved in between about 10% and up to 80% by weight of selected solvent and from about 1 to up to 20 millimoles of elemental sulfur or sulfur donating compound per mol of cyclic dichlorophosphazene oligomer, the polymerization being conducted at temperatures between about 140° C. and 225° C. in reactors which may be either glass lined stainless steel vessels or unlined stainless steel vessels, or glass vessels. 
     Briefly the present invention includes polymerization of cyclic dichlorophosphazenes in aromatic or cycloaliphatic hydrocarbon solvents by proceeding as follows. 
     All of the constituents, i.e. solvent, cyclic dichlorophosphazene oligomers and sulfur or sulfur donating compounds were charged into the reaction vessel. The sulfur was added as a solution in the hydrocarbon solvent, for ease in handling. 
     For solvents with boiling points below 150° C. pressurized systems were utilized with pressure from 0-1000 psig and a preferred range of 0-200 psig. 
     For solvents with boiling points between 150° C. and 250° C. reactions were carried out at atmospheric pressure. Dry Argon gas was passed over the reaction mixture. Care was taken at all points to prevent oxygen or moisture from being introduced into reaction mixture. Apparatus was preheated in 140° C. oven for 24 hours before use. 
     Stirring was by means of magnetic bars. Polymerizations were conducted for from 40-300 hours preferably at 200°-220° C. bath temperature. The polymerization reaction mixture was usually solid at completion of reaction. Unreacted cyclics and hydrocarbon solvent were stripped by repeated washing with dry hexane in inert, dry atmosphere. Recovered polymer was dissolved in benzene yielding a mostly soluble polymer, exhibiting dilute solution viscosities between from about 0.09 to 1.2 (DSV is equivalent to inherent viscosity stated in decaliters/gram). 
     Volatile material swept from the reaction mixture included H 2  S and HCl. 
     The same polymerization reaction carried out under the same reaction conditions to conversions of 60% or more except that it was conducted in the absence of sulfur or sulfur donating compounds produced products which were usually gelled and insoluble in benzene. The exact nature of the effect of the sulfur on the polymerization is not fully understood. 
     The invention will be further described in the following examples given by way of illustration and not intended to limit the invention. The data in Table I was obtained by proceeding as follows: 
    
    
     EXAMPLE 1 
     The indicated number millimoles of elemental sulfur dissolved in tetralin or other solvent was added to a solution of cyclic dichlorophosphazene trimer in the indicated solvent, in a glass or stainless steel reaction vessel. An Argon flow of 1 to 2 ml/minute was maintained across the surface of the solution. Temperature was maintained for the indicated times and yields of 25-60% conversion to polymer were obtained. 
     
                                           TABLE I                                 
__________________________________________________________________________
                Millimoles                                                
         % (NPCl.sub.2).sub.n                                             
                of Sulfur                                                 
                       Temp.                                              
                           Time                                           
                               % Conv-     Solvent                        
Solvent  in Solvent                                                       
                per (NPCl.sub.2)                                          
                       °C.                                         
                           Hrs.                                           
                               ersion                                     
                                    DSV/Gel                               
                                           Ratio                          
__________________________________________________________________________
Decalin* 52     6      200 70  45   .99/0                                 
Decalin* 26     10     205 68  49   .79/0                                 
Decalin  25     5      210 70  50   --                                    
Decalin  50     5      210 42  40   --                                    
Decalin* 75     5      210 42  56   .76/1.3                               
Decalin  34     7      210 96  26   --                                    
Decalin* 50     4      210 93  18   1.46/1.1                              
Decalin  49     2      190 45  .25  --                                    
Tetralin 49     0      210 70   9   .28/.5                                
Tetralin 50     0      210 70  19   .31/0                                 
Tetralin 50     0      210 70  15   .25/1.4                               
Tetralin 50     5      205 70  94   .25/0                                 
Tetralin 53     6      200 70  45   .31/0                                 
Tetralin 67     4      215 101 64   .21/0                                 
Tetralin 68     2      210 48  30   .17/0                                 
Tetralin 67     2      215 375 95   .23/0                                 
Naphthalene                                                               
         60     1      200 62  --   .16/0                                 
Naphthalene                                                               
         55     2      205 73  34   --                                    
Decalin/Tetralin                                                          
         60     3      205 148 60   1.00/0 1:1                            
Decalin/Tetralin                                                          
         50     4      205 160 49   .66/0  7:3                            
Decalin/Tetralin                                                          
         50     3      200 70  25   1.22/0 4:1                            
Decalin/Tetralin                                                          
         50     3      220 71  52   .24/0  4:1                            
Decalin/Tetralin                                                          
         50     3      220 70  36   .26/0  9:1                            
Decalin/Tetralin                                                          
         50     3      220 70  52   .18/0  4:1                            
Decalin/Tetralin                                                          
         50     3      220 70  43   .21/0  4:1                            
Decalin/Tetralin                                                          
         50     3      220 70  34   .20/0  4:1                            
Decalin/Tetralin                                                          
         50     3      200 140 27   .22/0  4:1                            
Prehnitene                                                                
         50     3      205 162 84   .42/0                                 
Prehnitene                                                                
         52     3      200 112 50   .58/0                                 
__________________________________________________________________________
 *1% ethanol added to benzene solutions to breakup slightly gelled polymer
 for DSV.                                                                 
 
    
     It can be seen from Table I that the phosphonitrile trimer can be polymerized using decalin as the solvent producing high molecular weight polymers, while various solvents such as tetralin, naphthalene, decalin/tetralin mixture, prehnitene produced polymers of somewhat different molecular weights. 
     EXAMPLE 2 
     For solvents with boiling points below 190° C., such as toluene it was necessary to use a pressurized system. An Argon blanket was maintained over the polymerization solution at 10-200 psig. Every 2-6 hours the system was flushed by lowering the pressure and repressuring with fresh Argon. Other conditions were as indicated in Table II. 
     
                       TABLE II                                                    
______________________________________                                    
                  Millimoles of                                           
       % (NPCl.sub.2).sub.n                                               
                  Sulfur per Temp. Time % Conv-                           
Solvent                                                                   
       in Solvent mol (NPCl.sub.2).sub.3                                  
                             °C.                                   
                                   Hrs. ersion                            
______________________________________                                    
Toluene                                                                   
       49         2.5        205   93   76                                
Toluene                                                                   
       60         3          205   46   33                                
______________________________________                                    
 
    
     EXAMPLE 3 
     Example 1 was repeated employing the solvents shown in Table III, resulting in 90-96% conversion of phosphonitrilic chloride oligomers to high molecular weight polymer. The resulting product after recovery was derivatized as described in U.S. Pat. No. 3,970,533 issued July 20, 1976 using C 2  and C 5  fluorinated alcohols and with o-allyl phenol for curative sites. Table III gives the final derivatized phosphonitrilic chloride polymer with trifluoro ethanol and octafluoropentanol with complete molecular weight determinations. The phosphonitrilic chloride polymer obtained from the examples discussed in Table I can be derivatized to useful fluoroalkoxide elastomers, in the same manner. 
     
                                           TABLE III                               
__________________________________________________________________________
Polymerization of PNCl.sub.2).sub.3 in Three Solvents and                 
Derivatization to C.sub.2 F,C.sub.5 F,OAP Polyphosphazenes                
                            Derivatized Polymer                           
          Monomer Conversion                                              
Solvent   Concentration                                                   
                  to Polymer                                              
                         DSV                                              
                            DSV                                           
                               Mn × 10.sup.-3                       
                                     Mw/Mn                                
__________________________________________________________________________
Tetralin  67%     95%    0.23                                             
                            0.33                                          
                               189.5 2.92                                 
1,2,3,4                                                                   
Tetramethylbenzene                                                        
          46%     96% -- 0.66                                             
                            295.2                                         
                               2.99                                       
2,4-Dichlorotoluene                                                       
          12.5%   90%    0.58                                             
                            0.47                                          
                               364.3 2.80                                 
__________________________________________________________________________