Quaternary ammonium catalyst system for the polymerization of 2-pyrrolidone
This is a division of application Ser. No. 687,968, filed May 19, 1978, now U.S. Pat. No. 4,098,774. 
    
    
     BACKGROUND OF THE INVENTION 
     Poly-2-pyrrolidone is produced by the alkaline-catalyzed polymerization of 2-pyrrolidone. The catalyst system may comprise a partially carbonated pyrrolidonate salt made, for example, by reacting an alkali metal hydroxide with 2-pyrrolidone, or by reacting a quaternary ammonium hydroxide with 2-pyrrolidone, dehydrating, and contacting the product with carbon dioxide (U.S. Pat. No. 3,721,652). Japanese Patent No. 47-26195 discloses a process for making a catalyst by reacting a non-water-forming alkali metal compound with 2-pyrrolidone and contacting the product with a quanternary ammonium halide under anhydrous conditions. In U.S. Pat. No. 3,835,100, the catalyst obtained by reacting an alkali metal alkoxide with a quaternary ammonium halide and contacting the product, ammonium alkoxide, with 2-pyrrolidone, also avoids the production of water. While both of the latter processes are anhydrous, they do not produce poly-2-pyrrolidone of very high molecular weight in ordinary reaction times. For many processes, it is advantageous to be able to rapidly produce poly-2-pyrrolidone having high molecular weight, e.g. in excess of 500,000 in good yield. 
     BRIEF SUMMARY OF THE INVENTION 
     A catalyst capable of producing a high-molecular-weight poly-2-pyrrolidone is made by contacting an alkali metal pyrrolidonate, certain quaternary ammonium halides and carbon dioxide the mol ratio of 1:0.1-2:0.1-0.5. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     While poly-2-pyrrolidone of 300,000 weight average molecular weight is producible over a polymerization period of less than 24 hours by using a partially carbonated potassium pyrrolidonate catalyst, the catalyst of the present invention is capable of producing poly-2-pyrrolidone having a weight average molecular weight in excess of 1,000,000 under the same conditions. The present catalyst also achieves high yields and high conversion rates without diminution of molecular weight. The catalyst does not require an anhydrous source of alkali metal pyrrolidonate. The pyrrolidonate may be made by contacting 2-pyrrolidone with the hydroxide, rather than by contacting it with an alkali metal or alkali metal alkoxide. 
     CATALYST SYSTEM 
     In the process of the present invention a catalyst for the polymerization of 2-pyrrolidone is made by contacting an alkali metal pyrrolidonate, certain quaternary ammonium halides and carbon dioxide in mol ratio of about 1:0.1-2:0.1-0.5, preferably in mol ratio of about 1:0.2-1.5:0.1-0.5, and most preferably in a mol ratio of about 1:1:0.3. 
     The reactants, the pyrrolidonate, the halide and carbon dioxide, may be contacted in any order beginning with the pyrrolidonate as one of the components. It is preferred, but not necessary, to add the quaternary ammonium halide to the previously carbonated pyrrolidonate salt. In a preferred embodiment, the catalyst of the present invention is formed in a solution of 2-pyrrolidone. An alkali metal hydroxide is added to an excess of 2-pyrrolidone, with which it reacts to produce a solution of the alkali metal pyrrolidonate and water in 2-pyrrolidone. The solution is dehydrated until it contains less than about 0.1-0.2 weight percent water. Then carbon dioxide is added in the required mol ratio to the pyrrolidonate in the solution at a temperature of about 25°-30° C. The quaternary ammonium halide is also added in the required mol ratio to the pyrrolidonate at about the same temperature. 
     Another method for preparing the catalyst systems of this invention involves the in situ preparation of tetraalkyl ammonium halide by the reaction of trialkylamine with an alkyl halide. For example, a portion of the 2-pyrrolidone monomer may be used as a solvent in which the above reaction is carried out. The reaction involves dissolving the trialkylamine in this pyrrolidone and then adding the alkyl halide while at the same time maintaining the temperature within the range of about 10 to 50° C. When the reaction is completed, the resulting solution is reacted with alkali metal pyrrolidonate and carbon dioxide as by addition to a pyrrolidone solution in which the carboxylated alkali metal salt has already been prepared by the usual procedures. 
     The quaternary ammonium halide is preferably a tetraalkyl ammonium halide, although alkylaryl ammonium halides, such as phenyl trimethyl ammonium halide and tolyl triethyl ammonium halide, are included within the scope of the ammonium halides of this invention. The tetraalkyl ammonium halide is preferably a tetra (C 1  -C 6 ) alkyl ammonium halide, and most preferably a tetra (C 1  -C 3 ) alkyl ammonium halide. Representative alkyl groups include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, t-butyl, hexyl, etc. The ammonium halide is preferably a chloride, bromide or iodide or more preferably a chloride or bromide, most preferably a chloride. The ammonium halide may be used as a combination of species, e.g., as a mixture of tetramethyl ammonium chloride and tetraethyl ammonium bromide. The ammonium halide should be substantially soluble under the alkaline conditions of catalyst system preparation in order to show an appreciable effect on the polymerization reaction. 
     The ammonium halides finding use within the scope of this invention include compounds such as R 1  R 2  R 3  (φCH 2 ) NX wherein φ represents a phenyl and R 1 , R 2  and R 3  may be the same or different alkyl groups or other aralkyl groups, and X is a halide. Consequently, ammonium halide, as used herein encompasses the aralkyl ammonium halides. These aralkyl groups will normally contain 7-12 carbon atoms e.g. benzyl, phenethyl etc. 
     The alkali metal pyrrolidonate is preferably sodium or potassium pyrrolidonate. For certain purposes, it may be advantageous to substitute for pyrrolidonate in whole or in part an alkali metal caprolactamate or the alkali metal salt of another low-molecular-weight lactam, but this is normally not preferred to the use of the pyrrolidonate. The alkali metal pyrrolidonate is preferably made by contacting the alkali metal hydroxide with excess 2-pyrrolidone, but other methods may be chosen, such as by reacting 2-pyrrolidone with an alkali metal or an alkali metal alkoxide. While it is preferable to contact the tetraalkyl ammonium halide, the pyrrolidonate and carbon dioxide in a 2-pyrrolidone solution, inert solvents may be used in whole or in part to replace the 2-pyrrolidone. Sulfur dioxide is believed to be an inferior substitute for carbon dioxide, but its use is not barred in the practice of the present invention. 
     In the catalyst system of the present invention, polymerization initiators and polymerization accelerators may also be used. Unexpectedly rapid polymerization to poly-2-pyrrolidone of satisfactorily high molecular weight is achieved in this catalyst system by the addition of 0.05-1.5 mol percent of acetic anhydride. Preferably 0.05-1.0 and most preferably about 0.05-0.5 mol percent of acetic anhydride is used. Suitable accelerators are also described in U.S. Pat. No. 3,721,652 and include N-acyl lactam, particularly the N-acyl pyrrolidones, preferably N-acetyl pyrrolidone and 1-(1-pyrrolin-2-yl)-2-pyrrolidone. 
     POLYMERIZATION CONDITIONS 
     The polymerization process of this invention is specifically applicable to the polymerization of 2-pyrrolidone to form a polymeric carbonamide of very high molecular weight in a reasonably short polymerization time for this reaction of 4-24 hours. Weight average molecular weights in excess of 1,000,000  have been attained. The high-molecular-weight polymer is capable of being formed into filaments having substantial orientation along the filamentary axis, high tensile strength and other properties suitable for making into textiles. It can be made into shaped articles and film by melt-molding or extrusion. 
     In order to produce high-quality poly-2-pyrrolidone capable of being formed into fibers, filaments and yarn of commercial textile quality, it is necessary that the 2-pyrrolidone be of high purity. Depending upon the process of manufacture, commercially available 2-pyrrolidone may contain appreciable amounts of various impurities, some of which are believed to interfere deleteriously with polymerization. Purification of the monomer to polymerization grade is achieved by known purification techniques, including distillation. 
     The process of the present invention is just as applicable to the production of polymers of C-alkyl-substituted pyrrolidone, such as 4-methyl-2-pyrrolidone and copolymers of 2-pyrrolidone, such as with caprolactam, as to the production of poly-2-pyrrolidone. Consequently, in general, and unless otherwise indicated, &#34;monomer&#34; denotes 2-pyrrolidone, subtituted 2-pyrrolidone, and any compound capable of copolymerizing with 2-pyrrolidone under the stated conditions of alkaline polymerization catalysis. 
     Preferably the catalyst system comprises about 0.5-30 mol percent or more of the 2-pyrrolidone-catalyst mixture, based on total 2-pyrrolidone, preferably about 5-20 mol percent, and most preferably about 10 mol percent catalyst. Total 2-pyrrolidone consists of 2-pyrrolidonate catalyst, including alkali metal pyrrolidonate and quaternary ammonium pyrrolidonate, as well as carbonated alkali metal pyrrolidone and carbonated quaternary ammonium pyrrolidonate, and 2-pyrrolidone provided as solvent to said catalyst, and any additional monomer charged to the mixture for polymerization reaction. The polymerization catalyst system is believed to principally comprise quanternary ammonium pyrrolidonate and carbonated quaternary ammonium pyrrolidonate, but substantial amounts of alkali metal pyrrolidonate and carbonated alkali metal pyrrolidonate (carboxypyrrolidonate) may also be present, depending upon the mol ratios chosen. Alkali metal halide is thought to be present, but it is believed to be inert towards the polymerization reaction. 
     In general, 2-pyrrolidone may be polymerized at a temperature from about 15° C. to about 100° C., preferably 25° C. to 70° C., and most preferably from about 40° C. to about 60° C., under a pressure ranging from subatmospheric to superatmospheric, in the presence of the catalyst system for a period from about 4 to about 100 hours or longer, preferably for about 8 to about 72 hours, and most preferably from about 8 to about 48 hours. In continuous operation, polymerization time refers to average residence under polymerization conditions. A small amount of water, not exceeding about 0.1-0.2 weight percent, based on total 2-pyrrolidone, is permissible in the reaction mixture, but less than 0.1 weight percent is preferred. 
     Preparation of polymers of 2-pyrrolidone, according to the normal process of this invention, can be carried out with various amounts of monomers, catalyst, inert nonsolvent liquids, initiators and other additives-- the amount of each being properly coordinated to produce the most effective polymerization. Although the preferred conditions and amounts of the components in the reaction have been given, it is understood that these are not intended to be limitations to polymerization, since it may be possible to achieve substantial polymerization outside the preferred ranges. 
     EXEMPLIFICATION 
     EXAMPLE 1 
     100 g of 2-pyrrolidone (1.175 M) was mixed with 1.55g of 85.5% KOH pellets (0.0236M) to make a 2 mol percent potassium pyrrolidonate solution which was dehydrated by heating to incipient distillation at 2 mm pressure for 11 minutes. To the dehydrated solution was added sufficient carbon dioxide to make a polymerizate containing 30 mol % CO 2  based on potassium. The carbonated 2-pyrrolidone solution was poured into 2 bottles, one of which was held at 50° C. for 8 hours, the other held at 50° C. for 22 hours. After these time intervals the contents of the bottles were chopped, washed with water, dried, weighed and subjected to viscosity measurement for molecular weight determination as described. The results are presented in Table I. 
     EXAMPLE 4 
     Same as Example 1 except for the addition of 2.59g (0.0236M) of tetra methyl ammonium chloride after carbonation by weighing the dried onium salt in a dry box and adding same under N 2  at room temperature to the polymerizate with stirring for 5 minutes. The remaining procedure was as in Example 1. 
     EXAMPLE 13 
     100 g (1.175M) of 2-pyrrolidone was mixed with 3.85g of 85.5% KOH pellets (0.0588M) to form a 5 mol percent potassium pyrrolidonate solution which was dehydrated by heating to incipient distillation at 2 mm pressure for 10 minutes. To the dehydrated solution was added 30 mol % carbon dioxide based on potassium. Then 0.12 g of acetic anhydride (0.1 mol percent based on total monomer) was added dropwise to the stirred polymerizate which was polymerized for 8 hours at 50° C. The product was chopped, washed, dried, weighed and subjected to molecular weight determination. The results are given in Table IV. 
     EXAMPLE 15 
     Same as Example 13 but 6.44 g (0.0588M) of tetramethyl ammonium chloride was added after carbonation to make a 5 mol % solution based on total monomer. Then 0.12g of acetic anhydride was added. The remaining procedure was as in Example 13. 
     The polymerization process of this invention produces a high molecular weight poly-2-pyrrolidone at a high rate of conversion without producing the unpleasant odors which are sometimes associated with the dehydration of quaternary ammonium hydroxide-2-pyrrolidone mixtures. The combination of carbon dioxide polymerization activation and quaternary ammonium halide as a source of polymerization catalyst produces extremely high molecular weight polypyrrolidone. The addition of about one mol percent acetic anhydride has the additional effect of greatly accelerating the rate of polymerization. 
     Table I shows several polymerizations in the presence of potassium pyrrolidonate and carbon dioxide (Py-K/CO 2 ) with and without tetramethyl ammonium chloride. The tetramethyl or tetraethyl ammonium chloride in combination with carbon dioxide and potassium pyrrolidonate are found to be capable of producing polypyrrolidone of an extremely high weight average molecular weight in excess of one million. All molecular weights are reported as the weight average molecular weight determined from the specific viscosity of 0.1 gram of polymer/100cc of m-cresol solution at 25° C. All reported percentages are mol percent unless otherwise indicated. 
     
         __________________________________________________________________________
TABLE I                                                                   
8 hours.sup.1                                                             
      % Py--K/CO.sub.2 2                                                  
              % (CH.sub.3).sub.4 NCl                                      
                      % Conversion                                        
                              Mw × 10.sup.-3                        
__________________________________________________________________________
Example 1                                                                 
      2       0       8.5     117                                         
Example 2                                                                 
      2       0       16.3    285                                         
Example 3                                                                 
      10      0       14.4    305                                         
Example 4                                                                 
      2       2       15.0    210                                         
Example 5                                                                 
      5       5       40.0    575                                         
Example 6                                                                 
      10      10      53.9    980                                         
22 hours.sup.1                                                            
Example 1                                                                 
      2       0       20.1    220                                         
Example 2                                                                 
      5       0       45.2    380                                         
Example 3                                                                 
      10      0       48.3    500                                         
Example 4                                                                 
      2       2       37.7    330                                         
Example 5                                                                 
      5       5       69.1    1050                                        
Example 6                                                                 
      10      10      59.42   820                                         
 .sup.1 50° C.                                                     
 .sup.2 30 mol % CO.sub.2 based on K.                                     
 
    
     
         TABLE II.sup.1                                                            
              Mol Ratio                                                   
      % (CH.sub.3).sub.4 NCl                                              
              (CH.sub.3).sub.4 NCl/K                                      
                      % Conversion                                        
                              Mw × 10.sup.-3                        
__________________________________________________________________________
Example 2                                                                 
      0       --      45.2    380                                         
Example 7                                                                 
      1       0.2     62.9    555                                         
Example 8                                                                 
      2       0.4     68.7    635                                         
Example 5                                                                 
      5       1.0     69.1    1050                                        
Example 9                                                                 
      7.7     1.5     69.2    605                                         
 .sup.1 22 hours, 50° C., 5 mol % Py-K/CO.sub.2 (30 mol% CO.sub.2  
 based on K)                                                              
 
    
     
         TABLE III                                                                 
      Carbonated                                                          
      Pyrrolidonate,                                                      
              Ammonium                                                    
      5%      Halide, 5%                                                  
                      % Conversion                                        
                              Mw × 10                               
__________________________________________________________________________
Example 1                                                                 
      Py--K--CO.sub.2.sup.1                                               
              0       45.2    380                                         
Example 5                                                                 
      &#34;       (CH.sub.3).sub.4 NCl                                        
                      69.1    1050                                        
Example 10                                                                
      Py--Na/CO.sub.2.sup.2                                               
              0       34.8    295                                         
Example 11                                                                
      &#34;       (CH.sub.3).sub.4 NCl                                        
                      48.5    380                                         
Example 12                                                                
      &#34;       (C.sub.2 H.sub.5).sub.4 NCl                                 
                      67.4    385                                         
 .sup.1 From KOH, 50° C., 22 hours, 30 mol % CO.sub.2 based on K   
 .sup.2 From Na-alkoxide, 50° C., 22 hours, 30 mol % CO.sub.2 based
 on Na                                                                    
 
    
     
         TABLE IV                                                                  
      %(CH.sub.3).sub.4 NCl                                               
                %Ac.sub.2 O.sup.1                                         
                          %Conversion.sup.2                               
__________________________________________________________________________
Example 2                                                                 
      0         0         16                                              
Example 13                                                                
      0         0.1       47                                              
Example 14                                                                
      5         0         40                                              
Example 15                                                                
      5         0.1       76                                              
__________________________________________________________________________
 .sup.1 Acetic anhydride, mol%                                            
 .sup.2 8 hours at 50° C., 5 mol% Py--K/CO.sub. 2 (30 mol % CO.sub.
 based on K)                                                              
 
    
     The examples of Table I show the remarkably high weight average molecular weights obtainable from the catalyst system of the present invention with good conversions of monomer to poly-2-pyrrolidone in remarkably short times for these molecular weights in this reaction. Percent conversion is calculated as 100× (weight of polymer)/(weight of total 2-pyrrolidone) and total 2-pyrrolidone has been defined heretofore. 
     The examples of Table II show the effect of mol ratio of alkali metal, e.g., potassium hydroxide, to tetralkyl ammonium halide. The highest molecular weights are believed to be achieved at about equimolar amounts (equivalent amounts) of the alkali metal pyrrolidonate and the tetraalkyl ammonium halide. 
     Table III shows the effect of an anhydrous source of alkali metal vs. a water-forming source, i.e., alkali metal hydroxide which reacts with 2-pyrrolidone to produce water, with and without 5 mol percent of the specified halide. In general, the hydroxide is a very satisfactory source of alkali metal for this catalyst system. 
     Table IV shows the remarkable effect produced on the rate of polymerization by the addition of a small amount of acetic anhydride to this catalyst system. 76% conversion is achievable after only 8 hours at 50° C., giving a product having a molecular weight of 175,000. 
     
                       TABLE V                                                     
______________________________________                                    
%Ac.sub.2 O.sup.1                                                         
             %(CH.sub.3).sub.4 NCl                                        
                        %Conversion.sup.4                                 
                                   M.sub.w × 10.sup.-3              
Example 16.sup.2                                                          
        0        0           4.1      35                                  
Example 17.sup.3                                                          
        0        5          35.4     165                                  
Example 18.sup.3                                                          
        0.6      5          48.2     150                                  
Example 19.sup.3                                                          
        1.11     5          75.7      56                                  
______________________________________                                    
 .sup.1 Mol percent acetic anhydride                                      
 .sup.2 10 mole percent KOH (K-pyrrolidonate), no CO.sub.2                
 .sup.3 5 mol percent, KOH (K-pyrrolidonate), no CO.sub.2                 
 .sup.4 22 hours, 50° C.                                           
 
    
     Table V shows the effect of omitting CO 2  from the catalyst system. Molecular weights are found to be lowered. While the addition of acetic anhydride gives high conversion at 22 hours the average molecular weight in the absence of carbon dioxide is still lowered. 
     
                       TABLE VI                                                    
______________________________________                                    
Ammonium Salt.sup.1                                                       
                %Conversion Mw × 10.sup.-3                          
______________________________________                                    
--              32.9        390                                           
(CH.sub.3).sub.4 NCl                                                      
                69.1        1050                                          
(CH.sub.3).sub.4 NBr                                                      
                35.0        420                                           
(CH.sub.3).sub.4 NI                                                       
                33          380                                           
(C.sub.2 H.sub.5).sub.4 NCl                                               
                73.2        1025                                          
(C.sub.2 H.sub.5).sub.4 NBr                                               
                51.6        610                                           
(C.sub.2 H.sub.5).sub.4 NI                                                
                33          390                                           
(C.sub.2 H.sub.5).sub.3 (0CH.sub.2)NCl                                    
                63.9        880                                           
(C.sub.2 H.sub.5).sub.3 (0CH.sub.2)NBr                                    
                49.1        510                                           
(C.sub.4 H.sub.9).sub.4 NI                                                
                23.9        410                                           
(CH.sub.3).sub.3 (CH.sub.2 CH.sub.2 OH)NCl                                
                17.9          95                                          
(CH.sub.3).sub.3 HNCl                                                     
                0           --                                            
H.sub.4 NCl     0           --                                            
______________________________________                                    
 .sup.1 In mol ratio K/ammonium salt = 1-1.3 5 mol% KOH (K-pyrrolidonate),
 1.5 mol % CO.sub.2 based on total 2-pyrrolidone                          
 .sup.2 22 hours at 50° C.,                                        
 
    
     Table VI shows the results obtained with a variety of ammonium halides including some remarkably high molecular weights for poly-2-pyrrolidone.