FIELD OF THE INVENTION 
     This invention relates to polyamides. More specifically, this invention relates to a catalytic process for the preparation of high molecular weight, thermally stable polyamides from omega-aminonitriles and water. This process utilizes an oxygenated phosphorus compound as a catalyst. In a preferred embodiment, this invention relates to the production of nylon-6 from 6-aminocapronitrile and water using oxyphosphorus acids or their derivatives as the catalyst. 
     DESCRIPTION OF THE PRIOR ART 
     It is known in the art that commercially available nylons may be prepared by polymerization of various monomers and combinations of monomers. For example various nylons may be prepared from the polymerization of diamines with dicarboxylic acids, the polymerization of dinitriles with diamines in the presence of water or the polymerization of lactams. 
     In part, the instant invention deals with nylons resulting from the polymerization of omega-aminonitriles. In U.S. Pat. No. 2,245,129 by Greenwalt, a method for producing a linear polyamide by heating an aminonitrile and water is described. The Greenwalt procedure consists of two stages. In the first stage the reaction mixture is heated in a closed reaction vessel until a low molecular weight polyamide is formed while in the second stage, this low molecular weight polyamide is converted to a high molecular weight polyamide upon additional heating. 
     In part, the instant invention also relates to the use of an oxygenated phosphorus compound as a polymerization catalyst. Recently Hoffmann et al., U.S. Pat. No. 4,436,898 disclosed an improved method for preparing a high molecular weight polyamide from a dinitrile, diamine and water by conducting the polymerization in the presence of an oxygenated phosphorus compound, such as phosphoric acid, or an ammonium salt or an ammonium alkyl salt of the acid. Also recently, Coffey et al., U.S. application Ser. No. 540,596, now U.S. Pat. No. 4,490,521, disclosed a method for the improvement of resin thermal stability through the use of metal salts of oxyphosphorus acids, such as disodium phosphite, as the catalyst. 
     Lastly, the instant invention relates to the production of nylon-6 from 6-aminocapronitrile. It has been recently reported, in EPC Application 7791, assigned to Allied Chemical Corporation of Morristown, N.J., that 6-aminocapronitrile may be selectively produced from adiponitrile by catalytic hydrogenation using a ruthenium catalyst on a basic support in the presence of ammonia, and subsequently cyclized to caprolactam which may be then polymerized to nylon-6. It is an objective of the process of the instant invention to produce high molecular weight nylon-6 directly from 6-aminocapronitrile without the intermediate isolation of 6-caprolactam. 
     SUMMARY OF THE INVENTION 
     Nylon-type polyamides suitable for fibers, plastics, films and molding compounds are produced in a process comprising contacting an omega-aminonitrile, water and an oxygenated phosphorus compound catalyst. In one embodiment, nylon-6 is produced by contacting 6-aminocapronitrile, water and an oxygenated phosphorus compound catalyst. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The Monomer 
     Any aminonitrile may be used as a monomer for this invention. Preferred are the omega-aminonitriles of the formula ##STR1## where R is a divalent organic radical and R&#39; is hydrogen or a univalent rganic radical. R can be a divalent aliphatic, alicyclic or aromatic radical and these radicals can bear one or more inert substituents. Similarily, R&#39; can be a hydrogen or a univalent aliphatic, alicyclic or aromatic radical and these radicals can also bear one or more inert substituents. By the term &#34;inert&#34; is meant that the substituent is essentially non-reactive with the reactants, catalysts and products of the process under process conditions. Typically, R is a divalent C 1  -C 20  aliphatic radical, a divalent C 5  -C 18  alicyclic radical or a divalent benzene radical and preferably R is a C 2  -C 8  straight chain alkylene radical. Typically, R&#39; is hydrogen or a C 1  -C 20  aliphatic radical, C 5  -C 7  alicyclic or a phenyl radical. Preferably, R&#39; is hydrogen or a C 1  -C 4  alkyl radical. 
     Representative aminonitriles include 6-aminocapronitrile, 3-aminopropionitrile, 4-cyanoaniline as well as the following: 
     H 2  N--(CH 2 ) 7  --CN 
     H 2  N--(CH 2 ) 16  --CN 
     H 2  N--(CH 2 ) 2  --CHCH 3  --(CH 2 ) 2  --CN 
     H 3  CHN--(CH 2 ) 6  --CN 
     CH 3  --CHNH 2  --(CH 2 ) 8  --CN 
     CH 3  --CH NH 2  --CH 2  --C(CH 3 ) 2  --CH 2  --CH 2  --CN 
     H 2  N--C 6  H 10  --CN (1,4) 
     H 2  N--CH 2  --C 6  H 4  --CN 
     H 2  N--CH 2  --C 6  H 4  --CH 2  CN 
     H 2  N--(CH 2 ) 3  --O--(CH 2 ) 2  --CN 
     H 2  N--(CH 2 ) 4  --S--(CH 2 ) 3  --CN 
     and H 2  N--CH 2  --C 6  H 4  --O--C 6  H 4  --CN. 
     Additionally, the instant invention is equally suited for the production of copolymers of aminonitriles with other polyamide forming monomers or polyamide forming monomer pairs such as dinitriles and diamines, diacids and diamines, aminoacids or lactams. 
     The Catalyst 
     The oxygen-containing phosphorus compounds suitable as catalysts for use in the invention include phosphorous acid; phosphonic acid; alkyl and aryl substituted phosphonic acid; hypophosphorous acid; alkyl, aryl and alkyl/aryl substituted phosphinic acid; and phosphoric acid; as well as the alkyl, aryl and alkyl/aryl esters, metal salts, ammonium salts, and ammonium alkyl salts of these various phosphorus containing acids. As used herein, &#34;alkyl/aryl&#34; refers to those combinations where there is more than one organic substituent. In the substituted acids, the alkyl or aryl group replaces the hydrogen connected directly to the phosphorous atom. The esters are formed conventionally with the alkyl or aryl group replacing the hydrogen of an -OH group comprising the acid. To clarify the nomenclature, the names of the acids are identified with the structural formulas as follows: ##STR2## where R&#34;&#39; is an alkyl or aryl or alkyl/aryl group and R IV  is hydrogen or an alkyl or aryl group. 
     Preferred catalysts are phosphorous acid, phosphoric acid, and the sodium, lithium or sodium/lithium dibasic salts of phosphorous or phosphoric acids. These salts may be added initially or generated in situ by the addition of NaOH and/or LiOH to a reaction mixture already containing the oxyphosphorus acid or a monobasic salt thereof at a time late in the reaction cycle. 
     Sufficient catalyst is employed to promote the polymerization of the omega-aminonitrile. A typical amount of catalyst suitable for the process is between 0.001 and 1 weight percent. based upon the total weight of the aminonitrile and water. Catalyst levels of about 0.01 to about 1 weight percent are preferred. 
     Process Parameters 
     High molecular weight, linear polyamides are prepared by forming a reaction mixture of the aminonitrile, water and catalyst. This reaction mixture can be formed by any one of a number of different methods. One method is the gradual addition, either continuously or incrementally, of the aminonitrile over the course of the hydrolysis portion of the reaction (polymerization). Typically, in this method less than 50 mole percent of the aminonitrile, preferably less than 5 mole percent, is initially present with the remainder of the aminonitrile added gradually over the course of polymerization. Another method and one preferred due to its simplicity of operation is a batch addition of all monomers and catalyst at the commencement of the reaction. 
     The reaction itself is preferably conducted in a batch mode. However, the reaction can also be conducted in a continuous mode, i.e. continual addition of the aminonitrile and catalyst with concomitant removal of product, if desired. An example of a continuous mode process is the use of a cascade reactor arrangement. 
     Water is necessary to the process as both a reactant and as an aid in formation of the polyamide. Consequently, a stoichiometric excess of water is typically employed during the polymerization. This does not mean a stoichiometric excess of water is present at all times during polymerization, it instead means that the total amount of water consumed by the polymerization plus the amount of water remaining at the end of polymerization is in excess of the amount of water required for full polymerization of the monomers. Preferably the initial water content of the reaction mixture does not exceed about 35 weight percent of the total weight of the reaction mixture, more preferably it does not exceed 15 weight percent. A typical polymerization might employ less than or about 15 weight percent water at the start of the reaction, then as the polymerization proceeds additional water is gradually added to the reaction mixture until water constitutes about 30 weight percent of the mixture. Although it is not desired, more water can be used. However, since the water will have to be removed from the reaction product at the end of the polymerization, preferably the amount of excess water is kept to a minimum (typically 30 weight percent or less) to facilitate ultimate removal. Moreover, the less water present during the polymerization generally means the less energy needed for the process and consequently, less expensive process equipment can be employed. The manner in which the water is initially introduced into the reaction mixture is not important to the practice of this invention and it can thus be either added alone or in combination with the aminonitrile. 
     Ammonia is a byproduct of the reaction of the aminonitrile and water. As a consequence, ammonia is constantly being generated within the reaction mixture. This ammonia typically enters the vapor phase and is preferably continuously removed from reaction zone (e.g. released through a pressure relief valve on the reaction vessel). The concentration of ammonia in the reaction mixture (which is a liquid) can vary from threshold detection limits up to about 5 weight percent of the total weight of the reaction mixture. Preferably, the concentration of ammonia in the liquid reaction mixture does not exceed 1 weight percent and more preferably, is kept as low as possible. 
     In one embodiment of this invention, high quality nylon-6 resin is prepared from 6-aminocapronitrile and water by continuously removing substantially all of the ammonia generated during the polymerization but while retaining all of the water. The water is eventually removed from the reaction system after the low molecular weight polyamides are formed, i.e. the hydrolysis of the aminonitrile is essentially complete as evidenced, for example, by the amount of ammonia that has evolved since the start of the reaction. 
     The polymerization of aminonitrile to form a high molecular weight, linear polyamide is best conducted over a temperature/pressure profile that varies over the course of the polymerization. The temperature/pressure profile will, of course, vary with the specific aminonitrile employed as well as with such factors as the nature and amount of catalysts, mode of operation (batch versus continuous), configuration of the reaction vessel, etc. For the manufacture of nylon-6 from an aminonitrile and water, a temperature/pressure profile comprising at least two stages is typically employed, both preferably conducted in the absence of oxygen. During the first stage of the polymerization, the temperature is maintained at 200°-300° C., preferably 240°-270° C., under autogenous pressure (typically in the range of about 200 to about 800 psig) for a period of time sufficient to form low molecular weight polyamides, e.g. polyamides having a weight average molecular weight of less than about 10,000, generally less than about 5,000 as measured by intrinsic viscosity. Ammonia is removed from the reaction vessel while maintaining the water concentration at a level sufficient for polymerization to proceed, typically in excess of 14 weight percent. At the completion of the first stage (which is the start of the second stage), the pressure is gradually reduced to atmospheric or subatmospheric pressure and the temperature is gradually increased, preferably to between about 260°-295° C. During this second stage, relatively low molecular weight polyamides are combined to form the high molecular weight polyamides that constitute the final product of the process. The second stage is typically concluded with a sweep or purge of the reaction vessel with a flow of inert gas, such as nitrogen. 
     In one embodiment of this invention, the molecular weight of the polyamide can be increased by performing the polymerization at a temperature just below the lowest temperature at which either the catalyst, monomers or final polyamide begin to degrade. 
     Although the polymerization is initially conducted at autogenous pressure with a later reduction to atmospheric or subatmospheric pressure, the process can be conducted at constant pressure by applying an inert gas pressure to the system and adjusting this pressure as the reaction proceeds. The pressure can be maintained with a gaseous reactant or a gas inert to the reaction or some combination of the two. However, since the reaction itself is conducted in the liquid phase, the presence of a gaseous reactant is for the purpose of maintaining reaction pressure, not for participating in the polymerization. 
     The Polyamide 
     The polyamides produced by this invention have a nylon structure, i.e. the polymer contains amide linkages (--CONH--) as an integral part of the polymer backbone, as opposed to polyacrylamides which have an essentially all carbon backbone. These polyamides are solid polymeric materials which can be used in any application calling for the use of a nylon-type polymer. For example, these polyamides can be used as fibers, plastics, films or molding compounds. 
     This invention is particularly well adapted to manufacturing high quality nylon-6 from 6-aminocapronitrile and water. Such a process would be an alternative to present commercial processes which produce nylon-6 from 6-caprolactam. 
     As a general observation polyamides produced by the instant invention using the salts of the various acids as catalysts have a lower weight loss as measured by thermal gravimetric analysis (TGA) than polyamides produced using esters of the acids or the acids themselves as catalysts. 
     SPECIFIC EMBODIMENTS 
     The following examples are provided in order to illustrate the instant invention. 
    
    
     EXAMPLE I--Polymerization of 3-aminobutyronitrile 
     Into two 45 ml Parr microreactors were placed 10 g of 3-aminobutyronitrile and 2.98 g of water. A catalyst of 0.014 g of H 3  PO 3  was added to one reactor and no catalyst was introduced into the remaining reactor. Both reactors were purged with nitrogen gas to remove air. The reactors were then heated at 260° C. for 3.5 hours during which time the ammonia byproduct pressure above 750 psig was continuously vented. After the 3.5 hours, the pressure was lowered to 100 psig and maintained there while the reactors were allowed to cool. 
     The products of each reactor were then identified as 3-methyl nylon 3, the viscosities of the product from each reactor were as follows: 
     
                       TABLE I______________________________________Experiment             Bulk ViscosityNumber        Catalyst (seconds)______________________________________Control-I     None     0.61             H.sub.3 PO.sub.3                  1.2______________________________________ 
    
     The bulk viscosities shown in the above table illustrate the relative molecular weight of the resulting polymers. As herein used, &#34;bulk velocity&#34; is defined as the time in seconds required for 0.415 grams of sample dissolved in 2.0 ml. of 90 percent formic acid to traverse 0.8 ml. in a 1.0 ml. pipette at room temperature. 
     EXAMPLE II Polymerization of 6-Aminocapronitrile 
     Table II describes several polymerizations using 6-aminocapronitrile which were conducted using the following procedures. Into a 450 ml. Parr autoclave were placed 100 g (0.89 moles) of 6-aminocapronitrile, water and 0.0013 moles of catalyst. The autoclave was then sealed and purged three times with nitrogen. The autoclave was then heated to 250° C. or 260° C. and held at that temperature for 3.5 hours. During this time pressure in the reactor was not allowed to exceed 750 psig. After the 3.5 hours the temperature was increased to or held at 260° C., while the pressure was slowly reduced over a 30 minute period to 0 psig. The autoclave was then purged with nitrogen again and allowed to cool under additional nitrogen purging. The product was identified as nylon-6. 
     EXAMPLE III Copolymers of omega-aminonitriles 
     Table III describes several polymerizations using 6-aminocapronitrile in conjunction with other polyamide forming monomers. The reaction procedure and reaction profile for these experiments was as described in Example 2. 
     
                                           TABLE II__________________________________________________________________________                    Intrinsic                          Bulk TGA %Experiment    wt. %             Temperature                    Viscosity*                          Viscosity                               weight lossNumber Catalyst         Water             (°C.)                    (dl/g)                          (seconds)                               (325-390° C.)__________________________________________________________________________Control-II none    23  250    0.11  0.90 7.42     H.sub.3 PO.sub.3         23  250    0.68  9.00 6.43     Na.sub.2 HPO.sub.3         23  250    0.89  15.20                               3.24     (CH.sub.3 O).sub.3 P(O)         23  250    0.63  9.60 7.45     H.sub.3 PO.sub.4         23  250    0.45  5.20 5.86     H.sub.3 PO.sub.3         40  250    1.13  25.80                               5.27     CH.sub.3 P(O)(OH).sub.2         40  260    0.92  16.10                               4.58     H.sub.3 PO.sub.3         40  260    1.34  40.34                               8.6__________________________________________________________________________ *Intrinsic Viscosity was measured using a CannonUbbelohde viscometer at 25° C. for samples dissolved in 90% formic acid. 
    
     
                                           TABLE III__________________________________________________________________________Experiment                             wt %                                      IntrinsicNumber Product  Monomers    Catalyst                           Temperature                                  water                                      Viscosity__________________________________________________________________________ 9    90/10 nylon-6/6,6          6-aminocapronitrile                      H.sub.3 PO.sub.3                           260° C.                                  23% 1.060          hexamethylene diamine          adipic acid10    90/10 nylon-6/6,6          6-aminocapronitrile                      H.sub.3 PO.sub.3                           260° C.                                  23% 1.090          hexamethylene diamine          adiponitrile11    90/10 nylon-6/3          6-aminocapronitrile                      H.sub.3 PO.sub.3                           260° C.                                  40% 0.147          3-aminopropionitrile__________________________________________________________________________ *Intrinsic viscosity was measured using a CannonUbbelohde viscometer at 25° C. for samples dissolved in 90% formic acid. 
    
     Since viscosity is proportional to molecular weight, Table I and Table II both illustrate that polyamides produced from omega-aminonitriles using the catalytic process of the instant invention have greater viscosities and consequently greater molecular weight than polyamides produced using an identical process without a catalyst. 
     Although only a few embodiments of this invention have been specifically described above, it should be appreciated that many additions and modifications can be made without departing from the spirit and scope of the invention. These and all other modifications are intended to be included within the scope of this invention, which is to be limited only by the following claims:
A catalytic process for the production of high molecular weight nylon-type polyamides from omega-aminonitriles has been discovered. The process comprises contacting an omega-aminonitrile, water and a catalytic amount of an oxygenated phosphorus compound catalyst.