Halophosphazene Polymers

This invention relates to the production of linear phosphonitrilic halide polymers. More particularly this invention relates to a process for producing halophosphazene polymers from readily-available, inexpensive oxygen-containing starting materials.

The customary method for the preparation of linear phosphonitrilic chloride polymers involves ring-opening polymerization of phosphonitrilic chloride trimer. Although workable, this method suffers from the fact that for satisfactory results to be achieved, highly pure cyclic phosphonitrilic chloride trimer must be used as the monomer. Such material is difficult and expensive to prepare.

Heretofore some work has been devoted to forming phosphonitrilic chloride polymers from lower molecular weight phosphonitrilic chloride oligomers. For example in J. Chem. Soc. 1960, 2542-7, Lund et al report an experiment in which a linear phosphonitrilic chloride oligomer of the formula (PNCl.sub.2).sub.11 PCl.sub.4.2 was heated with ammonium chloride in sym-tetrachloroethane under reflux. Polymerization occurred after 5.5 hours, at which time the amount of hydrogen chloride evolved corresponded to the composition (PNCl.sub.2).sub.10.6 PCl.sub.5. The rubbery product was extracted with light petroleum giving a significant quantity of a dark oil containing 10.5 percent PNCl.sub.2 trimer, the remainder of the oil consisting of cyclic polymers higher than the heptamer.

Moran in J. Inorg. Nucl. Chem. 30. 1405-13 (1968) investigated the thermal polymerization of the linear compound [Cl--(PCl.sub.2 .dbd.N).sub.3 -PCl.sub.3 ].sup.+ PCl.sub.6.sup.- in evacuated sealed tubes at 300.degree. C. for 5 hours and at 350.degree. C. for 5 hours. The phosphorus NMR spectrum of both samples indicated that polymers of other chain lengths were formed. The results in the 300.degree. C. case suggested to Moran that polymerization to the longer chain length compound [Cl--(PCl.sub.2 .dbd.N).sub.6 --PCl.sub.3 ].sup.+ PCl.sub.6.sup.- probably occurred. The NMR spectrum of the sample heated at 350.degree. C. indicated to Moran that polymers of both longer and shorter chain lengths were formed.

G. Allen et al in Polymer 11, 31-43 (1970) report attempts to prepare linear PNCl.sub.2 polymer by reacting PCl.sub.5 with ammonium chloride in ortho-dichlorobenzene, the ammonium chloride being introduced by stepwise addition to the reaction mixture. They were in hopes that the following reactions would occur:

However they obtained very low molecular weight polymer (intrinsic viscosity of trifluoroethoxy derivative was below 0.05 dL/g). When they tried to increase the molecular weight of the polymer product by reacting it with NH.sub.4 Cl in o-dichlorobenzene solvent, they obtained a crosslinked material.

U.S. Pat. No. 3,443,913 discloses a method wherein linear (PNCl.sub.2).sub.3-15 oligomers are heated at 240.degree.-260.degree. C. to produce linear phosphonitrilic chloride polymers having a molecular weight between 3,000 and 10,000. However, this process involves heating for long periods of time, the endpoint of the polymerization occurring about 40 to 60 hours after heating has been initiated. The product obtained via this process is reported to be a dark orange viscous oil. See also James M. Maselli, Thomas Bieniek and Rip G. Rice (W. R. Grace and Company), Phosphonitrilic Laminating Resins, Air Force Materials Laboratory, Technical Report AFML-65-314; Wright-Patterson Air Force Base, Ohio: June, 1965, pages 18-19, which describes this same process. At page 47 of this report Maselli et al describe an experiment wherein oligomeric phosphonitrilic chloride was placed in a resin kettle fitted with a nitrogen inlet, stirrer and exhaust tube condenser. The kettle was heated to 250.degree..+-.10.degree. C. for a total of 55 hours while the polymeric (PNCl.sub.2).sub.n was stirred under a blanket of dry nitrogen. Samples of the reaction material were taken at selected intervals of time during the heating for molecular weight determination. The resulting data were as follows:

According to the authors, when heating was continued for an additional 8 hours at temperatures in excess of 250.degree. C., the viscous, soluble oil (molecular weight 6900) was converted to the familiar insoluble "inorganic rubber".

In U.S. Pat. No. 3,545,942 which in part discloses a method of thermally stabilizing phosphonitrilic chloride oligomers by heating them in an inert atmosphere for 2 to 8 hours at 240.degree. to 260.degree. C., Rip G. Rice et al indicate that prolonged heating of the oligomer can result in the formation of an "inorganic rubber". A decade earlier Lund et al (op. cit.) referred to an experiment in which heating of a linear phosphonitrilic chloride oligomer in tetrachloroethane solution resulted in polymerization after 29 hours.

In prior applications Ser. No. 956,227 filed Oct. 30, 1978 and Ser. No. 176,926 filed Aug. 11, 1980, a distinctly superior thermal polymerization process is described wherein linear phosphonitrilic chloride oligomer is heated to 275.degree. to 350.degree. C. for 1 to 20 hours while concurrently withdrawing phosphorus pentachloride vapor from the liquid phase. A similar procedure is described in Japanese Laid-Open Application (Kokai) No. 55-27,344 published Feb. 27, 1980. In this case a linear phosphazene oligomer usually having a degree of polymerization of 3 to 15 is heated under reduced pressure (usually less than 20 mm Hg) to produce linear polymers. Heating for five hours or more at 100.degree.-300.degree. C. is suggested. Unfortunately, phosphorus pentachloride vapor is extremely corrosive at elevated temperatures--it tends to rapidly corrode even the most expensive corrosion-resistant metals used in the manufacture of corrosion-resistant chemical reactors.

Japanese Kokai No. 55-56,130 published Apr. 24, 1980 describes a method for producing phosphazene polymers in which a linear phosphazene oligomer is heated in the presence or absence of a solvent at 50.degree. to 300.degree. C. using a Lewis base such as urea, thiourea, polyurea or polythiourea as a catalyst for increasing molecular weight.

Japanese Kokai No. 55-56,129 published Apr. 24, 1980 discloses a process in which ammonium chloride is used as the catalyst in a reaction involving heating phosphazene oligomer at 150.degree.-350.degree. C. in a closed system. For example, a solution of linear and cyclic phosphonitrilic chloride oligomers in dichlorobenzene containing a small amount of ammonium chloride catalyst was heated at 255.degree. C. for 10 hours in a sealed tube to form the polymer.

Japanese Kokai No. 55-25,475 published Jan. 23, 1980 describes formation of phosphazene polymers by reacting a phosphorus source (e.g., P+Cl.sub.2 ; PCl.sub.3 +Cl.sub.2 ; PCl.sub.5) with a nitrogen source (e.g., NH.sub.3 ; NH.sub.4 Cl) in any of three reaction systems:

(1) In a solvent that does not dissolve the phosphazene polymers, such as an aliphatic hydrocarbon or alicyclic hydrocarbon that is resistant to halogenation.

(2) In an undiluted (concentrated) reaction system having a small quantity (250 mL or less per mole of P source reactant) of a solvent capable of dissolving the phosphazene polymers that is resistant to halogenation, such as a halogenated aromatic hydrocarbon.

(3) In a phosphazene oligomer as the solvent.

Japanese Kokai No. 55-65,228 published May 16, 1980 describes a method for producing phosphazene polymers in which a mixture of linear phosphazene oligomer, which has been stabilized with phosphorus pentachloride, hydrogen chloride or a metal halide, and cyclic phosphazene oligomer, is heated at 150.degree. to 350.degree. C. in a closed system having a solvent or non-solvent in the presence of a Lewis base catalyst. Urea, thiourea, polyurea, and polythiourea are examples of Lewis base catalysts used.

Japanese Kokai No. 55-50,027 published Apr. 11, 1980 discloses performing thermal ring-opening polymerization of cyclic phosphazene oligomers in the presence of linear phosphazenes stabilized with a metal halide, notably the linear oligomers formed as by-products when synthesizing the cyclic oligomers with metals or metal salts as catalysts. Such linear oligomers are indicated to have a degree of polymerization in the range of 2 to 100.

Japanese Kokai No. 55-60,528 published May 7, 1980 discloses a process wherein phosphazene polymers are formed by heating phosphazene oligomer at 150.degree. to 350.degree. C. in a closed system in the presence of a Lewis base such as urea, thiourea, polyurea or polythiorea. The phosphazene oligomer is a mixture of linear phosphazene oligomers (5 to 95 weight percent; stabilized with phosphorus pentahalide or hydrogen halide) and cyclic phosphazene oligomer.

Japanese Kokai No. 55-43,174 published Mar. 26, 1980 describes a process for producing phosphazene polymers in which cyclic phosphazene oligomers are subjected to thermal ring-opening polymerization in the presence of linear phosphazenes which have been stabilized by phosphorus pentahalides or hydrogen halides.

U.S. Pat. No. 4,374,815 describes a process wherein linear phosphonitrilic chloride polymers are produced from linear phosphonitrilic chloride oligomers by means of a two-step process. In the first step a mixture of linear phosphonitrilic chloride oligomer having an average degree of polymerization of at least four is heated with an excess of ammonia or ammonium chloride. Cyclic phosphonitrilic chloride oligomer formed during the course of the first step is removed from the reaction mixture. After the removal of the cyclic oligomer the reaction mixture is subjected to the second step which involves heating the mixture in an inert liquid solvent (optionally in the presence of ammonia or ammonium chloride) whereby the molecular weight of the linear phosphonitrilic chloride polymer is increased.

In copending application Ser. No. 447,720 filed Dec. 7, 1982, one of us (HML) describes an improvement in the process of U.S. Pat. No. 4,374,815 whereby the reaction rate when using ammonium chloride as the nitrogen source material in the first stage reaction is increased substantially by employing ammonium chloride having reduced particle size. For example, conversion of linear oligomer to linear polymer using ammonium chloride having a relatively small mean particle size of about 86 microns may be completed at a temperature of about 160.degree. C. after about one hour. On the other hand, when ammonium chloride having a relatively large mean particle size of about 115 microns is used, the polymerization rate at the same reaction temperature is slower and the yield of desired product is lower. The mean particle size of the ammonium chloride is preferably within the range of about 1 micron to about 100 microns and most preferably within the range of about 1 micron to about 90 microns.

In copending application Ser. No. 487,804 filed Apr. 22, 1983, one of us (HML) describes a process whereby linear phosphonitrilic chloride polymers are produced from phosphonitrilic chloride oligomers by means of a two-stage process. In the first stage a mixture of phosphonitrilic chloride oligomers enriched in linear oligomer is heated with at least a stoichiometric amount (and preferably an excess) of ammonia or ammonium chloride while concurrently removing hydrogen chloride to produce an intermediate reaction product. In the second stage the molecular weight of the reaction product is increased by heating the product to a higher temperature than the average temperature used in the first stage. Cyclic phosphonitrilic chloride oligomer may be present in the initial oligomer. Preferably, cyclic phosphonitrilic chloride oligomer is recovered from the reaction mixture after the start of the second stage. Most preferably it is recovered after completion of the second stage. At least a portion of the recovered cyclic oligomer may be used in a subsequent first stage reaction.

In copending application Ser. No. 487,805 filed Apr. 22, 1983, one of us (HML) describes a process whereby linear phosphonitrilic chloride polymers are produced from phosphonitrilic chloride oligomers by means of an improved process wherein a mixture of phosphonitrilic chloride oligomers enriched in linear oligomer is heated with at least a stoichiometric amount (and preferably an excess) of ammonia or ammonium chloride while concurrently removing hydrogen chloride to produce a higher molecular weight polyphosphonitrilic chloride product. Cyclic phosphonitrilic chloride oligomer may be and preferably is present in the initial oligomer. Preferably, cyclic phosphonitrilic chloride oligomer is recovered from the reaction mixture during (but more preferably after) completion of the polymerization reaction. At least a portion of the recovered cyclic oligomer may be used in a subsequent polymerization reaction.

In copending application Ser. No. 489,414 filed Apr. 28, 1983, one of us (HML) describes a process whereby halophosphazene polymers such as linear phosphonitrilic chloride polymers are prepared in two stages by (i) heating a nitrogen source (e.g., NH.sub.3 ; NH.sub.4 Cl; etc.) with an excess of a phosphorus source (e.g., PCl.sub.5 ; PCl.sub.3 +Cl.sub.2 ; P+Cl.sub.2 ; etc.) so that hydrogen halide is evolved and halophosphazene is formed, and (ii) heating at least a portion of the halophosphazene with at least a stoichiometric amount (preferably an excess) of a nitrogen source (e.g., NH.sub.3 ; NH.sub.4 Cl; etc.) so that hydrogen halide is evolved and halophosphazene polymer of higher molecular weight is formed. Preferably, a third stage is utilized wherein the resultant halophosphazene polymer is then heated, most preferably in an inert solvent or diluent which enhances the fluidity of the reaction mixture, to increase the molecular weight of the polymer.

This invention involves the discovery of a new, economical two-stage synthesis route for the production of halophosphazene polymers such as linear polyphosphonitrilic chloride polymers. In the first stage of this process a readily-produced inexpensive oxygen-containing starting material is used, namely one or a mixture of compounds of the formula:

wherein X is a halogen atom and n is an integer, usually below about 15. Such starting material is heated with a suitable phosphorus reactant such as PX.sub.5 ; PX.sub.3 +X.sub.2 ; P+X.sub.2 ; oligomer of the formula

wherein X is a halogen atom, R is PX.sub.6 or X, and n is an integer usually below about 15; or a mixture of two or more such phosphorus reactants so that a halophosphazene having a --PX.sub.3.sup.+ end group is formed as one of the co-products of the reaction. In other words, this co-product is a substance having the general formula

wherein X, R and n are as defined immediately above. The other co-product is phosphoryl trihalide, POX.sub.3. In the second stage the resultant reaction product is heated with at least a stoichiometric amount (preferably, an excess) of a nitrogen source (e.g., NH.sub.3 ; NH.sub.4 Cl; etc.) so that hydrogen halide is evolved and halophosphazene polymer of a suitably high molecular weight is formed.

Thus in accordance with one of its preferred embodiments this invention involves a process for the preparation of halophosphazene polymer which comprises heating at least one compound of the formula

wherein X is a halogen atom and n is an integer below about 15, most preferably from 1 to 5, with a phosphorus pentahalide (or its chemical equivalent such as PX.sub.3 +X.sub.2 ; 2P+5X.sub.2 ; 2P+PX.sub.3 +6X.sub.2 ; etc.) so that halophosphazene having a --PX.sub.3.sup.+ end group is formed, and then heating at least a portion of the resultant product with at least a stoichiometric amount (preferably, an excess) of a nitrogen source so that hydrogen halide is evolved and halophosphazene polymer of higher molecular weight is formed. Most preferably X is chlorine and the phosphorus pentahalide is phosphorus pentachloride.

In another of its preferred embodiments this invention involves a process for the preparation of halophosphazene polymer which comprises heating at least one compound of the formula

wherein X is a halogen atom and n is an integer below about 15, with an oligomer of the formula

wherein X is a halogen atom, R is PX.sub.6 or X, and n is an integer below about 15, so that halophosphazene having a --PX.sub.3.sup.+ end group is formed, and then heating at least a portion of the resultant product with at least a stoichiometric amount (preferably, an excess) of a nitrogen source so that hydrogen halide is evolved and halophosphazene polymer of higher molecular weight is formed. It is most preferred in this embodiment that X be at least predominantly chlorine.

In conducting the process of this invention an optional, but preferred, third stage operation described hereinafter may be utilized to further increase the molecular weight of the halophosphazene polymer.

A feature of this invention is that it is not necessary to remove the phosphoryl trihalide co-product from the first-stage reaction mixture before initiating the second-stage reaction between the resultant reaction product and the nitrogen source to produce the desired halophosphazene polymer. The phosphoryl trihalide does not materially interfere with this polymerization reaction.

Another feature of this invention is that the oxygen-containing starting material used in the first stage, i.e., one or a mixture of substances having the general formula:

can be readily prepared in high yields and in relatively short reaction periods from inexpensive raw materials, such as PX.sub.5 (usually PCl.sub.5) and (NH.sub.4).sub.2 SO.sub.4 or other ammonium salts containing an oxy-anion. See in this connection the paper by Emsley et al. in Journal of The Chemical Society, Part A, 1971, at pages 2863 and 2864, and references cited therein, for a description of suitable processes for preparing such starting materials. The Emsley et al. paper, which reports a procedure for obtaining dichlorophosphinylphosphorimidic trichloride (n=1, a preferred starting material) in almost quantitative yield, is incorporated herein by reference.

By virtue of this invention it is possible to produce halophosphazene polymers such as phosphonitrilic chloride polymers having relatively little, if any, crosslinking. Thus halophosphazenes of higher molecular weight with improved properties (e.g., lower gel content) can be readily produced by means of this invention. In fact, polymers of desired molecular weights (e.g., average degrees of polymerization in the range of 20 to 1000 or more) with low gel content can be formed in good yield and high purity at relatively moderate temperatures in relatively short reaction periods. And the resultant polymer has enhanced utility because of its greater linearity and reduced gel-forming tendencies.

As noted above, the initial oxygen-containing halophosphazene starting material employed in the first stage of the process is composed at least predominantly of one or more compounds of the formula

wherein X and n are as defined above. Preferably X is fluorine or bromine, or a mixture of both of them, or a mixture of either or both of them with chlorine, and most preferably is chlorine.

To free such halophosphazene from the --POX.sub.2 end groups and thereby produce in the first stage of the process a product eminently suited for use in performing the second-stage polymerization reaction, various processes may be used. Pursuant to one preferred embodiment of this invention the oxygen-containing halophosphazene is heated with phosphorus pentahalide (or the chemical equivalent thereof, e.g., PX.sub.3 +X.sub.2, 2P+5X.sub.2, or PX.sub.3 +2P+6X.sub.2, etc.) whereby the --POX.sub.2 end group is converted to the --PX.sub.3.sup.+ end group, apparently via the equation:

Another preferred embodiment of this invention involves removing the --POX.sub.2 end groups from the initial halophosphazene reactant by heating the halophosphazene with one or a mixture of substances of the formula

wherein X is a halogen atom, R is PX.sub.6 or X, and n is an integer below about 15. Preferably X is fluorine or bromine, or a mixture of both of them, or a mixture of either or both of them with chlorine, and most preferably is chlorine. In this reaction the --POX.sub.2 end group is believed to be transformed to a --PX.sub.3.sup.+ end group, presumably via either or both of the equations:

Other suitable processes for converting the --POX.sub.2 end group into a --PX.sub.3.sup.+ end group may now occur to those skilled in the art especially after a perusal of the present disclosure and appended claims.

The first stage of the process is conducted at a suitable elevated temperature, which ordinarily is in the range of between about 120.degree. to about 320.degree. C., and preferably in the range of between about 180.degree. to about 260.degree. C. It will of course be understood that this operation should be conducted in an essentially anhydrous, oxygen-free environment. Thus, the process may be conducted in a dry inert atmosphere such as anhydrous argon, anhydrous nitrogen, or the like. Similarly, the reaction may be conducted under vacuum in a dry environment.

Naturally, the first stage operation should be conducted for a period of time long enough for the halophosphazene reactant to be substantially freed of the --POX.sub.2 end groups. Periods of up to about 36 hours will ordinarily suffice, periods within the range of about 3 to about 24 hours being preferred. As a general rule higher reaction temperatures will involve use of shorter reaction periods, and vice versa.

The first stage operation may be conducted in a suitable closed or open reaction system so long as the system is kept essentially free of air and moisture. The operation may be performed on a batch, semi-continuous, or continuous basis.

The proportions of the reactants used in the first stage are not critical, although of course a sufficient amount of the PX.sub.5 or [X--(PX.sub.2 .dbd.N).sub.n --PX.sub.3 ].sup.+ [R].sup.- reactant should be present to enable substantially complete conversion of the --POX.sub.2 end groups to --PX.sub.3 end groups.

Upon the substantial conversion of the --POX.sub.2 end groups from the initial halophosphazene into --PX.sub.3.sup.+ end groups, the resultant halophosphazene of lower molecular weight is composed at least predominantly of one or more substances of the formula

wherein X is a halogen atom, R is X or PX.sub.6 and n is an integer below about 15.

The second stage of the process involves polymerizing at least a portion of the first-stage reaction product by heating the same with at least a stoichiometric amount (preferably an excess) of a nitrogen source material so that hydrogen halide is evolved. In this way polymerization is caused to take place along with the formation of hydrogen halide. Additionally, cyclic halophosphazene such as cyclic phosphonitrilic chloride oligomer tends to be produced. It is important to remove the hydrogen halide from the reaction mixture, preferably essentially as soon as it is formed. This is accomplished by performing the reaction in an open reaction system, "open" in the sense that the hydrogen halide is able to leave or be carried away from the reaction zone such as by a sweep of inert gas, application of a vacuum, or the like. The process may also be carried out under pressure, provided that the hydrogen halide is continuously or at least periodically bled from the reaction system.

It is also preferred to separate at least a portion of the cyclic phosphonitrilic halide oligomers from the heated reaction mixture during the course of the second stage reaction. Alternatively, such phosphonitrilic halide oligomers may be separated from the halophosphazene polymer after completion of the second stage reaction. In either case at least a portion of the recovered cyclic oligomer may be used as feed to the polymerization reaction, for example by recycle or otherwise. Still another alternative involves leaving all or a portion of the cyclic oligomers in the polymerization reaction product from the second stage reaction and subjecting this mixture with or without addition of another solvent or diluent to the optional, but preferred third stage operation described hereinafter.

To effect separation of cyclic halophosphazene oligomer (e.g., phosphonitrilic chloride oligomer) during the course of the second stage reaction, use may be made of several different process techniques. For example, all or a portion of the second stage may be performed in a boiling inert organic liquid whereby the liquid vapors drive off the cyclic oligomer. Another method is to perform all or a portion of the second stage reaction at a reduced pressure so that at the temperature employed the cyclic oligomer is distilled from the reaction mixture. Still another way of effecting the removal of the cyclic oligomer is to sweep the heated second stage reaction mixture with an inert vapor or gas either at subatmospheric, atmospheric or super-atmospheric pressures. In this way the entrained cyclic oligomer is carried away from the polymerization reaction zone during the course of the reaction. In all such cases it is desirable that the second-stage reaction mixture be suitably agitated both during the reaction and during the removal of the cyclic oligomer.

Instead of removing the cyclic oligomers during the course of the second stage reaction, the cyclic oligomers may be separated from the halophophazene polymer after completion of the second stage. While various methods may be used for effecting this separation, it is preferable to extract the polymeric reaction product formed in the second stage reaction with a suitable inert solvent such as hexane or the like.

The second stage of the process may be performed in the presence or absence of an inert organic liquid as diluent. In most cases it is preferable to conduct all or a portion of the second stage in bulk (i.e., in the substantial absence of added reaction solvent or diluent) as this reduces the size requirements for the polymerization reaction vessels.

When employing solvents in the second stage, use may be made of such materials as saturated cycloaliphatic hydrocarbons (e.g., cyclohexane, methylcyclohexane, 1,2-dimethylcyclohexane, etc.), aromatic hydrocarbons (e.g, toluene, xylenes, trimethylbenzenes, ethylbenzene, methylnaphthalenes, etc.), chlorinated hydrocarbons (e.g., 1,4-dichlorobutane, tetrachloroethane, chlorobenzene, dichlorobenzenes, etc.), and other similar inert solvents.

A wide variety of nitrogen source materials may be used in the second stage of the process. For example, use may be made of ammonia, ammonium chloride, ammonium fluoride, ammonium bromide, ammonium iodide, and the like. Use of ammonia or ammonium chloride (or both) is preferred.

When employing ammonium halide or like particulate materials as the nitrogen source material in the second stage reaction, the reaction time of this polymerization reaction can be significantly reduced by reducing the particle size of the ammonium halide used. For example, conversion of the first-stage reaction product to linear polymer using ammonium chloride having a relatively small mean particle size of about 86 microns may be completed at a temperature of about 160.degree. C. after about one hour. On the other hand, when ammonium chloride having a relatively large mean particle size of about 115 microns is used, the polymerization rate at the same reaction temperature is slower and the yield of desired polymeric product is lower. The mean particle size of the ammonium chloride is preferably within the range of about 1 micron to about 100 microns and most preferably within the range of about 1 micron to about 90 microns.

Additionally it is preferred that the ammonium halide be further characterized by satisfying additional particle size parameters. Such parameters may be represented by the following designations:

PH, which stands for 10 volume % of particles greater than the value of the microns stated

PM, which stands for 50 volume % of particles greater than the value of the microns stated

PS, which stands for 90 volume % of particles greater than the value of the microns stated.

For example, a PH of 145 microns, a PM of 83.8 microns and a PS of 37.4 microns means that the sample contains 10 volume % of particles greater than 145 microns, 50 volume % greater than 83.8 microns and 90 volume % greater than 37.4 microns, respectively.

Thus in accordance with these further preferred embodiments the ammonium halide (preferably ammonium chloride) employed has in addition to the foregoing Mean Values a PH below about 180 microns and most preferably below about 160 microns, a PM below about 90 microns and most preferably below about 85 microns, and a PS below about 45 microns and most preferably below about 40 microns.

Ammonium chloride having a relatively small particle size may be prepared, for example, by reacting hydrogen chloride gas with ammonia gas. If the ammonium chloride is formed and used in situ without first isolating the ammonium chloride, the particle size will have a Mean Value less than about 86 microns --i.e., a Mean Value as low as about 5 microns.

As noted above, at least a stoichiometric amount (preferably an excess) of the nitrogen source material is employed in the second stage of the process. Accordingly, the quantities in which the nitrogen source material is used are not critical provided that a sufficient amount is introduced into the reaction mixture to provide at least the stoichiometric amount relative to the quantity of halophosphazene product being reacted. The entire amount of the nitrogen source material being used in the reaction may be charged into the reaction vessel at the start of the reaction. However, in order to control the reaction the nitrogen source material may be introduced into the reaction mixture either periodically on an incremental basis or on a continuous basis during the course of all or a portion of the second stage.

The second stage is conducted at a temperature sufficiently high to cause the formation of hydrogen halide. Therefore, the temperatures employed will usually fall within the range of from about 100.degree. C. to about 350.degree. C. Preferably, the average temperature employed in the second stage will fall within the range of from about 120.degree. C. to about 280.degree. C.

The first and second stages may be conducted on a more or less continual basis simply by introducing the nitrogen source material at the proper time to the reaction mixture from the first stage.

Once the second stage reaction has been completed, the polymeric product may be recovered and purified by conventional procedures such as solvent extraction and the like.

In a particularly preferred embodiment, the product from the second stage is subjected to further processing in order to still further increase the molecular weight of the halophosphazene polymer. This optional but highly preferred third stage treatment is carried out by heating the resultant reaction mixture, preferably in an inert liquid solvent, at a suitably elevated temperature which on the average is usually (but not necessarily) higher than the average temperature employed in the second stage reaction, optionally in the presence of nitrogen source material of the type described above, preferably ammonia or ammonium chloride. The third stage is conducted for a time period sufficient to increase the molecular weight of the halophosphazene polymer. When ammonium chloride or like particulate nitrogen source material is used in the third stage, preferably it has a relatively small particle size as discussed above.

In the third stage as in the second stage of the process, a temperature high enough to cause the formation of hydrogen halide is employed. Thus, the third stage temperatures will generally fall within the range of from about 100.degree. C. to about 350.degree. C., and preferably the average temperature in the third stage will fall within the range of from about 120.degree. C. to about 280.degree. C. In one embodiment of this invention, a higher temperature is used in the third stage than in the second stage. The "higher temperature" used in the third stage is an average temperature that preferably is at least five and most preferably at least ten Centigrade degrees higher than the average temperature used in the prior second stage.

As in the case of the second stage, the third stage reaction should be performed so that hydrogen halide formed in the reaction is removed from the reaction system. This may be accomplished by performing the reaction in an open reaction system, "open" in the sense that the hydrogen halide is able to leave or be carried away from the reaction zone such as by a sweep of inert gas, application of a vacuum, or the like. The third stage processing may also be carried out under pressure, provided that the hydrogen halide is continuously or at least periodically bled from the reaction system.

Generally speaking, the longer the reaction time in the third stage, the higher the molecular weight of the resultant halophosphazene polymer. Accordingly, the reaction time for this reaction will depend to some extent on the desired molecular weight of the polymer and may be varied within relatively wide limits although ordinarily times in the range of about 1 to about 36 and preferably from about 4 to about 24 hours will usually be used. At least a portion of the third stage may be performed in the presence of a nitrogen source material of the type used in the second stage such as ammonia or ammonium chloride, or the like and such material(s) may be introduced into the reaction mixture at the start and/or during the course of the reaction. Alternatively, such material(s) may constitute residual nitrogen source materials remaining in the reaction mixture after completion of the second stage reaction.

A wide variety of inert solvents may be employed in the third stage. These include inert chloroaliphatic, cycloaliphatic, and aromatic solvents of various types, including mixtures of solvents. While various cycloalkanes, chloroalkanes and chlorocycloalkanes having appropriate boiling points are thus suitable for this operation, it is preferred to use an inert aromatic solvent such as aromatic hydrocarbons and chloroaromatic hydrocarbons having boiling points at least as high as the reaction temperature being used in the second stage. Preferred solvents of this type include toluene, xylenes, methylnaphthalenes, chlorobenzene, dichlorobenzenes, trichlorobenzenes, etc., as well as mixtures of such materials. The third stage preferably is conducted at elevated pressures with the hydrogen halide formed in the reaction being bled from the reaction system either continuously or at least periodically.

If desired, the second and third stages of the process may both be performed in the same solvent or mixture of solvents.

The amount of solvent used in the third stage is preferably regulated so as to keep the reaction mixture in a relatively concentrated solution while avoiding excessive gelation. Thus it is desirable to perform the third stage in a relatively concentrated reaction solution with periodic or continuous addition of solvent to maintain the reaction mixture in a fluid state as the reaction proceeds.

If it is desired to recover the linear phosphonitrilic chloride polymeric product from the reaction solvent used in the third stage, various techniques are available for use. For example, the solvent may be distilled off using an appropriate combination of reduced pressure and distillation temperature. Alternatively, the halophosphazene polymer may be precipitated from the solvent by the addition of the solution to a suitable non-solvent such as pentane or hexane. These and other similar techniques will be evident to those skilled in the art.

When it is desired to chemically convert the halophosphazene polymer into another type of phosphazene polymer, subsequent reactions with an appropriate reactant may be effected in the same reaction solvent as used in the third stage (or in the second stage, if a solvent is used therein). Indeed, in such cases it is unnecessary to isolate or recover the halophosphazene polymer formed in the second or third stages as the ensuing reaction(s) may be effected in the same solution. Alternatively, such subsequent reactions may be effected in a fresh solution and, if desired, in a different inert solvent.

In accordance with a preferred embodiment, the reaction mixture from the third stage reaction is extracted with a suitable inert solvent such as pentane, hexane or heptane in order to separate and recover the cyclic oligomers present in the reaction mixture. All or a portion of these recovered cyclic oligomers may be used in the second and/or third stage of the process.

The second and third stages may be conducted in separate reactors. A feature of this invention, however, is the fact that both stages may be performed in the same reaction vessel, provided of course that it is appropriately sized to handle the quantities of material involved in each stage. Thus in accordance with a preferred embodiment of this invention, both stages are conducted in the same reactor. It is further preferred to conduct the second stage in bulk and to introduce a solvent for the third stage into such reactor upon or near the completion of the second stage but otherwise perform the second and third stages as a more or less continual operation. Both of these stages (as well as the first stage) may be conducted in the presence of a solvent, if desired.

The foregoing and other procedures and process conditions for effecting polymer formation are fully set forth in U.S. Pat. No. 4,374,815, issued Feb. 22, 1983 and in co-pending applications Ser. No. 447,720, filed Dec. 7, 1982, Ser. No. 487,804, filed Apr. 22, 1983, Ser. No. 487,805, filed Apr. 22, 1983, and Ser. No. 489,414, filed Apr. 28, 1983, all assigned to the assignee of the present application. The disclosures of U.S. Pat. No. 4,374,815 and these commonly assigned applications are incorporated herein by reference.

The practice of this invention will become still further apparent from the following illustrative examples which are not to be construed in a limiting sense.

Examples I and II illustrate procedures by which the first stage of the process may be carried out.

A 4.3 g portion of Cl--(PCl.sub.2 .dbd.N).sub.n --POCl.sub.2 oligomer, prepared from phosphorus pentachloride (PCl.sub.5) and ammonium sulfate [(NH.sub.4).sub.2 SO.sub.4 ] by the procedure reported by Emsley et al. in Journal of The Chemical Society, Part A, 1971, at pages 2863 and 2864, and having an average degree of polymerization of 1.3, was combined in a glass ampoule with 3.3 g of PCl.sub.5. The sample was degassed via standard vacuum line freeze-thaw techniques and the ampoule sealed off with a torch. The ampoule was then affixed to a rocking device and submerged in a thermostated oil bath at 220.degree. C. for 24 hours. After 24 hours the ampoule was removed, opened in a nitrogen filled dry-box and sampled for .sup.31 P NMR analysis. The analysis showed the products to be [Cl--(PCl.sub.2 .dbd.N).sub.n --PCl.sub.3 ].sup.+ [PCl.sub.6 ].sup.- oligomer with an average degree of polymerization of 2.27 and phosphorus oxychloride (POCl.sub.3). There was no evidence of any residual Cl-(PCl.sub.2 .dbd.N).sub.n --POCl.sub.2 type reactant.

Into a glass ampoule was placed a sample of linear phosphazene oligomer, [Cl--(PCl.sub.2 .dbd.N).sub.n --PCl.sub.3 ].sup.+ [R].sup.- (where R was 72 mole % PCl.sub.6 and 28 mole % Cl), having an average degree of polymerization (n above) of 4.9 and contaminated with --POCl.sub.2 end groups. The sample was degassed via standard freeze-thaw vacuum line techniques and the ampoule sealed off with a torch. The ampoule was then affixed to a rocking device and submerged in a thermostated oil bath at 220.degree. C. for 24 hours. After 24 hours the ampoule was removed, opened in a nitrogen filled dry-box and sampled for .sup.31 P NMR analysis. The results are presented in the table below.

As the data show, the level of --POCl.sub.2 groups was reduced from 3.6 mole % of the total phosphorus to 0.4 mole %. There was a concomitant increase in the amount of phosphorus oxychloride (POCl.sub.3).

Comparative Example III is a typical illustration of the difficulties one experiences when attempting to practice the second stage of the process on phosphazene containing --POCl.sub.2 end groups.

Into a 500 mL four-neck creased flask fitted with a mcchanical stirrer, gas inlet and outlet and a thermometer were placed 194.6 g of the same oligomer used in Example II and 22.5 g of ammonium chloride. The contents of the flask were heated to 140.degree. C. and the evolved-hydrogen chloride and cyclic phosphazenes were swept from the head space by nitrogen gas. After seven hours no visible thickening (evidence of polymerization) had occurred. At this time the heat was removed and the reaction mass was allowed to stand overnight under nitrogen gas. The next morning the heat was reapplied and the temperature raised to 160.degree. C. After forty minutes the fluid reaction mass gelled suddenly to give an insoluble mass.

Examples IV and V are typical illustrations of the successful practice of the second and third stages, respectively, of the process on phosphazene which is essentially free of species containing --POCl.sub.2 end groups.

Using an apparatus similar to that described in Comparative Example III, 744 g of a linear phosphazene oligomer [Cl--(PCl.sub.2 .dbd.N).sub.n --PCl.sub.3 ].sup.+ [R].sup.- (R=99.6 mole % PCl.sub.6 and 0.4 mole % Cl) having an average degree of polymerization (n above) of 4.5 and being free of --POCl.sub.2 end groups (as evidenced by .sup.31 P NMR) was heated with 97 g of ammonium chloride at about 140.degree. C. for two hours and then at about 165.degree. C. for five hours while sweeping with nitrogen gas. At this time the reaction mixture had thickened considerably. The viscosity of the reaction mass was found to be 2430 cPs at 100.degree. C.

Into a two-liter pressure reactor were charged 100.0 g of the reaction mixture from Example IV, 3.5 g of ammonium chloride and 906 g of chlorobenzene. The oil jacket of the reactor was heated to 180.degree. C. while the contents were stirred at 70 rpm by means of an anchor and a turbine agitator. The pressure was held at 24 psig while the head space was swept with nitrogen gas at a rate of 0.1 cubic foot per hour. After twenty-three hours a gel-free phosphonitrilic chloride polymer having an intrinsic viscosity in chlorobenzene at 25.degree. C. of 0.68 was obtained.

The linear phosphonitrilic chloride polymers produced in accordance with this invention are useful for a variety of applications. By way of example these linear polymers when of relatively low molecular weight are useful as intermediates in the synthesis of hydraulic fluids, lubricants and flame retardants. In particular the linear phosphonitrilic chloride polymers preferably having average degrees of polymerization below about 50 may be substituted with aryloxy and/or alkoxy groups to form products useful as hydraulic fluids, lubricants and flame retardants. Methods for effecting such substitution are well known in the art and are described for example in U.S. Pat. No. 3,443,913; U.S. Pat. No. 3,856,712; U.S. Pat. No. 3,883,451; and U.S. Pat. No. 4,055,523. Alternatively, aryloxy and alkoxy substituted linear polymers of higher average degrees of polymerization containing ethylenic unsaturation can be compounded and cured by cross-linking to produce elastomers, coatings, adhesives, potting compounds, thermoset plastics and flexible or rigid foams. Note in this connection U.S. Pat. No. 4,264,531. Still other uses for the linear phosphonitrilic chloride polymers producible by the process of this invention will be apparent to those skilled in the art and are reported in the literature.