Catalytic process for the preparation of phosphazene polymers

Abstract

A process is disclosed for the catalytically-induced preparation of phosphazene polymers. Cyclic halophosphazenes are polymerized in the presence of a catalytically sufficient amount of a compound having the formula M(OR').sub.x where M is an alkali metal or alkaline earth metal, x is equal to the valence of the metal and R' is C.sub.1 to C.sub.10 linear or branched alkyl, C.sub.1 to C.sub.10 substituted linear or branched alkyl, the substituent selected from the group consisting of nitro, C.sub.1 to C.sub.10 alkyl, C.sub.1 to C.sub.10 alkoxy, C.sub.6 to C.sub.10 aryl and C.sub.6 to C.sub.10 aryloxy, or N.sub.3 P.sub.3 Cl.sub.z (OR').sub.6-z wherein R' is as defined above and z equals 0 to 5.BACKGROUND OF THE INVENTIONThe present invention relates to a process for the preparation of phosphazene polymers. More specifically, the present invention relates to a method for catalytically producing polyphosphazenes.DESCRIPTION OF THE PRIOR ARTPreparation of the polyphosphazenes has generally been recognized to be most readily accomplished by the technique of Allcock, et al as disclosed in U.S. Pat. No. 3,370,020. The preparation involves the use of the cyclic trimer, hexachlorocyclotriphosphazene as the sole starting material in what is essentially a melt polymerization technique. Purified trimer is thermally polymerized under sealed tube conditions at about 250.degree. C. for 20 to 48 hours to give substantially linear poly(dichlorophosphazene) and some unreacted trimer. The cyclic tetramer also is effective in this reaction. While the product of this reaction, poly(dichlorophosphazene), itself is a good elastomer of very high molecular weight, e.g. over one million, it suffers the severe disadvantage of undergoing relatively facile hydrolytic cleavage of the P-Cl bond followed by degradation of the polymer. The prior art has shown that attempts to increase the stability of the dichloropolymer by continued heating have proved ineffective since the highly crosslinked rubbery material produced by such heating is also hydrolytically unstable. Recent success for obtaining polyphosphazene polymers of good hydrolytic stability has been achieved by substituting all of the halogen on the linear polymer produced from the cyclic trimer by various organic species. The following scheme discloses the state of the prior art to date in which high molecular weight polyphosphazenes are produced by treating poly(dichlorophosphazene) I with a variety of organic nucleophiles, e.g. alcohols, phenols, and amines, to obtain the corresponding completely substituted polymers II, III and IV which are hydrolytically stable. ##STR1##The time period for accomplishing the ring opening polymerization reaction is economically disadvantageous, and considerable effort has been expended ascertaining what catalysts could be employed to successfully promote such ring opening polymerization. A variety of investigators have found that carboxylic acids, ethers, ketones, alcohols, nitromethane and metals such as zinc, tin or sodium, enhance the rate of polymerization of the cyclic trimer. The rate of enhancement is such that extensive polymerization is induced in 24 hours at 210.degree. C. compared to only 3% conversion to polymer in the same time in the absence of any catalyst. Comparable catalytic activity has also been shown by sulfur (at 215.degree.-254.degree. C.), by dialkyl paracresols and quinone or hydroquinone. See, for example, Allcock, "Phosphorus Nitrogen Compounds", Academic Press 1972, page 316 and following.The mechanism proposed for such catalytic enhancement in the conversion of the cyclic trimer to the linear polymer suggests that reagents that facilitate removal of chloride ion from phosphorus should be active catalysts. However, a variety of compounds, including those that should be good chloride acceptors, have been found to have no effect on the polymerization including carbon tetrachloride, chloroform, ligroin, benzene, biphenyl, cyclohexane, ethylbromide, phosphorus pentachloride, ammonia, water, mercuric chloride, aluminum chloride and stannic chloride.While the earlier discussed catalysts are effective for the purpose of catalyzing the ring-opening polymerization reaction, one of their major drawbacks is that the polymerization actually promotes the crosslinking reaction. Thus, while the disappearance of the cyclic trimer is enhanced, the resulting crosslinked polymer not only suffers the disadvantages of the prior art hydrolytically unstable crosslinked polychlorophosphazene V, but is additionally less reactive to the earlier mentioned nucleophiles due to its inherent insolubility. Further, there is no effective way to control molecular weight of the final product, such being so high as to make difficult any further manipulation such as melt or solution casting.There is, therefore, a need for effective catalysts to produce linear polyphosphazenes.SUMMARY OF THE INVENTIONIt is an object of the present invention to provide a novel process for the production of phosphazene polymers.It is a further object of the present invention to provide a novel process for the production of phosphazene polymeric foams.It is an additional object of the present invention to provide a novel process for the production of low molecular weight phosphazene polymers and foams from such low molecular weight polyphosphazenes.It is another object of the present invention to provide a novel process for the production of low molecular weight phosphazene polymers and foams from such low molecular weight polyphosphazenes which comprises thermally polymerizing a cyclic phosphazene with a catalytically sufficient amount of an alkali or alkaline earth metal compound.It is a further object of the present invention to provide a novel process for the production of low molecular weight phosphazene polymers and foams therefrom which comprises thermally polymerizing a cyclic phosphazene with a catalytically sufficient amount of an alkoxy-substituted cyclotriphosphazene.

Description

Claims

1. The process for preparing a phosphazene polymer which comprises heating a cyclic compound of the formula (NPCl.sub.2).sub.y where y is 3, 4, or mixtures thereof at 140.degree.-250.degree. in an inert atmosphere with a catalytically sufficient amount of a compound having the formula N.sub.3 P.sub.3 Cl.sub.z (OR').sub.6-z wherein R' is C.sub.1 to C.sub.10 linear or branched alkyl, C.sub.1 to C.sub.10 substituted linear or branched alkyl, the substituent selected from the group consisting of halogen, nitro, C.sub.1 to C.sub.10 alkyl, C.sub.1 to C.sub.10 alkoxy, C.sub.6 to C.sub.10 aryl and C.sub.6 to C.sub.10 aryloxy radicals and z equals 0 to 5.

2. The process in accordance with claim 1 wherein said thermal polymerization is carried out in an inert atmosphere for from 5 to 400 hours.

3. The process in accordance with claim 1 wherein said cyclic compound is hexachlorocyclotriphosphazene or octochloro cyclotetraphosphazene.

4. The process for preparing a phosphazene polymer which comprises heating hexachlorotriphosphazene at 140.degree.-250.degree. C. in an inert atmosphere with 0.05 to 5.0 mole percent of a compound having the formula N.sub.3 P.sub.3 Cl.sub.z (OR').sub.6-z, wherein R' is C.sub.1 to C.sub.10 linear or branched alkyl or C.sub.6 to C.sub.10 aryl-substituted C.sub.1 to C.sub.10 linear or branched alkyl and z is equal to 0 to 5.

5. The process in accordance with claim 4 wherein said heating is at a temperature of from about 175.degree. C. to about 250.degree. C. for a time from about 5 hours to about 300 hours.