The polyphenylene ethers and processes for their preparation are known in the art and described in numerous publications including Hay, U.S. Pat. No. 3,306,874 and 3,306,875, both of which are incorporated by reference. Other patents which show the preparation of polyphenylene ethers include Bennett and Cooper, U.S. Pat. Nos. 3,639,656, and 3,838,102, as well as Cooper and Bennett, U.S. Pat. Nos. 3,642,699, 3,661,848 and 3,733,299, which are also incorporated by reference.
The processes most generally used to produce the polyphenylene ethers involve the self-condensation of a monovalent phenol in the presence of an oxygen-containing gas and a catalyst comprising a metal-amine complex.
These processes are run in the presence of an organic solvent and the reaction is usually terminated by removal of the catalyst from the reaction mixture.
The product is usually recovered by adding an antisolvent to the reaction solution in an amount sufficient to precipitate the polymer. The antisolvent is soluble in the polymerization solvent, but does not dissolve the polymer.
It is known, for example, from A. Factor, Jr. Polymer Sci. B, 7, 205 (1969), that polyphenylene ethers dissolve readily in methylene chloride, but precipitate on standing in the form of a crystalline polyphenylene ether-CH.sub.2 Cl.sub.2 complex. The complex loses methylene chloride on heating or simply on air drying to yield amorphous polyphenylene ethers. Attempts have been made to make use of the unusual solubility behavior of poly(2,6-dimethyl-1,4-phenylene)ether in methylene chloride to isolate the resins from reaction mixtures without the use of an antisolvent, but polymers suitable for molding applications have not been obtained. For example, when the polymerization is carried out in methylene chloride solvent, the polymer precipitates before it reaches high molecular weight; furthermore, the precipitate carries with it a large portion of the copper component of the polymerization catalyst; and the copper cannot easily be removed from the polymer by washing with water, methanol, acetic acid, etc.
It has now been discovered that high molecular weight polyphenylene ether (I.V. as high as 0.70 dl./g.) can be produced in methylene chloride solution without precipitation of the complex by carrying out the polymerization at a temperature greater than 40.degree. C. Since this is above the atmospheric boiling point of the solvent, it is necessary to carry out the reaction in a closed system under oxygen pressure, to prevent loss of the solvent and to provide sufficiently high oxygen concentration in solution for the polymerization reaction. A pressure of 25 and, preferably, 30 psig is sufficient to produce high I.V. polymer is less than two hours at a xylenol: CuBr.sub.2 : DBA ratio of 300 : 1 : 45, if poly(2,6-dimethyl-1,4-phenylene)ether is the desired product.
Because the polymer does not precipitate from solution during the polymerization, and remains dissolved for at least a short time after the reaction mixture is removed from the reactor, the catalyst can be extracted from the hot solution (either with aqueous acid, which removes both the amine and metal ion catalyst components, or with a chelating agent such as salts of ethylenediaminetetraacetic acid (EDTA) or nitrilotriacetic acid (NTA) or the like, to remove only the metal ion). The solution can then be cooled, causing the polymer complex to precipitate out, and the polymer complex then can be isolated by filtration or centrifuging.
Essential features of the discovery are:
i. Polymerization in methylene chloride solution at a temperature of 40.degree. C. or above, under oxygen pressure; and PA1 ii. Cooling the polymer solution to room temperature or below to precipitate the polymer-methylene chloride complex. PA1 iii. Extraction of catalyst from hot solution; and PA1 iv. Separation of the complex by filtration or centrifuging and removal of the methylene chloride from the complex by heating.
Preferred features comprise:
The major advantage of the procedure is that no antisolvent is required for precipitation of the polymer, so that the substantial cost involved in separation of the solvent and antisolvent and recovery of the antisolvent by distillation is eliminated. A second advantage is that the polymer is amorphous, whereby polyphenylene ethers are more easily blended by coextrusion with other resins, such as polystyrene, than the partially crystalline polymers obtained by the usual commercial processes, in which an antisolvent such as methanol is added to the polymer solution to precipitate the polymer. The amorphous polyphenylene ether may be coextruded at lower temperatures and at higher rates than partially crystalline polyphenylene ethers, and the resulting blends, especially those containing relatively small proportion of the polyphenylene ether, have improved physical properties. See, e.g., U.S. Pat. No. 3,960,811, incorporated herein by reference.
The polymers obtained by the process have a yellow color, but blends with polystyrene have physical properties equal to those of blends made with polyphenylene ethers isolated by conventional antisolvent precipitation techniques, so that they are suitable for applications where color is not important. The color can be reduced to a level approximating that obtained by methanol precipitation by redissolving the powder in hot methylene chloride, then cooling and separating the polymer-methylene chloride complex.
As has been mentioned, the crude polymer obtained by the new process is highly colored by diphenoquinone and other colored byproducts of the reaction. The color can be reduced to an acceptable level by repeated washing of the precipitated complex with methylene chloride, but this results in a significant reduction in yield, as well as more than doubling the total amount of solvent which must be recovered by distillation. It is a further discovery that both of these disadvantages can be partially overcome by a two-cycle solvent system, and this also forms part of the present invention.
In this two-cycle solvent system, the polymer is filtered from the reaction mixture and washed with a small amount of methylene chloride, and methylene chloride filtrate and wash combined. The polymer is then washed with methylene chloride in an amount equal to the original solvent volume. The first methylene chloride fraction is then distilled to recover pure solvent for subsequent washing operations, while the "wash" methylene chloride is decanted to remove any aqueous phase and used without purification, as the solvent for the next polymerization. The polymer present in the wash liquid is thus recovered at the end of the polymerization; the small amount of quinones and other low molecular weight products have no apparent effect on the quality of the polymer produced.
Accordingly, it is an object of this invention to provide an improved process for the preparation of a polyphenylene ether resin with the aim of obtaining a polyphenylene ether resin with a reduced number of process steps and without the need to use an antisolvent.
It is also an object of this invention to provide a process which permits the removal of color producing compounds from polyphenylene ether and reducing the amount of solvent required.