Various methods have conventionally been disclosed regarding methods for producing a polyphenylene ether by oxidative polymerization of a phenolic compound.
Patent Documents 1 to 4 disclose, as catalysts effective for oxidative polymerization of phenolic compounds, a catalyst in combination of a copper compound and an amine, a catalyst in combination of a copper compound and a halogen compound, a catalyst using a primary amine, a secondary amine, a tertiary amine, a monoamine, a diamine or a polyamine, and a catalyst using a tetraalkyl-type diamine such as N,N,N′,N′-tetramethyl-1,4-butanediamine.
Patent Documents 5 and 6 disclose, as catalysts effective for the oxidative polymerization, a catalyst in combination of a copper compound, an iodine compound and a tetraalkyl-type diamine.
Patent Documents 7 to 9 disclose a catalyst in combination of a copper compound, a bromine compound, and a tertiary amine such as N,N′-di-t-butylethylenediamine or n-butyldimethylamine, and a catalyst in combination of a copper compound, a bromine compound, a tertiary amine and a secondary monoamine.
Patent Document 10 discloses a catalyst composed of a copper compound and a secondary aliphatic amine or a secondary aliphatic amine, an aniline having a special structure and N,N,N′,N′-tetramethyl-1,3-diaminopropane, and improved in water resistance.
Patent Documents 8 to 10 further describe that the use of a quaternary ammonium compound is advantageous.
Patent Documents 11 to 13 disclose that the blending of a polymerization catalyst and a phenolic compound in an inert gas atmosphere highly activates the catalyst and improves the productivity of a polyphenylene ether.
Patent Documents 14 and 15 disclose, as a method for separating a polyphenylene ether and a polymerization catalyst after terminating the oxidative polymerization, a method of a continuous liquid-liquid separation using a centrifugal separator.
On the other hand, in the oxidative polymerization of a phenolic compound, diphenoquinone is produced as a by-product. Since the diphenoquinone affects physical properties and color tone of a polyphenylene ether, the diphenoquinone needs to be disposed.
Patent Document 16 discloses, as a method of disposing diphenoquinone, a reduction removal method, in which after an oxidative polymerization of a phenolic compound is completed, an aminocarboxylic acid derivative and a reducing agent are added to the polymerization solution to separate diphenoquinone from polyphenylene ether.
Patent Documents 17 and 18 also disclose, as a method of disposing diphenoquinone, a method of reacting diphenoquinone with a polyphenylene ether.
However, a polyphenylene ether is polymerized using a catalyst disclosed in Patent Documents 8 to 10 and diphenoquinone is disposed, then the catalyst is tried to be removed from a polymerization solution by the method disclosed in Patent Documents 14 and 15, the interface between a water phase and an organic solvent phase does not stabilize and bubbles when the liquid-liquid separation is carried out, and thus without measures such as discarding an interface portion, a metal salt as a polymerization catalyst component cannot be separated completely from the polyphenylene ether in some cases.
Furthermore, a polyphenylene ether is produced using a favorable catalyst described in Patent Documents 8 to 10, and the same method as described above is applied, although a metal salt as a polymerization catalyst component can be separated from the polyphenylene ether, the obtained polyphenylene ether has a problem in mechanical properties.
That is, no technique has been disclosed in which a metal salt as a polymerization catalyst component is efficiently separated from a polyphenylene ether having been subjected to diphenoquinone treatment, and a polyphenylene ether excellent in mechanical properties is obtained.