Polyether ketone compounds obtained by reacting a compound represented by formula (1):

##STR00001##

wherein R.sup.1, X.sup.D, Y.sup.Dc, and n.sup.Dc are defined herein, with a compound represented by formula (2):

##STR00002##

wherein X.sup.e, Y.sup.e, Z.sup.e, and n.sup.e are defined herein, exhibit high thermal resistance.

Provided are a polymer electrolyte membrane used in fuel cells, and a method for producing the same, the method including a step of filling a crosslinkable ion conductor in the pores of a porous nanoweb support; and a step of crosslinking the ion conductor filled in the pores of the porous nanoweb support. The method for producing a polymer electrolyte membrane uses a relatively smaller amount of an organic solvent, can ameliorate defects of the support caused by solvent evaporation, and can enhance the impregnability of the ion conductor to the support and the convenience of the process.

The invention relates to a process for manufacturing a polyether ketone ketone, which involves placing in contact an aromatic ether which is diphenyl ether, 1,3-bis(4-phenoxybenzoyl)benzene, 1,4-bis(4-phenoxybenzoyl)benzene or a mixture thereof, an acyl chloride which is isophthaloyl chloride, terephthaloyl chloride, or a mixture thereof, a Lewis acid, and a first fraction s.sub.1 of a reaction solvent, so as to form a premix at a temperature T.sub.0 less than or equal to 25 C. and placing the premix formed at T.sub.0 in contact with a fraction s.sub.2 of the reaction solvent, the fraction s.sub.2 of reaction solvent being preheated, so as to form a reaction mixture heated to a temperature T.sub.1.

Poly(ether ketone ketone) (PEKK) composites include reinforcing fibers in a PEKK polymer matrix that includes a PEKK polymer. It was surprisingly discovered that by washing unneutralized PEKK polymer powder with an acid or base, the melt stability of the PEKK polymers can be improved, as compared with PEKK polymers prepared by conventional methods. The PEKK composites are especially well-suited for fabrication of thick composite parts where melt stability is especially important.