A polyaryl ether ketone copolymer [copolymer (PEDEK-PEEK)] comprising: - recurring units (R PEEK) of formula: and - recurring units (R PEDEK) of formula: wherein in above formulae and, each of R' and R", equal to or different from each other, is independently selected at each occurrence from a C 1 -C 12 group optionally comprising one or more than one heteroatoms; sulfonic acid and sulfonate groups; phosphonic acid and phosphonate groups; amine and quaternary ammonium groups; each of j' and k", equal to or different from each other, is independently selected at each occurrence from 0 and an integer of 1 to 4; wherein the said recurring units are comprised in a molar ratio (R PEEK):(R PEDEK) of 45:55 to 15:85, and wherein the said copolymer (PEDEK/PEEK): (j) comprises an amount of chemically bound chlorine of less than 2.0 μeq/g, as determined by microcoulometry, and (jj) possesses a narrow molecular weight distribution such that the following inequality is satisfied: MV kNs m 2 ≤ 6.62 ⋅ RV dl g 0.20 wherein: - MV is the melt viscosity measured pursuant to ASTM D3835 standard at 410°C and at a shear rate of 46.3 sec -1, using a conical die having a diameter of 1.016 mm a length of 20.32 mm and a cone angle of 120°C and expressed in kNs/m 2; and - RV is the reduced viscosity measured pursuant to ASTM D2857 standard at 25°C on 1.0 wt/vol % solution in concentrated H 2 SO 4 (96 wt%) and expressed in dl/g. - recurring units (R PEEK) of formula: and - recurring units (R PEDEK) of formula: wherein in above formulae and, each of R' and R", equal to or different from each other, is independently selected at each occurrence from a C 1 -C 12 group optionally comprising one or more than one heteroatoms; sulfonic acid and sulfonate groups; phosphonic acid and phosphonate groups; amine and quaternary ammonium groups; each of j' and k", equal to or different from each other, is independently selected at each occurrence from 0 and an integer of 1 to 4; wherein the said recurring units are comprised in a molar ratio (R PEEK):(R PEDEK) of 45:55 to 15:85, and wherein the said copolymer (PEDEK/PEEK): (j) comprises an amount of chemically bound chlorine of less than 2.0 μeq/g, as determined by microcoulometry, and (jj) possesses a narrow molecular weight distribution such that the following inequality is satisfied: MV kNs m 2 ≤ 6.62 ⋅ RV dl g 0.20 wherein: - MV is the melt viscosity measured pursuant to ASTM D3835 standard at 410°C and at a shear rate of 46.3 sec -1, using a conical die having a diameter of 1.016 mm a length of 20.32 mm and a cone angle of 120°C and expressed in kNs/m 2; and - RV is the reduced viscosity measured pursuant to ASTM D2857 standard at 25°C on 1.0 wt/vol % solution in concentrated H 2 SO 4 (96 wt%) and expressed in dl/g. (j) comprises an amount of chemically bound chlorine of less than 2.0 μeq/g, as determined by microcoulometry, and (jj) possesses a narrow molecular weight distribution such that the following inequality is satisfied: MV kNs m 2 ≤ 6.62 ⋅ RV dl g 0.20 wherein: - MV is the melt viscosity measured pursuant to ASTM D3835 standard at 410°C and at a shear rate of 46.3 sec -1, using a conical die having a diameter of 1.016 mm a length of 20.32 mm and a cone angle of 120°C and expressed in kNs/m 2; and - RV is the reduced viscosity measured pursuant to ASTM D2857 standard at 25°C on 1.0 wt/vol % solution in concentrated H 2 SO 4 (96 wt%) and expressed in dl/g. - MV is the melt viscosity measured pursuant to ASTM D3835 standard at 410°C and at a shear rate of 46.3 sec -1, using a conical die having a diameter of 1.016 mm a length of 20.32 mm and a cone angle of 120°C and expressed in kNs/m 2; and - RV is the reduced viscosity measured pursuant to ASTM D2857 standard at 25°C on 1.0 wt/vol % solution in concentrated H 2 SO 4 (96 wt%) and expressed in dl/g.