PPD was mixed with 36% hydrochloric acid (1 mol PPD–585.7 ml hydrochloric acid) in a three-necked flask (250 ml) equipped with a mechanical stirrer and thermometer. The reaction mixture was mixed at room temperature to dissolve PPD. A pink solution was obtained, cooled down to about 263K and treated with a small portions of sodium nitrite solution with continuous stirring
for about 40 min accompanied with gradual change of color to brown as the diazotization was going on. When the reaction was completed, the temperature was raised to 268 K, buffered at pH= 5 through addition of sodium acetate solution (1 mol PPD–428.6 g sodium acetate dissolved in 2.1 l water) and maintained at room temperature for 2 days. The precipitated polymeric powder was
collected by filtration, washed with distilled water, treated with 5% sodium hydrocarbonate aqueous solution, kept overnight, washed thoroughly with water till neutral reaction and dried in air. Then, soluble part of polymer was extracted by DMF and precipitated by water. The polymers obtained were dried in vacuo (323 K/2 kPa) and were stored in a desiccator over phosphorus pentoxide.