Process for preparing polyphosphonate

In a process for preparing a polyphosphonate which comprises polycondensing an aromatic diol having the formula (I) - (IV) ##SPC5##

• wherein Z represents chlorine or bromine atom or a lower alkyl group and m represents 0 or an integer of from 1 to 4 and n represents 0 or an integer of from 1 to 3;
• Y represents an alkylene group, cycloalkylene group, sulfonyl group, carbonyl group, or oxygen atom;
• with a dihalide of arylphosphonic or thiophosphonic acid having the formula (V) ##SPC6##
• wherein A represents oxygen or sulfur atom;
• X represents chlorine or bromine atom, and l represents 0 or an integer of from 1 to 5, the improvement which comprises the steps of
• 1. maintaining in a high pressure zone a solution of said polyphosphonate in an organic solvent at a temperature of 125.degree. - 300.degree.C;
• 2. discharging the solution of step (1) into a low pressure zone wherein all or a portion of the organic solvent is flashed and porous solid polyphosphonate is formed, the pressure of said low pressure zone being lower than the vapor pressure of said organic solvent and lower than that of said high pressure zone;
• and
• 3. recovering said porous solid polyphosphonate.