Best results are usually obtained if this final distillation or polycondensation is run at less than 1 mm pressure and 240??-260??C for less than 2 hours in the presence of antioxidants such as 1, 6-bis- [ (3, 5-di-tert-butyl-4- hydroxyphenol) propionamido] -hexane or 1,3, 5-trimethyl-2, 4,6-tris [3,5- ditertiary-butyl-4-hydroxybenzyl] benzene.