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.