Increasing the molecular weight of polycondensates

A process for increasing the molecular weight of a polycondensate, which comprises

• heating a polycondensate in the temperature range below the melting point and above the glass transition temperature in the solid phase of the polymer with the addition of at least one sterically hindered hydroxyphenylalkylphosphonic ester or monoester of formula I ##STR40## in which R.sub.1 is isopropyl, tert-butyl, cyclohexyl or cyclohexyl which is substituted by 1-3 C.sub.1 -C.sub.4 alkyl groups,
• R.sub.2 is hydrogen, C.sub.1 -C.sub.4 alkyl, cyclohexyl or cyclohexyl which is substituted by 1-3 C.sub.1 -C.sub.4 alkyl groups,
• R.sub.3 is C.sub.1 -C.sub.20 alkyl, or unsubstituted or C.sub.1 -C.sub.4 alkyl-substituted phenyl or naphthyl,
• R.sub.4 is hydrogen, C.sub.1 -C.sub.20 alkyl, unsubstituted or C.sub.1 -C.sub.4 alkyl-substituted phenyl or naphthyl;
• or
• is ##EQU2## M.sup.r+ is an r-valent metal cation, n is 1, 2, 3, 4, 5 or 6, and
• r is 1, 2 or 3.