Biodegradable polymers comprising a polyether ester P1 obtained by reactinga first mixture of20-95 mol% of adipic acid or ester-forming derivatives thereof or mixtures thereof,5-80 mol% of terephthalic acid or ester-forming derivatives thereof or mixtures thereof, and0-5 mol% of a compound containing sulfonate groups,and a second mixture of15-99.8 mol% of a dihydroxy compound selected from the group consisting of C.sub.2 -C.sub.6 -alkanediols and C.sub.5 -C.sub.10 -cycloalkanediols,85-0.2 mol% of a dihydroxy compound IHO--�(CH.sub.2).sub.n --O!.sub.m --H(n=2, 3 or 4, m=2-250)with0-5 mol%, based on the first mixture, of a compound D having at least three groups capable of ester formation,and a bisoxazoline C1, their preparation and their use in the manufacture of biodegradable molding materials.