Tin containing macrocyclic polylactones were prepared by di-n-butyl-2-stanna-1 ,3-dioxepane-initiated polymerizations of epsilon-caprolactone in bulk. The average ring size was varied from 10 to 100 monomer units via the monomer/initiator (M/I) ratio. Addition of terephthaloyl or sebacoyl chloride to the in situ prepared macrocycles yielded polycondensates under elimination of di-n-butyl tin dichloride. The molecular weights increased with the reaction temperature (e.g., 80-140 degrees C) and with the size of the macrocycles. Number-average molecular weights (M(n)s) up to 90,000 and polydispersities between 1.65 and 2.0 were obtained. Further polycondensations were conducted with isophthaloyl chloride, 4,4＇-biphenyldicarbonylchloride and 4,4＇-phenylenebisacryloylchloride. Several polycondensations were performed with macrocyclic poly(delta-valerolactone) and poly (beta-D,L-butyrolactone). In those cases the increase of the molecular weight was lower.