Abstract: Block and graft lactone copolymers are prepared by reacting a base polymer containing hydroxyl groups with an organometallic catalyst of the formula ##STR1## wherein R.sub.1 ', R.sub.2 ', R.sub.3 ', and R.sub.4 ' are alkoxy groups (alcoholate groups), M' is a trivalent metal and M is a divalent metal, whereby the catalyst becomes bonded to the base polymer by way of the hydroxyl groups. Then the remaining alkoxy groups are converted to acyl groups and thereafter the organometalic end-groups are reacted with a lactone to form the lactone polymer moiety. The resulting block and graft copolymers are particularly useful when blended with other polymers, as for example polyvinyl chloride.