PROCESS FOR PREPARING POLYETHERS IN WHICH ARE RETAINED AT LEAST TWO FREE HYDROXYL GROUPS OF THE STARTING ALCOHOLS

1. Process for the preparation of polyesters with at least two free hydroxyl groups being retained on the starting alcohol, the hydroxyl groups being separated from one another by at most three carbon atoms and the hydroxyl group at the end of the polyester chain being blocked, optionally reversibly, but stable towards the conditions of acetal or ketal splitting, characterized in that a) an oxirane of the formula see diagramm : EP0043966,P6,F1 in which R**1 denotes a hydrogen radical, an optionally halogenated hydrocarbon group with 1 to 30 carbon atoms or the group -CH2 OR**2, in which R**2 denotes an alkyl or alkenyl group, and wherein the groups R**1 within the polymeric molecule are identical or different, is added onto a trihydric or polyhydric alcohol in which at least two hydroxyl groups are acetalised or ketalised, b) the hydroxyl group of the resulting polyester-monool is blocked by reaction with a group, which is reactive in respect of this hydroxyl group, of a monofunctional compound which is stable under the conditions of acetal or ketal splitting, and c) the acetal or ketal is then split by the action of acid.