Preparation of polyetherols

Polyetherols based on solid initiator substances and liquid, hydroxyl-containing coinitiators are prepared by a catalyzed addition reaction of alkylene oxides by a process in which the initiator combination contains ethoxy structures, the ratio of the average number of hydroxyl groups per mole of initiator combination to the number of ethoxy structures in the polyetherol being from 1:0.2 to 1:1.8 and the ratio of the amounts by weight of the ethoxy structures to the average molecular weight of the polyetherol being from 1:2 to 1:15. The polyetherols prepared by this process are used for the preparation of PUR, in particular rigid PUR foams.