Batch process for preparing a polyether alcohol using a double metal cyanide catalyst

A batch process for preparing a polyether alcohol P having a hydroxyl number of equal to or lower than 115 mg KOH/g by reacting starter compound S1 and starter compound S2, which starter compounds have one active hydrogen atom or a plurality of active hydrogen atoms, with one or more alkylene oxides in the presence of a composite metal cyanide complex catalyst, comprising

• a) forming a starter mixture comprising starter compound S1 and the catalyst, followed by
• b) continuously adding an alkylene oxide;
• and
• c) continuously adding starter compound S2;
• wherein
• starter compound S1 has (I) a nominal functionality which equals the nominal functionality of polyether alcohol P and a hydroxyl number which is within 10% of the hydroxyl number of polyether alcohol P and/or (II) an equivalent weight of from 10 to 10,000 g/mol;
• starter compound S2 has an equivalent weight of from 10 to 300 g/mol;
• no alkylene oxide is added in step a) or between steps a) and b), and the continuous addition of alkylene oxide in step b) is not interrupted before the total weight of alkylene oxide needed to prepare polyether alcohol P has been added;
• and
• step c) starts before step b).