Process for preparing polyoxyalkylene polyester polyols

A process for preparing a polyoxyalkylene polyester polyol by reaction of a starter compound (1) having Zerewitinoff-active H atoms, a cyclic dicarboxylic anhydride (2) and a fatty acid ester (3) with an alkylene oxide (4) in the presence of a basic catalyst (5) comprising:

• (i) providing a mixture (i) comprising the starter compound (1) and the fatty acid ester (3), wherein the mixture (i) contains no cyclic dicarboxylic anhydride (2);
• (ii) reacting the mixture (i) from step (i) with a portion (v1) of the alkylene oxide (4), wherein at least 0.15 mol of alkylene oxide (4) per 1 mol of Zerewitinoff-active hydrogen atoms of the starter compound (1) is added to form a compound (ii);
• (iii) reacting the compound (ii) formed in step (ii) by addition of the dicarboxylic anhydride (2) to form a compound (iii);
• and
• (iv) reacting the compound (iii) from step (iii) with a portion (v2) of the alkylene oxide (4), wherein at least 1.15 mol of alkylene oxide (4) per 1 mol of the dicarboxylic anhydride (2) is added.