The invention provides linear polydimethylsiloxane-polyoxyalkylene block copolymers containing Si—C groups and carboxylic ester groups and having repeating (A1BA2B) units where A1 and A2 are two different polydimethylsiloxane units of the general formula (I) with a number of Si atoms N=(n+2) of 15≦N≦30 and NA1≠NA2 and B is a unit of the formula (II) —CH2—CH2—(CH2)a—CO[O—CH2—CH(R)]c—O-[D-O]e—[—CH2—CH(R)—O]d—OC—(CH2)b—CH2—CH2— (II) where R independently at each occurrence is H, —CH3 or —C2H5, a and b independently of one another are 0 to 20, c and d independently of one another are values≧1, e is 0 or 1 and D is a hydrocarbon radical having 2 to 10 C atoms, the overall molar weight of the unit of the formula II being from 600 to 10 000 g/mol, with the proviso that the average mixed molar weight of the polydimethylsiloxane units (A1+A2) is from >1665 to 0% to ≦50% by weight, based on the overall weight of [O—CH2—CH(R)] in B, a process for preparing them and their use.