Polyoxyalkylene-polysiloxane block polymers, a process for preparing them and their use as defoamers in aqueous dispersions

The invention relates to the polyoxyalkylene-polysiloxane block polymers of general formula (I) C-B-(AB)a-C1 (I), wherein A=a polyoxyalkylene block of the general formula -(CnH2nO)b-, B=a polysiloxane block of the general formula -(SiR2-O)c-, C, C1=the same or different alkoxypolyoxyalkylene groups of the general formula Z-O-[CH2-CH(R1)O]d-, R=the same or different C1-4 alkyl groups or phenyl groups, with the proviso that at least 90% of the groups R are methyl groups, R1=the same or different hydrogen, C1-12 alkyl groups or phenyl groups, Z=an alkyl, alkylene, aryl or alkylaryl group, a=1 to 20, preferably 2 to 10, b=an average value of 10 to 130, preferably 20 to 100, c=3 to 100, preferably 10 to 80, d=independently is in the range of C/C1 values of 2 to 20, preferably 2 to 12, with the proviso that the average value is in the range of 2 to