Block polymers usable as defoamers have the following formula ##STR1## in which R represents a glycerol or polyglycerol function having a degree of polymerization x of from 1 to 6 and the indices y is equal to (x-z+2), z is equal to (x-y+2), a is equal to 1 to 15, b is equal to 50 to 200 c is equal to 1 to 15, d is equal to 20 to 150 and e is equal to 1 to 15, with the proviso that the sums (a+c+e) is equal to 2 to 20, (b+d) is equal to 50 to 250 and (c+e) is equal to 1 to 15, and y has a value of at least 1.The block polymers are prepared by reaction of glycerol or polyglycerol having a degree of polymerization x with ethylene oxide, then with propylene oxide and, finally, again with ethylene oxide, preferably in the presence of alkaline etherification catalysts.