Defoamer composition comprising a compound based on polybutadiene having at least one repeat unit selected from the group consisting of the divalent radicals wherein A is in each case independently a monovalent organic radical or a hydrogen radical, preferably in each case independently selected from the group consisting of monovalent hydrocarbon radicals having 1 to 6 carbon atoms, more preferably in each case independently selected from the group consisting of alkyl radicals having 1 to 4 carbon atoms; B is in each case independently selected from the group consisting of radicals of the formula preferably in each case independently selected from the group consisting of radicals of the formula more preferably in each case independently selected from the group consisting of radicals of the formula R 1 is in each case independently selected from the group consisting of monovalent hydrocarbon radicals having 1 to 16 carbon atoms, preferably in each case independently an alkyl radical having 1 to 16 carbon atoms or a phenyl radical, more preferably in each case independently a methyl radical, an ethyl radical or a phenyl radical; R 2 is a radical of the formula -CH 2 -O-R 3; R 3 is in each case independently selected from the group consisting of monovalent hydrocarbon radicals having 3 to 18 carbon atoms; preferably each independently an allyl radical, a butyl radical, an alkyl radical having 8 to 15 carbon atoms or a phenyl radical that may be substituted by monovalent radicals selected from hydrocarbon radicals having 1 to 4 carbon atoms; more preferably a tert-butylphenyl radical or an o-cresyl radical; R 4 is each independently a monovalent organic radical having 1 to 18 carbon atoms or hydrogen, preferably hydrogen; and m, n, o, p and q are each independently 0 to 300, preferably 0 to 200, more preferably 0 to 100, with the proviso that the sum total of m, n, o, p and q is greater than 1, preferably greater than 5, more preferably greater than 10; the B radical has at least one repeat unit in accordance with including every permutation of the repeat units in the B radical. A is in each case independently a monovalent organic radical or a hydrogen radical, preferably in each case independently selected from the group consisting of monovalent hydrocarbon radicals having 1 to 6 carbon atoms, more preferably in each case independently selected from the group consisting of alkyl radicals having 1 to 4 carbon atoms; preferably in each case independently selected from the group consisting of monovalent hydrocarbon radicals having 1 to 6 carbon atoms, more preferably in each case independently selected from the group consisting of alkyl radicals having 1 to 4 carbon atoms; B is in each case independently selected from the group consisting of radicals of the formula preferably in each case independently selected from the group consisting of radicals of the formula more preferably in each case independently selected from the group consisting of radicals of the formula R 1 is in each case independently selected from the group consisting of monovalent hydrocarbon radicals having 1 to 16 carbon atoms, preferably in each case independently an alkyl radical having 1 to 16 carbon atoms or a phenyl radical, more preferably in each case independently a methyl radical, an ethyl radical or a phenyl radical; preferably in each case independently an alkyl radical having 1 to 16 carbon atoms or a phenyl radical, more preferably in each case independently a methyl radical, an ethyl radical or a phenyl radical; R 2 is a radical of the formula -CH 2 -O-R 3; R 3 is in each case independently selected from the group consisting of monovalent hydrocarbon radicals having 3 to 18 carbon atoms; preferably each independently an allyl radical, a butyl radical, an alkyl radical having 8 to 15 carbon atoms or a phenyl radical that may be substituted by monovalent radicals selected from hydrocarbon radicals having 1 to 4 carbon atoms; more preferably a tert-butylphenyl radical or an o-cresyl radical; R 4 is each independently a monovalent organic radical having 1 to 18 carbon atoms or hydrogen, preferably hydrogen; and preferably each independently an allyl radical, a butyl radical, an alkyl radical having 8 to 15 carbon atoms or a phenyl radical that may be substituted by monovalent radicals selected from hydrocarbon radicals having 1 to 4 carbon atoms; more preferably a tert-butylphenyl radical or an o-cresyl radical; R 4 is each independently a monovalent organic radical having 1 to 18 carbon atoms or hydrogen, preferably hydrogen; and m, n, o, p and q are each independently 0 to 300, preferably 0 to 200, more preferably 0 to 100, with the proviso that the sum total of m, n, o, p and q is greater than 1, preferably greater than 5, more preferably greater than 10;