A method for preparing a block copolymer comprising: - at least one block A not comprising any phosphonate group, and comprising at least one poly (alkylene oxide) group, and - at least one block B obtained by polymerization of a monomer B1 or of a mixture of monomers with ethylenic unsaturation comprising at least one monomer B1, wherein a monomer B1 is a monomer with ethylenic unsaturation comprising at least one phosphonate function comprising the steps of: b) providing a compound of the following formula: wherein: - block A is a block A not comprising any phosphonate group, and comprising at least one poly (alkylene oxide) group, - L is absent or is a binding group connecting covalently the block A and the group -S- R 10, - R 10 represents an alkyl, an arylalkyl, an aryl, an alkylaryl or a group OR 11, NR 12 R 13 or SR 14, - R 11, R 12, R 13 and R 14 represent independently an alkyl or an alkenyl, an arylalkyl, an aryl, an alkylaryl, wherein R 12 and R 13 may be connected together in order to form a ring with the nitrogen atom bearing them, c) controlled radical polymerization, or several successive controlled radical polymerizations, comprising the putting into contact: - of the compound of formula, - of a monomer B1 or a mixture of monomers with ethylenic unsaturation comprising at least one monomer B1, wherein a monomer B1 is a monomer with ethylenic unsaturation comprising at least one phosphonate function, and - of a source of free radicals. - at least one block A not comprising any phosphonate group, and comprising at least one poly (alkylene oxide) group, and - at least one block B obtained by polymerization of a monomer B1 or of a mixture of monomers with ethylenic unsaturation comprising at least one monomer B1, wherein a monomer B1 is a monomer with ethylenic unsaturation comprising at least one phosphonate function b) providing a compound of the following formula: wherein: - block A is a block A not comprising any phosphonate group, and comprising at least one poly (alkylene oxide) group, - L is absent or is a binding group connecting covalently the block A and the group -S- R 10, - R 10 represents an alkyl, an arylalkyl, an aryl, an alkylaryl or a group OR 11, NR 12 R 13 or SR 14, - R 11, R 12, R 13 and R 14 represent independently an alkyl or an alkenyl, an arylalkyl, an aryl, an alkylaryl, wherein R 12 and R 13 may be connected together in order to form a ring with the nitrogen atom bearing them, - block A is a block A not comprising any phosphonate group, and comprising at least one poly (alkylene oxide) group, - L is absent or is a binding group connecting covalently the block A and the group -S- R 10, - R 10 represents an alkyl, an arylalkyl, an aryl, an alkylaryl or a group OR 11, NR 12 R 13 or SR 14, - R 11, R 12, R 13 and R 14 represent independently an alkyl or an alkenyl, an arylalkyl, an aryl, an alkylaryl, wherein R 12 and R 13 may be connected together in order to form a ring with the nitrogen atom bearing them, c) controlled radical polymerization, or several successive controlled radical polymerizations, comprising the putting into contact: - of the compound of formula, - of a monomer B1 or a mixture of monomers with ethylenic unsaturation comprising at least one monomer B1, wherein a monomer B1 is a monomer with ethylenic unsaturation comprising at least one phosphonate function, and - of a source of free radicals. - of the compound of formula, - of a monomer B1 or a mixture of monomers with ethylenic unsaturation comprising at least one monomer B1, wherein a monomer B1 is a monomer with ethylenic unsaturation comprising at least one phosphonate function, and - of a source of free radicals.