Method for synthesizing at least N,N'-dimethyl-N,N'-di(2-hydroxyethyl)-1,6-hexanediamine of formula (I 2) below: comprising two reaction steps, and comprising at least a first reaction between a first halogen atom-free precursor compound and a second halogen atom-free precursor compound, said first precursor compound comprising a carbon-based backbone formed by a linear chain of 6 carbon atoms in which the central 4 carbon atoms are each bonded to 2 hydrogen atoms and the carbon atoms located in the alpha and omega positions are not bonded to a halogen atom, and in which neither of the two reaction steps comprises a halogenated organic reactant, the first precursor compound being 1,6-hexanediamine and the second precursor compound being ethylene oxide, said method comprising: - a first step of reaction between 1,6-hexanediamine and ethylene oxide to form at least one intermediate compound selected from the group comprising N-(2-hydroxyethyl)-1,6-hexanediamine, N,N'-di(2-hydroxyethyl)-1,6-hexanediamine, N,N-di(2-hydroxyethyl)-1,6-hexanediamine and N,N,N'-tri(2-hydroxyethyl)-1,6-hexanediamine; - a second step of methylation of the primary or secondary amine functions of said at least one intermediate compound to form at least N,N'-dimethyl-N,N'-di(2-hydroxyethyl)-1,6-hexanediamine, said second methylation step being carried out by reaction between said intermediate compound, formaldehyde and hydrogen in the presence of a hydrogenation catalyst, or by reaction between said intermediate compound, formaldehyde and formic acid according to the Eschweiler-Clarke reaction. comprising two reaction steps, and comprising at least a first reaction between a first halogen atom-free precursor compound and a second halogen atom-free precursor compound, said first precursor compound comprising a carbon-based backbone formed by a linear chain of 6 carbon atoms in which the central 4 carbon atoms are each bonded to 2 hydrogen atoms and the carbon atoms located in the alpha and omega positions are not bonded to a halogen atom, and in which neither of the two reaction steps comprises a halogenated organic reactant, the first precursor compound being 1,6-hexanediamine and the second precursor compound being ethylene oxide, said method comprising: - a first step of reaction between 1,6-hexanediamine and ethylene oxide to form at least one intermediate compound selected from the group comprising N-(2-hydroxyethyl)-1,6-hexanediamine, N,N'-di(2-hydroxyethyl)-1,6-hexanediamine, N,N-di(2-hydroxyethyl)-1,6-hexanediamine and N,N,N'-tri(2-hydroxyethyl)-1,6-hexanediamine; - a second step of methylation of the primary or secondary amine functions of said at least one intermediate compound to form at least N,N'-dimethyl-N,N'-di(2-hydroxyethyl)-1,6-hexanediamine, said second methylation step being carried out by reaction between said intermediate compound, formaldehyde and hydrogen in the presence of a hydrogenation catalyst, or by reaction between said intermediate compound, formaldehyde and formic acid according to the Eschweiler-Clarke reaction. - a first step of reaction between 1,6-hexanediamine and ethylene oxide to form at least one intermediate compound selected from the group comprising N-(2-hydroxyethyl)-1,6-hexanediamine, N,N'-di(2-hydroxyethyl)-1,6-hexanediamine, N,N-di(2-hydroxyethyl)-1,6-hexanediamine and N,N,N'-tri(2-hydroxyethyl)-1,6-hexanediamine; - a second step of methylation of the primary or secondary amine functions of said at least one intermediate compound to form at least N,N'-dimethyl-N,N'-di(2-hydroxyethyl)-1,6-hexanediamine, said second methylation step being carried out by reaction between said intermediate compound, formaldehyde and hydrogen in the presence of a hydrogenation catalyst, or by reaction between said intermediate compound, formaldehyde and formic acid according to the Eschweiler-Clarke reaction.