[Quark1]

Dear colleagues!
Prompt, please, in what conditions to better carry out a Mannich reaction with ethylenediamine, formaldehyde and sulfosalicylic acid. Found a technique, but in under such circumstances reaction does not go, and at the exit I receive the free salicylic acid or its salt with ethylenediamine. Also tried to neutralize salicylic acid and ethylenediamine and to replace ethanol with methyl cellosolve for larger heating, but without result.

Synthesis (Yael Albo, Nina Buzaglo, Eric Maimon, Israel Zilbermann & Dan Meyerstein
 Design of a ligand suitable for sensitive uranyl analysis in aqueous solutions, Journal of Coordination Chemistry, 68:17-18, 3079-3087, DOI: 10.1080/00958972.2015.1043287)

To 4.40 g (0.05 mol) of N,N-dimethylethylene-diamine in 100 mL ethanol, 10.0 g (0.1 mol) of formaldehyde (37% solution) was added. The mixture was stirred for 2 h and 25.4 g (0.1 mol) of 5-sulfosalicylic acid dihydrate was added to the solution. The mixture was refluxed for 12 h and allowed to cool to room temperature. Throughout the synthesis, the reaction vessel was covered with aluminum foil to prevent exposure to light. A white precipitate was isolated, washed in ethanol, and dried under vacuum. The precipitate is soluble in water.

Thank you in advance for your response

[pgk]

That way, it is possible that you have got the condensation polymer between N,N’-dimethylenediamine and formaldehyde, only.
So, try to put all the reagents together, N,N’-dimethylethylenediamine, formaldehyde and 5-sulfosalicylic acid and then continue, as described in the cited reference.
Alternatively, you can use acetonitrile as a solvent.