In the past I have conjugated an acid to polyethyleneimine with percentage conjugation of around 25%. I wanted to increase this percentage, and so I doubled the amount of acid, EDC and NHS before this solution was added to the polyethyleneimine. After allowing to react in methanol, with no solubility issues, after I precipitated the polymer as normal in diethyl ether, I have found it is now insoluble in water or methanol. However, there was no suggestion of the polymer "crashing out" of solution in methanol while the reaction was stirring. Why is the polymer precipitate suddenly insoluble when this has never been an issue previously? The acid I am using is a simple phenylboronic acid.
Lorcan Rooney
Polyethylene in water has a positive molecular charge. This is a positive ion. All your acids have a negative charge in water. Positive and negative charges interact according to Coulomb's law depending on the permittivity of the solvent. The greater the dielectric constant (in water), the lower the solubility compared to methanol, which has a lower dielectric constant. However, in addition to the electrostatic interaction in water, a hydrophobic interaction occurs between polyethyleneamine and phenylboronic acid. Therefore, the salt of polyethyleneamine and phenylboronic acid precipitates in water.
Yuri Mirgorod thanks for your help. Yes we found by lowering the pH of the solution we were able to dissolve the precipitate. Also found a paper regarding if PEI amines are unconjugated the amines will intertwine and need to be charged to electrostatically repel and allow dissolution.
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