Hyperbranched Polymers with Thermoresponsive Property Highly Sensitive to Ions

The salt effects on the water solubility of thermoresponsive hyperbranched polyethylenimine and polyamidoamine possessing large amounts of isobutyramide terminal groups (HPEI-IBAm and HPAMAM-IBAm) were studied systematically. Eight anions with sodium as the counterion and ten cations with chloride as the counterion were used to measure the anion and cation effects on the cloud point temperature (T-cp) of these dendritic polymers in water. It was found that the T-cp of these dendritic polymers was much more sensitive to the addition of salts than that of the traditional thermoresponsive linear polymers. At low anion concentration, the electrostatic interaction between anions and the positively charged groups of these polymers was dominant, resulting in the unusual anion effect on the T-cp of these polymers in water, including (1) T-cp, of these dendritic polymers decreasing nonlinearly with the increase of kosmotropic anion concentration; (2) the chaotropic anions showing abnormal salting-out property at low salt concentration and the stronger chaotropes having much pronounced salting-out ability; (3) anti-Hofmeister ordering at low salt conentration. At moderate to high salt concentration, the specific ranking of these anions in reducing the T-cp of HPEI-IBAm and HPAMAM-IBAm polymers was PO43- > CO32- > SO42- > S2O32- > F- > Cl- > Br-- > I-, in accordance with the well-known Hofineister series. At moderate to high salt concentration, the specific ranking order of inorganic cations in reducing the T HPELIBAm polymer was Sr2+ approximate to Ba2+ > Na+ approximate to K+ approximate to Rb+ > Cs+ > NH4+. Re, caz+ > Li+ approximate to Mg2+. This sequence was only partially simila: to the typical Hofmeister cation series, whereas at low salt concentration the cation effect on T-cp of the dendritic polymer was insignificant and no obvious specific ranking order could be found.