Preparation and characterization of thermoresponsive hyperbranched polyethylenimine with plenty of reactive primary amine groups

Certain amount of primary amine (NH₂) groups of hyperbranched polyethylenimine (HPEI) was first protected by Boc groups. Subsequently, the residual reactive amine groups were reacted with isobutyric anhydride to introduce isobutyramide (IBAm) groups to HPEI. Finally, Boc groups were deprotected to result in HPEI-IBAm-NH₂ with 18% of primary amine terminals on the periphery and 80% of IBAm terminal groups (abbreviated as HPEI-IBAm_0.80-NH₂). ¹H-NMR characterization proved the successful preparation of the product in each step. Compared with its spatial isomer HPEIIBAm_0.80 without primary amine groups, ¹H-NMR spectra verified that more IBAm groups were located in the interior of HPEI-IBAm_0.80-NH₂. The further modification of HPEI-IBAm_0.80-NH₂ and HPEI-IBAm_0.80 with p-nitrobenzaldehyde demonstrated that HPEI-IBAm_0.80-NH₂ was more reactive than HPEI-IBAm_0.80 due to its possession of primary amines. Turbidimetry measurements showed that HPEI-IBAm_0.80-NH₂ was thermoresponsive in water. In the pH range of 9.5–10 its cloud point temperature (T _cp) was constant, and it increased obviously upon decreasing the pH below 9.5. The thermoresponsive HPEI-IBAm_0.8 exhibited the similar trend, but the pH threshold to achieve the constant T _cp was around 8.5. Moreover, HPEI-IBAm_0.8-NH₂ showed higher T _cp and broader phase transition than HPEI-IBAm_0.8. The mechanism leading to the different thermoresponsive properties between HPEI-IBAm_0.8-NH₂ and its spatial isomer HPEI-IBAm_0.8 was discussed.