Acetamide(C2), propionamide(C3), butyramide(C4), isobutyramide(i-C4), isovaleramide(i-C5) and trimethylacetamide(t-C5) groups each were introduced to the terminals of hyperbranched polyethylenimine(HPEI) through the amidation reaction between HPEI and the corresponding anhydride. Moreover, HPEIs terminated with two kinds of amides were also prepared. The first amide was fixed to be i-C4 with 52% degree of amidation(DA), and the second amide varied from C2, C3, C4, i-C5 to t-C5. All the polymers were characterized by 1H-NMR. Turbidimetry measurements were performed for these polymers in water at different temperatures. With respect to the polymers bearing only one kind of amide group, except C2, all the other amide groups could render thermoresponsive properties to HPEI. The specific ordering of these amide groups to reduce the cloud point temperature(Tcp) was as follows: i-C5 > t-C5 > C4 > i-C4 > C3. Moreover, the more branched i-C4 and t-C5 were better groups than their less branched isomers C4 and i-C5 in the Tcp range of 12-51 °C to render the sharper phase transition to the thermoresponsive polymers. As for the polymers bearing two kinds of amide groups, the further introduction of C2, C3, C4, i-C5 or t-C5 could effectively endow HPEI bearing 52% of i-C4 with thermoresponsive properties. The specific ordering of these second amide groups to reduce the Tcp was as follows: i-C5 > C4 > i-C4 > C3 > C2. C4, i-C5 and t-C5 were all effective second amide groups to prepare the thermoresponsive polymers with sharper phase transition.
Optically active and inactive hyperbranched polymers with specific thermoresponsive behaviours in water were reported.Through two steps hyperbranched polyethylenimine(HPEI) polymers terminated with different amount of D-phenylalanine(D-Phe),L-phenylalanine(L-Phe) or DL-phenylalanine(DL-Phe) were prepared and characterized.The analyses on the solution properties by turbidimetry,dynamic light scattering,fluorescence probe and 1H-NMR demonstrated that all the polymers exhibited specific thermoresponsive behaviours in water,including:(1) In the dilute polymer concentration region,increasing the polymer concentration led to the increase of phase transition temperature;(2) The optically inactive thermoresponsive hyperbranched polymers showed a higher cloud-point temperature(Tcp) than their corresponding optically active ones in a relatively higher polymer concentration;(3) At the same polymer concentration the hydrophobic groups of the optically inactive HPEI-DL-Phe formed more perfect hydrophobic domain than those of the optically active HPEI-L-Phe and HPEI-D-Phe.
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-IBAm0.80-NH2). 1H-NMR characterization proved the successful preparation of the product in each step. Compared with its spatial isomer HPEIIBAm0.80 without primary amine groups, 1H-NMR spectra verified that more IBAm groups were located in the interior of HPEI-IBAm0.80-NH2. The further modification of HPEI-IBAm0.80-NH2 and HPEI-IBAm0.80 with p-nitrobenzaldehyde demonstrated that HPEI-IBAm0.80-NH2 was more reactive than HPEI-IBAm0.80 due to its possession of primary amines. Turbidimetry measurements showed that HPEI-IBAm0.80-NH2 was thermoresponsive in water. In the pH range of 9.5-10 its cloud point temperature(Tcp) was constant, and it increased obviously upon decreasing the pH below 9.5. The thermoresponsive HPEI-IBAm0.8 exhibited the similar trend, but the pH threshold to achieve the constant Tcp was around 8.5. Moreover, HPEI-IBAm0.8-NH2 showed higher Tcp and broader phase transition than HPEI-IBAm0.8. The mechanism leading to the different thermoresponsive properties between HPEI-IBAm0.8-NH2 and its spatial isomer HPEI-IBAm0.8 was discussed.
The influence of sodium dodecyl sulfate(SDS) on the cloud point temperature(Tcp) of the aqueous solution of thermoresponsive hyperbranched polyethylenimine derivative HPEI-IBAm was studied systematically. When p H was below 8.5, HPEI-IBAm was positively-charged. Initially, the Tcp of HPEI-IBAm decreased significantly, followed by an obvious increase with the increase of SDS concentration. The lower the p H was, the higher the SDS concentration was required to achieve the minimum Tcp. When p H was above 8.5, HPEI-IBAm was neutral and raising the SDS concentration led to the gradual increase of Tcp. Compared to linear poly(N-isopropyl acrylamide)(PNIPAm), the Tcp of the current hyperbranched HPEI-IBAm was more sensitive to SDS. The thermoresponsive HPEI-IBAm/SDS complex was used as host to accommodate the non-polar pyrene in water. The lowest SDS concentration for effectively enhancing the solubility of pyrene in water was around 6.4 mmol·L^(-1). When HPEI-IBAm was present, the SDS concentration threshhold was decreased to about 0.31 mmol·L^(-1). Fluorescence technique with pyrene as the hydrophobic probe demonstrated that the SDS concentration of 7.2 mmol·L^(-1) was required to form the hydrophobic domain to accommodate pyrene guests without HPEI-IBAm, while only 0.2 mmol·L^(-1) of SDS was required in the presence of HPEI-IBAm.
