Syntheses And Study On Thermoresponsive Properties Of Optically Active Hyperbranched Polymers

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.Then,the thermoresponsive properties of HPEI-Phes in water were studied.The details were as follows:1.Optically active hyperbranched polymers were designed and synthesized.FTIR spectra and ¹H-NMR characterization proved the successful preparation of HPEI-Phes.Polarimeter demonstrated that HPEI-Phes synthesized by D-Phe and L-Phe were optically active while others synthesized by DL-Phe were optically inactive.The degree of substitution?DS?of Phe groups in primary amine groups of HPEI could be deduced from elemental analysis.2.The thermoresponsive properties of HPEI-Phes in water were studied.Unusual thermoresponsive behaviors in water were unprecedentedly found in such polymers.These unusual thermoresponsive behaviours included:?1?In the heating and cooling process hysteresis cycle and anti-hysteresis cycle coexisted,unlike the other thermoresponsive polymers whose heating and cooling cycles are overlapped or had only a hysteresis circle;?2?In the dilute polymer concentration region,the increase of polymer concentration led to the increase of cloud point temperature(T_cp),whereas in other polymer systems,the increase of polymer concentration initially led to the decrease of cloud point temperature(T_cp)and then leveled off.Moreover,the cloud point temperature(T_cp)of the optically inactive HPEI-DL-Phe_0.88 was higher than those of the optically active HPEI-D-Phe_0.88 and HPEI-L-Phe_0.88 in the same condition.3.The analyses on the solution properties by dynamic light scattering,nuclear magnetic resonance and fluorescence probe demonstrated that the polymer molecules could self-assemble into aggregations in water.The critical aggregation concentrations?cac?,determined by dynamic light scattering and fluorescence probe,are all around 0.1mg/m L and the average hydrodynamic radius?R_h?of particles are nearly 13 nm.Moreover,it is clear that increasing the polymer concentration leads to the downfield movement of the chemical shift of the proton connected to the chiral center and the phenyl protons of HPEI-Phes according to the ¹H NMR.At the same polymer concentration,the proton connected to the chiral center shows a different chemical shift among HPEI-D-Phe_0.88,HPEI-L-Phe_0.88 and HPEI-DL-Phe_0.88 and the phenyl protons also show slight difference in chemical shift.These make the thermoresponsive behaviours of HPEI-Phes to be unusual.