Study On Synthesis And Performance Of CO₂ Responsive LCST Hyperbranched Polymer

Due to their controlled environmental responsiveness,the stimuli-responsive polymers are widely studied and applied in the various fields.These applications have greatly promoted the development of stimuli-responsive polymer-based advanced functional materials and meanwhile put forward higher demands on their intelligent performance.Because of structural constraints,linear functional polymers cannot satisfy all requirements of multidimensional molecular design and sensitivity.Hyperbranched polymers synthesized by simply one-step polymerization reactions have a series of excellent physical and chemical properties,such as a large amount of modified terminal groups,low viscosity,high rheological properties and good solubility,and so on.Therefore,stimuli-responsive hyperbranched polymers have been attracting more and more attention.In recent ten years,with the improvement of polymerization methods and the emergence of various novel stimuli-responsive groups,how to combine the external stimulus and the structure of polymers to synthesize stimuli-responsive hyperbranched polymers to meet the needs of human has become a hotspot at home and abroad.Therefore,this paper would design and synthesize a novel LCST-type hyperbranched polymer with sharp CO2-responsive behaviors,and study its properties.The main works were summarized as follows:Using cystamine dihydrochloride and poly(ethylene glycol)diglycidyl ether(PEGDGE,Mn=394)as raw materials,the target polymer of the hyperbranched poly(oligo(ethylene glycol)(HBPOEG)was prepared by the two-step epoxy ring-opening polymerization.The chemical structures of prebranched monomer and hyperbranched polymer HBPOEG were characterized by 1H-NMR and FT-IR.We used the calibration experiment of coumarin to determine the average branching number and unreacted epoxide number in every HBPOEG were 14 and 15,respectively.A series of tests on hyperbranched polymer solution were carried out by ultraviolet-visible spectrophotometer(UV-vis)and dynamic light scattering(DLS)to study the phase transition behaviors of HBPOEG in aqueous solution.The results indicated that HBPOEG in aqueous solution had concentration-dependant phase transition behaviors with excellent repeatability upon heating and cooling.Moreover,LCST of HBPOEG could be tuned by bubbling CO2 into the system.In addition,using the 1H-NMR spectra to track the hyperbranched polymer aqueous solution before and after bubbling CO2,1H-NMR analysis further elucidated the mechanism of the CO2 tunable LCST behaviour of HBPOEG in aqueous solution.The results indicated that the tertiary amines in HBPOEG could be protonated and the solubility of HBPOEG in aqueous solution increased by bubbling CO2 into the system,resulting in an increase of LCST of the hyperbranched polymer;Further the removal of CO2 in the system by bubbling N2 could return the LCST of HBPOEG to its original value reversibly.The epoxy values of hyperbranched polymer under different conditions were determined by hydrochloric acid-acetone titration to evaluate the stability of HBPOEG in the stimuli-responsive experiments.It turned out that the epoxy terminal groups might be stable in neutral aqueous solution,but could be hydrolyzed and ring-opened in CO2-rich water.This backbone-based hyperbranched LCST polymer with both CO2 and temperature responsiveness can be applied in application areas like drug delivery,gene transfection,functional coatings,etc.