The Study In The Synthesis And Characteristics Of Double Hydrophilic Star-like Copolymer （PEO-b-PAA）₄and Its Micelle Behavior

Owing to the easy transition from hydrophilicity to hydrophobicity undercertain condition, which equips the molecular with good structure adjustability andvarious self-assembly behavior, double hydrophilic block copolymer (DHBCs) haveattracted widely attention in recent years. Poly(ethylene oxide)(PEO) with goodbiocompatibility is a kind of typical and most studied hydrophilic polymer.Polyacrylic acid (PAA), a kind of environment sensitive polymers, can transformfrom hydrophilic to hydrophobic by changing pH. Therefore, the study of theapplication of DHBCs, composed of PEO and PAA chain segment, in biologicalmedicine area has been reported in many references. Compared with linear polymer,star-like polymer with complex structure has better drug inducibility, which resultsto potential applications in drug delivery.In this paper, we plan to combine the advantages of DHBCs and star-likepolymer by designing a new star-like copolymer with PEO and PAA segment.Anionic ring-opening polymerization and atom transfer radical polymerization(ATRP) were utilized to synthesize the target macromolecular. The results are asfollows.1. We synthesized (PEO)4with expected molar mass by anionic ring-openingpolymerization of ethylene oxide. Its structure was characterized by nuclearmagnetic resonance spectra (NMR) and infrared spectra (IR). The molecular weightand molecular weight distribution were measured by gel permeation chromatography(GPC). Then we synthesized four-arm-star-like copolymers by grafting differentmolecular weight PAA chain to the arms of (PEO)4. NMR, IR and GPCmeasurements indicate that the molecular weight of each arm of (PEO-b-PAA)4iscontrollable and its distribution is narrow, which meets the requirement ofcontrollable polymerization.2. DHBCs can self-assemble into micelles in aqueous solutions of certain pH. Because of different human organs being almost in acid environment, we studied themicellar behavior of polymers in phosphate buffer from pH2to pH8. We observedthe micellar hydrodynamic radius (Rh), scattering light intensity (Is) andpolydispersity index (PDI) at different pH by dynamic light scattering. At pH=2～4.5, the micellar particle distribution was very narrow (PDI≤0.4) and Rhand Iswerealmost no change, which demonstrated stable and narrow distributed nano micellesform under this pH value. We also observed the uniform spherical micellarmorphology by transmission electron microscope. At pH=5～6.5, PDI increased to0.65and Rhdecreased, which indicated the original polymer micelles with narrowparticle distribution decomposed into smaller ones with broad particle distribution.At pH≥7, we almost could not observe micelles. The critical micelle concentration(CMC) of the copolymers were determined by fluorescent probe method. Adowntrend of CMC value along with the increase of PAA molecular weight wasfound. At last, drug release behavior of the micelle entrapping indomethacin wasprimary investigated.