Sugar-bearing Block Copolymers Via ATRP: Synthesis And Self-Assembly Behavior

The sugar-bearing monomer 6-O-methacryloyl-1,2;3,4-di-O-isopropylidene-D- galactopyranose (MaIPG) was successfully synthesized in this work. Through atom transfer radical polymerization (ATRP), a series of amphiphilic block copolymers containing D-galactopyranose and different molecular weight PEG were prepared using PMDETA/CuBr as catalytic system and anisol as solvent. The chemical structure and composition of the resultant copolymers were verified by ~1H NMR and Infrared Spectroscopy (IR). The molecular weights and their polydispersities were characterized by gel permeation chromatography (GPC). The self-assembly behavior in aqueous solution was characterized by fluorescence spectrometer spectra (FS), Ultraviolet-visible spectrophotometer (UV-Vis) and transmission electron microscope (TEM). The results indicate that the polymerization follows the typical mechanism of ATRP.The macroinitiators were synthesized by the esterification between poly(ethylene glycol) with different number-average molecular weight and 2-bromoisobutyryl bromide. The amphiphilic block copolymers including AB-type diblock MPEG_m-b-PMaIPG_n and ABA-type triblock copolymers PMaIPG_n-b-PEG_m-b-PMaIPG_n were obtained via ATRP, which possess well-defined structure and narrow molecular weight distribution ( M_w/M_n<1.3).In selective solvent, these sugar-bearing amphiphilic block copolymers formed so-called"cut-crew"micelle structure with hydrophobic block PMaIPG as the core and hydrophilic block PEG loop chain as the corona. Ultraviolet-visible spectrophotometer (UV-Vis) and fluorescence spectrometer (FS) were employed for the measurements of critical water content (CWC) and critical micelle concentration (CMC). The CWC and CMC decrease with increasing the length of hydrophobic block PMaIPG. The higher solution concentration, the larger value of CWC and CMC, which indicates the solution of copolymer with high concentration was easier to form micelle. Under the same condition, AB-type copolymer had smaller value of CWC and CMC than ABA-type copolymer.The transmission electron microscope (TEM) can directly observe the morphologies of the aggregates. The micelle formed by the self-assembly behavior of sugar-bearing block copolymer were mainly spherical and vesicle. Along with the high ratio of chain length of PMaIPG block to PEG block, the morphology of micelle become vesicle with hollow structure and spherical when the ratio decreased to a certain degree, they were unable to form clear morphology. Compared with solution in low concentration, high concentrated solution was more likely to result in the occurance of sever adhesion phenomenon. AB-type block copolymer can form spherical and vesicle with clear cliff, while the ABA-type can only form unclear cliff under the same conditions.The absorbency of micelle formed by suger-bearing block copolymer was inspected using ultraviolet-visible spectrophotometer (UV-Vis) in different solutions. The micelle was sensitive to pH in inorganic acid solution, with decreasing of pH, micelle aggregated by hydrogen bonding interaction and the solution turned turbid. There was no obvious change of the micelle in low concentration of trifluoroacetic acid (TFA), while higher the concentration, isopropylidene, the protecting group of hydroxy was removed, the hydrophobic block PMaIPG became hydrophilic, the structure of micellar was destroyed, the solution turned transparent.