Synthesis of pH-responsive and thiol-degradable hollow microspheres

Abstract

Hollow microspheres were synthesized using an oil in water suspension system. The oil droplets were comprised of ethylene glycol dimethacrylate (EGDMA) and n-butyl acetate. Upon radical polymerization and gel formation, the polymer phase-separated from the solution and precipitated on the interface between water and organic solvent. The solvent n-butyl acetate was found to be crucial in the process as other solvent did not lead to hollow particles. The ratio between EGDMA and n-butyl acetate determined the structure of the microspheres with low EGDMA amounts leading to solid particles while increasing amounts of EGDMA results in bigger particles with larger shells. The crosslinking density of the microspheres could be adjusted by copolymerization of EGDMA with methyl methacrylate (MMA) or tert-butyl methacrylate (tBuMA). The latter allowed easy hydrolysis under acidic conditions to create pH-responsive hollow microspheres with methacrylic acid repeating units (MAA). Depending on the crosslinking density, the microsphere increased in size from around 75 μm in acidic conditions to more than 150 μm at high pH value creating hollow spheres that expand and collapse depending on the pH value. Replacing the stable EGDMA crosslinker with the cleavable crosslinker bis(2-methacryloyloxyethyl) disulfide (DSDMA) leads to the formation of hollow spheres held together by disulfide bridges. Degradation of hydrophobic (with MMA as comonomer) and hydrophilic (with MAA as comonomers) microspheres were tested in a reductive environment, which was simulated by the addition of thiols. While the hydrophilic hollow microspheres had a weight loss of more than 50–70% after 3 days, only 20 wt% of the hydrophobic microspheres were lost after one week.