Synthesis and Characterization of Monodisperse Sulfonic Acid-Functionalized Hollow Polymer Microspheres Using Precipitation Polymerization as Recyclable Solid Acid Catalysts.

Monodisperse crosslinked sulfonic acid-functionalized hollow polymer microspheres were prepared via the chemical cleavage of core-shell polymer microspheres containing a cleavable core. First, the core-shell polymer microspheres were synthesized through the precipitation polymerization of styrene and a styrene dimer bearing two phenylsulfonate groups, followed by seed polymerization of N,N'-1,3-phenylenedimaleimide. The effects of polymerization parameters on the yield, particle size, size distribution, and shell thickness were investigated. Smaller microspheres with thicker shells were obtained at lower monomer concentrations. The resulting core-shell polymer microspheres were then treated with an aqueous KOH solution, followed by solvent washing to remove the core polymer. Subsequent acid treatment regenerated the sulfonic acid groups, yielding sulfonic acid-functionalized hollow polymer microspheres. The sulfonic acid-functionalized hollow polymer microspheres were evaluated as solid acid catalysts for the esterification reaction of 2-phenylethanol with acetic acid. Compared with uniform-type polymer microspheres and commercially available solid acid resin, the hollow polymer microspheres exhibited higher reactivity. In addition, they could be readily recovered from the reaction mixture by simple centrifugation. The recovered catalysts were reused for five consecutive cycles without any significant loss of activity.