Green Synthesis of Soluble Polysilsesquioxane with Phthalimide Groups

Soluble polysilsesquioxane containing side-chain phthalimide groups (PSQ-PhI) was synthesized via a solvent- and catalyst-free hydrolytic polycondensation reaction using 2-[3-(triethoxysilyl)propyl]-1H-isoindole-1,3(2H)-dione. The composition and structure of polysilsesquioxane was confirmed via ¹H, ¹³C, and ²⁹Si NMR spectroscopy, Fourier transform infrared spectroscopy, gel permeation chromatography, thermogravimetric analysis, dynamic light scattering, X-ray diffraction analysis, and elemental analysis. The synthesized silsesquioxane showed a monomodal molecular weight distribution. The average molecular weight of polysilsesquioxane is 11,200 Da, and the polydispersity index is 1.10. ²⁹Si NMR analysis showed a half-peak width w_1/2 3.1 ppm at δ -68.3, which corresponds to the PhI(CH₂)₃SiO_3/2 unit and indicates an ordered structure in the polymer, with some defects caused by the presence of uncondensed silanol groups. PSQ-PhI showed good thermal stability (T_d5% decomposition at 345 °C). The polysilsesquioxane-based coating was transparent in the visible region.