Sulfur-containing aromatic silsesquioxane melting gels: network structure, thermal behavior, and post-gelation reactivity

Sulfur-containing substituents offer the possibility of modulating the structure, thermal behavior, and reactivity of silsesquioxane-based melting gels. Here, the first sulfur-containing aromatic reversibly softening silsesquioxanes (so-called melting gels) were prepared through the acid-catalyzed hydrolysis-condensation of 2-thienyltrimethoxysilane and 4-methylthiophenyltrimethoxysilane, yielding the transparent, glass-like oligosilsesquioxanes 2ThMG and 4MeSPhMG. These materials provide the first direct experimental comparison of how heteroatom-containing aromatic substituents (compact thienyl versus the more sterically bulky 4-methylthiophenyl) affect the condensation efficiency, the stabilization of silanol groups, and local network organization, as compared to previously described phenyl melting gels. Both materials retain the characteristic reversible softening behavior of melting gels, with softening temperatures of 32 and 58 °C, respectively. Solid-state ²⁹Si NMR, FTIR, SEC, MALDI-TOF, and X-ray powder diffraction reveal defect-rich, locally connected networks containing residual methoxy and silanol groups. The compact thienyl substituent allows for substantially higher degree of condensation (DC = 77%) than the sterically more demanding 4-methylthiophenyl group (DC = 67%), demonstrating a pronounced substituent effect on network formation and residual-group retention. Thermal treatment at 200 °C promotes further condensation and converts both materials into transparent thermosetting resins that retain their aromatic functionalities and remain thermally stable until temperatures exceed 380 °C in oxygen-containing atmospheres. The 4-methylthio group remains chemically addressable after gelation, undergoing partial oxidation without destruction of the siloxane backbone. Collectively, the results establish structure-property relationships that link the steric and electronic character of the aromatic substituent to residual silanol stabilization, softening temperature, and thermal decomposition pathways.