New low-temperature curable organic/inorganic hybrid polymers were designed and synthesized as gate dielectrics for organic thin-film transistors (OTFTs). Allyl alcohols were introduced to polyhedral oligomeric silsesquioxane (POSS) via hydrosilyation to produce an alcohol-functionalized POSS derivative (POSS-OH). POSS-OH was then reacted with hexamethoxymethylmelamine at carrying molar ratios at 80 °C in the presence of a catalytic amount of p-toluenesulfonic acid to give highly cross-linked network polymers (POSS-MM). The prepared thin films were smooth and hard after the thermal cross-linking reaction and had very low leakage currents (<10−8 A/cm2) with no significant absorption over the visible spectral range. Pentacene-based OTFTs using the synthesized insulators as gate dielectric layers had higher hole mobilities (up to 0.36 cm2/Vs) than a device using thermally cross-linked poly(vinyl phenol) and melamine as the gate dielectric layer (0.18 cm2/Vs). © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014