Base-catalyzed hydrolytic polycondensation of trialkoxymethylsilane was investigated to synthesize polymethylsilsesquioxanes (PMSs). The reaction of trimethoxy(methyl)silane and triethoxy(methyl)silane with tetramethylammonium hydroxide, tetrabutylammonium hydroxide, and also coline gave insoluble gels. Polymethylsilsesquioxane (PMS-IP) was obtained by the reaction of triisopropoxy(methyl)silane (MTIPS) with tetrabutylammonium hydroxide as a catalyst. PMS-IP was composed primarily of T 2 and T 3 units. The percentage of T 3 units and the molecular weight of PMS-IP increased with increases in the molar ratios of catalyst and water to MTIPS and with the reaction time. PMS-IP was soluble in organic solvents, except for methanol, and was separated by extraction with hexane and methanol into low- and high-molecular-weight fractions of M-W 2800-4000 and 7300-88,300, respectively. PMS-IP coating films were prepared by dip coating on the organic, inorganic, and metal substrates, using, the acetone-isopropyl alcohol solution of PMS-IP. Since PMS-IP solutions prepared with tetrabutylammonium hydroxide were hardly used because of the low content of hydroxy groups in the polymer, they showed low adhesion when compared with those solutions prepared with hydrochloric acid. The dielectric constant of the coating film prepared from the high-molecular-weight PMS-IP was 2.6. (c) 2005 Wiley Periodicals, Inc.