Polymethyl(alkoxy)siloxane copolymers, poly(MTES-co-TEOS), and poly(MTMS-co-TMOS), are prepared by acid-catalyzed controlled hydrolytic co-polycondensation of methyl(trialkoxy)silane MeSi(OR)(3) (R=Et (MTES) and Me (MTMS)) and tetra-alkoxysilane Si(OR)(4) (R=Et (TEOS) and Me (TMOS)), respectively. The products are purified by fractional precipitation to provide polymethyl(alkoxy)siloxane copolymers with molecular weight 1000-10,000 (poly(MTES-co-TEOS)) or 1700-100,000 (poly(MTMS-co-TMOS)) that are stable to self-condensation. These polymers are soluble in common organic solvents except for hexane, and form flexible and transparent free-standing films with a tensile strength of 4.0-10.0 MPa. The structure of the polymethyl(alkoxy)siloxane copolymers is thought to be a random or a block co-polymer. They are found to provide coating films with an adhesive strength up to 10, a refractive index of 1.36-1.40, and a dielectric constant of 3.5-3.6. The products also show better weathering stability than polyethoxysiloxane due to the hydrolytic polycondensation of TEOS. Field emission-scanning electron micrography analysis reveals that coating films are composed of a micro-phase separated structure. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4732-4741