Porous poly(methyl silsesquioxane) (PMSSQ) films were prepared from PMSSQ/amphiphilic block copolymer (ABC) hybrids, and this was followed by spin coating and multistep baking. The ABCs were poly(styrene-block-acrylic acid) (PS-b-PAA) and poly(styrene-block-3-trimethoxysilylpropyl methacrylate) (PS-b-PMSMA), which were synthesized by living polymerization. The chemical bonding between the ABCs and PMSSQ resulted in significant differences in the morphologies and properties of the hybrids and their porous derivatives. Both intramolecular and intermolecular hydrogen bonding existed in the PMSSQ/PS-b-PAA hybrid and led to macrophase separation. Through the modification of the chemical structure from the poly(acrylic acid) segment to PMSMA, covalent bonding between PMSSQ and PMSMA occurred and prevented the macrophase separation and initial pyrolysis of the ABC. Modulated differential scanning calorimetry results also suggested a significant difference in the miscibility of the two hybrid systems. The chemical bonding resulted in higher retardation of the symmetry-to-nonsymmetry Si-O-Si structural transformation for PMSSQ/PS-b-PMSMA than for PMSSQ/PS-bPAA according to Fourier transform infrared studies. The pore size of the nanoporous thin film from the PMSSQ/PS-b-PMSMA hybrid was estimated by transmission electron microscopy to be less than 15 nm. The refractive index and dielectric constant of the prepared porous films decreased from 1.354 to 1.226 and from 2.603 to 1.843 as the PS-b-PMSMA loading increased from 0 to 50 wt %, respectively. This study suggests that chemical bonding in hybrid materials plays a significant role in the preparation of low-dielectric-constant nanoporous films. (C) 2004 Wiley Periodicals, Inc.