Novel, randomly coupled, soluble, segmented polyimide-polydimethylsiloxane (PI-PDMS) copolymers were prepared from aminoalkyl-terminated polydimethylsiloxane (At-PDMS), 4,4'-oxydianiline diamine, pyromellitic dianhydride, and 4,4'-diphenylmethane diisocyanate (MDI). When At-PDMS was introduced into the polyimide chain, the polyimide copolymers exhibited lower dielectric constants and better moisture resistance and mechanical properties. The reductions in the dielectric constant of the PI-PDMS copolymers Could be attributed to the incorporation of polydimethylsiloxane (PDMS) into the polyimide chain and the nanopores in the film generated by carbon dioxide evolvement during the reaction. The lowest dielectric constant was 2.58 with 25 wt % PDMS and 5 wt % MDI. In addition, the water contact angles of the resultant copolymers increased from 51 to 109 degrees when the contents of PDMS increased from 0 to 25 wt %. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 1633-1641, 2009