In order to achieve a stable, long-term operation of flexible electronic devices, it is necessary to firmly fix semiconductive conjugated polymers to plastic substrates, thus preventing their damage against delamination or chemical treatments. Surface-initiated Kumada catalyst-transfer polycondensation of 2-bromo-5-chloromagnesio-3-alkylthiophene from photo-cross-linked poly(4-bromostyrene), PS(Br), films leads to covalent grafting of regioregular head-to-tail poly(3-hexylthiophene), P3HT. Herein, we investigate the grafting process in detail and elucidate the structure of the resulting composite films using ellipsometry, X-ray photelectron spectroscopy. Rutherford backscattering spectroscopy, and conductive mode atomic force microscopy techniques. In particular, we found that the grafting process is much more efficient if thick PS(Br) supporting layers are used. The maximal reachable thickness of the P3HT deposits is directly propotional to the thickness of the supporting PS(Br) layers. The obtained data suggest that the grafting process occurs not only at the PS(Br)/polymerization solution interface but also deeply inside the swollen PS(Br) films, penetrable for the catalyst and for the monomer. The process results into a kind of interpenetrated PS(Br)/P3HT network in which relatively short (similar to 10 nm) P3HT grafts emanate from long cross-linked PS(Br) chains. The films show good stability against delamination, high electrical conductivity in the doped state, and high swellability that might be exploited for construction of fully "plastic" electronic devices and sensors.