Based on density functional theory calculations, we have studied the self-assembled growth of thiophene substituted alkenes, [H2C═CH-(CH2)n-thiophene] on hydrogen-terminated H-Si(100)2×1 and H-Ge(100)2×1 surfaces into aligned one-dimensional (1D) molecular arrays which are chemically bonded to the surfaces via the alkane chain. The thiophene rings at the top end of the molecular arrays are situated side by side and can undergo an in situ polymerization reaction into polythiophene once radicals are introduced to the thiophene rings, thereby forming polyalkylthiophene-Si/Ge(100)2×1 surface-grafted polymers. Like most of conductive polymers, these surface single polymer chains exhibit semiconducting character and can be made conductive either by p-doping or by applying an external electric field. More importantly, both surface-grafted polymers and substrates retain their electrical properties, and the polythiophene chains are the sole conductive channels in the structures. Our findings put forth a new way to fabricate conductive polymeric molecular wires on traditional semiconducting substrates, and could find potential application in nanoelectronic devices.