Figure shows a representative constant current image of the dominant reaction product. Its building unit is clearly recognized as head-to-head d2, but the inter-unit coupling through the tails (methyl groups) is not trivial to determine even from high-resolution images with CO-functionalized probes in the repulsive interaction regime (Figure ), in part due to the non-planarity of the polymer carbon backbone within the reconstruction trenches. We have considered five different tail-to-tail linking motifs (Fig- ) and could determine the structure displayed in Figure as the actual product because it is the only one reproducing the electronic properties of the polymers that are described below. The two lobes of enhanced contrast at either side of the junction in constant current images (Fig. ) are thus assigned to the remaining sp methyl groups and their increased height, as occurred with the molecular structures on Au(111). with two examples (blue and red curves) obtained at the accordingly colored points in the inset (the pink curve is the reference spectrum on the substrate). In either case the spectra reveal broad resonances both for the occupied and unoccupied states. Conductance maps of occupied and unoccupied states are reminiscent of the HOMO and LUMO of d2 (Figure ), with a series of lobes along the molecular sides for the former and a large dominating signal at the position of the four-membered ring for the latter. The signal is additionally modulated by an asymmetry along the long molecular axis due to the alternating tilt of the different d2 units, whose sides nearest to the Au rows delimiting the trench are higher and thus appear with a stronger contrast.