We report the synthesis of well-defined poly(3,3'''-didodecyl-2,2':5',2 '':5 '',2'''-quaterthiophene) (PQT12) from a direct heteroarylation polymerization (DHAP) of 5-bromo-3,3'''-didodecyl-2,2':5',2 '':5 '',2'''-quaterthiophene (monomer A) and 5-bromo-3',4 ''-didodecyl-2,2':5',2 '':5 '',2'''-quaterthiophene (monomer B). Experiments with different catalysts, ligands, additives, and solvents have revealed that the utilization of Herrmann-Beller catalyst and P(o-NMe2Ph)(3) can lead to selective thiophene thiophene couplings. In this regard, solid-state optical and thermal measurements were particularly useful to detect the presence of beta-branching and indicate that minor molecular defects can induce important changes within the supramolecular organization. We also highlight the fact that steric protection around unsubstituted beta-positions of alpha-bromothiophene units is needed to obtain a good selectivity of the cross-couplings at the alpha-positions. This can be achieved by the presence of a substituent at an adjacent beta-position or the utilization of a bulky acidic additive (i.e., neodecanoic acid) in the catalytic system. These synthetic procedures applied to both monomers have led to PQT12 samples showing essentially the same optical and thermal properties and are comparable to those observed with their analogues prepared from chemical oxidation or Stine coupling.