The sequencing of a large set of polymers also allowed us to analyze the polymer ends, showing that in 84% of cases the polymers are terminated by a thiophene unit, and only in 16% by a PCT comonomer. Moreover, a similar statistical analysis of the polymer sequences demonstrated that, at variance with other cases of cross-coupling polymerizations, the synthetic approach used here for making poly(PCT-T) is highly defect-free, as only extremely rare examples of homocoupling defects were observed. Thiophene homocouplings (Figure and 4b) accounted for 0.2% of all measured bonds (over a total of 2585), while only a single case of PCT-PCT coupling was observed overall, corresponding to a relative frequency < 0.05% (Figure and). In this latter case, the two PCT macrocycles forming the defect appear distorted in the STM images, which is possibly a sign of a spatial deformation due to steric hindrance upon adsorption. Interestingly, a small percentage of polymer chains were seen to close over themselves (Figure ). While STM cannot directly prove the cyclization, the fact that these cycles are adsorbed across the herringbone reconstruction and are not seen to move under the STM probe during scanning suggests the presence of a strong intramolecular bond. These polymers or cycles are composed of 7 to 15 repeat units, displaying a whole distribution of values.