The fact that residual bromo groups are largely absent in the polymers may explain why no higher molecular weight polymers were obtained. Stille coupling reactions of halogenated phenyl units are known to suffer from reduced reactivity compared to more electron-deficient units, increasing the probability of dehalogenation reactions. The polymers did not precipitate during the polymerization reaction, indicating that precipitation is not the limiting factor for the molecular weight in this reaction. When testing other Stille polymerization conditions for the synthesis of poly(PCT-T), similar molecular weights were obtained (see Supporting Information section 2.4). However, both poly(PCT-T) and poly(PCT-2T) clearly showed the macroscopic behavior expected for conjugated polymers, including when spin coating and precipitating the polymers. Both polymers were found to be amorphous in powder X-ray diffraction (PXRD) measurements as well as in grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements of spin-coated polymer thin films (see Supporting Information section 6). The thermal properties of the polymers were studied by TGA and DSC, and both polymers showed featureless DSC thermograms, which is indicative of amorphous conjugated polymers (see Figure ). affected by aggregation issues and not limited to low-mass samples, unlike GPC or MALDI-MS. With an Mn of 3.3 kDa and an Mw of 4.5 kDa (Table right, corresponding to a polydispersity index of 1.36), the actual molecular weight was found to be considerably higher than that indicated by both GPC and MALDI-MS. The difference likely relates to more difficult ionization and desorption of higher molecular weight polymers in the MALDI-MS measurement, and to differences in hydrodynamic volume for poly(PCT-T) versus the polystyrene standard for GPC. Furthermore, while the number-(Xn) and weight-average (Xw) degree of polymerization of the 331 polymer chains analyzed by ESD-STM was 6.8 and 9.2, respectively, polymer chains with up to 16 repeat units were observed in STM images. The STM-derived mass distribution is shown in Figure .