We report the linear and nonlinear regions of the relationship between number average molecular weight determined by gel permeation chromatography (GPC) and 1H NMR end-group analysis for a series of o-tolyl-initiated poly(3-hexylthiophene)s (P3HTs). For conjugated polymers with chains that are 39–138 repeat units in length (6.5–23 kDa), GPC systematically overestimates the number average molecular weight (M n) by a factor of 1.3 ± 0.1 (standard error), and GPC and 1H NMR end-group analysis correlate in a linear manner. For chains 138–1130 (23–188 kDa) repeat units in length, we observe a nonlinear relationship between GPC and end-group analysis. Static light-scattering experiments confirm that at high molecular weight (>70 kDa) decreasing the catalyst loading does not appreciably increase the polymer chain length. Thus, we conclude that there is a molecular weight limit in the synthesis of externally initiated polythiophenes and a propensity for the growth of nonexternally initiated chains which increases as a function of M n. This is significant as external initiation has been reported to result in nearly 100% externally initiated chains as well as reduce the possibility of chain–chain coupling in a typical synthesis. Our data show that 100% external initiation only holds true for polymers that are less than 40 kDa and encourages caution when determining M n by NMR using this synthetic methodology at high molecular weights.