An Apparent Size-Exclusion Quantification Limit Reveals a Molecular Weight Limit in the Synthesis of Externally Initiated Polythiophenes

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 <i>o</i>-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 (<i>M</i>_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 <i>M</i>_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 <i>M</i>_n by NMR using this synthetic methodology at high molecular weights