Sterically Exposed Bromothiophene C–Br Bonds as Structural Warning Signs for Hidden Homocoupling in Migita–Stille-Derived DPP–DTE Polymers

Abstract

Here, we identify sterically exposed bromothiophene C–Br bonds in diketopyrrolopyrrole (DPP) monomers as monomer-level structural warning signs for hidden homocoupling defects in Migita–Stille-derived DPP–dithienylethene (DTE) polymers. High-temperature 1H NMR analysis revealed 11% DPP–DPP and 10% DTE–DTE linkages in a Stille-derived polymer represented by an alternating DPP–DTE repeat unit, whereas a locally hexyl-substituted structural comparator contained <1% and approximately 1%, respectively. Matched small-molecule Stille reactions reproduced the same substrate-dependent ordering in every catalyst system directly compared. Under Pd/P(o-tolyl)3 conditions, lowering the temperature decreased cross-coupling selectivity for the exposed substrate, but not the locally substituted analogue, thereby highlighting the elevated homocoupling susceptibility of exposed C–Br sites. Comparison of the homocoupling-rich Stille-derived polymer with a low-homocoupling Suzuki–Miyaura reference revealed distinct optical/electronic properties, thin-film organization, and transistor characteristics. These findings underscore the need to verify polymer linkage sequences experimentally rather than infer them from formally drawn repeat units or general expectations regarding coupling selectivity.