Understanding Broad Polydispersity in Step-growth Polymerization via Catalyst Competition Mechanism

Conjugated polymers are mainly synthesized by cross-coupling polymerizations catalyzed with transition metal (Pd, Ni) catalysts through step-growth polymerization (SGP) mechanism. According to the classical theory of SGP, the polymer dispersion index (Ð) of the synthesized polymers will never be higher than 2. However, the cases where conjugated polymers synthesized with Ð value far exceeding 2 are very common in reality, which severely limits their processing property, performance and applications. To investigate the reason behind the Ð value deviation from the theoretical value of SGP, direct arylation polycondensation (DArP) of 2-bromo-3-hexylthiophene (3HT) was chosen as the model reaction, and the reaction process was tracked using gel permeation chromatography analysis. When Pd(OAc)₂ was used as the catalyst, the Ð value linearly increased with the increase of the weight-average molecular weight (M_w) of polymer (P3HT) after a short period and reached up to 7.2 at prolonged reaction time. Scanning transmission electron microscopic images of the reaction mixture showed the fibril-like aggregation of P3HT and assembling of Pd species in P3HT aggregates. A catalyst competition mechanism was thus proposed, together with numerical calculation, giving a good fitting to the experimental results, which is believed to have far-reaching significance for guiding the design, synthesis and processing of conjugated polymers.