Suzuki–Miyaura catalyst-transfer polymerization: new mechanistic insights

Abstract

Controlled preparation of structurally precise complex conjugated polymer systems remains a major synthetic challenge still to be addressed, and this push is stimulated by the improved device performance as well as unique fundamental characteristics that the well-defined conjugated polymer materials possess. Catalyst-transfer polymerization (CTP) based on a Pd-catalyzed Suzuki–Miyaura cross-coupling reaction is currently one of the most promising methods towards achieving such a goal, especially with the recent implementation of N-methyliminodiacetic acid (MIDA) boronates as monomers in CTP. Further expansion and development of the practical applications of CTP methods will hinge on a clear mechanistic understanding of both the entire process and the particular steps involved in the catalytic cycle. In this work, we introduced Ag⁺-mediated Suzuki–Miyaura CTP and demonstrated that the presence of Ag⁺ shifted a key transmetalation step toward the oxo-Pd pathway, leading to the direct participation of MIDA-boronates in the transmetalation step and hence in the polymerization process, and resulting in the overall more efficient polymerization. In addition, we found that, under Ag⁺-mediated conditions, MIDA-boronates can also directly participate in small-molecule cross-coupling reactions. The direct participation of MIDA-boronates in Suzuki–Miyaura cross-coupling has not been envisaged previously and could enable new interesting possibilities to control this reaction both for small-molecule and macromolecular syntheses. In contrast to MIDA-boronates, boronic acid monomers likely undergo transmetalation through an alternative boronate pathway, although they may also be directed to react via the oxo-Pd transmetalation pathway in Ag⁺-mediated conditions. The interplay between the two transmetalation pathways, which are both involved in the catalyst-transfer polymerization, and the opportunity to selectively enhance one of them not only improves the mechanistic understanding of Suzuki–Miyaura CTP processes but also provides a previously unexplored possibility of gaining more effective control over the polymerization to obtain structurally better-defined conjugated polymers.