MIDA Boronate Monomers in Suzuki–Miyaura Catalyst-Transfer Polymerization: The Pros and Cons

Controlled polymerization for the synthesis of structurally precise conjugated polymers remains a challenging problem in polymer chemistry. Catalyst-transfer polymerization (CTP) based on Pd-catalyzed Suzuki–Miyaura cross-coupling is one of the promising approaches toward solving this challenge. Recent introduction of N-methyliminodiacetic acid (MIDA) boronates as monomers for Suzuki–Miyaura CTP has extended this approach toward a broader variety of monomer structures and led to improved control over the polymerization, particularly for heteroaromatic systems (such as thiophenes). Previously, we found that MIDA boronate monomer polymerization could be facilitated by Ag+-mediated reaction conditions due to shifting the Pd catalytic cycle toward a more efficient oxo-Pd transmetalation pathway where MIDA boronates could participate in transmetalation directly without prior hydrolysis to boronic acid. In this work, we continued studying this novel process and investigated the dual role of the MIDA boronate functional group in the case of less reactive fluorenyl (and potentially other all-carbon aromatic systems) monomers. With such monomers, the MIDA boronate group enables the controlled polymerization but also produces a hydrolysis byproduct, hindering the polymerization. We also investigated the role of Ag+ in counteracting this hindering effect. Steric bulkiness of the MIDA boronate functional group may also slow down the Suzuki–Miyaura CTP process. These complications could reduce the synthetic value of MIDA boronate monomers in Suzuki–Miyaura CTP, although better understanding of these implications and a proper choice of polymerization conditions and catalytic initiators could to some extent mitigate such problems. As part of this work, we also uncovered a "critical length" phenomenon, which results in a dual molecular weight distribution of the resulting conjugated polymer, both with MIDA boronate and boronic acid monomers. This phenomenon could account for the experimentally observed loss of polymerization control beyond formation of the polymer chains of a certain "critical length", even despite the formally "living" nature of the polymer chains. The generality of this phenomenon and whether it is restricted to using Pd catalytic systems based on Buchwald-type phosphine ligands remain to be studied. Overall, these new findings paint a sophisticated picture of the Suzuki–Miyaura CTP process with MIDA boronate monomers where the mere presence of a Pd center on the polymer chain is not sufficient to sustain the polymerization (even if a chain could be considered "living" in the sense of possessing a Pd center), and the choice of the phosphine ligand on the Pd center is an effective tool to overcome the "critical length" restriction.