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

Controlled\npolymerization for the synthesis of structurally precise\nconjugated polymers remains a challenging problem in polymer chemistry.\nCatalyst-transfer polymerization (CTP) based on Pd-catalyzed Suzuki–Miyaura\ncross-coupling is one of the promising approaches toward solving this\nchallenge. Recent introduction of <i>N</i>-methyliminodiacetic\nacid (MIDA) boronates as monomers for Suzuki–Miyaura CTP has\nextended this approach toward a broader variety of monomer structures\nand led to improved control over the polymerization, particularly\nfor heteroaromatic systems (such as thiophenes). Previously, we found\nthat MIDA boronate monomer polymerization could be facilitated by\nAg<sup>+</sup>-mediated reaction conditions due to shifting the Pd\ncatalytic cycle toward a more efficient oxo-Pd transmetalation pathway\nwhere MIDA boronates could participate in transmetalation directly\nwithout prior hydrolysis to boronic acid. In this work, we continued\nstudying this novel process and investigated the dual role of the\nMIDA boronate functional group in the case of less reactive fluorenyl\n(and potentially other all-carbon aromatic systems) monomers. With\nsuch monomers, the MIDA boronate group enables the controlled polymerization\nbut also produces a hydrolysis byproduct, hindering the polymerization.\nWe also investigated the role of Ag<sup>+</sup> in counteracting this\nhindering effect. Steric bulkiness of the MIDA boronate functional\ngroup may also slow down the Suzuki–Miyaura CTP process. These\ncomplications could reduce the synthetic value of MIDA boronate monomers\nin Suzuki–Miyaura CTP, although better understanding of these\nimplications and a proper choice of polymerization conditions and\ncatalytic initiators could to some extent mitigate such problems.\nAs part of this work, we also uncovered a “critical length”\nphenomenon, which results in a dual molecular weight distribution\nof the resulting conjugated polymer, both with MIDA boronate and boronic\nacid monomers. This phenomenon could account for the experimentally\nobserved loss of polymerization control beyond formation of the polymer\nchains of a certain “critical length”, even despite\nthe formally “living” nature of the polymer chains.\nThe generality of this phenomenon and whether it is restricted to\nusing Pd catalytic systems based on Buchwald-type phosphine ligands\nremain to be studied. Overall, these new findings paint a sophisticated\npicture of the Suzuki–Miyaura CTP process with MIDA boronate\nmonomers where the mere presence of a Pd center on the polymer chain\nis not sufficient to sustain the polymerization (even if a chain could\nbe considered “living” in the sense of possessing a\nPd center), and the choice of the phosphine ligand on the Pd center\nis an effective tool to overcome the “critical length”\nrestriction.