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Controlled preparation of structurally precise complex conjugated polymer systems remains to be 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 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 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 presence of Ag+ shifted a key transmetalation step toward the oxo-Pd pathway, leading to 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 mechanistic understanding of Suzuki-Miyaura CTP process but also provides a previously unexplored possibility to gain more effective control over the polymerization to obtain structurally better-defined conjugated polymers.
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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 towards 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 monomers 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, MIDA-boronate group enables the controlled polymerization but also produces a hydrolysis byproduct hindering the polymerization. We also investigated the role of Ag+ acting to counteract 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 remains 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 a sense of possessing a Pd center), and the choice of phosphine ligand on the Pd center is an effective tool to overcome the "critical length" restriction.
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- PAR ID:
- 10549952
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- Macromolecules
- Volume:
- 57
- Issue:
- 16
- ISSN:
- 0024-9297
- Page Range / eLocation ID:
- 7847-7861
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acs.macromol.4c01303
