Precision Synthesis of Conjugated Polymers Using the Kumada Methodology

Since the discovery of conductive poly(acetylene), the study of conjugated polymers has remained an active and interdisciplinary frontier between polymer chemistry, polymer physics, computation, and device engineering. One of the ultimate goals of polymer science is to reliably synthesize structures, similar to small molecule synthesis. Kumada catalyst-transfer polymerization (KCTP) is a powerful tool for synthesizing conjugated polymers with predictable molecular weights, narrow dispersities, specific end groups, and complex backbone architectures. However, expanding the monomer scope beyond the well-studied 3-alkylthiophenes to include electron-deficient and complex heterocycles has been difficult. Revisiting the successful applications of KCTP can help us gain new insight into the CTP mechanisms and thus inspire breakthroughs in the controlled polymerization of challenging π-conjugated monomers.In this Account, we highlight our efforts over the past decade to achieve controlled synthesis of homopolymers (p-type and n-type), copolymers (diblock and statistical), and monodisperse high oligomers. We first give a brief introduction of the mechanism and state-of-the-art of KCTP. Since the extent of polymerization control is determined by steric and electronic effects of both the catalyst and monomer, the polymerization can be optimized by modifying monomer and catalyst structures, as well as finding a well-matched monomer-catalyst system. We discuss the effects of side-chain steric hindrance and halogens in the context of heavy atom substituted monomers. By moving the side-chain branch point one carbon atom away from the heterocycle to alleviate steric crowding and stabilize the catalyst resting state, we were able to successfully control the polymerization of new tellurophene monomers. Inspired by innocent role of the sterically encumbered 2-transmetalated 3-alkylthiophene monomer, we introduce the treatment of hygroscopic monomers with a bulky Grignard compound as a water-scavenger for the improved synthesis of water-soluble conjugated polymers. For challenging electron-deficient monomers, we discuss the design of new Ni(II)diimine catalysts with electron-donating character which enhance the stability of the association complex between the catalyst and the growing polymer chain, resulting in the quasi-living synthesis of n-type polymers. Beyond n-type homopolymers, the Ni(II)diimine catalysts are also capable of producing electron-rich and electron-deficient diblock and statistical copolymers. We discuss how density functional theory (DFT) calculations elucidate the role of catalyst steric and electronic effects in controlling the synthesis of π-conjugated polymers. Moreover, we demonstrate the synthesis of monodisperse high oligomers by temperature cycling, which takes full advantage of the unique character of KCTP in that it proceeds through distinct intermediates that are not reactive. The insight we gained thus far leads to the first example of isolated living conjugated polymer chains prepared by a standard KCTP procedure, with general applicability to different monomers and catalytic systems. In summarizing a decade of innovation in KCTP, we hope this Account will inspire future development in the field to overcome key challenges including the controlled synthesis of electron-deficient heterocycles, complex and high-performance systems, and degradable and recyclable materials as well as cutting-edge catalyst design.

Isolation of Living Conjugated Polymer Chains

Living polymerizations currently play a central role in polymer chemistry. However, one feature of these polymerizations is often overlooked, namely, the isolation of living polymer chains. Herein we report the isolation of living π-conjugated polymer chains, synthesized by catalyst-transfer polycondensation. Successful preservation of the nickel complex at polymer chain ends is evidenced by nuclear magnetic resonance spectroscopy, end group analysis, and chain extension experiments. When characterizing living chains by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, we discovered a unique photoionization-photodissociation fragmentation process for polymers containing a nickel phosphine end group. Living chains are isolated for several types of conjugated polymers as well as discrete living oligomers. Additionally, we are able to recycle the catalysts from the isolated polymer chains. Catalyst recycling after π-conjugated polymerization has previously been impossible without chain isolation. This strategy not only exhibits general applicability to different monomers but also has far-reaching potential for other catalytic systems.

The role of halogens in the catalyst transfer polycondensation for π-conjugated polymers

Catalyst transfer polycondensation is the only method to prepare π-conjugated polymers in a chain-growth manner, yet several aspects that underlie this polymerization are not fully understood. Here, we investigate the nickel-catalyzed polymerization mechanisms of a series of thiophene monomers bearing different halogen functionalities (Cl, Br, I). We have discovered the significant role that halogens and magnesium salts play in this polymerization. More specifically, the catalyst resting state changes depending on the type of halogenated monomer. For chlorinated monomers a mixture of Ni(ii)-dithienyl and dissociated Ni(phosphine) complexes are the resting states, which results in uncontrolled polymerization. For brominated monomers, a Ni(ii)-dithienyl complex is the resting state, which leads to controlled polymerization. For iodinated monomers, a Ni(ii)-thienyl iodide complex is the resting state, and notable inhibition by magnesium salt by-products is observed. The catalyst resting state changes to a Ni(ii)-dithienyl complex when a turbo Grignard reagent (i-PrMgCl·LiCl) is used. These findings are used to guide the design of a new monomer, 2-bromo-3-(2-ethylhexyl)-5-iodotellurophene, which enables the first controlled polymerization of a tellurophene monomer containing a sterically encumbered 2-ethylhexyl side chain. These insights are crucial for deepening the mechanistic understanding of Kumada cross coupling reactions and the controlled synthesis of π-conjugated polymers.