Letter Cite This: ACS Macro Lett. 2018, 7, 767−771
pubs.acs.org/macroletters
Dual-Catalytic Ag−Pd System for Direct Arylation Polymerization to Synthesize Poly(3-hexylthiophene) Jason Albert Lee† and Christine K. Luscombe*,†,‡ †
Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, United States Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195-2120, United States
‡
S Supporting Information *
ABSTRACT: Direct arylation polymerization (DArP) has gained interest in materials chemistry as a method to synthesize conjugated polymers with minimal use of harsh reagents and additional steps. Traditional DArP conditions do not readily yield ideal polymerization characteristics, including chain-growth and low dispersities. It would be of great utility to advance DArP methodology to become competitive with traditional conjugated polymerization techniques. We have developed conditions for a dual-catalytic Ag−Pd system for the synthesis of poly(3-hexylthiophene) (P3HT) that exhibits chaingrowth kinetics, low dispersities, and catalyst chain association by Pd. Specifically, the presence of Ag-carboxylate additives plays a beneficial role in the polymerization as a C−H activating agent, while PEPPSI-iPr is used as the Pd source for C−C coupling. The addition of pyridine is necessary to inhibit Pd-mediated C−H activation in the interest of catalyst orthogonality, which can lower dispersities.
D
Scheme 1. General Reaction Conditions for Reported P3HT Syntheses and Dual-Catalytic DArP (This Work)
irect arylation polymerization (DArP) has gained increasing popularity as a methodology for the synthesis of π-conjugated polymers.1−5 The ability to directly and selectively activate the typically inert C−H bond in conjugated monomers allows for novel atom-economical polymerizations, eliminating prefunctionalization of monomers with activating groups that are lost as waste via traditional C−C coupling or condensation reactions. Current syntheses of conjugated polymers include C−C coupling reactions such as Stille, Suzuki, and Kumada-type.6−9 These methods require additional preparative steps to synthesize the active monomer, which may require harsh conditions, extensive purifications, and stoichiometric amounts of organometallic reagents, contributing to synthetic complexity and increased cost. Bypassing these undesirable processes aid in lowering both the environmental footprint and production cost to allow implementation of these materials in research and industrial scales to be more amenable.10,11 DArP has been demonstrated on a variety of heterocyclic monomers and comonomers, but there still remain areas to improve the methodology. DArP was built upon seminal direct arylation (DA) studies12,13 intended for small molecule crosscoupling. As such, important polymerization characteristics such as chain-growth kinetics, low dispersity, and molecular weight control remain challenges with few DArP reports attempting to achieve them.14 This arises from the mechanistic understanding of DA reactions, known as concerted metalationdeprotonation (CMD), which yields less desirable stepgrowth.15 One-pot conditions whereby chain-growth and low dispersities could be achieved would advance the utility of DArP, which is our aim in this work (Scheme 1). © XXXX American Chemical Society
To overcome the limitations of CMD, namely step-growth leading to high dispersities, we propose using a dual-catalytic system of Ag and Pd (Scheme 2). Pd is widely used throughout DA literature and Ag has been used as an additive to promote C−H activation.16−19 Studies by Larossa and Sanders have suggested that Ag is involved in the rate-limiting C−H activation of electron deficient substrates, which can then Received: June 5, 2018 Accepted: June 6, 2018
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DOI: 10.1021/acsmacrolett.8b00429 ACS Macro Lett. 2018, 7, 767−771
Letter
ACS Macro Letters Scheme 2. Proposed Dual-Catalytic DArP Cycles for P3HT Synthesisa
a
In this work, My is Ag and Mx is Pd.
undergo transmetalation by Pd.20,21 We chose to develop conditions for the synthesis of poly(3-hexylthiophene) (P3HT), a ubiquitous polymer in the field of organic electronic, which has been already well-characterized. Electron-deficient thiophenes, among other heterocyclic aromatic monomers, have been shown to undergo C−H activation via coinage metals.22,23 By incorporating both Ag for C−H activation and Pd for transmetalation and subsequent C−C coupling, we are able to show chain-growth kinetics with low dispersities (
