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The use of polythiophenes as conducting polymers has attracted a great deal of interest for applications in many organic electronic devices, such as field-effect transistors (FETs), thin-film transistor (TFT) circuits, sensors, electrochemical capacitors, light emitting diodes (LEDs) and, more importantly, polymer photovoltaics for electrical energy production.[1] The optical and electronic properties of these polymers are very sensitive to conformation and morphology changes. High purity, regioregularity and molecular weight are crucial for best electrical performance. Poly(3‑alkylthiophenes) (P3AT) are the most used polymers in organic photovoltaics because of their numerous electronics properties.[2] Several methods have been reported for the synthesis of highly regioregular P3AT.[3]^,[4],[5] The Grignard metathesis method (GRIM) remains the most convenient and efficient. Recently, the controlled chain-growth polymerization of P3HT (H = hexyl) from an external halobenzene initiator was reported independently by the groups of Kiriy[6]  and Luscombe.[7]

The initial goal of this project was the development of (NHC)-Pd systems for the polymerization of thiophenes using Suzuki-Miyaura cross-coupling reactions.[8] We wanted to address two main limitations of the GRIM process: low functional group tolerance and the need of preparing the Grignard reagent in situ, which requires the stoichiometry of the reaction to be precisely controlled in order to achieve high molar mass material. To obtain a preliminary confirmation of the positive effect of the use of NHC as ligands for the synthesis of polythiophenes, we decided to test first our premise in a GRIM (Ni-catalyzed) polymerization. In our previous report, we described how by modifying Kiriy's conditions⁶ using N-heterocyclic carbene ligands (NHC)[9] instead of the classical phosphines, we had been able to prepare high molecular weight P3HT with one of the lowest values of polydispersity reported to date for this molecular weight range (1.09). Our optimized system makes use of Ph‑Ni(PPh₃)₂‑Br as the pre-catalyst (2.5 mol%) in the presence of a small excess of the imidazolium salt 1,3‑bis(2,6‑diisopropylphenyl)imidazol-2‑ylidene chloride (IPr·HCl) (precursor of the NHC) at 0 °C for 3 hours (Scheme 1). Our following step was to carry out experiments to determine the catalytic active species, with the goal of designing a well-defined NHC-bearing complex to act as a pre-catalyst for these polymerizations. We were also interested in synthesizing chlorothiophene monomers to attempt the first polymerization of that type of substrates.

Scheme  SEQ Scheme \* ARABIC 1. Thiophene polymerization using an (NHC)-Ni catalyst

Technical problems with our gel permeation system (GPC-120, Agilent –former Polymer Labs), currently out of service, have prevented us from a more exhaustive look into our polymerization systems, and we have focused our efforts on corroborating the results of our optimized system. In collaboration with Prof. Michael Turner from The University of Manchester, UK, whose group performed GPC characterization of new samples, we have been able to at least make sure that our initial results are reproducible and not false positives due to our malfunctioning GPC. This collaboration with Prof. Turner has also derived in a review recently accepted for publication in Current Organic Chemistry, and in which support from the ACS-PRF is acknowledged.

These preliminary results have also provided us the basis for a full proposal that will be submitted this fall to the National Science Foundation in order to address all the goals that we wanted to achieve with this ACS-PRF grant and to expand these couplings to a variety of substrates.