Role of the transition metal in Grignard metathesis polymerization (GRIM) of 3-hexylthiophene

Mahesh P Bhatt,Harsha D Magurudeniya,Mihaela C Stefan,Benjamin G Janesko,Richard D Mccullough,Malika Jeffries-El,Elena E Sheina,Prakash Sista

doi:10.1039/c3ta13258g

Abstract

Regioregular poly(3-alkylthiophene)s are widely used in organic electronics applications such as solar cells and field effect transistors. Nickel, palladium, and platinum diphenylphosphinoethane complexes were tested as catalysts for the Grignard metathesis (GRIM) polymerization of 2,5-dibromo-3-hexylthiophene and 2-bromo-5-iodo-3-hexylthiophene. Nickel-mediated polymerization generated regioregular, low-polydispersity poly(3-hexylthiophene) with well-defined molecular weight consistent with a “living” chain-growth mechanism. By contrast, palladium-mediated polymerization proceeded by a step-growth mechanism and generated polymers with less than 80% head-to-tail couplings. Platinum-mediated polymerization gave very low molecular weight products. Kinetic and computational results suggested that the nickel catalyst acts as an initiator and remains associated with the growing polymer chain, while palladium dissociates from the growing chain. Computational and experimental evidence was provided for various side reactions of dissociated Pd(0) catalyst, which could yield a step growth mechanism and lower regioirregularity.

Highlights

Poly(3-alkylthiophene)s are the most used semiconducting polymers for organic electronics applications.[1,2] The asymmetric 3-alkyl-2,5-dihalothiophene monomer can generate multiple intra-monomer orientations: “head–head”, “head–tail”, and “tail–tail”
Palladium, and platinum diphenylphosphinoethane complexes were tested as catalysts for the Grignard metathesis (GRIM) polymerization of 2,5-dibromo-3-hexylthiophene and 2-bromo-5-iodo-3-hexylthiophene
The Grignard metathesis (GRIM) polymerizations was reported by McCullough's group in 1999 and allows the synthesis of regioregular P3HT in large scale without the use of cryogenic reaction aUniversity of Texas at Dallas, Department of Chemistry, 800 West Campbell Road, Richardson, TX 75080, USA

Summary

Introduction

Scheme 1 shows the accepted mechanism for the nickel mediated GRIM polymerization. The cyclic chain growing reaction begins with a M(II)–halide complex bound to the growing chain end (7). A 2 M solution of alkyl magnesium chloride (2.5 mL, 5.0 mmol) in diethyl ether (Et2O) was added via a deoxygenated syringe, and the reaction mixture was reacted at room temperature for 2 hours. At this time an aliquot (0.5 mL) was taken out and quenched with water. Before the addition of the catalyst, 40 mL of anhydrous THF was added to the reaction mixture, followed by the addition Ni(dppe)Cl2 (0.04 g, 0.075 mmol). M06 self-consistent eld calculations use Gaussian keyword “SCF 1⁄4 Tight”, and use a numerical integration grid with 99 radial and 590 angular points per atom (Gaussian keyword “Integral(Grid 1⁄4 UltraFine)”)

Results and discussion

Conclusions

Handbook of Thiophene-Based Materials