Extremely regio‐regular poly (3‐alkylthiophene)s from simplified chain‐growth Grignard metathesis polymerisations and the modification of their chain‐ends

Abstract

A simplified route to regio-regular poly(3-alkylthiophene)s (P3AT) with predetermined molecular weights and low molecular weight distributions based on the chain-growth GRIM polymerisation of 2,5-dibromo-3alkylthiophenes is detailed. It is proposed by way of in-depth 13 C-, 1 H-and two-dimensional NMR characterisations that the resulting P3ATs are quasi-100% regio-regular except for one regio-irregular pair at one chain-end. It is probable that chain-end groups exhibit reduced conjugation with the rest of the polymer. A comparison of the preparation of poly(3-hexylthiophene) and poly(3-butylthiophene) (P3BT) is presented. New methods required for the hydrogenation, formylation and bromomethylation of the chain-ends of P3BT, necessary to overcome its poor solubility and low reactivity, are described.

High Molecular Weights, Polydispersities, and Annealing Temperatures in the Optimization of Bulk-Heterojunction Photovoltaic Cells Based on Poly(3-hexylthiophene) or Poly(3-butylthiophene)

“…[26] Characterization of P3HT-1 using 1 H and 13 C NMR spectroscopy suggested that the integrals of the main-chain and chain-end hydrogens give a good indication of the molecular weight, provided that it is not too high. [25] Using this technique, a coefficient of 1.6 was determined for P3HT-1. Dividing 4500 by 1.6 gives M n = 2800 g mol -1 (equivalent to around 17 units), which is in close agreement with that predicted.…”

“…In addition, P3BT was not reactive enough to be debrominated by metathesis. [25] P3HT-6 was expected to have a molecular weight of around 40 000 g mol -1 but was indicated by GPC to have M n = 33 400 g mol -1 , which equates to a 'realistic' molecular weight of 33 400/2 ≈ 17 000 g mol -1 . P3HT-7, prepared in diphenyl ether (DPE), was not subject to treatment with LiAlH 4 as it had a very low concentration of chain ends.…”

“…P3HT-7 was found to be insoluble in THF, chloroform, dichloromethane (DCM), and toluene, but well soluble in hot chlorobenzene and o-dichlorobenzene, which indicated that its molecular weight was above 100 000 g mol -1 . [28] Formylation via the Vilsmeier technique [25] was attempted in order to improve its solubility for characterization. However, this either yielded a polymer with too slight a degree of formylation to make any difference to its solubility or led to a breakdown of the polymer chains.…”

“…2). [25,28] Important minor peaks that can be distinguished are those at 120.4 and 119.5 ppm, which are due to the chain-end carbons in positions 2 and 4, as denoted in Figure 5. a-and b-methylene carbons on regio-irregular chainend groups at 30.9 and 29.5 ppm, respectively, are also distinguishable.…”

“…2). [24,25] These techniques have been extended to prepare the extremely high molecular weight samples of P3HTs required for study in P3HT:PCBM cells. Beneficially, the timeconsuming technique of polymer fractionation is no longer necessary to obtain monodisperse samples of P3HTs.…”