Influence of the Carboxylic Acid Additive Structure on the Properties of Poly(3-hexylthiophene) Prepared via Direct Arylation Polymerization (DArP)

Abstract

Carboxylic acids are known to have a profound effect on the rate of direct arylation. Despite this, little attention has been paid to them as additives for direct arylation polymerization (DArP). Here we report the influence of the carboxylic acid chemical structure on the properties of poly(3-hexylthiophene) (P3HT) prepared via DArP. We study the effect that acid pK a, steric bulk, and backbone cyclization have on the reactivity of the DArP catalytic system. We found that pK a values do not correlate with DArP reactivity in the pK a range of 4.76–5.05. The increase of acid size in the classes of linear, secondary, and tertiary acids leads to a continuous increase in polymer yield and molecular weight, whereas for the case of cyclic secondary acids the trend is reversed and a decrease of acid size leads to an increase in polymer yield and molecular weight in an alternating (zigzag) fashion depending on whether the acid contains an even or odd number of carbons. A profound enhancement of reactivity was found for the case of the smallest cyclopropanecarboxylic acid, which yields P3HT with remarkably high molecular weights (M n 33.4 kDa and M w 207.1 kDa), which is expected to benefit the development of direct arylation protocols for inherently less reactive substrates.

Structural Analysis of Poly(3‐hexylthiophene) Prepared via Direct Heteroarylation Polymerization

“…Replacing all β‐protons with methyl groups circumvented the unwanted branching reactions leading to a well‐defined polymer. β‐branching and homocouplings were also pointed out by Rudenko and Thompson in some P3HT syntheses …”

“…In addition to the possibility of homocouplings, the activation of β CH bonds on thiophenes can also lead to branching. Based on model compounds, 1 H NMR attributions for β‐branching can be detected in the range of 2.18–2.38 ppm and resulted in broad signals . No evidence of such defects was found when looking at P1 1 H NMR spectra (see the Supporting Information).…”

En Route to Defect-Free Polythiophene Derivatives by Direct Heteroarylation Polymerization

“…Recently, Rudenko and Thompson have shown that the nature of the aliphatic chain (linear, secondary, tertiary or cyclic) of the acidic moiety could also have a positive influence on the selectivity of the couplings. 32 Therefore, we investigated the effects of a carboxylic acid additive in the conditions developed by Itami et al (entries A5 and B5). Although Rudenko and Thompson used a wide range of carboxylic acids, we focused our investigation on neodecanoic acid (NDA) since this one was reported to lead to P3HT free of β-branching.…”

Steric effect of carboxylic acid ligands on Pd-catalyzed C–H activation reactions

“…Therefore, it is interesting to see how steric effect of the carboxylic acids (carboxylates) influences the C-H bond activation reactions. Recently, Thompson investigated the effect of various carboxylic acids on the Pd-catalyzed direct arylation polymerization with 3-hexylthiophene [20].…”