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

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 pKa, 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 pKa 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.