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.