posted on 2022-02-01, 01:03authored byLiwen Xing, Ji-Ren Liu, Xin Hong, Kendall N. Houk, Christine K. Luscombe
The
Carothers equation
is often used to predict the utility of
a small molecule reaction in a polymerization. In this study, we present
the mechanistic study of Pd/Ag cocatalyzed cross dehydrogenative coupling
(CDC) polymerization to synthesize a donor–acceptor (D–A)
polymer of 3,3′-dihexyl-2,2′-bithiophene and 2,2′,3,3′,5,5′,6,6′-octafluorobiphenyl,
which go counter to the Carothers equation. It is uncovered that the
second chain extension cross-coupling proceeds much more efficiently
than the first cross-coupling and the homocoupling side reaction (at
least 1 order of magnitude faster) leading to unexpectedly low homocoupling
defects and high molecular weight polymers. Kinetic analyses show
that C–H bond activation is rate-determining in the first cross-coupling
but not in the second cross-coupling. Based on DFT calculations, the
high cross-coupling rate in the second cross-coupling was ascribed
to the strong Pd-thiophene interaction in the Pd-mediated C–H
bond activation transition state, which decreases the energy barrier
of the Pd-mediated C–H bond activation. These results have
implications beyond polymerizations and can be used to ease the synthesis
of a wide range of molecules where C–H bond activation may
be the limiting factor.