An Exception to the Carothers Equation Caused by the Accelerated Chain Extension in a Pd/Ag Cocatalyzed Cross Dehydrogenative Coupling Polymerization

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.