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 crosscoupling 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 crosscoupling 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 Pdmediated 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.

Automated summary

In this study, the mechanistic study of Pd/Ag cocatalyzed cross dehydrogenative coupling (CDC) polymerization reveals that the second chain extension cross-coupling is more efficient than the first cross-coupling, which goes against the Carothers equation. Kinetic analyses and DFT calculations provide insights into the reasons behind this phenomenon, which has implications for the synthesis of molecules where C-H bond activation may be the limiting factor.