In recent years, direct arylation polymerization (DArP) has attracted increasing interest as a method to prepare conjugated polymers in contrast to conventional cross-coupling polymerizations. The most appealing aspects of DArP are reduced organometallic waste and improved step economy as no organometallic prefunctionalization is required. DArP is distinguished as a sustainable and atom-economic approach for constructing C-C bonds over traditional coupling methods by features including generating benign by-products and only requiring functionalization of one component with routine and bench-stable halogens. However, DArP is not without issue – specifically, selectivity and control over the polymerization can be difficult to achieve.

Synergistic catalysis involves the use of more than catalyst to activate different substrates to enable reactions that were not achievable before. In light of the aforementioned issues related to DArP, our group has been investigating the use of the combination of Pd/Ag or Au/Ag as synergistic catalysts.

Our studies began with the development of a controlled DArP to achieve polymers with targeted molecular weights and low dispersities. Since we were unable to achieve controlled DArP using a single catalyst, we chose to use two metals (i) one that would perform the C-H activation followed by (ii) one that would perform the controlled polymerization. While the achieve molecular weights were quite low, nevertheless, we were able to show some living characteristics for the polymerization.

We then tackled the synthesis of donor-acceptor copolymers using cross dehydrogenative coupling (CDC). While standard DArP involves the cross-coupling between C-Br and C-H groups, CDC entails cross-coupling between C-H and C-H groups. As such, selectivity becomes a very important issue and there must be a way to ensure that only cross-coupling products are obtained and not homocoupling products. In this regard, the Pd/Ag cocatalyzed CDC reaction has been reported to be highly effective, and we were able to uncover the origin of this efficacy. We 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. 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.

In recent years, direct arylation polymerization (DArP) has attracted increasing interest as a method to prepare conjugated polymers in contrast to conventional cross-coupling polymerizations. The most appealing aspects of DArP are reduced organometallic waste and improved step economy as no organometallic prefunctionalization is required. DArP is distinguished as a sustainable and atom-economic approach for constructing C-C bonds over traditional coupling methods by features including generating benign by-products and only requiring functionalization of one component with routine and bench-stable halogens. However, DArP is not without issue – specifically, selectivity and control over the polymerization can be difficult to achieve.

Synergistic catalysis involves the use of more than catalyst to activate different substrates to enable reactions that were not achievable before. In light of the aforementioned issues related to DArP, our group has been investigating the use of the combination of Pd/Ag or Au/Ag as synergistic catalysts.

Our studies began with the development of a controlled DArP to achieve polymers with targeted molecular weights and low dispersities. Since we were unable to achieve controlled DArP using a single catalyst, we chose to use two metals (i) one that would perform the C-H activation followed by (ii) one that would perform the controlled polymerization. While the achieve molecular weights were quite low, nevertheless, we were able to show some living characteristics for the polymerization.

We then tackled the synthesis of donor-acceptor copolymers using cross dehydrogenative coupling (CDC). While standard DArP involves the cross-coupling between C-Br and C-H groups, CDC entails cross-coupling between C-H and C-H groups. As such, selectivity becomes a very important issue and there must be a way to ensure that only cross-coupling products are obtained and not homocoupling products. In this regard, the Pd/Ag cocatalyzed CDC reaction has been reported to be highly effective, and we were able to uncover the origin of this efficacy. We 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. 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.