A DFT study on Pd-catalyzed Suzuki cross-coupling polycondensation of aryl bromide monomers

Pd-catalyzed Suzuki cross-coupling polycondensation is a versatile tool to synthesize various conjugated polymers. Recently, two catalysts (PtBu3)Pd(Ph)Br and (PtBu3)Pd(pC6H4Br)I have been reported to give polymerization products with a high molecular weight and low polydispersity index (PDI). The mechanism of this polycondensation was studied with DFT calculations. Among the three steps of the mechanism, i.e. oxidative addition, transmetallation, and reductive elimination; transmetallation is the rate determining step. The barriers of the transmetallation of initiation are +19.5 kcal/mol for (PtBu3)Pd(pC6H4Br)I catalyst and +20.2 kcal/mol for (PtBu3)Pd(Ph)Br catalyst, respectively. The barrier of propagation is +19.7 kcal/mol. Thus the PDI of the polycondensation catalyzed by (PtBu3)Pd(pC6H4Br)I is lower than that of PhPd(PtBu3)Br because the initiation of the former is faster than the latter. On the other hand, the decomposition of the catalyst is negligible because the following transmetallation is faster. In addition, ortho-bromophenyliodide and perfluorophenyliodide are predicted to be more efficient initiators for the polycondensation because their barriers of transmetallation are even lower than that of (p-BrC6H4)Pd(PtBu3)I. The boronic ester monomer is found to have a lower barrier of transmetallation than the boronic acid in this polycondensation.