We have reported that conventional Suzuki-Miyaura polycondensation of diboronate monomer and dibromo monomer in the presence of a Pd catalyst such as ^tBu₃PPd, which undergoes intramolecular transfer on the -face of aromatics, affords high-molecular-weight polymer with boronate at both ends even when an excess of dibromo monomer is used.^1,2 The Flory principle, in which the reactivities of monomer and polymer are assumed to be equal, does not apply in this case, because successive substitution of dibromo monomer with two equivalents of diboronate monomer takes place through intramolecular Pd catalyst transfer on the dibromo monomer. We expected that if this chemistry were applied to polycondensation of diboronate monomer and tribromo monomer (A₂ + B₃ polycondensation), hyperbranched polymer with boronate ends would be obtained through intramolecular Pd catalyst transfer on the tribromo monomer, even when an excess of tribromo monomer is used. Since all bromines in this hyperbranched polymer react with the diboronate monomer or the boronate ends of oligomers formed during polymerization, the degree of branching (DB) of the obtained hyperbranched polymer should be 100%. Furthermore, successive tri-substitution of tribromo monomer with the boronate monomers and oligomers would result in a higher degree of polymerization (DP) before the critical gel point than would be expected on the basis of the Flory-Stockmayer (F-S) theory, which assumes the reactivities of monomer and polymer to be equal, as in the case of the Flory principle. Herein, we report unstoichiometric Suzuki-Miyaura polycondensation of excess tribromo monomer and 1.0 equivalent of diboronate monomer in the presence of ^tBu₃PPd precatalyst, affording hyperbranched polymer with 100% DB and a much higher DP than that predicted from the F-S theory.

References

1. M. Nojima, K. Kosaka, M. Kato, Y. Ohta, T. Yokozawa, Macromol. Rapid Commun. 2016, 37, 79.
2. T. Yokozawa, Y. Ohta, Polym. Sci. C 2020, 62, 78.