Gelation-free synthesis of high-molecular-weight hyperbranched aromatic polymers containing silicon by Suzuki-Miyaura polycondensation of tri- or tetra(bromoaryl)silane with arylenediboronate

Abstract

Suzuki-Miyaura polycondensation of tri- or tetra(bromoarylsilane) 1 with arylenediboronic acid (ester) 4 in the presence of tBu3PPd precatalyst 3 was investigated for the synthesis of silicon-containing hyperbranched aromatic polymers by means of simple A2 + Bx (x = 3, 4) polycondensation without gelation. The key reaction in this polycondensation, successive substitution of 1 with 4 through intramolecular Pd catalyst transfer on the silyl group and π-face of 1, was investigated by means of model reactions of 1 with a variety of arylboronates 2 in the presence of 3 at room temperature. The Suzuki-Miyaura reaction of tri(bromophenyl)silane 1a, tri(bromofluorenyl)silane 1b, and tetra(bromophenyl)silane 1c with phenylboronic acid and carbazoleboronate yielded exclusively tri- and tetrasubstituted products, even though the bromine sites in 1 were present in excess relative to the boronic acid (ester) sites in 2. The polycondensation of 1a with phenylenediboronate 4a, fluorenylenediboronate 4b, thienylenediboronate 4c, and carbazolylenediboronate 4d afforded high-molecular-weight silicon-containing aromatic hyperbranched polymers without gelation. The use of 1c instead of 1a resulted in the formation of some polymers with low solubility, but the polycondensations of 1c with phenylenediboronic acid 4a’ having a branched side chain and with 4b yielded the corresponding high-molecular-weight polymers. In the polycondensation of 1b with 4, 4a afforded a moderate-molecular-weight polymer (Mn = 7610), whereas 4b-d yielded polymers with Mn = 64700-80700. It turned out that donor and/or extensively conjugated compounds 4 were particularly effective for the Suzuki-Miyaura polycondensation of extensively conjugated tribromide 1b. The UV-vis absorption and emission spectra in solution of the silicon-containing hyperbranched polyphenylenes, obtained by the polycondensation of 1a or 1c with 4a, were similar to those of hyperbranched polyphenylene with no silicon (HBPP), whereas the fluorescence quantum yields were considerably higher (50-53%) than that of HBPP (16%).