Nonstoichiometric Suzuki–Miyaura Polycondensation for the Synthesis of Aromatic Polymers

This mini-review deals with the nonstoichiometric Suzuki–Miyaura polycondensation of excess dibromoarylene with arylenediboronic acid (ester) in the presence of t-Bu3PPd precatalyst and CsF/18-crown-6 as a base. Since t-Bu3PPd(0), generated from precatalyst with the assistance of base, has a propensity for intramolecular catalyst transfer on the π-electron face of aromatics, successive reaction of dibromoarylene with 2 equivalents of arylenediboronic acid (ester) takes place through intramolecular catalyst transfer of t‑Bu3PPd(0) on dibromoarylene, affording high-molecular-weight polymer, even though an excess of dibromoarylene is used. High-molecular-weight poly(tetraalkoxy-substituted stilbene), polyphenylene, polyfluorene, poly(thiophene-alt-phenylene), poly(fluorene-alt-benzthiadiazole), poly(phenylene-alt-benzothiadiazle), poly(cyclopentadithiophene-alt-benzothiadiazole), and polyphenylene containing C≡C, N=N, CH2, CO, N-Bu, O, SO2 in the backbone have been obtained under nonstoichiometric conditions. The obtained polymers have a boronic acid (ester) moiety at both ends, which can be converted to other functional groups by means of Suzuki–Miyaura coupling reaction with another reagent, oxidation, bromination, and so on. In the synthesis of poly(tetraalkoxy-substituted stilbene), the abnormal nonstoichiometric polycondensation, affording high-molecular-weight polymer with a boronic acid ester moiety at both ends, can be switched to normal nonstoichiometric polycondensation, affording low-molecular-weight polymer with bromine at both ends, simply by the addition of unsubstituted stilbene, which promotes intermolecular catalyst transfer due to catalyst trapping. When at least one of the two monomers has a kinked structure, such as m-dibromophenylene or m-phenylenediboronic acid (ester), many kinds of cyclic polymers can be selectively obtained even under nonstoichiometric conditions.