Low-Valent Niobium Catalysis for the Polymerization of Electron-Rich Diphenylacetylenes

A low-valent niobium (Nb) species formed in situ from NbCl5 with PhSiH3 induces the polymerization of diphenylacetylenes having electron-donating groups to afford the corresponding poly(diphenylacetylene)s (PDPAs) with high molecular weights in good yield. An insertion ring-expansion mechanism would be involved in the polymerization and would produce cyclic PDPAs, as suggested by results of atomic force microscopy (AFM) observations and NMR analysis of the obtained polymers. The present polymerization method was applicable to the synthesis of symmetrically substituted PDPAs having chiral alkoxy pendants at para- and meta-positions, and their chiroptical properties were investigated.