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

A low-valent niobium (Nb) species formed in situ fromNbCl(5) with PhSiH3 induces the polymerizationof diphenylacetyleneshaving electron-donating groups to afford the corresponding poly(diphenylacetylene)s(PDPAs) with high molecular weights in good yield. An insertion ring-expansionmechanism would be involved in the polymerization and would producecyclic PDPAs, as suggested by results of atomic force microscopy (AFM)observations and NMR analysis of the obtained polymers. The presentpolymerization method was applicable to the synthesis of symmetricallysubstituted PDPAs having chiral alkoxy pendants at para- and meta-positions, and their chiroptical propertieswere investigated.