Understanding the Polymerization of Diphenylacetylenes with Tantalum(V) Chloride and Cocatalysts: Production of Cyclic Poly(diphenylacetylene)s by Low-Valent Tantalum Species Generated in Situ.

A systematic investigation of the polymerization of representative diphenylacetylenes with TaCl and cocatalysts suggested that low-valent Ta species, which are formed by in situ reduction of TaCl by the cocatalysts, are involved in the polymerization and that the polymerization reaction proceeds by an insertion ring expansion mechanism via the formation of tantalacyclopentadiene intermediates, rather than the previously considered metathesis mechanism. This polymerization mechanism indicates the production of unprecedented -stereoregular cyclic poly(diphenylacetylene)s. Indeed, the possibilities of a cyclic structure and high -stereoregularity of the resulting polymers were reasonably supported by the results of their detailed atomic force microscopy (AFM) and NMR analyses, respectively.