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 TaCl5 and cocatalysts suggested
that low-valent Ta species, which are formed by in situ reduction
of TaCl5 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 cis-stereoregular cyclic poly­(diphenylacetylene)­s. Indeed, the possibilities
of a cyclic structure and high cis-stereoregularity
of the resulting polymers were reasonably supported by the results
of their detailed atomic force microscopy (AFM) and NMR analyses,
respectively.