Polyacetylenes are an important class of materials with interesting electronic properties. A tetraanionic pincer-ligand supported tungsten (VI) alkylidene polymerizes acetylenes with the highest activity among alkyne polymerization catalysts, yielding high molecular weight polymers in quantitative yields. These polymers exhibit a cyclic topology and are likely formed via Ring Expansion Polymerization (REP). Early experiments suggested the alkylidene is merely a spectator and propagation proceeds by repeated alkyne coordination to the W(VI) and insertion into the W-C bond of the metallacycle. DFT calculations suggest high reorganization barriers necessary for insertion into the metallacycle render the mechanism unfeasible. Investigating other mechanisms supports  insertion of the alkylidene into the metallacycle with subsequent insertion of an alkyne monomer  forming  a metallacylooctatetraene. Ring slippage easily generates a W-cycloheptatrienyl complex that  easily reduces to W(V) via W-C bond homolysis. The resulting “naked” W(V), merely maintaining the pincer ligand, exhibits extremely low alkyne polymerization barriers and ring re-organization barriers. Subsequent experiments (EPR etc.)  support involvement of a doublet W species.

Mechanism of tungsten-catalyzed cyclic polyacetylenes synthesis

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