Iron catalysts ring-opening polymerization

For the polymerization of cyclobutene, Natta and coworkers [37] reported binary catalytic systems containing titanium, vanadium, chromium, and tungsten to be the most active, those of molybdenum less active, and systems derived from cobalt, iron, manganese, and uranium totally inactive. Catalysts based on vanadium and chromium yield preferentially polycyclobutylene by addition polymerization, those with molybdenum and tungsten give polybutenamer by ring-opening... 

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