Stereospecific Ring‐Opening Metathesis Polymerization of Norbornene Catalyzed by Iron Complexes

Developing well‐defined iron‐based catalysts for olefin metathesis would be a breakthrough achievement in the field not only to replace existing catalysts by inexpensive metals but also to attain a new reactivity taking advantage of the unique electronic structure of the base metals. Here, we present a two‐coordinate homoleptic iron complex, Fe(HMTO) 2 [HMTO=O‐2,6‐(2,4,6‐Me 3 C 6 H 2 ) 2 C 6 H 3 ], that is capable of performing ring‐opening metathesis polymerization of norbornene to produce highly stereoregular polynorbornene (99 % cis, syndiotactic). The use of heteroleptic Fe(HMTO)(RO) [RO=(CH 3 ) 2 CF 3 CO, CH 3 (CF 3 ) 2 CO, or Ph(CF 3 ) 2 CO] prepared in situ significantly increases the polymerization rate while preserving selectivity. The resulting polymers were characterized by 1 H and 13 C NMR spectroscopy and gel‐permeation chromatography.