Stereoselective Ring-Opening Metathesis Polymerization with Molybdenum Imido Alkylidenes Containing O‑Chelating N‑Heterocyclic Carbenes: Influence of <i>Syn</i>/<i>Anti</i> Interconversion and Polymerization Rates on Polymer Structure

The structures of the first insertion products (<i>syn</i>/<i>anti</i>, <i>cis</i>/<i>trans</i>) in the ring-opening metathesis polymerization (ROMP) of norbornene derivatives using both neutral and cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) complexes based on an O-chelating NHC, i.e., [Mo­(<i>N</i>-2,6-Me₂-C₆H₃)­(<i>N</i>-mesityl-<i>N</i>′-2-<i>O</i>-1-C₆H₄-imidazolin-2-ylidene)­(CHCMe₂Ph)­(OTf)] (<b>1</b>), [Mo­(<i>N</i>-2,6-^<i>i</i>Pr₂-C₆H₃)­(<i>N</i>-2,6-^<i>i</i>Pr₂-C₆H₃-<i>N</i>′-2-O-1-C₆H₄-imidazolin-2-ylidene)­(CHCMe₂Ph)­(OTf)] (<b>2</b>), and [Mo­(<i>N</i>-2,6-^<i>i</i>Pr₂-C₆H₃)­(<i>N</i>-2,6-^<i>i</i>Pr₂-C₆H₃-<i>N</i>′-2-<i>O</i>-1-C₆H₄-imidazolin-2-ylidene)­(CH₃CN)­(CHCMe₂Ph)⁺ (B­(Ar^F)₄^–)] (<b>3</b>), have been identified. Also, <i>syn</i>/<i>anti</i> interconversion rates of catalysts <b>1</b>–<b>3</b> have been determined in acetonitrile. Correlation of these values with the rate constants of polymerization revealed the importance of a balanced ratio between these two values. Disrupting that balance by changing the solvent or the monomer or by switching to a similar, but more ROMP-active catalyst leads to significant changes in the <i>cis</i>/<i>trans</i> contents of the resulting polymers. Despite the chelating and bulky nature of the ligands, a mechanism that entails inversion at the metal center of the catalyst during polymerization is proposed. Thus, highly <i>cis-syndiotactic</i> ring-opening metathesis polymerization-derived polymers have been prepared with the aid of <b>3</b>. On the basis of our results, we propose a comprehensive mechanism for the formation of <i>cis-</i> and <i>trans-</i>configured polymers with molybdenum imido complexes containing an O-chelating NHC