3 years ago

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

Stereoselective Ring-Opening Metathesis Polymerization with Molybdenum Imido Alkylidenes Containing O-Chelating N-Heterocyclic Carbenes: Influence of Syn/Anti Interconversion and Polymerization Rates on Polymer Structure
Wolfgang Frey, Christina Lienert, Michael R. Buchmeiser
The structures of the first insertion products (syn/anti, cis/trans) 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(N-2,6-Me2-C6H3)(N-mesityl-N′-2-O-1-C6H4-imidazolin-2-ylidene)(CHCMe2Ph)(OTf)] (1), [Mo(N-2,6-iPr2-C6H3)(N-2,6-iPr2-C6H3-N′-2-O-1-C6H4-imidazolin-2-ylidene)(CHCMe2Ph)(OTf)] (2), and [Mo(N-2,6-iPr2-C6H3)(N-2,6-iPr2-C6H3-N′-2-O-1-C6H4-imidazolin-2-ylidene)(CH3CN)(CHCMe2Ph)+ (B(ArF)4)] (3), have been identified. Also, syn/anti interconversion rates of catalysts 13 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 cis/trans 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 cis-syndiotactic ring-opening metathesis polymerization-derived polymers have been prepared with the aid of 3. On the basis of our results, we propose a comprehensive mechanism for the formation of cis- and trans-configured polymers with molybdenum imido complexes containing an O-chelating NHC.

Publisher URL: http://dx.doi.org/10.1021/acs.macromol.7b00841

DOI: 10.1021/acs.macromol.7b00841

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