Mechanistic Insights in Alternating Ring-Opening Metathesis Copolymerization

Synthesis of alternating copolymers via ring-opening polymerization represents a challenging field, still object of development. Since N-Heterocyclic Carbene (NHC)-Ru based complexes bearing unsymmetrical substitution on the nitrogen atoms revealed as more selective and efficient catalysts than symmetrically substituted complexes in alternating polymerization, research efforts have been devoted to enhance the selectivity by modifying N-substituents.1,2 Recently, we have reported on ruthenium bearing uNHCs with syn phenyl groups on the backbone (Figure 1), highly selective toward alternating copolymerization of norbornene (NBE) with cyclooctene (COE) or cyclopentadiene (CPE).3 Although several complexes were reported in literature in recent past, searching for best catalyst architecture able to promote highly efficient and selective catalysts toward alternating copolymerization via ring-opening polymerization, extensive molecular modelling studies that rationalize the influence of NHC ligand on catalyst selectivity still lack. Herein, we report a DFT study relative to the copolymerization NBE-CPE in the presence of NHC-Ru based catalyst, to interpret how different NHC design can influence the reaction selectivity. (1) Vehlow, K.; Wang, D.; Buchmeiser, M.R.; Blechert, S. Alternating Copolymerizations Using a Grubbs- Type Initiator with an Unsymmetrical, Chiral N-Heterocyclic Carbene Ligand. Angew. Chem. Int. Ed. 2008, 47, 2615. (2) Paradiso, V.; Grisi, F. Ruthenium-Catalyzed Alternating Ring-Opening Metathesis Copolymerization of Norborn-2-ene with Cyclic Olefins. Adv. Synth. Catal. 2019, 361, 4133. (3) Troiano, R.; Costabile, C.; Grisi, F. Alternating Ring-Opening Metathesis Polymerization Promoted by Ruthenium Catalysts Bearing Unsymmetrical NHC Ligands. Catalysts 2023, 13, 34.