Unraveling the Compounded Interplay of Weakly and Strongly Coordinating Ligands in G3-Catalyzed Living Metathesis Polymerization: Toward Well-Defined Polynorbornene at Ambient Temperature

Ambient temperature ring-opening metathesis polymerization(ROMP)of unhindered cycloolefins is crucial for industrial applicationsfrom a sustainable perspective. However, balancing the reactivityof propagating species and polymer molecular structure remains a challengedue to competing secondary metathesis at ambient temperature. We reporta facile ligand-competition strategy for achieving excellent controlover 3-bromopyridine-coordinated Grubbs' third-generation catalyst(3-bropyr-G3)-catalyzed ROMP of unsubstituted norbornene(NB). The addition of external pyridine with a strongcoordinating affinity to regulate the catalytic metathesis activityis capable of tuning polymerization rates. Importantly, our strategyallows well-controlled ROMP of NB at both low (i.e.,-20 & DEG;C) and ambient (i.e., 20 & DEG;C) temperatures, yieldingPNB with quantitative M (n) andlow D (& LE;1.07). Synergetic experimentaland computational investigations suggest that stably coordinated pyridinecan prevent the Ru center from attacking the backbone C UC doublebonds by breaking the interaction between the original weakly coordinatingligand and Ru carbene and thus mitigates the intermolecular chaintransfer. This study guides the design of new G3-catalyzedROMP systems to regulate the balance of metathesis reactivity andpolymer molecular structures.