Macrocyclic Ruthenium Ring Expansion Metathesis Polymerization Initiators Exhibit Tether Length-Dependent Behavior

Polymers with a cyclic architecture are invaluable constructs for a variety of materials science and engineering applications. Due to their macrocyclic nature, these plastics are imbued with enhanced mechanical durability; a single scission event along the backbone keeps the polymer intact, and the molar mass remains unchanged. Hence, polymer topology can be envisioned as a strategy to prolong the useful lifetime of a material before failure. With a growing interest in synthesizing such cyclic polymers using ring-expansion metathesis polymerization (REMP), there remains an unmet need to further understand the structure-activity relationships of the requisite organometallic initiators. While analogous Ru-based initiators for ring-opening metathesis polymerization (ROMP) have been widely studied, this same focus has not been applied to REMP. Herein, we report the synthesis and activity of CBX cyclic Ru-benzylidene initiators (X = number of carbon atoms in the tether, 4-6). Through mechanistic studies, we note a marked difference in REMP molar mass control that is reinforced via analyses using solid-state X-ray crystallography and DFT calculations. Overall, these studies provide insight into the relationship between the CBX tether length and its propensity for productive secondary metathesis, a key mechanistic facet for modulating cyclic polymer molar mass.