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.