Concentration-Driven Ring Expansion Metathesis Polymerization via Tunable Ring Transfer Processes

Ring expansion metathesis polymerization (REMP) is a robust and versatile method used to access polymeric cyclic architectures for applications in biomedicine, electronics, and performance engineering. Cyclic Ru-benzylidene REMP catalyst CB6 demonstrates higher stability and polymerization rates compared to other Ru-based systems. However, CB6 also exhibits an unusual molar mass evolution profile where high-molar-mass cyclic polymers are produced at early time points followed by a gradual decrease in molar mass. For broad cyclic polymer applications to be fully realized, a mechanistic understanding of REMP is crucial. In this work, we investigate the polymerization profiles of CB6 using a series of mechanistic studies to probe the requisite ring transfer steps envisaged for such a polymerization profile. Furthermore, our studies reveal an intricate relationship between reaction concentration and experimental molar mass. These collective studies demonstrate CB6's role not only as an initiator but also as a catalytic ring transfer agent. Overall, we showcase a new toolkit by which to control REMP that will allow further optimization of catalyst design and the creation of novel cyclic materials.