User Guide to Ring-Opening Metathesis Polymerization of endo-Norbornene Monomers with Chelated Initiators

The development of facile synthetic strategies to access well-defined polymers promises to provide advanced soft materials with functionality that rivals that observed from nature. To this end, ring-opening metathesis polymerization (ROMP) presents a compositionally simple and rapid strategy for controlled polymerization, yet it has received far less attention relative to radical counterparts. This limited attention arises in part from scattered reports on optimization strategies and a narrow monomer scope. Contemporary ROMP methods favor the use of exo-norbornene derivatives and highly reactive nonchelated Ru-alkylidenes, such as Grubbs Catalysts. In contrast, endo-norbornene derivatives, from which analogous exo-forms are often generated, present a more accessible alternative, yet examples of their utility in ROMP remain scarce. Herein, a systematic examination of ROMP with endo-norbornene monomers using stable chelated Ru-alkylidene initiators is presented. Through initiator screening and polymerization optimization, the ROMP process is shown to be versatile and robust, providing rapid access to polymers with excellent molecular weight control, low dispersities (D < 1.1), good functional group tolerance, and high chain-end fidelity that enabled the preparation of block copolymers via sequential monomer addition. Furthermore, the process is oxygen-tolerant, allowing for ROMP to be performed under ambient conditions on the bench, which was showcased in synthesizing mechanically robust endo-norbornene imide thermoplastics with high glass transition and decomposition temperatures. This report provides a comprehensive overview of the scope and limitations of endo-norbornene ROMP with chelated initiators, serving as a user guide for the polymer chemistry community to develop well-defined next-generation functional plastics.