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

Abstract

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 (Đ < 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 endonorbornene 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.

Rapid Controlled Synthesis of Large Polymers by Frontal Ring-Opening Metathesis Polymerization

“…We observed that the reaction purged with air has a slightly higher M n compared with the ones purged under nitrogen and vacuum; however, the difference is not statistically significant (Figure S4). The high tolerance to oxygen when initiator ligands are used during ROMP was also recently observed . Moreover, we suspect the slightly higher M n is caused by small amounts of oxygen in the monomer that can degrade the initiator during the reaction.…”

“…However, access to linear polymers with degree of polymerizations ( DPs ) higher than 1000 and low dispersity ( Đ ) remains a challenge, as seen in Figure . The synthesis of high polymers with low Đ via ROMP are mostly performed under an inert atmosphere with reaction times that can exceed several hours (depending on the catalyst). , …”

A modular low-cost automated synthesis machine demonstrated by ring-opening metathesis polymerization

“…Thus, the first initiated polymers have longer synthesis times at a higher monomer concentration, compared to the last initiated polymers in the vial. The narrow standard deviation in Đ for A n indicates precise and reproducible syntheses when using the ASM, however, the polymers exhibit a larger Đ than found in literature ( Đ = 1.16–1.30 for similar synthesis using G3 37 ). Since the Đ of the manual ROMP (1.4) is comparable to those of the machine, the larger dispersities can be ascribed to the synthesis procedure or the GPC.…”

Long-Range Kinetic Effects on the Alternating Ring Opening Metathesis of Bicyclo[4.2.0]oct-6-ene-7-carboxamides and Cyclohexene

“…In order to accurately measure the forward rate of the metathesis reaction: ring opening of cyclohexene by cyclohexylidene 1 -Ru, triphenylphosphine PPh 3 , a strong σ-donating, labile ligand that is inexpensive and easy to handle, was added to the reaction mixture. We anticipated that PPh 3 will bind to the Ru center , of 1 - 2 -Ru and form a more stable PPh 3 ligated complex ( 1-2 -Ru­(PPh 3 )), thus suppressing the reverse ring closing reaction (Figure ). When 10 equiv of PPh 3 was added to the reaction mixture, the rate of formation of 1 - 2 -Ru­(PPh 3 ) increased significantly, consistent with suppression of the reverse reaction.…”