Unusual Superior Activity of the First Generation Grubbs Catalyst in Cascade Olefin Metathesis Polymerization

Recently, we reported a new cascade ring-opening/closing metathesis polymerization of monomers containing two cyclopentene moieties. Several Ru catalysts were tested, but the best polymerization results were unexpectedly obtained using the first-generation Grubbs catalyst (<b>G1</b>). This was puzzling since the second- and third-generation Grubbs catalysts are well-known for their higher activities compared to <b>G1</b>. In order to explain the unique and superior activity of <b>G1</b>, we conducted a series of kinetics experiments for the polymerization of 3,3′-oxydicyclopent-1-ene, a representative monomer of this cascade polymerization, as well as the competition polymerization with cycloheptene using the various Grubbs catalysts. Based on our results, we propose a model in which the differences in the steric hindrance between the different ligands and the monomer determine the selectivity of the catalyst approach to the monomer and, therefore, the extent to which the productive pathway leads to successful cascade polymerization. In short, <b>G1</b> with the smaller ligand showed a high preference for the productive pathway

Tandem Ring-Opening/Ring-Closing Metathesis Polymerization: Relationship between Monomer Structure and Reactivity

Monomers containing either cycloalkenes with low ring strain or 1-alkynes are poor monomers for olefin metathesis polymerization. Ironically, keeping two inactive functional groups in proximity within one molecule can make it an excellent monomer for metathesis polymerization. Recently, we demonstrated that monomer <b>1</b> having cyclohexene and propargyl moieties underwent rapid tandem ring-opening/ring-closing metathesis (RO/RCM) polymerization via relay-type mechanism. Furthermore, living polymerization was achieved when a third-generation Grubbs catalyst was used. Here, we present a full account on this tandem polymerization by investigating how various structural modifications of the monomers affected the reactivity of the tandem polymerization. We observed that changing the ring size of the cycloalkene moieties, the length of the alkynes, and linker units influenced not only the polymerization rates but also the reactivities of Diels–Alder reaction, which is a post-modification reaction of the resulting polymers. Also, the mechanism of tandem polymerization was studied by conducting end-group analysis using ¹H NMR analysis, thereby concluding that the polymerization occurred by the alkyne-first pathway. With this mechanistic conclusion, factors responsible for the dramatic structure–reactivity relationship were proposed. Lastly, tandem RO/RCM polymerization of monomers containing sterically challenging trisubstituted cycloalkenes was successfully carried out to give polymer repeat units having tetrasubstituted cycloalkenes