Seven-Membered Ring-Forming Cyclopolymerization of 1,8-Nonadiyne Derivatives Using Grubbs Catalysts: Rational Design of Monomers and Insights into the Mechanism for Olefin Metathesis Polymerizations

Studies into the cyclopolymerization (CP) of diyne derivatives using metal carbenes have focused on the formation of five- and six-membered rings because these small rings can be easily synthesized while the preparation of medium-sized seven-membered rings are more difficult. For the first time, we achieved the CP forming challenging seven-membered rings as repeat units using Grubbs catalysts by novel design of 1,8-nonadiyne monomers. The key to the successful CP was the introduction of the appropriate aminal and acetal groups, which have short C–N and C–O bonds, and low rotational barriers, thus greatly enhancing the cyclization efficiency. During our mechanistic investigation, we directly observed an actual 14-electron Ru propagating carbene by ¹H NMR spectroscopy for the first time during olefin metathesis reaction, presumably because the great steric hindrance from the propagating carbene containing a larger seven-membered ring than five- or six-membered ring retarded the coordination of ligands. We also observed decomposition of the catalysts to ruthenium hydrides during polymerization for the first time. Kinetic studies revealed three interesting features of this 1,8-nonadiyne CP: (i) in contrast to conventional polymerizations, the rate-determining step for the CP of 1,8-nonadiynes was the cyclization step; (ii) the intrinsic reactivity of the acetal monomers was higher than that of the aminal monomers; but (iii) the overall polymerization efficiency of the aminal monomers was higher than that of the acetal monomers because of the higher stability of their carbenes. Finally, we achieved a controlled CP of the aminal monomers using a fast-initiating third-generation Grubbs catalyst. This allowed the synthesis of not only the diblock copolymer containing five- and seven-membered rings but also the triblock copolymer containing five-, six-, and seven-membered rings.