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

Abstract

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.

Catalytic living ring-opening metathesis polymerization with Grubbs’ second- and third-generation catalysts

“…The molecular weight dispersities obtained for polymers P8-P11 resemble those of early Grubbs' initiators 48 . However, virtually identical molecular weight dispersites have been reported for cyclopolymerizations of diynes 49 or the polymerization to yield conjugated polyenynes 31 with the G3 initiator in a conventional living ROMP. Other examples include the G3-initiated copolymerization of norbornene derivatives carrying peptides, chromophores and poly(ethylene gylcol) chains in a living fashion, leading to slightly broader molecular weight dispersites 30 or the polymerization of chromophore-labelled monomers with the G3 complex 11 .…”

Controlled Cyclopolymerization of 1,5-Hexadiynes to Give Narrow Band Gap Conjugated Polyacetylenes Containing Highly Strained Cyclobutenes

“…Cyclopolymerization (CP) of α,ω -diyne derivatives is a powerful tool for preparing substituted polyacetylenes. − Early studies regarding CP used ill-defined catalysts, including Ziegler–Natta, MoCl 5 , and WCl 6 catalysts, − until the first breakthrough by the Schrock group and later by the Buchmeiser group demonstrating living CP using well-defined Mo-based olefin metathesis catalysts. − Subsequently, the utility of CP broadened when user-friendly Ru-based catalysts were shown to promote living CP. − Extensive studies have been conducted using 1,6-heptadiyne monomers, yielding five- or six-membered rings (in a random or highly regioselective fashion) in the conjugated backbone via α - ,− or β -addition, ,− respectively. Moreoever, recent studies have expanded the scope of CP where 1,7-octadiyne − and 1,8-nonadiyne derivatives were used to successfully produce polyacetylenes containing six- and seven-membered rings via α -addition (Scheme ). However, in contrast to the cyclization to form thermodynamically stable five- to seven-membered rings, which is relatively easy, cyclization to make cyclobutenes is far more challenging due to their extremely high ring strain energy (29.5 kcal/mol), making the process thermodynamically disfavored.…”

Living β-selective cyclopolymerization using Ru dithiolate catalysts

“…The whole field expanded rapidly when Ru-based Grubbs11–14 and modified Grubbs catalysts ( e.g. , Hoveyda–Grubbs15,16 and Buchmeiser catalysts17–22) showed excellent activity, selectivity, and stability for CP.…”