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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
journal contribution
posted on 2017-03-29, 00:00 authored by Jung-Ah Song, Tae-Lim ChoiStudies 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.