A first-generation Grubbs catalyst has been shown to not only catalyze the ring opening metathesis polymerization (ROMP) of cyclopropene derivatives but also differentiate E- and Z-double bonds in intermediates by different reaction patterns, leading to stereospecific synthesis of poly(methylene-E-vinylene). The intermediate with the Z-double bond formed during the course of ROMP of cyclopropene is sensed by the catalyst, chopped by the catalyst, and removed as the cyclohexadiene derivative, and in the meantime, a ruthenium–carbene species is regenerated for continuing polymerization. When a second-generation Grubbs catalyst is used, a cyclic polymer with all double bonds in the trans configuration is obtained as the sole polymeric product, in addition to the same cyclohexadiene derivative as described above. The degree of polymerization in the latter reaction is independent of the loading of the catalyst. The formation of cyclic polymer is not common and the mechanism thereof remains unclear. One possibility would be a change in conformation of the alkenyl substituents on azetidine rings that would result in alteration of the folding nature of the polymer, leading to a cyclic polymer via ring closure metathesis.
The Grubbs G-I or G-II catalyst gives the ruthenium ethoxy carbene complex, which catalyzes ring-opening cross metathesis (ROCM) of a strained cyclic alkene to give a diene where one of the two alkene moieties in the product contains an ethoxy substituent. No polymeric products are detected. Hydrocarbons such as parent norbornene or substituted cyclopropenes can proceed with the reaction smoothly. Tertiary amines, N-alkylimides, esters, and aryl or alkyl bromides remain intact under the reaction conditions. In addition to vinyl ethers, vinylic esters can also be used. The time required to reach a 50% yield of the ROCM product t 50 varies from 0.01 to 140 h depending on the strain and nucleophilicity of the double bond. Anchimeric participation of an electron-rich group would result in significant enhancement of the reactivity, and the t 50 could be as short as several minutes. A similar substrate without such a neighboring group shows a much slower rate. An exo-norborne derivative reacts much faster than the corresponding endo-isomer. Alkenes with poor nucleophilicity are less favored for the ROCM process, so is less strained cyclooctene.
Stromaphane is a new type of two-dimensional covalent organic framework (2D-COF), constituting of multiple layers of ladderphanes where two adjacent ladderphane motifs share a common polymeric backbone. The linkers for each ladderphane moiety are perpendicular to the polymeric backbones and staggered with the linkers in the immediate neighbouring ladderphane layers. The framework of the stromaphane contains numerous slit-shaped pores with a width in subnanometres and length in nanometers. This 2D-COF is synthesized by Grubbs G-I catalyst-mediated stereospecific ring opening metathesis polymerisation of a biscyclopropene derivative having a benzene triad linker. The overall process can be considered as a self-intercalation polymerisation of a biscyclopropene. The structure of the 2D-COF has been proved by XRD measurements, DFT simulation, and STM images. This 2D-COF is composed of substituted poly(methylene-E-vinylene) (PMEV) backbones and benzene triad linkers to form a two-dimensional framework with multiply symmetrically distributed subnanometric slit-shaped pores.
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