Reactivity of Ruthenium-Based Initiators

Compared to molybdenum- or tungsten-based Schrock catalysts, the reactivity of ruthenium-based systems is different. While reactivity slightly increases in the order I Br Cl [170] it may more efficiently be enhanced via the phosphines than by the nature of the alkylidene moiety [168] or by substitution of the chlorides by other, more electron-withdrawing, groups [171]. The thermodynamics and in par- 

While the decomposition products of molybdenum-based initiators may conveniently be removed as molybdates after any metathesis reaction by addition of aqueous base, ruthenium tends to form RUO2, a black insoluble product. The removal of decomposed Ru-sites from any product is best performed by the use of a water-soluble phosphine such as tris(hydroxymethyl)phosphine [182]. Alternatively, addition of triphenylphosphinoxide or DMSO followed by filtration over sihca was reported to be advantageous [183]. For purposes of completeness, the investigations of the actual catalytic cycle in the reaction of Cl2Ru(CHPh)(PCy3)2 with cyclic olefins carried out with substituted cyclobutenes shall be mentioned [184]. 

This percentage appears to be basically independent of temperature and solvent Regioselectivity for unsymmetrically substituted NBFs such as exo-methyhdene-NBE is generally low, only 7-substituted NBDFs are polymerized with anti-selectivity, that is with the substituent pointing away from the initiator. Usually, a strong isotactic bias is found for NBE-based monomers, NBDE-based analogues are usually less isotactic [190]. 

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