Ring-opening metathesis characteristics

Whereas only limited stereoselectivity is characteristic of the metathesis of acyclic olefins, ring-opening metathesis polymerization of cycloalkenes may be highly stereoselective provided the proper catalysts and reaction conditions are selected. Cyclopentene, for instance, is transformed to either all-cis [Eq. (12.25)] or all-frans polypentenamers [Eq. (12.26)] in the presence of tungsten catalysts 21 92... 

A characteristic feature of cycloolefin ring-opening metathesis polymerisation is alteration of the metal-carbon active bonds from the metal carbene a, n bond into metallacycle a bonds, and vice versa, as polymerisation progresses. It is worth mentioning, in this connection, that metallacyclobutanes can be successfully used as catalysts for this polymerisation [36,37]. 

Scheme 1.34 summarizes the most characteristic metathesis reactions. The following abbreviations are used CM (cross metathesis), RCM (ring-closing metathesis), ROM (ring-opening metathesis), ROMP (ring-opening metathesis polymerization) and ADMET (acyclic diene metathesis polymerization). 

Reactivity characteristic of alkylidene complexes of tantalum is that the a-carbon is susceptible to electrophilic attack, in contrast to the electron-deficient a-carbon of Fischer-type carbene complexes of group 6 transition metals [62]. Based on this unique property of the alkylidene metal-carbon double bond, a range of new types of reactions has been developed. The discovery of the alkylidene complexes of tantalum was a key to understanding the mechanism of olefin metathesis, and they continue to play important roles in C—H bond activation, alkyne polymerization, and ring-opening metathesis polymerization. 

Amir-Ebrahimi, V. Corry, D. A. Hamilton, J. G Thompson, J. M. Rooney, J. J. Characteristics of RuCl2(CHPh)(PCy3)2 as a catalyst for ring-opening metathesis polymerization. Macromolecules 2002, 33, 717-724. 

Poly(norbornenes) containing pendant triarylamines have also been synthesized by ruthenium catalyzed ring-opening metathesis polymerization [102]. By varying the polarity and the length of the linker between polymer backbone and the triarylamine functionality the device characteristics could be tuned (Scheme 39). 

The highlighted peak at 968 cm-i characteristic of ring-opened pwlyfIXZPD) is paesent only in the brown spectrum. It dearly evidences that the embedded DCPD is active in the metathesis reaction. As shown by the FT/IR analysis reported in figure 4, the amoimt of DCPD encapsulated is a suffident quantity to adivate the ROMP readion [32 - see Supp. Inf.]. 

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