Ring well-defined transition metal-based

A major advance in polymerization via metathesis occurred when Schwab et al7 and Schrock unveiled well-defined transition metal catalysts. Molybdenum- and rathenium-based catalysts are highly active in olefin metathesis reactions. The general mechanism that underlies their reactivity is outlined in Figure 2. The catalyst reacts with the alkene via a (2 + 2] cycloaddition reaction to afford a metallocyclobutane intermediate. Cycloreversion with concomitant ring opening relieves strain and unleashes a new metal carbene that reacts with subsequent monomers to yield the polymer chain. 

Ni, Y, Rulkens, R., and Manners, I. (1996) Transition metal-based polymers with controlled architectures Well-defined poly(ferrocenylsilane) homopolymers and multiblock copolymers via the living anionic ring-opening polymerization of silicon-bridged [l]ferrocenophanes. Journal of the American Chemical Society, 118,4102. 

Though this book focuses on late transition metals, tlie few well-defined Group IVA- or VA-based systems that exist shall be outlined briefly. Among the first transition metal alkylidenes that were observed in ROMP was the bimetallic compound (Cp2TiCH2ClAlMe2) [29, 30]. Upon reaction with norborn-2-ene (bicyclo[2.2.1] hept-2-ene, NBE) at ambient temperature it forms the rnetallacycle shown in Fig. 5.1 which opens at 65 °C under ring-opening [31]. 

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